Pool Backwash Line Collapse: The Highveld Thunderstorm Problem

A problem unique to Gauteng and the Highveld

Most Gauteng pool installations bury the backwash discharge line in a shallow trench (300–450 mm deep) running from the pump house to a stormwater channel, soakaway, or street kerb. That line is typically 50 mm uPVC, glued, with one or two 90° bends.

It works fine for years. Then one February afternoon a 60-mm-per-hour Highveld thunderstorm dumps water into clay soil that's been baking dry since November, the soil swells, the trench shifts, and the line cracks or pulls apart at a joint.

You only notice the next time you backwash the pool.

The classic symptoms

• A soggy patch on the lawn between pool and street that won't dry, often with chlorinated-smelling water.
• Backwash takes much longer than it used to, with weaker discharge at the outlet.
• The pump runs louder during backwash (the line is partially blocked with soil).
• The outlet at the kerb runs clear instead of cloudy when you backwash — water is escaping before it gets there.
• A small sinkhole above the buried line — a serious warning sign that needs immediate attention.

Why it's worse on the Highveld

• Highveld clay (especially in suburbs like Sandton, Edenvale, Boksburg, Centurion) is expansive — it swells dramatically with moisture and shrinks dramatically when dry.
• The annual wet/dry cycle stresses every buried pipe joint.
• Hailstorms compact and erode the trench cover, removing the protective load-spreading layer.
• Termites — yes, really — chew through softened uPVC at joints in some areas.

Why you can't just ignore it

A leaking backwash line:

• Wastes 600–1 000 litres of treated pool water per backwash cycle.
• Saturates the soil under garden walls and paving — sinkholes and wall collapses follow.
• May breach municipal by-laws (most metros require backwash to reach the stormwater system, not a soakaway near a boundary wall).

How a plumber finds and fixes it

• Smoke test — a low-pressure smoke generator pushed into the pump end shows exactly where the line is broken (smoke comes up through the lawn).
• Tracing dye in the backwash water — quick, cheap, very effective.
• Excavation only at the failure point — modern repair uses a slip-fix or rubber coupling rather than re-glueing.
• Repair in HDPE for the failed section. HDPE flexes with the clay; uPVC does not.

Prevention

• Bed the pipe in river sand, not clay backfill, for the full length.
• Add a cleanout at the pool end so you can rod the line clear without digging.
• Avoid 90° bends — use two 45° bends instead.
• Don't plant deep-rooted trees (poplars, oaks, syringas) within 4 m of the line.