The influence of meso-structure on the mechanical behaviour of a marly clay from low to high strains

On October 31, 2002, a ML=5.5 earthquake struck the Molise region in Southern Italy. The strongly non-uniform damage distribution observed in the town of San Giuliano di Puglia suggested that site amplification significantly affected the seismic response of the Toppo Capuana marly clay formation. The geotechnical laboratories of both the University of Napoli and the Technical University of Bari were entrusted by the Department of Civil Protection of the Italian Government with the experimental investigation of the geotechnical properties of the marly clay, in order to develop seismic microzonation studies. Laboratory tests were carried out on undisturbed borehole samples taken at depth along two orthogonal sections crossing the town center. The testing programme consisted of standard classification tests, one-dimensional and isotropic compression tests at medium-high pressures, cyclic and monotonic triaxial tests and both cyclic and dynamic torsional shear tests carried out at variable frequencies. The geotechnical investigation identified the presence of three main geotechnical units within the Toppo Capuana formation: grey marly clays at depth, overlayed by a few metre thick stratum of intensely fissured tawny clay, and a thin cover of softer soils disturbed by repeated sliding; the laboratory tests were developed on samples from each of these units. The paper discusses the main experimental results and the consequent interpretation of the stress-strain behaviour of the tested soils, characterizing in particular the dependency of the soil pre-failure behaviour and strength on its stress state and history, strain level and strain rate. The influence of the different meso-strucures was not recognisable on the physical properties, but clearly affected the mechanical behaviour of tawny and grey clays, which exhibited more pronounced scale effects. The experimental data permitted to evaluate the effects of the degree of fissuring on compressibility, strength and small strain behaviour; in particular, the different meso-structures of the tawny and grey clays were reflected by the comparison between yielding stresses in one-dimensional compression, the peak strength values, as well as by the small strain stiffness and damping ratio. All such effects are conceptually consistent, and significantly reflect on the subsoil modelling for seismic response analyses of the subsoil to the seismic sequence.