Bridge site factors and tunnel drainage

Factors for selecting the bridge site

A bridge site is chosen so that the bridge is safe and economical, and serves both the road alignment and the river with the least disturbance. Factors to consider (IRC:5 / IRC:SP:13 and Nepal DoR bridge practice):

1. Stream/river behaviour: a straight, stable reach with a well-defined, non-meandering channel; avoid bends, confluences, and braided and aggrading reaches.
2. Waterway and flood data: a narrow, stable crossing with high banks; adequate waterway for the design flood (usually a 100-year return period for major bridges as per IRC:5 and DoR, 50-year for small bridges and culverts as per IRC:SP:13); reliable HFL, discharge and velocity records.
3. Foundation conditions: firm strata or rock at a reasonable depth; the foundation must be taken below the scour depth (Lacey's regime theory, IRC:78) at an affordable cost; avoid soft, liquefiable or landslide-prone soils.
4. Approaches and alignment: the bridge should be square (normal) to the river flow; approach roads should be straight, of limited height and not on unstable slopes. For major bridges the road alignment is adjusted to suit the best bridge site, while for minor bridges and culverts the bridge follows the road alignment.
5. Navigation and clearance: vertical and horizontal clearance for boats, floating logs or boulders; vertical clearance between HFL and the soffit of 150 mm to 1.5 m depending on the discharge (IRC:5), with more in Nepal's boulder-carrying rivers.
6. Economy: total cost of bridge, approaches and river training works; nearness of construction materials and labour, and access for equipment.
7. Geology and seismicity: avoid active faults and steep gorges with rockfall; follow the seismic provisions of NBC 105 and IRC:6.
8. Existing structures and utilities: distance from other bridges, dams and weirs (backwater/draw-down effects), settlements, and land acquisition.
9. Environmental and social aspects: minimum afflux, no damage to property upstream, aesthetics, and environmental clearance requirements.
10. Construction convenience: space for temporary works and diversion, and a low-flow period for working.

Drainage in tunnel construction

Water entering a tunnel from rock joints, springs or the portals makes excavation unsafe, softens the ground, damages the lining and reduces working efficiency, so drainage is planned at all stages.

During construction

• Gravity drainage: if the tunnel is driven up-gradient (rising heading), water flows out by itself through a side ditch/gutter or pipe at the invert. The gradient is commonly 0.2-0.5%.
• Sumps and pumping: if it is driven down-gradient (falling heading), water collects at the face in a sump and is pumped out in stages through pipes with booster pumps.
• Pre-grouting/probe holes: probe drilling ahead of the face detects water in advance, and cement or chemical grout is injected to seal water-bearing zones.
• Dewatering of the ground: well points or deep wells and drainage holes in poor ground.
• Sealing: shotcrete, waterproof membrane and concrete lining to control leakage.

Permanent drainage (road tunnel)

• Side/central drains with covered manholes and cleaning chambers, and sufficient cross-fall (usually 1-2%) on the carriageway.
• Drainage behind the lining: perforated pipes and geotextile fleeces behind the lining carry seepage to the invert drain, so that hydrostatic pressure does not act on the lining.
• Waterproof membrane (PVC/HDPE) between the primary and final lining.
• Portal drainage: cut-off drains above the portal to divert surface water, and a catch drain.
• Fire/oil separators for drained liquids in road tunnels, in line with IRC:SP:91.