Analysis of concrete retaining wall design is usually considered using the appropriate codes and design methods. Concrete retaining walls are useful within the built environment, especially at the bridge sites, riverbank area, and even within the house in a sloppy terrain. Swimming pools are common features in houses and more often than not concrete retaining walls are used to retain liquid either in the form of overhead or surface tanks.
Concrete retaining wall design is intended to provide retaining walls that can adequately resist lateral loads and hold back soil materials in a sloppy area, therefore a concrete retaining wall should be able to support earth backfill without possible lateral movement of the soil, and the surface of the backfill must not settle unduly. All these are properly checked in the design of concrete retaining walls.
Design Consideration of Concrete Retaining Walls
In the choice of and design of concrete retaining walls, the following must be considered:
- Materials for construction: The materials, labor, and technology available for construction must be considered in the concrete retaining wall design. Where technology may not support a particular type of concrete retaining wall other types may be adopted, in the case of basement walls, the materials, and technology for waterproofing is of utmost importance in concrete retaining wall design.
- Design technique: This involves the manipulation of design principles to achieve a more economical design. For instance, the use of curved or folded plan shape to save cost or extend the foundation to a deeper depth to reduce sliding and overturning.
- Construction technique: where very soft or loose soil is encountered underneath the retaining wall pilling may be used to support the walls at intervals. the wall will be erected on the coping beam serving as the pile cap. In addition erosion on the face of the wall should be prevented through property protection. joints, where applicable, should be properly designed and constructed not to become the source of weakness to the entire structure.
- Drainage: In concrete retaining wall design, weep holes must be provided in the wall to prevent hydraulic pressures build-up which can otherwise increase loads unduly. Adequate drains should be provided in front of the walls to collect water dripping from the weep holes.
- Service: For a perfect concert retaining wall design, provision must be made for services to penetrate the walls without loss of strength or functionality especially in waterproofing and in cases where walls are used as the basement to houses.
Analysis and Design of Concrete Retaining Walls
The analysis and design of the concrete retaining walls are in stages which are outlined as follows:
- The determination of the horizontal pressure of the earth, to be supported by the wall.
- Assumption of the width of the base and the determination of the factors of safety against overturning and sliding forward.
- The calculation of the pressure on the ground under the base and comparing this with permissible bearing pressure.
- Calculation of the bending moment on the cantilever wall or for walls with counterfort, calculation of the bending moment on the slab spanning horizontally and vertically, and the bending moments and shearing forces on the counterforts.
- Calculation of the bending moments and shearing forces on the base
- Determination of the thickness and the reinforcement of the wall and base, and the size of the counterforts if any, are required to provide resistance to the bending moments and shearing forces.
The following forces tend to act on a typical concrete retaining wall:
- Earth pressure: Giving rise to active earth pressure and is calculated from
P= ka γH
- ka – coefficient of active pressures calculated from ka =tan2 (45-ɸ/2)
- ɸ- the angle of internal resistance of the soil (or loosely called, angle of repose)
- γ- bulk density of the soil and
- H- wall height.
where passive pressure is to be taken advantage of, it is calculated from P= kp γh where kp =1/ka and h- the height of the earth on the wall or beam offering passive resistance.
- Water pressure: In the case where the back part of the wall is submerged in water, two pressure diagrams are produced. The first is due to the earth (using γsub – submerged soil density) and the second is due to the water (using γwat – water density).
- Surcharge pressure: This may be due to moving loads or permanent loads imposing surcharge weight behind the wall. Surcharge pressure is obviously rectangular since the surcharge is constant throughout the height of the wall.
These constitute the main horizontal forces and sliding forces (except passive) acting against the wall and causing damage. The following vertical forces tend to resist these forces:
- Wall own weight: Usually calculated from the wall geometry and multiplied by the earth fill materials’ own unit weight, including the wall and base weight.
- Earth own weight: Where the wall has an appreciable heel, the earth load on the top of the heel is calculated. This also adds to the vertical forces. Earth loads on top of the toe area, however, ignored.
Conditions for Equilibrium
For a concrete retaining wall to be stable, the effect of the balancing forces must outweigh that of sliding forces. The overturning moment generated by the sliding forces must also be resisted by the moments generated by the counter-balancing forces. In addition to these, the pressure either at the heel or toe of the base must not exceed the soil-bearing pressure. If any of these are violated, the size of the wall (usually the base) must be increased. Alternatively, a heel beam and anchorage are provided to tie the wall back.
Since a concrete retaining wall is subjected to lateral forces, the foundation is usually eccentrically loaded. The eccentricity, e of the resultant force on the base, measured from the center of the wall base is determined by taking moments at some convenient point in the plane of the base of all the forces acting above the plane of the base.
If the resultant lies within the middle third of the base the pressures on the base are:
Pmax =∑w/BD [ 1+6e/D]
Pmin =∑w/BD [1-6e/D]
Usually, B = 1.0m, and D is the width of the base.
if the resultant is outside the middle third, the soil pressure diagram is triangular across alength 3(D/2-e) of the base and the maximum pressure is
Pmax =2∑w/3B(D/2 -e).