Guide to Retaining Walls: Which One Is Right for Your Property?

For properties with sloping terrain or uneven backyards, a retaining wall is often essential for maximizing usable space. Whether your goal is to have a level lawn for family activities, create a flat driveway for your cars, or to prevent soil erosion in a garden area, selecting the appropriate type of retaining wall is crucial.

While these walls can enhance the aesthetic appeal of a property, they must be engineered to withstand specific weather conditions and frequent ground movement. Below is a clear overview of the most common types of retaining walls found in residential building sites in New Zealand.

Working of Retaining Walls

Retaining walls function through a strategic combination of mobilizing soil strength at their base and utilizing gravity. The retained soil, water, and surcharge exert pressure on the wall, attempting to displace it from the fill. While these forces aim to overturn the wall, the wall's self-weight and the weight of the retained soil are typically the primary factors providing stability against overturning.

To prevent overturning, walls must be proportioned appropriately, and they must be designed to possess sufficient strength and stiffness to withstand these pressures.

Even the most robust wall can fail if ground conditions are unsuitable. One of the most significant risks is a sliding failure, which occurs when the entire wall is pushed forward by the soil behind it. Sliding resistance is often achieved through friction between the wall base and the ground and by mobilizing the passive resistance of the soil base, either by embedding the wall or by incorporating a shear key.

Need of Competent Professionals to Design Retaining Walls

A skilled professional is crucial for bringing your vision to life while ensuring safety, cost-effectiveness. Each project comes with its own unique site constraints and preferences, best managed by an experienced expert. When an engineer designs a wall, they are not just focusing on its appearance; they are customizing it to meet your specific site needs. Furthermore, they ensure adherence to the performance standards mandated by the Building Code, including:

• Ultimate Limit State (Safety): The primary requirement is to prevent the wall from collapsing in a way that endangers people. In the event of a significant earthquake or storm, the wall is engineered to remain standing, even if it sustains some damage.

• Serviceability (Functionality): The wall should not tilt or shift to the point where it no longer functions properly. This is particularly important for walls attached to dwellings, where even minor movements could cause windows to stick out, doors to jam, or pipes to crack.

• Protection of Neighboring Property: A wall must be designed to ensure that its movement or failure does not damage neighboring land or buildings.

The New Zealand Standard NZS 3604 for timber-framed buildings provides prescriptive designs for foundation walls that do not retain more than 600mm of soil/fill. All other retaining walls require a specific engineering design (SED) by a qualified engineer.

Retaining Walls are Restricted Building Work items

Retaining walls are significant structures and are typically classified as Restricted Building Work (RBW), necessitating consent. However, Schedule 1 of the Building Act 2004 provides exemptions for certain retaining walls from requiring building consent:

• Exception 20: Retaining walls that do not retain more than 1.5 meters of ground depth and do not support any surcharge or additional load beyond the weight of the ground itself (such as vehicle loads).

• Exception 41: Retaining walls designed or reviewed by a Chartered Professional Engineer in a rural zone are exempt from requiring building consent if the retained height does not exceed 3 meters and the distance between the wall and any legal boundary or existing building is at least the height of the wall.

Common Types of Retaining Walls

Timber Pole Retaining Walls (Post and Lagging)

This wall type is the most favored choice for residential areas. It features robust wooden poles embedded deeply into the ground, with horizontal planks positioned behind them.

Applications: Ideal for creating garden terraces, leveling lawn areas, or bordering driveways on sloped terrain.

Advantages

• Cost-Effective: Typically the most economical option for homeowners.

• Efficient Construction: An experienced contractor can install these walls relatively quickly.

• Adaptable: Constructed from timber, they are slightly flexible during the ground movement without breaking.

Limitations

• Deep Excavation Required: The holes for the poles often need to be quite deep, usually 1-1.5 times the height being retained.

• Rot Resistance: It is essential to use "H5" treated timber for the poles to prevent rotting in wet soil conditions.

Concrete Block Retaining Walls (Masonry)

These structures are constructed using hollow concrete blocks, which are subsequently reinforced with steel bars and filled with concrete. This results in an exceptionally robust and permanent solution.

Applications: These are ideal for basement walls, walls adjacent to a house, or any location requiring maximum strength within a limited space.

Functionality: Concrete block walls depend on a substantial concrete footing and internal steel reinforcement. The steel reinforcement integrates the wall with the base, forming a rigid structure capable of resisting the high lateral pressure exerted by the retained earth.

Advantages

• Strength and Durability: These structures are highly robust and can support considerable weights from buildings or driveways.

• Aesthetic Appeal: They can be plastered or painted to seamlessly match the exterior of your home.

Limitations

• Cost: These structures are more expensive than timber due to higher material and labor expenses.

• Drainage Requirements: They necessitate highly effective drainage systems to prevent water pressure from accumulating behind the heavy blocks.

Crib Retaining Walls (Timber and Concrete)

A crib wall resembles a large wooden or concrete "crate" or "grid" filled with rocks. It utilizes its own weight (gravity) to stabilize the hillside.

Applications: Commonly used on steep slopes where significant water runoff occurs or where a very high wall is necessary.

Mechanism: These are known as "gravity walls." The interlocking framework forms a heavy, stable mass when filled with rock. The wall is slightly inclined towards the hill, allowing its weight to exert pressure against the soil to maintain stability.

Advantages

• Excellent Drainage: The rock filling allows water to flow through the wall's surface.

• Durability: They can adjust and move slightly with the ground without incurring structural damage.

Limitations

• Large Base Area: They occupy considerable space at the base, making them less suitable for confined areas.

Gabion Retaining Walls

These structures are composed of wire cages filled with durable stones and are becoming increasingly popular in contemporary landscaping.

Applications: Commonly utilized for garden features, boundary walls, or areas adjacent to waterways for erosion control.

Mechanism: Similar to crib walls, they function based on "gravity." The weight of the stones within the baskets provides sufficient mass to prevent soil displacement. The wire mesh secures the stones while allowing the wall to maintain significant flexibility.

Advantages

• Natural Aesthetic: They offer a rugged, "eco-friendly" appearance.

• Superior Drainage: Water passes through them as effortlessly as through a sieve.

• Flexibility: Highly resistant to ground movement.

Limitations

• Corrosion Risk: In coastal areas, the wire must be PVC-coated to prevent corrosion from salt air.

Reinforced Concrete Cantilever Walls

These are "L-shaped" walls, resembling a large concrete boot, with the "foot" portion embedded beneath the soil.

Applications: Ideal for very steep slopes where space is constrained, yet a permanent solution is necessary.

Mechanism: These walls rely on the weight of the soil for stability. The retained soil rests atop the buried "heel" of the L-shape, anchoring the wall and preventing it from toppling.

Advantages & Disadvantages: They are efficient space savers but necessitate professional engineering and skilled construction to ensure proper steel reinforcement placement.

Mechanically Stabilized Earth (MSE) Walls

These walls incorporate layers of durable mesh (geogrid) embedded deep into the soil behind the wall face to cohesively bind the earth.

Applications: Commonly used for large-scale projects or on steep hillsides.

Mechanism: Instead of relying on a heavy wall to retain loose soil, the mesh layers generate friction within the soil, transforming it into a solid, reinforced mass capable of supporting itself.

Advantages

• Seismic Strength: They are considered highly safe during earthquakes, as the reinforced soil behaves as a single, flexible unit.

• Height: Can be constructed significantly taller than many other types of residential walls.