0.00 0.10 0.20 0.30 0.40 0.50 0.60 0 CRRM7.5 Liquefaction No 3 1 2 4 Liquefaction Increase CRR: 1. Increase Soil Density 2. Increase Lateral Stress 3. Provide Mechanism for Rapid Dissipation of Excess Pore Pressures Decrease CSR: 4. Soil Reinforcement/Shear Stress Redistribution 10 20 30 N 1,60cs Figure 2. Liquefaction mitigation mechanisms for compacted gravel piles permeabilities of installed compacted gravel piles was needed. In previous studies, often the permeability of compacted gravel piles was assumed to be the same as the permeabilities reported in literature for gravel drains. However, while compacted gravel piles and gravel drains are both stone in columnar form, to a large extent the similarity ends there. This is due to the differences in the installation procedures. Gravel drains, as the name implies, primarily act as drains to rapidly dissipate excess pore pressures (i.e., 3 in Figure 2). Towards this end, the gradation of the stone and installation procedure are selected or designed to minimize the infiltration of soil into drain during installation. In contrast, compacted gravel piles primarily mitigate liquefaction by densifying the soil during installation, increasing lateral stresses, and, to a lesser extent, redistribute shear stresses, as written about by R.A. Green et al., in 2008; and D. Rayamajhi et al., in 2012. As a result, the selection of the gradation of the stone and the installation techniques are not selected/designed to minimize the infiltration of sand into the compacted gravel piles during installation, or during the occurrence of earthquake-induced liquefaction, for that matter. To determine the permeabilities of compacted gravel piles, full scale permeability tests on Impact Piers were 48 • DEEP FOUNDATIONS • SEPT/OCT 2012 performed by Dr. David White and students at Iowa State University, and large scale laboratory tests were performed by the author and students at Virginia Tech. The test results showed that the ratio of the 40 50 parametric study for a range of earthquake scenarios (i.e., earthquake magnitude and site-to-source distance) and pile-to-soil permeability ratios are shown in Figure 3. In this figure, the initial spacing of the compacted gravel piles was determined to achieve a desired FS against liquefaction, without consideration of the rapid dissipation of excess pore pressures. An iterative procedure was then used wherein the FS was recomputed with dissipation effects taken into account, and the spacing of the piles was increased until the desired FS was again achieved. The percent increase in the spacing of the piles with and without dissipation effects taken in to account for a desired FS is shown on the vertical axis in Figure 3. As shown in this figure, there is a benefit from dissipation of excess pore pressures through the compacted gravel piles, but this benefit is minimal. As a result, the current practice of ignoring the mechanism of rapid dissipation of excess pore pressure through compacted gravel piles when determining the initial and final spacing of the piles is likely prudent. Figure 3. Results from a finite element parametric study for a range of earthquake scenarios and pile-to-soil permeability ratios (K /K ) pile h soil permeabilities of compacted gravel piles to the host soil is approximately 2 to 8, as opposed to 200+ that is expected for gravel drains. This relatively low permeability of compacted gravel piles limits their ability to rapidly dissipate excess pore pressures. Some of the results from the finite element The author gratefully acknowledges the contri- butions of the following individuals to this study: Dr. Kord Wissmann, Brian Metcalfe, Allen Minks, Dr. David White, Steve Gyurisin, Kevin Foster, Sam Lasley and Theresa White. Funding for this study was provided by Geopier Foundation Company. CSRM7.5
