Trial pits

Hand-dug trial pits are expensive, take time to excavate and are not always as informative as expected. They do, however, expose a formation so that it can be examined in detail. This may be important if thin layers of weak clay or pre-existing shear zones are suspected below ground. The starting size for a pit depends on the depth it is to be sunk. If required to a depth of 5 m for  instance, it will have to be started between 3 and 3.5 m square, because the supports to it will have to be brought in twice, and the reduced area at the bottom of the pit must be large enough for the men to work in, with a crane skip present and also possibly a pump.

Before starting a trial pit it is necessary to decide the depth and information to be sought and whether other means, such as augering or a borehole, would produce the information quicker and at less cost. If the requirement is simply to find rock level, or to ascertain whether soft material lies below hard (such as a boulder), a boring may be a better option. If one is looking for clay, silt or soft material, a most important matter is whether the pit is to find the full depth of such material or just penetrate into it. The former can be much more difficult and expensive than the latter and may prove impossible without groundwater lowering.
If undisturbed samples are to be taken it is necessary to know whether they are to be taken horizontally into the sides of the pit, or vertically from the bottom. Pushing a 100 mm diameter sampling tube horizontally into the side wall of a trial pit often involves the use of jacks, and digging the tube out is no easy matter.

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