Cyclic Shear Strain Needed for Liquefaction Triggering and Assessment of Overburden Pressure Factor Kσ

Abstract

This paper has two objectives. The first is to evaluate the cyclic shear strain needed to trigger liquefaction, γcl, 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, σv0′, on the value of CRR. The first objective is accomplished mainly by using an equation relating γcl 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 Mw=7.5, γcl≈0.03%, with this value increasing to γcl≈0.3% (in some cases to 0.6%) in denser and overconsolidated, preshaken, and compacted sands. These small values of γcl 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 γcl is either constant with σv0′ or increases slowly with it. Therefore, in the second objective the simplest assumption is made in the paper that the small γcl, valid for the low confining pressures covered by the liquefaction charts [σv0′<200kPa (≈2 atm)] can be extrapolated without change to confining pressures as high as σv0′=800 or 1,000 kPa (≈8 or 10 atm). This assumption allows derivation of a simple expression for the overburden pressure factor, Kσ. The expression predicts a decrease in Kσ with confining pressure that is very similar to some curves of Kσ versus σv0′ proposed in the literature. Kσ curves are also calculated assuming that γcl is proportional to (σv0′)β, where 0≤β≤0.5. Additional experimental research is needed to establish the exact variation of γcl with σv0′ for different sands and different parts of the liquefaction charts and to clarify some of the remaining discrepancies between the different approaches.