Tight. Work goes on as membrane seals treatment zone. (Photo by Peter Reina for ENR)

To speed up ground settlement at the site of a Vietnamese powerplant, French engineers are vacuuming water from a 16-meter-deep layer of compressible clay. Final ground levels are due to be reached in under half the 18 months needed when using conventional gravity drainage, claims the contractor.

Under a $4.3-million subcontract, 90,000 sq m of low-lying site is being urgently prepared for a new powerplant next to the Cai Tau River near Ca Mau, in Vietnam’s south. Another, less urgent 110,000 sq m of the site is due for conventional treatment, costing about 25% less, says Marc Lacazedieu, deputy general manager of subcontractor Menard-Soltraitement S.A., Nozay, France.

Menard is basically pumping water and air out of drains in the clay, which is sealed from the atmosphere by a tough plastic membrane on the surface. Initial settlement of up to 4 m is forecast at zones of highest loading from generating equipment. Overburden spread to corresponding depths during treatment will simulate the differing loads.

In August, Menard began installing 150,000 vertical flat drains on 0.9-m-sq grid across the site. Within a 1.5-m-deep drainage layer placed by the main civil contractor, Campenon Saigon Builders Ltd., Ho Chi Minh City, Menard is installing a grid of circular horizontal drains. These lead to about 50 specialized pumps able to handle air and water, says Lacazedieu. The covering membrane is protected with a shallow sand layer.

To prevent air leakage around the membrane, the site’s five treatment zones, corresponding to areas of different final loading, are surrounded by trenches cutting into the underlying clay. These trenches allow the membrane to be keyed into the clay. And keeping the trenches filled with water prevents peripheral clay from cracking and creating air cracks.

Menard must leave the site 2.2 m higher than it was and guarantee maximum settlement of 10 cm over 10 years, says Lacazedieu. The firm also bears the risk of importing more fill, should it be necessary.

The technique is one of several that exploit a vacuum. Menard developed its system and first used it in 1989 on a 400-sq-m plot in France, says Lacazedieu. It has since exported it to South Korea, Thailand and now Vietnam. In the U.S., "they are showing interest," he adds.

Reel Estate. Workers install vertical drains. (Photo by Peter Reina for ENR)

Other techniques achieve the vacuum differently. The Dutch BeauDrain, for example, sets the horizontal drains within the clay layer, eliminating the membrane, says Hans Van Eijk, managing director of the technique owner COFRA B.V., Amsterdam. The Dutch IFCO method places horizontal drains at the bottom of narrow sand-filled trenches in the clay, says Robert Van Dorp, of the developer IFCO Funderingsexpertise B.V., Waddinxveen.

Menard’s main vulnerability is the need to keep its membrane airtight, says David Potts, head of soil mechanics at London University’s Imperial College. But that is not hard to do, he adds. "We have to do this for our waste disposal sites and they have to last many years," he says.