Digging post holes for a retaining wall in hard clay is one of the more physically demanding DIY jobs around the home, but with the right tools and a bit of know-how it's absolutely achievable. The key is choosing the correct equipment for your soil conditions, understanding how deep your holes need to be, and knowing when the job is better left to a professional retaining wall builder.
Understand the Rules Before You Dig
Before you pick up a shovel, check whether your retaining wall needs a building consent. In New Zealand, retaining walls over 1.5 metres in height (or any height if they support a surcharge such as a driveway or building) generally require consent under the NZ Building Code. Walls under 1.5 m may be exempt under Schedule 1 of the Building Act — but always confirm this with your local council before starting, as rules can vary by district and site conditions.
For consented walls, the design must meet the requirements of MBIE Building Performance and will typically need to be designed by a structural engineer. Even for exempt walls, following good practice protects your property and your neighbours.
Tools You'll Need
Hard clay is dense and sticky — standard garden spades will wear you out fast. Here's what actually works:
- Manual post-hole digger (clamshell digger): Good for occasional holes in moderately hard clay. Tiring but effective.
- Breaker bar / digging iron: A heavy steel bar used to break up compacted clay before scooping. Essential in very hard ground.
- Petrol or electric post-hole auger (hired): The most efficient option for multiple holes. A 200–300 mm auger bit suits most timber retaining wall posts. Available from hire shops around NZ.
- Tow-behind tractor auger: Worth considering for large projects or very deep holes.
- Bucket of water: Pouring a small amount of water into the hole and letting it soak in for 10–15 minutes can soften clay enough to make digging significantly easier.
You can hire augers and breaker bars from most tool hire companies. Bunnings and Mitre 10 also stock manual post-hole diggers and digging bars if you prefer to buy.
How Deep Should Post Holes Be?
Post depth depends on the wall height and your engineer's or manufacturer's specifications. As a general guide:
| Wall Height | Minimum Post Embedment Depth |
|---|---|
| Up to 600 mm | ~600 mm |
| 600 mm – 1.0 m | ~900 mm – 1.0 m |
| 1.0 m – 1.5 m | ~1.2 m – 1.5 m |
Always follow your engineer's design or your timber supplier's specifications — these are guides only. Embedment depth also depends on soil bearing capacity, which in clay can vary significantly. BRANZ has useful guidance on soil conditions and foundation design.
Step-by-Step: Digging in Hard Clay
1. Mark Out Your Post Positions
Use a string line and stakes to mark the wall line accurately. Measure and mark each post position — typically 1.2 m to 1.8 m centres for timber retaining walls, but confirm with your design.
2. Call Before You Dig
Always contact Dial Before You Dig (0800 248 344 or dialbeforeyoudig.co.nz) to locate any underground services — power cables, water pipes, and gas lines can all be buried in residential sections.
3. Start with a Breaker Bar
Drive the breaker bar into the clay repeatedly to break up the surface. Work in a circular pattern to loosen a column of soil roughly the diameter of your intended hole.
4. Use the Clamshell Digger or Auger
Once the clay is broken up, use a clamshell digger to scoop out the loosened material. If using a petrol auger, keep the bit moving steadily — stopping in clay can cause it to bind. Reverse the auger periodically to clear clay from the flighting.
5. Add Water to Soften Stubborn Sections
For particularly hard layers, pour in a litre or two of water, wait 15 minutes, then continue digging. Don't overdo it — you don't want the base of the hole to become waterlogged before you pour concrete.
6. Check Depth and Diameter
Use a tape measure to confirm depth. The hole diameter should be at least 2–3 times the post width to allow for concrete backfill.
Drainage Matters in Clay
Clay is poorly draining, which is a major issue for retaining walls. Water build-up behind a wall dramatically increases the load on your posts. Always include agricultural drain coil (ag pipe) and free-draining gravel backfill behind the wall. Weep holes through the wall face also help relieve hydrostatic pressure. Poor drainage is one of the most common reasons retaining walls fail in NZ.
Concrete Backfill
Once your post is positioned plumb and braced, backfill with a concrete mix — a standard 20 MPa mix is commonly used. Allow it to cure for at least 24–48 hours before loading the post. PlaceMakers and Mitre 10 stock bagged concrete mixes suitable for post setting.
When to Call a Professional
Call a qualified retaining wall builder or structural engineer if:
- Your wall is over 1.0 m high or retains a driveway, slope, or building load
- You hit rock, large boulders, or unknown fill that an auger can't penetrate
- The site is near a boundary, waterway, or existing structure
- You're unsure about soil stability or drainage
- A building consent is required
For restricted building work and consented walls, you may need a Licensed Building Practitioner (LBP) to carry out or supervise the work. Find experienced local retaining wall builders on tradedirectory.co.nz.
Safety note: Never enter or lean into a deep post hole — clay walls can collapse without warning. Keep bystanders and children well clear of the work area, and follow all WorkSafe NZ guidelines for excavation safety.
Licensed Building Practitioner (LBP)
