Understanding effects from overburden drilling of piles – a rational approach to reduce the impacts on the surroundings

This paper presents two case studies dealing with undesirable impacts of overburden drilling of casings for end bearing piles to bedrock. Monitored pore-water pressures and ground settlements are used to document and assess the influence from rotary percussive drilling with "down-the-hole" (DTH) hammers. The studies show that drilling with high-pressure air driven DTH hammers may cause considerable erosion and soil volume loss adjacent to the drill bit and along the casing resulting in settlements of the surrounding ground. The risk of soil volume loss increases when the drilling is carried out in erodible soils like silt and fine sands. The volume loss is found to be caused by a combined air-lift pump effect and a Venturi suction effect. Monitoring pore pressures in the vicinity of the drilling may be used to reduce soil volume loss and prevent damaging settlements. Results from drilling with water-driven DTH hammer showed significantly less ground settlements and influence on pore pressures compared to using an air-driven hammer. The study suggests that the drilling parameters flow rate and penetration rate, and the cross-sectional area of the pile casing can be combined in a non-dimensional methodology to assess the mass balance of drill cuttings when drilling with water flushing. A design framework is suggested to guide overburden drilling in urban settings to reduce potential impact on the surroundings.