Chapter 11 Soils: Surveying and Exploration/Classification/Field Identification Topics

This lesson introduces geological and pedological (ped-l-oj-i-kuhl) surveys, and present various methods used for this type of surveying. It will also further your understanding of soils exploration and explain how to classify soils based on their textural and plasticity-compressibility characteristics using the Unified Soils Classification System. Finally, it will detail various field tests that are useful for expedient soil classification. The material in this lesson assumes that you, as a senior EA, are now knowledgeable of the physical properties of soils, and experienced with laboratory testing procedures that are necessary for accurate soil identification and classification such as mechanical analysis and Atterberg limits. If necessary, you may find it helpful to review EA Basic Chapter 16 Materials TestingSoil and Concrete before beginning this lesson.

When you have completed this lesson, you will be able to:

1. Identify the support requirements for geology and pedology surveys.
2. Describe the objectives of soil surveys.
3. Identify the different types of soil classifications.
4. Identify the different types of field identification.

Contents

1.0.0 Survey Support for Geology and Pedology

2.0.0 Soil Surveys

3.0.0 Soil Classification

4.0.0 Field Identification

Summary

Review Quiz


1.0.0 SURVEY SUPPORT FOR GEOLOGY and PEDOLOGY

This section will provide a brief familiarization with the topics of geological and pedological surveying and mapping. Rather than including them in a separate discussion in a topographic surveying lesson, they are included here because both are closely related to soil exploration and investigation.

1.1.0 Geological Surveys

Surveys supporting geology are essentially topographic surveys, but you must also be aware of other specialized data that the geologist or soil engineer may require when you are collecting data for the engineering studies of naval construction projects. Producing a topographic map is the end goal for most topographic surveys. However, in geology or other related sciences, the topographic survey is only the first part of a series of interrelated surveys. In geological surveys, the goal is a map that contains not only topographical information but also other specialized data keyed to it. In geologic surveys, a geologist makes systematic observations of the physical characteristics, distribution, geologic age, and structure of the rocks as well as the groundwater and mineral resources within the rocks. Geologic maps (usually with text) reflect these observations. (Figure 11-1) The objective of the geological survey is to portray, in plan or in cross section, geological data required for subsequent constructions or for other uses. Figure 11-1 — Typical geological survey map and text. Pure geological data has little direct application to blue water or littoral naval problems; however, if the raw field information is interpreted into specialized lines in plan or profile, it can be considerably useful in Naval Construction Force (NCF) planning and operations. NCF requirements may necessitate on different scales: regional geological study and mapping; surveys of more limited areas; or development of detailed geological data at a construction site.

1.1.1 Methods of Geological Surveying

A field examination of rocks provides most geological data, and an examination of detailed maps or aerial photographs provides considerable supplementary data for drainage and relief patterns, as well as rock structure and distribution. A geologist in the field surveys rock exposed at the surface, called an outcrop, then systematically records the physical characteristics of the rock, thickness of exposure, inclination of the rock, inclination of rock bedding, and development of joints or fractures. In addition, the age of the rock is determined from fossils or the layered sequence of rock units. Deeper-seated rocks are examined by using samples from auger or boreholes. Information gathered is placed on a map base by plotting the rock types in color and with other data included with symbols or annotations. Amplifying the map data, additional complete descriptions of outcrops are entered in notebooks with entries keyed to the field map. Surveyors support geologists by preparing basic topographic maps on which geologists plot the results of investigations. Surveyors then make such tie in measurements to geological features as the geologist may require. (Figure 11-2) Figure 11-2 — Example of geological map and associated topographical map. The geologist uses simple survey methods to plot geological features on a field map. When an outcrop can be located relative to a cultural or relief feature, it is usually plotted by spot recognition. In other cases, a magnetic compass, and pacing or taping are used to determine direction and distance of a geological feature to a recognizable topographic feature. A clinometer (klī-nŏm'ĭ-tər) or hand level is used to measure slope or small differences in elevation, and an altimeter is used where there are large differences in elevation. When the geological survey is keyed to a large-scale plan, the geologist generally uses a plane table and plots data with accuracy commensurate with the accuracy of the base plan. For low accuracy requirement, Seabees use Transit Stadia to collect topographic data.  11-5

1.1.2 Base Map Surveys

The survey for the base map should normally take place before the geological survey because the geologist uses the map in the field to determine his position by identifying topographic details and then plotting his data. If aerial photographs are available, the geologist can use them as a plotting base and later transfer the data to a surveyed base map. (Figure 11- 3) However, if possible, the survey base map should be prepared in advance since the number of aerial photographs needed to cover an area is generally too large for useful field application. Figure 11-3 — Example of aerial map of Figure 11- 2. In the absence of detailed instructions, the following specifications are generally satisfactory: 1. Base direction — To determine a base direction, take from a known base a side in a triangulation net or a course of a basic control traverse. 2. Local horizontal control — Use Transit table traverses run in closed circuits or between known control stations of a higher order of accuracy. 3. Local vertical control — Where the terrain is relatively level, carry elevation along traverses by vertical angle, adjusting elevations on closure at a basic control station. For rugged terrain mapped at one of the larger contour intervals or trigonometric leveling is suitable. 4. Sights — Use telescopic alidade. 5. Distance measurements — Use stadia or graphical triangulation to locate points and stations. Certain measurements can be made most conveniently by pacing or rough taping. 6. Contouring — Locate and determine the elevations of controlling points on summits, in valleys and saddles, and at points of marked change of slope. Interpolate and sketch contours in the field, using these elevations for control.  11-6 7. Accuracy — Distance measurements by stadia should be accurate to 1 part in 500. Side-shot points located by pacing or other rough measurements should be accurate to within 25 feet. Take sights for traverse lines or graphical triangulation with care to obtain the maximum accuracy inherent in the telescopic alidade. At any point on the finished map, elevation error should not exceed one half of a contour interval. In heavily timbered country, topography surveying by stadia measurements from transitstadia traverse may be more convenient than by plane table but plotting time will increase. The specifications listed above are generally applicable. Also:

• Read horizontal angles on traverses to 1 minute and horizontal angles for side shots that will be plotted by protractor to the nearest quarter of a degree.

• Read vertical angles for elevation determination to 1 minute or use the stadia arc.

• Keep complete and carefully prepared stadia notes and sketches to assure correct plotting. Either plane table or compass traverses are appropriate when the geologist indicates that a map of a lower order of accuracy will suffice.

1.1.3 Use of Aerial Photographs

A geologist will generally use aerial photographs instead of a map if they are available. Large-scale photographs, 1:15,000 or larger, provide the most satisfactory results. Some topographic features, such as shallow ravines, rocky knobs, or sinkholes, are too small to be shown on maps. With aerial photographs, these and other, larger topographic features, such as stream channels and swamps, can be observed directly. If the aerial photographs are in color, additional geographic details are more easily discernable. (Figure 11-4) The photos can also be used to prepare a base map for portrayal of the field data by tracing planimetric details from an uncontrolled mosaic with spot elevations added from field surveys. Figure 11-4 — Example of color aerial map of Figure 11-3.  11-7 The geologist may satisfactorily use contact prints of aerial photographs in place of the base map except where large-scale plans for engineering purposes are to be the base. For engineering purposes, the distortion within an aerial photograph does not permit plotting of geological data commensurate with the accuracy needed for the final plan.

1.1.4 Map Base for Detailed Geological Surveys

Detailed geological surveys generally cover a specific map area’s geographic region from scales of 1:62,500 to 1:600 or larger. In general, the very large scales are used for specific engineering or mineral development problems.

1.4.1 Site Plans and Profiles Geological data affecting the design of construction site foundations are typically plotted on plans drawn to scales of 1 inch = 50-, 100-, 200-, or 400-feet. Contour intervals may range from 1- to 10-feet, depending upon the roughness of the terrain. In addition to a topographical plan, a geologist may require profiles be drawn along selected lines, or boring logs of test holes be plotted to suitable scales.

1.

1.4.2 Using a Topographic Map as a Base Map A detailed geological survey’s base map must be a complete topographic map or plan with relief expressed by contours. Simple colors and symbolization of basic details should be used so they will not conflict with the geological information overlay, which is shown by colors and symbols. (Figure 11-5) Use published topographic maps where suitable. A geological survey is expedited if the map base is from a quarter to double the scale of the intended final presentation map. Enlargements of the base map generally satisfy these requirements. This permits the direct reduction of geological data to the scale of the final map with a minimum amount of drafting. Figure 11-5 — Example of topographical map with geological map overlay.  11-8 When existing maps are unavailable or unsuitable, a base map must be prepared from detailed topographic surveys. Culture and relief (contours) should be shown in the greatest possible detail. The survey for the base should conform to third-order accuracy where large geographic areas are concerned. In place of field surveys, maps made from aerial photographs by precise instrument methods can be used. Altitude (or elevation) of the intersection of boreholes and the surface should be accurate to the nearest one-half foot.

1.2.0 Pedological Surveys

Sometimes there is a requirement for pedological mapping to locate the limits of sand or gravel deposits suitable for concrete aggregates, road materials, or other construction operations. In such a case, the surveyor’s mission would be to support the soils engineer’s objective. In a pedological survey, the engineer’s objective is to prepare data in plan and profile symbolizing soils and outcropping on maps, overlays, and sketches for subsequent engineering uses. (Figure 11-6) Figure 11-6 — Example of geological map in plan and profile with text. A soils survey operation may use the following approaches:

• Aerial photography — used when an extensive area is to be surveyed; usually no survey measurements are required.  11-9

• Maps of an area that extend several square miles — required when an initial study or technical reconnaissance is needed; low-order survey measurements usually suffice for a reconnaissance sketch; the soils engineer can plot the pertinent data.

• A sketch — frequently required by the soils analyst before construction planning can be initiated, of an airfield, for example; the surveyor applies low-order measurements to prepare a sketch (1 inch = 100, 200, or 400 feet) upon which the soils engineer plots the results of soil tests and findings.

1.2.1 Aerial Photography

Photo coverage of the area under consideration aids in the establishment of control for the pedological survey. In the planning phase of outlining ground control, using vertical aerial photographs will speed the survey regardless of the size of the area to be covered. If controlled photographs are available, the survey engineer can locate points by pricking or keying them to the photographs. (Figure 11-7) An uncontrolled photograph may be satisfactory for surveys of low-order accuracy requirements. Figure 11-7 — Example of aerial photograph with keyed geological points. Terrain analysts are responsible for pedological interpretation of aerial photographs and the survey party chief prepares maps or overlays for plotting the controls and ties them to the pedological features according to the soils analyst’s instructions.  11-10

1.2.2 Traverse Using Transit

As previously mentioned, when maps of lower accuracy are acceptable, transit traverse (and compass traverse) are suitable. The transit traverse is best adapted to relatively open country for the preparation of the basic sketch for the soils engineer to plot pertinent data. In the absence of detailed instructions from the soils engineer, the following procedures are generally satisfactory for preparing a sketch of an area of several square miles (3 miles by 3 miles maximum for initial exploration): 1. Scale — 1:12,500 or 1:25,000. 2. Traverse control — Run in circuits or between known positions of a higher order of accuracy. 3. Sighting — Use a peep sight or telescopic alidade. 4. Distance measurements — Pace or obtain a rough measurement with tape. When a telescopic alidade is available, use stadia measurements where possible (to reduce the time required for the survey, rather than to increase the accuracy). 5. Base direction — To determine a base direction, select known bases: railroad or highway tangents, recognizable features, or reliable topographic maps. In the absence of these known bases, use magnetic north as determined by compass observations. 6. Compass — Use military compass, forestry compass, or pocket transit. 7. Distance between basic control points — Maintain 3 miles as the extreme maximum distance between stations. 8. Accuracy — Distances should be measured in such a manner that points can be plotted within 25 feet. For the scales suggested, measurements to 1 part in 100 will suffice. Take sights with peep-sight alidade carefully to maintain directions of accuracy comparable to distances. 9. Topography — Topography is usually not required on reconnaissance surveys for pedology, particularly in areas of low relief. Where suitable deposits of sand, gravel, or stone have been located, route surveys from the site to the point of use may be required for the location of haulage roads, conveyors, or other means of transporting the material. In hilly terrain, a rough topographic map, obtained by clinometer, pocket transit, or stadia, may be required to make the location of a favorable route easier.

1.2.3 Compass Traverse

Compass traversing is more convenient than Transit Stadia traversing for heavily wooded areas; however, more time is required for plotting by compass traverse. Keep traverse lines between stations long to reduce the number of observed bearings. Locate points between stations by offsets from the traverse lines. Where local attraction affects compass readings, plot points by intersection. Survey readings may be plotted in the field and notes should be kept in case the traverse must be retraced.  11-11

1.2.4 Field Sheets and Site Plans

The survey engineer must furnish suitable maps, overlays, and sketches for plotting pedological data to the soils analyst. After preparing a reconnaissance field sheet of several square miles, the soils analyst may require a sketch of a particular site in which many samples are taken for a more detailed study. In the absence of detailed instructions, the surveyor then prepares a sketch on a scale of 1 inch = 400 feet and provides ranges and reference points to aid in plotting or tying in specific positions of auger holes, drill holes, and lines of exposed rock or other pedological features. The soils analyst may require the surveyor provide a basic plot on a scale of 1 inch = 100 feet or of 1 inch = 200 feet for plotting the data of a range, cross section, or series of boreholes and survey measurements need to be conducted accordingly.

 

Test Your Knowledge

1. Surveys supporting geology are essentially _____ surveys.

A. plane table traverse
B. compass traverse
C. aerial photographic
D. topographic

 2.0.0 SOIL SURVEYS

A survey of soil conditions is vital to both the planning and execution of military construction operations. It provides information about the nature, extent, and condition of soil layers; the position of the water table; drainage characteristics; and sources of possible construction materials.

2.1.0 Objectives of a Soil Survey

A soil survey’s objective is to explore and gather as much information as possible of engineering significance pertaining to surface and subsurface conditions in a specified area. Soil surveyors collect laboratory test samples to determine if the existing soil conditions are capable of supporting the type of structure planned. If not, the project may need to relocate or add material for stabilization. The exploration uses specific procedures to determine the following information:

• Location, nature, and classification of soil layers

• Condition of soils in place (density and moisture content)

• Drainage characteristics

• Groundwater and bedrock

• Overall soil profile

2.1.1 Location, Nature and Classification of Soil Layers

The types and depths of soil must be known in order to design an appropriate and economical soils addition/removal and foundation. By classifying the soils (discussed later in this lesson), you can predict the probability and extent of problems concerning drainage, frost action, settlement, stability, and similar factors. You can initially estimate  11-12 the soil characteristics by field observations, but for laboratory testing, you need to obtain samples of the major soil types as well as any less extensive deposits that may influence design.

2.1.2 Condition of Natural Soils

A soil’s moisture content and density plays an important part in design and construction. In its natural state, the moisture content may be so high as to require a different site. If it is sufficiently dense and meets the required specifications, no compaction of subgrade is required. If extremely dense, soil lying in cut sections maybe be difficult to excavate with ordinary tractor-scraper units and may need to be scarified or rooted before excavation.

2.1.3 Drainage Characteristics

Both surface and subsurface drainage characteristics greatly affect a soil’s strength. A combination of factors controls this characteristic:

• void ratio

• soil structure and stratification

• soil temperature

• depth to water table

• extent of local disturbance by roots and worms Coarse-grained soils have better internal drainage than fine-grained soils.

2.1.4 Groundwater and Bedrock

All structures must be constructed at an elevation that ensures they will not be adversely affected by the groundwater table. If a structure’s proposed grade line lies below the elevation of the water table, the grade line must be raised or the water table must be lowered by artificial drainage. An unexpected discovery of bedrock within the limits of an excavation can greatly increase the time, equipment and cost of excavation. If the amount of rock is extensive, a change in grade or proposed site may be cost and time effective.

2.1.5 Field Notes and Soil Profile

The person in charge of a survey, either the soils engineer or EA, must keep accurate field notes and logs such as numbering and recording each boring, test pit, or other exploration investigation.  11-13 Figure 11-8 — Typical boring log. A detailed field log must be kept of each test hole, auger boring or test pit. It should show the depth below the surface (or the top and bottom elevations) of each soil layer, the field identification of each soil present, and the number and type of each sample taken. The log should also include other items of information: density of each soil, changes in moisture content, depth to groundwater; depth to rock. Figure 11-8 shows a typical boring log. When the survey is complete, the information in the separate logs needs to be consolidated. You should classify and show the:

• depth of soil layers in each log

• natural water contents of fine-grained soils (when possible) along the side of each log

• elevation of the groundwater table o This elevation is simply the level of any free water standing in the test hole. For a more accurate measurement, allow 24 hours to permit the water to reach maximum elevation before measuring it. A soil profile is a graphical representation, vertically through the soil layers, showing the location of test holes, any encountered ledge rock, natural ground profile to scale, field identification of each soil type, thickness of each soil stratum, profile of the water table, and profile of the finished grade line. (Figure 11-9)  11-14 Figure 11-9 — Typical soil profile. Use standard soil symbols to indicate the various soil layers, and standard procedure is to add the proper color symbols representing the various soil types you discover. A soil profile has many practical uses in the location, design, and construction of roads, airfields, dams, and buildings.

• It greatly influences the location of the finished grade line o These should be located to take full advantage of the best soils available.

• It shows whether planned excavated soils are suitable for use in embankments or whether borrow soils are required

• It may show the existence of undesirable soils, such as peat or other highly organic soils

• It may show the existence of bedrock encroaching into planned foundations

• It aids in planning drainage capabilities since these are planned to take advantage of well-draining soils

• It identifies any needed considerations relating to frost action when frostsusceptible soils are shown on the profile  11-15

2.2.0 Sources of Information

You may be able to secure published information and previous soil analyses without field exploration to locate, within a large general area, small areas that you want to investigate further. However, for final site selection, you must make field investigations. Published information resources can include engineer intelligence reports, geologic and topographic maps and reports, agricultural soil maps and reports, and air photographs. Intelligence reports that include maps and studies of soil conditions are usually available for areas in which military operations have been planned. Among the most comprehensive of these are the Terrain Intelligence Folios prepared by the Intelligence Branch of the U.S Army Corps of Engineers, in cooperation with the U.S. Geological Survey. (Figure 11- 10) Figure 11-10 — Terrain Intelligence Folio. The U.S. Geological Survey publishes folios with geologic maps and brief descriptions of regions or quadrangles. Geologic maps usually indicate the extent of formations (the smallest rock unit mapped) by means of letter symbols, color, or symbolic patterns. (Figure 11-11) Letter symbols on the map may indicate the location of sand and gravel pits; sometimes the back of the map sheet has a brief discussion entitled “Mineral Resources,” describing the location of construction materials. Figure 11-11 — Typical geologic map.  11-16 Ordinary topographic maps can be used in conjunction with geologic maps and may be of some use in estimating soil conditions. Observing contour lines provides clues to drainage patterns, which in turn can provide clues to the nature of rocks, depth of weathering, soil erosion, and drainage rates. (Figure 11-12) Figure 11-12 — Typical topographic map. Agricultural soils maps and reports are available for many of the agricultural areas of the world. Primarily concerned with surface soils to a depth of about 6 feet, the information can include topography, drainage, vegetation, temperature, rainfall, water sources, and rock location. Soils are usually classified according to texture, color, structure, chemical and physical composition, and morphology (topographic features produced by erosion). (Figure 11-13) Figure 11-13 — Typical agricultural soils map.  11-17 A trained observer can identify some soil types from clues in an aerial photograph such as landform, slopes, drainage patterns, vegetation, erosion characteristics, soil color or “tone,” and land use. (Figure 11- 14) Your use of aerial photographs to show and identify soils is based upon your ability to recognize typical patterns formed under similar conditions. The land’s configuration in different types of soil deposits is one definite characteristic that can be identified on aerial photographs; for example, a characteristic dune shape in desert areas indicates sand subject to movement by wind. Figure 11-14— Example of aerial photograph identifying soils. Prevailing ground slopes are also clues to the texture of the soil.

• Steep slopes are characteristic of granular materials.

• Relatively flat and smoothly rounded slopes may indicate soils that are more plastic. Drainage patterns tend to reflect underlying rock structure.

• The absence of surface drainage or a very simple drainage pattern often indicates pervious soil.

• A highly integrated drainage pattern often indicates impervious soils that are plastic and usually lose strength when wet. Erosion patterns often provide clues to the character of the soil. For instance, the cohesiveness of the soil controls the cross section or shape of a gully. Each abrupt change in grade, direction, or cross section indicates a change in soil profile or rock layers.

• Short, V-shaped gullies with steep gradients are typical of noncohesive soils.

• U-shaped gullies with steep gradients indicate deep, uniform silt deposits.

• Round, saucer-shaped gullies are a sign of cohesive soils. The color of soil is shown on aerial photographs by shades of gray ranging from almost white to almost black.

• Soft, light colors or tones generally indicate pervious, well-drained soils.  11-18

• Large, flat areas of sand are frequently indicated by uniform, light gray color tones, a flat appearance, and a lack of conformity; this indicates a natural surface drainage.

• Clays and organic soils frequently appear as dark gray to black areas.

• A sharp change in color tones usually represents a change in soil texture. The character of the vegetation may reflect the surface soil type; however, its significance is often difficult to interpret because of the effects of climate and other factors. Knowing how agricultural land is used often helps in soil identification.

• To those with local experience, both cultivated and natural vegetation cover are good indicators of soil type.

• Orchards require well-draining soils; therefore, the presence of an orchard implies a sandy soil.

2.3.0 Field Observations

You can use various types of published information and aerial photographs to explore a general area and narrow the selection of site down to several smaller areas suitable for further investigation. Available time and resources will determine the extent and method of collecting detailed information by field observations. When conditions do not permit a complete or deliberate soil survey, a rapid ground (hasty) observation along a proposed highway or airfield location may yield valuable preliminary information. Scrape off loose surface soils before examining and making any field identification. Observe the soil profile along the natural banks of streams, eroded areas, bomb craters, road cuts, or other places where you can observe the stratified areas. These may indicate the types and depths of soil layers. You can expose surface soils by pick and shovel, particularly in areas of questionable soils or at critical points in the location, to make preliminary determinations, and locate your identified hasty survey soils on the field sketches, available maps, or photographs. Samples may be taken from exposed soils for testing in a field laboratory; however, sampling and testing are normally at a minimum in this type of soil survey.

2.4.0 Methods for Collecting Samples

Deliberate investigations are carried out when you need a more thorough investigation of the subsoil and time and equipment are available. Test pits and test holes are the two most commonly used methods of obtaining soil samples for deliberate investigations. A test pit is an open excavation large enough for a man to enter and study the soil layers in the pit walls in an undisturbed condition. It is the preferred method for observing and collecting soil in its natural undisturbed condition. A test pit is usually dug by hand but if the equipment is available, excavation can be expedited by dragline, clamshell, bulldozer, backhoe, or a large 24-inch (diameter) power-driven earth auger.  11-19 Excavations below the groundwater table require the use of pneumatic caissons or the lowering of the water table. (Figure 11- 15) Load-bearing tests can also be performed on the soil in the bottom of a test pit. Figure 11-15 — Example of a pneumatic caisson. For test holes, a hand auger is the most common method of digging, principally at shallow depths. Best suited to cohesive soils, a hand auger can also be used on cohesionless soils above the water table, provided the bit clearance of the auger is larger than the diameter of the individual aggregate particles. By adding a pipe extension, an earth auger may reach a depth of about 30 feet in relatively soft soils. The sample is completely disturbed but is still satisfactory for determining soil profile, classification, moisture content, compaction capabilities, and similar properties. Wash boring is probably the most common commercial method used to make deep test holes in all soil deposits except rock or other large obstructions. The test hole is made by a chopping bit fastened to a wash pipe inside a 2-, 4-, or 6-inch (diameter) steel casing. The wash pipe is churned up and down while the bit, with water flowing under pressure, loosens the soil. The water then carries the soil particles to the surface where they can be collected. (Figure 11-16) Figure 11-16 — Example of a wash boring.  11-20 From the wash water’s appearance, an experienced operator can detect a change in the type of soil being penetrated. Samples taken directly from the wastewater (wash samples) are so disturbed, however, that their value is limited. This sampling method should not be used if any other means is available. Dry-sample boring uses wash boring only to sink the hole. Washing is stopped when a change of soil type occurs, or at specified depths, and the bit is replaced by a sampler. A sampler (an open-end pipe) is then driven into the minimally disturbed soil in the bottom of the hole to extract a sample. The sample is removed and preserved for laboratory testing. (Figure 11-17) Figure 11-17 — Example of a dry-sample boring. Undisturbed sampling, best obtained from relatively cohesive soils, is used to obtain samples with negligible disturbance and deformation for testing of shear strength, compressibility, and permeability. Methods to obtain undisturbed samplings are discussed in EA Basic. Core boring is used to obtain samples from boulders, ground rock, frozen ground, and highly resistant soils. Cutting elements may be diamond, chilled shot, or steel-tooth cutters. The drill cuts a ring in the rock around a central core. The core in the barrel of the drill is retained when the drill is removed. This is the best method for determining the characteristic and condition of subsurface rock. Figure 11-18 — Typical core boring cutters.  11-21

2.5.0 Planning Field Explorations

Soil tests from test holes or test pits should be made on samples that are representative of the major soil types in the area; therefore, their location depends upon the particular situation. The first exploration step in determining the representative soils is to develop a general picture of the subgrade conditions. Make full use of all existing data, from field reconnaissance to study landforms and soil conditions in ditches and cuts, to aerial photographs where techniques have been developed for delineating areas of similar soil conditions.

2.5.1 Subgrade Areas

For road or for airport facilities such as runway, taxiway, and apron construction, the next step after field reconnaissance is usually to make preliminary borings at strategic points to determine subgrade conditions. Arbitrarily spacing borings at uniform intervals is not recommended; that process does not give a true picture. Instead, you should strategically space preliminary borings to obtain the maximum amount of information with a minimum number of borings. Take soil samples in these preliminary borings for classification purposes. After classifying the preliminary samples, develop soil profiles and select representative soils for detailed testing. Following your soil profiles, excavate test pits or larger diameter borings to obtain the samples needed for laboratory testing or to permit in-place tests. The types and number of required samples will depend on the characteristics of the subgrade soils. In areas of proposed pavement, subsoil investigations must include measurements of in-place water content, density, and strength to determine the required compaction depth, and to ascertain the presence of any soft layers.

2.5.2 Borrow Areas

When planning to borrow material from adjacent areas, take borings samples 2 to 4 feet below the anticipated depth of the borrow. Classify and test samples for water content, density, and strength. Explore areas within a reasonable haul for select material to use as subbase. Exploration procedures are similar to those described for subgrade areas, but for gravelly materials you will need test pits or large borings drilled with power augers.

2.6.0 Recommended Procedures for Soil Surveys

When conducting soil surveys, use the following guide and step-by-step procedures to assist you in the process:

• Considerations to include: o soil types o securing the samples o density and moisture content of soil in place o drainage characteristics o depth to groundwater and bedrock

• Use published information including: o geological and topographic reports with maps  11-22 o agricultural soil bulletins with maps (These require careful interpretation and knowledge of local terms.) o aerial photographs to predict subsurface conditions o previous explorations for nearby construction projects

• Gather field information: o general observation of road cuts, stream banks, eroded slopes, earth cellars, mine shafts, and existing pits and quarries o test holes made with a hand auger or a power auger o test pits where hand auger cannot penetrate or large samples are required

• Gather information from local inhabitants: o trained observers such as contractors, engineers, quarry workers 2.6.1 Preparation Planning for the project’s general layout will determine the areas and the extent of earthwork that may occur to disturb the various soil types, both vertically and laterally. Large cuts and fills are the most important areas for detailed exploration. For airfield exploration, place borings at high and low spots, wherever a soil change is expected, and in transitions from cut to fill. There is no maximum or minimum spacing requirement between holes; however, the number of holes must be sufficient to give a complete and continuous picture of the soil layers throughout the area of interest. Typically, flat terrain with uniform soil conditions requires fewer exploration borings than terrain where the soil conditions change frequently. Conduct exploration borings at points of interest and disburse them to get the maximum value for each boring. This may require exploration borings in the centerline as well as edges of runways or roads, but do not employ a specific pattern except perhaps a staggered or offset pattern to permit the greatest coverage. Accepted policy is to conduct exploration borings at the edge of existing pavements unless these pavements have failed completely; in that case, the reason for the failure should be found. For depth exploration, if possible, take a cut section sample 4 feet below subgrade, a fill section sample 4 feet below original ground level, and make the effort locate the groundwater table.

Procedures

  1. Log the exploration holes or pits.
  2. Locate and number the samples.
  3. Determine the elevation and exact location of each hole and tie the data results into the site layout.
  4. Technical Soils Report A good program for soils testing requires:
  5. • Careful and complete tests
  6. • Test completion as quickly as possible
  7. • Test data be clearly and accurately presented in a technical soils report

How you present a soils report is very important. It must be well-organized, and presented in a logical and concise format with emphasis on technical conclusions.

 

Test Your Knowledge

2.  Soil surveys, vital to the planning element of military construction operations, are completed before the execution phase and unnecessary during construction.

  • A. True
  • B. False
  • 3.0.0 SOIL CLASSIFICATION

    Soil classification is a process intended to enable the prediction of engineering properties, and thus the behavior of a soil, based on a few simple laboratory or field tests. The test results help to identify a soil and classify it as a group of soils that have similar engineering characteristics. Regional authorities throughout the world have established several different methods of soil classifications; the United States military has adopted the Unified Soil Classification System (USCS).  11-24 Table 11-1 — Unified Soil Classification System (USCS) Major Divisions Group Symbols* Typical Names Coarse-Grained Soils More than half of material is larger than 200 sieve size. Gravels More than half of coarse fraction is larger than No. 4 sieve size Clean Gravels GW Well-graded gravels, gravel-sand mixtures, little or no fines. GP Poorly graded gravels, gravel-sand mixtures, little or no fines. Gravels With Fines GM Silty gravels, gravel-sand-silt mixtures. GC Clayey gravels, gravel-sand-clay mixtures. Sands More than half of coarse fraction is smaller than No. 4 sieve size Clean Sands SW Well-graded sands, gravelly sands, little or no fines. SP Poorly graded sands, gravelly sands, little or no fines. Sands With Fines SM Silty sands, sand-silt mixtures. SC Clayey sands, sand-clay mixtures. Fine-Grained Soils More than half of material is smaller than 200 sieve size. Silts and Clays Liquid limit less than 50 ML Inorganic silts and very fine sands, rock flour, silty or clayey fine sands or clayey silts with slight plasticity. CL Inorganic clays of low to medium plasticity, gravelly clays, sandy clays, silty clays, lean clays. OL Organic silts and organic silty clays of low plasticity. Silts and Clays Liquid limit greater than 50 MH Inorganic silts, micaceous or diatomaceous fine sandy or silty soils, elastic silts. CH Inorganic clays of high plasticity, fat clays. OH Organic clays of medium to high plasticity, organic silts. Highly Organic Soils Pt Peat and other highly organic soils. * Boundary classifications. Soils possessing characteristics of two groups are designated by combinations of group symbols, for example GW-GC, well graded gravel-sand mixture with clay binder. Soils seldom exist naturally separated as sand, gravel, or any other single component. They are usually compounds of varying proportions of different size particles with each component contributing to the mixture’s characteristics. Textural or plasticitycompressibility characteristics indicate how a soil will behave as a construction material; this forms the basis for the USCS’s classifications. The USCS classifies all soils by three major divisions: (1) coarse-grained, (2) finegrained, and (3) highly organic. From previous studies, you should recall that coarsegrained and fine-grained soils are distinguished by the amount of material retained or passing a No. 200 sieve.  11-25 If 50 percent or more of the soil by weight is retained on a No. 200 sieve, the soil is coarse-grained. If 50 percent or more of the soil by weight passes a No. 200 sieve, the soil is finegrained. Highly organic soils can generally be identified by visual examination. The major divisions are further subdivided into soil groups. The USCS uses 15 groups and each group is distinguished by a descriptive name and letter symbol, as shown in Table 11-1. The letter symbols are derived either from the terms descriptive of the soil fractions (G, S, M, C, O, Pt) the relative value of the liquid limit (L, H), or the relative gradation of the soil (W, P). Note in Table 11-1 group symbol notes how the letters are used in combinations to form the 15 soil groups.

    3.1.0 Coarse-Grained Soils

    Refer to Table 11-1. Coarse-grained soils are further divided into two major divisions: gravels and sands. If more than half of the coarse fraction by weight is retained on a No. 4 sieve, the soil is a gravel; if more than half of the coarse fraction is smaller than a No. 4 sieve, it is classed as a sand. There is no clear-cut boundary between gravelly and sandy soils, and the exact point of division is relatively unimportant as far as behavior is concerned. To classify or describe a soil where a mixture occurs, use a noun to name the predominant fraction and an adjective for the minor fraction. For example, a sandy gravel is a mixture containing more gravel than sand by weight, whereas a gravely sand contains more sand than gravel. To systematize the classifications, coarse-grained soils are further divided into groups based on the amount of fines (materials passing a No. 200 sieve) they contain. (Figure 11-19)  11-26 3.1.1 GW, GP, SW, and SP Groups Coarse-grained soils with less than 5- percent fines may fall into the groups GW, GP, SW, or SP. The shape of the grain size distribution curve on DD Form 1207 determines the second letter of the symbol. 3.

    1.1.1 GW and SW Groups The GW group contains well-graded gravels and sandy gravels with little or no fines. The SW group contains well-graded sands and gravelly sands with little or no fines. The fines must not noticeably change the strength characteristics of the coarsegrained fraction or interfere with its freedraining characteristics. 3.

    1.1.2 GP and SP Groups The GP group contains poorly graded gravels and sandy gravels with little or no fines. The SP group contains poorly graded sands and gravelly sands with little or no fines. These soils will not meet the gradation requirements established for the GW and SW groups. The poorly graded groups are predominately composed of one particle size or range of sizes with some intermediate sizes missing. Figure 11-19 — USCS with fines criteria. 3.1.2 GM, GC, SM, and SC Groups Coarse-grained soils containing more than 12-percent fines may fall into the groups designated GM, GC, SM, and SC. These symbols, M (silt) and C (clay), are based upon the plasticity characteristics of material passing the No. 40 sieve.  11-27 The liquid limit (LL) and plasticity index (PI) are used to specify the laboratory criteria for these groups. Note the plasticity chart shown in Figure 11-20 which is based upon established relationships between the liquid limit and plasticity index for many different finegrained soils. The symbol M indicates that material passing the No. 40 sieve is silty in character, usually designating a finegrained soil of little or no plasticity. The symbol C indicates that the binder soil is predominately clayey in character. Figure 11-20 — Example of a plasticity chart. 3.

    1.2.1 GM and SM Groups

     Silty gravels and gravel-sand-silt mixtures are typical soils included in the GM group. Similarly, silty sands and sand-gravel silt mixtures fall into the SM group. For both of these groups, the Atterberg limits must plot below the A-line, and be less than 4 on the plasticity index. (Figure 11-20) 3.

    1.2.2 GC and SC Groups The GC group includes clayey gravels and gravel-sand-clay mixtures. Similarly, SC includes clayey sands and sand-clay mixtures. For both of these groups, the Atterberg limits must plot above the A-line with a plasticity index for more than 7. (Figure 11-20) 3.1.3 Borderline Soils Coarse-grained soils containing 5 to 12 percent of material passing the No. 200 sieve are classed as borderline, and given a dual symbol such as GW-GM. Figure 11-21 — Example of course-grain symbol criteria.  11-28 Moreover, coarse-grained soils containing more than 12 percent material passing the No. 200 sieve, but whose limits plot in the shaded portion of the plasticity chart of Figure 11-21, are also classified as borderline and require dual symbols, such as SM-SC. In rare instances, it is possible for a soil to fall into more than one borderline zone. In such a case, using appropriate symbols for each possible classification would result in a multiple designation of three or more symbols; this approach is unnecessarily complicated. Use only a double symbol in these cases; select the two that you believe to be most representative of the soil’s probable behavior and use the symbols representing the poorer qualities of the possible groupings. For example, a well-graded sandy soil with 8 percent passing the No. 200 sieve, an LL of 28 and a PI of 9 would be designated as SW-SC. If the Atterberg limits of this example indicated a plot in the shaded portion of the plasticity chart (for example, LL 20 and PI 5), the soil can be designated either SW-SC or SW-SM; depending on the judgment of the engineer from the standpoint of the climatic region in which the soil is located.

    3.2.0 Fine-Grained Soils

    The fine-grained soils are classified based on plasticity and compressibility rather than grain size distribution, that is, the relationship between the liquid limit and plasticity index as designated in the plasticity chart in Figure 11-20. This chart was established by the determining of limits for many soils, together with an analysis of the effect of limits upon physical characteristics. Fine-grained soils are defined by two major groupings; the L groups, which have liquid limits less than 50, and the H groups, which have liquid limits equal to or greater than 50. The symbols L and H have general meanings of low and high compressibility, respectively. Fine-grained soils are further divided with relation to their position above, clay (C), or below, silt (M), the A-line of the plasticity chart.

    3.2.1 ML and MH Groups

    The ML (low compressibility) and MH (high compressibility) groups are inorganic silts, all of which plot below the A-line of the plasticity chart. The ML group includes very fine sands, rock flours (rock dust), and silty or clayey fine sand, or clayey silts with low plasticity. Loess (loh-es) type soils usually fall into this group. Diatomaceous (dahy-uh-tuh-mey-shuhs) and micaceous (mahy-key-shuhs) soils usually fall into the MH group but may fall into the ML group when the liquid limit is less than 50. Plastic silts fall into the MH group. 3

    .2.2 CL and CH Groups

    The symbol C stands for clay, with L and H denoting low or high liquid limits. Clay soils plot above the A-line and are principally inorganic clays. The CL group includes gravelly clays, sandy clays, silty clays, and lean clays. The CH group includes inorganic clays of high plasticity.  11-29

    3.2.3 OL and OH Groups

    The symbol O indicates the presence of organic matter, and all of these soils generally plot below the A-line. The OL group includes organic silts and organic silt-clays of low plasticity, while organic clays of high plasticity plot in the OH zone. Along the lower reaches of the Atlantic seaboard, many of the organic silts, silt-clays, and clays deposited by rivers have liquid limits above 40 and plot below the A-line. Peaty soils may have liquid limits of several hundred percent and plot well below the Aline because of their high percentage of decomposed vegetation. However, a liquid limit test is not a true indicator in samples that contain a considerable portion of material other than soil, that is, organic matter.

    3.2.4 Borderline Soils

    Borderline cases and are given dual symbols, such as CL-ML. for fine-grained soils with limits that plot in the shaded portion of the plasticity chart. There are several soil types exhibiting low plasticity, where no definite boundary between silty and clayey soils exists, that plot in the general region of the shaded portion on the chart.

    3.3.0 Highly Organic Soils

    Pt is a special classification reserved for highly organic soils, such as peat, which have characteristics unsuitable for foundations or use as construction material. Particles of leaves, grass, branches, or other fibrous vegetable matter are common components of these soils. Since these highly organic soils can be readily identified in the field by their distinctive color, odor, spongy feel, and fibrous textures, there is no established laboratory criteria.

    3.4.0 Coefficient of Uniformity

    Note in Figure 11-22, that wellgraded gravels (GW) and wellgraded sands (SW) must meet certain requirements with regard to Cu and Cc . Cu means the coefficient of uniformity with regard to the plotted grain size curve for the sample material. The following example demonstrates how to determine coefficient of uniformity, Cu . Figure 11-22 — Example of classification criteria.  11-30 Example: The sieve analysis of a soil sample identified as FT-P1-1 is as follows: Sieve % Passing 3/8 100.0 No. 4 85.8 10 74.4 20 51.2 40 30.2 100 16.3 200 3.1 Plot these values on a DD 1207 form like the one shown in Figure 11-23. This form is a logarithm type of graph layout. Horizontal coordinates are sieve sizes (top) and grain sizes in millimeters (bottom); vertical coordinates are percent passing (left), percent retained (right). Figure 11-23 — Grain size distribution chart (DD 1207) for soil sample FT-P1-1.  11-31 The formula for determining Cu is: D D Cu 10 60 = where D60 is the indicated grain size, in millimeters, at the 60-percent passing level and D10 is the indicated grain size, in millimeters, at the 10-percent passing level. In Figure 11-23, follow the 60-percent passing line to the point where it intersects the gradation curve for FT-P1-1, then drop down and read the grain size in millimeters indicated below. It reads about 1.25mm. Now, similarly follow the 10-percent passing level, then drop down and read the grain size in millimeters indicated below. It reads about 0.11mm. For this sample, then, Cu is 11.0 25.1 or about 11.4. 3.5.0 Coefficient of Curvature Cc means coefficient of curvature of the gradation curve. [Sometimes Cg (coefficient of gradation) is used instead ofCc ] The formula for determining Cc is: ( ) DD D C x C 10 60 2 30 = where D30 is the indicated grain size, in millimeters, at the 30-percent passing level. In Figure 11-23, follow the 30-percent passing line to the point where it intersects the gradation curve, then drop down and read the grain size in millimeters indicated below. It reads about 0.35mm. For this sample, then, ( ) 25.111.0 35.0 2 C x c = or 1375.0 1225.0 C =c or about 0.89. FT-P1-1 is: 1. a sand, since more than half of its coarse fraction passes the No. 4 sieve. (Refer to Figure 11-19) 2. a clean sand, since less than 5 percent of it passes the No. 200 sieve. (Refer to Figure 11-21) 3. not a well-graded sand (SW), because although its Cu is greater than 4, its Cc is less than 1, the minimum prescribed for SW. (Refer to Figure 11-22) Therefore, FT-P1-1 is in the SP (poorly graded sands, gravelly sands, little or no fines) category. 3.6.0 Sample Classification Problems The following soil classification problems show how the soil classification chart is used to classify soils. (Figure 11-24)  11-32 Figure 11-24 — Typical soil classification chart.  11-33 Sample Problem A. Sieve analysis shows a Cu of 20, a Cc of 1.3, 12-percent gravel, 88-percent sand, and no fines (smaller than No. 200). 1. What is the first letter of the symbol, is the sample coarse-grained or finegrained? a. To be Coarse-grained, a soil must have less than 50-percent fines. b. This soil contains no fines; therefore, it is coarse-grained with the first letter either G (gravel) or S (sand). c. It contains more sand (88 percent) than gravel (12 percent). Therefore, the first letter is S. 2. What is the second letter of the symbol, is the sample well-graded, poorlygraded, or have plasticity characteristics? a. To have plasticity characteristics, a soil must contain fines. b. This soil contains no fines, therefore, it has no plasticity characteristics, so the second letter of the symbol must be W (well-graded) or P (poorlygraded). c. The soil has a Cu greater than 4 and a Cc between 1 and 3; it meets the criteria for well-graded. Therefore, the symbol for this soil is SW, meaning “well-graded sand.” Sample Problem B. Sieve analysis shows 50-Percent gravel, 20-percent sand, and 20-percent fines. Plasticity tests show an LL of 35 and a PI of 8, on the portion passing the No. 40 sieve. 1. What is the first letter of the symbol, is the sample coarse-grained or finegrained? a. To be Coarse-grained, a soil must have less than 50-percent fines. b. This soil contains less than 50-percent fines, therefore, it is coarse-grained with the first letter either G (gravel) or S (sand). c. Gravel predominates over sand. Therefore, the first letter is G. 2. What is the second letter of the symbol, is the sample well-graded, poorlygraded, or have plasticity characteristics? a. Does the soil contain more than 12-percent fines? The answer is yes (sieve analysis shows 20-percent fines), so the second letter in the symbol must be either C (clay) or M (silt) and well- or poorly-graded designation does not apply. b. Is it nonplastic? The answer is no, an LL and PI have been obtained. Potting LL 35 and PI 8 on the plasticity chart finds the plotted point lies below the A-line. Therefore, the symbol for this soil is GM, meaning “silty gravel.”  11-34 Sample Problem C. Sieve analysis shows 10-percent sand, 75-percent fines. Plasticity tests show an LL of 40 and a PI of 20 on the portion passing the No. 40 sieve. 1. What is the first letter of the symbol, is the sample coarse-grained or finegrained? a. To be Coarse-grained, a soil must have less than 50-percent fines. b. This soil contains more than 50-percent fines; therefore, it is a fine-grained soil with the first letter in the symbol is either O (organic), M (silt), or C (clay). c. Assume the soil shows no indication of being organic (principal indications are black color and musty odor), so the first letter must be either M or C. d. Potting LL 40 and PI 20 on the plasticity chart finds the plotted point lies above the A-line. Therefore, the first letter in the symbol is C. 2. What is the second letter of the symbol, does the sample have plasticity characteristics? a. Potting LL 40 and PI 20 on the plasticity chart finds the plotted point lies to the left of the B-line. b. The liquid limit is less than 50. Therefore, the second letter of the symbol is L (low plasticity or compressibility). Therefore, the symbol for this soil is CL, meaning “inorganic clays of low to medium plasticity/compressibility.”

     

    Test Your Knowledge

     3. Regional authorities throughout the world have established several different methods of soil classifications; the United States military has adopted the _____.

    A. Natural Resources Conservation Service (NRCS)
    B. Unified Soil Classification System (USCS)
    C. National Cooperative Soil Survey (NCSS)
    D. American Association of State Highway and Transportation Officials (AASHTO)

    4.0.0 FIELD IDENTIFICATION

    In military construction, lack of time and facilities may make laboratory soil testing impossible, but even when laboratory tests will follow, field identification tests must be made during soil exploration to keep duplicate laboratory testing to a minimum. This section will describe several simple tests for use in field identification with a minimum of time and equipment. However, consider classifications derived from these tests as approximations. The number of tests used will depend on the type of soil being tested and the experience of the individual performing the field tests. Experience is the greatest asset in field identification, and learning the technique from an experienced technician is the best method of acquiring the skill. If an experienced technician is unavailable, you can gain experience yourself by getting the “feel” of the identified soil during laboratory testing.  11-35 You can make an approximate identification by spreading a dry sample on a flat surface and examining it. Pulverize the lumps until individual grains are exposed but not broken; breaking grains changes the character of the soil. Use a rubber-faced or wooden pestle if possible, but for an approximate identification, you can mash a sample underfoot on a smooth surface. You may perform field tests with little or no equipment, other than a small amount of water, but accuracy and uniformity of results will be greatly increased by properly using certain equipment such as the following:

    • Sieves — A No. 40 U.S. standard sieve, or any screen with about 40 openings per lineal inch, may be the most useful item. You can make an approximate separation by sorting the materials by hand. (Note: a No. 4 sieve is used to separate gravel and sand, and a No. 200 sieve for fines.)

    • Pioneer tools — Essential for hand excavation. Use a pick and shovel or a set of entrenching tools for collecting samples, a hand auger if you need samples from more than a few feet below the surface.

    • Stirrer — A mess kit spoon can serve to mix materials with water to the desired consistency, or aid in collecting samples.

    • Knife — A combat knife or pocketknife is useful for collecting samples and trimming them to the desired size.

    • Mixing bowl — A small bowl with a rubber-faced pestle to pulverize the finegrained portion of the soil. Both may be improvised, such as a canteen cup and wood pestle.

    • Paper — Several sheets of heavy paper for rolling samples.

    • Pan and heating element — A pan and heating element to dry samples.

    • Scales —Balances or scales to weigh samples. As you have learned already, the Unified Soil Classification System, as shown in Figures 11-23 & 11-24, considers three soil properties: 1. percentage of gravel, sand, or fines 2. shape of the grain size distribution curve 3. plasticity However, you should also include other observed properties in a soil description, whether made in the field or in the laboratory. The following are typical samples of characteristics used in describing soil:

    • “Dark brown to white.” (or any suitable color shade description)

    • “Coarse-grained, maximum particle size 2 ¾ inches, estimated 60-percent gravel, 36-percent sand, and 4-percent fines.” (passing through No. 200 sieve)

    • “Poorly graded.” (gap-graded, insufficient fine gravel)

    • “Gravel particles subrounded to rounded, predominately gravel.”

    • “Nonplastic.”

    • “Mostly sand with a small amount of nonplastic fines.” (silt)

    • “Slightly calcareous, no dry strength, dense in the undisturbed state.”  11-36

    4.1.0 Visual Examination

    Visual examination should establish color, grain size, grain shapes (of the coarsegrained portion), some idea of the gradation, and some properties of the undisturbed soil. Color is helpful in distinguishing between soil types, and may be useful in identifying the particular soil type. With experience, by recognizing colors, you may be able to identify the presence of certain chemicals. The color of a soil often varies with moisture content, so you should include the moisture content at the time of color identification. The following are some of the more familiar color properties:

    • Generally, colors become darker as the moisture content increases and lighter as the soil dries.

    • Some fine-grained soils with dark drab shades of brown or gray, including almost black, (OL, OH) contain organic colloidal matter.

    • Clean, bright looking shades of gray, olive green, brown, red, yellow, and white are associated with inorganic soils.

    • Gray-blue or gray- and yellow-mottled colors frequently result from poor drainage.

    • Red, yellow, and yellowish brown indicate the presence of iron oxides.

    • White to pink may indicate considerable silica, calcium carbonate, or aluminum compounds. Always estimate (if not measured) the maximum particle size of each sample, this establishes the upper limit of the gradation curve. Gravels range down to the size of peas; sands start just below this size and decrease until individual grains can barely be seen by the naked eye. The eye can normally see individual grains about 0.05mm in size or about the size of the No. 200 screen. Thus silt and clay particles (which are smaller than this dimension) are not detected as individual grains. While examining the grains for sizes, determine the grain shapes as well. Sharp edges and flat surfaces indicate an angular shape; smooth, curved surfaces are associated with a rounded shape. Particles may not be completely angular or completely rounded; these are called subangular or subrounded, depending on which shape predominates. When you need accurate grain size distribution, you must perform laboratory analysis, but you can approximate the distribution by visual examination with the following steps: 1. Separate the larger grain particles from the rest of the soil sample by picking them out one at a time. 2. Examine the remainder of the soil and estimate the proportion of visible individual particles (larger than the No. 200 sieve) and the fines. 3. Convert these estimates into percentages by weight of the total sample. If the fines exceed 50 percent, the soil is considered fine-grained (M, C, or O); if the coarse material exceeds 50 percent, the soil is coarse-grained (G or S). 4. Examine the coarse-grained soil for gradation of particle sizes from the largest to the smallest. A good distribution of all sizes without too much or too little of any  11-37 one size means the soil is well-graded (W).Overabundance or lack of any size means the material is poorly graded (P). 5. Estimate the percentage of the fine-grained portion of the coarse-grained soil. If nonplastic fines are less than 5 percent of the total, the soil maybe classified either as a GW, GP, SW, or SP type, depending on the other information noted above. 6. If the fine-grained portion (Step 5 above) exceeds 12 percent, the soil is either silty (M) or clayey(C) and requires further testing to identify. 7. Fine-grained portions (Step 5 above) between 5- and 12-percent (nonplastic fines or fines not interfering with drainage, or 0 to 12 percent plastic fines) total are borderline and require a double symbol (GW-GM or SW-SM). 8. Fine-grained soils (M, C, or O) from Step 3 above require other tests to distinguish them further. Note: Grain size distribution of fine portions is normally not performed in field identification. However, if necessary, you can approximate the grain size distribution of fines by shaking them in a jar of water and allowing the material to settle. The material will settle in layers of different sizes, then you can estimate the proportions. Keep in mind that gravel and sand settle into a much denser mass than either clay or silt. Using the characteristics determined up to this point, you can evaluate the soil as it appeared in place (undisturbed). Gravels or sands can be described qualitatively as loose, medium, or dense; clays may be hard, stiff, or soft. The ease or difficulty of removing the sample from the ground is a good indicator. Cultivated or farmed soils can be further evaluated as loose and compressible; highly organic soils can be spongy and elastic. Furthermore, the soil’s moisture content always influences the in-place characteristics and you should recognize and report this condition with the undisturbed soil properties.

    4.2.0 Breaking or Dry Strength Test

    Perform the breaking test only on material passing the No. 40 sieve. Use it, as well as the roll test and the ribbon test, to field measure the soil’s cohesive and plastic characteristics. Make the test on a small pat of soil about ½-inch thick and about 1½-inches in diameter. Prepare the pat by molding a portion of the soil, in a wet-plastic state, into the size and shape desired, then allow the pat to dry completely. You may also test samples for dry strength in their natural condition as they are found in the field. However, do not depend too much on such tests because of the variations that exist in the drying Figure 11-25 — Typical breaking (dry-strength) test.  11-38 environment under natural field conditions. You may approximate the dry strength by such a test however, and verify it later by a carefully prepared sample. After the prepared sample is thoroughly dry, attempt to break it using the thumbs and forefingers of both hands (Figure 11-25). If you are able to break it, then try to powder it by rubbing it with the thumb and fingers of one hand. Typical reactions obtained for various types of soils are described below.

    • Very highly plastic soils (CH). The pat cannot be broken or powdered by finger pressure.

    • Highly plastic soils (CH). The pat can be broken with great effort, but cannot be powdered.

    • Medium plastic soils (CL). The pat can be broken and powdered with some effort.

    • Slightly plastic soils (ML, MH, or CL). The pat can be broken quite easily and powdered readily.

    • Nonplastic soils (ML, MH, OL, or OH). The pat has little or no dry strength and crumbles or powders when picked up.

    4.3.0 Roll or Thread Test

    Perform this test only on the material passing a No. 40 sieve. Mix a representative portion of the sample with water until you can be mold or shape it without sticking to your fingers. You refer to this moisture content as being just below the sticky limit. Prepare a nonabsorbent rolling surface by placing a sheet of glass or heavy wax paper on a flat or level support. Shape the sample into an elongated cylindrical shape and place it on this surface. Then attempt to roll the cylindrical sample rapidly into a thread approximately 1/8 inch in diameter (Figure 11-26). Figure 11-26 — Typical roll or thread test. If it rolls into a thread, it has some plasticity. The number of times it can be reassembled and rolled into a thread without crumbling is an indicator of the degree of plasticity. Materials that cannot be rolled in this manner have extremely low plasticity or are nonplastic.  11-39 Typical reactions obtained for various soils include:

    • High plasticity (CH). The soil can be molded into a ball or cylinder and deformed under firm finger pressure without crumbling or cracking.

    • Medium plasticity (CL). The soil can be molded, but it cracks or crumbles under finger pressure.

    • Low plasticity (CL, ML, or MH). The soil cannot be lumped into a ball or cylinder without breaking up.

    • Organic material (OL or OH). The soil forms a soft, spongy ball or thread when molded.

    • Nonplastic soil (ML or MH). The soil cannot be rolled into a thread at any moisture content. The cohesiveness of the material near the plastic limit may also be identified from the thread test and described as weak, firm, or tough. The higher the soil is on the plasticity chart, the stiffer the threads are as they dry out, and the tougher the lumps are if the soil is remolded after rolling.

    4.4.0 Ribbon Test

    Perform the ribbon test only on the material passing the No. 40 sieve. Prepare the sample for this test with a moisture content slightly below the sticky limit, and form a roll about ½-inch to ¾-inch in diameter about 3- to 5-inches long. Place it in the palm of your hand and, starting at one end, squeeze it between your thumb and forefinger to flatten the roll and form a ribbon 1/8 to 1/4 inch thick. (Figure 11- 27). Handle the sample carefully to form the maximum length of ribbon that can be supported by the cohesive properties of the material. Typical reactions obtained for various soils indicate: (CH) The material is considered both highly plastic and highly compressive if the soil sample holds together for a length of 6 to 10 inches without breaking. (ML or MH) It is nonplastic if the soil cannot be ribboned. (CL) The soil is considered to have low plasticity if it can be ribboned only with difficulty into short lengths. The roll test and the ribbon test complement each other in giving a clearer picture of the degree of plasticity of soil. Figure 11-27 — Typical ribbon test.

    4.5.0 Wet-Shaking Test

    Perform the wet-shaking test only on the material passing the No. 40 sieve. Prepare a sample of enough material to form a ball about ¾- inch in diameter moistened with water just below the sticky limit. Placed the sample in the palm of your hand and shake it vigorously by jarring the hand on the table or some other firm object, or by jarring it against the other hand. The soil has given a reaction to this test if, when shaken, water comes to the surface producing a smooth, shiny appearance. This appearance is frequently described as livery (Figure 11- 28). Figure 11-28 — Typical wet-shaking test. Then squeeze the sample between the thumb and forefinger of the other hand and the surface water quickly disappears with the surface becoming dull. As pressure continues the sample becomes firm, resists deformation, and cracks occur, finally crumbling like a brittle material. The vibration caused by shaking tends to reorient the soil grains, decrease the voids, and force water, held within the voids, to the surface. Pressing the sample between fingers tends to disarrange the soil grains and increase the void spaces with the water redrawn into the soil. If the water content is still adequate, shaking the broken pieces will cause them to liquefy again and flow together allowing the complete cycle to be repeated. This process can occur only when the solid grains are bulky in shape and noncohesive in character. Very fine sands and silts fall into this category and you can readily identify them by the wet-shaking test. Fine sands and silts rarely occur without some amount of clay mixed with them, so there are varying degrees of reaction to this test. Even a small amount of clay tends to retard this reaction greatly. Some of the descriptive terms applied to the different rates of reaction to this test are as follows:

    4.6.0 Odor Test

    Organic soils of the OL and OH groups usually have a distinctive, musty, slightly offensive odor. With experience, you can use this odor, especially apparent from fresh samples, as an aid in identifying these groups. The odor gradually reduces when exposed to air but can become effective again when heating a wet sample. Organic soils are not desirable as foundation or base course material and are usually removed from the construction site as waste material.

    4.7.0 Bite or Grit Test

    The bite or grit test is a quick and useful method used to identify sand, silt, or clay. For this test, grind a small pinch of solid material lightly between your teeth. You can then identify the soils as follows:

    4.8.0 Slaking Test

    Use the slaking test to assist in determining the quality of certain soil shales and other soft rocklike materials. For this test, dry the soil in the sun or in an oven then allow it to soak in water for at least 24 hours. Following this, examine the strength of the soil. Certain types of shale disintegrate completely and lose all strength.

    4.9.0 Acid Test

    Use the acid test to determine the presence of calcium carbonate by placing a few drops of hydrochloric acid on a piece of soil. A fizzing reaction (effervescence) indicates the presence of calcium carbonate, normally desirable because its cementing action adds to stability, and the degree of reaction indicates the concentration. Note: Some very dry non-calcareous soils appear to effervesce after they absorb the acid. This effect can be eliminated in all dry soils by moistening the soil before applying the acid. Normally, calcium carbonate’s cementing action develops only after a long curing period; you cannot count on it for strength in most military construction. The primary use for this test is to provide a better value of fine-grained soils tested in place.

    4.10.0 Shine Test

    The shine test is another means of field measuring the plasticity characteristics of clays. When rubbed with a fingernail, pocketknife blade, or any smooth metal surface, a slightly moist or dry piece of highly plastic clay displays a definite shine. On the other hand, a piece of lean clay remains dull, not exhibiting any shine.

    4.11.0 Feel Test

    The feel test is a general-purpose test requiring experience and practice before you can obtain any reliable results. You can determine two characteristics by the feel test: consistency and texture. Consistency — The natural moisture content of a soil has value as an indicator of the drainage characteristics, proximity to the water table, or other factors that may affect its properties. Feel test a piece of undisturbed soil by squeezing it between the thumb and forefinger to determine its consistency. Consistency is described by such terms as hard, stiff, brittle, friable, sticky, plastic, or soft. Remold the soil by working it in your hands and observe changes, if any. With experience, you can use the feel test to estimate natural water content relative to the soil’s liquid or plastic limits. Clays that turn almost liquid on remolding are probably near or above the liquid limit. If the clay remains stiff and crumbles during remolding, the natural water content is below the plastic limit. Texture — Texture, as applied to the fine-grained portion of a soil, refers to the degree of fineness and uniformity. Texture is described by such expressions as floury, smooth, gritty, or sharp, depending upon the sensation produced by rubbing the soil between the fingers. You can increase your sensitivity to this determining sensation by rubbing some of the material on a tender skin area such as the wrist. Fine sand feels gritty. Typical dry silts will dust readily and feel relatively soft and silky. Clay soils are powdered only with difficulty but become smooth and gritless like flour.

    Summary

    Geological and pedological field surveying encompasses a wide and diverse range of resources. To identify suitable terrain for military construction you should gather information from multiple sources that range from aerial maps to smell and taste testing. Each source will provide its own piece of information to the pedology puzzle, sometimes by revealing what is in the soil being sampled, other times by clarifying what is not in the soil. Relative to soils, your goal as a senior Engineering Aid should be to become experienced and comfortable while interpreting all available local sources such as photographs, maps, surveys, and be able to apply the techniques of field identification, such as visual inspections, break- roll- and ribbon-testing, to identify quickly the needs and requirements for any further laboratory testing. Using all resources and applying your field-testing experience will expedite the decision making process for building the soil foundation for the project’s foundation.

    Review Questions 

    1. To obtain which of the following data are geological surveys conducted? A. To locate rock formations in the field and determine their physical characteristics B. To determine rock age and distribution C. To determine the types of rocks and their mineral content D. All of the above 2. How can a geologist determine the approximate age of a rock formation? A. By examining the sequence of rock units B. From data obtained by seismic surveys C. By the presence of certain organic particles D. From data obtained by nuclear density surveys 3. Which of the following survey methods might a geologist use in plotting features on a field map? A. Reference an outcrop to a relief feature B. Reference an outcrop by establishing direction with a compass C. Measure the difference in elevation with an altimeter D. Each of the above 4. By performing which of the following tasks, does the surveyor support the geologist? A. By examining the borehole samples B. By plotting the results of the geological surveys C. By preparing the basic topographic map D. Both % and & above 5. Which of the following steps should you take to establish your base direction? A. Use an established base line from a triangulation net B. Run a control traverse C. Begin from an established monument D. Perform a triangulation survey 6.  Distance measurements should be obtained as accurately as possible for a base map survey. A. True B. False  11-44 7. Measurements made by stadia during a geological survey must be accurate to 1 part in _____. A. 200 B. 300 C. 500 D. 1,000 8. What is the maximum allowable error in elevation when plotting data for a geological map? A. One half of the contour interval B. One contour interval C. Two contour intervals D. 25 feet 9. Horizontal angles that are plotted on a geological topography map should be read to the nearest _____. A. 1 Minute B. 15 minutes C. 30 minutes D. 1 degree 10.  Aerial photographs may be used in place of a base map that will be used for engineering purposes. A. True B. False 11. Which of the following topographic features are more clearly shown on aerial photographic maps? A. Intermittent streams B. Sinkholes C. Heavily wooded swamp areas D. Abrupt contour changes 12. The plotted elevations of the intersection of core borings and the surface of the earth should be accurate to the nearest ________. A. 0.1 ft B. 0.5 ft C. 1.0 ft D. 5.0 ft 13. To what degree of precision should geological surveys conform? A. First order B. Second order C. Third order D. Fourth order  11-45 14. What type of drawing is prepared for a pedological survey? A. Mosaic B. Base map C. Plan and profile D. Preliminary survey map 15. To what degree of precision can pedological surveys conform? A. Low order B. Second order C. Third order D. Fourth order 16. What should be used for an established base direction, in the absence of known bases? A. Railroad tracks B. Magnetic north C. Highway center lines D. True north 17. In what manner should you measure distances in order to reduce survey time? A. Pacing B. Rough chaining C. Stadia D. EDM 18. When you are not given specific instructions for the preparation of sketches of the pedological survey, what scale should you use? A. 1 in. = 200 ft B. 1 in. = 300 ft C. 1 in. = 400 ft D. 1 in. = 500 ft 19. Which of the following information is provided by a soil survey of a proposed construction site? A. The condition of the soil layers B. The drainage characteristics C. The source of possible construction materials D. All of the above 20. Which of the following types of soil has better internal drainage? A. Well-graded gravel B. Inorganic clay C. Silty sand D. Organic clay  11-46 21. When you discover that a proposed grade line is below the groundwater table, which of the following actions must be taken? A. Change the bedrock B. Lower the grade line C. Lower the water table by mechanical means D. Install a water barrier during the construction 22. At what time interval should the measurement for the groundwater table be taken in a test hole? A. As soon as water is located B. At high tide C. 24 hours after the hole is bored D. 36 hours after the highest water level is reached 23. Which of the following information does a soil profile provide? A. Location of ledge rock B. Location of the water table C. Identification of the soil layers D. All of the above 24.  The soil profile does NOT provide information that is useful in determining the finished grade location. A. True B. False 25. Which of the following sources of information would provide you with a location of construction materials, as well as locations of sand and gravel pits? A. Intelligence reports B. Topographic maps C. Agricultural maps D. Geologic maps 26. To what maximum depth does an agricultural soils map provide information on soils? A. 72 inches B. 36 inches C. 12 inches D. 6 inches 27. What type of soil is indicated when you are reviewing aerial photographs and observe areas with smoothly rounded slopes? A. Granular B. Plastic C. Bedrock D. Silt deposits  11-47 28. What is indicated by a sudden change in color to dark gray and black when reviewing aerial photographs? A. Diversion ditches B. Rock formations C. Clay and organic soil D. Gullies 29.  With proper study of maps and photographs, there should be no need for any field investigation. A. True B. False 30. Which of the following requirements is true of a test pit excavation? A. Must be large enough for a man to enter B. Must be made with power-driven equipment C. Must be below the water table D. Load-bearing tests must be performed on soil samples taken every 18 inches 31. Test holes are best performed on what type of soil? A. Cohesiveless soil below the water table B. Cohesiveless soil above the water table with large aggregate C. Cohesive soil D. Bedrock 32. For which of the following test purposes are soil samples obtained by test holes? A. Soil classification B. Compaction C. Moisture Content D. Each of the above 33. What method is commonly used commercially to make deep test holes? A. Wash boring B. Core boring C. Drilling D. Auger boring 34. Which of the following qualities of soil is tested using undisturbed samples? A. Saturation point B. Cohesiveness C. Shear strength D. Load-bearing strength  11-48 35. You must determine subgrade conditions for construction of a new road. What is the next step once the field reconnaissance has been completed? A. Develop soil profiles B. Classify the soil C. Obtain samples for laboratory testing D. Perform preliminary borings at appropriate locations 36. When performing soils investigation on possible borrow areas, you should make borings to what depth? A. 10 feet B. The depth of planned excavation C. 2 - 4 feet below anticipated excavation D. Same depth as all other borings 37. Which of the following sources should you use to obtain information pertinent to the area when performing soils surveys? A. Local contractors B. Existing mine shafts or earth cellars C. Eroded slopes D. All of the above 38. At what type of site should detailed soil explorations be performed? A. Proposed center line B. Proposed large cut location C. Extreme grade shift D. Proposed pavement location 39. What minimum spacing, if any, is required between boring holes? A. 25 feet B. 50 feet C. 100 feet D. None 40. By what manner is highly organic soil identified? A. More than 50 percent passing a No. 200 sieve B. 50 percent or more retained on a No. 200 sieve C. Determining that the sample is neither a fine-grained nor coarse-grained soil D. Visual inspection  11-49 41. How many groups does the Unified Soil Classification System use for soil classification? A. Five B. Seven C. Fifteen D. Thirty 42. Coarse-grained soils are divided into what divisions? A. Silt and sand B. Clay and gravel C. Sand and gravel D. Clay and silt 43. What sieve is used to classify a coarse-grained soil? A. 1/4 inch B. No. 4 C. No. 50 D. No. 200 44. What additional characteristic(s) is(are) used to classify coarse-grained soils with more than 12-percent fines? A. Cohesiveness B. Liquid limit C. Plasticity index D. Both 2 and 3 above 45. For a soil sample to be classified as silty gravel, the plasticity index should be _____. A. more than 7 B. between 4 and 7 C. less than 4 D. unmeasurable 46. In what manner are coarse-grained soils with between 5- and 12-percent fines classified? A. By dual symbols B. As clayey silts C. As nonplastic, nonliquid soils D. As silty clays 47.  A borderline soil may meet more than one zone requirement. A. True B. False  11-50 48. On what basis are fine-grained soils classified? A. Plasticity B. Grain-size distribution C. Percentage of organic material D. Liquid limit 49. What group designation do plastic silts have? A. MW B. MP C. ML D. MH 50. In what manner is peat identified? A. By grain-size distribution B. By liquid limit determination C. By odor D. By plasticity index determination 51. C is defined as the coefficient of _____. c A. uniformity of the grain-size curve B. gradation C. curvature of the gradation curve D. distribution 52. What information must you have to determine the coefficient of uniformity? A. Percent retained on the No. 10 and No. 60 sieves B. Percent passing the No. 10 and No. 60 sieves C. Grain size, in centimeters, at 10- and 60-percent passing levels on the gradation curve D. Grain size, in millimeters, at 10- and 60-percent passing levels on the gradation curve 53. Why is it also necessary to perform field identification tests on soils even when laboratory tests are required during soil explorations? A. To determine which laboratory tests will be omitted B. To minimize the duplication of laboratory tests samples C. To provide duplicate results for positive identification D. To ensure there are no errors in the laboratory tests 54. What is the best way to gain the necessary skills required for field-testing? A. By working with experienced technicians B. By receiving formal soils training C. By getting the “feel” of the soil during the laboratory tests  11-51 55. What is the most useful tool for performing field identification tests? A. A hand auger B. A scale or balance C. A No. 40 sieve D. A No. 200 sieve 56. Which of the soil properties should you include in the description of the soil when identifying soils in the field? A. Estimated percentage of sand B. Maximum particle size C. Particle shape D. Each of the above 57. Which of the following properties of the soil uses visual examination to establish? A. Color B. Grain distribution C. Cohesiveness of the soil D. Grain shape of the fines 58. When the color of a soil has been identified through a visual examination, what other data should you note regarding the soil condition at the time of identification? A. Temperature B. Maximum particle size C. Chemical content D. Moisture content 59. What can you conclude about the soil from an observation during visual examination when you notice a yellow color? A. Organic material is present B. Iron oxides are present C. The soil has poor drainage capabilities D. Aluminum compounds are present 60. What is the first step in approximating grain-size distribution in field identification? A. Separating the larger particles B. Examining the coarse-grained soil for gradation distribution C. Estimating the percentage of fine-grained soil D. Performing the sieve sampling  11-52 61. You have determined the soil to be coarse-grained and estimated the fines to be 4 percent. What is the soil classification of the soil? A. GW-GM B. SW-SM C. GC D. Gravel or sand, depending on additional information 62. Which of the following field tests can be used to determine the cohesiveness of a soil? A. Ribbon B. Roll C. Breaking D. Each of the above 63. The breaking, ribbon, and wet-shaking tests are performed on material passing the A. No. 40 sieve B. No. 60 sieve C. No. 100 sieve D. No. 200 sieve 64. What is the classification of the soil if you have performed the dry-strength test, and the sample cannot be powdered, but will break with difficulty? A. CL B. CH C. ML D. MH 65. What tests complement each other in giving a clearer picture of the plasticity of the soil? A. Wet shaking and roll B. Ribbon and breaking C. Roll and ribbon D. Wet shaking and breaking 66. What is the size of the sample used for the wet-shaking test? A. A roll of soil 1/2 inch in diameter and 3 inches long B. A pat of soil 1/2 inch thick and 1 1/2 inches in diameter C. A ball of soil 3/4 inch in diameter D. A ball of soil 1 3/4 inches in diameter  11-53 67. How will a small amount of clay present in a sample affect a wet-shaking test? A. Causes no reaction B. Causes a sudden reaction C. Retards the reaction D. Assists in identifying the sands and silts 68. What type of soil is the odor test effective in identifying? A. Organic B. Cohesive C. Clayey D. Oily 69. What field test can readily identify soil as containing sand, silts, or clay? A. Acid B. Feel C. Bite D. Shine 70. To prevent a false test result when performing the acid test, you should prepare your sample in what manner? A. By heating B. By adding moisture C. By wet-sieve washing D. By adding lime 71. A positive result of the shine test indicates the _____. A. lack of clay in the soil B. presence of highly plastic clay C. presence of peat D. lack of plasticity of the sample 72. To determine the texture of the soil, it is recommended you rub the soil _____. A. on the back of your hand and allow the sample to dry B. between slightly oiled fingers C. between dry fingers D. on a tender skin area, such as the wrist.