There is no single number that suits every project. The number and depth of boreholes depend on the site and the structure, and are set by the geotechnical engineer in coordination with the consultant and in line with the Saudi Building Code.
What decides the number and depth?
- The area and shape of the site.
- The type of structure, its loads, number of floors and basement depth.
- Variability of the ground: the more the layers change from point to point, the more boreholes are needed.
- The presence of weak soil, rock or groundwater.
- The requirements of the consultant and the permitting authority.
As a principle, a borehole should go deeper than the zone under the foundations that is influenced by the building loads, and reach a layer suitable for founding.
How are boreholes drilled?
- Rotary drilling with a tri-cone bit, using water or air for cooling.
- Sampling of clay soils with Shelby tubes.
- Coring of rock with double-tube core barrels fitted with a diamond bit.
What is the SPT?
The Standard Penetration Test is carried out inside the borehole at successive depths. It gives an N-value that roughly indicates the density of sandy soil or the stiffness of clay. It is a common field test used together with laboratory results to estimate bearing capacity.