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Foundation on hard rock

Short answer

For hard rock, recommend a shallow foundation, either a strip or spread footing placed directly on cleaned, levelled rock. The rock has a safe bearing capacity of about 3240 kN/m², negligible settlement, and this option is economical because no deep foundation is required.

What type of foundation will you recommend for the foundation on hard rock? Justify.

For a foundation on hard rock, a shallow foundation is recommended: a simple strip or spread footing (or a footing directly on the cleaned, levelled rock), with the rock benched or stepped if the surface is sloping.

Justification

  1. High bearing capacity: Hard sound rock has a safe bearing capacity of about 3240 kN/m² (IS 1904), far more than the pressure from ordinary building loads, so no deep foundation is needed.
  2. Negligible settlement: Rock is nearly incompressible, so differential settlement is not a problem.
  3. Economy: Deep excavation, piles or rafts are not needed, so the cost and time are lower.
  4. Excavation: Only the top weathered layer is removed. If the rock is sloping, it is benched into steps, or anchor bars (dowels) are grouted into the rock to prevent sliding.
  5. Bonding: The cleaned, roughened rock surface is bonded with a PCC bed or the footing concrete for firm contact; any cracks or fissures are grouted.
   column
    | 
  [footing] 
 ///rock/// (benched, cleaned)

If the rock is weathered or fractured, the excavation is deepened until sound rock is reached, or the cracks are grouted.

✓ Answer checked by Claude Opus 5.5 (high effort)

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