Shoring up a trench is one of the most important safety steps you can take when digging on your property. In New Zealand, trench collapses kill and seriously injure workers and homeowners every year — even a shallow trench can collapse without warning and trap a person under hundreds of kilograms of soil. Before you pick up a spade, understand your legal obligations and the right shoring methods for your situation.
Why Trench Safety Is So Serious
Soil looks solid, but it can behave like a liquid under pressure. A cubic metre of average NZ soil weighs roughly 1,500–1,800 kg. A collapse gives you almost no time to react. WorkSafe NZ treats excavation as a high-risk activity, and their guidelines apply to homeowners carrying out work on their own property, not just professional contractors.
As a general rule: any trench deeper than 1.5 metres must be shored, battered back, or otherwise made safe before anyone enters it. Some soil types (loose sand, fill, waterlogged ground) require protection at shallower depths — sometimes from as little as 600 mm.
NZ Legal and Regulatory Context
Under the Health and Safety at Work Act 2015, you have a duty to manage risks to yourself and anyone else who could be affected by your work. WorkSafe's excavation guidelines set out minimum requirements. If you are hiring any workers or contractors to help, additional PCBU (Person Conducting a Business or Undertaking) obligations apply.
For building-related trenches (e.g., foundations, drainage), you may also need a building consent depending on the scope of work. Schedule 1 of the Building Act lists some exemptions, but always confirm with your local council before you start — rules vary by region and by what the trench is for.
Drainage and plumbing trenches that connect to the public system involve restricted work under the Plumbers, Gasfitters and Drainlayers Act. Check with the Plumbers Gasfitters & Drainlayers Board about what you can legally do yourself.
Assessing Your Soil Type
Before choosing a shoring method, assess your soil:
- Stable cohesive soil (firm clay): holds a cut face reasonably well but can still collapse, especially when wet.
- Loose or granular soil (sand, gravel, fill): very high collapse risk; requires shoring from the moment you start digging.
- Waterlogged or disturbed ground: treat as unstable regardless of depth.
- Ground near existing structures, trees, or previous excavations: extra caution required — seek professional advice.
If you are unsure of your soil type, consult a geotechnical engineer or a qualified excavation contractor. You can find local excavation professionals on tradedirectory.co.nz.
Main Shoring Methods for DIY Trenches
1. Battering (Sloping the Sides)
The simplest method — cutting the trench walls back at a safe angle so they are self-supporting. The required angle depends on soil type, but as a rough guide, stable cohesive soils may be battered at 1:1 (45°), while loose soils may need 1:2 or flatter. This method requires significantly more space and spoil management.
2. Timber Shoring (Sheeting and Struts)
The traditional DIY-friendly approach for narrower trenches:
- Timber planks or sheets are placed against the trench walls.
- Walers (horizontal timbers) hold the sheets in place.
- Struts (cross-bracing timbers) span the trench width and push the walers against the soil.
- Timber must be structurally sound and sized appropriately for the depth and soil load — this is not a job for off-cuts.
3. Hydraulic or Mechanical Trench Boxes (Drag Boxes)
A trench box (also called a drag box or trench shield) is a steel or aluminium box that sits inside the trench and protects workers. These are commonly hired from equipment hire companies. They are the safest and most practical option for deeper trenches and are widely used by professional contractors. Check with your local hire company about availability and correct sizing.
| Method | Best For | DIY Feasibility | Key Risk |
|---|---|---|---|
| Battering | Open sites, shallow trenches | High | Needs space; soil-dependent |
| Timber shoring | Narrow trenches, <2.5 m deep | Moderate | Must be correctly sized |
| Trench box (hired) | Deeper or longer trenches | Moderate–High | Correct installation critical |
Key Safety Rules for Any Trench
- Never enter an unshored trench deeper than 1.5 m — no exceptions.
- Keep spoil (excavated soil) at least 600 mm back from the trench edge to avoid surcharge loading.
- Have a second person present whenever anyone is in a trench.
- Check the trench condition after rain, frost, or vibration (e.g., nearby traffic or machinery).
- Know where underground services are before you dig. Call Before U Dig (0800 248 344) or visit their website to request a service location — this is free and essential.
- Have a rescue plan in place before anyone enters.
When to Call a Professional
Some situations are beyond safe DIY scope. You should engage a licensed excavation contractor if:
- The trench will exceed 2.5 metres in depth.
- You are working in unstable, waterlogged, or unknown fill material.
- The trench is near existing buildings, retaining walls, or large trees.
- You need to connect to public drainage or water supply (restricted work under NZ law).
- You are unsure about soil conditions or the correct shoring design.
A qualified tradesperson will have the right equipment, training, and insurance. Find local excavation contractors on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
