Geotechnical Drilling – A Tool in Soil Investigation

Written by: Mayowa Adeseko – Geotechnical Engineer, Fundamental Integrated Site Appraisal Services (FISAS) Limited, Abuja

Movements of subsurface material impact/affect infrastructural development. Such impact includes slope stability, ground subsidence, landslides, sinkholes, ground settlements due to collapsible soils, liquefaction, erosion, among others. It is therefore important that the properties and characteristics behaviour of subsurface materials are determined in order to mitigate their potential adverse impacts on the built structure on the ground.

 

Drilling/boring is one of the primary methods used to access and appraise the physical and chemical properties of the subsurface material (soil/rock). Drilling is also used in the search for mineral deposits and to determine the thickness/depth of overburden over a mineral deposit.

 

Drilling to investigate engineering properties of soil/rock (geotechnical drilling) is performed nowadays mainly with drill/core bit attached to a rotating machine (rotary drilling) or with shell attached to a belt-driven engine (cable percussion drilling). The choice of rotary or cable percussion drilling method in geotechnical appraisal depends on the terrain and depth of investigation.

 

Geotechnical investigation drilling activities includes transfer of drilling rig with its accessories to designated investigation site and setting up of the unit. Thereafter, boring commences by using the drill/core bit or shell to advance the hole. As the hole advances in depth, drill casings are tripped into the hole to prevent collapse of the borehole wall. Fluids (water, chemicals, air, etc.) may be added as necessary.

Measurements (groundwater level, depth to soil layer, etc.), in-situ testing (standard penetration test, in-situ vane shear test, permeability test, pressuremeter test, etc.) are carried out at intervals and samples of soil/materials encountered are also collected during geotechnical drilling. The sample may be disturbed or undisturbed. Disturbed samples are collected either from the shell or by using window sampler. Undisturbed samples are collected by means of a shelby tube sampler pushed into the soil.

 

The samples collected are labelled, waxed top and bottom (undisturbed sample) and stored under-controlled conditioned to prevent loss of moisture. The observed physical properties/conditions of recovered material; depth to groundwater; date drilled; clients name; borehole number; and project number are documented in the Log Book.

 

Upon attaining the required depth of investigation, drilling is terminated. All casings used to protect the holes from collapsing are retrieved from the borehole. The hole is then backfilled with sand/cuttings recovered from the hole or bentonite/cement mixture.

 

The recovered samples are appropriately packaged and transferred to the laboratory for further testing. Chain-of-Custody form usually accompany the samples from field to the laboratory. The laboratory upon receipt of the sample makes necessary preparations and performed required tests/analyses on the samples.

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