| Principal Investigators and Institutions
Sabanayagam Thevanayagam, University at Buffalo
Geoffrey R. Martin, University of Southern California
Objective
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.
Approach
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.
Products
- A report summarizing current design methods and results of
parametric studies, using the newly developed numerical model, of the
influences of the design input parameters on densification achievable
for silty soils subjected to dynamic compaction.
- Guidelines for design of dynamic compaction in silty soils.
- Design guidelines for stone columns.
Technical Challenges
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. |