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5.3 DESIGN OF SOIL MITIGATION
The design of soil mitigation strategies involves investigating the cost/benefit ratio, the seismic
performance, and the effect of the mitigation technique(s) on adjacent structures. The mitigation
methods listed in Table 5.1 can be generally categorized in terms of their affect on the soil:
compaction, drainage, or cementation. The design for each of these remediation categories is
briefly discussed below. A more in-depth discussion on the design of soil mitigation strategies
and procedures may be found in the reference Handbook on Liquefaction Remediation of
Reclaimed Land (PHRI 1997).
Compaction remediation methods densify the soil with vibration or impact (examples include
methods 1, 2, 3, 4 and 14 from Table 5.1). Compaction methods are more suitable for use in
saturated, cohesionless soils with a limited percentage of fines. They cause noise and vibration
during installation, and also increase horizontal earth pressures against adjacent structures. This
increase in pressure is the major disadvantage of using compaction methods in close proximity to
retaining walls. The major advantage of compaction methods is the relatively low cost/benefit
ratio. The necessary degree of compaction can be evaluated using penetration resistances that
have been back-calculated from an acceptable factor of safety against liquefaction (Chapter 3).
Drainage remediation methods enhance the rate of excess pore pressure dissipation. The most
common methods use gravel, sand or wick drains. Drains are suitable for use in sands, silts or
clays. One of the greatest advantages of drains is that they induce relatively small horizontal
earth pressures during installation. Therefore, they are suitable for use adjacent to sensitive
structures. In the design of drains, it is necessary to select a suitable drain material that has a
coefficient of permeability substantially larger than the in situ soils.
Cementation remediation methods increase soil strength by adding a cementatious material (i.e.
cement, grout, lime, chemicals, asphalt). Cementation techniques (methods 5, 8, 9, 10, 12, and 14
from Table 5.1) can be used with any type of soil. They are advantageous because the installation
methods are relatively quiet and induce relatively small vibrations as compared to compaction
methods. The induced horizontal earth pressures are smaller than with compaction methods, and
are larger than with drainage methods. Their disadvantage is the relatively high cost/benefit ratio
as compared to compaction and drainage methods.
The relative performance of the specific improvement method also is of concern in the design of
a mitigation program. Experience has demonstrated that compaction and cementation techniques
reduce the liquefaction susceptibility of soils to a larger extent than drainage methods alone.
The influence of ground treatment on existing structures is a primary design consideration. The
construction methods may lead to increased horizontal earth pressures, which can result in
deformations of embankments, walls and pile foundations. Mitigation methods also may induce
excess pore pressures and vibration, which will affect sloping embankments and retaining
structures. Therefore, a mitigation strategy design may include the combination of two or more
improvement techniques in order to take advantage of robust treatment methods in free field
areas and less disturbing methods close to existing structures. In general, compaction techniques
have the largest impact on adjacent structures, followed by controlled cementation techniques,
and then drainage methods, which generally are the least disruptive.
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