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.
Greater London, among many large cities, was subject to bombing by the German military in both the World Wars and was the target of many air raids during the Second World War (WW2). This was particularly the case during the Blitz, September 1940-May 1941, when over 28,000 high explosive bombs and parachute mines were dropped on London. Post war research conducted in 1949 estimated that approximately 12,750t of bombs, including V1 and V2 rockets, were dropped on London. The night of 16 th-17 th April 1941 was one of the worst bombing raids, when 446t of bombs were dropped on London and over 58t did not detonate. Unexploded bombs remain buried underground today, as they were unidentified at the time or abandoned owing to difficulties in recovering them. Uncharted bombs continue to pose a potentially significant hazard for developments around London. This paper considers the probability of discovering unexploded ordnance (UXO), particularly WW2 ordnance, during intrusive groundworks in London. The prevalence of unexploded ordnance has been assessed using data obtained from governmental organisations to estimate the likelihood of discovery in London.
Concrete piles have become common high-load-bearing foundation solutions providing end bearing and frictional resistance along the shaft. They are typically used for founding commercial or residential blocks with a design life of approximately 50 years. Following this the superstructure is decommissioned and may be demolished. However, piles are difficult to remove and therefore future developers can incur significant expense and programme delays in preparing the site to avoid obstructions. If removed, concrete piles are required to be broken down, which is a slow and laborious process. However, a foundation solution has been developed that allows foundations to be installed and extracted with relative ease while still achieving a similar, if not improved capacity. This solution has been defined as a hybrid foundation comprising deep sheet piles for shaft resistance and a pile cap as a shallow foundation. The hybrid pile offers significant advantages over concrete piles, including ease of installation, extraction, reuse and economy. The axial capacity of individual sheet piles is low; however, geometrically arranging sheet piles was shown to offer comparable or improved capacity over conventional concrete piles. The results from a series of centrifuge tests are presented in this paper.
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