Micro Piles: Foundations for Tight Sites and Underpinning

Micro piles, also written micropiles, are small-diameter drilled piles reinforced with steel and filled with grout. They are typically under 300 mm in diameter, yet because they are built with high-strength steel and develop strong grout-to-ground bond, each one can carry a significant load. Their real advantage is access: they can be installed with compact rigs in basements, beside existing walls, under low headroom and through obstructions where conventional piling equipment cannot go.

This article explains how micro piles work, how they are installed, where they are used, how they are tested and what their limitations are. It sits under our wider guide to pile foundation types.

What micro piles are

A micropile is a drilled replacement pile in which most of the load is carried by steel reinforcement, with grout transferring load to the ground along a bonded length. A typical pile consists of a drilled hole, a steel element such as a high-strength threaded bar or a steel casing (or both), and cement grout placed by gravity or under pressure.

The concept originated in Italy in the early 1950s, when engineer Fernando Lizzi and the contractor Fondedile developed the palo radice, or root pile, to underpin historic buildings damaged over time and during the Second World War. The idea was a network of small piles that would bind into the ground and the existing structure. Since then the technique has spread worldwide, and the European execution standard EN 14199 now covers micropile construction.

How micropiles carry load

Unlike large bored piles, micropiles carry very little in end bearing because their base area is small. Almost all of the capacity comes from the bond between the grout and the surrounding soil or rock along the bonded length. The structural capacity, meanwhile, comes mainly from the steel, since the grout section is small.

This has several consequences:

Ground-grout bond governs geotechnical capacity: it depends heavily on ground type and on the grouting method. Pressure grouting, including post-grouting through sleeved pipes, can increase bond substantially.
Steel governs structural capacity: high-strength bars and casings allow high loads in a slim section.
Tension and compression: micropiles can carry both, which makes them useful for uplift, overturning and anchoring.
Buckling and lateral load: in very soft soils or voids, the slender pile may need checking for buckling, and lateral capacity is limited unless the pile is cased near the top or installed in inclined groups.

Installation

Micropiles are usually installed with compact rotary or rotary-percussive drilling rigs, some small enough to pass through a doorway or work under headroom of only a few metres. The typical sequence is:

Drill to the design depth, using temporary or permanent casing, drilling fluid or a hollow self-drilling bar depending on the ground.
Insert the steel reinforcement, with centralisers to keep it in the middle of the hole.
Grout from the base upward by tremie to displace drilling fluid, then apply pressure grouting or post-grouting if required.
Connect the pile head to the new foundation or the existing structure, often through a new pile cap, needle beam or collar.

Self-drilling hollow bar systems combine drilling, reinforcing and grouting in one operation: the bar acts as the drill rod with a sacrificial bit, and grout is pumped through its core during drilling. They are fast in collapsing ground but require careful control of grout quality and cover.

Where micropiles are used

The typical applications follow from the compact equipment and low disturbance:

Underpinning: strengthening or deepening foundations of existing buildings that have settled or are affected by adjacent excavation. Our guide to underpinning foundations covers the wider options.
Addition and alteration works: new columns, lift pits or extensions within an existing building, where large rigs cannot gain access. Such A&A works need structural design and approval in their own right.
Low headroom and restricted access: basements, under bridges, beside railways and within plant rooms.
Difficult ground: fill with obstructions, boulders or karst, where drilling can advance through material that would stop a driven pile.
Tension and anchoring: resisting uplift on basements and tanks, or anchoring towers and masts.
Slope stabilisation and ground reinforcement: as inclined or networked elements.

Testing and verification

Because capacity depends so heavily on grout-to-ground bond, load testing is an integral part of micropile works. Typical practice includes preliminary test piles loaded beyond working load to confirm the design bond values, and proof tests on a proportion of working piles. Both compression and tension tests may be specified depending on the loads. Grout is sampled and tested for strength, and records of drilling, strata, grout volumes and pressures are kept for each pile.

For underpinning, monitoring of the existing structure during installation is just as important. Drilling and pressure grouting can cause heave or settlement, and connecting new piles to an old foundation needs careful sequencing so that the structure is never left unsupported.

Limitations of micro piles

Micropiles are not a universal substitute for larger piles. Their individual capacity is lower than large-diameter bored piles, so heavy loads require many piles and substantial caps. Lateral load capacity is limited. Cost per kilonewton carried is generally higher than for conventional piling on an open site, and productivity is lower. Corrosion protection of the steel needs attention in aggressive ground or in permanent tension piles, using adequate grout cover, sacrificial steel thickness, coatings or encapsulation.

They come into their own where access, headroom, vibration or obstructions make conventional piling impractical. On open sites with good access, bored piles or driven piles are usually more economical. The decision is one for the foundation engineer, weighing loads, ground, access and the sensitivity of neighbouring structures.

Frequently asked questions

What is the difference between micro piles and normal piles?

Micropiles are much smaller in diameter, typically under 300 mm, and carry load mainly through grout-to-ground bond and high-strength steel reinforcement. Conventional piles are larger and rely on their concrete section and a combination of shaft friction and end bearing.

How much load can a micropile carry?

It depends on the ground, the bonded length, the grouting method and the steel element, so capacity is set by design and confirmed by load testing. Individual micropiles can carry substantial loads for their size, but heavy structures still need groups of them.

Are micro piles suitable for underpinning houses?

Yes. Underpinning is one of their main uses because compact rigs can work inside or close to existing buildings with low vibration. The scheme must be designed by a structural engineer, approved where required, and sequenced so that the existing foundations remain supported throughout.

Do micropiles cause vibration?

Drilling generates far less vibration than driving piles, which is why micropiles are used next to sensitive structures. Rotary-percussive drilling and pressure grouting can still affect very sensitive buildings, so monitoring is normally specified.

Can micropiles resist uplift?

Yes. Because their capacity comes mainly from bond along the shaft, micropiles work well in tension as well as compression. They are commonly used to resist uplift on basements and tanks and to anchor towers and other structures subject to overturning.

Considering micro piles for underpinning or an A&A scheme and need the design reviewed? Talk to our consultancy or get in touch.