Vertical drain in peat bog

GEO-TECHNICS AMERICA, INC

Vertical wick drains dewater peat bog under parking lot
for new high rise building

A12-story office tower went up on schedule in suburban Chicago despite an unexpected peat bog that threatened to delay the entire project. Vertical drains and a heavy earth surcharge squeezed the water out and avoided the added cost and delay of removing and replacing the soft subsoils.

James McHugh Construction Co., Chicago, constructed the 12 story office tower--the first stage of a commercial building complex--under a multimillion dollar contract with the Hamilton Partners, a major developer active in the Chicago area. McHugh's 12-month contract covered site development, the building, and its 600-vehicle parking lot on the initial 8 acre site, and access, drainage and utilities for the entire 38 acre complex.

Needle-like mandrel vibrates vertical drain through filter fabric and hard crust of frozen ground, then pushes drain as much as 30’ to bottom of bog in less than a minute. The peat bog was covered with filter fabric and a 9" lift of crushed stone before wicks were installed.

The problem

The unexpected problem soils were about 60,000 sq. ft. of peat and organic clays extending some 14 to 22 ft. below grade near the northeast corner of the office building. Those soils either had to be removed and replaced or stabilized.

"Excavation and backfilling of the soft soils would have cost an estimated $250,000," said John Jurewicz, assistant superintendent for McHugh. "It would have also delayed the project at least 30 days because of curtailed access, and perhaps longer if the removal undercut the building's foundation.

The Solution

"By contrast, stabilization would cost about $110,000, but it could be done independently of the building work and avoid the delay that would otherwise occur," he added. "That's what we did."

Stabilization would include spreading a woven geotextile across the still-frozen ground surface and placing a 9 in. lift of crushed stone on the filter fabric to act as a drainage layer. That drainage layer would also distribute the weight of the equipment used to insert the vertical drains to the full depth of the compressible soils.

A herringbone system of drain pipe would be installed within the drainage layer, which would be topped with a 9-ft.-deep surcharge of heavy clay to squeeze water out of the compressible soils. As much as 24 in. of settlement was expected.

Once the plan was resolved, work proceeded quickly. McHugh’s crews hand-spread more than 100,000 sq ft of Exxon GTF 200 woven filter fabric, lapping all seams at least 3 ft.

About 2500 tons of crushed 3/4 in. stone was dumped along one edge of the filter fabric and dozed forward over the fabric by a Komatsu D65P low ground pressure dozer to form the 9 in. drainage blanket. Placement of the fabric and stone took only three days.

More than 31,000 ft. of AMERDRAIN^Ò vertical drains were installed in the next three days. The drains consist of a flat, 4-in.-wide sleeve of nonwoven filter fabric surrounding a core of longitudinally corrugated plastic. The subcontractor -- Geotechnics America, Inc., Matthews, NC -- used their vibro-static installation rig on 36-in.-wide, low ground pressure crawlers and carrying a 62-ft.-high mast.

A needle-like, 50-ft.-long, hollow steel mandrel in the mast vibrates through the stone and filter fabric and pushes vertically through the soft soils to the bottom of the peat. A piece of rebar strung through the looped lower end of the drain anchors the drain in position as the mandrel withdraws.

When the mandrel is out, the drain is cut and the new end doubled over another rebar at the bottom of the mandrel. The boom is swiveled to the next location and the next drain inserted to a depth of as much as 30 ft.

Placing each drain takes only 45 to 60 seconds. Moving and positioning the excavator for the next drain takes longer.

In plan view, the drains are inserted in a 5 ft. by 5 ft. diamond-shaped pattern. In practice, three drains are placed from one position by simply swiveling the excavator. It then moves forward 5 ft. and places the next three drains. About 1500 drains were installed in three days.

A McHugh crew followed a day after the vertical drain work installing the horizontal drain system to collect water from the vertical drains. The crew used a small tractor/loader/backhoe to install 750 ft. of 6-in. spine and 4-in. laterals in 12-in. trenches. A perforated heavy wall PVC pipe was used.

The surcharge

A subcontractor placed 24,000 cu. yd. of heavy clay surcharge in one week's time using six Terex TS-14 scrapers. The first 2 ft. lift was spread with a Caterpillar D5 low ground pressure dozer. The scrapers placed the rest of the surcharge in 9 in. lifts. Surcharge material came from the excavation for an on-site retention basin less than 1000 ft. away. Placement of the surcharge was delayed a month while the ground thawed sufficiently for excavation of the basin to start. Settlements up to 24" were expected. The surcharge was removed and spread as site grading before paving the parking lot.

Project Manager Bud Firstenberger, Project Superintendent Greg Engleman and Assistant Superintendent Jurewicz directed McHugh's crews of 40 to 50 and coordinated the efforts of another 10 to 20 subcontractor personnel. Most crews worked five 10-hour-days a week. STS Consultants, Northbrook, Ill., did the geotechnical investigations and developed the settlement plan. []


Herringbone-like horizontal drain system is installed to collect water squeezed into the crushed stone blanket.		A 9’ thick surcharge of heavy clay is placed atop vertical and horizontal drain systems to squeeze water out of the bog and compact the soft subsoils.

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