Liquefaction Resistance Curves Research Articles

The seismic analysis of sandy sites

This paper presents an approach for performing one dimensional effective stress site response analyses for sandy sites, including the evaluation of liquefaction potential. This type of analysis differs from the more common total stress response analyses in that induced pore pressures in saturated sandy soils are accounted for, including the resulting influence on soil properties. This analytical method has been refined to the point where the need for complex and expensive laboratory soil testing is no longer required, a factor which has traditionally held back developments in the effective stress area. The effective stress analysis requires the determination of five soil specific parameters. A trial and error backfitting procedure was developed to successfully determine these parameters from traditional site investigation data rather than detailed laboratory testing. This procedure was investigated using two case studies, the Edgecumbe earthquake of 1987 and the Loma Prieta event of 1989, which both exhibited significant liquefaction damage. The Edgecumbe analysis produced useful results. The predicted ground acceleration required to initiate liquefaction was 2.8 m/s2 (0.29g) which is close to the estimated value of 3 m/s2 (0.31g). This was a good result as a reasonable amount of estimated and correlated data had to be used due to a lack of specific site data. The case study of Treasure Island, in the San Francisco Bay area, also produced encouraging results with both the prediction of liquefaction and surface response spectra in good agreement with recorded data. Both case studies used liquefaction resistance curves determined empirically from SPT blow count data. While this data proved acceptable it was discovered that care must be taken in the use of such overseas derived empirical data, particularly if no corroborating site specific information is available.

Re‐assessing liquefaction resistance curves

A major part of the US‐Japan Co‐operation Programme concerns earthquake research; results are presented here of a study of field practices in those countries and elsewhere of the Standard Penetration Test, in which the equipment and procedures used led to anomalies in energy ratios and N‐values. Accordingly a standard energy ratio of 60 percent is recommended, and this has been applied in a re‐evaluation of the accumulated data base relating SPT to liquefaction susceptibility.

Influence of SPT Procedures in Soil Liquefaction Resistance Evaluations

The purpose of this paper is to clarify the meaning of the values of standard penetration resistance used in correlations of field observations of soil liquefaction with values of N1 measured in SPT tests. The field data are reinterpreted and plotted in terms of a newly recommended standard, (N1)60, determined in SPT tests where the driving energy in the drill rods is 60% of the theoretical free‐fall energy. Energies associated with different methods of performing SPT tests in different countries and with different equipment are summarized and can readily be used to convert any measured N‐value to the standard (N1)60 value. Liquefaction resistance curves for sands with different (N1)60 values and with different fines contents are proposed. It is believed that these curves are more reliable than previous curves expressed in terms of mean grain size. The results presented are in good accord with recommended practice in Japan and China and should, thus, provide a useful basis for liquefaction evaluations in other parts of the world. Finally, suggestions are made concerning the significance of the term “liquefaction” as it is often used in conjunction with field evidence of this phenomenon.