Retaining walls are structures designed to retain soils at different levels where there is insufficient space to allow stable slopes to be provided (generally 1 in 1.5 to 1 in 2).

Retaining walls greater than 1m high should be designed by a civil or structural engineer who is familiar with site and ground conditions.

Please find further information on the different types of retaining walls below, but for more detail please call one of our local offices to speak to an engineer.
retaining-wall

Types of Retaining Wall

Gravity walls depend on their mass  (stone, concrete or other heavy material) to resist pressure from behind and may have a ’batter’ setback to improve stability by leaning back toward the retained soil.  For short landscaping walls, they are often made from mortarless stone or segmental concrete units (masonry units).  Dry-stacked gravity walls are somewhat flexible and do not require a rigid footing in most areas.

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).

Cantilevered retaining walls are made from an internal stem of steel-reinforced, cast-in-place concrete or mortared masonry (usually in the shape of an inverted L or T).  These walls transfer the lateral overturning movements to a large, structural footing, converting horizontal pressures from behind the wall to vertical pressures on the ground below.

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.

 

Sheet pile retaining walls are usually used in soft soils and tight spaces.  Sheet pile walls are made out of steel, vinyl or wood planks which are driven into the ground.

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.

 

An anchored retaining wall can be constructed in any of the aforementioned styles but also includes additional strength using cables or other stays anchored in the rock or soil behind it.

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

Soil nailing is a technique in which soil slopes, excavations or retaining walls are reinforced by the insertion of relatively slender elements – normally steel reinforcing bars.

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.

 

 

A number of systems exist that do not simply consist of the wall itself, but reduce the earth pressure acting on the wall by charging the soil composition.

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).

 

 

This type of soil strengthening, often also used without an outside wall, consists of wire mesh ’boxes’ which are filled with roughly cut stone or other material.

The mesh cages can support most heights of ground, and also reduce erosion adjacent to water course and outfalls.

 

 

Mechanically stabilized earth, also called MSE, is soil constructed with artificial reinforcing via layered horizontal mats (geosynthetics) fixed at their ends.

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.