27-12-2012, 02:23 PM
Liquefaction mitigation in silty soils using composite stone columns and dynamic compaction
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Abstracts:
The objective of this study is to develop an analytical methodology to evaluate the effectiveness ofvibro stone
column (S.C.) and dynamic compaction (D.C.) techniques supplemented with wick drains to densify and mitigate liquefaction
in saturated sands and non-plastic silty soils. It includes the following: (i) develop numerical models to simulate and analyze
soil densiflcation during S.C. installation and D.C. process, and (ii) identify parameters controlling post-improvement soil
density in both cases, and (iii) de'~elop design guidelines for densification of silty soils using the above techniques. An
analytical procedure was developed and used to simulate soil response during S.C. and D.C. installations, and the results
were compared with available case history data. Important construction design parameters and soil properties that affect the
effectiveness of these techniques, and construction design choices suitable for sands and non-plastic silty soils were identified.
The methodology is expected to advance the use of S.C. and D.C. in silty soils reducing the reliance on expensive field trials
as a design tool. The ultimate outcome of this research will be design charts and design guidelines for using composite stone
columns and composite dynamic compaction techniques in liquefaction mitigation of saturated silty soils.
Introduction
Liquefaction is one of the primary causes of lateral
spreading, failures of bridge foundations, embankments,
and ports and harbor facilities during earthquakes (e.g.
1964 Alaska earthquake, 1995 Kobe earthquake). Soil
densification techniques using vibro-stone column
(Fig.l(a)) and deep dynamic compaction (Fig.2(a)) are
proven ground improvement methods for liquefaction
mitigation in loose saturated sands containing less than
15% non-plastic silts and less than 2% of clay particles
(FHWA 2001, Mitchell et a/.,1995, Andrus and Chung,
1995). Silty sands containing excessive fines have
been considered difficult to densify using the above
densification methods. However, recent case histories
show that provision of pre-installed supplementary wick
drains around the vibro-stone columns (Figs.l(b)) and
impact locations (Fig.2(b)) help densification of silty
soils during vibro-stone column installation or dynamic
compaction (Andrews, 1998; Disc et al., 1994; Luchring
et al., 2001 ).
Semi-theoretical framework
Densification of saturated sands and silty soils
by vibro-stone column and dynamic compaction is
essentially a process involving vibration of the soil
causing excess pore pressure development, liquefaction,
and consolidation of the soil leading to concurrent
densification. Vibro-stone column also involves
expansion of a zero cavity and associated pore pressures
and densification of the soil. This paper presents a
methodology to simulate pore pressure developments
in the soil due to vibratory energy imparted during
installation, and subsequent consolidation of the soil
and densification. Simple attenuation relationships
are used to estimate the energy dissipated in the soil.
Experimental data based on energy principles is used to
estimate the pore pressures generated as a function of
the energy dissipated.
Energy radiation and attenuation
Consider vibro-stone column (S.C.) (Fig.3) and
dynamic compaction (D.C.) impact (Fig.4) processes.
The energy delivered at the source by the vibratory
probe and by a falling weight propagates through the
surrounding soil as body waves (compressional and
shear waves) in case of S.C., and body waves and surface
waves (Rayleigh waves) in the case of D.C., repectivcly.
Field observations indicate that the ground vibrations
caused by S.C. is in the range of 30 to 50 Hz (FHWA,
200l) and between 2 to 20 Hz (Mayne 1985) for D.C. A
solution for energy dissipated (per unit volume of soil),
the associated pore water pressures, and densification
at any point in the soil requires a reasonably accurate
quantification of energy partitions in the above three
categories and their spatial attenuation relationships.
The problem is complex due to non-uniformity in stress
field, stress and density dependent soil properties, and
changes in the stress field, pore water pressures~ and
soil densities in the ground during and immediately
following the energy delivery. In order to circumvent
this problem, as a first order approximation, models for
energy partition in elastic half-space coupled with field--
observation based attenuation models are used herein.
Simulations and f i e l d comparisons
Vibro-stone column
Two sets of numerical simulations were conducted
to study densification process of soils during stone
column installation. In the first set of simulations, the
effect of cavity expansion was neglected and the effect
of vibration induced pore pressure generation and
dissipation was considered. Based on experimental data
available, hydraulic conductivity k was obtained as a
function of silt content. In the second set of simulations,
the effect of cavity expansion was included and the
vibration induced pore pressures were neglected. In both
set of simulations vertical dissipation was neglected
in order to reduce the computational time. These
simulations are presented below.