Hostel Soil Stack Without Venting and a Silted 30 m Drain
A three-storey boys' hostel with 24 WCs and 18 wash basins is drained by a single 100 mm diameter soil stack at the rear; there is no vent stack. Complaints are: the top-floor WC clears slowly; ground-floor traps gurgle after a top-floor flush; a sewer smell returns to the ground-floor toilet two days after the traps are refilled; the ground-floor floor trap measures only a 30 mm water seal and is partly choked with hair and soap. The canteen kitchen sink discharges into the same stack through a 40 mm pipe that has no trap. Site drainage runs 30 m from the last manhole to the municipal sewer at a gradient of 1:200, with three 90° bends; during the last monsoon sewage backed up into the ground floor. The roof is a 150 mm RCC slab with a 1.0 m parapet.
1. Explain the chain of events by which the ground-floor traps lose their water seal, and why refilling them does not cure the smell.
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The stack is acting as a single-stack system with no vent, so every flush is a piston in a closed pipe.
- A top-floor WC discharges perhaps 6–9 litres in a few seconds into a 100 mm stack. The slug of water falling down the stack compresses air ahead of it (positive pressure at the bottom) and leaves a partial vacuum behind it (negative pressure above and at branch connections).
- With no vent to admit air, the pressure swings travel to the lowest branch - the ground-floor trap - which is the weakest, shallowest seal in the system.
- The 30 mm seal is induced to siphon out on the negative phase and to bubble through on the positive phase. The gurgling the users hear is exactly this pressure equalisation. A 30 mm seal is below the minimum of 50 mm anyway, so it is already marginal.
- The untrapped 40 mm canteen sink connection is a second, permanent leak path - grease and food waste enter the stack, and sewer gas flows straight back through the open pipe.
Refilling the trap restores water but not the pressure balance, so the next flush from two floors up empties it again. Until the stack is vented and the untrapped connection is corrected, the smell will keep returning within days. The slow top-floor WC points to the same cause plus possible partial blockage or inadequate fall.
2. Set out the remedial scheme with pipe sizes, gradients and fittings, and show how you would check that the 30 m drain can achieve the required fall.
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Stack and venting: retain the 100 mm stack but add a 50 mm diameter vent stack carried up beside it, connected to the top of the stack and re-venting the branch groups on each floor, and taken at least 0.6 m above the roof/parapet and away from any window. Fit 50 mm deep water-seal P-traps to WCs and 50 mm seal gully traps at floor level; replace the 30 mm trap body. Provide a 40 mm trapped connection with a 50 mm seal for the canteen sink and a grease trap before it joins the stack.
Drain: the minimum self-cleansing gradient for a 100 mm drain is about 1:60. Over 30 m that is 500 mm of fall. Measure the invert level of the last manhole and the crown/invert of the municipal sewer connection point. If the available fall is 500 mm or more, re-lay at 1:60. If it is less, do not force a flatter drain - either re-lay at 1:80 with a flushing arrangement and more frequent inspection chambers, or install a pumping sump with a duty/standby pump and non-return valve.
Bends: replace each 90° bend with two 45° bends and an inspection chamber so the line can be rodded. Provide inspection chambers at the head of the run and at every change of direction, and a rodding eye at the 30 m midpoint.
Check: a straight-edge and level survey of each pipe length before backfilling, recorded as invert levels in a simple table.
3. What tests and records would you require before accepting the drainage system?
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Acceptance must be evidence-based, since almost everything is buried after backfilling:
- Air or smoke test on the stack and branches: plug the lowest point, charge the system to a specified test pressure (typically 38–50 mm water gauge) and confirm it holds for at least 3 minutes with no drop; smoke testing is better for finding open, untrapped connections such as the old sink pipe.
- Water test on the drain: fill each length between manholes and hold for 30 minutes, checking for leaks at joints and bends before backfilling.
- Seal depth check: physically measure the water seal of every new trap with a dipstick and record it (50 mm minimum).
- Flow test: five consecutive full flushes from the top floor while an observer watches the ground-floor traps for gurgling or seal loss - this directly tests the fix.
- Invert level record and CCTV survey of the 30 m run after laying, with the survey report attached to the completion file.
- As-built drawing showing vent termination height, trap positions, inspection chambers and the gradient actually achieved.
Do not release retention until one full monsoon has passed, because the original failure only appeared in the rainy season.