Underpinning existing foundations

One-line orientation

Underpinning strengthens or deepens an existing foundation — needed for added loads or adjacent deep excavation — and the exam’s favorite trap is confusing it with shoring (which retains a new excavation face, not an existing footing).

Key points

• What it is: strengthening or extending an existing foundation, typically deeper or wider, so it can carry more load or stay supported when adjacent ground is disturbed.
• Why underpin:
  • Renovation / added load — the structure becomes heavier (e.g. adding stories).
  • Insufficient existing foundation that needs strengthening.
  • Adjacent sitework — nearby construction or excavation has weakened the foundation.
• Code trigger: when an excavation reduces support of an adjacent foundation, IBC Chapter 18 requires protecting that foundation with underpinning, bracing, excavation retention, or another approved engineered measure.
• Ways to improve the foundation: extend deeper to stable strata · widen the footing to spread load · improve the soil (fill/grout) · new piles or mini-piles under/through the existing footing.
• Temporary support during the work: needle beams (horizontal beams through the foundation carrying load) and trench support (intermittent shoring of the underpinning excavation).

Confusions / comparison

	Underpinning	Shoring
Acts on	An existing foundation	The soil face of a new excavation
Goal	Strengthen / deepen / widen footing	Hold back soil so the cut doesn’t collapse
When	Added loads; adjacent dig undermines it	Any excavation too deep/tight to lay back
Permanence	Permanent foundation work	Usually temporary; may remain permanently when designed for that service

Related

→ Shoring systems (this module): the new-excavation counterpart · Excavation bracing (this module): crosslot/tiebacks supporting shoring · Deep foundations (Systems/Construction): piles and mini-piles used to underpin.