About RCC box culverts
A box culvert is an RCC tunnel constructed below a roadway, railway, or embankment to carry water (storm drainage, irrigation canal, stream) or to provide a small underpass for pedestrians or livestock. The box section consists of a top slab, bottom slab, and two side walls — all rigidly connected at corners. For wider spans, intermediate walls divide the opening into 2 or 3 cells.
IRC SP 13:2004 (Guidelines for the Design of Small Bridges + Culverts) is the primary reference for small bridges + culverts up to 30 m span in India. IS 456:2000 governs the RCC structural design; IRC 6:2017 specifies live loads (Class A or 70R vehicle loadings); IRC 78:2014 covers sub-structure + foundations. For typical highway projects, the MoRTH (Ministry of Road Transport + Highways) standard drawings are also referenced.
Compared to slab + pipe culverts, the box culvert offers: (a) higher discharge capacity for similar opening area (rectangle is more efficient than circle), (b) greater earth-fill tolerance (the rigid frame distributes load well), (c) better structural performance under seismic loads, (d) easier construction with cast-in-situ or precast options.
Design loadings
- Dead load: self-weight of slab + walls + earth fill above top slab + any superimposed dead load (roadway pavement).
- Live load: IRC Class A (3-axle vehicle, 27t total) or 70R wheeled (70t) — per IRC 6:2017. Calculated with impact factor (5-25% depending on cover depth).
- Earth pressure on side walls: Coulomb or Rankine active earth pressure plus surcharge from live load on adjacent ground.
- Water pressure (if applicable): Hydrostatic load when full of water — design for maximum operating + flood conditions.
- Soil reaction below bottom slab: Uniform pressure (rigid base assumption) or modulus of subgrade reaction.
Common box culvert issues
- Hydraulic capacity inadequate — undersized opening for design discharge; flooding upstream; downstream scour. Calculate based on 50-100 year return period rainfall + Mukherjee or Dicken formula.
- No haunch at internal corners — high stress concentration at right-angle corners; cracking. Haunches of 150-300 mm size + reinforcement detail per IRC SP 13.
- Wing walls + return walls inadequate — required at inlet + outlet to prevent erosion + scour of approach earth. Designed for active earth pressure with FOS of 1.5-2.0.
- No apron + cutoff wall — at inlet + outlet, protects against scour. Apron length typically 2-3 × culvert depth; cutoff wall depth based on scour calculation per IRC SP 13.
- Joint detailing in long culverts — culverts longer than 25-30 m need expansion joints to handle thermal + shrinkage; missed in many designs.
- Inadequate foundation — soft soil under bottom slab; differential settlement; cracks. Plate Load Test (PLT) or SBC verification mandatory.
- Mid-wall load not properly analysed — for multi-cell culverts, the mid-wall is a critical element carrying double the load + needs careful design.
- Live load impact factor wrong — for shallow earth cover (< 600 mm), live load impact is 25%; misapplied for deeper cover by some designers.
Related references
- IRC SP 13:2004 — Guidelines for Design of Small Bridges + Culverts (this drawing's primary parent)
- IS 456:2000 — Plain + Reinforced Concrete code (Cl. 34.5 for haunch + corner detailing)
- IRC 6:2017 — Standard Specifications + Code of Practice for Road Bridges — Loads + Stresses
- IRC 78:2014 — Standard Specifications + Code of Practice for Road Bridges — Foundations + Sub-structure
- IRC 22:2015 — Composite Construction (relevant for combined RCC + steel)
- MoRTH Standard Drawings (Vol 1) — Standard box culvert types BX-1 to BX-6
- IS 13920:2016 — Seismic detailing for ductile behaviour
- InfraLens CAD Library — more parametric DXF generators
- Isolated Footing Generator — for box culvert foundations on suitable soils