Heavy rain and retaining walls
The landslides in Brazil over the last day or so are an indication of the consequence of a build up of water within the soil.
The friction between the granules in sand, or the platelets in clay, create the “angle of repose”. If you emptied a bag of sugar onto the table it would form a cone, the angle of which would be the angle of repose, typically 30 to 40 degs. Amongst the factors that change the angle of repose is the water content of the soil as it lubricates the elements within the soil.
If you were feeling particularly messy, you could get several bags of sugar and add water to them in increasing amounts and pour them onto the table, noting the angle of repose getting more and more shallow. Eventually the sugar will become saturated, flow across the table and drip onto the floor where it will be licked up by the dog.
A similar mechanism occurred in Rio de Janeiro, and periodically occurs in this country. When the rainfall becomes so heavy that the ground water has no chance to disperse, the soil becomes increasingly saturated until all internal friction is gone. When this soil is the side of a hill a mudslide occurs. The forces generated under such conditions are immense, which is why structures originally designed to retain the slope have no chance of doing so.
It is rare for Structural Engineers to have to design for such large forces, but due cogniscance is often taken of the possibility. In commercial situation, a site investigation will reveal the necessary parameters to design appropriate structures to resist most conditions, whether this is bored piles beneath a building or a horribly expensive retaining wall.
On a much smaller scale, the same mechanism can cause problems in residential garden retaining walls. These animals are relatively cheap to build, so a site investigation (or a Structural Engineer’s fees – bah!) is often considered uneconomic. Such walls often survive for decades (many Victorian examples still stand) and if they do decide to start giving up, there is usually plenty of warning in the shape of horizontal displacement or cracking.
Many builders overlook the need to allow the water behind the retaining wall to escape. In normal conditions, the water disperses naturally, but in unusually heavy rain the water pressure builds up behind the wall.
There are proprietary products that can be used when it is known the water table is a problem, but usually simply leaving the vertical mortar joints (perpends) open or installing small plastic tubes within the wall is enough to allow the water to percolate the wall and disperse.
The photo was taken from the BBC website at http://news.bbc.co.uk/1/hi/world/americas/8605386.stm
Keith
There’s a wall nearby that’s clearly suffering from the recent rain, and has started to list heavily – as it’s next to a busy cut-through path, it’s probably rather dangerous, so can you let me know what type of person at the local council to inform about it?
Thanks very much.
The first port of call would be Building Control. If you are in the same area as me you need Central Bedfordshire Council and their number is 0300 300 8635, but your own Council’s website will have the local number. You mention a public footpath, so if this is an adopted highway so further consents may be required.
Once you have reported your concerns to Building Control, they will probably visit the site and have a look. As it is a public thoroughfare they can visit at will. If they consider further investigation is required they will send a letter to the owner asking them either for further and better particulars or to take action. It is the owner’s responsibility to maintain his property, but Building Control will keep a watching brief on the structure and issue a Dangerous Structures Notice if one is warranted. Ultimately they can arrange for the wall to be taken down and the bricks stacked neatly to one side and send the bill to the owner, but there are many steps that are much less Draconian before this.
I hope this helps. If you know the person concerned, they are welcome to contact me and I can discuss it with them further.
Keith