9. RETAINING WALLS

All retaining walls must be properly designed and construction details provided by the Engineer. Construction must be in accordance with the specified requirements. Take particular care with backfilling ,and back drainage which can cause severe instability if not carried out properly. The wall must not be surcharged during or after construction unless so designed. An outline of the principle forms of retaining walls is given below:

(a)Reinforced concrete Generally expensive for low wall situations, therefore normally used for high structures or where there are problems with limited space for the works. RC walls rely on adequate soil foundation and the vertical loading onto the base from the retained material for stability.

(b) Mass concrete These rely on the soil foundation and their own mass to retain the soil behind. They are infrequently used nowadays as cheaper alternatives are available.

(c) Brickwork

There are significant differences between low brick retaining walls and higher walls with heavy loading. Generally it is cheaper to construct a high reinforced concrete wall with a brick facing and therefore major brick retaining walls are no longer built.

(d) Reinforced block work

This is an economical form of construction for low to medium height retaining walls. Care in construction to properly tie in reinforcement and infill with concrete at specified levels is most important. Specified heights must not be exceeded by adding an extra course of blocks.

(e) Crib wall

This is made from interlocking pre-cast concrete or treated timber components which form a three dimensional grid. This is then In filled with non-cohesive soil and/or stone to form a mass type retaining wall. Various sizes are available for different heights and the width of the wall can be varied with height by reducing the number of grids. Although these types of walls are easy to assemble and modify, it is essential that they are built in accordance with a proper design and specification.

(f) Reinforced earth

This technique is used where the material to be retained is yet to be positioned. It is ideal for reinforcing a road embankment where a battered slope would occupy too much space. It is constructed by placing a row of interlocking pre-cast concrete panels on a strip foundation, galvanised metal strips are then bolted onto the back face of the panels and compacted in with the layers of fill material. The wall height can be increased by adding successive panels, strips and fill material (typically hoggin). A composite structure is formed and remains stable due to the friction between the reinforcing strips and the fill material being greater than the outward force the fill material exerts on the rear of the panel.

(g) Systems such as the “Tensar Geogrid" use a polypropylene mesh  interwoven between successive layers of soil, this acts as reinforcement between the soil layers producing a composite mass. Vegetation will grow on the exposed face to bind the soil further.

(h) Paving slab

The paving slab on edge is a very cheap way of retaining up to, say 500mm where space is too restricted for a bank. Where the retained height varies or runs down a slope, the top edge of paving should be cut to the slope and not stepped which gives an unsightly appearance.