Geology in bridge site selection

A bridge site must have a safe foundation for piers and abutments, stable banks and a stable river channel.

Geological considerations

1. Foundation rock: sound, massive, strong bed rock at shallow depth is the best; avoid deep soft alluvium or weak rock.
2. River behaviour: a straight, stable reach with narrow channel; avoid meanders, braided reaches and confluences; assess scour depth and flood level.
3. Geological structure: avoid faults, shear zones and highly jointed rock; bedding and joints should not dip out of the abutment slopes toward the river.
4. Abutment slopes: stable, no landslides, rock fall or debris; low erosion.
5. Weathering and overburden thickness.
6. Groundwater: water table and seepage affect excavation of foundations.
7. Seismicity: active faults and liquefiable sand near the site.
8. Construction material for concrete and approach roads nearby.
9. Approach alignment should be stable.

Investigation of the bridge foundation

• Geological mapping of both banks and river bed.
• Boreholes at each pier and abutment location, with core drilling in rock to at least 3 m into sound rock.
• Test pits; SPT and sampling in soil.
• Geophysical profiles (seismic refraction/resistivity) across the river.
• Laboratory tests for bearing capacity, scour and shear strength.
• Hydrological data for flood and scour assessment.