Civil Engineering Reference
In-Depth Information
However lateral earth pressure is not a unique value, varying over a range between
the extreme limits of active and passive pressure. It is advisable to design for pressure
at rest (
K
o
), and to check the sensitivity of the design to reasonable variations in the
pressure. Heavy vehicles over the haunches will increase the lateral earth pressure on
the arch as well as the vertical pressure. Such eccentric loading will cause the arch to
sway sideways, away from the load, creating a countervailing earth pressure on the
unloaded side. This may be simulated in analysis by modelling the soil as a series of
springs. The arches also had to be designed for the situation near the portals where the
embankment ramped down on one side.
The structures were precast in half arches 3.2 m long, and erected in pairs without
falsework, Figure 17.10. They were placed in sockets on the foundations, on carefully
levelled packs, and then grouted in. The precast units were then stitched at the crown
to complete the structure. At the request of the client, on New Cowdens they were
also stitched all the way down the arches to create a monolithic roof. Both arches were
entirely covered with a waterproof membrane.
At the portals it is necessary to retain the earth above the arch. This may be done
by building a conventional spandrel wall. However, this causes some diffi culties, as
the arch ring is likely to be too slender to provide moment fi xity for a cantilevered
retaining wall. One solution is to allow the spandrel wall to span horizontally between
counter forts over the arch springings. Alternatively, the spandrel wall may be made of
reinforced earth. There is also a need for wing walls to contain the fi ll in the approach
to the arch.
Figure 17.10
Maryville arch under construction (Photo: Benaim)
