Which method involves driving piles along both sides of an existing wall and inserting needles, acting as pile caps, through the existing wall?
Try it first: pick an option, then read the explanation.
Asked in SSC JE 09/10/2023 (Morning)
Detailed Explanation
Underpinning means strengthening or deepening the foundation of an existing building — for example when it has settled, when loads are being increased, or when a deeper excavation is being made next to it.
Why “Pile method” Is Correct
In the pile method, piles are driven along both sides of the wall. Holes are cut through the wall above its foundation and steel or concrete needle beams are passed through, resting on the piles on each side like pile caps. The wall load is transferred through the needles to the piles. It suits clayey and waterlogged soils and heavy walls.
| Underpinning method | Idea |
|---|---|
| Pit method | Wall is supported on needles while pits are dug under it in short sections and filled with new foundation |
| Pile method | Piles on both sides of the wall; needle beams through the wall act as pile caps |
| Miscellaneous | Grouting, chemical consolidation, jacked piles, freezing, etc. |
Why the Other Options Are Wrong
- A. Pit method: In the pit method, pits are dug beneath the wall in sections; piles are not driven on both sides.
- C. Vibro-flotation: Vibro-flotation densifies loose sand with a vibrating probe; it is ground improvement, not underpinning.
- D. Miscellaneous method: Not a specific method.
Key Point
Underpinning by piles: piles both sides + needle beams through the wall.
Key Concepts for Students
- Shoring supports walls temporarily during underpinning.
- Underpinning is done in short alternate sections to keep the wall stable.
- Jacked (mega) piles are another way to underpin with minimum vibration.
Practice more: this is question 15 in Shallow and Deep Foundation MCQs – Part 1. Try the whole set and check every answer instantly.