General
Many retaining wall failures can be attributed to the ingress of ground water and a failure to remove it from behind the wall.
Access Behind Wall
If there is insufficient clearance behind a retaining wall during construction, it will be difficult to install the drainage system. This is often a problem for retaining walls constructed on a boundary. In such a case, position the drainage system before completing the stem of the retaining wall.
Wet Concrete Behind Retaining Walls
Some innovative designs call for the placing of concrete, either with or without reinforcement, behind retaining wall facings. Until it has had a chance to stiffen, such wet concrete exerts on the face of the wall, a pressure that is significantly higher than the normally expected active pressures. In this case, the following precautions must be observed to ensure that the wall does not collapse under the pressure of wet concrete.
- Ensure that wall is securely braced to a stable anchorage before placing any concrete behind the wall.
- Do not fill more than three blocks high in any concrete lift.
- Wait until concrete has stiffened and the wall is stable before proceeding with further concrete filling. This may require the delivery of concrete in more than one load.
- The use of no-fines concrete, which has a low slump, will reduce the risk of overturning. See below.
No-fines Concrete Behind Retaining Wall
No-fines concrete has a low slump, and therefore applies lower forces on the retaining wall than fluid concrete. Care is needed to ensure that the wall does not overturn before the no-fines concrete has hardened, although the risk is substantially reduced.
Granular Fill behind Retaining Walls
The purpose of granular fill behind a retaining wall is twofold. It provides a medium for conducting ground water to the drainage pipe and it exerts a lesser force on the wall than clay, silt and similar materials. To prevent the granular fill from becoming contaminated with the in-situ soil, enclose it in an envelope of geofabric which will allow the water to pass but trap the soil. A layer of granular fill, at least 300 mm wide at its narrowest section, should extend up to a position that is at least half the height of the wall or to within 1 metre of the top.
Weep Holes
Ground water must be able to pass through the face of a retaining wall in the event of a failure of the drainage system. The preferred method is to provide 50 mm diameter weep holes through a concrete hob at 1.2 m centres. Alternatively, weep holes may be placed through solid or grouted masonry walls using special flush-ended clean-out blocks in the bottom course.
Inappropriate Concrete Blocks
In reinforced concrete masonry cantilever retaining walls, both vertical and horizontal reinforcement are grouted into hollow concrete blockwork. It is a common incorrect practice to use flush-ended two-celled blocks (commonly designated 2042 or 2020 units). Use of these blocks requires horizontal reinforcement to pass through a 10 mm wide air space that is incapable of being filled with grout at the perpendicular joints, and is a potential source of corrosion. The correct blocks to use are H blocks (2048) or Double U blocks (2091).
Location Of Steel Reinforcement
In reinforced concrete masonry cantilever retaining walls, the position of the vertical reinforcement is critical to the strength of the wall. It is preferable to construct a concrete up-stand or hob as part of the footing, thus permitting the vertical starter bars to be accurately positioned. This also enables the footing to be sloped longitudinally, permitting the agricultural pipe to achieve the required fall. Weep holes can also be positioned through the hob.
Position of Keys
Large retaining walls often require a concrete key into the foundation material to restrict forward sliding. It is preferable to place the key towards the rear (heel) of the base, since this will simplify the excavation process. The key is excavated first, then the rest of the base, dragging the excavated material forward (towards the toe) and thus away from the key excavation.
Tightness Of Geogrids
In reinforced soil walls, geogrids must be pulled tight using stakes before the placing and compacting of fill. If this is not done, they will be ineffective in reinforcing the soil.
Depth Of Timber Posts
Where timber posts are required to resist overturning, they must be founded to a depth stated in the drawings and surrounded with concrete. It is advantageous to widen the diameter of the surrounding concrete close to the soil surface, thus increasing the area of soil resisting overturning. It is essential that the ground in front of the posts remain undisturbed.
Ground Anchors
If rock impedes the driving of ground anchors, the design should be modified to ensure that the equivalent load carrying capacity at the same depth of embedment is achieved.
Building Posts
On sloping sites, the downhill creep of the upper soil strata will slowly cause movement in retaining walls and any building supports and posts in contact with the soil. The layout of retaining walls and adjacent building supports or posts should be as follows.
- Building supports and posts should be founded below the base of any adjacent retaining wall or on sound rock foundations.
- Building supports and posts should be isolated from the soil fill by a space of at least 200 mm to permit future soil movement without bearing on the supports.
- Bearers and bracing should run in the direction of the site slope.
- Building posts should be tied together by galvanised steel ties encased in concrete beams (running down the slope between the posts) with smooth sides that permit the soil to slip past the posts without applying any load.
Large Reinforced Soil Retaining Walls on Poor Quality Foundations
When soils of low bearing capacity support high reinforced soil retaining walls, the length of the geogrids must be increased, effectively increasing the length of the reinforced soil block. Alternatively, a more economic solution may be to improve the quality of the foundation material by excavating the poor soil and replacing it with an enlarged levelling pad of either 15 MPa concrete or Class 1 compacted gravelly sands, controlled crushed sandstone or gravel fills.
Wind, Earthquake And Impact Loading On Retaining Walls
Where appropriate, retaining walls must designed to resist impact on the guard rails and wind or earthquake loadings on supported structures such as fences and buildings, in addition to these forces acting on the face of the wall.
Boulder Walls
The shape of the boulders affects the stability of the walls. Boulders that are shaped similar to cubes are much more stable than boulders that are shaped like spheres.
Determining Soil Properties to Australian Standard AS 4678
The specification of soil properties should be "conservative estimates of the means" in accordance with AS 4678.
Treatments to Reduce Water Penetration
The rear face of retaining walls may be coated with a water proofing membrane or similar. Refer to the specification on painting for details.
