Vertical Strip Drains Increase Dredged Material Storage Area

March 04, 1999

CHAMPAIGN, Ill. -- Dredging the bottoms of harbors and shipping channels to permit the passage of existing and larger ships may keep commerce flowing smoothly, but it also creates huge mounds of mud that must be disposed of. Once confined containment areas are full, more must be built, at high cost.

Now, a team of researchers -- led by a University of Illinois engineer -- has found that using prefabricated vertical strip drains offers an economical way to significantly increase the storage capacity of existing facilities.

"Vertical strip drains were originally developed to help consolidate the soft clay deposits often found under building sites and road construction," said Timothy D. Stark, a U. of I. professor of civil and environmental engineering. "They consist of thin plastic strips -- with grooves to help channel water flow -- surrounded by a geotextile filter that prevents soil from clogging the drains."

Punched far into the ground, the drains provide a path for trapped water to escape, Stark said. And, as the excess water pressure is removed, the ground is free to settle or consolidate.

To test their idea, Stark and his colleagues -- Thomas Williamson of GeoSyntec Consultants, Jack Fowler of GEOTEC Associates, and David Pezza and Yvonne Gibbons of the U.S. Army Corps of Engineers -- installed vertical strip drains in a 600-by-400-foot test section of the Craney Island Dredged Material Management Area. This man-made storage site -- located near Norfolk, Va. -- is the placement area for material dredged from the channels and ports in Virginia's Hampton Roads area.

"Because vertical drains had never been installed in an active disposal area before, we encountered a number of challenges," Stark said. "Not only was the 160-foot drain length close to the longest drain ever installed, but the installation equipment had to operate on the surface of soft, saturated dredge material."

When the installation was completed, the test section was outfitted with monitoring equipment to evaluate the drains' effectiveness in consolidating the dredge material.

"After only 30 months, the test section had settled more than 8 feet," Stark said. "Multiply that by the 2,000 acres available in the containment area, and you have greatly increased both the storage capacity and the service life of the facility."

For comparison, the measured settlement rates prior to vertical-drain placement averaged about 4 inches per year, Stark said. "Thus, the installation of vertical drains significantly accelerated the rate of consolidation of the dredged fill and underlying clay."

Stark and his colleagues published their findings in the February issue of the ASCE Journal of Performance of Constructed Facilities.

University of Illinois at Urbana-Champaign

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