Through the use of the various types of published information and aerial photographs, the exploration of a general area maybe narrowed down to several smaller areas suitable for further investigation. The extent and method of collecting more detailed information by field observations depends on the time available.

Rapid ground observation along the proposed highway or airfield location may yield valuable information when conditions do not permit you to make a complete or deliberate soil survey. Observe the soil profile along the natural banks of streams, eroded areas, bomb craters, road cuts, or other places where you see stratified areas. Such observations may indicate types of soil and depths of layers. Scrape off loose surface soils before you examine and make field identification. 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. Surface soils may be exposed by the use of pick and shovel, particularly in areas of questionable soils or at critical points in the location. Soils identified in the hasty survey may be located by field sketches or on available maps or photographs.

METHODS FOR COLLECTING SAMPLES

A deliberate investigation is made when time and equipment are available and when a more thorough investigation of the subsoil is needed than can be obtained by hasty field observations. The two most commonly used methods of obtaining soil samples for deliberate investigations are test pits and test holes.

A test pit is an open excavation that is large enough for a man to enter and study the soil in its undisturbed condition. This method provides the most satisfactory means for observing the natural condition of the soil and the collection of undisturbed samples. The test pit is usually dug by hand; however, power excavation by

The use of the hand auger is the most common method of digging test holes. 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. By adding a pipe extension, you may use the earth auger 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 properties. Auger borings are principally used for work at shallow depths.

Wash boring is probably the most common method used commercially 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, from which water flows under pressure, loosens the soil. The water then carries the soil particles to the surface where they collect inside the casing. An experienced operator can detect from the appearance of the wash water when a change in the type of soil being penetrated has occurred. Wash samples are samples taken directly from the wastewater. They are so disturbed, however, that their value is limited. This method of sampling should not be used if any other means is available.

Dry-sample boring makes use of the wash boring method to sink the hole. When a change of soil type occurs or sometimes at specified depth intervals, the washing is stopped and the bit is replaced by a sampler. The sampler (an open-end pipe) is driven into the relatively disturbed soil in the bottom of the hole to extract a sample. The sample is removed and preserved in a sample bottle until tested in the laboratory.

The undisturbed sampling process is used to obtain samples with negligible disturbance and deformation for testing for shear strength, compressibility, and permeability. These samples can best be obtained from relatively cohesive soils. Methods that you can use to obtain undisturbed samples are discussed in the EA3 TRAMAN.

The core boring process is used to obtain samples from boulders, sound rock frozen ground, and highly resistant soils. The cutting element may consist of diamonds, chilled shot, or steel-tooth cutters. The drill cuts an angular ring in the rock leaving a central core which enters the core barrel of the drill and is retained by a holding device when the drill is removed from the hole. This is the best method for determining the characteristic and condition of subsurface rock

The location of test holes or test pits depends upon the particular situation. Soil tests should be made on samples that are representative of the major soil types in the area. In view of this, the first step in exploration is to develop a general picture of the subgrade conditions to assist in determining the representative soils. Field reconnaissance should be made to study landforms and soil conditions in ditches and cuts. Techniques have been developed whereby aerial photographs can be used for delineating areas of similar soil conditions. Full use should be made of all existing data.

To determine subgrade conditions in an area to be used for road or for airport runway, taxiway, and apron construction, the next step after field reconnaissance is usually to make preliminary borings at strategic points. An arbitrary spacing of these borings at uniform intervals does not give a true picture and is not recommended. Intelligent use of various procedures permit strategic spacing of the preliminary borings to obtain maximum information with a minimum number of borings.