Retaining Wall Engineering

Dec 17, 2019 | retaining wall engineering | 0 comments

Retaining wall engineering involves the assessment and design to create a wall to retain a height of material. In some situations, an engineering design is legally required to satisfy local council requirements.

Retaining walls come in all sizes from 0.2m to 10m high and many different styles. There are moss rock walls, steel post/concrete sleeper walls, stacked block walls, steel-reinforced concrete-filled block walls and many more.

And of course, retaining walls come with many different prices. They are generally expensive, and the longer and taller they are, the more costly the total price.

Image courtesy Outback Sleepers

But what type of retaining wall is best for you?

What are the main types of retaining walls?

Post and Sleeper Retaining Wall

The most common type of wall used in residential cases is the post and sleeper wall. This can either be:

  • Timber post and sleeper, with the posts set in a concrete fill bored pier. These are not recommended as the timber rots or is eaten by termites, reducing the lifespan of the wall. If you get 15 years out of a wall like this, you are doing well.
  • A steel post/concrete sleeper wall with the posts set in concrete-filled bored piers. These walls should last at least 40 to 50 years and are a similar price to the timber version above.

These walls can have metal sheet fences installed over.

The timber sleepers come in many different colours and textures and don’t have to be concrete grey and flat.

These tend to be the cheapest type of retaining wall and are very versatile.

As the wall is supported by a concrete pier in the ground, these walls may not be suitable for sites with shallow rock, as the cost of boring into rock is high. If rock is suspected or known to be shallow in the area, it is best to take some soil logs to determine upfront if this type of wall is the best option or not.

This type of wall is suitable for wall heights of 200mm to 5m. Obviously, the greater the level change, the greater the cost of the wall.

This type of wall is suited to placing a retaining wall across an easement, where it may be necessary to excavate and replace a pipe sometime in the future. In this case, the sleepers in one bay can be removed, giving easier access to the pipe under that area. The general requirements are for the in-ground concrete piers to be at least 1m away from the pipe.

Moss Rock Retaining Wall

This type of wall is basically a battered earth slope with large rocks stacked over it, in an engineered layout.

This type of wall does not extend far below ground level and hence is suitable for sites with shallow rock, but it has a very wide footprint due to the batter slope and the size of the rocks. This width is typically as wide as the wall is high, or wider.

These walls have a very natural look compared to other types of retaining walls.

Steel reinforced, concrete fill block walls

These retaining walls are made from concrete besser block (or similar) and contain steel reinforcement. Cavities in the blocks are concrete filled. They are narrow (about 200mm wide generally) and can be several metres high.

They must be founded on either:

  • a concrete strip beam with reinforced concrete piers under,
  • a concrete slab footing.

The concrete slab footing makes these walls suitable for sites with shallow rock. However, the concrete slab will extend out some distance in front of the wall.

If the wall is to be installed on a site which is to be filled, the slab footing can extend back under the fill, which makes the wall more efficient and the slab footing narrower.

As the walls are blockwork, they are often finished by render and/or paint to improve their appearance.

These walls tend to be more expensive than post and sleeper walls, as they require several trades:

  • an excavator
  • a concreter for the slab footing
  • a bricklayer for the blockwork

Stacked Block Walls

These are walls constructed of special proprietary engineered blocks, such as keystone or AB blocks.

They are suitable for lower height garden walls (up to 1m, depending on the type of block) and they are:

  • founded on a rubble base
  • consist of stacked blocks, which may be unfilled, filled with screenings or concrete
  • may be hand-
  • assembled by owners.

Walls higher than 800mm (depending on the block type) may be engineered by including geotechnical tie backs to tie the wall to the soil mass behind.

Engineering Requirements for Walls

All the different types of retaining walls discussed above require structural engineering design to ensure they are suitable for their intended application.

An engineer should be engaged to design your wall for your particular situation.

We would be happy to assist with your selection of retaining wall type and the design of it to ensure your wall is cost-efficient and long-lasting.

Give us a call today to discuss your retaining wall requirements

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