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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. Key words: model tests, drilling, piles, anchors, settlement, pore pressure. Le forage de pieux de fondation et d'ancrages d'arrimage dans le sol a l'aide d'un tubage continu pour soutenir le trou de forage est souvent appele << forage dans les morts-terrains >>. Les donnees de surveillance provenant de plusieurs etudes de cas montrent que le forage de morts-terrains peut provoquer des tassements considerables du sol a court terme, indiquant une perte de volume de sol autour des tubages. Toutefois, il est necessaire de mieux comprendre les mecanismes qui regissent le forage des morts-terrains. De nouveaux essais sur modele physique ont ete realises pour etudier les effets de divers parametres tels que le milieu de rincage (eau ou air), le debit et le taux de penetration sur la force de penetration, les changements de pression interstitielle, les deplacements de sol et le transport des deblais de forage. Les essais avec rincage a l'eau indiquent une relation claire entre le debit et le taux de penetration et l'influence consequente sur le sol environnant. L'augmentation des debits a entraine des pressions de pores plus importantes a des distances radiales plus grandes et a genere plus de deblais de forage en exces par rapport au volume theorique du tubage. Les resultats obtenus ont ete traduits dans un cadre non dimensionnel pour estimer les parametres de rincage optimaux dans des conditions similaires. Les essais de chasse d'air ont ete considerablement limites par les contraintes de modelisation. Une reduction notable de la pression des pores adjacents au tubage indique un effet de pompe a air qui peut entrainer des mouvements importants du sol, comme on l'a observe sur le terrain. [Traduit par la Redaction] Mots-cles: essais sur maquettes, forage, pieux, ancrages, tassement, pression interstitielle., 1. Introduction In areas where soft soil deposits of limited depth are overlying competent bedrock so-called overburden drilling is often carried out to install tieback anchors for sheet pile walls [...]

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 such as 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. [ABSTRACT FROM AUTHOR]