Trial pits and borings - Soils - Foundations.

Trial pits are generally used for geotechnical category 1 investigations They are useful for examining the quality of weathered rocks for shallow foundations Trial pits extended to trenches provide the most reliable means of assessing the state of deposition and characteristics of filled ground

Hand and mechanical auger borings are also suitable for category 1 investigations in soils which remain stable in an unlined hole When carefully done augering causes the least soil disturbance of any boring method Light cable percussion borings are generally used in British practice The simple and robust equipment is well suited to the widely varying soil conditions in Britain, including the very stiff or dense stony glacial soils, and weathered rocks of soil-like consistency

Large-diameter undisturbed samples (up to 250 mm) can be recovered for special testing

Rotary open hole drilling is generally used m USA, Middle East, and Far Eastern countries The rotary drills are usually tractor or skid-mounted and are capable of rock drilhng as well as drilling in soils Hole diameters are usually smaller than percussion-drilled holes, and sample sizes are usually limited to 50 mm diameter

Bentonite slurry or water is used as the drilling fluid, but special foams have been developed to assist in obtaining good undisturbed samples

Wash borings are small-diameter (about 65 mm) holes drilled by water flush aided by chiselling Sampling is by 50 mm internal diameter standard penetration test equipment (see below) or 50—75 mm open-drive tubes

Wash probings are used in over-water soil investigations They consist of a small-diameter pipe jetted down and are used to locate rock head or a strong layer overlain by loose or soft soils, for example in investigations for dredging There is no positive identification of the soils and sampling is usually impracticable