Large-diameter piles in chalk – part 1, ground investigation

Deep bored piled foundations were planned to support a new 279 m tall tower in east London. The piles were founded in chalk, and therefore detailed stratigraphical logging of the chalk strata was carried out using known marker bands such as flints bands and marl seams. This helped to develop a ground model, including the separation of the various chalk formations into their respective beds. Other investigation methods were employed to assist in developing the chalk stratigraphy, including downhole geophysical techniques, fluid temperature, fluid flow, resistivity, density, caliper and natural gamma. Other downhole tools, such as downhole optical imaging, were also used. Critical to the design of the pile was understanding the behaviour of the bentonite support fluid when used in chalk. Owing to concerns regarding the possible interaction of the bentonite with the chalk, bespoke laboratory tests were carried out on core samples using two commonly used bentonite types. The tests were carried out at two bentonite concentrations with chalk samples from three depths. Control samples were also tested without chalk. This paper presents the detailed investigation of the chalk and its interaction with the bentonite carried out prior to the commencement of the pile design.