Deep excavations in soft clay with an underlying aquifer lead to major concerns of ground deformation, soil-structure interaction and base instability due to hydraulic uplift. Given vast amount of case histories reported in the literature, it is still difficult to differentiate and understand ground deformations induced by excavations in densely built urban areas and greenfield sites. Moreover, reliable measurements of earth pressure on walls in excavations in soft clay are rarely reported. In spite of frequently reported base instabilities due to hydraulic uplift, the initiation and failure mechanism of base instability are not systematically investigated and well understood. Existing calculation methods against base instability may be too conservative and deserved to be improved....[ Read more ]

Deep excavations in soft clay with an underlying aquifer lead to major concerns of ground deformation, soil-structure interaction and base instability due to hydraulic uplift. Given vast amount of case histories reported in the literature, it is still difficult to differentiate and understand ground deformations induced by excavations in densely built urban areas and greenfield sites. Moreover, reliable measurements of earth pressure on walls in excavations in soft clay are rarely reported. In spite of frequently reported base instabilities due to hydraulic uplift, the initiation and failure mechanism of base instability are not systematically investigated and well understood. Existing calculation methods against base instability may be too conservative and deserved to be improved.

This research aims at investigating ground deformation, soil-structure interaction and base instability of deep excavation in soft clay subjected to hydraulic uplift. Three methodologies, namely field monitoring, centrifuge modelling and finite element analysis, are adopted. To study ground deformation and soil-structure interaction, a diaphragm wall panel constructed at a greenfield site in Shanghai soft clay was comprehensively instrumented and monitored. Coupled-consolidation finite element analysis was conducted to back-analyse the case history. Moreover, a parametric study was carried out to investigate, understand and quantify key factors governing relatively small excavation-induced ground deformation in Shanghai. To study base instability due to hydraulic uplift, a new experimental setup was developed to simulate in-flight multi-propped excavation de-stabilised by hydraulic uplift in centrifuge. Centrifuge model tests and three-dimensional coupled-consolidation finite element analyses were carried out to investigate the initiation and failure mechanism of base instability due to hydraulic uplift in soft clay and effectiveness of using ‘anti-uplift piles’ for base stabilisation. Together with dimensional analysis and numerical parametric studies, key variables affecting the initiation and failure mechanism of base instability are indentified.

In the instrumented greenfield site, both magnitude and influence zone of ground settlements are about 20% larger than those in densely built areas in Shanghai. Numerical analyses reveal that a key factor affecting excavation-induced ground deformation in Shanghai is jet grouted prop, which reduces ground deformation by providing passive resistance and hence decreasing soil yielding in front of the wall.

It is found that the initiation and failure mechanism of base instability due to hydraulic uplift is mainly governed by a ratio of excavation width over the thickness of soft clay inside excavation (B/D). As an excavation becomes narrower (i.e., B/D decreases), the hydraulic pressure (P_i) required to initiate uplift movement of clay inside excavation increases significantly, due to increased effect of downward shear stress acting along soil-wall interface on basal resistance. Compared to the effect of B/D, effect of undrained shear strength of clay (c_u) on P_i is much less significant. At basal failure, the dominant failure mode changes from simple shear in relatively narrow excavations (i.e., B/D<3) to a combined mode of triaxial compression and triaxial extension in relatively wide excavations (i.e., B/D>3). A new calculation chart is developed for estimating P_i of excavations with varied B/D and c_u accordingly. Installaton of ‘anti-uplift piles’ inside excavation does not alter P_i, but can reduce uplift movement by 80%, by mobilising c_u along soil-pile interface.