Retaining Walls
Retaining wall systems include cantilevered round or sawn timber, mass wall and crib wall construction.

Retaining walls up to 1.0 m in height are generally simple in design and can be constructed using standard proprietary designs. Walls greater than 1.0 m high are required to be engineer designed, including the footings, drainage and timber strength.

Drainage behind retaining walls is critical to the long term stability and durability of the wall. Drainage reduces the water pressure behind the wall allowing economical design and construction. In order to reduce hydrostatic loads (water pressure) from the retaining wall, a free draining granular fill is used to back fill the area immediately behind the wall. A geotextile filter fabric is also incorporated between the soil and the fill to minimise the silting up of this granular fill.

Mass Walls
Mass walls rely on the mass of timber to resist overturning pressure. As timber is relatively light, only low walls are constructed in this way. Typically, sleepers are spiked together in a staggered pattern.
 

Figure 1: Typical timber crib wall

Timber Crib Walls
Timber crib walls are generally made up of scantling (small) sized, treated hardwood which is placed in parallel rows and interlocked at frequent intervals to each other. Free draining granular fill is packed into each layer of the crib, forming a mass of stable earth.
 

Figure 2: Cantilevered Retaining wall

Cantilevered Walls
Cantilevered walls are usually constructed using round or sawn timbers that are generally buried one third of their length into the ground and cantilever out to support timber walling. These are placed at close centres to support sawn timber that spans between them.

This type of wall design is well suited to larger scale retaining situations and require engineer design.