Foundations: Footings, Driven Piles & Drilled Shafts

Every highway structure eventually transfers its load into the ground, and the foundation is the one piece of the structure an inspector can never re-inspect once it's built over. Whether the design calls for a simple spread footing or a deep foundation reaching far below grade, verification has to happen while the excavation or the shaft bottom is still visible.

Shallow Foundations: Footings

A spread footing is excavated down to the bearing elevation identified in the geotechnical report. The inspector confirms the exposed material actually matches what the borings predicted, not just the design assumption, and looks for soft spots, loose or disturbed soil, or standing water in the excavation bottom before any concrete is placed. Some contracts call for a proof roll or a plate load test to confirm bearing capacity in the field. Footings are formed, or placed against a neat, undisturbed excavation line, with dimensions and reinforcing matching the plans.

Deep Foundations: Driven Piles

Driven piles, timber, steel H-piles, or open/closed-end steel pipe piles, sometimes precast/prestressed concrete, transfer load to depth by being hammered into the ground rather than excavated for. Piles are driven to a specified tip elevation, a required driving resistance, or both, confirmed through a dynamic driving formula, wave equation analysis, or direct measurement with a pile driving analyzer (PDA). The inspector's pile driving log records, for every pile: hammer type and energy, blow counts per foot (or per inch, near refusal), plumbness and batter, any splices made, and any sign of damage.

Driving criteria are set by a specified tip elevation, a specified driving resistance (minimum blow count over the final increment of penetration), or both. Near the end of driving, blows are counted per foot and then per inch; a sudden jump in blow count with little added penetration is treated as practical refusal. Where required, a pile driving analyzer estimates mobilized capacity in real time, beyond what blow count alone shows. Brooming or crushing at the pile tip or head signals over-driving or a mismatched hammer/cushion and calls the pile's capacity into question. Once driven, piles are cut off at the design elevation and embedment into the cap is verified.

Deep Foundations: Drilled Shafts

Drilled shafts (sometimes called caissons) are large-diameter holes excavated with an auger or rotary drill, often stabilized with temporary casing or slurry (mineral or polymer) where soils are unstable or groundwater is present. Unlike a driven pile, a drilled shaft's capacity depends heavily on tip (bottom) cleanliness, loose cuttings, sediment, or slurry cake left at the base reduce end-bearing capacity and can cause excessive settlement under load. The inspector confirms the base has been cleaned (with a cleanout bucket, airlift, or similar method) and verifies cleanliness immediately before concrete placement, not on trust.

Shaft depth is confirmed against the boring logs and plans, the shaft must extend into the bearing stratum identified by the geotechnical investigation, and the inspector cross-checks the driller's log and measured tip elevation against the design. The reinforcing cage is lowered and centered with spacers or wheels to maintain cover, and held down against buoyancy from the fluid concrete if the design requires it.

Placing Concrete in Wet or Cased Holes

Where a shaft excavation contains water or slurry, concrete is placed through a tremie pipe kept embedded in the rising fresh concrete, displacing water or slurry upward rather than letting concrete fall through it and segregate. Free-fall placement is permitted only in a clean, dry, stable hole. Drilled shaft concrete mixes typically specify a higher slump, a highly flowable, self-consolidating mix, than ordinary structural concrete, since the mix must flow around a closely spaced rebar cage without the benefit of vibration below the tremie.

Pile Splices and Cutoffs

Piles that must extend deeper than a single mill length are spliced, mechanically or by welding, using a manufacturer-approved splice detail that restores the pile's full design strength across the joint. Welded pile splices follow the same qualified-welder and inspection expectations as structural steel welding elsewhere on the project. Once a driven pile reaches its final tip elevation or driving resistance, the excess length above cutoff is removed cleanly, without cracking or shattering the remaining pile, and the inspector confirms the cutoff elevation and any exposed reinforcing or dowels match what the pile cap detail requires for embedment.

Documentation

For both driven piles and drilled shafts, the inspector maintains an installation log, keeps material certifications on file, and records as-built tip elevations. Any anomaly encountered during installation, an unexpected obstruction, artesian water pressure, caving soil, or a driving resistance far outside the expected range, gets reported to the engineer of record for evaluation before work continues, since it may signal the foundation isn't behaving as the geotechnical report predicted.