Base and shaft post-grouting of bored piles

How bridges get built · Bored piles and deep foundations · Base and shaft post-grouting of bored piles

Also called: post-grouted drilled shaft, PGDS, base grouting, tip grouting, pile-end post-grouting, shaft grouting

How it is done

Choose the system with the designer: sleeve-port U-tubes (tube-à-manchette) or a flat-jack cell at the toe for base grouting, and sleeve-port pipes up the cage for shaft grouting; set target pressure, volume and head-uplift limits for each pile.
Fix the grout circuits to the cage, usually two or more independent U-loops crossing the base with sleeved ports, alongside the CSL tubes; pressure-test the lines before the cage is lowered.
Build the pile as normal. While the concrete is still young, open the ports by flushing each circuit with water at high pressure to crack the cover, then flush the circuits clean.
Grout once the concrete has gained strength (commonly 2–7 days after casting), starting with a thin mix (w/c 0.7–0.9) and stepping down to 0.4–0.6, recording pressure, volume and pile-head uplift continuously.
Stop each stage when the target pressure is held, the planned volume is placed or the uplift limit is reached; if pressure will not build, let the grout set and grout again in a further stage.
Grout shaft circuits after the base, from the bottom up, keeping pressures below the value that would fracture the ground or lift the pile.
Review each pile's pressure, volume and uplift record against the acceptance criteria (hold point before the cap), and verify the gain on instrumented or bi-directional test piles, ideally comparing grouted and ungrouted piles.

Key numbers

Grout mixfinal w/c 0.4–0.6, starting at 0.7–0.9 to prime the circuits
Termination pressure (JGJ 94)about 1.2–4 MPa in saturated soils; 3–10 MPa in weathered rock and unsaturated cohesive soils
Cement quantity (JGJ 94 estimate)about 1.5–1.8 t per metre of pile diameter for the base, plus 0.5–0.7 t per metre of diameter for each shaft-grouting level
Timinggrouting commonly starts 2–7 days after concreting
Uplift limita few millimetres at the pile head; uplift shows the base reaction has been mobilised
Gainlargest in loose to medium clean sands and gravels, small in stiff clay and sound rock

Where it fits

Bored piles and barrettes founded in sand, gravel or weak weathered rock; long marine and river piles drilled under slurry where base cleanliness cannot be proved; designs that need end bearing at small settlement; projects that can verify the gain on test piles.

Where it does not

Piles in stiff clay or sound rock, where the gain is small.
Sites with sensitive structures or services nearby that hydrofracture or grout travel could reach.
Using grouting to rescue a poorly built pile; it does not cure necking or weak concrete higher up the shaft.

Choosing it

Choose post-grouting over a longer or larger pile when the toe sits in sand or gravel and a slurry-drilled base cannot be proved clean; it turns a soft toe into a stiff, pre-loaded one.
Choose it on long piles in deep sands to cut the scatter in capacity and settlement between piles of a group.
Prefer a measured clean base without grouting in sound rock or stiff clay, where gains are small and the circuits add cost and risk.
Prefer a longer shaft or socket where uplift or shaft friction governs; base grouting mainly improves end bearing.

Plant, pace and money

PlantColloidal grout mixer and agitator, high-pressure grout pump (to about 10 MPa) with flow meter and pressure recorder, sleeve-port U-tubes or toe cells fixed to the cage, packers, and dial gauges or levels for pile-head uplift.

ProductivityOne to four hours per grouting stage per pile; one grout plant serves several rigs. Grouting runs a few days behind piling and must be finished before the cap is cast.

CostIndicative: USD 5,000–20,000 per pile for base grouting of 1.5–2.5 m shafts, including circuits; shaft grouting adds a similar sum. The cost is often recovered through shorter or fewer piles.

Risks and controls

What goes wrong

Circuits blocked by concrete or not opened in time.
Grout escaping up the shaft or into adjacent piles and services instead of building pressure at the toe.
Excessive uplift damaging the pile, or grouting after the cap is cast and cracking it.
Relying on predicted gains without load tests on the site's ground.

Quality assurance

Pressure test of circuits before concreting, record of port opening, grout tests (density, Marsh viscosity, cubes), continuous pressure, volume and uplift logs for every pile checked against the acceptance criteria, and bi-directional or static load tests on grouted and ungrouted piles.

Origins

Bolognesi and Moretto reported multi-stage base grouting in 1973 on two Paraná River bridges in Argentina, with 1–2 m shafts about 76 m long in sand; sleeve-port and flat-jack systems followed in Europe and the Middle East in the 1980s, and Florida DOT research around 2000 brought post-grouted shafts into US highway practice.

Examples

Used on large bored piles since the early 1970s: Paraná River bridges in Argentina, Florida and Louisiana highway bridges in the US, the Rędziński Bridge pylon in Poland, and widely in Chinese practice for long river and sea bridge piles of 2.5–3 m diameter.

Case studies

Paraná River bridgesArgentina · early 1970sOne of the first documented uses: shafts of about 1–2 m diameter and about 76 m long in sand, base-grouted in multiple stages (Bolognesi and Moretto, 1973).

PGA Boulevard grade separation, Palm Beach County, FloridaUSA · 2003 (reported)Post-grouted drilled shafts in sand replaced the specified driven piles; two instrumented test shafts, one grouted, calibrated the design, set a project grouting criterion and refined production tip levels.

Huey P. Long Bridge widening, New OrleansUSA · 2009Base grouting on 2.8 m shafts about 61 m deep in Mississippi alluvial sands, built in full-length casing and proved by an 80 MN bi-directional test.

Rędziński Bridge, WrocławPoland · 2011Bases of the 160 pylon piles (1.5 m × 18 m) were cement-injected with grout pressure and pile heave monitored; in static tests the injected piles performed better than an uninjected one.