Tag: Retaining Wall

Retaining Wall Drainage

I’ve previously posted about the importance of Drainage Behind a Retaining Wall to minimise the forces acting on the wall…. here is some advice on how to make sure it works.

You should aim to properly drain surface water away from the wall.

Then you need to consider drainage of water in the soil.

There are two sorts of retaining walls as far as drainage is concerned:

Permeable Walls (water can drain through the wall)

Examples of permeable walls are:

With permeable walls the water should seep out anywhere on the face.You still need to put a drainage layer behind the walls. This will ensure that there is a clear path to the seepage points and that any dirt is filtered out rather than staining the face of the wall. It can be worth using a drainage pipe behind the wall as extra security against water pressure building up.

Impermeable Walls (water cannot flow through the wall)


Examples of impermeable walls are:

  • Reinforced concrete walls.
  • Block Work or Brickwork Walls (see this link: Besser Brick Walls)
  • Sleeper walls…. With a plastic backing.

With these walls you need to make special provision for drainage.

Either:

  • Weep holes which are specially constructed holes through the structure at 1m, or less, spacing.

or

  • An  Agricultural Pipe(a perforated or slotted pipe see this Link Aggi Pipe) discharging to a surface water drain. A good idea is where possible have discharge points with silt pits (see this link: Silt Pit) at both ends of the wall.

Drainage Layer

  • For the drainage layer I would recommend  at least 100mm thickness of 10mm size, or smaller gravel.
  • If you are going to have a pipe the width of the drainage layer should be 50-100mm greater than the pipe diameter.
  • Isee lots of people use a geotextile round the pipe. In my experience thegeotextile can get clogged with silt and stop the drainage occuring.

For responsibility for Boundary Retaining Walls see Fairness

See Why Aggi Pipes Have Slots In The Base to learn more

 

Understanding Retaining Walls

Its quite usual when you have a sloping site to need a retaining wall to make the land you build on level.

You might need a retaining wall just to have a flat garden.

Retaining walls are not simple structures.

They have to withstand significant loads and need to be properly designed and constructed to avoid failure.

Loads On A Retaining Wall

The sketch above shows a retaining wall. There are three main loads on this wall:

  1. A wedge of soil (shown as a grey triangle) tending to slide down. The size of the soil wedge depends on angle ‘A’ which will vary depending on the soil. Typically the loading will be over 1 tonne/m length for a 1m wall. The taller the wall the greater the load.
  2. The additional weight of a car (or any other above ground load, such as piled up dirt) which will be adding to the soil load. Say another tone per m
  3. If water is allowed to build up behind the wall the load is increased by the water pressure. Almost another tonne of force.

Methods of Failure

There are 3 ways simple retaining walls typically fail:

  • Toppling Over
  • Sliding forward on the foundations
  • Breaking with the top separating from the bottom

As one of the main factors in holding up the wall is the strength of the soil at the foot of the wall:

    • DO NOT excavate in this area or steeply slope the soil away from the wall without checking with a designer.
    • DO provide proper Drainage Behind The Wall.
    • DO slope the surface below the wall gently away from the wall. You want to prevent the soil supporting the wall becoming soft and failing.

Regulations

Usually all retaining walls over a certain height (‘H’ in the sketch above) require a permit from the council, who will want to see that the wall has been properly designed.(Depending on the Council this height can be as low as 600mm)
‘H’is the difference in height between the upper and lower LEVEL areas.

NB The following walls all have an effective height of MORE THAN 0.9m.

In the case of the tiered wall above you would need to set the walls at least 2 x H apart (Even more in bad ground) before the walls could be considered as separate.

The reason why you don’t need a permit for smaller walls is to save the councils extra work, as they think that damage from a smaller wall failing will be fairly small.

It doesn’t mean you don’t need to make sure its properly designed if you want the wall to last.

What Can You Do

This is NOT  a lesson on how to design retaining walls just helping you to understand the loads involved so:

If you are uncertain its best to get it all checked by an engineer!

 

For advice on who pays see Retaining Wall Fairness

 

Retaining Walls – Gabbions

Gabbions are big wire baskets filled with rocks.

The baskets are typically available for retaining wall use in 1m x 1m x 2m units. and you arrange the rock fill yourself.

Gabbion walls can be described as a Permeable, Steel Reinforced, Low Skill, Dry Stone Wall.

Low Skill however doesn’t mean no skill so don’t just tip the rocks in. For best results the rocks need to be adjusted to pack them tightly into the baskets.

In my view the best looking gabbions are the square mesh ones like the photo above.

If appearance is not an issue then there are these standard units, on the right, that have a more basic mesh.

Although the wall is permeable and doesn’t need an aggi drain behind it I would still advise a layer of fine gravel. This prevents fine soil particles being washed through the gabbion, as has occurred in the lower section of this wall.

 

To know more about retaining walls follow this link: Understanding Retaining Walls

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