An experimental study has been carried out to investigate the effects of soil partial saturation on the behaviour of laterally loaded piles. The proposed study has been conducted by means of centrifuge tests at 100×g, where a single vertical pile has been subjected to a combination of static horizontal load and bending moment. The study has been conducted on a silty soil characterized with laboratory testing under saturated and unsaturated conditions. During flight, two different positions of water table have been explored. The influence of density has been investigated compacting the sample with two different void ratio. Finally, the effects of a variation of saturation degree on the pile response under loading have been studied rising the water table to ground surface. Data interpretation allows drawing different considerations on the effects of partial saturation on the behaviour of laterally loaded piles. As expected, compared to saturated soils, partial saturation leads always to a stiffer and resistant response of the system. However, the depth of the maximum bending moment is related to the position of water table and the bounding effects induced by partial saturation appears to be more important for loose soils.

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This paper presents the results of a numerical study aiming at simulating the response of an unsaturated fine-grained soil under wetting and gravitational loading processes. This study is based on the results of some centrifuge tests carried out to assess the influence of partial saturation on the laterally loaded pile response. The hydro-mechanical behaviour of the silty soil is described using a constitutive model adapted to unsaturated conditions. The model predictions are compared with the measurements provided by LVDTs and laser transducers in the first phases of the experimental study. Besides validating the model, the numerical study aimed at investigating the influence of the after-compaction conditions on both the displacement field and the evolution of the more significant state variables during imbibition and gravitational loading processes. Finally, an additional analysis is conducted to determine the effects of the pile installation on the soil response.

In this paper, selected aspects of an experimental study conducted in a geotechnical centrifuge are discussed. The tests aimed to explore the effects of partial saturation of soil on the response of a single pile subjected to a combination of lateral force and bending moment under drained conditions. The soil used in the experiments is a low plasticity silty soil, named B-grade kaolin, characterized by a relatively high permeability compared to the typical values for clayey soils. Two different elevations of the water table and its effects on the pile response under loading are studied. The data show a marked influence of soil partial saturation on the pile response, both under working loads and ultimate loads. In particular, under working loads, the displacement of the head of the pile is appreciably lower than that measured under saturated conditions.

The interaction between a laterally loaded pile and the surrounding soil is typically limited to the shallower soil layer. Often, this zone is above the water table and therefore the interaction takes place under unsaturated conditions. The available evidence is scarce but suggests that unsaturated conditions play a major role on the pile’s response. The actual mechanisms governing the soil–pile interaction under unsaturated soil conditions are not understood entirely, and this paper provides a useful insight on this topic. The analysis is carried out with a fully coupled three-dimensional numerical model, the soil behaviour is simulated with a Modified Cam Clay Model extended to unsaturated conditions. The model accounts for the increase in stiffness and strength of unsaturated soils as well as the volumetric collapse upon wetting. The constitutive model is calibrated on the laboratory data and validated against centrifuge data with satisfying agreement. The results highlight the substantial differences in the soil reaction against the pile depending on different water saturation profiles. The study also shows that the influence of unsaturated conditions on the pile response increases as the pile’s flexibility increases. Comparing the findings with currently available design methods such as the p–y curves, it is found that these do not adequately describe the unsaturated soil reaction against the pile, which opens the door for new research in the field. The proposed numerical model is a promising tool to further investigate the mechanisms underlying the soil–pile interaction under unsaturated soils.

An experimental study into the ultimate capacity of an "impression" pile in clay

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.

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