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Sitework & Earthworkaka: foundation underpinningaka: helical piersaka: push piers

Underpinning

In Plain English

Reinforcing the foundation of an existing building from below — used to fix settling foundations or allow digging nearby.

Definition

The process of strengthening or deepening the foundation of an existing structure, typically to increase bearing capacity, address settlement, or allow excavation adjacent to the structure. Methods include mass concrete underpinning (sequential bays), helical piers, push piers, and jet grouting. Underpinning is used when adding stories, correcting settlement, or excavating near existing buildings.

Why It Matters in Bidding

Underpinning is one of the highest-risk line items in an estimate because it involves working beneath a loaded, occupied structure where unknown soil and existing footing conditions drive cost. Estimators must qualify the bid with assumptions about method, sequencing, and access, since scope creep on underpinning quickly erodes margin and triggers change orders. Specialty geotechnical subs and engineering coordination should be locked in before pricing.

Example

An estimator pricing a basement addition includes 14 sequential mass-concrete underpinning bays at roughly $4,500 each, noting in the bid that the unit assumes the existing footing depth shown on the survey and excludes dewatering.

Related Terms

ShoringExcavationSoil BoringRetaining Wall

Frequently Asked Questions

They base pricing on the geotechnical report and exploratory data, then attach explicit assumptions and exclusions covering obstructions, groundwater, and existing footing depth. Unknowns are often carried as allowances or unit prices per bay or per pier so actual quantities can be reconciled, protecting margin if field conditions differ from the bid basis.
Unit pricing per bay, per linear foot, or per pier is preferred because depth and quantity often change once excavation exposes actual conditions. A lump sum shifts quantity risk entirely to the contractor. Many GCs combine a base lump sum for known scope with unit prices for additional bays discovered during sequencing.
Responsibility depends on contract delivery. On design-bid-build, the owner's structural engineer typically details the sequence and bay sizing. On design-build or specialty-sub scopes, the underpinning contractor's engineer provides shop drawings and calculations. Estimators must confirm which party carries engineering cost so it is not double-counted or omitted from the bid.

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