Limitation of damage from overburden drilling for piles and tieback anchors

Abstract

Several cases with unexpected damages related to excessive ground movements induced by initial and secondary effects from overburden drilling to install tieback anchors and piles from inside deep excavations were observed over the last couple of decades in Norway. Overburden drilling is characterized by continuous permanent casings that are drilled through varying soils (i.e. overburden) and with an embedment in bedrock. Based on the lack of knowledge regarding the effects of drilling on the surrounding ground, and the associated risks of excessive ground settlements, the objectives of this PhD study are to: a. Identify and analyze effects on pore pressure changes and ground settlements caused by overburden drilling for piles and tieback anchors installed in different soil conditions by using different drilling methods and procedures. b. Establish recommendations and guidance for execution of overburden drilling to limit negative installation effects and risk of damage on surrounding structures or utilities.

A full-scale field test program investigating drilling for tieback anchors through soft marine clay and into bedrock was conducted to study the impacts on pore pressures and ground settlements. Five different rotary percussive duplex drilling methods including air- and waterdriven down-the-hole (DTH) hammers and top hammer systems were investigated. The field tests showed that drilling in clay with high penetration rates (>1 m/min) combined with water flushing caused excess pore pressures in the surrounding clay with a larger influence zone than experienced with driven closed-ended piles of the same dimensions. Further, the tests showed that drilling with the DTH air hammer caused larger impacts on both pore pressures and settlements than the other methods, indicating a higher risk of unwanted effects on the surrounding ground.

Although the full-scale tests provided new insight into overburden drilling, the complex drilling processes involved made it difficult to investigate in detail the mechanisms and drilling parameters affecting the surrounding ground. Therefore, a series of novel small-scale pile drilling tests in saturated sand were performed. The main objective of this physical model was to explore the role of drilling parameters such as flushing media (water or air) and flow and penetration rates on the surrounding soil. The results from the water flushing tests showed a distinct 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. Based on the test data, a framework of normalized flow rate and normalized mass of drill cuttings was introduced, which could be used to derive ideal drilling parameters in similar conditions. The air flushing tests were considerably limited by modelling constraints; thus, no clear conclusions could be drawn from those tests. However, notable reductions in pore pressures adjacent to the casing indicated an airlift pump effect that can lead to extensive ground settlements as observed in the field. Five well-documented case studies of overburden drilling for foundation piles are provided. The field data illustrates that rotary percussive duplex drilling with air flushing (top hammers and DTH air hammers) frequently caused an air-lift pump effect at the front of the drill bit that may lead to significant local erosion and loss of soil mass (i.e. cavities). This effect was pronounced when drilling in erodible soils like silt and sand, which are typically for glacial tills (e.g. moraine material) often found below marine deposits in Norway. Drilling through confined aquifers with artesian pore-water pressures and a high recharge of ground water further increased the risk of excessive erosion which may result in considerable ground displacements and damage in surrounding areas. Drilling with water-driven DTH hammers greatly reduced the risk of excessive soil volume loss and ground displacements compared to drilling with air hammers. Practical recommendations were derived from the research carried out to guide practitioners in the design, execution, and evaluation of overburden drilling. Hence, this work will reduce the risk of damage caused by overburden drilling and thus realize potential cost savings in the building, construction, and property sector.