Basements are very common in many developments. It is a floor of a building which is partly or entirely built below ground level. The reasons for constructing a basement can arise from simply a restriction on building height by the building agencies to the need to provide utility space for car parks, storage space and plant rooms. Constructing a basement can also be necessitated by sites with abrupt changes in elevation. Whatever the case, constructing a basement provides an efficient way of making use of the available land by providing more usable floor area.
Structurally, there are two elements that must be considered in the design and construction of a basement; basement slab and basement walls. This article is only concerned with the latter.
Designing a Basement Wall
Basement walls are essentially retaining walls which in most instances, in addition to the lateral forces from soil, pore pressure and surcharge must also carry axial forces due to gravity loads from slabs. At the preliminary design stage, the designer of a basement wall must make a decision between designing a basement wall either as a free cantilever or propped cantilever, or both. This is basically contingent on construction, particularly the construction sequence.
In their final condition, basement walls are invariably propped by the floor the support. Even where large openings exist adjacent to the walls, it is still possible to prop the wall at intervals to the walls, for example, by utilizing beams that frame into the wall. Hence there is a likely consensus that basement walls should be designed as propped cantilevers, however, careful attention must be given to the construction sequence. In many instances basement walls are erected and fully back-filled before the floor slabs are constructed, this makes the assumption, that they should be designed as free cantilevers equally valid.
However, designing a basement wall as simply free cantilevers could result in total disaster, since basement walls are invariably propped in their final condition. The result is a stress reversal, that sees the basement wall produce a positive moment somewhere within the span. Where the designer simply considered them as a free cantilever, he would be over reinforcing the wrong side in the final condition. Propped cantilever retaining walls would have both main reinforcement in the Near face (N.F) & Far Face (F.F) of the wall due to positive and negative moment acting respectively, as opposed to free cantilever walls where the main reinforcement is provided only at the far face of the wall and minimum area of steel provided at the near face to satisfy serviceability issues.
With respect to economy, designing a basement wall as propped cantilever retaining wall is economically better and cheaper to construct, aside the need to ignore the basic stability checks such as overturning and sliding which often leads to larger footing areas in free cantilevers walls, the flexural reinforcement is lesser as a consequence of the reduced bending forces in propped cantilevers.
Thus, basement walls are best designed as propped cantilevers and designers should always do well to leave a footnote explaining the construction sequence, particularly the need for the floor slabs assumed to be propping the basement walls to be constructed first before back-filling. Where the construction sequence is in doubt, the basement walls should first be designed as a free cantilever and then a propped cantilever, to fully capture the most unfavourable loading arrangement.
Actions on Basement Walls
The loads acting on a basement walls can be broken down into two types, the lateral pressures from, the earth retained materials, surcharges and pore water pressure and the vertical actions from the floor slabs applied on the wall. Typically, the following loads should be considered in the design of a basement wall.
- Lateral earth pressure (Active, Passive & At rest)
- Lateral Pore water pressure
- Lateral pressure from surcharges
- Lateral pressure due to loads from adjoining structure
- Vertical loads from superstructure.
How to derive these actions has being covered in a previous article please (See: Structural Analysis of Retaining Walls).
Where a basement wall is to be designed as a free cantilever in it temporary stages, the wall’s stability is a primary concern. In other to achieve stability, the following proportioning rules should be followed as much as possible.
- The width of the footing should be 2/3 of the retained height for most design conditions
- It is usually more advantageous to have more of the footing on the heel side of the retaining wall
- Should there be any property line restriction on the heel side, use the maximum heel width possible
- If a key is required, the length should be kept within one-fourth of the retained height
- If there’s a property line restriction on the toe, the footing should be increased slightly because soil pressures are usually greater at the toe.
Only two elements need to be designed; the stem and the base. Both of them can be subjected to very high bending moments that must be addressed. To design the base, the basement wall is considered to be slab with it maximum moment occurring at the point where it interfaces the stem. The stem is designed either as a propped cantilever or a free and propped cantilever depending on the proposed construction sequence.
In designing a basement wall, the following procedures should be followed:
- Establish the construction sequence and compute all the applied forces acting on the wall, lateral earth pressures, water pressures, surcharges etc.
- Choose a trial section, determining the sizes of the stem and footings.
- Check stability of the retaining wall against overturning and sliding and bearing capacity, if the wall is considered a free cantilever in it temporary stage, otherwise ignore this check.
- Verify bearing pressures
- Design the stem for flexure and shear assuming the wall to be a propped cantilever beam/slab subjected to the at-rest earth pressure, water pressure and horizontal components of surcharges in its permanent stage or in addition, a cantilever beam subjected to the active earth pressure, water pressures and other horizontal components in its temporary stage.
- Design the base as a typical pad/strip foundation which must resist the effect of soil pressure with maximum moment occurring at the face/centreline of the stem.
The following detailing rules should be followed when providing the reinforcing bars in the basement wall.
- The minimum area of steel required in a retaining wall should not be less than:
- Vertical reinforcement: 0.002Ac with the smallest bar being 12mm in each face of the wall
- Horizontal reinforcement: 25% of the vertical reinforcement or 0.002Ac (whichever is greater) in each face
- The maximum area of steel possible should not be greater than 0.04Ac
A Basement wall is required for a newly proposed residential development to retain a 4.0m height of granular material soil. A subsurface investigation has been carried out and results showed that the water table is below the retained soil. In addition to the lateral loads, from the granular soil and surcharges from moderate traffic, the wall is also required to also sustain actions transferred from floor slabs. Design this wall completely using C20/25 concrete and 460Mpa rebars.
|Soil unit weight = 18kN/m3||Surcharge Action = 10kN/m2|
|Shear resistance angle = 30°||Floor Slab gk=45kN ; qk =18kN|
|Coefficient of friction = 0.55|