Has anyone noticed that periods of large downpours of rain often coincide with an increase in retaining wall failures? Of course unless there is a flood of biblical proportions, water has only acted as the straw which broke the camel’s back. Rainfall is expected, and the retaining wall has to be designed to withstand such a condition.
The causes of retaining wall failures are many and varied. These can range from the way the wall was designed in the first place, to how it was built, and the loads which are eventually applied to the wall.
When it comes to retaining wall design, the principle is quite simple. That is, the load which is applied to the wall, which are both horizontal and vertical loads, has to be resisted by the wall itself. These “resisting forces’ include the bearing resistance of the material underneath the wall footing, the weight of the wall and footing, the width of the footing and the location of the footing with respect to the wall, the horizontal and vertical strength of the wall, the strength of the connection between the wall and the footing, and others .
To throw in another factor, the loads which are applied to the wall generally consist of two (sometimes three) components. The first is the pressure of the soil itself. This will depend on the type of soil (sand, clay, etc) which in turn dictates the soil "strength"?.
The second is water or hydrostatic pressure. This is produced generally by rain, but can also be caused by burst water pipes or even garden watering. The amount of water can vary depending on whether it’s raining (and how hard), or whether a dry period is being experienced. Hydrostatic pressure is rarely constant, and as such the wall should be designed for the worst case scenario.
The third load which is sometimes applied to a retaining wall is what is termed a surcharge load. This could include a vehicle being driven immediately adjacent to the top of the wall, the upward slope of the terrain which the wall is attempting to retain, or stockpiles of soil which may be placed adjacent to the top of the wall.
Most permanent retaining walls are designed to drain water from behind the wall before it has a chance of applying the hydrostatic pressure against the wall. In this way the wall strength (and therefore the design and wall cost) can be significantly less than otherwise would be required. In order to do this drainage material (an open aggregate of some description such as blue metal) needs to be placed behind the wall, as well as some means of allowing the water to escape from behind the wall. This can include such means as an agricultural pipe, weep holes through the wall, or vertical wall drains. Some are more effective than others.
The general concepts of building a retaining wall is well known by most builders. This though does not necessarily lead to an understanding of what these concepts relate to, and how they may affect a wall’s performance. Some years ago I was at a construction site where a brand-new concrete block retaining wall had failed. The builder couldn’t understand why this had happened. The wall had been designed in accordance with engineers’ instructions, there was plenty of aggregate behind the wall for drainage purposes, and a 75 mm diameter agricultural pipe had been placed at the base of the wall. The problem here, however, was that the agricultural drain lead nowhere. It wasn’t plugged into the drainage system to allow the water to escape. Therefore the water which entered into the agricultural pipe stayed there, and was able to build up against the wall, and apply a hydrostatic load for which the wall was not designed to withstand.
“Oh” said the builder.
There are many types of retaining walls. These will vary depending on the height of the soil block being retained, the amount of room one has to construct the wall, aesthetics required, and location. They can be generally divided into two camps, being gravity and cantilevered. This is not to be confused with shoring and inclusions such as anchoring, soil nails, or mechanical stabilisation.
A different type of retaining wall is constructed using reinforcing strips or grid which is attached to the wall and extends back into the soil block behind the wall. These act as a type of anchor to hold the wall back. They are "smarter than the average wall" because they use the material which they are actually retaining to keep the wall in place. This was pioneered many years ago and was known as “reinforced earth” (being the company that first applied the technology). There are now many types of similar designs which are now available, some less risky than others. The main problem with these types of wall is the detail to construction that is required, including the soil parameters, which is often a cause of failure.
In order to work out why a wall fails one needs to look at all of those loads which are being applied to the wall. With this you need to be able to determine proper design parameters, such as the soil strength and the associated force being applied by the soil, strength of the steel reinforcing (where applicable), size of the footing, bearing capacity of the ground on which the footing is founded, and many other parameters which have to be taken into consideration. Clearly not all of these are required for every failure; this depending on the type of failure which is exhibited. For example, a sliding wall failure will require different parameters to be analysed than that of a rotational wall failure.
A wise lecturer once told me that once a retaining wall starts to fail, there is no stopping it even though it may take some time to actually collapse. I have found this to be absolutely correct. Although that metaphorical “straw�? may have occurred, it very rarely is the only cause of failure.
B.E.(Hons), M.I.E.Aust., F.C.L.A., ANZIIF (Fellow), F.C.I.L.A., A.I.A.M.A.
Chairman & Executive Adjuster
Technical Assessing Pty Limited