Retaining walls Explained
As part of an RICS Home Survey Report, assessments are made of the ground levels around a property. Where ground levels change significantly, we often come across retaining walls. The principal design of a retaining wall must be such that it can cope with holding back the higher ground level without overturning or sliding. However, it should also be able to able to resist the pressure build up caused by groundwater or, better still, be designed to enable groundwater to be safely discharged away from the wall via weep holes or drainage channels.
Mass or Gravity Retaining Walls
There are two main types of retaining wall. The first is known as a mass or gravity retaining wall and it relies upon its own mass together with the friction underneath its footings to prevent collapse or overturning. It is probably the most common type of retaining structure found on residential properties particularly in rear gardens which are tiered. Economic constraints in the amount of masonry required mean that they are only viable up to a height of approximately 1.5 metres. Due to exposure and relatively high levels of dampness they are generally constructed of dense engineering bricks or of mass concrete. There is, however, a very simple type of mass or gravity retaining wall known as a Gabion wall which is growing in popularity. The word Gabion is derived from the Italian ‘Gabbione’ which literally translates as cage. They comprise rectangular metal baskets which are filled with stones. The first use of Gabion baskets Can be dated back to World War One when they were constructed as protection against artillery fire. Their use is becoming ever more widespread in the residential sector, not because of a need to deflect machine gun fire, but due to the ease and speed of construction, plus a desire for a modern but industrial feel to urban architecture.
Cantilever Retaining Walls
The cantilever retaining wall provides stability and prevents overturning from the lateral pressure of the soil that is holding back based on a principle known as leverage. The heel (or base) of the cantilever retaining wall is constructed to be large enough so that the force of the soil acting downwards on it is utilised to, in effect, increase the overall mass of the wall and therefore prevent overturning.
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