Construction glossary · Concrete and masonry
What is caisson in construction?
A caisson is a deep foundation element: a large diameter shaft drilled into the ground and filled with reinforced concrete so that building loads bypass weak soils and bear on rock or another competent stratum. On US commercial projects the term is used interchangeably with drilled shaft and drilled pier. It differs from a pile, which is typically a slender driven element installed in groups, while a caisson is a single large drilled shaft, often one per column.
Updated June 2026 · Reviewed by the Ruh construction team
Caisson section to bearing
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Book a walkthroughIn US commercial work, caisson, drilled shaft, and drilled pier all describe the same element: a hole drilled to a specified diameter, often belled at the bottom for extra bearing area, with a rebar cage set and concrete placed full depth. You will find caissons on the structural foundation plan with a caisson schedule listing mark, shaft diameter, bell diameter, top elevation, estimated bottom (tip) elevation, and design bearing pressure. The geotechnical report establishes the bearing stratum, and spec section 31 63 29 (drilled concrete piers and shafts) governs installation. A specialty drilling subcontractor installs them, but the GC's estimator still owns the gaps: concrete overpour, spoils haul-off, temporary casing, slurry, and rock sockets. New estimators make predictable mistakes. They price the schedule's estimated depths as if guaranteed, miss bell volume, which can add several cubic yards of concrete per caisson, and lump earth drilling with rock drilling even though the unit costs are very different. Treat every depth on the schedule as the baseline for unit prices, not a fixed quantity.
Caissons are counted per each by mark and diameter, then broken into unit price quantities. Drilling is measured in vertical linear feet (VLF), split into overburden (earth) drilling and rock socket drilling because production rates and unit costs differ sharply. Concrete is taken off in cubic yards (CY) from shaft volume plus bell volume, with an overpour allowance added (10 to 15 percent is a typical starting point in stable ground, more where soils cave or hold water). Rebar cages are quantified in pounds or tons, casing in linear feet by diameter, and spoils haul-off in CY. Quantities come from the caisson schedule and the geotech borings, and the bid form usually carries unit prices per VLF and per CY so actual drilled depths can be adjusted both ways.
Worked example
Take a 48 in diameter caisson with an estimated depth of 40 ft. Shaft area: 3.14 × 2 ft × 2 ft = 12.6 sq ft. Design volume: 12.6 sq ft × 40 ft = 504 cu ft, and 504 / 27 = 18.7 CY. Carry a 10 percent overpour allowance and you are pricing about 20.5 CY of concrete. Now the rig finds suitable bearing 8 ft deeper than the boring suggested. Extra drilling is 8 VLF, and extra concrete is 12.6 sq ft × 8 ft = 101 cu ft, or 3.7 CY. At illustrative unit prices of $120 per VLF for earth drilling and $250 per CY for concrete in place, the overrun is (8 × $120) + (3.7 × $250) = $960 + $925 = about $1,885 on one caisson. Spread similar overruns across a 60 caisson job and depth variance is serious money, which is why caisson bids carry unit prices tied to estimated tip elevations and every foot past them is change order territory.
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How Ruh handles caisson
Ruh reads the foundation plan and caisson schedule from the drawings, takes off each caisson by mark, diameter, and estimated depth, and turns that into drilling VLF, concrete CY including bells and your overpour factor, and rebar weight. It then prices those quantities against your own price book, your real unit costs for earth drilling, rock sockets, casing, and concrete, and hands the estimator a line-item draft. The estimator still reviews the assumed tip elevations against the geotech report, adjusts the allowances, and signs off, because depth risk is a judgment call, not a takeoff output.
See concrete estimating softwareCaisson: frequently asked questions
What is the difference between a caisson and a pier?+
In commercial building work there is usually no difference: caisson, drilled pier, and drilled shaft are regional names for the same drilled, cast-in-place element. Caisson is the common term in Chicago and much of the Midwest, while drilled pier and drilled shaft show up elsewhere. The confusion comes from bridge work, where a pier is the visible substructure column, and from light construction, where a pier can be a shallow pad or masonry support. Confirm against the drawings and the spec section rather than the label alone.
What is the difference between a caisson and a pile?+
Size, installation method, and load path. A caisson is drilled at large diameter, commonly 30 in to 96 in, cast in place, and usually placed one per column to carry load in end bearing on rock or hardpan. Piles are slender elements, often 12 in to 18 in, driven or augered in groups under a pile cap, and they develop capacity through skin friction, end bearing, or both. The takeoff differs too: caissons price per VLF of drilling plus CY of concrete, while driven piles price per LF installed plus the cap.
When are caissons used instead of piles?+
Caissons make sense when column loads are heavy and a competent bearing layer sits at a reachable depth, because one big shaft can replace a whole pile group and its cap. They also win on vibration-sensitive urban sites, since drilling avoids the impact and noise of pile driving next to occupied buildings. Driven or augercast piles tend to win where bearing strata are very deep, where friction capacity in soft soils does the work, or where a lighter rig and lower mobilization cost fit the job.
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Figures on this page are illustrative. Construction estimates depend on project-specific conditions, source documents, market pricing, and professional judgment. Ruh's AI assists the estimator and does not replace professional review: your team reviews, validates, and approves every estimate, bid, and pricing decision.