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.
Technical section of the system, from inside (left) to outside (right).
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.
Why it works
Friction and injectionThe micropile carries almost everything by skin friction along the shaft, not by its tiny base. The key is the injection: grout pushed under pressure penetrates and keys into the soil around the hole, forming a bulb that «stitches» the pile to the ground — the higher the pressure, the more friction is mobilised. Small and low-vibration, it is drilled with compact rigs even inside buildings, and a cap hangs the existing foundation onto the new piles: consolidation without demolition.
Suitability for underpinning the existing
Comparison · insulantsNodal details
Critical junctions · sectionsA reinforced-concrete cap embraces the existing footing, tied to it by drilled-and-grouted dowels; from the cap a pair of inclined micropiles descends, so the old foundation is «hung» on the new piles and the load reaches firm soil at depth.
- Existing footing
- Connecting cap
- Connection dowels
- Inclined micropiles
- Tubular reinforcement
- Bearing soil
In section the shaft is the injected grout that fills the hole and keys into the soil, wrapped around the steel tube; inside the tube, more grout. It is the friction on this grout surface, not a base, that carries the pile.
- Soil
- Injected grout (shaft)
- Steel tube reinforcement
- Grout inside the tube
- Bond with the soil (friction)
Installation controls
Specification · checklist01 · Survey & setting out
02 · Drilling
03 · Tubular reinforcement
04 · Injection
05 · Connection to footings
Recurring defects
Diagnostics · siteComponent materials
The network · materialsReference regulations
2 norms- D.P.R. 380/2001Consolidated Building Act (Testo Unico Edilizia)In force
- D.M. 1444/1968Mandatory limits of density, height, distance between buildings and urban standardsIn force
Informational links to the regulatory framework. Always verify the current text on the official source.