Retaining walls greater than 1m high should be designed by a civil or structural engineer who is familiar with site and ground conditions.
Types of Retaining Wall
The wall face is often of pre-cast concrete units that can tolerate some differential movement. The reinforced soil’s mass, along with the facing, then acts as an improved gravity wall. The reinforced mass must be built large enough to retain the pressures from the soil behind it. Gravity walls usually must be a minimum of 50 to 60 percent as deep or thick as the height of the wall, and may have to be larger if there is a slope or surcharge on the wall.
Early in the 20th century, taller retaining walls were often gravity walls made from large masses of concrete or stone. Today, taller retaining walls are increasingly built as composite gravity walls such as: geosynthetic or with precast facing; gabions (stacked steel wire baskets filled with rocks); crib walls (cells built up log cabin style from precast concrete or timber and filled with soil); or soil-nailed walls (soil reinforced in place with steel and concrete rods).
Sometimes cantilevered walls are buttressed on the front, or include a counterfort on the back, to improve their strength in resisting high loads. Buttresses are short wing walls at right angles to the main length of the wall. These walls require rigid concrete footings taken to below the depth affected by seasonal changes. This type of wall uses much less material than a traditional gravity wall.
The penetration of the pile is usually twice the supported height, but this may be altered depending on the environment. Taller sheet pile walls will need a tie-back anchor, or “dead-man” placed in the soil and tied to the wall, usually by a cable or a rod.
Technically complex, this method is very useful where high loads are expected, or where the wall itself has to be slender and would otherwise be too weak.
Alternative Retaining Techniques
The bars are usually installed into a pre-drilled hole and then grouted into place or drilled and grouted simultaneously. They are usually installed untensioned at a slight downward inclination. A rigid or flexible facing (often sprayed concrete) or isolated soil nail heads may be used at the surface.
These are usually used in combination with one of the other wall types, though some may only use it as facing (i.e. for visual purposes).
The mesh cages can support most heights of ground, and also reduce erosion adjacent to water course and outfalls.
These mats provide added internal shear resistance beyond that of simple gravity wall structures. Other options include steel straps, also layered.
This type of soil strengthening usually needs outer facing walls (S.R.W.’s – Segmental Retaining Walls) to fix the layers to and vice versa.