Micropiles (underpinning)

Small-diameter piles, drilled and grouted around a tubular reinforcement or bars: they are born to consolidate and underpin the existing. Executed with compact rigs, even inside buildings, they transfer to depth the loads of structures that are settling or being raised, without demolition. They are the prime tool for strengthening foundations and soils.

The micropile is a small-diameter drilled pile (typically 10–30 cm), reinforced with a steel tube or bars and filled with a cement grout injected under pressure. It is born for a specific task: to consolidate and underpin existing structures, where large piles could not reach and where demolition is not possible. It is the technology of foundation strengthening.

The small diameter is its strength: it is executed with compact rigs, in tight spaces, inside cellars, close to existing structures, with little vibration. Many closely-spaced micropiles replace one large pile where it would be impossible to build. This makes them irreplaceable in cities, on historic fabric and in emergency consolidations.

The micropile carries almost everything by skin friction along the shaft, not by the base. The key is the injection: the grout, pushed under pressure, penetrates and keys into the soil around the hole, creating a bulb that «stitches» the pile to the ground. The higher the injection pressure, the more friction is mobilised. The tubular reinforcement can also serve as a duct to repeat the injections.

To underpin, the micropiles are placed in pairs or as trestles on either side of the existing footings and connected to them with a reinforced-concrete beam or cap that «hangs» the old foundation on the new piles. They can be inclined to resist horizontal actions too. Success depends on the soil, the cleanliness of the hole and the control of the injections.