During heavy rain events, the following may occur: collapse of retaining structuressuch as walls And bulkheadswhose aim is to contain the ground behind them. In fact, there is a direct link between rains and collapses… but why? In this article we will first clarify what is meant by support works, what are the main types and why they could collapse following heavy rains.
What are retaining works: the main types
We are talking about support work whenever we find ourselves in front of a vertical wall that acts as a containment to an elevated area of land compared to the height at which one is located. The works are carried out following the excavations necessary for lower the campaign plan, or for contain an elevated area. They are “supportive” because, literally, they support the ground behind themavoiding that this – following movements – can to collapse. These are works often used in the infrastructural field, as they are widely used in the construction of new road or railway infrastructures. However, applications can also be found in ordinary construction, for example for the construction of basements of buildings.
They are mainly distinguished in bulkheads and retaining walls. There is a big difference between them:
- The bulkheads they support the ground behind them thanks to the help of the ground at their feetthrough a designed depth of insertion: it means that, compared to the part of the work that we see above ground, there is also another one important part that is underground.
- THE walls instead, the supporting structures are based on the valley soil and they support the ground behind them by their own weight. They do not have a part “drowned” inside the valley floor (except for any deep foundations in cases where the foundation soil is particularly poor).
How support works work
In the absence of a retaining wall or bulkhead, the vertical excavation face, above a certain excavation height, would tend to collapse, as the mechanical characteristics of the ground are not suitable for it to remain vertical. This also depends on the loads that are applied upstream of the excavation face (for example, the presence of buildings). The support structure therefore serves to restore the conditions of equilibrium which, otherwise, would be lost. Since the ground is impeded in its horizontal movements, then this will exert a push on the work which will depend, among other things, on the weight of the earth behind the work. This must therefore be designed in such a way that it is able to absorbwith a good safety margin, the pushing forces that the ground will exert.
But what does water have to do with it? Just like the soil, even water pushes sideways if placed inside a container. Let’s imagine the support structure as just the wall of a container. Since the ground has its own permeability, when it rains the water can start to filter inside the ground and accumulate inside this container, which already contains the soil. If this water has no other way to move away from the container, then here it is start pushing – like the ground – on the side walls, therefore on the retaining structure. The result? The total thrusts exerted on the walls they grow frighteningly. They can even more than double in size due to the presence of water! So, if in ordinary conditions we have an overall force that pushes 10, then when it rains a lot we can reach overall thrusts of 20, 30. External forces increase disproportionately and stress the wall or bulkhead, possibly causing it to collapse if these are not designed to absorb them.
Prevention measures
But if this phenomenon is known, why don’t they take it? measures suitable? The design of the retaining structure takes into account the presence of water in the ground only in the case in which the ground itself already has, in ordinary conditions, water inside it. This refers to all those cases in which the aquifers are at a level such as to stress the work even in favorable weather conditions (when it is sunny, to be clear). In all other cases, the presence of water is not contemplated because it would lead to an oversizing of the work that is not economically advantageous. In these cases, however, the project of the work must include appropriate drainage systems that remove this excess water, avoiding filling this container. What can generate the collapse is a possible occlusion of these drainage systems, due to lack or poor maintenance.
The collapse is always the combination of multiple factors: for example, there may be problems related to design errors, usually associated with a poor knowledge of the mechanical behavior of the soils that are to be contained. However, the thrust of the water has a high importance on the stability of the work, which is why if there is a problem latentSurely It will manifest itself when it rains a lot.
