The technical feasibility of a new liquefaction mitigation technique is investigated by introducing small amounts of gas/air into liquefaction-susceptible soils. To explore this potential beneficial effect, partially saturated sand specimens were prepared and tested under cyclic shear strain controlled tests. A special flexible liquefaction box was designed and manufactured that allowed preparation and testing of large loose sand specimens under applied simple shear. Partial saturation was induced in various specimens by electrolysis and alternatively by drainage-recharge of the pore water. Using a shaking table, cyclic shear strain controlled tests were performed on fully and partially saturated loose sand specimens to determine the effect of partial saturation on the generation of excess pore water pressure. In addition, the use of cross-well radar in detecting partial saturation was explored. Finally, a setup of a deep sand column was prepared and the long-term sustainability of air entrapped in the voids of the sand was investigated. The results show that partial saturation can be achieved by gas generation using electrolysis or by drainage-recharge of the pore water without influencing the void ratio of the specimen. The results from cyclic tests demonstrate that a small reduction in the degree of saturation can prevent the occurrence of initial liquefaction. In all of the partially saturated specimens tested, the maximum excess pore pressure ratios ranged between 0.43 and 0.72. Also, the cross-well radar technique was able to detect changes in the degree of saturation when gases were generated in the specimen. Finally, monitoring the degree of partial saturation in a 151 cm long sand column led to the observation that after 442 days, the original degree of saturation of 82.9% increased only to 83.9%, indicating little tendency of diffusion of the entrapped air out of the specimen. The research reported in this paper demonstrated that induced-partial saturation in sands can prevent liquefaction, and the technique holds promise for use as a liquefaction mitigation measure.
The liquefaction response of partially saturated loose sands was experimentally investigated to assess the effect of partial saturation on the generation of excess pore water pressures. An experimental setup including a cyclic simple shear liquefaction box was devised and manufactured. The box includes pore pressure and displacement transducers as well as bender elements and bending disks to monitor the response of partially saturated specimens. Uniform partially saturated specimens with controlled density and degree of saturation were prepared by wet pluviation of powdered sodium perborate monohydrate mixed with Ottawa sand. The reaction of the sodium perborate with pore water released minute oxygen bubbles, thus reducing the degree of saturation of the specimens. The uniformity of a specimen was confirmed with S wave velocity measurements and a high-resolution digital camera. The P wave velocity measurements could only confirm the presence of partial saturation but not the degree of saturation. Partially saturated specimens with varying relative densities and degrees of saturation when tested under a range of cyclic shear strains do not achieve initial liquefaction defined by maximum pore pressure ratio (r u,max) being 1.0. For a given degree of saturation and cyclic shear strain amplitude, the larger the relative density, the smaller is r u,max. For a given degree of saturation and relative density, the larger the shear strain amplitude, the larger is r u,max. The excess pore pressure ratio (r u) can be significantly smaller than r u,max depending on the number of cycles of shear strain. Tests on the sustainability of partial saturation under upward flow gradient and base excitation led to the conclusion that the specimens remained partially saturated without significant change in the degree of saturation. Based on the experimental test results presented in this paper, an empirical model for the prediction of r u in partially saturated sands under earthquake excitation is presented in a companion paper.
Partial saturation in sands attributable to the presence of gas bubbles (not capillarity) can be encountered naturally in the field because of the decomposition of organic matter, or it can be induced for liquefaction mitigation. An empirical model (RuPSS) was developed to predict the excess pore pressure ratio (r u) in partially saturated sands subjected to earthquake-induced shear strains. The model is based on experimental test results on partially saturated sands. Cyclic simple shear strain tests were performed on specimens prepared and tested in a special liquefaction box. Excess pore pressures were measured for a range of degrees of saturation 40% , S , 90%, relative densities D r 5 20 2 67%, and cyclic shear strains g 5 0:01 2 0:2%. The test results demonstrated that partially saturated sands achieved a maximum excess pore pressure ratio (r u,max) when large enough cycles of shear strain were applied. The excess pore pressure ratio (r u) that partially saturated sand can achieve under a given earthquake-induced peak shear strain and the number of equivalent cycles of application can be significantly smaller than r u,max. Therefore, the empirical model was developed in two stages. In the first stage, r u,max was related to S, D r , and shear strain (g). In the second stage, a model was developed relating r u to r u,max , shear strain amplitude (g), effective stress (s9 v), and earthquake magnitude (M). This paper presents the equations that define the predictive models for r u,max and r u. Through these equations, plots for r u,max and r u are provided for ranges of soil and earthquake parameters for ease of use in engineering applications. To illustrate the implementation of the empirical model for predicting r u,max and r u , an example is presented in which a partially saturated sand layer experiencing a peak earthquakeinduced shear strain was analyzed, and the pore pressure response of the sand was evaluated using both the predictive equations and the plots.
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