Vertical drain for marine terminal

GEO-TECHNICS AMERICA, INC

Vertical (wick) drains cut a year from marine terminal construction time

More than 3,000,000 linear feet of vertical drain, the largest such installation ever in this country at that time, and 300,000 sq. yd. of an extra heavy geotextile saved a year of construction time on a new marine terminal in Baltimore.

The 113-acre disposal area for spoil from the I-95 tunnel under Baltimore harbor was consolidated for development as the SeaGirt container facility. C.J. Langenfelder & Son, Inc., Baltimore, was the general contractor on the $10.9-million, 600-calendar-day project for the Maryland Port Administration.

"It is all on a fast track schedule," said Ron Lange, project engineer for the Administration. "The heavy duty geotextile is essential to support the equipment installing the wick drains. If we didn't use the wicks, the surcharge would have to remain for a settlement period of at least an additional year to adequately consolidate the spoil."

The pumped spoil in the disposal site was almost liquid in most areas. The surface crust would support a man, but not construction machinery. Some areas would not even support a man, and equipment was safe only in a few areas along the old shoreline.

An extra heavy woven geotextile placed on top of the spoil distributed loads to the point that a man was safe, even in the softest areas, where movement felt like walking on a water bed. A 2 ½’ blanket of sand was placed over the geotextile and capped with a 6" layer of crushed slag to provide barely adequate support for heavy equipment.

Equipment works on slag surface

Subcontractor Geotechnics America, Inc. worked off the slag surface while installing the vertical drains with one of their vibro-static drain installation rigs. It had 42-in.-wide crawler tracks to reduce its ground pressure to 6 psi. A mandrel in the mast acted like a needle while inserting the wick drain from 16 to 50 ft. down into the soft soil.

The toughest part of the installation was punching through the heavy geotextile, which was usually done with the rig's 15-ton static force. On occasions, a short burst of vibratory power helped. Once the pointed mandrel tip pierced the geotextile, the mandrel was pushed almost effortlessly to the bottom of the spoil and withdrawn leaving the drain behind.

After the mandrel was withdrawn, a laborer cut the drain material with hand-held hedge trimmers, doubled the loose end back into the eye of the mandrel and inserted a 9" length of ½" rebar to hold the drain snugly in position for the next insertion.

The GTA installation rig with wide-tracks and 56’ mast still sinks into soft qround while installing more than three million feet of vertical drains to drain the wet dredgings under new port site. Mandrel is withdrawn from soft goo and top of drain is cut from reel with hand shears.

One wick every 30 seconds

GTA regularly installed one drain every 30 seconds as the rig rotated from side to side inserting two or three rows of drains from a single position. The drains were spaced about 5’ apart in a diamond-shaped pattern. Daily production, including downtime to change reels of drain material or for major movement of the machine, ranged from 10,000 to 18,000 linear feet installed per 10-hour day.

The GTA installation rig has a self-erecting mast will permits quick set-up and movement from one part to the job to another. The mast can be lowered to pass under utility lines. With the mast lowered, the 70,000 lb., 11 ½’-wide rig can be walked onto GTA’s special four-axle, hydraulic beam trailer, jacked up to provide 6" of clearance under its tracks, and moved over the highway as a 112,000 lb., special-permit load when moving from job to job.

The AMERDRAIN^Ò 407 vertical wick drain used on the job was manufactured by American Wick Drain Corporation. Each roll consists of 1000’ of a flat, 4"-wide sleeve of nonwoven filter fabric surrounding a core of longitudinally corrugated plastic.

Water is squeezed out

Once the drains were installed and a 7-9’ earth surcharge was placed, water is squeezed out of the spoil through the filter fabric of the vertical drains and up the drain core for discharge into the sand blanket. An underdrain system in the sand blanket collected the water and conveyed it to a sump for pumping into a settling basin prior to discharge in the bay.

The underdrain is a 6" diameter corrugated, perforated polypropylene pipe installed within the 2 1/2' sand blanket by one of Langenfelder's crews using a Vermeer V-430 trencher.

Horizontal drains are installed in 2 ½’ sand blanket to collect water to be discharged from vertical drains after a heavy, earth surcharge is placed.

The extra heavy geotextile spread over the surface of the spoil was Nicolon's 62809 woven fabric consisting of cords of polypropylene in one direction and polyester in the other. The fabric weighs nearly 2 lb. per sq. yd. and has a tensile strength of more than 1,000 lb. per lineal inch in each direction. The fabric, which comes in 1,200 lb. rolls about 16½’ wide and 270’ long, was towed to the site by wide track dozers, unrolled and hand-spread by laborers. Seaming was done with a special heavy-duty electric sewing machine hung on a small Steiner farm tractor. The sewing machine is powered by a Homelite HG 1400 portable generator mounted on the rear of the tractor. The longitudinal joints are double J-stitched in which two layers of geotextile are lapped, folded over, and the four thicknesses of material (about a third of an inch) stitched together with a heavy polyester "thread." In practice, this takes a crew of six to eight laborers to support and shape the fabric seams for the sewing machine operator.

Sand spread over geotextile

About 250,000 cu. yd. of sand was spread over the seamed geotextile to a 2½' depth. The sand was hauled in from off-site locations by lngram Trucking, Co., Baltimore, and spread by two small Caterpillar D3B and one Komatsu D31P wide-track dozers. Slag was placed in a similar manner after the sand course had been leveled by the small dozers dragging their blades. After the vertical drains and underdrains were installed, the 210,000 cu. yd. surcharge was built up in 1’ lifts to provide the load to squeeze water out of the subsoils. "Sud" Cockey was project manager for Langenfelder coordinating a work force of 30 to 35, plus truck drivers, on the project. Crews typically work five 10-hour days a week.

A 2 ½’ layer of sand and 6" of slag are spread over geotextile by wide-track dozers.

For additional information about the SeaGirt project, other GTA contracting services, or for assistance with your next ground improvement project, call us at (770) 719-9495, fax us at (770) 719-1907, or email us at RussJoiner@msn.com