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Scaffolding, underpinning, shoring; underpinning methods

Short answer

Scaffolding is a temporary framework for workers and materials at height. Underpinning strengthens or deepens existing foundations to firmer strata. Shoring provides temporary support to unsafe structures to prevent collapse. Underpinning methods include pit method (excavating alternate sections), pile method (piles with cross beams), pretest method (hydraulic jack testing), chemical grouting, cantilever needle beam, and jet grouting.

Define the terms scaffolding, underpinning and shoring. Explain the methods to carry out underpinning work.

Definitions

  • Scaffolding: a temporary framework of timber, bamboo or steel erected beside a building so that workers and materials can be placed at height while the work is done.
  • Underpinning: 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.
  • Shoring: the temporary support given to an unsafe wall or structure by inclined, horizontal or vertical members to prevent it from collapsing.

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

  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.

✓ Answer checked by Claude Opus 5.5 (high effort)

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