To make an examination of the subsoil on a building site, trial pits or boreholes are excavated. Trial pits are usually excavated by machine or hand to depth of 2 to 4 m and at least the anticipated depth of the foundations. The nature of the subsoil is determined by examination of the sides of the excavations. Boreholes are drilled by hand auger or by machine to withdraw samples of soil for examination. Details of the subsoil should include soil type, consistency or strength, soil structure, moisture conditions and the presence of roots at all depths. From the nature of the subsoil the bearing capacity, seasonal volume changes and other possible ground movements are assumed. To determine the nature of the subsoil below the foundation level it is either necessary to excavate trial pits some depth below the foundation or to bore in the base of the trial hole to withdraw samples. Whichever system is adopted will depend on economy and the nature of the subsoil. Trial pits or boreholes should be sufficient in number to determine the nature of the subsoil over and around the site of the building and should be at most say 30 m apart.
Ground movements that may cause settlement are:
(1) compression of the soil by the load of the building
(2) seasonal volume changes in the soil
(3) mass movement in unstable areas such as made up ground and mining areas where there may be considerable settlement
(4) ground made unstable by adjacent excavations or by dewatering, for example, due to an adjacent road cutting.
(4) ground made unstable by adjacent excavations or by dewatering, for example, due to an adjacent road cutting.
It is to anticipate and accommodate these movements that site investigation and exploration is carried out.
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