Stone columns (vibro-replacement)

How bridges get built · Ground improvement · Stone columns (vibro-replacement)

Also called: vibro stone columns, gravel piles

How it is done

Check suitability from CPTs and boreholes: undrained strength, fines and organic content, and layering; plan geotextile-encased columns if the soft layer is weaker than about 15 kPa.
Design the grid and diameter for the purpose: settlement improvement (Priebe method), drainage (unit cell), or liquefaction mitigation by densification, drainage and stiffness.
Build a granular working platform that can carry the rig, and set out the column positions.
Penetrate to the design depth by vibration with water flush (wet top-feed) or with air and stone delivered to the tip through a tremie tube (dry bottom-feed).
Build the column in lifts of about 0.5–1.0 m: lift, feed stone, re-penetrate to compact and expand it, using amperage or hydraulic pressure and stone consumption to confirm compaction.
Record every column automatically (depth, power, time, stone volume) and manage the spoil and water from wet methods.
Place a compacted granular load-transfer platform, often geogrid-reinforced, and test with plate or zone load tests and CPTs between columns before building the fill or footing.

Key numbers

Diameter 0.6–1.2 m; spacing 1.5–3.0 m; area replacement ratio about 10–35 %.
Depth typically 5–15 m, reaching about 20–25 m with standard equipment.
Settlement improvement factor (Priebe)typically 1.5–3.
Stoneabout 40–75 mm for wet top-feed, 10–40 mm for dry bottom-feed.
Minimum undrained strength about 15–20 kPa without encasement.
Outputabout 150–300 m of column per rig per shift.

Where it fits

Approach embankments, MSE walls and small abutment footings on soft clay with undrained strength above 15 kPa, and on loose sands.
Liquefaction mitigation around piles.

Where it does not

Very soft clay (cu below 15 kPa) where the column bulges and the gravel is lost.
Organic soils.

Choosing it

Choose stone columns over vibro-compaction when fines exceed about 10–15 %: the stone does the work the soil cannot.
Prefer DCM or a piled embankment when the clay is weaker than about 15 kPa, organic, or must not settle next to a fixed abutment.
Choose stone columns over PVDs alone when strength and stiffness must rise quickly as well as drainage; combine them with PVDs in thick clay.
Use them under abutments and around piers for liquefaction where drainage and densification are enough, and prove the result with CPTs.

Plant, pace and money

PlantVibroflot on a crane or a purpose-built rig, gravel feed, water or air supply.

Productivity20-40 columns of 10-15 m per rig per shift.

CostUSD 40-80 per metre of column.

Risks and controls

What goes wrong

Column bulging in very soft layers; contaminated spoil in wet methods.

Quality assurance

Depth and gravel consumption per column, plate load tests on treated ground, CPT between columns.

Origins

Vibroflotation was developed in Germany by the Keller company in the 1930s (Steuerman and Degen) to densify sand; adding gravel to treat silts and clays, vibro-replacement, followed in the late 1950s, and the dry bottom-feed method in the 1970s.

Examples

Widely used for road approaches in Southeast Asia and the Gulf; often paired with PVDs.

Case studies

Quisibis River BridgeCanada · 1999361 columns of 1 m diameter at 3 m centres at the embankment toes and pier locations in about 15 m of very loose silt, mainly to drain earthquake pore pressures; CPT dissipation times fell from 10–30 minutes to 1–6 minutes.

Penny's Bay reclamationHong Kong, China · early 2000sStone columns were installed through soft marine deposits beneath the reclamation, with field performance documented in published case histories.