Construction methods · Bored piles and deep foundations

Micropiles

Small-diameter (150-300 mm) drilled piles with a central steel bar or tube grouted under pressure, installed with rigs that fit under an existing bridge. Individually modest (300-1,500 kN) but installed in numbers, they underpin, retrofit and reach into rock where large rigs cannot go.

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

Also called: minipiles, root piles, pali radice

Concept diagram (TheBridgeEng)

How it is done

Drill a cased hole with a small rotary or DTH rig; through fill and boulders if needed.
Insert the reinforcing bar or pipe; grout from the bottom, then pressure-grout the bond zone as the casing is withdrawn.
Connect to a new cap or an existing footing through drilled and grouted dowels.

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.

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.

Examples

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