D21 Report on the SFSI, foundation performance and vulnerability assessment for permanent ground displacement

Study of the foundation type effect

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A number of recent large magnitude earthquake events have been characterized by extensive outcropping of the causative fault. Permanent ground displacements associated with surface fault rupture may pose a serious risk to historic structures and monuments. The vulnerability of old masonry buildings subjected to tectonically induced ground distress is studied here through analysis of a simple but representative soil–foundation–masonry wall system.

A series of 3D FE analyses have been conducted to investigate the response of a single masonry wall supported on isolated footings standing upon a 8 m deep layer of dense sand, which is subjected to dip-slip (normal and reverse) tectonic displacement. A validated nonlinear soil constitutive model has been employed which is able of simulating fault rupture–soil–foundation interaction with sufficient accuracy. Apart from the nonlinear soil material response, the numerical method has also accounted for the nonlinearities expected to take place at the soil foundation interface (i.e. sliding, uplifting).

Results provide valuable insight on the different mechanisms of response taking place for different positions of the structure with respect to the fault. The associated foundation performance, judged on a displacement basis, is discussed and found to depend dramatically on the exact position of the structure. The masonry wall response was modelled in a very simplifying manner, assuming elastic behaviour, but assessment of its performance is attempted in view of the magnitudes of stresses developed in it. The distress of the masonry wall due to the tectonic deformation of the ground is also evaluated with respect to the distortion of its foundation system and the amount of uplifting (loss of contact with the supporting soil) and sliding taking place at the soil–foundation interface.

Furthermore, a parametric investigation of the effect of the foundation type on the response of the wall–foundation system is carried out considering three different foundation alternatives: isolated footings, continuous raft foundation, and embedded foundations.