Task E2-1: Liquefaction and Remediation in Silty Soils
Subject Area: Geotechnical and Foundation Engineering
|Principal Investigators and Institutions
Sabanayagam Thevanayagam, University at Buffalo
The objective of this task is to develop an improved remediation technique and design method to mitigate liquefaction hazards in silty soils using dynamic compaction and stone columns, supplemented with wick drains. Significant progress was made in understanding the liquefaction and post-liquefaction densification process in silty soils during Research Year 1; during Research Year 2, a numerical model was developed to simulate pore pressure generation, dissipation, and densification during stone column installation and during an earthquake. This will be further verified and refined using field test data during first half of Research Year 3.
The Research Year 3 task will also focus on using the knowledge gained from Research Years 1 and 2 to develop a design method and guidelines for dynamic compaction that is supplemented with wick drains. Development of the methodology involves extending the numerical model developed for stone columns to include dynamic compaction, parametric studies on influence of design parameters, field verification and development of final design guidelines.
Subtask 1 - Numerical Studies (pore pressure generation, liquefaction, and dissipation / densification during dynamic compaction) Current approach for design of dynamic compaction involves past experience at similar sites, field trials, or simple numerical analyses using wave equations similar to those employed in pile driving analysis. Extrapolations of past field experiences and analysis using wave equation approximations (assuming drained conditions) work fairly well for highly permeable sands where the pore pressures developed during dynamic compaction dissipate rapidly with concurrent densification. In silty soils, slight variations in grain characteristics cause dramatic changes in permeability and dissipation rates, and therefore adversely affects expected performance in a significant way. Simple wave equation analysis without due consideration for coupling of pore pressures also does not work well for such soils. A field trial-and-error approach is often used as a way to determine compaction energy, effective compaction grid spacing, supplementary wick spacing, and time lag between each compaction stage.
Subtask 1 will first involve a literature study of the various design methods currently in use for clean sands. Critical review of this work will be used to determine how the current analytical methods can be extrapolated to silty soils. This task will quantify the effects of grid spacing, input impact energy, wick drain spacing, and time lag between impacts on achievable densification, and the resulting increase in resistance to liquefaction. It will also quantify the effects of soil permeability, compressibility, and fines content, which will then serve as the basis for a comparison of field performance data, and development of design guidelines after further refinements following field performance comparisons.
Subtask 2 - Field Performance Data Collection This subtask involves collection of field performance data in terms of pore pressures and accelerations induced during compaction, and densification following compaction. In discussions with the staff of Hayward Baker Inc., it was indicated that there are plans to use deep dynamic compaction with supplementary drains in a new project in a silty soil site within the next year. A field dynamic compaction project was completed at the Port of Los Angeles (POLA) in mid-2001. Instrumentation and measurement of pore pressures and accelerations due to impact vibrations in the ground will be conducted at Treasure Island, California, with NSF support, in 2002. This offers another opportunity to obtain field data on acceleration and pore pressure distributions induced in the ground due to impact vibrations.
During Subtask 1, the soil characteristics, compaction energy, grid spacing, relevant for the POLA site will be used as input parameters for preliminary analysis and parametric studies. It is possible that another site will also be instrumented to collect data on ground accelerations, pore pressures, dissipation-time data, and densification relevant for the various spacing of supplementary drains and grid patterns under real field trial conditions. This will be used to verify and refine the work resulting from Subtask 1. The results will then be used to develop design guidelines for dynamic compaction in silty soils.
Subtask 3 - Completion of Stone Column Design Guidelines In Research Years 1 and 2, progress was made in understanding the liquefaction and post-liquefaction behavior of silty soils, development of a numerical model for simulation of densification during stone column installation, and parametric numerical studies using available field data (at the Salman Lake Dam site) where stone columns supplemented with wick drains were used. Subtask 3 will involve coordination with Hayward Baker Inc. to collect detailed field data for verification and refinement of the numerical model, and completion of the design guidelines for stone columns in silty soils.
Subtask 4 - Guidelines for Design of Dynamic Compaction for Densification This will be prepared based on the results of Subtasks 1 and 2.
The primary challenge in this task hinges on the development of a model for the extent of pore pressure distribution and liquefaction surrounding the impact zone. Although prior work offers a way to address this issue, it needs to be further developed and verified. Another possible challenge is the need for timely coordination with field activities; communications with appropriate field organizations are already in place, so it is anticipated that this will be successfully completed.
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