Observed Performance of Multipropped Excavation in Stiff Clay

Abstract

A 10-m-deep multipropped excavation in the overconsolidated and fissured gault clay has been monitored. Unusual features of the field monitoring were to measure strut loads and earth pressures as well as wall deflections and rebar stresses at one of the diaphragm wall panels and to monitor both vertical subsurface movements and pore pressures in the center of the site. In addition, overall ground response to construction activities was recorded. The results of the field monitoring have shown a significant reduction in lateral stresses associated with small ground movements after wall installation. As a result of low horizontal stresses in the ground after wall installation, the observed strut forces, wall deflections, and earth pressures were substantially lower than conventional design predictions. In the center of the site, unusual pore pressures and soil swelling behavior were observed. Following each stage of excavation a rapid rise of pore pressures occurred accompanied by general swelling of the clay throughout the stratum. The pore pressures in the fissure system appeared to be very different from those in the soil lumps and the observed vertical subsurface strain distribution with depth does not seem to fit with classical swelling theory. A new displacement ratio is defined as the ratio of total horizontal-to-vertical surface ground movement. For the top-down construction in gault clay, this ratio was >1 during the wall installation and piling. But it gradually reduced to ∼0.6 at the end of excavation.