Prefabricated vertical drains with surcharge

How bridges get built · Ground improvement · Prefabricated vertical drains with surcharge

Also called: wick drains, PVD and preload, band drains

Prefabricated vertical drains with surcharge — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Characterise the soft layer: CPTu with dissipation tests for the horizontal consolidation coefficient, oedometer or CRS tests on good samples for compressibility and stress history, and the drainage boundaries above and below.
Design drain spacing for the target degree of consolidation in the time available (Barron and Hansbo theory with smear and well resistance), and stage the embankment, with berms or basal reinforcement where stability requires.
Place a sand drainage blanket of about 0.5–1.0 m with outlets, then install settlement plates and piezometers before the drains go in.
Install PVDs with a closed mandrel and anchor plate on a square or triangular grid to the base of the soft layer, recording depth and position for every drain.
Build the embankment in stages, holding each until pore pressures have dissipated enough, then add a surcharge of 1–3 m above final level.
Monitor settlement and pore pressure and fit Asaoka or hyperbolic curves to estimate final settlement and the degree of consolidation reached.
Remove the surcharge only when the target consolidation (typically 90 % or more) and the residual settlement limit are met; then build the pavement and approach slab and keep monitoring into the defects period.

Key numbers

Band drains about 100 × 3–5 mm, equivalent diameter about 50–65 mm.
Spacing 1.0–2.5 m; installed depth typically 10–30 m, with long-mast rigs reaching about 40–50 m.
Surcharge1–3 m of fill above final level.
Time to 90 % consolidationcommonly 3–12 months at 1.0–1.5 m spacing in soft marine clay.
Smear zoneabout 2–3 times the mandrel equivalent diameter, with permeability reduced several times.
Output3,000–8,000 m of drain per rig per day.

Where it fits

Approach embankments and MSE abutments on soft clay 5-30 m thick.
Projects with a year of programme to spend before the pavement goes down.

Where it does not

Peat and very organic soils, which creep after primary consolidation.
Programmes that cannot wait: use piled embankments instead.

Choosing it

Choose PVD and surcharge over piled embankments when there are 6–12 months to spend and fill is available: it is typically several times cheaper.
Prefer vacuum consolidation, alone or with fill, when fill is scarce or staged loading would be unstable; the vacuum adds no outward shear stress.
Prefer piled embankments, DCM or lightweight fill beside abutments where residual and creep settlement must be close to zero, and in peat.
Combine PVDs with stone columns or DCM when both drainage and early stiffness are needed.

Plant, pace and money

PlantPVD installation rig (mast 20-35 m), earthmoving fleet, instrumentation.

Productivity3,000-8,000 m of drain per rig per day; consolidation 4-12 months.

CostUSD 2-5 per metre of drain installed; the surcharge fill is often the larger cost.

Risks and controls

What goes wrong

Embankment instability during staged loading.
Smear around the mandrel slowing drainage.
Removing the surcharge too early: residual settlement damages the approach slab and the expansion joint.

Quality assurance

Settlement plates every 50 m, piezometers in each soft layer, Asaoka plots to confirm the degree of consolidation before release.

Origins

Sand drains were patented by Daniel Moran in the 1920s and first used by the California Division of Highways in 1934; Kjellman introduced cardboard wick drains in Sweden in 1937, Barron (1948) and Hansbo (1979–81) provided the theory, and plastic band drains replaced sand drains from the 1970s.

Examples

Incheon and Busan reclamations, Bangkok and Ho Chi Minh City approaches, the Gulf sabkha coasts.

Case studies

Muar Flats trial embankmentsMalaysia · 1989Instrumented trial embankments built by the Malaysian Highway Authority on soft marine clay compared vertical drain schemes and other treatments on the same ground, with measured performance set against predictions.

Changi Airport second runwaySingapore · 1977Vertical drains installed as deep as about 43 m to consolidate thick soft marine clay under reclaimed land.

Suvarnabhumi AirportThailand · 2006PVDs with surcharge, and vacuum-assisted trials, consolidated soft Bangkok clay beneath the airfield; the field data are widely used to calibrate drain design.