Weston Place Project
Digging Deep for 12-Story Condo

by Elaine Schmidt

There's more to Madison's Weston Place condominium development than its simple exterior.

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Four levels of underground parking, which required an excavation into bedrock and a soil-nailed system of earth retention, added complexity to the project for Madison-based general contractor Stevens Construction Corp.

Work began on the 121-unit, $23 million structure in mid-March 2004. The core and shell of the 12-story structure in the Hilldale neighborhood near downtown are scheduled for completion in August.
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Dealing with Uneven Bedrock

Geoff Vine, vice president of Stevens, said the 25,000 cu. yds. of concrete required on the project and the tight, urban site have presented time and logistical issues.

"We don't have much space just a small lay-down area on part of the site," Vine said. "A lot of our stuff has to be stage in our yard and brought to the site on the say use and lifted up onto the building."

But the project's biggest hurdle was encountered below grade in an uneven layer of bedrock, which varied in strength from area to area throughout the site. Excavating the 135 ft., 260 ft. hole to a depth of 50 ft. required a significant earth retention system.

"We had about 20 to 25 ft. of overburden over bedrock," said Eric Bahner vice president of Edward E. Gillen Co. of Milwaukee, the firm that designed and built the earth-retention system. Although the soil overburden could be excavated somewhat simply, the bedrock was another matter.

"Our real problem was evaluating the bedrock," he said. "We had soil borings, but they gave completely different pictures of how competent the rock was depending on where the boring was taken."

This meant that it was impossible to get a clear picture of how prone the rock would be to raveling and destabilization during excavation.

"The work required jack hammers on the ends of excavators in some places," Vine said. "Workers were constantly hammering and chiseling to get to the final footing depth."

But areas held rock that was no better than the overburden soil. As a result, some areas of the excavation required some fairly technical stabilization.

"We looked at this geology and decided that soil nailing was the system best suited to this set of conditions," Bahner said. Even so, the varying conditions required almost daily evaluations to determine the specific course of action for each individual area.

Soil Nailing Done

The soil-nailing method employed on the site's weakest areas involved soil anchors that were screwed into the active zones to reach a passive, or more stable, zone behind it. The soil anchors were placed at close spacing to create a block of reinforced soil. A protective layer of shotcrete was then applied to the face of the nailed soil.

This method eliminated both the possibility that ongoing excavation would undermine the already excavated areas above, as well as the possibility of rock falls from the excavated areas, which would pose a hazard to those working below.

Bahner, who has done some writing on soil-nailing recently, said it is a method of earth retention that came to this country in 1976 from from France and Germany, where it had its roots in different types of tunneling techniques. First employed in Portland, Ore., its use has slowly spread eastward. Soil nailing was first used in Wisconsin in late '80s and early '90s.

"There are probably only two companies in Wisconsin using soil nailing and maybe six or eight in the upper Midwest," Bahner said. He added that the success of a soil-nailing process is heavily dependent on the workers on the jobsite.

"Soil nailing is one of the most craftsmanship-intensive systems we build," he added. "It's not only me drawing lines and doing calculations, but it's the guys who work in the field to build the system for what we say it will cost and keep people safe."

Bahner added that other issues on the Weston Place site included proximity of the dig to a bank, an apartment building and a city street. The drive-through lane of the bank ran alongside the excavation.

"We took every possible safety precaution to keep the public safe," he said.

"But I'm sure our excavation got the attention of the bank customers in the drive-through lane."

For Vine, the complexity and size of the excavation presented another set of concerns.

"By the time we went down to the bottom of the hole and came back up again, we had gone through half the schedule and at least half of the budget," Vine said. "That can be hard for an owner to understand because it doesn't look like a lot of progress."

Above-Grade Issues

The 400,000-sq.-ft. building clad in precast panels is a shell above ground. But erecting a post-tensioned concrete structure requires significant planning and plenty of workers.

"We have about 50 Stevens people onsite forming and pouring concrete," Vine said. He added that crews began pouring concrete on the site in June and will continue through this month.

A flying-form system known as "minitables" allows the same forms to be reused from floor to floor. But pouring concrete through four seasons of Wisconsin weather demands additional considerations.

"We enclose the floor below each pour using a blanket system that encloses the outside of the building," Vine said. "Then we supply temporary heat in the space underneath the slab we are pouring."

That heat, combined with the heat generated by the exothermic reaction within the concrete, makes it possible to pour throughout the winter. Then comes the issue of the pulling of the tension material.

"The concrete has to reach 3,000 psi on all areas of the slab before we can pull the post-tension material," Vine said. A special mix of concrete, combined with the heat from the floor below the slab, keeps the concrete reaction uniform throughout the slab.

Constantly watched in-slab monitoring systems keep track of temperature changes within the slab, providing an indicator of the psi. Test cylinders, monitored by a testing firm, are also used.

Vine said that experience on this job has shown that the concrete has reached sufficient strength for the pull within 24 to 36 hours.

The floors require a good deal of layout work before each pour. The plumbing sleeving for the various units (13 units per floor on the lower floors) has to be laid out and encased by the pour and any penetrations for electrical and HVAC work have to be included as well. All of the rebar and post-tension materials also have to be laid in place before each pour.

Condos in the building will range in size from 8,000 sq. ft. to 2,600 sq. ft. The "white box" prices - costs before customization and build-out - range from $162,000 to $1.5 million. Build-out on the individual units is slated to begin in June and will continue for the next three years as the units are sold.