Civil Engineering Reference
In-Depth Information
2.
Liquefaction-prone soils:
If the liquefaction-prone soils are shallow and the ground-
water table can be temporarily lowered, then these soils can be removed and replaced with
different soil during the grading operations. Another option is to remove the potentially liq-
uefiable soil, stockpile the soil and allow it to dry out (if needed), and then recompact the
soil as structural fill.
3.
Earthquake-induced settlement:
As discussed in Sec. 7.3, one approach for level-
ground sites that can be used to reduce the potential for liquefaction-induced ground dam-
age, such as surface fissuring and sand boils, is to add a fill layer to the site. This operation
could be performed during the grading of the site. It should be mentioned that this method
will provide relatively little benefit for sloping ground since it will not prevent structural
damage and surface fissuring due to lateral spreading.
4.
Volumetric settlement and rocking settlement:
Loose soils and those types of soils
that are susceptible to plastic flow or strain softening can be removed and replaced during the
grading operations. Another option is to remove the soil, stockpile the soil and allow it to dry
out, and then recompact the soil as structural fill.
Instead of removing and recompacting the soil during grading, another approach is to
use precompression, which is often an effective method of soil improvement for soft clays
and organic soils. The process consists of temporarily surcharging the soils during the grad-
ing operations in order to allow the soils to consolidate, which will reduce their compress-
ibility and increase their shear strength.
5.
Earthquake-induced bearing capacity:
Similar to the options for settlement, poor
bearing soils can be removed and replaced or surcharged during the grading operations.
6.
Drainage and dewatering systems:
Drainage systems could be installed during the
grading operations. Drainage and dewatering are discussed in Sec. 12.4.
12.3
OTHER SITE IMPROVEMENT METHODS
12.3.1 Soil Replacement
As discussed in the previous section, soil replacement typically occurs during grading. As
indicated in Table 12.1, there are basically two types of soil replacement methods: (1)
removal and replacement and (2) displacement. The first method is the most common
approach, and it consists of the removal of the compressible soil layer and replacement with
structural fill during the grading operations. Usually the remove-and-replace grading
option is only economical if the compressible soil layer is near the ground surface and the
groundwater table is below the compressible soil layer, or the groundwater table can be
economically lowered.
12.3.2 Water Removal
Table 12.1 lists several different types of water removal site improvement techniques.
If the site contains an underlying compressible cohesive soil layer, the site can be sur-
charged with a fill layer placed at ground surface. Vertical drains (such as wick drains
or sand drains) can be installed in the compressible soil layer to reduce the drainage path
and to speed up the consolidation process. Once the compressible cohesive soil layer
has had sufficient consolidation, the fill surcharge layer is removed and the building is
constructed.
