Category: Retaining Walls

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

 

Retaining Wall Fairness

In Western Australia subdivisions are usually completed with retaining walls in place on the boundary so that all blocks are level.

For the rest of us if we are faced with a sloping site a retaining wall may be needed before you can build…………..but who pays?

Here are a few examples

Block A is sloping down towards the boundary and Block B is fairly flat

Block A has to build up their block and should be the only one to pay. The wall should be within their block.

Block A is fairly flat and Block B Falls away from the Boundary

Block B has to excavate their land and should be the only way to pay. The wall should be within their block.

The slope affect both Blocks A and B

As one block has to be build up and the other has to be excavated both should share the cost. This however can a fairly complex with some of the issues being:

  • Should the overall cost be apportioned if one site has to be built up by 1m while the excavation needed is only 0.5m.
  • The location of the wall particularly on slopes.
  • Fences on the wall
  • What happens if you are keen to build but the other block is unsold or the owner is in no rush to build.

Each block having their own wall may be a solution, however these can’t be too close together (See this link for more: Retaining Wall) which may be an issue if you need to site the house near the boundary.

In cases like this you really need to make sure your lawyer sews up a watertight agreement on retaining walls before you buy the block.

This link: Understanding Retaining Walls gives an understanding of the technical issues

 

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

 

Geotextile around Drainage Pipes

Many articles about installing agricultural drains recommend that you place a geotextile in the trench before the initial gravel surround and then wrap the geotextile over the gravel afterwards.

You can also buy agi pipe with a geotextile “sock’ around it like this:

The idea is that the geotextile prevents the gravel,or the pipe getting clogged with fine materials.

Its something I used to do, but . . . I DO NOT  recommend this anymore.

The Reason

Practical experience has shown that what happens is:

  • If you wrap the gravel with geotextile the geotextile actually collects all the  particles on its surface and gets clogged up. This stops the water getting to the gravel and on through to the pipe.

or

  • If you wrap the pipe and put the sock around the pipe its only the very fine particles that get through the gravel but they then get caught in the ‘sock” in front of the slots. This again stops water getting into the pipe and draining away.

Recomendation

It far better to just go for a 10mm or smaller gravel surround.

This stops the larger particles.

Any fine material that gets through the gravel will be that fine it will easily pass through the slot and likely to get washed down the pipe.

 

See Agricultural drains for more information.

Front Fence Failure

Generally you don’t have to go far to find a front brick fence falling over like this one.

Normally they fall over in the direction of the street.

So why is this failure so common?

Reasons For Failure

  1. People think “It’s only a brick fence” so don’t put a good foundation in, and only make it a single brick thick.
  2. Simple brick construction is fairly brittle when any stress is placed at 90 degrees to the wall face.
  3. Frequently the soil is built up on the garden side of the wall making the wall a retaining structure. with the consequent stresses which can be significant.
  4. If the wall is built on reactive clay the street side of the wall is kept dry by the pavement while the garden side gets water seeping through and can be subject to soil heave.

What You Can Do

  • Construct a substantial foundation
  • Make the wall at least two bricks thick with steel reinforcement built into the wall.

or

  • Consider alternatives like panel fences or hedges

 

Fall Regulations

When you take the keys for your new home the house should comply with all regulations relating to falls. . . . . but after you move are you planning to do some improvements.

If you are planning a deck for entertaining or modifying the garden with retaining walls you need to think about protecting your family, and guests.

You can do a lot of damage with a fall from from a relatively low height.

If someone falls and you haven’t provided appropriate barriers you could be personally liable and the insurance company has the opportunity to avoid paying compensation.

National Construction Code

Here is a summary of the current regulations  on protecting from falls:

Where people could fall 1 m from a floor, or through an opening a barrier must be provided.

The barrier must be continuous and extend for the full extent of the hazard; and be high enough to protect people from accidentally falling

The construction shall prevent people from falling through the barrier, including small children, and should be strong enough to withstand thee impact of people; and the pressure of people pressing against it.

 

 

 

 

 

Alternatives to Retaining Walls

When it comes to a change of gradient in your land most people just thing ‘Retaining wall’.

Perhaps you should think about some alternatives using a slope.

What you need to consider is how to make sure at that erosion is prevented when rainfall gets washed down the slope.

A great example is this slope has been protected by large boulders carefully placed to appear like a natural rock outcrop.

Another approach is to construct something like a drystone wall but laid back at an angle of around 45 degrees like this second example.

For more inspiration just try Google Search ‘Rockery’ for hundreds of ideas.

Sleeper Wall – Appearance


Have you thought about about concrete for your sleeper wall?

As well as plain concrete it’s possible to have a wide variety of other finishes, and colours.

Here is an interesting ‘Stacked Stone’ appearance wall I saw the other week.

Some of the other options include ‘Exposed Aggregate’ and ‘Split Stone’ and even ‘Wood Grain’.

Just do a ‘GOOGLE Images‘ Search for “Concrete Sleeper Retaining Walls” and you should find a range of styles.

An advantage of concrete sleepers is they don’t rot, which means a quality lasting finish with no maintenance!

Prices start at around $25 for a 2.00m x 200mm panel, rising to $50 for some of the more attractive finishes.

 

See Retaining Walls for more information

 

Silt Pits

Any drainage system should have silt pits at regular intervals.

This includes both stormwater systems and agricultural drains.

In the case of the above photo this is a combination drain and grate  silt pit. (As the water  can become stagnant this is a potential breeding ground for mosquitos!)

The pit may be either circular or square, with a base below the level of the inlet and outlet pipes. This means any sand or silt that gets into the drainage system will get washed into the silt pit where it falls to the bottom and can be easily removed.

Collecting the silt in the pit stops it being washed further along the system eventually causing a blockage,

The following diagram illustrates how the silt pit works.

Part of your ongoing property maintenance, after you move in, should to check the silt pits every year. If the silt gets up to the outlet pipe the pit will need to be cleaned.

 

For more posts see Drainage

 

Placing Fill

Of so you want to place some fill. . . .perhaps behind a retaining wall to level a garden . . . So what do you need to understand?

Here are some things to consider.

Final Volume – Loose Volume

Most people underestimate the volume of material they need because they measure the volume they have to fill, and then quote that volume in the order.

The problem is that the material for delivery is measured by the loader bucket, or the truck load, which is when the material is loose.

Typically the loose volume will compact down by around 10% so if you are filling a large volume to advise the supplier you may need more than the measured volume and you will advise them you may need to add to your order as the job proceeds.

 Consolidation and/or Compaction

There are two ways of getting to the final volume:

  • Consolidation This is letting the material compact under its own weight. This can work quite well for sand, especially if it is ‘washed in’. For other materials it takes much too long (0ften many years) which means you will be forever topping up and re-levelling the top surface.
  • Compaction This is making extra effort to pack the fill down. Considerations in compaction are:
    • Compact in layers the thinner the layer the better the compaction. (Layers should be no more than 150mm)
    • Even compaction will give better result (Covering the whole area several times with a vibrating plate or roller will give a much better result  than running a bob cat up and down a few times on each layer)

For DIY jobs you can hire a vibrating compactor for around $60/day.

If you are engaging a contractor to do the fill ask people giving you a price how they intend to compact the fill. (The cheapest price will be to place the whole lot, level, and run the machine over it. . . .a recipe for an area that will remain soft and continually sink)
 

If you are planning to build on the filled area you really need to have ‘Controlled Fill‘ professionally placed and tested.

 

 

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