Monitoring an excavation in an urban area

During the construction of a new building in the centre of Salerno (Italy), a buried two-level car park needed to be constructed close to existing buildings. The excavation depth was 8 m below the ground level and diaphragm walls retained the soil. They consisted of jointed cast-in-place, 0.60 m thick and 18 m long reinforced concrete panels. During the excavation the induced displacements and the water table depth were monitored in order to control the effects on the surrounding structures. The ground conditions comprised about 20 m of alternated silty, sandy and gravelly layers and suggested they were continuous over the area to be excavated. Hence the concern for the bottom instability due to a possible high hydraulic gradient was not negligible. For this reason, the water table level inside and outside the excavation was measured during each construction phase, together with the displacements behind one of the diaphragm walls. To this purpose standpipe piezometers and wells at different locations were used to measure the water table level. Two inclinometer tubes were also installed behind the diaphragm wall to allow measuring the horizontal displacements. Finally, a net of targets for optical survey was installed in the area surrounding the excavation and particularly on the closest buildings. These measurements have been compared to the empirical predictions, as obtained following the classic literature suggestions. Some of these comparisons are shown in the paper.