Building a retaining wall on a steep or difficult site is a very different proposition from a simple flat-section job. In New Zealand, where hilly terrain, expansive clay soils, and high rainfall are common, experienced retaining wall builders use a combination of careful planning, engineering know-how, and the right construction methods to get the job done safely and in compliance with the NZ Building Code.
Initial Site Assessment
Before any work begins, a good builder will carry out a thorough site assessment. This involves:
- Measuring the slope gradient and identifying any unstable ground
- Checking soil type — clay, sandy, or rocky ground each behaves differently under load
- Identifying existing structures, trees, and underground services nearby
- Assessing access for machinery and materials
- Reviewing any previous earthworks or fill on the site
On very steep sites, a builder may recommend a geotechnical engineer assess the ground before design work starts. This is especially important where there is a history of slipping or where the wall will be retaining significant height.
Engineering and Design
In New Zealand, retaining walls over 1.5 metres in height generally require a building consent from your local council, and walls under that height may still need consent depending on the surcharge (load) behind them, proximity to boundaries, or site-specific conditions. Always check with your council — don't assume a wall is exempt under Schedule 1 of the Building Act.
For complex or tall walls, a Licensed Building Practitioner (LBP) or structural engineer will typically be involved in the design. Licensed Building Practitioners are registered to carry out and supervise restricted building work, which includes retaining walls that require consent — though note that not all consented retaining wall work is classified as restricted building work; check with MBIE or your LBP for the specific scope.
Engineered designs will specify:
- Wall type (timber, concrete block, concrete panel, gabion, or boulder)
- Foundation depth and type
- Reinforcing requirements
- Drainage design
- Tie-back or deadman anchor systems where needed
Managing Access and Machinery on Difficult Sites
Steep sites often limit what machinery can safely get in. Builders use a range of approaches:
- Mini excavators and compact machinery that can work on slopes and in tight spaces
- Hand excavation for areas where machinery simply can't reach
- Crane lifts or hiab trucks to place heavy materials like concrete panels or large boulders
- Temporary access tracks cut into the slope to allow safe movement of equipment and materials
- Staging materials at the top or bottom of the slope and moving them in sections
Worksafe NZ requirements around working on slopes and near excavations must be followed. WorkSafe NZ sets out the obligations for safe excavation work, including shoring and battering of trenches.
Drainage — The Most Critical Factor
Poor drainage is the number one cause of retaining wall failure in New Zealand. On steep sites, water builds up behind walls quickly, creating enormous hydrostatic pressure. A competent builder will always include:
- Coarse aggregate (gravel) backfill directly behind the wall
- Ag pipe (agricultural drainage pipe) at the base of the wall to carry water away
- Weep holes through the wall face at regular intervals
- Directing drainage to a legal discharge point — often a stormwater drain or soakage area
BRANZ provides technical guidance on retaining wall drainage that both builders and homeowners can reference.
Choosing the Right Wall Type for the Site
Different wall systems suit different conditions:
| Wall Type | Best For | Key Consideration |
|---|---|---|
| Timber (H5 treated) | Moderate heights, good access | Lifespan varies; check treatment rating and manufacturer guidance |
| Concrete block | Versatile, strong | Needs solid foundation; heavy materials |
| Concrete panel | Tall walls, engineered sites | Requires crane; engineer design essential |
| Gabion baskets | Steep, rocky, or wet sites | Flexible; allows drainage naturally |
| Boulder/rock | Natural look, rural sites | Heavy machinery needed; site-specific |
For steep sites, tiered walls (a series of smaller walls with flat terraces between them) are often preferred over one very tall wall. This spreads the load, improves drainage, and is often easier to build safely.
Earthworks and Stability During Construction
During construction on a steep site, the builder must manage the risk of ground movement or collapse. This includes:
- Cutting and filling in stages rather than all at once
- Propping or shoring any exposed cuts
- Monitoring the site after rain before continuing work
- Ensuring spoil (excavated material) is stockpiled safely away from the edge
If the site involves significant earthworks, a resource consent from the local council may also be required under the Resource Management Act — your builder or a planner can advise.
When to Call a Professional
You should always engage a qualified retaining wall builder or structural engineer when:
- The wall will be over 1.0 metre high (and certainly over 1.5 metres)
- The site is steep, unstable, or has a history of slipping
- The wall is near a boundary, building, or driveway
- There is significant soil or water pressure expected behind the wall
- Building consent is required
Attempting to build a retaining wall on a difficult site without the right expertise can result in wall failure, property damage, and serious injury. Find a qualified retaining wall builder in your area through tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
