Civil Engineering Reference
In-Depth Information
When the groundwater table lies above the bottom of the invert or base
slab of a subsurface structure, an upward pressure - buoyancy - on the
bottom of the base slab equal to the piezometric head at that level must
be accounted for. This can be countered by the weight of the tunnel section,
which can be increased by thickening the walls, roof and base slab or
increasing the width of the base slab. Alternatively, tension piles or tie-
down anchors can be provided.
The design of the excavation side supports (shoring systems) must minim-
ize movements and hence subsidence to adjacent structures and services.
Any temporary or permanent lowering of the groundwater must also be
considered in terms of the effects on adjacent structures and services.
5.8.4 Excavation support methods (shoring systems) for the
sides of the excavation
There are many methods that can be used to support excavations and only
some of these will be described in this section. For further details on this
subject the reader is directed to Macnab (2002) for instance. The design
of the excavation support system will depend on many factors, which
include (after Kuesel and King 1996):
•
the physical nature of the ground in which the excavation is to take
place, including below the final excavation level;
•
the position of the groundwater table during construction;
•
the width and depth of the excavation;
•
the configuration of the subsurface structure to be constructed and
whether the excavation supports will be incorporated into this structure
or not;
•
the proximity of the excavation to adjacent structures;
•
the number, size and type of utilities crossing the proposed excavation
and also adjacent to the excavation;
•
the surcharge loading adjacent to the excavation from traffic or
construction equipment;
•
noise restrictions in urban areas.
The excavation support methods described in this section are:
•
sheet piles with walings and struts, or ground anchors;
•
ground anchors;
•
soldier or king piles and horizontal poling boards (lagging);
•
slurry trench walls;
•
large diameter bored piles, contiguous or overlapping ('secant' piles).
