Borehole Layout.

Three bores are the minimum necessary to determine the dip of a plane strata (where known with confidence to be plane) and as a rough guide this is the minimum for a proposed investigation (it is almost self-evident not to have too many!). The more bores drilled then the more is known about the soil and the risks of meeting difficulties and the greater surety and economy of the foundation design. But obviously once enough is known to design an economical foundation then any further bores are an added-on cost to the project. This assumes, of course, that the stratum  are accurately recorded, described and positioned, etc. by  a competent supervisor during the drilling operations.

Inadequate or inexperienced supervision could lead to expensive errors.

On large sites, say for an industrial estate, when the positions of structures have not been defined it is advisable to establish a grid of boreholes as shown in Fig. 3.1 (c). The spacing of the grid depends upon the site study and reconnaissance. A common grid spacing is about 30 m but, if the site is well-known and of uniform strata, the spacing may be increased. If the site is unknown, suspect and variable, the spacing should be decreased. Where the findings are not uniform and difficulties are unknown, or are expected, then the grid centres should be closed up. Where the site has been mined, an irregular grid is advisable since the
workings may be on a regular grid.

The boreholes enable soil profiles (cross-sections) to be drawn noting the strata classification, thickness and level, and samples taken from the borehole enable the properties of the soil in each strata to be examined. The bores can also enable observations to be made on groundwater levels and variations. The depth of the borehole depends on:

(1) The foundation load. Light, single-storey structures founded on known firm ground of thick strata need
investigation to a depth of about 3 m – and this can be done effectively by trial pits. Tall, heavily loaded structures may need bores taken down to proven firm soil of adequate strata thickness.

(2) The width of the structure. At a depth of 1.5 times the width of the structure the vertical pressure on the soil can be about 20% of the foundation contact pressure.

Closely spaced (i.e. at centres less than about 4 times their width) strip or pad foundations due to pressure
distribution overlap would have the same pressure effect at such a depth as a raft foundation. The wider the structure the deeper the effect of vertical pressure (see Fig. 3.2) and it may be necessary to bore down to
1.5 times the width of the structure.

(3) Whether there is a possible need for piling. Then the bores should be taken down to 3 m below preliminary estimated pile base level.

(4) Whether there is a possible need for foundations to be taken down to bedrock. It is advisable to prove that it is in fact bedrock and not boulders (in glacial or flood deposits or quarry backfill) or relatively thin layers of cemented rock-hard soils (shales in mining areas). This can mean that drilling should continue for at least 3 m into the rock. There have been a number of spectacular failures in mistaking isolated boulders as bedrock.

Fig. 3.1 Typical borehole layouts for (a) multi-storey
flats, (b) factory building, (c) large development area
where building layout is not decided.

Fig. 3.2 Vertical pressure at a depth of 1.5 times
foundation width.