Micropiles

The defining distinction is install method, which drives every other tradeoff. Driven piles are forced into the ground by impact or vibratory hammers, displacing soil radially and developing capacity through end bearing and skin friction on the displaced surface. Micropiles are drilled and grouted, with no soil displacement, and develop capacity through grout-to-ground bond. Driven piles are typically lower cost per ton of capacity in open sites with simple stratigraphy and no vibration constraints. Micropiles are preferred where confined access, sensitive adjacent structures, vibration-prone soils, or complex stratigraphy with cobbles, boulders, or weathered rock would prevent driving. Driven piles often cannot work in karst terrain or interbedded soils, where micropiles handle variable conditions routinely.

Both systems are drilled, but the geometry and capacity model differ sharply. Drilled shafts, also called caissons, are large-diameter elements, typically 24 to 120 inches, reinforced with a rebar cage and tremied with concrete, developing very high single-element capacity primarily through end bearing on rock or dense soil. Micropiles are small-diameter elements, typically 5 to 12 inches, reinforced with high-strength steel and pressurized grout, developing high unit capacity from a much smaller cross-section. A drilled shaft requires open-air drilling with large equipment and substantial site footprint. Micropile groups deliver comparable total capacity from low-headroom rigs in basements, on bridge decks, and in any tight-access setting where a drilled shaft cannot be installed.

Both are small-diameter deep foundation elements, but the engineering envelope differs by an order of magnitude. Helical piers are screw-in steel shafts with helical bearing plates, installed with a hydraulic torque head, with single-pier capacities typically in the 20 to 50 kip range and most installations in the 5 to 30 foot depth band. They are the standard for residential underpinning, light commercial additions, and tower foundations on soft to medium soils. Micropiles develop capacities from 50 to 300 tons and reach 100+ feet through the entire stratigraphy, including bedrock. Helical piers install faster and cleaner with no spoil. Micropiles cost more per pile but cover the heavy commercial, infrastructure, seismic retrofit, and rock-bonded foundation envelope that helicals cannot serve.