The construction of buildings, dams, reservoirs, docks and other types of construction works, soil investigation will require deep and closely spaced borings.

Borings will reveal the types of soil layers beneath the proposed foundation, and whether or not it will be suitable to carry the proposed structure safely.

Borings should be dug in a regular pattern in such a way that it gives a clear picture of all significant variations of the soil within the site.

Investigative boreholes should be at least 1.5m deep. Three main factors will govern the depth of exploration:

Depth to which the foundation load will act on beneath the base of the foundation.

Depth to which weathering will affect the soil.

Depth at which impervious strata occur.

Some investigative borings  can be up to 6m deep.

The standard depth for building structures is up to 1.5 times the breadth of the foundation.

How soil is stressed under foundation loading

Load of the structure is transferred to the foundation and then to the surrounding soil beneath the foundation. The soil stress beneath the foundation is reduced with depth.

The diagram above shows the soil stresses below a pad foundation. Notice that at 1.5b below the foundation level, the bulb stress is 0.2 of the original stress just under the foundation. This value is higher for a strip foundation as shown below.

The reason for this is because strip foundation is continuous while pad foundation is isolated.