SECTION II. SOIL SURVEYS AND SAMPLING
Surveying soil conditions at proposed military construction sites provides information about the nature, extent, and condition of soil layers; the position of the water table and drainage characteristics; and the sources of possible construction materials. A soil survey is vital to planning and executing military construction operations. The information obtained from a soil survey is the basis for a project’s success.
Types of Soil Surveys
A soil survey consists of gathering soil samples for examining, testing, and classifying soils and developing a soil profile. The two types of soil surveys commonly associated with military construction are the hasty and deliberate surveys.
A hasty survey—made either under expedient conditions or when time is very limited—is a type of survey that usually accompanies a preliminary site analysis. A deliberate survey is made when adequate equipment and time are available. When possible, a hasty survey should be followed by a deliberate survey.
A hasty survey should be preceded by as careful a study of all available sources of information as conditions permit. If aerial observation is possible, a trained person may observe soil conditions in the proposed construction area. This gives a better overall picture, which is often difficult to secure at ground level because important features may be hidden in rough or wooded terrain. Rapid ground observation along the proposed road location or at the proposed airfield site also yields useful information. The soil profile may be observed along a stream’s natural banks, eroded areas, bomb craters, and other exposed places. As construction proceeds, additional soil studies will augment the basic data gained through the hasty survey and will dictate necessary modifications in location, design, and construction.
A deliberate survey does not dismiss the fact that the time factor may be important. Therefore, the scope of a deliberate survey may be limited in some cases. A deliberate survey is often performed while topographical data is being obtained so that the results of the soil survey may be integrated with other pertinent information. The principal method of exploration used in soil surveys for roads, airfields, and borrow areas is soil samples obtained either by using hand augers or by digging a test pit. Other methods that may be used are power-driven earth augers, sounding rods, or earthmoving equipment under expedient conditions to permit a hasty approach to the underlying soil.
Objective of a Soil Survey
The objective of a soil survey, whether hasty or deliberate, is to explore and gather as much information of engineering significance as possible about the subsurface conditions of a specified area. The explorations are conducted to determine the—
- Location, nature, and classification of soil layers.
- Condition of soils in place.
- Drainage characteristics.
- Groundwater and bedrock.
Location, Nature, and Classification of Soil Layers
Information regarding the location, nature, and classification of soil layers is required for adequate and economical earthwork and foundation design of a structure. By classifying the soils encountered, a prediction can be made as to the extent of problems concerning drainage, frost action, settlement, stability, and similar factors. An estimate of the soil characteristics may be obtained by field observations, but samples of the major soil types and the less-extensive deposits that may influence design should be obtained for laboratory testing.
Condition of Soils in Place
Soil conditions, such as moisture content and density of a soil in its natural state, play an important part in design and construction. The moisture content may be so high in some soils in place that the selection of another site should be considered for an airfield or other structure. If the natural soil is dense enough to meet the required specifications, no further compaction of the subgrade is required. Very compact soils in cut sections may be difficult to excavate with ordinary tractor-scraper units, so scarifying or rooting may be needed before excavation.
Drainage characteristics in both surface and subsurface soils are controlled by a combination of factors, such as the void ratio, soil structure and stratification, the temperature of the soil, the depth to the water table, the height of capillary rise, and the extent of local disturbances by roots and worms. Remolding a soil also may change its drainage properties. Coarse- grained soils have better internal drainage than fine-grained soils. Observations of the soil should be made in both disturbed and undisturbed conditions.
Groundwater and Bedrock
All structures must be constructed at such an elevation that they will not be adversely affected by the groundwater table. The grade line can be raised or the groundwater table lowered when a structure may be adversely affected by capillary rise or by the groundwater table itself. Bedrock within the excavation depth tremendously increases the time and equipment required for excavation. If the amount is very extensive, it may be necessary to change the grade or even the site location.
Sources of Information
There are many sources of information available to soils engineers, and they should all be used to the fullest extent to eliminate as much detailed investigation as possible. These sources can be used to locate small areas within a large general area that are suitable for further investigation. Field information requires general observation of road cuts, stream banks, eroded slopes, earth cellars, mine shafts, and existing pits and quarries. A field party must obtain reliable data rapidly, since final decisions on site selection are based on field observations. These sources include—
- Intelligence reports.
- Local inhabitants.
- Aerial photographs.
Intelligence reports that include maps and studies of soil conditions usually are available for areas in which military operations have been planned. Among the best and most comprehensive of these are the National Intelligence Surveys and Engineer Intelligence Studies. These reports are a source of information on geology, topography, terrain conditions, climate and weather conditions, and sources of construction materials.
Local inhabitants (preferably trained observers), such as contractors, engineers, and quarry workers, may provide information to supplement intelligence reports or provide information about areas for which intelligence reports are unavailable. Data obtained from this source may include the possible location of borrow material, sand and gravel deposits, and peat or highly organic soils, as well as information on the area’s climate and topography.
Maps provide valuable information, especially when planning a soil survey. Maps showing the suitability of terrain for various military purposes, prepared by enemy or friendly foreign agencies, may be useful. Some of the maps that provide different types of information about an area under investigation are—
- Geological maps.
- Topographic maps.
- Agricultural soil maps.
Geological maps and brief descriptions of regions and quadrangles are available from the US Geological Survey, 1200 South Eads Street, Arlington, Virginia 22202. Generally, the smallest rock unit mapped is a formation, and a geological map indicates the extent of the formation by means of symbolic letters, colors, or patterns. Letter symbols on the map also indicate the locations of sand and gravel pits. The rear of the map sheet sometimes has a brief discussion entitled “Mineral Resources” that describes the location of construction materials.
Ordinary topographic maps may be helpful in estimating soil conditions, but they give only a generalized view of the land surface, especially when the contour interval is 20 feet or more. Therefore, they should be used with geological maps. An inspection of the drainage pattern and slopes can provide clues to the nature of rocks, the depth of weathering, soil characteristics, and drainage. For example, sinkholes may indicate limestone or glacial topography; hills and mountains with gently rounded slopes usually indicate deeply weathered rocks; and parallel ridges are commonly related to steeply folded, bedded rock with hard rock along the ridges. Features such as levees, sand dunes, beach ridges, and alluvial fans can be recognized by their characteristic shapes and geographic location.
Agricultural Soil Maps
Agricultural soil maps and reports are available for many of the developed agricultural areas of the world. These studies are concerned primarily with surface soils usually to a depth of 6 feet and are valuable as aids in the engineering study of surface soils. For example, if the same soil occurs in two different areas, it can be sampled and evaluated for engineering purposes in one area, and the amount of sampling and testing can then be reduced in the second area. Maps are based on field survey factors that include the careful study of the soil horizons in test pits, highway and railway cuts, auger borings, and other exposed places. Information on topography, drainage, vegetation, temperature, rainfall, water sources, and rock location may be found in an agricultural report. Soil usually is classified according to its texture, color, structure, chemical and physical compositions, and morphology.
Aerial photographs may be used to predict subsurface conditions and previous explorations for nearby construction projects. The photographs aid in delineating and identifying soils based on the recognition of typical patterns formed under similar conditions of soil profile and weathering. Principal elements that can be identified on a photograph and that provide clues to the identification of soils to a trained observer are—
- Drainage patterns.
- Erosion patterns.
- Soil color.
- Agricultural land use.
The landform or land configuration in different types of deposits is characteristic and can be identified on aerial photographs. For example, glacial forms such as moraines, kames, eskers, and terraces are readily identifiable. In desert areas, characteristic dune shapes indicate areas covered by sands subject to movement by wind. In areas underlaid by flat-lying, soluble limestone, the air photograph typically shows sinkholes.
Prevailing ground slopes usually represent the soil’s texture. Steep slopes are characteristic of granular materials, while relatively flat and smoothly rounded slopes may indicate more plastic soils.
A simple drainage pattern is frequently indicative of pervious soils. A highly integrated drainage pattern frequently indicates impervious soils, which in turn are plastic and lose strength when wet. Drainage patterns also reflect the underlying rock structure. For example, alternately hard and soft layers of rock cause major streams to flow in valleys cut in the softer rock.
Erosion patterns provide information from the careful study of gullies. The cross section or shape of a gully is controlled primarily by the soil’s cohesiveness. Each abrupt change in grade, direction, or cross section indicates a change in the soil profile or rock layers. Short, V-shaped gullies with steep gradients are typical of cohesionless soils. U-shaped gullies with steep gradients indicate deep, uniform silt deposits such as loess. Cohesive soils generally develop round, saucer-shaped gullies.
Soil color is shown on photographs by shades of gray, ranging from white to black. Soft, light tones generally indicate pervious, well-drained soils. Large, flat areas of sand are frequently marked by uniform, light-gray tones; a very flat appearance; and no natural surface drainage. Clays and organic soils often appear as dark-gray to black areas. In general, sharp changes in the tone represent changes in soil texture. These interpretations should be used with care.
Vegetation may reflect surface soil types, although its significance is difficult to interpret because of the effects of climate and other factors. To interpreters with local experience, both cultivated and natural vegetation cover may be reliable indicators of soil type.
Agricultural Land Use
Agricultural land use also facilitates soil identification. For example, orchards require well-drained soils, and the presence of an orchard on level ground would imply a sandy soil. Wheat is frequently grown on loess-type soils. Rice is usually found in poorly draining soils underlain by impervious soils, such as clay. Tea grows in well-draining soils.
A field investigation consists of the sampling operation in the field.
The extent and methods of sampling used depend on the time available. Military engineers obtain samples from—
- The surface.
- Excavations already in existence.
- Test pits.
- Auger borings or holes.
In a hasty survey, the number of test pits and test holes is kept to a minimum by using existing excavations for sampling operations. In a deliberate survey, where a more thorough sampling operation is conducted, auger borings or holes are used extensively and are augmented by test pits, governed by the engineer’s judgment. The following paragraphs describe this method of sampling.
A test pit is an open excavation large enough for a person to enter and study the soil in its undisturbed condition. This method provides the most satisfactory results for observing the soil’s natural condition and collecting undisturbed samples. The test pit usually is dug by hand. Power excavation by dragline, clamshell, bulldozer, backhoe, or a power-driven earth auger can expedite the digging, if the equipment is available. Excavations below the groundwater table require pneumatic caissons or the lowering of the water table. Load-bearing tests can also be performed on the soil in the bottom of the pit. Extra precaution must be taken while digging or working in a test pit to minimize potentially fatal earth slides or cave-ins. The walls must be supported or sloped to prevent collapse. A good rule of thumb for sloping the pit sides is to use a 1:1 slope. For additional guidance on excavation, refer to Engineering Manual (EM) 385-1-1, Section 23B.
A hand auger is most commonly used for digging borings. It is best suited to cohesive soils; however, it can be used on cohesionless soils above the water table, provided the diameter of the individual aggregate particles is smaller than the bit clearance of the auger. The auger borings are principally used at shallow depths. By adding pipe extensions, the earth auger may be used to a depth of about 30 feet in relatively soft soils. The sample is completely disturbed but is satisfactory for determining the soil profile, classification, moisture content, compaction capabilities, and similar soil properties.
The location of auger holes or test pits depends on the particular situation. In any case, the method described in the following paragraphs locates the minimum number of holes. The completeness of the exploration depends on the time available. A procedure is described for road, airfield, and borrow-area investigations. Make soil tests on samples representing the major soil types in the area.
First, develop a general picture of the subgrade conditions. Conduct a field reconnaissance to study landforms and soil conditions in ditches and cuts. Techniques using aerial photographs can delineate areas of similar soil conditions. Make full use of existing data in agricultural spill maps for learning subsurface conditions.
Next, determine subgrade conditions in the area to be used for runway, taxiway, and apron construction. This usually consists of preliminary borings spaced at strategic points. Arbitrary spacing of these borings at regular intervals does not give a true picture and is not recommended. Using these procedures (especially the technique of identifying soil boundaries from aerial photographs) permits strategic spacing of the preliminary borings to obtain the most information with the least number of borings. In theater-of- operations (TO) cut areas, extend all holes 4 feet below the final subgrade elevation. In TO fill areas, extend all holes 4 feet below the natural ground elevation. These holes usually result in borings below the depth of maximum frost penetration (or thaw in permafrost areas). Where the above requirements do not achieve this result, extend the borings to the depth of maximum frost (or thaw in permafrost areas).
Obtain soil samples in these preliminary borings. After classifying these samples, develop soil profiles and select representative soils for detailed testing. Make test pits (or large-diameter borings) to obtain the samples needed for testing or to permit in-place tests. The types and number of samples required depend on the characteristics of the subgrade soils. In subsoil investigations in the areas of proposed pavement, include measurements of the in-place water content, density, and strength. Use these to determine the depth of compaction and the presence of any soft layers in the subsoil.
In borrow areas, where material is to be borrowed from adjacent areas, make holes and extend them 2 to 4 feet below the anticipated depth of borrow. Classify and test samples for water content, density, and strength.
Select material and subbase from areas within the airfield site and within a reasonable haul distance from the site. Exploration procedures for possible sources of select material and subbase are similar to those described for subgrades since the select material and subbase usually are natural materials (unprocessed). Test pits or large borings put down with power augers are needed in gravelly materials.
Base and pavement aggregates are materials that generally are crushed and processed. Make a survey of existing producers plus other possible sources in the general area. Significant savings can be made by developing possible quarry sites near the airfield location. This is particularly important in remote areas where no commercial producers are operating and in areas where commercial production is limited.
The engineer in charge of the soil survey is responsible for properly surveying, numbering, and recording each auger boring, test pit, or other investigation. Keep a log of each boring, showing the elevation (or depth below the surface) of the top and bottom of each soil layer, the field identification of each soil encountered, and the number and type of each sample taken. Include other information in the log that relates to the density of each soil, the changes in moisture content, the depth to groundwater, and the depth to rock.
Obtaining Representative Soil Samples
Planning the general layout determines the extent of the various soil types (vertically and laterally) within the zone where earthwork may occur. Large cuts and fills are the most important areas for detailed exploration. See Chapter 4 for procedures on obtaining soil or aggregate samples from a stockpile.
Place borings at high and low spots, in places where 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. As a general rule, the number of exploration borings required on a flat terrain with uniform soil conditions is less than in a terrain where the soil conditions change frequently.
Conduct exploration borings at the point of interest and locate them in a manner to get the maximum value from each boring. This may require exploration borings in the centerline as well as edges of runways or roads, but no specific pattern should be employed except perhaps a staggered or offset pattern to permit the greatest coverage. Exploration borings may be conducted at the edge of existing pavements, unless these pavements have failed completely. In this case, find the reason for the failure.