Skip to main content

Indirect sub-surface methods: tunnel and dam

Short answer

For tunnels, indirect subsurface methods include seismic refraction to assess rock quality and faults, electrical resistivity to locate water-bearing fractures and cavities, and borehole geophysical logging. For dams, seismic refraction determines bedrock depth, resistivity identifies seepage paths, gravity/magnetic surveys detect buried faults, complemented by packer permeability tests.

Describe indirect methods of sub-surface site investigation for tunnel alignment and dam site.

Indirect sub-surface methods estimate underground conditions from measured physical properties, mainly by geophysics, often in boreholes.

Tunnel alignment

  • Seismic refraction survey along the alignment: gives rock velocity and so rock quality, depth of weathered layer and bed rock, and fault or shear zone (low velocity zones).
  • Electrical resistivity profiling/tomography: detects water-bearing fractures, clay-filled shear zones, karst cavities (low resistivity for water and clay, high for dry cavity).
  • Ground penetrating radar for shallow cover at portals.
  • Borehole geophysical logging (gamma, resistivity, sonic, caliper) and cross-hole/down-hole seismic for stiffness and fractured zones.
  • Seismic reflection/tomography to predict ahead of the face (TSP) during construction.

Dam site

  • Seismic refraction for depth to sound bed rock, thickness of overburden and rippability on abutments and river bed.
  • Resistivity for sand-gravel thickness, groundwater and seepage paths; cavities in limestone.
  • Gravity/magnetic methods for buried faults and dykes.
  • Borehole logging and packer tests for permeability, with cross-hole seismic for the rock mass between holes.
  • Spontaneous potential and tracer methods for seepage.

These are checked with a few cored boreholes.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Engineering Geology II chapter 4 questions →