Tunnel alignment survey and ideal bridge location

Steps in tunnel alignment survey

A tunnel alignment survey fixes the centre line and levels of the tunnel on the ground and then transfers them underground so that headings driven from both ends meet correctly.

1. Reconnaissance - study of topographic maps, aerial photographs and geological information; walk-over survey to select the possible routes and portal locations.
2. Preliminary (route) survey - topographic map of the area at suitable scale; geological and geotechnical investigation (boreholes, geophysical surveys, rock mass rating) along the alternatives; select the best alignment (shortest, stable rock, safe portals, minimum cover problems).
3. Surface (final location) survey - set up a network of triangulation stations or a precise traverse using total station/GPS joining the two portals; mark the centre line by pillars on each side of the hill; measure the baseline precisely; fix reference marks outside the portals.
4. Levelling - precise levelling from benchmarks to the portals and shafts to find the invert levels and gradient (minimum gradient about 1 in 250-500 for drainage); check by double levelling.
5. Transfer of alignment underground - from the portal by theodolite sighting on the two reference pillars; through shafts by two plumb wires or optical/laser plummets, or by gyro-theodolite; then extend the centre line along the heading with laser guidance.
6. Check and control - repeat the survey from both ends, correct closing errors (permissible error in meeting of headings is a few centimetres), and keep checking lines and levels while the tunnel advances.

Features of an ideal bridge location

An ideal bridge site has the following features:

1. Straight, narrow, stable river reach - well-defined banks, parallel flow, stable regime (no meandering, braiding, heavy aggradation or degradation), and no bends, confluence or islands close to the site.
2. Firm foundation - rock or hard strata at a reasonable depth, so that foundations are economical and safe against scour.
3. High, stable banks - above the highest flood level (HFL) so that long high approaches are avoided; no landslides on the banks.
4. Right-angled crossing - the bridge axis normal to the direction of flow, avoiding skew.
5. Good approaches - straight alignment, easy gradients and minimum cut and fill; approach roads should not be flooded.
6. Minimum waterway length and obstruction to flow - small afflux and low scour, with adequate linear waterway.
7. Construction facilities - availability of materials, labour, space for stacking, access to the site, and nearness to the road network and population.
8. Economy - least total cost of bridge, approaches and training works.
9. Free from other limitations - no hindrance from existing utilities or from land acquisition, and in Nepal, away from active landslides, gullies and debris flow paths.
10. Suitable for river training - banks that can be protected economically.