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Site investigation for dam and reservoir

Short answer

Direct methods include geological mapping, core drilling, test pits, adits and in-situ Lugeon tests for permeability. Indirect methods are seismic refraction for bedrock depth, electrical resistivity for water and weak zones, and aerial photo interpretation. Reservoir investigations check leakage paths, slope stability and siltation.

Describe the geological investigation methods (direct and indirect) for the site of a large dam and its reservoir.

A dam is a huge structure that must stand on rock able to carry the load, resist sliding and stay watertight, and its reservoir must hold water without leakage or slope failure. Investigation is therefore done at the dam site and the reservoir area.

What is to be found out

  • Rock type, strength and weathering of foundation and abutments.
  • Structures: dip and strike of beds, joints, faults, shear zones, folds.
  • Permeability and groundwater conditions.
  • Depth to sound bedrock and thickness of overburden.
  • Slope stability of reservoir rim; sedimentation; seismicity.
  • Availability of construction materials.

A. Direct methods

  1. Geological mapping of the dam site (scale 1:500 to 1:2000) and reservoir (1:5000 to 1:25000).
  2. Test pits, trenches across suspected faults and in overburden.
  3. Core drilling at the axis, abutments, spillway and stilling basin; core recovery, RQD and logging; permanent record of the rock.
  4. Drifts/adits in abutments to see rock at depth and carry out in-situ tests.
  5. In-situ tests: Lugeon (packer) water pressure tests for permeability, plate load test, shear tests and borehole dilatometer tests.
  6. Laboratory tests on cores: compressive strength, density, porosity, durability.

B. Indirect methods

  1. Seismic refraction: depth to bedrock, rock quality and the fractured zones.
  2. Electrical resistivity: depth of water table, weak and saturated zones, buried channels.
  3. Gravity and magnetic surveys: large buried features.
  4. Aerial photographs and satellite images: lineaments, landslides and reservoir rim features.
  5. Geophysical logging of boreholes.

Reservoir-specific checks

  • Leakage paths through permeable beds, solution cavities in limestone, open joints and faults.
  • Stability of reservoir slopes (old landslides which could be reactivated).
  • Watertightness of the reservoir rim and divide.
  • Silting and seismic activity, including reservoir-induced seismicity.

The data are combined into a geological report with plans, sections and treatment recommendations such as grouting and cut-off.

✓ Answer checked by Claude Opus 5.5 (high effort)

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