MODELLAZIONE FISICA DI PALI TRIVELLATI IN SABBIA

The dissertation deals with the modelling of the behaviour of bored pile, embedded in a coarse grained soil, axially loaded in tension and compression, with particular attention to the mobilization of the lateral and the base capacities. A series of almost 60 centrifuge tests has been carried out on free-headed and close-ended rough model piles, instrumented and not, whose dimensions varies from 8 mm to 32 mm in diameter and 160 mm to 320 mm in length. The tests have been carried out at various acceleration levels between 30g and 160g, thus simulating a large series of prototype dimensions. The “ideally non-displacement” model piles are embedded in a very fine silica sand, called FF, pluvially deposed at a medium and a high density. To analyse the load direction effect on the mobilisation of the skin friction, the results of tension tests are compared with those gathered in compression at same state conditions. A series of almost 150 direct shear constant normal stiffness tests, on interface between FF sand and rigid aluminium plates of various surface roughness has been performed. All centrifuge and direct shear constant normal stiffness tests have been performed at the ISMGEO laboratories (Istituto Sperimentale Modelli Geotecnici in Seriate BG, Italy), which totally financed the experimental program. The conceptual framework of the physical modelling of bored pile foundation in centrifuge is described in the first part of the thesis, underlining the limitations mainly connected to the scale effects. The experimental set up, the model piles dimensions and characteristics, the mechanical properties of the FF sand, the sample deposition technique, the test instrumentations and procedures are also outlined. The common empirical correlations for the prediction of the shaft resistance are summarised and applied to the centrifuge test results; a new formulation to evaluate the normal stress variation, acting on the pile shaft at peak friction, is also proposed, which take into account for the density of the sand. The scale effects on the ultimate shaft resistance and on the correlated settlement are analysed so as the displacement scaling law which has to be applied on the interface behaviour to scale the settlement of model pile to prototype scale. The critical and ultimate base resistance of model non-displacement piles loaded in centrifuge tests are evaluated using the most common practical correlations. New formulations for the evaluation of the critical base resistance from a cone penetration test and the sand state parameters are presented. The main conclusions derived from the centrifuge loading tests on model piles, from the direct shear constant normal stiffness interface tests and form some literature data, are summarised in the last chapter together with future developments of the research.