The gathering, or collecting, of soil samples in the field for testing is called SOIL SAMPLING. The three principal methods of sampling are the taking of samples from the surface, from already existing excavations, and from test pits and test holes. The extent and methods used will be dependent upon the time available.

The method that provides the most satisfactory results for both studying the natural soil conditions and for collecting undisturbed soil samples is the taking of samples from test pits. A test pit is an open excavation that is large enough for a per-son to enter. Usually, these pits are dug by hand; however, when power equipment is available, power excavation by clamshell, dragline, bull-dozer, backhoe, or a power-driven earth auger can expedite the digging. Excavations below the groundwater table require the use of pneumatic caissons or the lowering of the water table. Additionally, excavations that extend to 5 ft or more in depth may require adequate shoring and bracing to prevent cave-ins, as discussed in the previous chapter. Load-bearing tests can also be performed on the soil in the bottom of the pit.

Test hole exploration, with the use of the hand auger, is the most common method of digging test holes. It is best suited to cohesive soils but 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. Auger borings are usually used for work at shallow depths, but if pipe extensions are added, the earth auger may be used to a depth of about 30 ft in relatively soft soils. Samples obtained by this method are completely disturbed but are satisfactory for determining the soil profile, classification, moisture content, compaction capabilities, and similar soil properties. In a hasty soil survey, which is made under expedient conditions or when time is limited, the number of test pits and test holes is kept to a minimum by the use of existing excavations for soil sampling. In a deliberate survey, where time and conditions allow a more thorough sampling operation, test holes are used extensively and are augmented by test pits, governed by the judgment of the engineering officer.

Table 15-2.-Methods of Underground Exploration and Sampling

Figure 15-3.-Sketch showing locations of soil exploration points.

Table 15-2 shows methods of underground exploration and sampling in a condensed form.