Cyclic shear strain needed for liquefaction triggering and assessment of overburden pressure factor K_σ

This paper has two objectives. The first is to evaluate the cyclic shear strain needed to trigger liquefaction, γ_cl, in clean and silty sands in the field during earthquakes as an alternative to the currently used cyclic resistance ratio (CRR). The second objective is to explore the effect of a high effective overburden pressure, σ'_v0, on the value of CRR. The first objective is accomplished mainly by using an equation relating γ_cl and CRR valid for shear wave velocity-based liquefaction charts. This equation is supplemented with laboratory results from undrained cyclic strain-controlled tests as well as large-scale and centrifuge model shaking experiments. It is shown that for recent uncompacted clean and silty sand deposits and earthquake magnitude M_w = 7.5, γ_cl ≈ 0.03%, with this value increasing to γ_cl ≈ 0.3% (in some cases to 0.6%) in denser and overconsolidated, preshaken, and compacted sands. These small values of γ_cl in the field are controlled by two factors: excess pore pressure buildup in the soil due to the cyclic straining and the redistribution of excess pore pressures and upward water flow that occurs during shaking. The available evidence suggests that γ_cl is either constant with σ'_v0 or increases slowly with it. Therefore, in the second objective the simplest assumption is made in the paper that the small γ_cl, valid for the low confining pressures covered by the liquefaction charts [σ'_v0 < 200 kPa (≈ 2 atm)] can be extrapolated without change to confining pressures as high as σ'_v0 = 800 or 1,000 kPa (≈ 8 or 10 atm). This assumption allows derivation of a simple expression for the overburden pressure factor, K_σ. The expression predicts a decrease in K_σ with confining pressure that is very similar to some curves of K_σ versus σ'_v0 proposed in the literature. K_σ curves are also calculated assuming that γ_cl is proportional to (σ'_v0)β, where 0 ≤ β ≤ 0.5. Additional experimental research is needed to establish the exact variation of γ_cl with σ'_v0 for different sands and different parts of the liquefaction charts and to clarify some of the remaining discrepancies between the different approaches.