Principal Investigator(s) and
Institution(s)
Sabanayagam Thevanayagam, University at Buffalo
Objective
This task will develop an improved remediation technique and design method to mitigate
liquefaction hazards in silty soils using stone columns. It involves laboratory studies on
the behavior of silty soils, numerical studies on pore pressure generation, dissipation,
densification of silty soils during stone-columns installation, and performance evaluation
using scale-models. The end product from this research will be a report summarizing the
improved design method with typical design charts and guidelines for installation of stone
columns.
Approach
Many of the past earthquake-induced ground failures and large deformations observed in the
built environment have occurred mostly in sites containing non-plastic silty soils. The
need to develop new methods to assess liquefaction potential of these soils and to
cost-effectively remediate such sites to mitigate earthquake induced damages has been
widely recognized. Experience indicates that silty soils behave differently than clean
sands. Silty soils have low permeability and are relatively more compressible. Their
liquefaction (pore pressure) behavior is also different. There is consensus among
engineers and contractors that the traditional remediation techniques (densification,
reinforcement, or solidification by grouting or admixtures) used for sandy soils need to
be modified to suit the need for silty soils. The approach to the solution is two-fold, as
described below.
The first issue that needs to be addressed is the matrix behavior of silty soils and what
constituents control their behavior. Recent studies have shed some light on this issue. It
is now recognized that a silty soil can be judiciously characterized in terms of the
behavior of the host silts and/or host sands. At fines contents less than typically 20 to
30%, the intergranular void ratio controls the behavior. Beyond that the silt primarily
controls the behavior with minor reinforcement effect by the sand grains. Further work is
still needed to quantify the matrix effects. However, with this framework, it is now
possible to analytically study and develop appropriate techniques for remediation of silty
soils.
Among potential techniques, vibratory stone columns has appeal. It combines densification,
reinforcement, and provision for drainage of excess pore water pressures. But, how to
account for these effects in silty soils is the question. The central issues are the
effects of low permeability and compressibility characteristics of the silt matrix on pore
pressure generation, dissipation, densification of the soil during stone-column
installation, and dissipation during earthquakes. These issues need to be understood and
incorporated in the analysis in order to appropriately design and install stone-columns,
and provide confidence that they will mitigate liquefaction damage in silty soils.
As a result, this task will be comprised of the following subtasks:
Subtask 1 - A laboratory study of silts under undrained and drained loading conditions
will be performed aimed at characterizing liquefaction (pore pressure) behavior,
permeability, and compressibility characteristics of these soils at conditions relevant
during installation and earthquakes. This will increase our understanding of liquefaction
behavior and post-liquefaction pore pressure dissipation phenomenon in natural silty
soils. To this effect, natural silty soils recovered from Oregon, the San Fernando Dam
site, the Memphis, Tennessee area (a mid-America site), and other (eastern) silty sites
will be used. They will be subjected to undrained monotonic and cyclic loading followed by
drained dissipation conditions. Their liquefaction, pore pressure generation,
permeability, and compressibility characteristics will be determined at density (silt void
ratio) levels relevant for detrimental field conditions. Effects of sand content will also
be studied. This will provide input variables for numerical studies in subtask 2.
Subtask 2 - Numerical and semi-theoretical studies of pore pressure generation,
dissipation, and densification of silty soils during stone-column installation and
earthquakes will be conducted. This subtask will quantify the effects of stone-column
design variables (installation procedure, diameter, stone drainage characteristics, etc.)
and silt characteristics on the level and extent of pore pressure generation, dissipation
and densification achievable in the surrounding silty soils during installation, and
assess the effectiveness of dissipation during earthquakes. The use of wick drains to
enhance dissipation of the installation pore pressures will also be studied.
Semi-theoretical numerical analyses will be carried out addressing the effects of the
above design variables and material characteristics on the effectiveness of stone columns.
Subtask 3 - Experimental evaluations of stone column performance and assessment of their
effectiveness in silty soils will be performed. This will provide an assessment of the
effectiveness of stone columns and their performance during shaking using scale-model
experiments.
Subtask 4 - Guidelines for design and installation of stone columns will be developed. The
results from the above three subtasks will provide the necessary insight and information
needed to develop a design method and guidelines for installation of stone columns in
silty soils.
Anticipated Start Date and Duration
January 1, 1999 - 24 months
9/24/99 |