Examination of the Kσ Overburden Correction Factor on Liquefaction Resistance

The overburden correction factor (Kσ) is used to account for the curvature of the cyclic strength envelope with increasing consolidation stress, and it provides part of the basis for extrapolation of liquefaction-triggering correlations based on standard penetration test (SPT) and cone penetration test (CPT) to larger depths than covered by current databases of liquefaction case histories. This paper presents an updated database of laboratory test results defining Kσ effects. Laboratory test results included in previous databases are reexamined in light of current understanding of factors that can affect laboratory measurements of cyclic strengths, including the effects of increasing density with increasing consolidation stress, variable overconsolidation ratios, and other factors. The updated database is used to examine potential biases in the Kσ relationships used in two SPT-based liquefaction triggering procedures. The first relationship was found to be conservative with respect to the data for clean sands and less conservative for sands with fines contents between 7 and 35%. The second relationship was found to provide a reasonably good fit to the data for clean sands and to be slightly unconservative for sands with fines contents between 7 and 35%. Implications for practice are discussed.