RCC Storm Drain BOQ — Worked Example for a 50 m Covered U-Section Drain

RCC storm drains are the backbone of urban and rural drainage. PMGSY road-network rules mandate side-drains alongside every rural road; AMRUT 2.0's stormwater component co-funds urban drain networks; every panchayat does sullage and storm drains. The CPWD-standard configuration is a U-section RCC drain — M20 reinforced concrete raft and walls, optionally covered with an RCC top slab for footpath or vehicle access. This article walks through a complete BOQ for a 50 m × 600 × 600 mm covered drain along an urban society road.

Project Scenario

You're estimating a storm drain along a 50 m frontage on an urban society's internal road. The drain runs alongside the footpath, so it needs a top slab (pedestrians walk over it). Spec: 600 × 600 mm internal section, 150 mm walls and raft, 150 mm cover slab, 200 mm of cover below ground level.

Design specification at a glance

• Length: 50 m
• Internal width: 600 mm
• Internal depth: 600 mm
• Wall thickness: 150 mm (M20 RCC)
• Raft thickness: 150 mm (M20 RCC)
• Cover slab: 150 mm (M20 RCC) — covered drain
• Depth below NGL: 200 mm (drain top is 200 mm below ground level)
• Steel ratio: 85 kg per m³ RCC (light-medium duty)

The Complete BOQ — 10 Items in CPWD DSR 2023

The live BOQ Builder evaluates these formulas in real time — change internal section, depth below NGL, wall thickness, or toggle off the cover slab for an open drain, and the quantities update instantly.

How Each Quantity Was Computed

1. Earthwork — Item 2.8.1

Trench needs 150 mm working space on each side for shuttering access, plus PCC depth:

External drain width = 0.6 + 2 × 0.15 = 0.9 m
Trench width = 0.9 + 0.3 = 1.20 m
Total trench depth = depth below NGL + wall height + cover slab + PCC
= 0.20 + 0.75 + 0.15 + 0.10 = 1.20 m
Excavation volume = 50 × 1.20 × 1.20 = 72 m³

2. PCC bed — Item 4.1.8

PCC width = external drain width + 150 mm overhang = 0.9 + 0.15 = 1.05 m
PCC volume = 50 × 1.05 × 0.10 = 5.25 m³

3. RCC raft slab — Item 5.1.2

Raft volume = length × external width × raft thk
= 50 × 0.9 × 0.15 = 6.75 m³

4. RCC walls — Item 5.2.2

Two walls, each at full wall-height (internal depth + raft thickness):

Wall height = 0.6 + 0.15 = 0.75 m
Wall volume = 2 × 50 × 0.15 × 0.75 = 11.25 m³

5. RCC cover slab — Item 5.3

Slab volume = 50 × 0.9 × 0.15 = 6.75 m³

6 – 8. Centering & shuttering

• Raft (5.9.1): under-side area only = 50 × 0.9 = 45 m²
• Walls (5.9.2): 4 faces total (2 walls × 2 faces each) = 4 × 50 × 0.75 = 150 m²
• Slab (5.9.3): under-side area only = 50 × 0.9 = 45 m²

9. Reinforcement — Item 5.22.6

85 kg per m³ of RCC — typical for light-medium duty storm drains (≈ 1.1 % by volume per IS 456). For drains under vehicle-access cover slabs, revise upward to 100-120 kg/m³.

Total RCC volume = 6.75 + 11.25 + 6.75 = 24.75 m³
Steel = 24.75 × 85 = 2,103.75 kg (~2.1 tonnes)

10. Backfill — Item 2.25

The trench above the drain (200 mm below NGL) is back-filled with excavated earth:

Backfill = trench volume above drain − drain external volume above drain
= 50 × 1.20 × 0.20 − 50 × 0.9 × 0.20
= 12 − 9 = 3 m³

Open vs Covered RCC Drain — Which to Use

Open drain (no top slab) is the default for rural side-drains, garden / society storm drains, and anywhere the drain needs to be visually accessible for desilting. Costs ~30 % less than covered (no slab volume, no slab shuttering, lower steel).

Covered drain (with RCC top slab + grating openings) is used along urban footpaths, vehicle accesses, parking lots, and anywhere pedestrian traffic crosses the drain line. The top slab is 150-200 mm thick RCC with M20 grade for footpath; bump to M25 + 250 mm for regular vehicle access. Always provide manhole / inspection openings at 20-30 m intervals on covered drains for desilting — these are quoted separately (≈ ₹4,500 each for a 600 × 600 mm CI cover with frame).

For carriageway crossings, do NOT use a covered drain — use a box culvert instead. The slab in a covered drain is not designed for IRC Class A/AA wheel loads.

Cost Benchmark by Section (Open Drains)

Per-metre rate scales non-linearly because larger internal sections need disproportionately more wall and raft RCC. Adding a cover slab adds ~30 %. State-PWD rates run 8-15 % below DSR; municipal corporation SORs are typically cheaper still.

What This BOQ Excludes

• Gully grating + frame — cast iron 24 × 24 inch ~ ₹3,500 each, one per 10-15 m
• Inlet / inspection chambers at intervals (typically 30 m c/c)
• Manhole covers for desilting access on covered drains
• Expansion joints every 30 m linear (asphalt-filled or PVC water stop)
• Benching / step-irons inside the drain for desilting personnel access
• Connection to existing storm-water main — open trench plus T-junction chamber

Common Estimation Mistakes

• Using 85 kg/m³ steel under a vehicular cover slab — that slab needs 110-130 kg/m³ to take 5-tonne wheel loads. Under-steeling causes cracking within 6 months of opening.
• Forgetting backfill — when the drain top is below ground, the trench above needs back-filling. Most quick-estimates skip this 3-10 m³ item.
• Treating raft + walls + slab as one RCC item — they have different DSR rates (5.1.2 raft < 5.2.2 walls < 5.3 slab in cost per m³). Lumping them under one rate under- or over-estimates the BOQ by 8-12 %.
• Centering for raft = under-side only, not walls + under-side. The raft sits directly on PCC bed; no side shuttering needed. Some estimators double-count by including 4 raft sides.
• Skipping the 100 mm PCC bed on the assumption that "the GSB is enough" — PCC provides a smooth, clean casting surface for the raft. Without it, raft underside is uneven and steel cover is unreliable.

What Changes for a Different Drain

• Open drain (no cover slab) — set "covered" parameter to false. Drops slab volume (6.75 m³), slab shuttering (45 m²), and ~15 % of steel.
• Larger section (e.g., 1200 × 1200 mm) — quadruples internal area, ~3× the RCC volume. Use 200 mm walls and 200 mm raft.
• Vehicular cover slab (parking lot, driveway crossing) — bump slab thickness to 200-250 mm, M25 grade, steel to 110-130 kg/m³, and ideally use a box-culvert-style design instead.
• Below-water-table location — add waterproof admixture to the concrete + 50 mm extra cover to steel + dewatering during construction.
• State-PWD SOR — switch from CPWD DSR to your state SOR from the same dimensions in the Builder.

Get Your Drain's Cost in 30 Seconds

References & Companion Reading

• IS 456:2000 — Plain and Reinforced Concrete (concrete grades, cover, mix proportions, water-cement ratios)
• IS 1200 Part 2:1974 — Concrete measurement
• IS 1200 Part 5:1982 — Earthwork measurement
• IS 4111 Part 1:1986 — Code of Practice for Ancillary Structures in Sewerage System (drains, manholes)
• IS 1786:2008 — TMT Fe-500D reinforcement
• CPHEEO Manual on Storm Water Drainage — sizing methodology (rational method, Manning hydraulics)
• Sister tool: Box Culvert BOQ Builder — for carriageway crossings
• CPWD DSR 2023 — full SOR with rates by item