Influence of initial stress distribution on liquefaction-induced settlement of shallow foundations

Abstract

During earthquakes, saturated sandy soils may generate significant excess pore pressures and approach a state of liquefaction. Structures founded on shallow foundations above such soils may consequently undergo large settlements. Recent case history analysis has shown that the stress imposed by the foundation is a key factor in the estimation of such settlements. However, the case history data showed that although increasing bearing pressure caused an increase in settlements as expected, this was only true up to a point, and that very heavy structures appeared to settle less than some lighter structures. This work aims to investigate these counter-intuitive results by means of controlled experimental testing using a geotechnical centrifuge. Results of the centrifuge tests show that the trend derived from case histories is correct and that liquefaction-induced settlements peak for a given bearing stress (90 kPa for the models tested) and reduce for greater applied stresses. Further, by analysis of excess pore pressure distributions beneath the foundations it is shown that the main factor inhibiting pore pressure generation beneath the footings is not so much the confining pressure as the in-situ static shear stress around the edge of the foundation. This is supported by element test data from the literature. When this initial static shear stress is so high that the applied cyclic shear stress cannot exceed it (i.e. the direction of shear stress does not reverse) then pore pressure generation is greatly reduced, thus causing the observed reduction in expected settlements.

Structure–soil–structure interaction (SSSI) of adjacent buildings with shallow foundations on liquefiable soil

“…Adamidis and Madabhushi 8 further demonstrated that the developing deformation mechanisms are a function of the depth of the liquefiable layer and the foundation bearing pressure. Employing centrifuge modeling, Bertalot and Brennan 9 experimentally confirmed their previoulsy discussed empirical conclusion (based on the case histories), showing that the settlement increases with q up to a certain stress level, above which a reduction should be expected.…”

Performance of vertical drains in liquefaction mitigation under structures

“…For both soil regions, the greatest impact in the generation of pore pressures was produced during the start of the shaking at t = 5 s. In the unmitigated region (Fig. 4b), instruments placed at the edge of the foundation (PPT198) show lower excess pore pressures values compared to the central axis (PPT110) during all the shaking phase as there was an absence of initial shear stress in this central area (Bertalot and Brennan 2015). In the soil with drains (Fig.…”

Effect of soil permeability on soil–structure and structure–soil–structure interaction of low-rise structures

“…Soils with differing particle size distributions and relative densities in granular soils can see different amounts of excess pore water pressure (EPWP) rise resulting in either full or partial liquefaction, which will affect near-surface ground motions and seismic foundation bearing capacity. Previous studies have found that the behaviour of liquefiable soil beneath simple shallow foundations is different to that in free-field soil through field observation (Yoshimi and Okimatsu, 1977;Adachi et al, 1992) and experiments (Liu and Dobry, 1997;Bertalot &Brennan, 2015;Dashti et al, 2010), though analytical and numerical simulations struggle to replicate behaviours of buildings on liquefied soil accurately, and there is no widely accepted simplified method to evaluate the damage to existing foundation on different types of soil.…”