How to Safely Excavate a Sump Pit for a Submersible Pump

Excavating a sump pit for a submersible pump is a practical way to manage groundwater, stormwater, or seepage around your home's foundation. Done correctly, it's a manageable DIY project — but it does require careful planning, an understanding of NZ regulations, and strict attention to safety. Here's a step-by-step guide to help you get it right.

What Is a Sump Pit and Do You Need One?

A sump pit (also called a sump well) is a below-ground chamber, typically 300–600 mm in diameter and 600–1200 mm deep, that collects water which is then pumped away by a submersible sump pump. They're commonly installed in:

• Basements or subfloor spaces prone to water ingress
• Low-lying sections where stormwater pools
• Areas with high water tables

Before you start digging, check whether your project requires a building consent. Under the NZ Building Code and Schedule 1 of the Building Act 2004, minor drainage work may be exempt — but rules vary by council and by how the sump connects to your drainage system. Always confirm with your local council before proceeding. You can find guidance at MBIE Building Performance.

Step 1: Plan Your Location and Check for Services

Choosing the right spot is critical. The pit should sit at the lowest natural point of the area you want to drain. Before any digging:

• Call Before You Dig (0800 248 344) — this is essential. Underground services including power cables, gas lines, water pipes, and telecoms can be present anywhere. Striking one can be fatal.
• Check your property plans and talk to your local council about stormwater and drainage connections.
• Confirm the discharge point — where will the pump send the water? It must comply with your council's stormwater bylaws and the NZ Building Code (refer to Clause E1 – Surface Water).

Step 2: Gather the Right Tools and Materials

For most residential sump pits, hand digging is sufficient. You'll need:

• Spade and long-handled shovel
• Mattock (useful in clay soils common in many NZ regions)
• Tape measure and spirit level
• Timber or plywood for temporary shoring (if deeper than 1.5 m — see safety section)
• A pre-formed sump liner or perforated plastic pit (available from Bunnings or Mitre 10)
• Drainage metal or gravel for backfill around the liner
• Geotextile filter fabric to prevent silt ingress

Step 3: Excavate Safely

This is the most important phase. Follow these guidelines:

Excavation Depth and Trench Safety

• For most sump pits, you'll be digging 600–1200 mm deep — manageable by hand.
• If you need to go deeper than 1.5 metres, WorkSafe NZ requires that the excavation be benched, battered back, or shored to prevent collapse. Soil collapse is a leading cause of serious injury and death on worksites. Do not enter an unshored excavation deeper than 1.5 m.
• NZ soils vary widely — Auckland's heavy clays behave very differently from Canterbury's gravels. Clay can appear stable but fail suddenly, especially when wet.
• Keep excavated spoil at least 600 mm away from the edge of the pit to reduce surcharge loading.
• Never work alone in or near a deep excavation.

Digging the Pit

1. Mark out the pit diameter — typically 100–150 mm wider than your liner on all sides.
2. Dig straight down, checking vertical alignment regularly.
3. Once at depth, compact or level the base.
4. Add a 100 mm layer of drainage gravel to the base.

Step 4: Install the Liner and Backfill

• Lower the pre-formed sump liner into the pit. Many are made from perforated PVC or polyethylene.
• Wrap the outside with geotextile fabric to filter out fine particles.
• Backfill around the liner with drainage metal (gravel), compacting in 150 mm layers.
• At the surface, seal around the liner with concrete or compacted soil to prevent surface water bypassing the filter layer.
• Fit a lid or grate over the pit to prevent debris entry and reduce injury risk.

Step 5: Connecting the Pump — Restricted Work Considerations

Installing the submersible pump itself is generally DIY-friendly, but connecting it to your home's electrical supply is restricted work under the Electricity Act 1992. Any fixed wiring — including installing a dedicated power outlet near the pit — must be carried out by a registered electrician. Check the Electrical Workers Registration Board (EWRB) to verify your electrician's registration.

Similarly, if the pump discharge connects to your household drainage or stormwater system, that pipework connection may constitute restricted sanitary plumbing or drainlaying work, which must be done by a licensed tradesperson registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB).

Safety note: Water and electricity are a dangerous combination. Always use a residual current device (RCD) protected circuit for any pump near water, and never handle electrical connections in wet conditions.

When to Call a Professional

Consider hiring a qualified excavator operator or drainage contractor if:

• The excavation needs to go deeper than 1.5 m
• You encounter unexpected groundwater, rock, or unstable soil
• The site is near existing foundations, retaining walls, or structures
• You're unsure about drainage connections or consent requirements
• A larger machine is needed to work efficiently in tight or difficult ground

A licensed building practitioner can also advise on whether your project needs consent. Find one at Licensed Building Practitioners, or search for a local excavation or drainage specialist on tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance – Building Consents
• WorkSafe NZ – Excavation Safety
• PGDB – Licensed Drainlayers
• EWRB – Registered Electricians
• BRANZ – Moisture and Drainage Guidance