Vertical drains underwater

GEO-TECHNICS AMERICA, INC

GTA Installs Vertical (Wick) Drains Underwater

Some 25,000 vertical wick drains were installed for the U.S. Navy's Home Port Facility at Pascagoula, Mississippi. It took just four months to install the 1.3 million linear feet of drains. More than 4000 of the drains were placed underwater, the first such installation in North America.

Geo-Technics America, Inc. (GTA), Matthews NC, installed the drains under its subcontract to National Projects Inc., Mobile, Ala., the general contractor for the port. About 800,000 feet (244,000 m) of the wick drain was installed under 12 building sites located over soft ground. The remaining 500,000 feet (152,400 m) went into the foundations for a new berthing pier.

The AMERDRAIN^Òvertical wick drain used on the job is manufactured by American Wick Drain Corporation. Each roll holds 1000 feet (305 m) of a flat, 4"(100 mm) wide sleeve of nonwoven filter fabric surrounding a core of longitudinally corrugated plastic.

Once the drains are installed and a 20-foot (6 m) thick sand surcharge placed over them, water is squeezed out of the subsoil through the filter sleeve and up the drain into the sand surcharge.

Working on land, large crane reaches out as much as 90 feet (27 m) as a 95 feet (29 m) long hanging mast installs wick drains underwater.

Underwater vertical drains

For the underwater work, GTA used a 95’ (29 m) long mandrel and hanging mast suspended from a 150-ton Manitowoc 4000 Vicon crane. The crane had a 400HP diesel power pack mounted above the counterweight to power the hydraulically-operated crowd chain and vibrator for the mandrel. The crane worked on timber mats laid out along the shoreline parallel to the 4’ (1.22 m). square pattern of the wick drains. Two rows of stakes and laths beside the timber mats indicated the location of each line of drains.

Drain installation started with the outermost row located 90’(27 m) from the crane. A steel tape was used to establish the 90’(27 m) spacing, which was accomplished by changing the boom angle and locking it at the correct position. The crane then simply walked the full length of the outer line of drains (nearly 500 ft.), stopping every 4 feet (1.22 m) over the laths to insert each drain.

Operator sights over row of stakes and laths between timber mats and shoreline to align each row of drains. Crane boom is pre-positioned and locked to reach a specific row of drains. One laborer on a small barge cuts and reloads wick drain, while two others man engines and the PVC spuds used to hold barge in position.

The boom was then repositioned for an 86’(26 m) reach, and the second row of drains was installed as the crane tracked down the timber mats. The mandrel and hanging mast assembly weighed 24,000 lb. That much crowd can be applied to push the chain-driven mandrel through the soft bottom silts into an underlying layer of sand. The bottom of the drain stays anchored in that sand as the hollow mandrel is lifted. Once the mandrel clears the water surface, a laborer standing on a 20’(6 m)-ft.-long barge cuts the drain. The laborer then laps the new free end of the drain around a 9" length of ½" rebar (230x12 mm) and back into the tip of the mandrel for the next insertion. The crane then moves forward to the next pair of laths and reinserts the mandrel at the next location. Each cycle takes about three minutes. The barge supporting the laborer was positioned and held parallel to the row of drains being installed by lightweight spuds. Those spuds were 6" (150 mm) PVC pipe pushed into the silty bottom. After five drains were positioned, the barge was moved forward another 20’(6m) by two 50HP (37 KW) outboard engines and the spuds repositioned in the silts. "On this job, the work was close to shore and the water was shallow. Had the drains been further from the shore and in deeper water, we would have walked the crane along a 300’(100 m) barge floating in the deep water," Russ Joiner of GTA said

Other wick drains

GTA installed 30-65"(9-20 m) long drains under various building sites using two other rigs. One has a 70‘(21 m) high mast attached to a modified Koehring 266 hydraulic excavator. The other, also a 70’ high fixed mast, is attached to a Caterpillar 235 hydraulic excavator. The additional weight and horsepower of the Caterpillar rig made it a favorite for production, but the modified Koehring was the more versatile due to its light weight and lower ground pressure - less than 6 psi (41 kPa) versus about 9 psi (62 kPa) for the Cat. Each of these rigs was able to place two drains from a single position before advancing the machine, rather than having to move the crane between each location. That reduction in travel time increased the production of the excavator-mounted units to about 12,000’ (3660 m) of drain per 10-hr. day. The crane-mounted unit, by comparison, averaged about 8000’ (2440 m). of drain per 12-hr. day. The crane-mounted rig was also used on nearly 3000 deep drains required along the shoreline close to the berthing pier. These drains, the majority of which ranged from 70-72’ (21-22 m) long, exceeded the practical depth capabilities of the two excavator-mounted rigs.

Modified hydraulic excavators with 70' high masts install shorter drains under building sites. The crane installed the deeper land drains.

For additional information about the Pascagoula project, E-QUAKE Drains, 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