Browsing by Subject "Ground improvement"

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Effect of prefabricated vertical drains on pore water pressure generation and dissipation in liquefiable sand
(2010-05) Marinucci, Antonio; Rathje, Ellen M.; Stokoe II, Kenneth H.; Wilson, Clark; Gilbert, Robert; Zornberg, Jorge
Soil improvement methods are used to minimize the consequences of liquefaction by changing the characteristics and/or response of a liquefiable soil deposit. When considering sites with previous development, the options for soil improvement are limited. Traditional methods, such as compaction and vibratory techniques, are difficult to employ because of adverse effects on adjacent structures. One potential method for soil improvement against soil liquefaction in developed sites is accelerated drainage through in situ vertical drains. Vertical drains expedite the dissipation of excess pore water pressures by reducing the length of the pore water drainage path. For more than thirty years, vertical gravel drains or stone columns have been employed to ensure the excess pore water pressure ratio remains below a prescribed maximum value. In recent years, the use of prefabricated vertical drains (PVDs) has increased because the drains can be installed with less site disruption than with traditional soil improvement methods. To date, little-to-no field or experimental verification is available regarding the seismic performance of sites treated with PVDs. The effectiveness of PVDs for liquefaction remediation was evaluated via small-scale centrifuge testing and full-scale field testing. A small-scale centrifuge test was performed on an untreated soil deposit and on a soil deposit treated with small-scale vertical drains. Compared to the untreated condition, the presence of the small-scale vertical drains provided numerous benefits including smaller magnitudes of excess pore water pressure generation and buildup, smaller induced cyclic shear strains, reduced times for pore pressure dissipation, and smaller permanent horizontal and vertical displacements. In addition, full-scale in situ field experiments were performed in an untreated soil deposit and in a soil deposit treated with full-scale PVDs using a vibrating mandrel as the dynamic source. In the untreated test area, the maximum induced excess pore pressure ratio reached about 0.95. In the treated test area, the vibratory installation of the first few drains generated significant excess pore pressures; however, significant excess pore pressures were not generated during the vibratory installation of additional drains because of the presence of the adjacent drains. Additionally, the vibratory installation of the drains caused significant settlement and significantly altered the shear wave velocity of the sand. Dynamic shaking after installation of all of the drains induced small accelerations, small cyclic shear strains, and negligible excess pore water pressures in the soil. The results of the field experiment indicate that the prefabricated vertical drains were effective at dissipating excess pore water pressures during shaking and densifying the site.
Field evaluation of large-scale, shallow ground improvements to mitigate liquefaction triggering
(2017-10-26) Roberts, Julia Nicole; Stokoe, Kenneth H.; Andrus, Ronald D.; Cox, Brady R.; Rathje, Ellen M.; Wilson, Clark R.
Much of the devastation wrought by the 2010-2011 Canterbury Earthquake Sequence (CES) in Christchurch, New Zealand, was caused by extreme levels of liquefaction-induced damage to structures with shallow foundations. In response to this disaster, the New Zealand Earthquake Commission (EQC) funded a large study known as the Ground Improvement Trials to evaluate and identify shallow ground improvement methods that are not only effective at increasing the soil’s resistance to soil liquefaction, but are also cost effective and practical to build for lightweight structures. Of the nine ground improvement methods included in the trials, three were selected for extensive analysis in this dissertation. These three ground improvement methods are the Rapid Impact Compaction (RIC), the Rammed Aggregate PiersTM (RAP), and the Low-Mobility Grout (LMG). At three test sites along the Avon River in Christchurch neighborhoods that were among the worst hit by liquefaction-related damage, full-scale test panels of natural soil and ground-improved soil were constructed and evaluated using a variety of in situ test methods. The analysis in this dissertation primarily relies on data from excavation trenching, cone penetrometer testing (CPT), direct-push crosshole testing (DPCH), and shake testing with T-Rex. These tests capture changes in density and stiffness, and therefore liquefaction resistance, due to the ground improvement methods in comparison to the natural soil. Shake testing with T-Rex is further able to define the relationship between cyclic shear strain and the generation of excess pore pressure that ultimately determines whether or not a soil will liquefy under cyclic loading. Under this framework, the effectiveness of each of the three ground improvement methods is evaluated and discussed.