Borehole investigation is done by the service provider who is an expert on it. After finalizing the borehole layout, an investigation of the condition of the ground is done. As discussed in the article “How to select number of boreholes“, the number of boreholes are drilled to get the recommendation for foundations.
Commonly, the standard penetration method (SPT) is used to find the condition of the ground. It is a method that provides the stiffness of the soil based on the penetration when a constant pressure applied.
Based on the SPT values and observations made on the samples taken during the coring, allowable bearing capacity for foundations is provided after doing necessary calculation by taking into account the types of soil layers present and the overburden where the foundations are rested.
For deep foundations, drilling the ground up to the fresh rock and coring in the fresh rock are done to check the soil and rock profiles. The depth of the coring in the rock depends on the condition of the rock. If the rock is weak, the depth of the coloring could be increase to examine the condition of the groud.
There are several parameters required for designing a deep foundation.
- Skin friction of the soil above the weathered rock
- Skin friction of Weathered rock
- Negative skin friction of soil if any
- Skin friction of the rock
- End bearing of rock
The above values will be provided in the geotechnical investigation report with a necessary recommendation for the foundation that includes the depth of socketing. The friction of the soil can be calculated based on the soil parameters or based on the SPT values. There are many methods to evaluate the skin friction of the soil.
End bearing of the rock is decided commonly based on the uniaxial compressive strength o the rock which is evaluated from the test done form the rock samples taken during the coring.
Usually, measurements are taken at ta depth of 1m intervals. SPT values are tested and recorded at each 1m depth of drilling. The following figure indicates the sample borehole log.
The time in which the hammer bouncing is considered as the SPT is grader than 50. When there are very stiff layers this can be observed. The borehole log will be indicated as shown above figure at a depth around 6m when the hammer is bouncing.
The article published in Wikipedia “Standard Penetration Test” states the ranges of SPT values based on the relative density of soil. The same article could be referred for further reading on the standard penetration test.