Importance of Support Fluids
Fluids used in foundation drilling serve two purposes, support and groundwater counterbalancing. As the excavation becomes deeper, the lateral earth and groundwater pressures increase resulting in an increased chance of hole collapse or water infiltration occurring. Soil conditions can exacerbate these problems further, making it much more difficult to use large diameter bores and reach the required depths.
Support & Fluid Loss
Sloughing and hole collapse can be avoided by using a high-density support fluid. The weight of the fluid column exerts a greater force on the bore wall counteracting the lateral earth pressures caused by the weight of the overburden. By incorporating a fluid loss control additive into the support fluid, a filter cake can develop on the bore wall. The filter cake binds together the loose and porous material making up the bore wall, further stabilizing the formation. Fluid loss control additive are especially useful in non-cohesive formations such as sand, silt and gravel where the fluid can be readily lost into the formation.
Groundwater can accumulate in the bottom of the bore and reduce bore stability. In saturated soils groundwater will move to the point of lowest pressure. In most cases this is the open bore. This leads to groundwater accumulation in the bottom of the excavation. To counteract this, the column pressure creates a zone of high pressure, making water migration into the bore unfavorable. The filter cake adds a second layer of defense, creating a low-permeability seal on the bore wall further disincentivizing water infiltration into the bore.