Retaining walls are subjected to a mixture of lateral pressures and they must always be designed and constructed to withstand them. The principal pressure is normally formed by the force exerted by the volume of soil supported but this is often increased by further applied forces. These may include groundwater pressure, loads from adjoining buildings and loads from traffic on nearby roads.
The higher the ground being retained, the thicker the wall, although it can be of stepped width -retained heights up to 900mm should have a 225mm thick brick wall, which is required to become progressively thicker as the height of the wall increases. The addition of piers or buttresses may allow a thinner wall to be formed, but never less than 225mm.
A retaining wall that is too slender to withstand the pressures being applied against it will tend to bow or lean outwards. This is a defect that is commonly observed in garden walls that are retaining soil. It is often as a result of too little thought being given to the design implications of any external landscaping. A further cause can be the close proximity of trees. The growth of the root system over a number of years can lead to increasing lateral pressure on any adjoining retaining wall.
Finally, all retaining walls require the provision of drainage points at frequent intervals in their length and height. Such provision is a common omission and a frequent cause of movement due to the pressure of the groundwater behind the retaining wall.
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