--- title: "Ideal bridge characteristics; tunnel drainage" subject: "Transportation Engineering II" chapter: 5 key: "semester_7/transportation_engineering_ii:ideal-bridge-characteristics-tunnel-drainage" source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/ideal-bridge-characteristics-tunnel-drainage site: BCT Engineering --- # Ideal bridge characteristics; tunnel drainage Transportation Engineering II · Chapter 5: Introduction to Bridge and Tunnel Engineering - Asked in: 2081 Bhadra, 8 marks - Video: [Tunnel Drainage](https://www.youtube.com/watch?v=Ke2DoRPpKdQ) — Priyanka P Dange - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/ideal-bridge-characteristics-tunnel-drainage - Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-5 ## Question Explain the major characteristics of Ideal Bridge. How drainage are managed while tunneling? ## Short answer An ideal bridge is safe under IRC loads and NBC 105, serviceable with adequate width and waterway, economical, durable, easy to build, and aesthetically suited. Tunnel drainage uses gravity side drains when driving upgrade, sump pumping for downgrade, probe holes and grouting to seal water, relief holes, temporary ditches, and permanent weep holes with waterproof membrane behind the lining. ## Answer #### Major characteristics of an ideal bridge An ideal bridge is one that serves its purpose safely and economically throughout its life. Its major characteristics (IRC:5 and Nepal DoR bridge standards) are: 1. **Safety and structural adequacy** - designed for IRC loads (Class A, 70R) and for flood, earthquake (NBC 105), wind and temperature effects with the required factors of safety. 2. **Serviceability** - adequate carriageway width, vertical and horizontal clearance, a smooth riding surface, and limited deflection and vibration. 3. **Economy** - minimum total cost including initial cost, maintenance and approaches; use of local materials; standard designs where possible. 4. **Adequate waterway and freeboard** - passes the design flood (50 to 100 years return period) with little afflux and scour; foundations below the scour level. 5. **Durability and low maintenance** - good materials, drainage, bearings and expansion joints that need little upkeep. 6. **Ease of construction** - suits the available equipment, labour and construction period; minimum temporary works. 7. **Aesthetics** - a pleasing appearance that fits the surroundings. 8. **Environmental compatibility** - minimum disturbance to the river and the surrounding environment. 9. **Suitable site and good approaches**, as described under the ideal bridge site. #### Drainage during tunnelling Water in the tunnel heading (seepage, groundwater and drilling water) disturbs work and can destabilise the rock. It is managed as follows: 1. **Gravity drainage** - when the tunnel is driven upgrade from the portal (positive gradient of 0.2-0.5 percent), water flows back to the portal through a side or central drain, so no pumping is required. 2. **Pumping from sumps** - when driving downgrade (or in shafts), water collects in sumps at the face and is pumped in stages to the outlet through pipes. 3. **Probe holes and pre-grouting** - holes are drilled ahead of the face (probing) to detect water, and cement or chemical grout is injected to seal fissures before excavation. 4. **Drainage holes and dewatering wells** - relief holes drilled into the rock lower the water pressure; wellpoints or deep wells from the surface are used for shallow tunnels. 5. **Temporary drains** - ditches along the invert, covered or piped, carry the water away from the face. 6. **Permanent drainage after lining** - weep holes and drain pipes behind the lining, longitudinal side drains, catch pits and cross-drains, and a waterproof membrane to stop leakage; a drainage layer prevents water pressure on the lining. 7. **Portal and cut drainage** - catch drains above the portals and slope drainage stop surface water from entering the tunnel. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. An ideal bridge should pass the design flood with: A. High afflux and scour B. Little afflux and scour, foundations below scour level C. No freeboard D. Shallow footings Answer: B. Adequate waterway and foundation depth below scour level are required. (Key point: Waterway) 2. Gravity drainage in tunnelling works when the tunnel is: A. Driven downgrade B. Driven upgrade from the portal (gradient 0.2-0.5%) C. Vertical D. Under water Answer: B. Water flows back to the portal through the drain, so no pumping is needed. (Key point: Gravity drainage) 3. When driving downgrade, water is handled by: A. Sumps at the face and staged pumping B. Natural flow C. Closing portal D. Ventilation fans Answer: A. Water collects in sumps and is pumped out in stages. (Key point: Sumps) 4. Probe holes and pre-grouting are used to: A. Detect water ahead and seal fissures before excavation B. Ventilate the face C. Light the tunnel D. Place lining Answer: A. Probing detects water; grout seals water-bearing zones. 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