Monday, January 7, 2013

Retaining Walls.

Retaining walls in relation to structural foundations are dealt with here in general terms. Such walls are a necessary part of many foundations where changes in level occur.

They may be used to retain earth or other material within, below or around the foundation and can be constructed in numerous forms from a number of materials. Common materials used are plain, reinforced and prestressed masonry, and plain, reinforced and prestressed concrete (see Fig. 9.50 for some examples).

The walls may be acting as pure cantilevers, propped cantilevers, tied cantilevers, simply supported or continuous spanning slabs, etc. (see Fig. 9.51). They may be stiffened by shaping into fins, counterforts, diaphragms, zig-zag, and many other profiles (see Fig. 9.52). They can be mass filled, reinforced or post-tensioned (see Fig. 9.53). The engineer should apply skill and ability in arriving at the most suitable and economic form for each individual situation.

The design of retaining walls in relationship to foundations does mean that the normal design to retain earth can become secondary to or parallel to the overall foundation behaviour. For example, where the building is constructed on a raft foundation and the retaining wall becomes part  of the raft, then continuity of raft stiffening ribs are most critical to the design and detail (see Fig. 9.54).

The location of settlement or other movement joints through foundations which embrace the retaining walls can be critical to or dictate the structural behaviour of the wall, for example, by effectively removing the prop/tying action of the upper floor slab of a change in level (see Fig. 9.55).

In mining areas the need to relieve horizontal ground  stress by allowing the foundation to move relative to the sub-strata can conflict with the need to resist lateral loads in a retaining situation. On sloping sites this conflict can often be overcome by the detail shown in Fig. 9.56.

Where a basement is required on a flat mining site the conflict is more difficult and much greater forces have to be resisted by the building foundations (see Fig. 9.57).

Fig. 9.50 Basement retaining walls.

 Fig. 9.51 Retaining walls – design approach.

Fig. 9.52 Masonry retaining walls/plan forms.

Fig. 9.53 Masonry retaining walls – structural forms.

 Fig. 9.54 Retaining wall/raft slab.

 Fig. 9.55 Retaining wall/movement joint.

Fig. 9.56 Mining raft slab/retaining wall.

Fig. 9.57 Mining (basement) raft /retaining wall.

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