Effects of stiffness nonlinearity on E′ standard penetration test N correlations for analysing wall deflections in Hong Kong excavations

Soil stiffness is one of the vital soil parameters governing lateral wall movements of an excavation. For the design of excavation and lateral support (ELS) works in Hong Kong (HK), it is a common practice to idealise soil as an elastic material, whose stiffness is characterised by a constant called Young's modulus (E′), that is then empirically correlated to uncorrected standard penetration test (SPT) N values through a correlation factor, f. Although soil stiffness is well recognised to decrease with an increase in strain nonlinearly, most existing E′-SPT N correlations do not consider stiffness nonlinearity explicitly. In order to account for the influence of strain non-linearity on soil stiffness, 2 new sites were instrumented and monitored, and 12 other relevant case histories in HK were interpreted and back-analysed by using FREW and Plaxis 2D. It is revealed and verified that reduction of soil stiffness with an increase in strain can be correlated with an increase in final excavation depth (H_f). This is because an increase in H_f mobilises larger soil strains to result in a decrease in soil stiffness. For practical purposes, some moderately conservative H_f-dependent E′-SPT N correlations are established for fill and completely decomposed granite and they are verified with independent field measurements.