1. Nature of ground
In clayey soils, borings are suitable for deep exploration and pits for shallow exploration. In sandy soils, boring is easy but special equipment should be used for taking representative samples below the water table. Such samples can however, be readily taken in trial pits provided that, where necessary, some forrn of ground water lowering is used.
Borings are suitable in hard rocks while pits are preferred in soft rocks. Core borings are suitable for the identification of types of rock but they cannot supply data on joints and fissures which can only be examined in pits and large diameter borings.
When the depth of exploration is large, and where the area of construction site is large, geophysical methods (specially the electrical resistivity method) can be used with advantage. However, borings at one or two locations should be carried out, for calibration purposes. In soft soil, sounding method may also be used to cover large area in relatively shorter duration.
In hilly country, the choice between vertical openings (for example, boring sand trial pits) and horizontal openings (for example, headings) may depend on the geological structure, since steeply inclined strata are most effectively explored by headings and horizontal strata by trial pits or borings. Swamps and areas overlain by water are best explored by borings which may have to be put down from a floating craft.
For deep exploration, borings are usual, as deep shafts are costly. However, if the area is vast, geophysical methods or sounding methods may be used in conjunction with borings. For shallow exploration in soil, the choice between pit and borings will depend on the nature of the ground and the information required for shallow exploration in rock; the cost of boring a core drill to the site will only be justified if several holes are required; otherwise trial pits will be more economical.