Backfill a finished spring box by placing clean gravel of 20-40 mm at least 30 cm thick behind the permeable back wall, then covering the whole box with clay or compacted earth, mounded above ground level so rain runs off it. Test the box for leaks and let the concrete cure before covering it, keep the hatch and the pipe ends above the mound, and fence and divert the surface water so that the box stays clean.
Part of Spring Capture
Backfill is the soil and gravel placed round a completed spring box. It supports the walls, keeps the spring’s water on its path into the box, and is the last barrier between the water and the surface.
The Backfill Has Two Different Zones With Opposite Jobs
Behind the permeable back wall the fill must let water through: clean 20-40 mm gravel, at least 30 cm thick, with the graded layers against the soil (Inlet Wall). Everywhere else, and over the top, the fill must keep water out: clay or compacted earth, shaped to shed rain. The two meet at the top of the gravel, where a clay cap seals it so that surface water cannot run straight down into the gravel.
What You Need
- Clean washed gravel, 20-40 mm, and the finer grades for the filter layers
- Clay or earth — puddled or well compacted
- A rammer — to compact in thin layers
- Clean water and disinfectant — to flush and disinfect before use
- Posts and fencing — for the protected zone (Fencing)
- Turf or grass seed — to cover the mound
Method: Test, Fill In Layers, Mound, Protect
Step 1: Let the walls cure. Concrete and mortar should be damp-cured before they carry the earth load (Curing). Do not backfill against green masonry.
Step 2: Test the box. Fill it with clean water, plug the outlet, and look for leaks round the walls, floor and pipes for a day. Fix what leaks before it is covered.
Step 3: Place the gravel behind the permeable wall in layers, as the filter grading requires, up to the top of the wall, at least 30 cm thick.
Step 4: Cap the gravel with clay, rammed, so that surface water cannot run down into it.
Step 5: Backfill the sides and the top in thin layers, each compacted, bringing earth up evenly on both sides so the walls are not pushed over. Do not drop large stones against the walls.
Step 6: Mound. Finish the earth over the box higher than the ground around it, so that rain runs away on every side, with the hatch lip and the overflow end clear above it.
Step 7: Cover it. Seed or turf the mound so that rain does not wash it off.
Step 8: Flush and disinfect. Open the drain and flush the box; disinfect it with chlorine as described in Well Development (the procedure there is for a well, and the same steps of dosing, standing and flushing apply); then have the water tested before use (Community Water Testing).
Protect The Finished Box From Its Surroundings
- Keep the gravel dry from the top. The clay cap and the mound do it.
- Divert uphill water (Uphill Diversion).
- Fence out stock and people (Fencing).
- Keep trees trimmed back near the box (Vegetation Management).
- Lock the hatch, and inspect the box regularly for cracks, leaks and silt.
Common Mistakes
- Backfilling green concrete. The walls crack under the load.
- Soil, not gravel, behind the permeable wall. The spring is blocked.
- No mound. Rain pools over the box and seeps into the gravel.
- Hatch below ground level. Rainwater runs in.
- Burying the overflow end. The box backs up.
Safety
Open cuts and chlorine
The excavation round a half-filled box is still an open cut: keep it sloped or shored if deeper than 1.2 m, keep spoil at least 0.6 m back, and do not work alone (Excavation). Chlorine products irritate skin and lungs: handle them outdoors, never mix them, and keep them away from children. Dosing amounts are those of the disinfection procedure referred to above, not set here.
What’s Next
- Learn next: Fencing — the protected zone round the box
- Combine with: Piping from Spring — carrying the water away