Vacuum consolidation (vacuum preloading)

How bridges get built · Ground improvement · Vacuum consolidation (vacuum preloading)

Also called: vacuum preloading, vacuum-assisted consolidation, Menard Vacuum, atmospheric preloading

How it is done

Investigate the soft layer and its boundaries: thickness, sand lenses or seams that would leak, the crust, the underlying aquifer and the groundwater level, since the drains and the seal depend on them.
Install PVDs at close spacing in the soft layer, stopping short of any permeable base so the vacuum is not lost to it, and lay horizontal collector drains in a sand blanket.
Seal the area: excavate a perimeter trench through the blanket and any permeable crust into the clay and bury the membrane edges in bentonite slurry or clay, or use capped drains connected directly to the pipework without a membrane.
Lay one or more geomembrane layers, protect them with water or soil cover, and connect the collector pipes to vacuum pumps with water-air separation and standby power.
Pump down to about 60–80 kPa beneath the membrane and hold it without interruption; place fill on top as well when the design load exceeds what the vacuum provides.
Monitor vacuum under the membrane and at depth in the drains, settlement plates, piezometers and inclinometers at the edges, where the ground moves inward; repair leaks at once.
Stop pumping when settlement and pore pressure show the target consolidation, then remove the pumps, cut the membrane and build the embankment or pavement.

Key numbers

Vacuum under the membranetypically 60–80 kPa, equivalent to about 3.5–4.5 m of fill.
PVD spacing about 0.8–1.5 m, closer than for surcharge alone.
Treatment depth commonly 10–30 m; the effective vacuum falls with depth and drain length.
Pumping period typically 3–6 months.
Perimeter cut-off trench typically about 1.5–3 m deep, through the permeable crust into the clay.

Where it fits

Bridge approaches and reclamations on very soft clay where fill is scarce or staged loading would be slow or unstable.
Sites beside existing structures, where the outward movement caused by a fill surcharge would do harm.

Where it does not

Soft clay with permeable sand layers, shell beds or a pervious crust that cannot be sealed.
Loads well above about 80 kPa without added fill; peat with large secondary compression.

Choosing it

Choose vacuum over a fill surcharge when fill is scarce or expensive, when staged fill on very soft clay would be unstable, or when outward movement would harm neighbours: vacuum pulls the ground inward.
Combine vacuum with fill when the design load exceeds about 70–80 kPa.
Prefer a fill surcharge alone where sand layers or a shallow aquifer would leak the vacuum and cannot be cut off at reasonable cost.
Prefer DCM or a piled embankment where residual and creep settlement at the abutment must be near zero, or the programme cannot spare 3–6 months.

Plant, pace and money

PlantPVD rig, excavator for the cut-off trench, geomembrane welding crew, vacuum pumps with water-air separation and standby generators, and instrumentation.

ProductivityDrain installation as for surcharge schemes (3,000–8,000 m per rig per day); treatment in sectors of a few thousand to a few tens of thousands of m²; 3–6 months of pumping.

CostIndicative only: about USD 15–50 per m² of treated area for 15–25 m of soft clay, including drains, sealing and pumping, before any fill.

Risks and controls

What goes wrong

Loss of vacuum through sand lenses, the crust or a damaged membrane, reducing the effective load.
Inward lateral movement and tension cracks near the edges, affecting nearby services.
Power failure stopping the pumps, letting pore pressures recover.
Relying on vacuum alone for loads above about 80 kPa and leaving the clay under-consolidated.

Quality assurance

Continuous vacuum readings under the membrane and at depth, settlement plates and piezometers in every sector, leak inspection, and back-analysis of settlement (Asaoka) before pumping stops.

Origins

Proposed by Walter Kjellman in Sweden in 1952, developed at large scale in China (notably at Tianjin port) from the 1980s, and commercialised in Europe as Ménard Vacuum in 1988; membraneless systems with directly connected drains spread in Asia from the 2000s.

Examples

Used on port and airport reclamations in China and Southeast Asia and on road embankments over soft estuarine clay where imported fill is costly, often combined with a reduced fill surcharge.

Case studies

Tianjin PortChina · 1980s onwardsLarge-area vacuum preloading of dredged fill and soft marine clay; one of the two field cases analysed in the vacuum-and-drain studies of Indraratna and co-workers (2012).

Suvarnabhumi AirportThailand · 2006Vacuum-assisted PVD consolidation of soft Bangkok clay was trialled and monitored at the airport site; the field data are a standard reference for predicting vacuum consolidation.