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

This paper has two objectives. The first is to evaluate the cyclic shear strain needed to trigger liquefaction, gamma(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, sigma'(v0), on the value of CRR. The first objective is accomplished mainly by using an equation relating gamma(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, gamma(cl) approximate to 0.03%, with this value increasing to gamma(cl) approximate to 0.3% (in some cases to 0.6%) in denser and overconsolidated, preshaken, and compacted sands. These small values of gamma(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 gamma(cl) is either constant with sigma'(v0) or increases slowly with it. Therefore, in the second objective the simplest assumption is made in the paper that the small gamma(cl), valid for the low confining pressures covered by the liquefaction charts [sigma'(v0) < 200 kPa (approximate to 2 atm)] can be extrapolated without change to confining pressures as high as sigma'(v0) = 800 or 1,000 kPa (approximate to 8 or 10 atm). This assumption allows derivation of a simple expression for the overburden pressure factor, K-sigma. The expression predicts a decrease in K-sigma with confining pressure that is very similar to some curves of K-sigma versus sigma'(v0) proposed in the literature. K-sigma curves are also calculated assuming that gamma(cl) is proportional to (sigma'(v0))(beta), where 0 <= beta <= 0.5. Additional experimental research is needed to establish the exact variation of gamma(cl) with sigma'(v0) for different sands and different parts of the liquefaction charts and to clarify between the different approaches. (C) 2015 American Society of Civil Engineers.