Environmental Engineering Reference
In-Depth Information
Figure 2.8.
Cross-section along the grout cap foundation at Mangrove Creek Dam. (MacKenzie &
McDonald, 1985).
Figure 2.9.
Upper surface of Unit E sandstone at Mangrove Creek Dam valley bulge, showing joints
open as much as 100 mm, clay-filled in part.
and the underlying unit D (interbedded) contained zones of crushed rock and clay up to
600 mm thick, apparently produced by overthrust faulting (
Figure 2.10
). This unit also
contained gaping joints parallel to bedding.
To provide a stable foundation for the grout cap and to prevent possible erosion of the clay
filling from joints, cable anchors were installed and a concrete diaphragm wall 30 m long and
15 m deep was constructed beneath the valley floor (MacKenzie and McDonald 1985).
Although usually not as pronounced as in Figure 2.8 to Figure 2.10, destressing effects are
invariably found at and near the floors of valleys in strong to extremely strong rocks, in areas
where high horizontal stresses are known to exist. The effects include sheet joints and gaping
or soil-infilled joints of tectonic origin. These joints usually occur both in the valley walls
and beneath the floor. It is clear from
Figure 2.11a
that large stress concentrations are
