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Polk Parkway Paving Slips on Mining Slime

Wed March 22, 2000 - Southeast Edition
George Graham

Road building contractors are all too familiar with the problems encountered in Florida — shallow, sandy soil, igneous limestone, environmental restrictions and water, water everywhere. But problems facing the state Department of Transportation in constructing the recently completed Polk Parkway were different.

Before they could begin construction of the 40.2-kilometer (25 mi.) toll road circling the city of Lakeland, engineers had to figure out how to overcome soil conditions resulting from decades of phosphate mining.

“It was a very special challenge,” said Turnpike District Construction Engineer Charles Wegman. “With problems such as water, you call in subcontractors who specialize in that kind of work. They’ve been doing it for years. With phosphate slime, we faced a different set of circumstances.”

“Until the mid-l970s, when phosphate companies were required to start reclamation, they scooped out fingers with dams between as a means of disposing of tailing sand and clay,” said project manager Neal Penny of Parsons Brinckerhoff Construction Services. “After the area was mined out, the company moved dirt over the fingers, which resulted in unstable sand and water being trapped beneath the covering layer of sand and dirt. This slime and water shifts horizontally beneath the top layer, so while it is fine land on which to graze cattle or grow oranges, it was not suitable for other uses.”

It certainly wasn’t suitable for road building.

“The builders were dealing with unknown soil conditions, as records were not required on where or how deep the settling areas were,” said Penny. “The clay prevents the water from being reabsorbed into the aquifer, accounting for the unstable soil condition. This is a special problem encountered by developers and builders in Polk County and hardly anywhere else in the world.”

The slime deposits encountered in the project were sometimes 30 to 40 feet deep.

The turnpike’s designers decided on three main approaches. They completely removed the muck and one large slime pond; designed a 426.7 -meter (1,400 ft.) bridge over one deep pocket of clay to replace the 60.9-meter (200 ft.) bridge that would normally have been built; finally, they used surcharging and wicking to consolidate the most stubborn slime deposits.

“Two methods of surcharging slime materials were included in the design,” said engineer Murray Yates of Metric Engineering Inc., who supervised the project for the Department of Transportation. “A 15-foot reinforced embankment and a 5-foot non-reinforced embankment were used.”

Yates said the RECo design was used for reinforcing the surcharge embankment system, and RECo provided geosynthetic material manufactured by Nicolon Mirapi. The reinforcement design called for one or two layers of geosynthetic fabric having strength requirements for total fill heights of 8.5 to 9.1 meters (28 to 30 ft.).

As a special design feature, wick drains were used.

A wick is a vertical strip drain about 10.2 centimeters wide and .32 to .64 centimeters (4 in. wide and .125 to .25 in.) thick. Wicks come in rolls of about 304.8 meters (1,000 ft.), and consist of flat sleeves of nonwoven filter fabric surrounding a core of longitudinally corrugated plastic.

Turnpike contractors had earlier used the technique to consolidate muck pockets for a 19.3-kilometer (12 mi.) expressway in Seminole County. The muck pockets were old sink holes that had filled in over hundreds of years.

“The use of wick drains was a controversial design issue,” Yates said. “The turnpike’s consultant construction staff fought to include wick drains in the design. Results indicate the wick drains have played a significant role in the rate of fall in pore pressure, allowing an accelerated settlement to occur.”

To monitor the settlement, engineers used settlement plates. To measure pore pressure in the soil, they used pizzometers. And to measure horizontal movements along the perimeter of the surcharge during construction, they used inclinometers. This equipment was installed by Bromwell and Carrier Inc. of Lakeland.

“Simple surcharging as a soil consolidation method would have taken three to five years,” Yates said.

“The design called for the surcharge to remain in place for two years. And we were able to cut that time in half. This was primarily attributed to the use of the wick drains.”

Yates described the operation this way:

“The contractor, Granite Construction Co. of Watsonville, California placed 491,000 cubic yards of surcharge materials. They used Hitachi EX 700 excavators to excavate the surcharge. To haul the surcharge, they employed Volvo A-40 articulated off-road dump trucks with a capacity of 29 cubic yards.

The surcharge material was then spread with a Caterpillar D6 dozer and compacted into 1-foot lifts using Ingersoll-Rand Model SD 100 self-propelled rubber and steel vibratory compactors.”

Nilex Corp. of Englewood, CO, installed the wicks.

“The wicks were installed vertically,” Yates said. “The tops of the wicks terminated in a 2-foot layer of sand, allowing the water to flow laterally out of the embankment. The wicks were installed by employing a Cat 325 hydraulic backhoe.

“Each wick was enclosed in a tubular steel mandrel measuring 2 by 5 inches. A small steel anchor plate was attached to the drain at the bottom of the needle-like mandrel. Nilex technicians pushed the mandrel down through this sticky material and anchored it in the underlying soil.

“When the desired depth was reached, the mandrel was extracted. The anchor plate retained the drain in the soil. When the mandrel was fully extracted, the drain was cut off, a new anchor plate was installed and the process began again,” continued Yates.

It took about nine months to build the two large surcharge areas, Yates said.

Once the road builders had solved the problems inherited from phosphate mining, construction could proceed on the $471-million parkway, linking major Polk County cities to each other and to Interstate 4.

“The entire project was an example of successful partnering, including design, production, construction and consulting engineers, including geotech Dick Hawkins,” Penny said. “The Turnpike District was lucky to engage the services of Metric Engineering as CEI and Granite as contractor. These two companies continued the cooperative partnership through completion of construction.”

The project’s completion was a long-anticipated event. Christa Deason, the parkway’s information officer, said the dream was born nearly two decades ago when local business and political leaders visualized a county road that would link the major Polk municipalities and provide the easy access so vital to commerce and development. But a critical element was missing — funding.

With the formation of the Blue Ribbon Imperial Parkway Task Force in 1988, the county intensified its efforts to realize the dream, Deason said. Then when the bonds for building the original Florida Turnpike were paid off, the Florida Legislature decided to use the turnpike’s toll revenue for badly needed road-building projects throughout the state.

Deason credits the Blue Ribbon Imperial Parkway Task Force with making such a compelling case for Polk County that the long-dreamed-of access road was included in the projects approved by the Legislature.

The Polk Parkway was born.

In 1996, contractors broke ground for the 40.2-kilometer (25 mi.) toll road. The first section, from I-4 at County Line Road to US 98, was completed in August 1998. The next stage, from US 98 to US 92, was opened in August 1999. Finally, the remaining 12.1-kilometer (7.5 mi.) segment from US 92 to I-4 at Mount Olive Road in Polk City, opened to traffic on Dec. 12.

By 2001, the Florida Department of Transportation, using toll revenue and Florida’s Turnpike bonding capability, will have added more than 100 miles of new roads to Florida’s Intrastate Highway System. The collected toll revenue has also funded the construction of 11 new interchanges and additional lanes on the Turnpike’s mainline, improving access and traffic flow.

Florida’s Turnpike District is one of eight districts within the Florida Department of Transportation, overseeing a 401-mile system of limited-access toll highways. Its “Main Street” passes through 11 counties from a junction with Interstate 75 in north central Florida.

Before construction of the Turnpike, most travelers through Central Florida used US 27. Today, Florida’s Turnpike handles the bulk of traffic between Central Florida, the east coast and South Florida.

Florida’s Turnpike system is funded entirely by tolls. The Turnpike District receives no state gas tax or federal tax revenue and the state is facing a $22-billion shortfall in funding identified transportation improvements through the year 2010, according to official projections.

For information on turnpike construction, call 850/488-4671.

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