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Direct methods of sub-surface investigation

Short answer

Direct sub-surface methods expose or sample ground: test pits, trenches, shafts, adits, and boreholes (auger, percussion, rotary core drilling). They yield soil/rock profiles, RQD, undisturbed samples, and permit in-situ tests like SPT and Lugeon, but are point-wise and costly.

Describe the direct methods of sub-surface site investigation.

Direct sub-surface methods expose or sample the ground directly so that rock and soil can be seen, tested and logged.

  1. Test pits (trial pits): open excavations, 1-3 m wide and up to 3-5 m deep; give a clear view of the soil profile, bed rock, water table; allow undisturbed block samples and in-situ tests. Used for shallow foundation and borrow areas.
  2. Trenches: long narrow excavations across slopes, faults or landslides to expose structure and thickness of overburden.
  3. Shafts: vertical excavations (up to 100 m) to examine rock at depth, used at dam sites and tunnels.
  4. Adits, drifts and exploratory tunnels: horizontal tunnels into abutments of dams; allow geological mapping and in-situ tests such as shear, plate load and flat jack test on rock.
  5. Boring and drilling:
    • Auger boring in soft soil;
    • Wash boring and percussion boring in soil;
    • Rotary core drilling in rock, giving cores for logging, RQD and lab tests; diamond bits used in hard rock. Samples taken: disturbed (SPT split spoon) and undisturbed (thin-walled Shelby tube).
  6. In-situ tests in boreholes: standard penetration test, packer (Lugeon) permeability test, pressuremeter, borehole camera.
  7. Water-level observation in holes.

Limits: these methods are costly and time-consuming and give information only at points, so they are supplemented with geophysics.

✓ Answer checked by Claude Opus 5.5 (high effort)

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