Micropiles

How bridges get built · Bored piles and deep foundations · Micropiles

Also called: minipiles, root piles, pali radice

Micropiles — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Survey the existing structure, services and headroom; select the micropile type (FHWA Types A–D by grouting method) and the connection to the cap or footing, and agree the test programme.
Drill a cased hole with a rotary or rotary-percussive (DTH) rig using water, air or polymer flush; core through existing footings first where piles pass through them.
Clean the hole and insert the reinforcement (threaded bar, often hollow, a steel pipe, or both) with centralisers.
Tremie grout from the toe with neat cement grout (w/c about 0.45–0.5); for pressure-grouted types, pull the casing over the bond zone while grouting under pressure, or post-grout through sleeve ports.
Make the connection: grouted sleeve or bearing plate through the existing footing, or embedment in a new cap; preload where the design requires it.
Load test sacrificial verification piles before production and proof test a share of the production piles, recording creep.
On retrofits, monitor the existing structure for movement during drilling (in real time where piers are sensitive) and adjust the sequence if it moves.

Key numbers

Drilled diameter100–300 mm (cased micropiles commonly 178–273 mm casing)
Capacity300–1,500 kN typical; over 2,000 kN with large casings and rock sockets
Ultimate grout-ground bond (FHWA)roughly 35–360 kPa in soils, about 200–4,000 kPa in rock, by type and ground
Testing (FHWA)verification to about 2.5× design load; proof tests to about 1.67× on about 5 % of production piles
Output5–15 piles per rig per shift

Where it fits

Retrofit and seismic strengthening of existing piers, scour repairs, foundations under low headroom, sites with boulders or karst where drilling small holes is safer, steep slopes.

Where it does not

New heavy foundations where a few large shafts are cheaper; soft clay with little bond.

Choosing it

Choose micropiles over large shafts where headroom, access or existing footings rule out big rigs, as under the Route 92/280 connectors and the 130-year-old Nipigon rail bridge.
Choose them for underpinning while the structure stays in service; at Nipigon the owner expected about 20 % saving against replacement.
Prefer drilled shafts for new heavy foundations: per kN micropiles are expensive, and their slender section can buckle in very soft soil or scour.
Where piers rock in earthquakes, design for tension cycling and keep displacement at the tension design load close to the bar's elastic elongation (a lesson from the Route 92/280 retrofit).

Plant, pace and money

PlantSmall drilling rig, grout mixer and pump.

Productivity5-15 piles per shift.

CostUSD 150-400 per metre; high per kN but low mobilisation.

Risks and controls

What goes wrong

Grout loss in fissured rock, bar corrosion without encapsulation, buckling in soft soils.

Quality assurance

Grout volume per pile, load tests on 5 % including creep tests, bond zone length verified.

Origins

Fernando Lizzi of Fondedile developed the 'pali radice' (root piles) in Naples in the early 1950s to underpin war-damaged buildings; the method spread worldwide from the 1970s, and FHWA research and manuals of the 1990s–2000s made it standard for US bridge retrofit.

Examples

Seismic retrofit of Californian and Japanese piers; scour repairs under river bridges.

Case studies

CPR Nipigon bridge, OntarioCanada · 2007 (reported)Underpinning of a 130-year-old rail bridge on masonry piers and overstressed timber piles: 273 mm cased micropiles with a 69 mm bar for 1,200 kN service load, tested to 3,000 kN in compression without failure.

Route 92/280 interchange connectors, San Francisco Bay AreaUSA · 2013 (reported)Caltrans seismic retrofit with micropile groups at five bents instead of CIDH piles, because of poor rock, high groundwater and low headroom.

Liberty Bridge, Greenville, South CarolinaUSA · 2004Curved cable-stayed footbridge whose drilled-pier design was replaced by micropiles for compression and anchor piles for tension, because of difficult access and ground.

King and Queen Street bridges, West Don Lands, TorontoCanada · 2012–15Remedial rock-socketed micropiles battered at about 1H:5V under existing piers; drilling-induced movement of the bridge forced stoppages, and the work was completed under real-time monitoring with a revised sequence.