Assessing Probabilistic Methods for Liquefaction Potential Evaluation — An Update

This paper presents an assessment of existing probabilistic methods for liquefaction potential evaluation. Emphasis is placed on comparison of probabilities of liquefaction calculated with four different methods. Two of these methods are based on SPT, and the other two are based on CPT. In both SPT- and CPT-based evaluations, logistic regression and Bayesian techniques are applied to map factor of safety to probability of liquefaction. The present study shows that the Bayesian approach yields more conservative results than does the logistic regression approach, although results from the two approaches are quite comparable. Discussion of the procedure for risk-based liquefaction potential evaluation is also presented

Probability-Based Liquefaction Evaluation Using Shear Wave Velocity Measurements

Three preliminary probability-based models and one artificial neural network model for evaluating soil liquefaction potential using shear wave velocity measurements are presented and compared with the deterministic curves developed by Andrus et al. The probability models are developed using logistic regression and Bayesian techniques applied to the same case history data used to develop the deterministic curves. The case history data consists of in situ shear wave velocity measurements at over 70 sites and field performance data from 26 earthquakes. The artificial neural network model is a high-order function capable of tracking the irregular boundary separating individual liquefaction and no liquefaction case histories. From the logistic regression and Bayesian models, the deterministic curve is characterized with a probability of about 30 %. This finding indicates that the shear wave-based deterministic curve and the SPT-based deterministic curve exhibit similar conservatism. The results provide a method for liquefaction risk analysis

Liquefaction Mitigation Using Air Injection

Soils most susceptible to liquefaction are loose, non-plastic and saturated. Because the compressibility of air is orders of magnitude greater than the compressibility of water, un-saturation or partial saturation can significantly increase the liquefaction resistance of a soil deposit. Nakazawa et al. (2004) have shown that cyclic strength in laboratory test specimens can be more than twice as high in partially saturated soil than fully saturated soil. It is hypothesized that sufficient de-saturation to increase the liquefaction resistance can be induced by injecting air or gas into the subsurface. Simple, qualitative shake-table experiments demonstrate the increase in liquefaction resistance as a result of de-saturation from air injection. Air sparging is a widely used environmental remediation method that involves the continuous injection of air into soil to promote volatilization of contaminants. This method can be readily adapted for use as a liquefaction mitigation technique. Although air sparging relies on a continuous flow of gas, Okamura et al. (2006) present data that indicate de-saturation from air injection can last for years or more after an initial, short-term injection period. In summary, intermittent or periodic air injection over the life of a structure may be useful in increasing the liquefaction resistance. This method would be particularly well suited for the protection of existing structures founded on soils susceptible to liquefaction

Estimating Liquefaction Potential if a 200,000-Year Old Sand Deposit Near Georgetown, South Carolina

A geotechnical experimentation site is being developed near Georgetown, South Carolina, to study the effect of soil age on liquefaction resistance. The site is located in an area called Hobcaw Barony on a 200,000-year-old beach to barrier-island sand deposit. Initial investigations conducted at the site include seismic cone penetration tests with pore pressure measurements, standard penetration tests with energy measurements, seismic crosshole tests, dilatometer testing, and fixed-piston sampling. Shear-wave velocities calculated from seismic cone test results are based on the true-interval method. The near-surface sand deposit extends from the ground surface to a depth of 8.5 m. The groundwater table is located at a depth of 2.4 m. Measured shear-wave velocities from the near-surface sand deposit are, on average, 47% higher than velocities of 10 year-old sand deposits with similar penetration resistances. The sand deposit at the Hobcaw Barony site is found to be susceptible to liquefaction, but ground shaking levels during the 1886 Charleston earthquake were not sufficient to cause liquefaction. This finding supports the observation that no surface manifestations of liquefaction occurred in the area

Influence of source-to-site distance and diagenesis on liquefaction triggering of 200,000-year-old beach sand

Liquefaction triggering of the outcropping 200,000-year-old Ten Mile Hill beds sand facies (Qts) near Charleston, South Carolina is characterized in this paper. The characterization includes evaluating the relative susceptibility to liquefaction of Qts assuming a uniform seismic load and using seismic cone penetration testing at 46 sites, the ratio of measured small-strain shear wave velocity to estimated small-strain shear wave velocity (MEVR), and the liquefaction potential index (LPI). The computed LPI values indicate a slight decrease in the liquefaction susceptibility of Qts moving away (5 to 30 km) from the likely source of the 1886 Charleston earthquake (Mw ~ 7). Also characterized is the liquefaction potential of Qts during the Charleston earthquake and a range of other earthquake loadings. The results for the range of other earthquake loadings are summarized in terms of liquefaction probability curves, which provide a method for mapping liquefaction potential in Qts. A back-calculated, lower-bound diagenesis correction factor of 1.11 for areas of Qts where liquefaction surface manifestations were not observed agrees well with the published data