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Structuralaka: deep foundation system

Deep Foundation

In Plain English

A foundation that extends far into the ground to reach solid soil or rock capable of supporting the building.

Definition

A foundation system that transfers structural loads to deeper, more competent soil or rock layers far below the surface. Deep foundations are used when near-surface soils are too weak or compressible to support the structure. Piles and drilled piers are the most common types.

Why It Matters in Bidding

Deep foundations are one of the largest swing items in a bid because their cost depends heavily on subsurface conditions that are often poorly defined at bid time. Estimators must rely on the geotechnical report, anticipated pile lengths, and rig mobilization, and they carry significant risk if actual refusal depths or obstructions differ from assumptions. Misjudging this scope can erase an entire project's margin.

Example

Reading the geotech report, an estimator prices 120 drilled piers at 45-foot average depth, then adds a unit-price line for over-drilling so the owner pays per linear foot if piers must go deeper to reach competent rock.

Related Terms

PilePierFootingFoundation

Frequently Asked Questions

They typically bid on the geotechnical engineer's estimated tip elevations, then add unit prices for additional depth or obstruction removal. This converts hidden subsurface risk into measurable add/deduct line items, so the owner absorbs cost variation rather than the contractor eating it as a lump-sum loss.
Piling and drilled-pier work require specialized rigs, crews, and engineering that most GCs don't own. Estimators solicit pricing from geotechnical or foundation subs, coordinate mobilization, load testing, and spoils disposal, then carry the sub's number plus markup and any GC-furnished items like surveying or excavation support.
Review the geotechnical report for soil profiles and recommended capacities, the structural drawings for pile loads and counts, and the specifications for testing requirements. Also check addenda for revised boring logs. Missing or limited borings increase risk and may justify qualifying your bid or adding contingency.

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