Micropiles

Foundations provide support for structures, transferring their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics. Very broadly, foundations can be categorised as shallow foundations or deep foundations.

Pile foundations are deep foundations. They are formed by long, slender, columnar elements typically made from steel or reinforced concrete or sometimes timber. A foundation is described as piled when its depth is more than three times its breadth.

Micropiles are piles that have a relatively small diameter, typically in the range of 100-250 mm. They can also be described as:

• Minipiles (generally mini piles are smaller than micro piles)
• Pin piles.
• Needle piles.
• Root piles.
• Lightweight piles.

The size of the pile is determined by the load-bearing capacity of the ground and the size of rig that is able to access the piling location.

They can be driven or screw piles and tend to be used where access is restricted, for example underpinning structures affected by settlement. They were first used in Italy in the 1950s in response to the demand for innovative underpinning techniques that could be used for historic buildings and monuments.

They are particularly suited to:

• Shallow bedrock.
• Boulders and cavities.
• Immediate hard strata.
• Where underpinning is required, such as for foundations adjacent to planned excavations.

Micropiles can also be used in combination with other ground modification techniques where complex site conditions and design specifications are present.

In a typical installation process a high-strength steel casing is drilled down to the design depth. A reinforcing bar is inserted and high-strength cement grout pumped into the casing. The casing may extend along the full length of the pile, or it may only extend along part of the length of the pile, with the reinforcing bar extending along the full length.

Drilling may be achieved by a removable bit, or by a sacrificial head to the steel casing.

In some cases, the steel casing may be removed, or partially removed, and further grout pumped in at pressure.

They can be installed in restricted access sites where there is low headroom by the use of lagging. This is where wood, steel or precast concrete panels are inserted behind the pile as the excavation proceeds so as to resist the load of the retained soil and transfer it to the pile. Greater capacity can be achieved by post-grouting within the bond length to increase frictional forces with surrounding soils.

There are advantages of micropiles include:

• They are small and relatively light.
• They are relatively inexpensive.
• They can be installed through almost any ground condition, making them suitable for installation in environmentally-challenging conditions, such as for wind turbine towers.
• The limited vibration and noise causes little disturbance.
• Piling rigs can be low-emission or even electrically driven.
• They can be installed while avoiding existing utilities, meaning that expensive utility re-routing is not required.
• They can be installed close to existing walls with limited headroom and congested site conditions.
• They have a high load capacity and an ability to resist compressive, tensile and lateral loads.

[edit] Related articles on Designing Buildings

• Bored piles.
• Caisson.
• Cofferdam.
• Continuous flight auger piles.
• Driven piles.
• Footings.
• Foundations.
• Geothermal pile foundations.
• Ground anchor.
• Groundworks.
• Grouting in civil engineering.
• Pad foundation.
• Pile cap.
• Pile foundations.
• Raft foundation.
• Retaining walls.
• Screw piles.
• Sheet piles.
• Types of pile foundation.
• Underpinning.
• Vibro-compaction.
• Vibro-replacement.