A retaining wall is a structure that acts like a wall holding back soil. A retaining wall might be to keep soil from flooding into your home with heavy rains, or it might be to hold back soil and create a patio or outdoor space if your home is built into a slope. 

Retaining walls come in all different types, so there will be a style of retaining wall to compliment your home and budget. Retaining walls can also be used to create garden beds and terraced style gardens.

What are the different types of retaining walls?

The four main types of a retaining walls are;

1. Cantilever
2. Gravity
3. Soil Reinforcement
4. Anchor retaining
   
   

1. Cantilever retaining walls

This is built using reinforced concrete, with an L-shaped or inverted T-shaped foundation.
Cantilever retaining walls are further broken up into different types;

• King Post
  Steel UC columns driven or cast into augured post holes. Concrete panels or timber sleepers are slid down the channels to fill the gap between the posts.

• Sheet Pile
  Steel sheet piles driven in the ground, then tied back using ground anchors.

• Secant Walls
  A continuous line of bored and concrete fill piles, common when building large basements.

• Precast concrete L shaped
  Can be bolted down or cast in a concrete foundation, the units are made in a precast concrete factory and brought to site. 

• Hollow block L Shaped
  Blocks are laid on top of a reinforced raft with started bars, then cement is used to fill the cavity of the block.

2. Gravity retaining walls

These walls use their own weight to hold back the earth, the four main types of gravity retaining walls are;

• Gabion basket
  Steel baskets filled with various size stone.

• Criblock
  Constructed from timber or concrete frames and filled with stone.

• Concrete block
  Interlocking concrete blocks.

• Masonry
  Walls built using bricks.

3. Soil Reinforced retaining walls

Created by excavating layers of soil and placed on top of a geogrid. Geogrid sheets reinforce the soil and the soil becomes the retaining structure. Concrete blocks wall are connected to the geogrid layers.

4. Anchor Retaining Walls

A wall is constructed and then given strength to the wall by  using cables or steel bars anchored in the rock or soil behind by being bored into the ground.

What is the cheapest way to build a retaining wall?

The cheapest retaining walls to hold a massive amount of earth back would be masonry bricks that are filled with concrete as well as gabion baskets filled with stone.

If the retaining wall is for a garden bed so not as much earth, then wood would suffice. The wood will need to be pretreated to withstand the elements of being semi buried into the earth. These can be wood blocks or even wood sleepers.

How much will it cost to build a retaining wall?

The cost of building a wall is of course the labour time of the builder, but then the expense of the materials. Building a brick retaining wall versus a stone retaining wall might be a similar amount of labour time but the material costs will vary greatly. Before speaking with the builder do your research on which style wall will best suit your property and be strong enough for the amount of soil the wall needs to hold back. Then take into account the access to the area and location on your property. If living in a terraced house could a small digger be taken around to the back of your garden or will the foundation have to be dug by hand adding days of labour onto the cost of the retaining wall?

The height and width of the wall will depend on how deep the foundation needs to be dug and there will be a cost involved for the removal of the excavated soil. The ground conditions will affect the price as sandy soil equals better drainage than clay soil, and this too will influence the materials needed for the construction of the retaining wall as will the extent of the slope the wall needs to hold back.

Does my retaining wall need a foundation?

In short, yes. Another term for foundation of a retaining wall is footing.  The footing is important as over time the ground will shift causing the wall to shift and this can result in the wall falling down. The wall also needs good drainage to avoid a pressure being created against the wall from water which also can eventually push the wall down. The depth of the foundation will depend on how tall the retaining wall needs to be, e.g.; a four foot wall needs 9 inches of foundation with an inch being crushed rock to keep the retaining wall stable.

The foundation should also be wider than the wall and double the depth, for added stability.

As well as ensuring a foundation, the retaining wall needs to be suitably thick to hold back the earth. The least thick a wall should be is the depth of a house brick. A formula to work out best thickness is taking the thickness as a quarter of the depth of the soil, e.g. 150cm deep divided by 4 (1 quarter) = 37.5cm thick.

How often do I need to replace my retaining wall?

If the builder has built your retaining wall with good drainage and a solid foundation, depending on the material used, the wall should see you through 25-100 years. Over the years, check on the health of  your retaining wall, noticing any signs of flooding, deep cracks, bending or loose mortar.  Certain cracks or loose mortar a builder might just be able to repair and maintain the wall to stop the wall from crumbling. If drainage is an issue, see if a builder can retrofit some drainage points or weep holes.

If the retaining wall is buckling, speak to a builder about strengthening the foundation, possibly adding deeper anchors to secure the wall. It is a lot easier to repair a retaining wall than the expense of removing the wall to build a new retaining wall. The only time the builder will want to replace a wall is if there is no other option to save the existing retaining wall.

How many years will a retaining wall last?

This question is dependent on the type of material used to construct the retaining wall and as already touched on, the amount of drainage and solid foundation. A tough material like stone might not last 10 years, nevermind a 100 years if it has no way of draining out the water.

Average life span for different types of material:

Stone; 50 – 100 years
Stone makes for an attractive and sturdy wall without the concern of insects infesting in the wall, however certain stone can be very costly and this wall comes with limited colour choices.

Wood; 25  -40 years
An inexpensive way to create a retaining wall with a natural look but will not last as long as other retaining walls and must be weather treated as well as pest treated.

Brick; minimum of 100 years
An easy to create, pretty option that complements many gardens and is weather resistant but there are not many colour options.

Concrete;  a 100 years
This is a good mid-range costing wall, allowing many design options to create a feature in the garden aside from the practicality point of a retaining wall. The builder will need to factor in the need for drainage as this does not naturally occur like with stone.

Gabion;  50 -100 years
Gabion walls are easy to install without the need for deep foundations and have a great lifespan and require little or no maintenance. Plants can be grown to flower over the wall and the vegetation can help strengthen the wall. This style however does not suit all properties.