Physical modelling of pile drilling in sand

Abstract

Drilling for foundation piles and tieback anchors through soils using a continuous casing to support the borehole is often referred to as "overburden drilling". Monitoring data from several case studies show that overburden drilling may cause considerable short-term ground settlements indicating a loss of soil volume around the casings. However, further insight is required to understand the mechanisms that govern overburden drilling. Novel physical model tests were carried out to investigate the effects of varying parameters such as flushing media (water or air), flow and penetration rate on the penetration force, pore pressure changes, soil displacements and drill cutting transport. Tests with water flushing indicate a clear relation between the flow and penetration rate and the resulting influence on the surrounding ground. Increasing flow rates caused larger excess pore pressures at greater radial distances and generated more excess drill cuttings compared to the theoretical casing volume. The obtained results were translated into a non-dimensional framework to estimate optimal flushing parameters in similar conditions. The air flushing tests were considerably limited by the modelling constraints. Notable reduction of pore pressures adjacent to the casing indicate an air-lift pump effect that can lead to extensive ground movements as observed in the field.