Design method for the ‘impression pile’

Abstract

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.

Forces required to create impressions in clay

“…For the currently available jacking force (35kN) and geometry (d = 126mm), the nodules can be embedded into a clay with s u ≤230kPa. In London Clay, this s u -value can be found around a depth − 14moD (metres Ordnance Datum) (Martin et al,2016) whilst the nodules have been successfully impressed on piles up to 24m long (Lalicata et al, 2022).…”

“…The push force depends on embedment length b, that varies between 0.5d and 0.6d (Lalicata et al, 2022). Consequently, N c =12.0 can be assumed where circular nodules with a attened cone tip are installed, Fig.…”

“…Impression piles out-perform traditional bored piles in stiff clays providing additional shaft capacity from impressions that generate a nodular pile surface (Lalicata et al, 2022(Lalicata et al, , 2023. The performance of the impression piles depends on the quality of indentations.…”