Evaluation of the observed performance of a deep top down excavation

Abstract

In densely populated cities, due to lack of free constructable space, the need for deep excavations is increasing day by day. Owners tend to have more and more basement floors which are more energy efficient for reducing the operating costs and gain extra space. The evaluation of the risks and the design is not only a pre-construction activity, but it continues also during construction by installing a proper instrumentation system. The Verdi Hotel excavation project was constructed using the top-down construction method, in which a 1.20m thick diaphragm wall was supported by six concrete floor slabs. The 25m deep excavation is monitored by ten inclinometers in the diaphragm wall, ten pressure cells pushed into the soil, and piezometers. This thesis work presents the performance of the top-down excavation by comparing the finite-element analysis of the system and the observed values and state of the art of top-down excavation at Verdi Hotel. Results obtained from observations are correlated with construction activities and finite element analysis results and the following conclusions are made: the top-down excavation at Verdi Hotel had no adverse effect on nearby structures; no post excavation wall movements were observed; The sea side at each floor level had the greatest deflection; The wall deflection increased slightly while the excavation depth remained the same; Maximum lateral movements occured near to the excavation bottom; The curing time for the floor slabs affects the wall deflections; Thermal expansion of the slabs should be taken into account in design; Dewatering of the excavation pit should be done right before the floor slab construction at each stage.