Environmental Engineering Reference
In-Depth Information
FIGURE 3.7
Continuous trenching and filling of reactive media. (Reproduced from University of Waterloo.)
no open trench at any time. Continuous trenching is the most rapid PRB
installation method. Continuous trenches can be dug to install a treatment
zone from 0.3 to 0.9 m wide to a depth of 11 m. An arrangement of parallel
trenches can be made to install thick walls. Although the cost of the continu-
ous trenching equipment is relatively high, the fast trenching rate may make
the overall installation cost economical for a larger barrier.
3.3.1.3 Unsupported and Supported Excavation
PRBs can be constructed without any sidewall supports (including biopoly-
mer methods) if the trench can remain open for about 4 h without any sig-
nificant caving in. This method is typically limited to depths of 6 m or less.
A standard backhoe can be used for the excavation. If a trench wall collapses
it is not possible to rectify it and a new trench needs to be constructed a few
meters away, which may not be an ideal location for the PRB. Caution must
therefore be exercised during unsupported excavation. Supported excava-
tion for trench side walls can be constructed by trench boxes or hydrau-
lic shores. The depth of the trench should be <6 m. Successful installation
depends on an effective site layout, construction sequence and the selection
of heavy equipment, flexibility in the construction method to accommodate
unforeseen conditions, an understanding of backfill materials, the potential
impact on the community and the environment, and the season for the con-
struction work.
3.3.1.4 Sheet Piling
During trench construction sheet piles are driven around the perimeter of the
PRB and the soil. Typically, internal bracing is required as depth increases.
After backfilling is completed the sheet piling is removed and the GW is
allowed to flow through the treatment zone, also known as “the cofferdam
approach.” A crane with a vibrating hammer is used to install the imperme-
able barrier sheet pile for the construction of the funnel of a funnel-and-gate
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