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Define underpinning; one method

Short answer

Underpinning strengthens or deepens an existing foundation to a firmer stratum or enlarges its base for safe load carrying. The pit method divides the wall into 1-1.5 m sections, excavates alternate pits below the old footing to firm soil, fills with concrete or masonry leaving a 75 mm gap packed with dry mortar, then repeats for remaining sections.

Define underpinning and explain any one method of it.

Underpinning is the process of strengthening or deepening the foundation of an existing building by extending it to a firmer or deeper stratum, or enlarging the base, so that it can carry the load safely.

Pit method (pier method)

  1. The wall is divided into sections of 1-1.5 m width.
  2. A pit is dug under the old foundation in alternate sections, and the wall is supported if needed.
  3. The pit is dug down to a firm stratum and filled with PCC or masonry in cement mortar (1:4) up to about 75 mm below the old footing.
  4. The gap is packed with dry cement-sand mortar (1:1) or slate pieces, rammed, and left 3-4 days to set.
  5. The same is repeated for the intermediate sections, then the pits are backfilled.
   wall
   |||||| old footing
   ||||||===| pit
          | new concrete
          | 75 mm packing
          |_ firm soil

✓ Answer checked by Claude Opus 5.5 (high effort)

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