Generally ﬁve or more pits are necessary.
Trial pits yield such information as soil classiﬁcation, how well the sides of the excavation stand up, the position of the water-table, whether seepage of groundwater will be a problem, the ease of level, ram and trim, the invert of the excavation, possible deterioration of the soil on exposure to the atmosphere, the presence and depth of ﬁlls, and the ease or difﬁculty of excavation. (Boreholes can discover sandstone, for example, which contractors will tend to price with high excavation rates yet the trial pit excavator may well be able to excavate the rock easily.) Percussion boring may compress thick layers of peat into thin slices and it is not uncommon to receive descriptions such as ‘sand with traces of peat’ when trial pits would disclose the layer of peat within the stratum of sand. For this reason it is good practice to excavate several trial pits in the vicinity of proposed boreholes so as to check the correlation of the ﬁndings of the two techniques. It is easier to take good undisturbed soil samples from a trial pit than a borehole; to carry out in situ tests (such as the standard penetration test and shear vane test) and to give the soil the apocryphal kick with the heel to estimate its strength.
Trial pits should be excavated down to at least the expected excavation level and on difﬁcult sites (subject to thorough boring, sampling and testing) the information obtained can be used as a useful additional aid to foundation design and construction. They can also provide a visual check on the likely reliability of test information. If the sides of the pit are liable to collapse and access is required, then propping should be carried out to protect the investigator, or the sides should be battered or stepped by the excavator.
Where the site is open to access by people or animals, the pits should be backﬁlled or protected at the end of each day. Where it is necessary to check, over a period of time, seepage or deterioration, the pit should be planked over and covered with tarpaulins or otherwise adequately protected.
The position, ground level and invert level of the pits should be noted together with the ﬁndings of soil classiﬁcation, properties and levels of the strata. Colour photographs of the sides of the pits can be useful and the photographs have increased value if a ranging rod is included to conﬁrm the scale. Where the presence of services is suspected, trial pits can be used to detect them, preferably by careful hand-digging.
Where, from past experience, the ground is known to be ﬁrm clay or dense gravel of considerable depth, then trial pits may be all that is necessary to investigate the suitability of the site for a lightly loaded structure. They must be dug to an adequate depth, to prove the stratum and to detect soft lenses or layers likely to be affected by the foundation loading. Trial pit information is also invaluable in determining the borehole grid layout.