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Underpinning: necessity, procedure, methods

Short answer

Underpinning strengthens or deepens an existing foundation. It is needed when the foundation has settled, loads increase, adjacent excavation occurs, or soil is weakened. The pit method (most common) excavates alternate 1-1.5 m sections, fills with concrete to required depth, and packs the 75 mm gap with dry mortar; other methods include piling, pretest, grouting, and needle beams.

Define underpinning. Describe its necessity and the methods (types and procedure) of underpinning.

Definition

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

  • Foundation has settled or is weak due to poor soil or an overload.
  • A building is to be raised (extra storeys) and loads increase.
  • Adjacent deep excavation or new basement lowers the support (e.g. neighbouring building).
  • A change in water table, scouring or earthquake damage reduces bearing capacity.
  • Faulty or shallow original foundation, or damage by roots, mining, or subsidence.

Methods of underpinning

  1. Pit method (pier method): The wall is divided into 1-1.5 m sections. Pits are dug below the old footing in alternate sections (the others stay in place), the soil is excavated to the required depth and filled with concrete or masonry, and a gap of about 75 mm is left under the old foundation, then packed with dry mortar or wedges. This is repeated with the next sections.
  2. Pile method: Piles (bored or driven) are installed on both sides of the wall and connected by cross beams (needles) below the wall. Useful when good soil is deep.
  3. Pretest method: Hydraulic jacks apply a test load to piles, then wedges are inserted to avoid later settlement.
  4. Chemical (grouting) method: Cement or chemical grout is injected into weak soil to increase bearing capacity.
  5. Cantilever/needle beam method: Needle beams carry the wall's load so the footing can be rebuilt below.
  6. Jet grouting / micropile method for tight spaces.

Procedure of the pit method (most common)

  1. Study the soil and the structure, and support the building by shoring if necessary.
  2. Dig the first pit below the footing, in sections about 1-1.5 m wide, leaving alternate sections untouched.
  3. Dig to the design depth, with timbering support if needed.
  4. Fill with PCC or masonry in cement mortar (1:4) up to about 75 mm below the old foundation.
  5. Pack the gap with dry cement-sand mortar (1:1) or wedges and leave to set for 3-4 days.
  6. Repeat for the intermediate sections, in a planned order.
  7. Backfill and compact.
   wall
   ||||||  old footing
   ||||||====|  pit (section 1)
          |  new masonry/concrete
          |  packing (75 mm gap)
          |__ firm stratum

✓ Answer checked by Claude Opus 5.5 (high effort)

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