Full-scale load tests on instrumented micropiles

In September 2001, following an exceptional rainfall event with an estimated return time of about 500 years, a number of ancient buildings located in a restricted area in the historical centre of Napoli underwent significant settlement and heavy structural damage. Structural collapse on saturation of the pyroclastic soils and loss of bearing capacity by submersion are believed to be the origin of the observed phenomena. After the settlements came to a standstill, a subsoil investigation was carried out and a remedial solution was designed, based on a massive underpinning of the damaged buildings by micropiles. The size of the project and the well-known dependence of the behaviour of micropiles on technological details suggested that some preliminary load tests on piles should be undertaken. The results of two load tests on instrumented piles installed with different procedures are reported and discussed. A new technique to install embedded vibrating wire gauges was tested, and was revealed to be successful. The experimental findings are in good agreement with the Bustamante and Doix method. Also, the simplified approach proposed by Randolph and Wroth to evaluate the stiffness of the soil layer was checked, providing good results. The experimental finding about the mobilisation of some negative skin friction along the pile shaft, probably due to the large grouted volumes, is confirmed by several finite element analyses.