Liquefaction Risk Mitigation Liquefaction is a phenomenon that occurs in saturated soils that involves the transfer of the effective overburden load from the soil grains to the pore fluid, with the commensurate reduction in effective stress, causing a reduction in soil strength. Excess pore pressure can be dissipated through compacted gravel piles, as shown by recent tests completed at Iowa State University and Virginia Tech. In earthquake-induced liquefaction, transfers are initiated in sandy soils by the collapse of the soil skeleton due to earthquake shaking. Research on evaluating liquefaction potential during earthquakes began in earnest in 1964 following the Anchorage, Alaska, and Niigata, Japan earthquakes. However, liquefaction had been observed during earthquakes well before 1964, and was even linked to the destruction of Sodom and Gomorrah detailed in the Book of Genesis. Also, liquefaction has been documented in almost all, if not all, significant earthquakes since 1964 and is responsible for much of the damage and economic loss in Christchurch, New Zealand during the recent 2010-2011 Canterbury earthquake sequence. In U.S. practice, liquefaction potential is most commonly evaluated using the “simplified” procedure that was developed independently by Robert V. Whitman in 1971, and H.B. Seed and I.M. Idriss, the same year. This procedure is semi- empirical and is largely based on laboratory and field observations. The procedure has been continually refined and updated as a result of newer studies and the increased number of earthquake liquefaction case histories. However, the basic form of the procedure has remained unchanged and provides a factor of safety (FS) against liquefaction wherein the load imposed on the soil by the earthquake is quantified in terms of cyclic stress ratio (CSR) and the ability of the soil to resist liquefaction in terms of cyclic resistance ratio (CRR). Figure 1 shows the simplified liquefaction evaluation procedure chart in which both CSR and CSR are normalized to a magnitude 7.5 earthquake. CSR is M7.5 Accordingly, liquefaction can be miti- 0.00 0.10 0.20 0.30 0.40 0.50 0.60 CRRM7.5 Liquefaction CSRM7.5 gated by increasing CRR and/or decreasing CSR, with approaches to achieve this listed in Table 1. Almost all soil improvement techniques developed to mitigate liquefaction entail more than one of the approaches listed in Table 1. For example, CRRM7.5 No Liquefaction 0 10 20 30 40 N 1,60cs Figure 1. Simplified liquefaction evaluation procedure chart proportional to the intensity of shaking at a site and CRR is proportional to M7.5 penetration resistance of the soil at the site, with penetration resistance being quantified in the figure in terms of the standard penetration test’s (SPT) corrected blow count (i.e., N ). If a given 1,60cs earthquake scenario at a site is such that it plots above the CRR curve, liquefaction M7.5 is expected to occur, while if it falls below the curve, liquefaction is not anticipated. More often than not, considerations other than geotechnical determine whether a site will be developed, relying on geotech- nical engineers to design and implement strategies to mitigate against the risk of liquefaction at the site, if liquefaction is anticipated to occur. Per the simplified liquefaction evaluation procedure, the FS against liquefaction is defined as FS = ______ CSRM7.5 CRRM7.5 50 displacement stone columns and Impact Piers) densify the soil, increase lateral stress, provide a mechanism for rapid dissipation of excess pore water pressures, and reinforce the soil/redistribute shear stresses; this is illustrated in Figure 2. However, the initial spacing of compacted gravel piles is often determined by considering only soil reinforcement/ seismic shear stress redistribution, according to J.I.Baez (1995). The final spac- ing is often determined by considering only increases in lateral stress and soil density, and the mechanism to rapidly dissipate excess pore pressures is generally not considered. Capacity C1) Increase Soil Density C2) Increase Lateral Stress C3) Prevent Collapse of Soil Skeleton i) Bond Soil Particles Together ii) Fill Voids with Grout C4) Provide Mechanism for Rapid Dissipation of Excess Pore Pressures Demand D1) Soil Reinforcement/Seismic Shear Stress Redistribution D2) Increase Vertical Effective Stress D3) Shift Fundamental Period of Soil Profile Table 1. Approaches to mitigate liquefaction To quantify the contribution to AUTHORS: Russell A. Green, Department of Civil and Environmental Engineering, Virginia Tech Blacksburg, VA Jongwon (John) Lee Paul C. Rizzo Associates, Inc. Pittsburgh, PA liquefaction mitigation by rapidly dissipating excess pore pressures through compacted gravel piles, a parametric study was performed using the finite element code FEQDrain, documented by J.M. Pestana et al., in 1997. However, before any meaningful numerical analyses could be performed, an accurate estimate of the DEEP FOUNDATIONS • SEPT/OCT 2012 • 47 compacted gravel piles (e.g., vibro- ® CSRM7.5
