Civil Engineering Reference
In-Depth Information
reinforced concrete wall that becomes part of the permanent reinstatement
of the river wall. In this way, a seal is provided all around the element,
enabling the approaches to be pumped dry.
As the approaches are pumped out, the end of the last element is exposed
and loses the hydrostatic force that is being applied to it. In such circum-
stances, if there is not enough frictional restraint to prevent movement of
the tunnel element, the overall compression on the gaskets between the
elements is lost as the gaskets expand with the moving of the last element.
Frictional restraint around the elements will limit this, but this type of
movement must not be allowed to occur at all. Thus, it is normal to provide
the end of the last element with some sort of bracing or temporary prop
to prevent any longitudinal movement and retain the compression in the
gaskets. This can be as simple as concrete wedges inserted between the end
of the element and the floor of the approach. The Medway Tunnel in the
United Kingdom was constructed like this and used the base slab of the
approaches to brace against (Figure 8.5).
Medway Tunnel
secondary closure
Figure 8.5
Medway Tunnel end closure detail. (Courtesy of BAM Nuttall.)
