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- 2067 Mangsir (old course) · 8 marks
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Classification of cross drainage per NRS
Short answer
NRS classifies: culverts (span ≤6m) - pipe, box, slab, arch; causeways/drifts; minor bridges (span >6m); major bridges. Pipe: small flow, flat terrain, sufficient cover. Box: moderate-large flow, weak foundation, high fill. Slab: up to 6m span, firm foundation. Arch: hill roads, stone available, strong foundation. Selection based on discharge, foundation, materials, cost.
Classify cross drainage structures as per NRS. (Describe the suitability of construction of pipe, box, slab and arch culverts.)
Classification
Cross-drainage structures carry streams and road-side drains across the road. As per NRS 2070 (Nepal Road Standard) and IRC practice, they are classified by span and discharge as:
- Culverts (clear span up to about 6 m):
- pipe culvert (single or multiple barrels),
- box culvert (RCC, single or multi-cell),
- slab culvert (RCC slab on masonry or concrete abutments),
- arch culvert (masonry or concrete arch).
- Causeways / drifts (submersible crossings): vented (with pipes) or unvented (fords, Irish bridges).
- Minor bridges (span above about 6 m).
- Major bridges, and special structures such as aqueducts and inverted siphons where canals meet the road.
Suitability
| Culvert | Suitable where | Notes |
|---|---|---|
| Pipe | Small, steady discharge; flat terrain; sufficient cover of 0.6 to 1.0 m or more; need for a quick, cheap crossing | Min. diameter about 600 mm (900 mm preferred). Multiple pipes for larger flows. Can choke with debris. |
| Box | Moderate to large discharge; soft or poor foundation; high embankment or low headroom; uncertain flow | RCC; rigid frame spreads the load over the base; also good in seismic areas; formwork is costly. |
| Slab | Small to medium span (up to about 6 m); firm foundation or rock; stable banks; little height available | Open waterway, easy to clean; needs abutments and wing walls. |
| Arch | Hill roads where stone is plentiful; high fill; strong abutments and good foundation; aesthetic needs | Masonry in compression, long life; slow to build; skilled labour needed. |
General rules: use a pipe where the flow is small, a slab where a clear opening is needed, a box where the soil is weak, and an arch where stone is available and the foundation is rock or hard soil. The culvert should follow the natural stream line, with minimum skew, and have a gradient that prevents silting.
✓ Answer checked by Claude Opus 5.5 (high effort)
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