Underpinning is best described as

strengthening or deepening an existing foundation

covering walls with finishing material

Which is a cause needing underpinning?

Damage by flood or earthquake or a change in the water table

Which list gives methods of underpinning?

Pit, pile, pretest and chemical grouting

In the pit method, why is a gap of about 75 mm left under the old footing before packing?

To allow dry mortar packing to be rammed in for tight contact

2066 Jestha (old course) · 2.5 marks

Short note: Underpinning

Short answer

Underpinning strengthens an existing foundation by extending it to firmer soil or enlarging the base to safely carry load. It is needed for settled foundations, extra loads, nearby excavation, water table changes or damage. Common methods include the pit method (concrete in alternate sections), piles, pretest and chemical grouting.

Write a short note on underpinning.

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.

Necessity: weak or settled foundation, extra load on the building, nearby deep excavation, damage by flood or earthquake, or change in the water table.

Methods: pit method, pile method, pretest method and chemical grouting.

Pit method: pits are dug below the footing in alternate sections about 1-1.5 m wide, filled with concrete to about 75 mm below the old footing, the gap is packed with dry mortar, then the remaining sections are done.

   wall
   |||||| old footing
   ||||||==| new pier
          | 75 mm packing

✓ Answer checked by Claude Opus 5.5 (high effort)

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