Micropiles

Micropiles, also called micro-piles or minipiles, are small-diameter, reinforced grout columns constructed using drilling or driving methods to transfer loads from structures to deeper, more competent soil or rock. They typically consist of one or more steel reinforcement bars enclosed in cementitious grout and are installed with limited access and relatively low disturbance, making them suitable for retrofit and foundation underpinning.

Construction methods: Drilled micropiles are created by drilling a borehole, placing reinforcement, and injecting grout to fill the annulus; sometimes casing is left in place. Driven micropiles use a hollow steel tube or casing driven into the ground, after which the reinforcement is grouted and extracted, or the pile is grouted in place.

Load transfer occurs primarily through bond between grout and soil, and through friction along the grout column, with the steel reinforcement providing tensile capacity. Design depends on soil conditions, grout strength, reinforcement area, and connection details to the superstructure.

Advantages include suitability for restricted access sites, soft or deformed ground, and minimal vibration; fast installation; ability to underpin existing structures with limited excavation; and potential to install near utilities and foundations.

Limitations include typically lower axial capacity per unit length compared with larger-diameter piles and higher skill/quality-control requirements to ensure grout integrity and corrosion protection.

Applications: underpinning existing buildings, foundations on poor soils or near slopes, seismic retrofitting, tiebacks and retention systems, and ground improvement in crowded urban environments.

Standards and practice vary; design is typically performed by geotechnical or structural engineers using applicable codes and test methods, including load testing and non-destructive evaluation.