Retaining walls hold back soil to create a split-level landscape. There are many mechanisms by which the landscape is held back. The amount of load on the wall is defined by a wedge of soil that is above the ‘natural angle of repose’. An engineer can determine the load from this wedge by finding out the friction angle of the soil. Horizontal earth pressure on the wall increases from zero at the surface to the maximum value at the base uniformly.
How Retaining Walls Work?
There are several types of retaining wall systems, and all work on the same principle. The earth pressure (can be simplified as acting one-third up the wall) must be counteracted by a reactive force opposing it. Here are some explanations of different retaining walls systems:
- The self-weight of the wall holds the soil back. These have no fixings into the soil, and are prone to toppling easily.
- Gravity walls can be constructed from concrete, masonry, or stone.
- Long piles are driven into the earth and form the basis of the retaining wall. As the piles are founded in earth in all directions, the moment generated by the load is countered by the pile soil. The piles must be able to resist bending forces, and are typically able to support heavy loads.
- These piles can include earth anchors, reinforcing beams, and shotcrete layers.
- Cantilevered walls look like an inverted T that are usually heavily reinforced, sometimes with external bracing. Cantilevered walls rely on the weight of the soil acting upon it to keep itself upright.
- These typically use less material than gravity walls, as they rely on the soil’s weight rather than their own.
- Sheet piles are typically used with soft soils and in coastal or fluvial areas. The piles are commonly made from interlocking steel sheets, but can also be made from timber.
- Most of the pile is driven into the soil using vibration, with the remaining pile left to bear the load. Tall sheet piles usually require dead-man anchors to resist bending.
- Anchoring can be incorporated in any of the previous designs. Anchoring involves the use of cables secured to the wall anchored into the material being retained. This anchor can be a large concrete block, or a steel plate/auger. Reinforcing bars can also be drilled and grouted into the soil in a process called ‘soil nailing’.
- Anchoring can be retrofitted to existing walls that are showing signs of damage or bowing.
- Gabions are cages that are filled with loose rocks. These are free-draining, and resist erosion.
- These are popular for landscaping purposes.
- Gabions work as a gravity wall, but are less rigid.
Why Retaining Walls Fail?
There are many causes of retaining wall failure:
- Saturation of the soil increases load on the wall through filling air voids in the soil with water. In clay soils, the weight of soil increases more than in sandy soils, which drain more freely.
- If the structure begins to tilt, the loading will increase due to larger ‘wedge’ of retained material. For a tilt of 10°, the load can increase by 20%.
- Stacked layers of retaining walls placed too close together can cause failure of soil. The passive reaction force at the bottom of pier footings can extend laterally many times the height of the wall.
- Use of incorrect design. This is a common issue with DIY retaining walls, as without engineering design, the forces on the wall and required retaining cannot be accurately determined.
- Use of incorrect materials. To cut corners, cheaper materials will sometimes be used. For example, a masonry block wall costs around 40% less than a reinforced concrete wall, but with a 75% reduction in capacity.
What to Look For: Early Warning Signs
- Bowing of wall
- No water coming from weep holes after rain
- Subsidence in surrounding areas
How to Avoid Failures?
- Adequate drainage from base of retaining wall. Well-drained fill such as gravel is commonly used behind a retaining wall to improve drainage characteristics. Agricultural pipe is often laid at the base of this gravel to take moisture away from the retaining material.
- Compaction of the soil beneath and behind the retaining wall ensures the soil is close to maximum density when being constructed. This means further saturation of the soil will have little effect, as the voids in the soil are minimised. The soils should be compacted in layers, and often reinforcing mesh can be added between layers to engage soil with the wall.
- Reinforcement of both the wall and the retained wall will increase bearing capacity of the wall. Existing walls can be retrofitted with reinforcement like anchor bolts to rectify bowing.
- Ensure your local council requirements are met for retaining walls. For walls over a certain height, an engineer must be engaged to design the retaining wall, along with a building surveyor to oversee the building process.