Geology in road alignment selection

Geology controls the cost, stability and maintenance of a road, especially in the hills. The alignment is chosen after a geological study.

Engineering geological parameters

1. Rock and soil type: strong, massive rocks (quartzite, granite, gneiss) give stable cut slopes; phyllite, schist, shale, Siwalik mudstone and loose colluvium are weak and landslide-prone.
2. Geological structure: relation of bedding, foliation and joints to the road cut.
   • Dip into the hill (against the slope) is favourable.
   • Dip out of the slope (daylighting) gives planar and wedge failures; avoid it.
   • Avoid alignments parallel to faults and shear zones.
3. Slope and topography: gentle, stable slopes; avoid steep slopes, gullies and scarps; prefer ridges and stable benches.
4. Landslides and unstable ground: avoid old landslides, debris slopes and eroding areas.
5. Weathering and overburden thickness: deep soil and highly weathered rock mean large cut volumes and frequent failures.
6. Groundwater and springs: avoid wet areas, seepage zones and swampy ground.
7. Streams and floods: minimise crossings; stay above flood level; choose stable stream banks.
8. Seismic hazard: avoid active faults.
9. Construction materials: quarry, aggregate and borrow areas along the route.
10. Foundation of structures: bridge and culvert sites.

Investigation

Reconnaissance with air photos, geological mapping, test pits, trenches and geophysics at critical sections; slope stability assessment by kinematic (stereonet) analysis. Select the alignment that has the least geological risk, and then design cuts, drainage and protection works.