Abstract
Demand for underground space in the urban environment for infrastructure schemes has increased drastically in recent years. Diaphragm walls are often implemented to support the sides of deep excavations and it is becoming increasing necessary to install these close to existing structures. This type of cast-in-situ permanent retaining structure installation can result in substantial soil deformations and also cause damage to adjacent structures. A better understanding of this soil-structure interaction problem is urgently needed so that safe and economic designs can be achieved.
The installation effects of diaphragm walls on an adjacent single pile in dry dense fine sand were studied by means of a series of small-scale model tests in the geotechnical centrifuge. A model construction system was developed to simulate the construction sequence (excavation and casting) of diaphragm wall panels. Instrumented model piles, capable of measuring axial force, bending moment and radial stress along their length, were driven in-flight and a constant force was maintained at the pile heads to simulate axially loaded driven piles. The load distributions recorded during pile installation were similar to the general observations, suggesting that the prototype situation was modelled realistically.
Miniature pressure cells were used to monitor soil stress changes during wall construction and subsequent reduction in slurry level. Stress redistribution around the trench was clearly observed and its direction and magnitude depended on the relative changes in soil stresses caused by disturbances in trench pressures. Soil settlement was also recorded during the reduction in trench pressure or vice versa. A progressive development of craters occurred when the slurry head was being dropped. The presence of a pile reduced the size of the crater, however, destabilisation of the soil behind the pile was observed after the pile had started moving significantly. Thin layers of coloured sand were placed at equal depth intervals to locate the soil failure surfaces. Distinct soil failure zones were identified at the end of the tests, i.e. shallow, triangular wedge-shaped and end-bearing failure zones. Their development depended on the layout and sequence of the wall construction.
Diaphragm walling also induced substantial changes in pile behaviour, the magnitude of which depends on the trench pressure, pile location relative to the trench, the trench length and the presence of adjacent pre-installed concreted panels. Based on the test results, a progressive failure mechanism for this soil-structure interaction problem is proposed and the associated soil-pile interface stress path is generalised into three phases. A simplified three-dimensional limit equilibrium analysis was carried out and good agreement between the predictions and the centrifuge test results was found. By adopting a critical state soil friction angle, this approach should provide a safe way of assessing the failure load of a pile installed at various slurry levels and offset distances away from a single trench, of which the length also varies.
In general, the stability of the system could be improved by reducing the trench length, increasing the pile offset distance away from the trench, or pre-constructing a pair of adjacent stiff panels either side of the panel to be constructed.