Slab Culvert / Minor Bridge BOQ — Worked Example for a 6 m Span × 7.5 m Deck
Slab culverts fill the gap between box culverts (up to 3 m span) and major bridges (12+ m span with T-beam superstructure). The IRC SP 13 + IRC 5 + IRC 21 standard configuration for 4-12 m spans: RCC abutments + RCC deck slab + brick masonry wing walls + RCC parapet + RCC approach slabs + stone-pitched apron. Standard for PMGSY high-discharge cross-drainage, state-PWD minor bridges over canals + small streams, urban service-road crossings. This article walks through a complete BOQ for the most common configuration — 6 m span × 7.5 m deck length.
Project Scenario
You're estimating a slab culvert under a PMGSY road crossing a perennial stream that's too wide for a box culvert. Span: 6 m clear between abutments. Deck length: 7.5 m (5.5 m carriageway + 1 m extension each side). Abutments 2.5 m high (NGL to deck soffit). Wing walls 3 m at 45° splay, RCC parapet on deck edges, stone-pitched apron downstream.
Design specification at a glance
- Clear span (between abutments): 6.0 m
- Deck length (carriageway crossing): 7.5 m
- Deck slab thickness: 600 mm RCC M20
- Abutment height (NGL to deck soffit): 2.5 m
- Abutment wall thickness: 500 mm
- Abutment foundation: 0.9 m deep, 1.7 m wide
- Wing walls: 4 nos × 3 m each, 345 mm brick
- Parapet: 150 mm × 1.1 m RCC on both deck edges
- Approach slabs: 2 nos × 7.5 × 3.5 × 0.30 m at both ends
- Apron + backfill: standard
The Complete BOQ — 14 Items in CPWD DSR 2023
| # | DSR Code | Item Description | Unit | Quantity |
|---|---|---|---|---|
| 1 | 2.8.1 | Earthwork in abutment foundation trench (both abutments) | m³ | 32.40 |
| 2 | 4.1.8 | PCC 1:4:8 below abutment foundation | m³ | 2.89 |
| 3 | 5.1.2 | RCC M20 in abutment foundation strips | m³ | 12.75 |
| 4 | 5.2.2 | RCC M20 in abutment walls (vertical) | m³ | 18.75 |
| 5 | 5.3 | RCC M20 in deck slab, 600 mm thick | m³ | 29.70 |
| 6 | 5.1.2 | RCC M20 in approach slabs (2 nos × 7.5 × 3.5 × 0.30) | m³ | 15.75 |
| 7 | 5.9.1 | Centering & shuttering for abutment foundation | m² | 25.50 |
| 8 | 5.9.2 | Centering & shuttering for abutment walls (both faces, 2 abutments) | m² | 75.00 |
| 9 | 5.9.3 | Centering & shuttering for deck slab (under-side, scaffolded) | m² | 49.50 |
| 10 | 5.22.6 | TMT Fe-500D reinforcement (cut, bent, placed) | kg | 9,180.00 |
| 11 | 6.4.2 | F.P.S. brick masonry — 4 wing walls (45° splay) | m³ | 7.25 |
| 12 | 5.2.2 | RCC M20 in parapets (both deck edges) | m³ | 2.48 |
| 13 | 7.1.1 | Stone pitching apron (both ends, 2 m × full width × 300 mm) | m³ | 9.00 |
| 14 | 2.25 | Backfill behind abutments + approach embankment | m³ | 56.25 |
Total RCC volume = 61.2 m³ split across abutment foundations + walls + deck + parapets + approach slabs. At 150 kg/m³ that's ~9.2 tonnes of steel. The live BOQ Builder evaluates these formulas in real time — change span (4-12 m), deck length, abutment height, deck thickness, wing wall length, parapet toggle.
How Each Quantity Was Computed
Geometry
Foundation depth = 0.9 m below NGL
Foundation size = abut_thk + 1.2 = 0.5 + 1.2 = 1.7 m wide (per abutment)
Total deck width = span + 2 × 0.3 m bearing seat = 6.0 + 0.6 = 6.6 m
Deck cross-section per metre of length: 6.6 × 0.60 = 3.96 m²
1-2. Excavation + PCC
Excavation (2 abutments × foundation pit each): 2 × (7.5+0.6) × (1.7+0.3) × (0.9+0.1)
= 2 × 8.1 × 2.0 × 1.0 = 32.40 m³
PCC bed = 2 × (7.5+0.3) × (1.7+0.15) × 0.10 = 2 × 7.8 × 1.85 × 0.10 = 2.89 m³
3-4. Abutment foundation + walls
Foundation (RCC strip, 2 abutments × deck length × foundation width × 0.5 m thick):
= 2 × 7.5 × 1.7 × 0.5 = 12.75 m³
Walls (2 abutments × deck length × wall thk × full height):
= 2 × 7.5 × 0.5 × 2.5 = 18.75 m³
5. Deck slab — Item 5.3
Deck volume = (span + 0.6) × deck_length × deck_thk
= 6.6 × 7.5 × 0.60 = 29.70 m³
Deck thickness 600 mm is standard for 6 m span per IRC 21. The Builder defaults match the IRC tabulated values: 4 m → 400 mm, 6 m → 500-600 mm, 8 m → 600 mm, 10 m → 700 mm, 12 m → 800 mm.
6. Approach slabs
Approach (2 ends × deck_length × 3.5 m × 0.30 m):
= 2 × 7.5 × 3.5 × 0.30 = 15.75 m³
Approach slabs prevent settlement transition between approach embankment and bridge deck — a "jump" forms over time without them. Standard 300 mm RCC × 3.5 m long for IRC Class A loading.
7-9. Centering & shuttering
- Abutment foundation (5.9.1): 2 × (deck_l × foundation_width) = 2 × (7.5 × 1.7) = 25.50 m²
- Abutment walls (5.9.2): 2 abutments × 2 faces × deck_l × abut_h = 2 × 2 × 7.5 × 2.5 = 75 m²
- Deck slab (5.9.3): under-side only (scaffolded from natural ground or temporary props) = 6.6 × 7.5 = 49.50 m²
10. Reinforcement
Total RCC = 12.75 + 18.75 + 29.70 = 61.2 m³
Steel = 61.2 × 150 = 9,180 kg (~9.2 tonnes)
150 kg/m³ is typical for slab culverts under IRC Class A loading. For IRC Class AA / 70R tracked, push to 180-200 kg/m³.
11. Wing walls
Wing wall avg height = abutment_h × 0.7 = 1.75 m
Brick masonry = 4 × 3.0 × 0.345 × 1.75 = 7.25 m³
12. Parapet
Parapet volume = 2 (sides) × deck_length × 0.15 m thk × 1.1 m height
= 2 × 7.5 × 0.15 × 1.1 = 2.48 m³
1.1 m height is the IRC safety standard. Parapets can be solid RCC (current spec), open-rail steel (cheaper but less crash protection), or precast crash-barrier sections.
13-14. Apron + backfill
Stone apron = 2 ends × 7.5 m × 2.0 m × 0.30 m = 9.0 m³
Backfill behind abutments = 2 × 7.5 × 1.5 × 2.5 = 56.25 m³
When Slab Culvert Beats Box Culvert vs Bridge
| Span range | Recommended structure | Why |
|---|---|---|
| ≤ 3 m | Box culvert | Monolithic raft + walls + slab is most economical at small spans |
| 3 - 4 m | Box culvert (still economical) OR slab culvert | Crossover zone — site-specific |
| 4 - 12 m | Slab culvert (current article ⤴) | Open-bottom design saves raft concrete; natural stream bed preserved |
| ≥ 12 m | T-beam or PSC girder minor bridge | Slab self-weight too high; need beam-supported deck |
Construction Sequence (IRC 21 Compliant)
- Earthwork in foundation trench (both banks)
- PCC bed below abutment foundation
- RCC abutment foundation (raft strip)
- RCC abutment walls (vertical or slightly battered)
- Curing of abutments — minimum 7 days
- Centering of deck slab (props from natural ground or temporary scaffold spanning the stream)
- RCC deck slab (span + 0.3 m bearing seat each side)
- Curing of deck — 14 days minimum per IRC 21, ideally 28 days
- Wing walls cast / built
- Approach slabs at both ends
- Parapets on deck edges
- Stone-pitched apron at base of abutments
- Backfill around abutments + approach road tapering
- Removal of centering only after 28-day cube test ≥ design strength
What This BOQ Excludes
- Bearing pads / elastomeric bearings at deck-abutment junction (₹3-5k each × 4 nos)
- Expansion joint sealant at deck-approach slab transition
- Reflective markers + parapet end-caps
- Signage (warning, restriction) — IRC 67
- Approach road tapering + transition to existing carriageway
- Dewatering during foundation work — typical extra 15-25 % in waterlogged sites
- Pile foundation if SBC < 100 kN/m² (separate scope — significantly increases cost)
- Curtain walls (cut-off walls upstream/downstream) — for scour-prone sites, add ~₹40-60k as 5.2.2 RCC of 300 × 450 mm cross-section
- Crash barriers in addition to parapet (NH/SH requirement)
Common Estimation Mistakes
- Using M20 grade for spans > 6 m — IRC 21 requires M25 minimum for spans 6-9 m, M30 above. Bump grade and re-rate using DSR 4.20.X design-mix items.
- Under-sized abutments — abutments take both vertical load (deck + traffic) AND horizontal load (earth pressure from backfill). 400 mm walls undersized for 2.5 m high abutments; default to 500 mm.
- Skipping approach slabs — without them, a 50-100 mm "jump" forms at the deck-approach interface within 6-12 months from backfill settlement. Dangerous for high-speed traffic.
- Open-rail parapet on rural road — IRC standard is solid RCC parapet for bridges with traffic speed > 40 kmph. Open-rail is acceptable only for low-speed urban service road.
- Wing walls in brick when RCC is needed — for spans > 8 m or NH/SH applications, RCC wing walls (5.2.2) are recommended over brick (6.4.2). More durable; resists scour.
- Forgetting cushion/curtain walls — at the discharge end, water exiting at high velocity scours the apron. Curtain walls (300 × 450 mm RCC, 600 mm deep) anchor the apron + headwall foundation.
- Treating deck slab as one-way slab — for spans 6+ m it's two-way bending; reinforcement must be designed bothways with main steel along short span.
What Changes for a Different Slab Culvert
- 4 m span (smaller, possibly box-vs-slab borderline) — deck 400 mm, abutment height down to 1.5-2 m. Total cost ~50 % less.
- 10 m span (large stream) — deck 700 mm, M25 design mix per IRC 21, steel ratio 180 kg/m³, deeper abutment foundations.
- 12 m span (max for slab culvert) — deck 800 mm, M30 grade, often paired with T-beam superstructure as alternative. Borderline economic case.
- Two-lane carriageway (10-12 m deck length) — proportional scale-up of all quantities except span-related items.
- High-discharge perennial stream — add deeper curtain walls + larger apron + scour protection
- NH / SH grade — M25 grade everywhere, RCC wing walls (5.2.2 instead of 6.4.2), crash-barrier in addition to parapet, full traffic management overlay
- Pile foundation for soft soil — adds 20-40 % to abutment cost
Get Your Slab Culvert Cost in 30 Seconds
Change span (4-12 m), deck length + thickness, abutment height + thickness, wing wall length, parapet toggle — the Builder regenerates the BOQ + downloads Excel with formulas embedded. Customize for Your Project →
References & Companion Reading
- IRC 5:2015 — Standard Specifications for Road and Bridge Works (General Features of Design)
- IRC 6:2017 — Standard Specifications for Loads & Stresses on Bridges
- IRC SP 13:2004 — Guidelines for the Design of Small Bridges and Culverts
- IRC 21:2000 — Standard Specifications for Cement Concrete Road Bridges
- IS 456:2000 — Plain and Reinforced Concrete
- IS 1786:2008 — TMT Fe-500D reinforcement
- IS 1200 Part 5:1982 — Concrete measurement
- Sister article: RCC Box Culvert BOQ
- Sister article: Hume Pipe Culvert BOQ
- CPWD DSR 2023 · MoRTH SOR