Waterproofing Retaining Walls

How to Build a Waterproof Retaining Wall: A Step-By-Step Guide

Retaining walls are becoming more common as higher density housing is built. A retaining wall allows for the lands natural slopes to be utilised for practical and aesthetic reasons.
Often they are needed for boundary fencing or to to terrace a sloping site. Creating usable space, or simply add profile and interest to an otherwise flat and featureless garden.
Regardless of the reason for its construction, a retaining wall must be well built to remain intact, safe and trouble-free.

There is a high percentage of sloping land in Australia, which can be challenging when designing gardens with usable space.
Using a qualified builder or structural landscaper is highly recommended to ensure that you have a retaining wall that is designed and engineered to be a sturdy , long-lasting and safe retaining wall.

With Brisbane’s insane rainfall at the start of 2022, we saw many retaining walls and garden areas be washed away or become unsafe as the extra weight of water in the ground.
Water and moisture is always an ongoing problem when dealing with outside structures and waterproofing a retaining wall is a big part of ensuring that you have a well built and compliant feature on your property.

Read on for some practical tips on designing, building, and waterproofing a retaining wall and information on the issues that can arise if the wall isn’t built correctly.

What is a retaining wall?

As the name suggests, when there is a drastic elevation change, a retaining wall holds the soil to the side and prevents it from spilling over onto the level below. Retaining walls can range from simple raised beds to large walls several feet high that are built to prevent erosion, divert water and prevent landslides.

A retaining wall is usually needed when a slope has lost its natural integrity or is made of naturally unstable gravel or sand. 

Types of retaining walls

The materials from which retaining walls are made vary significantly. The different types of retaining walls include:

• A rock wall is a large wall made of stones and boulders that use the existing natural slope but at a less acute angle. It can be sewn with natural vegetation to increase strength and erosion control.
• Sleeper wall – one of the most famous walls due to its cost, advantages and ease of installation. However, wood rots over time, so concrete sleepers may be a more durable option.
• Masonry wall – a crushed stone or concrete base on which layers of plastered blocks or bricks are stacked and topped with a finished capping plaster. These walls can be built higher than other retaining walls but require a solid foundation and good drainage.
• Gabion Wall – Crushed stone, rock, or pebbles are carefully selected and placed into reinforced steel cages (similar to a dry stone wall in a cage). These walls are highly porous, making them ideal for drainage or soil problems sites.
• Dry stone wall – a mortarless retaining wall constructed of carefully selected rocks and stones. It requires a vast base to provide strength and stability (at least one-third the height of the wall).

How is a retaining wall waterproofed?

Waterproofing a retaining wall is an important aspect of any building project, and if not correctly designed and executed, it can become a dangerous hazard.

1. Check the regulations

Depending on your residence in Australia and how high your proposed retaining wall is, you may need council approval to build it.
In some areas, any wall that is more than 600mm high or costs more than $5,000 will require council approval, and if it costs more than $16,000, you may also need the builder’s warranty insurance.
Often you will need to use a professional service such as Brisbane Certifier to ensure that the wall is built to specification

Since you’ll be digging for your wall’s foundation, you’ll also need to check existing plumbing and electrical wiring to make sure no pipes or wires are running under the site. (cutting public utility lines is not only dangerous but can also result in hefty fines for repairs)

If your wall is being built near your property lines, you’ll also need to consult with your neighbours to make sure they agree with your plans and that your wall won’t pose a danger to their property from erosion or water runoff. And if your wall is to support large trees or load-bearing structures, it is advisable to hire an engineer to build it.

2. Check the soil type

When designing your retaining wall, it’s important to consider the nature of the slope it will intercept. This is because different materials have varied “angles of repose,” such as clay and sand. (the steepest angle to which they can be backfilled without sagging).

3. Create a solid base

Creating a sturdy foundation is one of the most crucial foundations for excellent wall construction. A retaining wall’s base should be below ground level and should consist of compacted soil and at least 150 mm of compacted sand and gravel.

The wall will remain flat as a result of this, which means more contact between the materials used for construction and, therefore, more friction and ultimately more strength.

4. Ensure good drainage

Adequate drainage is essential for a retaining wall; otherwise, water pressure, known as hydrostatic pressure, will build up behind the wall, which can cause buckling or cracking.

Ways to ensure good drainage include installing a perforated pipe inside the wall that discharges into a storm drain and placing small soakaways in the wall for water to drain through.

5. Consider function and height

Retaining walls hold back soil when there is a drastic elevation change. They are usually necessary on steep lots to create safe, usable space for gardens, buildings and driveways. Many Australian homeowners also opt for low retaining walls to delineate different garden areas for functional and aesthetic reasons. 

Generally, the higher the wall, the lower it starts in the ground. A good rule of thumb is that one-tenth of the height of the wall should be below the ground surface. For a 900 mm high wall, the base course should be at least 90 mm below ground level. Posts for projecting walls should be 100 mm below ground level for every 100 mm of wall height. Also, remember to excavate another 300 mm behind the wall for backfill with gravel.

6. Obtain the consent of the municipality

Poorly built retaining walls can buckle, crack or lean and become an eyesore. In worse cases, they can also topple. That’s why officials want to see your plans before you start any major work. Always check with your local government to see if you need a permit to build a retaining wall. They can provide you with guidance on waterproofing retaining walls to Australian standards. If your building is near a property line, you may also need to consult with neighbours.

7. Check plumbing and wiring

Check your plans against plumbing and electrical plans to ensure you don’t have any nasty surprises when you start digging. It’s also important to choose the right digging equipment. If it is a large project, there are many different excavators to consider. However, if it is a small residential wall, you can simply choose from a variety of mini excavators that are suitable for the project.

8. Design

A well-designed retaining wall will not tip over and will support the soil behind it and any loads that may act on it, such as those from cars and buildings. It also prevents water from accumulating behind the wall, increasing the wall’s lateral load. 

9. Use the force

Use gravity to your advantage by having the wall recede toward the ground. Consider your backfill in this context. Retaining walls look like they need to hold back large masses of earth, but they only need to hold back a small wedge.
Don’t fill that wedge with moisture-loving soil that could compromise your wall, but instead with sandy, gravelly material that can be easily compacted and allow water to drain down and away from the wall. The pressure is directed downward and not against the wall by compacting the fill.
You will need at least 300 mm of gravel or similar material directly behind the wall for proper drainage. Provide at least 150 mm of topsoil above the gravel if you plan to plant vegetation behind the wall.

10. Waterproofing of the retaining wall

A retaining wall must be waterproofed and have proper drainage to prevent hydrostatic pressure from building up. Waterproofing also protects the wall’s back from moisture that may remain after the water has drained and slowly seeped through the wall, eventually discolouring the surface and possibly affecting the wall’s integrity.

Therefore, it is essential to use a waterproofing membrane on the back of your retaining wall, just as you would for a basement wall where the same conditions occur.
There are several types of waterproofing membranes available on the market. The membrane you select must be able to withstand standing water. Verify with your supplier that it meets this requirement.

• Waterproofing membrane

A waterproofing membrane, a thin layer of material, is designed to prevent water from coming into contact with another material over which it is placed. Waterproofing membranes can be glued or simply placed on the material intended to protect.

Qld Waterproofers is here to help and guide you if you need waterproof retained walls. We are reliable, honest, experienced, and professional in all our undertakings and always do our best to guarantee 100% client satisfaction.