Methods of surface and subsurface site investigation

Site investigation is the study of the geological and ground conditions at a proposed site so that a safe and economical structure can be built. It is done in stages (phases), and the methods are of two groups: surface and subsurface.

Phases of investigation

1. Desk study / reconnaissance: collect existing maps, aerial photographs, satellite images, reports and records; walk over the area to see main features.
2. Preliminary investigation: geological mapping, a few boreholes and pits, and geophysical profiles to compare alternative sites or alignments.
3. Detailed investigation: closely spaced drilling, trenches, in-situ tests, laboratory tests and geophysics on the selected site to supply design parameters.
4. Construction stage: inspection of excavations and foundations, and checking of assumptions.

A. Surface methods

• Geological mapping: rock types, bedding, folds, faults, joints, springs and slope stability recorded on a topographic base, giving an engineering geological map.
• Aerial photographs and remote sensing: show lineaments, landslides, drainage and rock boundaries.
• Geophysical methods (done from the surface):
  • Seismic refraction gives depth of bedrock and rock quality.
  • Electrical resistivity gives depth of water table, bedrock and soil layers.
  • Others are gravity, magnetic and ground-penetrating radar.

B. Subsurface methods

• Test pits and trenches: shallow, direct view of soil and weathered rock; give undisturbed samples; trenches across faults and shear zones.
• Drifts and adits: tunnels driven into a hillside to inspect rock at depth (dam abutments, tunnels).
• Boring and drilling:
  • Auger boring for soft soils.
  • Wash boring and percussion boring for soils.
  • Rotary core drilling for rocks, giving continuous cores.
  • Borehole logs record strata, core recovery and RQD.
• Sampling: disturbed and undisturbed samples for laboratory tests.
• In-situ tests: standard penetration test, plate load test, permeability (Lugeon) test, and shear tests.
• Borehole geophysics and logging for fractures and water.

 Desk study -> Reconnaissance -> Preliminary
        -> Detailed -> Construction stage

The results are combined into a report with maps, sections and recommendations.