Environmental Engineering Reference
In-Depth Information
Figure 16.27.
Effect of poorly located drain holes on uplift pressures (FERC, 2000).
Figure 16.28.
Effect of aprons on uplift pressures. (a) upstream apron reduces uplift pressure, (b) down-
stream apron increases uplift pressure (FERC, 2000).
Many stilling basins or spillway chutes have drains constructed at the concrete-rock
contact. These may, if they are well constructed and maintained, reduce the pressure at the
interface, but as discussed in Section 16.6.2.6, they will not be effective for potential fail-
ure surfaces within the rock foundation.
16.6.2.6
“Contact” or “box” drains
Many older, smaller concrete dams and spillways do not have a drainage gallery and drain
holes bored into the foundation, but are provided with a gravel filled drains at the contact
between the concrete and the rock.
It is the authors' experience that too much reliance has been put on these drains to
reduce uplift pressures.
Provided they are well constructed, and can be cleaned or inspected to ensure they are
not clogging, contact drains should reduce uplift pressures at the concrete-rock contact.
However as pointed out by EPRI (1992) they will not reduce pressures in the rock,
because the flow of water in the foundation is controlled by the fracture system. If the
