Prefabricated Vertical Drains (PVD) to accelerate consolidation of soft clay
Prefabricated Vertical Drains (PVD), also called wick or band drains, are flat plastic cores wrapped in a filter fabric, installed vertically through soft, compressible clay on a regular grid. Water squeezed out of the clay by a surcharge or preload travels sideways a short distance to the nearest drain and up to a drainage blanket, instead of a long way vertically through the clay. Consolidation settlement that would take years happens in months, before construction, and the clay gains strength as it consolidates.
Suitable ground conditions and applications
Can be considered where…
- Soft, saturated clays and silts with large expected consolidation settlement.
- Reclamation, embankments, roads, tank farms and storage yards.
- Programmes that cannot wait for natural consolidation.
- Combined with a surcharge or vacuum preload.
Limitations
- Drains speed up consolidation; a surcharge, preload or vacuum is still needed to cause it.
- Settlement still occurs and must be allowed for in levels and services.
- Smear and well resistance reduce drain efficiency and are allowed for in design.
- Obstructions, stiff crusts or sand layers may need pre-drilling.
- Secondary compression (creep) continues after primary consolidation.
Suitability is confirmed from the site investigation and the performance requirement.
How it works
A drainage blanket of sand or granular material is placed as a working platform. A mandrel mounted on a stitcher rig pushes the drain through the soft layer to the design depth; an anchor plate holds the drain in place as the mandrel is withdrawn, and the drain is cut above the blanket.
Drains are installed on a square or triangular grid. The surcharge or preload is then placed in stages so the clay is not overloaded, and settlement and pore water pressure are monitored until the target degree of consolidation is reached before the surcharge is removed.
Design inputs
- Site investigation: clay thickness, consolidation and permeability parameters, CPTu dissipation tests.
- Required settlement and time to reach it.
- Drain spacing and pattern, depth and drain type.
- Surcharge height, staging and stability.
- Monitoring layout and acceptance criteria.
Trial and calibration
- Trial section with settlement plates and piezometers to confirm the rate of consolidation.
- Installation trial to confirm penetration and the anchor system.
- Back-analysis of monitoring to confirm or adjust spacing and surcharge time.
Execution controls
- Drain position, depth and verticality recorded for each drain.
- Continuous drain material, splices to the manufacturer's method.
- Mandrel penetration resistance logged to identify obstructions.
- Surcharge placed in the approved stages.
- Settlement and pore pressure readings at the agreed frequency.
Verification and acceptance
Design intent, production control and acceptance verification are kept separate, so it is always clear what was required, what was done and what was proven.
What is required
Target degree of consolidation, residual settlement and time agreed with the consultant.
What was done
Drain installation records and as-built grid.
What was proven
Settlement plates, extensometers and piezometers against the predicted curves before surcharge removal.
Deliverables
- Method statement and risk assessment
- Drain layout plan
- Installation records
- Surcharge staging plan
- Monitoring records and consolidation analysis
- Completion report
Discuss whether Prefabricated Vertical Drains (PVD) fits your ground conditions.
Share the site location, the investigation report and the performance requirement.