What Is a Deep Foundation vs. Shallow Foundation?
Shallow foundations (spread footings, mats) bear on soil near the surface and are used when competent soil is shallow; deep foundations (driven piles, drilled shafts/piers) extend through weak surface soils to bear on competent strata below. The geotechnical report decides: adequate bearing within ~10 feet favors shallow; fill, expansive clay, liquefiable layers, or high loads push to deep. Cost roughly doubles to triples going from shallow to deep.
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Shallow foundations
Spread footings under columns and strip footings under walls are the economical default — simple to design, simple to build, easy to inspect. Mat (raft) foundations spread the whole building's load over one thick slab where soils are marginal but uniform. Shallow systems need competent soil within economical excavation depth and are sensitive to the water table and frost depth.
Deep foundations
Driven piles (concrete, steel, timber) are hammered to bearing or refusal — fast in the right soils, noisy, with vibration neighbors feel. Drilled shafts (piers) are augered and concreted in place — quieter, better for urban sites and large loads, but slower and weather-sensitive. Micropiles thread through tight access and existing structures. Each type has its testing and inspection regime (see pile load tests, CSL testing for shafts).
How the decision gets made
The geotechnical engineer compares bearing capacity, settlement estimates, and constructability for both options and recommends — sometimes both work and cost decides, sometimes only deep works. High-rise, heavy industrial, and anything on fill or liquefiable soil go deep. The structural engineer then designs the pile caps or grade beams connecting the deep elements. Never value-engineer a deep foundation to shallow without the geotechnical engineer's written approval.
The bottom line
Shallow = footings on good near-surface soil (cheap, simple). Deep = piles/piers through bad soil to bearing (2-3x cost). The geotechnical report decides; never downgrade without its author's approval.
Torch Engineering PLLC · (480) 490-0064 · info@torchengineering.com — PE stamping backed by Apex Grid Engineering.