Tuesday, February 19, 2013

Retaining walls and basements.

In commercial developments occupying congested city centre sites it has become common to utilize deep basements to provide accommodation for plant room, car parking and other areas. The depth of these basements requires careful consideration of the aspects of design and construction in order to achieve a satisfactory engineering solution. For the engineer requiring a full explanation of  the approach to design and the methods of construction  of deep basements, reference should be made to the IStructE publication where this topic is dealt with in comprehensive detail.

Retaining walls and peripheral walls to basements are subject to lateral (i.e. horizontal) pressure from retained earth, liquids or a combination of soil and water. They are normally made, in structural work, of concrete or brick (plain, reinforced or prestressed).

Basements are relatively expensive to construct (the cost per square metre is higher than for normal floor construction) so the client should be advised to carry out a cost  evaluation of, say, adding a further storey to the structure and eliminating the basement. However, basements can  be made cost-effective when they are used as cellular buoyancy rafts or where increased height is restricted by planning.

The walls are basically vertical cantilevers, either free or propped (at the top by a floor slab). Where the ground floor slab can be made continuous with the top of the wall (and not merely be propped) the basement can be designed as a continuous box. The walls can be constructed with either a base slab extending under the retained earth (see Fig. 15.1 (a)), which is generally the more economical form for  cuttings, or projecting forward (see Fig. 15.1 (b)), the more  economical form for basements.

 Typical retaining walls.
Fig. 15.1 Typical retaining walls.

While propped cantilevers (e.g. basement wall propped by ground floor slab) have a maximum bending moment (for a udl) of  pH^2 /8, compared to that of a free cantilever of  pH^2 /2, they are not frequently used in building structures.

This is because the wall must either be temporarily propped, or not backfilled, until the ground floor can act as the prop.

However, in the authors’ experience it can be worth considering the use of the more economical propped cantilever, especially for design-and-build contracts, where a close relationship is developed with the contractor from an early stage, and construction methods can be programmed into the design. It is important to provide a clear route for  the propping force through the substructure and to take account of any out-of-balance lateral forces, such as those resulting from sloping backfill on one side of the structure.

The bending moment diagrams for triangular pressure (i.e. no surcharge) for the three cases: free cantilever, propped cantilever and cellular (fixed), are shown in Fig. 15.2.

Bending moment diagrams for retaining walls.
Fig. 15.2 Bending moment diagrams for retaining walls.

It can be seen that partially filling a basement with water can equalize the external earth pressure on the basement wall. The authors’ practice has used this method of temporary propping, raising the water level as backfill is placed. Where the basement is con- structed in waterlogged ground, filling the basement in this way can also be utilized to avoid flotation before the weight of the rest of the building is added.

Walls to swimming pools are a special case since they can be subject to reversal of stress. With the pool empty, the wall is subject to earth/water pressure on its earth face and with the pool full and earth pressure absent (either due  to shrinkage of backfill or water testing for leaks, before backfilling), the wall is subject to water pressure alone on its water face (see Fig. 15.3).

Walls to culverts can similarly be subject to reversal of stress under the two conditions of earth pressure acting alone or when the water pressure is acting alone. Service ducts, boiler houses, inspection chambers and similar excavated substructures can unwittingly be subject to internal water pressure acting alone, which needs to be designed for. This has happened when heavy rainfall during con- struction has flooded and filled the substructures with water before the backfill has been placed.

Pressures acting on swimming pool walls.
Fig. 15.3 Pressures acting on swimming pool walls.

1 comment:

  1. Nice article. Thank you.
    Load reversal on swimming pool walls comes with another issue worth mentioning - flotation.
    We try to brace propped cantilever retaining walls during construction especially since the compactive effort on the soil increases loads on the wall.