Underground Water Tank / Sump Generator

About RCC underground water tanks & sumps

An underground water tank (sump) is a buried RCC liquid-retaining structure used to store municipal / borewell water at the ground level, from which water is pumped up to an overhead tank (OHT) for gravity distribution. It is also used as a fire-water sump or a flushing-water reservoir. Design is governed by IS 3370 (Part 1):2021 (general requirements) and IS 3370 (Part 2):2009 (reinforced concrete — design + reinforcement), read together with IS 456:2000 for general RCC detailing. IS 1893 (Part 2) applies for seismic design of liquid-retaining tanks.

Covered vs open sump

• Covered tank: an RCC top slab seals the tank with a 450–900 mm access manhole. Use for potable / domestic water storage — keeps out debris, light (algae control) and contamination. The slab also carries any superimposed load (landscaping, light traffic).
• Open sump: no top slab. Used for fire-water sumps, pump-house collection sumps, flushing reservoirs where contamination is not a concern and frequent access / inspection is needed. Lower cost; provide a railing / grating for safety.

A typical building water system uses an underground sump at ground level (filled from the municipal main / borewell) plus a pump set that lifts water to an overhead tank; distribution is by gravity from the OHT. The sump buffers intermittent municipal supply.

IS 3370 design basis

Liquid-retaining structures are designed for serviceability (no leakage), not just strength. IS 3370 Part 2 permits two approaches: the older no-crack (working stress) method with low permissible tensile stresses in concrete, or the limit-state method with a crack-width check (permissible crack width 0.2 mm for severe / very severe exposure on the liquid face). The environment is treated as severe-to-very-severe, which drives the M30 minimum grade, the 45 mm minimum cover on the water face, and tight bar-spacing / smaller-diameter bars to control crack width.

Design steps (what the generator does)

1. Capacity: required volume from demand (e.g. half-day to one-day storage). Net capacity = Li × Bi × Hw; shown in litres and m³.
2. Hydrostatic pressure: triangular pressure on the wall, zero at top and maximum p = γ_w·H_w at the base (γ_w = 9.81 kN/m³).
3. Wall design: for a buried rectangular tank the wall is analysed as a vertical cantilever (or propped between base + top slab when covered); the indicative cantilever moment M ≈ γ_w·H³/6 per metre run is shown for sizing.
4. Base raft: designed as a slab on soil, with two-way mesh top + bottom; uplift / flotation governs when the tank is empty and the water table is high.
5. Crack-width check: spacing + bar diameter limited so the calculated surface crack width ≤ 0.2 mm (IS 3370 Part 2 Appendix).
6. Detailing: continuous vertical bars anchored into the raft, hoop (horizontal) bars on the water face, haunches at the wall-base junction, PVC water-bar at all construction joints.

Common mistakes

1. Using M20 concrete — fails the durability / crack-control requirement of IS 3370 Part 1. Water-retaining concrete needs M30 minimum (denser, lower permeability, controls cracking).
2. 25 mm cover on the water face — IS 3370 Part 1 requires 45 mm minimum on the liquid face. Thin cover leads to early rebar corrosion + spalling + leakage.
3. No water-bar at construction joints — the kicker / wall-to-base and lift joints are the first to leak. A PVC water-bar (or swellable strip) is mandatory at every construction joint.
4. No haunch at the wall-base junction — a sharp 90° internal corner is a stress concentration that cracks. Provide a 150 × 150 mm splay haunch to spread the moment.
5. Ignoring uplift / flotation when empty — an empty buried tank in a high water table can float (buoyancy > self-weight). Check flotation with a factor of safety ≥ 1.2; thicken the raft, project the raft as an anti-flotation toe, or anchor down.
6. No waterproofing / internal plaster — even sound RCC is not fully watertight. Provide an internal 20 mm CM 1:3 plaster (with waterproofing compound) and an external membrane / coating before backfilling.
7. Inadequate free board — no air gap above water level leads to overflow seepage at the wall-slab junction; provide 150–300 mm free board and an overflow + scour pipe.

Related references

• IS 3370 (Part 1):2021 — Concrete structures for storage of liquids: general requirements
• IS 3370 (Part 2):2009 — Reinforced concrete structures (design + reinforcement)
• IS 3370 (Part 4):1967 — Design tables for liquid-retaining structures
• IS 456:2000 — General RCC code (durability, cover, detailing)
• IS 1893:2016 — Earthquake-resistant design (Part 2 covers liquid-retaining tanks)
• IS 1786:2008 — HYSD / TMT rebar specification
• Septic Tank Generator — companion buried-tank generator (IS 2470)
• InfraLens CAD Library — more parametric DXF generators
• InfraLens Handbook — engineering quick-reference notes