Behaviour of a Large Anchored Basement in Dense Sands and Gravels

Prior to the excavation of half a million cubic meters of dense gravelly material within the city centre of Milan, a 24 to 27m long diaphragm wall was built to provide a 17m retained height to a four-level basement. Temporary support was offered by two to four rows of ground anchors with the exception of a portion adjacent to an existing multi-level cark park where a post-tensioned capping beam was installed to protect the car park and prevent it from being subject to sway. Other design challenges were posed by the proximity of a buried channel and of an urban rail tunnel. An extensive monitoring system was set up including inclinometers, load cells and topographical survey points to reveal lateral movements in the order of 10mm or less, negligible variation of anchor loads as the excavation progressed and heave behind the wall. In response to a general lack of case histories and design guidance on deep excavations in dense coarse material, especially in the region, a backanalysis of diaphragm wall monitoring data was carried out and its main results are commented on in the present paper. Lateral movements are best reproduced with pseudo-FE or full FE software if a soil stiffness compatible with the relevant shear strain level is adopted