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.

 

 

Retaining Walls – Besser Blocks

If  you want a rendered surface on your retaining wall one of the best ways of achieving this is by using ‘Besser Blocks’.

These are hollow concrete blocks which are designed to incorporate steel reinforcement within the block.

A few  issues when building these walls are:

  • Don’t skimp on the foundation. Even a 600mm high wall will need a concrete foundation 600mm wide by 250mm deep.
  • Make sure that the cement grout is well packed around the steel reinforcement.
  • Buy some of the yellow safety caps you see here to put over  the reinforcement bars and ‘Keep Yourself Safe.
  • Don’t backfill behind the wall for at least a week


As well as free standing retaining walls Besser Blocks are also used as basement walls and to provide structural strength for external walls when building against a slope.

This photograph shows a typical example where the Besser Block wall will provide the external wall of a garage. ( You can also see the builders plastic which will form part of the ‘tanking’ to keep damp from passing through the wall)

 

See Retaining Walls for other  solutions to slopes

Block Retaining Wall – Loose Lay

Loose lay retaining wall blocks are concrete blocks that have a nib cast on the underside. (see photo) This stops each block slipping forward of the block below.

There are a number of different manufacturers and most produce more than one type of block. You will normally find several different types of block in any of the big DIY Stores.

The sketch below shows a typical construction detail for this type of wall. (check the manufacturer’s web site as most provide comprehensive design and installation guides)

The most critical aspect of building a wall is to get the foundation and the first course right. Although you can use gravel I would probably go for a concrete foundation and then lay the first course on mortar to allow for adjustment.

Although this type of wall will be more expensive than a sleeper wall it can be laid in curves as well as straight lines.

 

See: Retaining Walls  for more posts

 

 

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

Retaining Walls – Sleeper Wall

Retaining walls are created to hold soil behind them. They can be made to control soil erosion due to hard rains, surround a garden, create a terraced yard, or retain soil along a highway. 

When designing a retaining wall, take note that there are several factors that will affect the type of wall and material you build.

These include the following:

Location – it’s best to understand your property lines and your underground as well as above-ground utilities, including stormwater irrigation and management systems.

Soil – the soil that makes the base or foundation of your wall must be checked to determine if it meets the strength needed to support the wall. The bearing capacity of the soil, its type, friction angle, and stress parameters of the soil used for the foundation should be determined. Generally, wet soils, like clay, aren’t suggested for infilling. In locations with freezing temperatures, wet soil can contract or expand, possibly damaging the wall. Sandy soils provide excellent draining. 

Design – To start the design, you should calculate the footprint sizes, wall heights, slopes, and setback angle, which depends on the site grade and elevation. 

Drainage – Since water is usually the primary reason why retaining walls fail, it’s crucial to ensure that your wall has excellent drainage and there won’t be water buildup behind the wall. It’s best to check the site for drainage patterns and create a drainage system behind the wall. For excavation services, you may contact professionals, such as Hammer Excavations Melbourne, to help you. A good drainage system can include the use of drain pipes, backfilling, or using ‘weep’ holes to let water pass through a wall.

Details

Here are some details I would be using to build a retaining wall with galvanized steel posts up to 800mm high. This is not a guarantee that these details will be suitable for your application.

Ground

I would want to be building it in reasonable ground such as Hard Clay, Compacted Coarse Sand, or Gravel.

It is possible to build walls in poorer ground such as fine sand, soft clay and more loamy soil but specialist advice will be required.

Post Hole

A minimum diameter hole of 300mm dia holes at 1250 spacing (so sleepers cut in half will fit) between posts. Depth to be equivalent to wall height plus 100mm for a gravel layer at the bottom of the hole.

Posts

The minimum post lengths to be twice as long as the final height of the wall.

Aim to slope the post back at a gradient of 10-20mm for each sleeper height. Sometimes when a wall is loaded it does move slightly which compacts the ground. If you make the wall vertical and it then tilts  a little after backfilling it really looks bad. When the wall is built with a back slope it just finishes up a bit more vertical.

Set the posts a minimum of 5mm below the planned finished height. It’s safer with the hard steel edges below the softer timber.

Spend some time making sure the tops of the posts are level as any mistakes really show up.

If I was using timber, rather than galvanised steel posts, I would have them on the front face of the wall so the whalings would press against the post.

Drainage

Use an Aggi Pipe with a 100mm gravel surround, maximum stone size of 10mm. Continue the gravel up to just below the surface at least 100mm thick behind the wall. This is to relieve water pressure on the face of the wall. (not to drain surface run off in a storm)

Make sure the ground slopes away from the base of the wall as it helps to keep the soil around the post holes from becoming waterlogged and softening.

See Understanding Retaining walls for more information