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Sub-soil exploration, improving bearing capacity

Short answer

Sub-soil exploration investigates ground layers, strength, water table and bearing capacity by borings, sampling and tests. Bearing capacity can be improved by a sand cushion: excavate weak soil to 1-1.5 m depth and beyond footing width, lay clean sand in 200-300 mm layers, sprinkle water and compact thoroughly.

What is sub soil exploration? Explain a method of improving bearing capacity of soil.

Sub-soil exploration

Sub-soil exploration (soil exploration) is the investigation of the ground below a site to find the soil layers, their thickness, strength, water table and bearing capacity, so as to choose and design the foundation. It includes reconnaissance, boring or pits, sampling, in-situ tests (SPT, plate load, cone penetration), and lab tests. The result is a soil report with borehole log, soil profile and recommended SBC.

Method of improving bearing capacity: Sand cushion (replacement)

This is the method commonly used for light buildings.

  1. The weak soil under the footing is excavated to a depth of about 1 to 1.5 m (or until firm soil), and over a width larger than the footing (extending beyond it by the thickness of the cushion).
  2. Clean, well graded sand, gravel or murrum is laid in layers of 200 to 300 mm.
  3. Each layer is sprinkled with water and compacted thoroughly by rammers or rollers up to the required density.
  4. The foundation is then constructed on the cushion.

The sand layer spreads the load over a larger area of the weak soil below, reducing pressure, and also drains the water.

        [ footing ]
     /               \
    /  compacted sand  \
   /     cushion        \
   weak soil below

Other methods: compaction, drainage, sand piles, stone columns, grouting, stabilisation, and increasing the footing depth.

✓ Answer checked by Claude Opus 5.5 (high effort)

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