Urban development in congested cities requires a better exploitation of the available surface, leading to taller structures. These buildings are usually founded on piles that have to be increased in dimension to accommodate the larger loads, resulting in increases in both the cost and the carbon dioxide footprint of the pile foundation. An alternative option is to improve pile performance by enhancing the shaft capacity, which is commonly the most important factor in determining the ultimate capacity of a pile constructed in a clay soil subjected to axial load. For piles in stiff clays such as London Clay, the soil–pile friction may be increased by profiling the side walls of a bored cast in situ pile with small discrete ‘impressions’ such that the latter form nodules on the shaft of the concreted pile. Centrifuge tests carried out at City, University of London, and field trials undertaken by Keltbray Piling across different London sites showed an increase in the shaft capacity of around 40%. In this work, a simple design method based on experimental evidence and an existing plastic failure mechanism was derived for ‘impression’ piles. The proposed method shows good agreement with data and enables a direct prediction of the increase in capacity for future designs.
The availability of space above ground decreases as cities expand, causing a demand for very deep underground structures so developments must mitigate the risk of damaging adjacent buildings. This is especially critical in soft clays where ground movements are considerable and can extend far beyond the excavation site. This paper investigates the efficacy of a shallow lime stabilised clay layer on reducing heave and the settlement profile behind an embedded retaining wall. Centrifuge modelling at 160g was used to observe surface and subsurface soil movements of a 12m deep excavation (H) supported by a retaining wall of 8.8m embedment at prototype scale. Since this research focussed on measures used to minimise heave the model comprised a high stiffness, fully supported 'rigid wall' to eliminate ground movements attributed to wall deformation. A direct comparison between a reference test, with no improvements and a test comprising H/2 thick 5% lime stabilised layer indicated that the lime treatment increased the excavation stability by a factor of three.
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