Meridian City Hall

Story by Stephanie Worrell | September 28, 2010

In the regular world, surprises can be fun. In the construction world, most often ... they are not.

That was exactly the situation when construction manager Petra Inc. (Meridian, Idaho) discovered a few surprises during site preparation for the Meridian City Hall building that began back in April 2007.

Contaminated ground, a higher-than-expected water table and unusual weather set the aggressive construction schedule of the $16-million, 100,000-square-foot, three-story project back a couple of months.

In spite of these adverse conditions, Petra will be completing the project five weeks ahead of the contract requirements of 18 months.

The project also is gunning for a silver LEED certification, which added special considerations throughout construction — including concrete placement and material.

"We knew there was some contamination on the construction site that used to be an old creamery," said Adam Johnson, project engineer for Petra. "And, we also knew the water table was going to be a little high. But when we got in there, we found that both of these issues had been underestimated."

Approximately 16,000 cubic yards of the site was contaminated with petroleum-based material and unsuitable soil. The material was free-flowing and presented special consideration during the cleanup to keep the contaminated material from entering the groundwater, and still dewater the site for the basement excavation. A unique situation presented itself when an impermeable clay layer was discovered on top of the groundwater.

Through value engineering efforts of Petra, Lombard Conrad and the city of Meridian, it was decided to raise the structure four feet, creating a grand entrance to the building, while maintaining the impermeable clay layer beneath the basement and protecting the groundwater of Meridian from the contaminated material on the site.

"We had to work with the EPA through remediation to ensure the site was clear of contamination," said Jack Vaughan, Petra's project superintendent. "We were able to get it cleaned up and certified safe within a couple of months, which is pretty remarkable."

Structural fill was brought in, and a geo-fabric barrier was utilized because the water table was so high on the entire area planned for the building's pad — about 18 feet below the roadway.

Seven wells — four artesian — also were discovered. Storm lines that ran down the adjacent roadway also added to the water issues. More structural fill was trucked in to account for the higher water table. The site is now almost five feet higher than called for in the original plans.

"We didn't know the volume and extent of the old open-trench irrigation system that ran alongside the site," said Johnson. "The alternative to bringing in fill was massive pumping, which would have set us back even further.

"Water stop was used extensively in the basement footings to the floor connection; this provides an envelope the water cannot easily penetrate. The exterior membrane filters or wicks the water from the walls to the drainage system, leading to the storage tank."

Pads and Footings

Only delayed by a couple of months, the pads and footing work began in June. Crews then faced one of the coldest fall and winter seasons in recent Idaho history. A special recycled concrete made with fly ash, which adds to the recycled content for LEED criteria, was utilized. The mix includes a waterproofing agent, a mid-range water reducer and 3/4-inch stone aggregate. The 18- to 21-inch-thick mat slab sits on pilings.

"Fly ash for this project was not a massive increase in cost; it was specified by the structural engineer for its concrete-enhancing abilities (early strength and stronger bond) and it just so happened to benefit in the LEED category," said Johnson.

In addition, a waterproof membrane beneath the concrete foundation provides an extra layer of protection. With water stop and membrane to deal with the water table, Petra engineers had to figure out how to pour the floor without having any joints in the membrane. Access to the site was also an issue related to damaging the membrane, so back fill and 4 by 4 by 8 economy blocks were utilized to hold things in place until the floor was poured.

A honeycomb membrane surrounding the foundation storm drain systems keeps everything in one place and protects against future issues if the water table rises.

Sidewalk's LLC (Meridian, Idaho) called in a fleet of Schwing 52-meter truck-mounted concrete pumps from Brundage Bone Concrete Pumping (Boise) to place 28,000 cubic yards of concrete needed for the project. One of the larger pours came in July 2007, when the Schwing truck-mounted concrete pumps placed approximately 186 yards for foundation floor in six hours.

A Somero laser screed was used to assist the placement of the basement slab in coordination with manpower, which takes a lot of time on hands and knees floating it out.

Tenting was used to block off sections of deck from above, and decks were covered with plastic and followed by concrete blankets with heat underneath to keep things above from freezing — and, a few times, it was a lost cause because cold weather placement was delayed. During delays, steel erection and mechanical rough-ins continued.

Concrete in Cold Weather

Because concrete is essential to almost every project, one of the challenges in winter construction is how to work with concrete in a season when temperatures are lower than ideal. Common industry standards say that concrete cannot be below 40 degrees Fahrenheit when it is poured or curing. And its optimum temperature is between 60 degrees and 80 degrees. Making concrete work properly in cold weather begins with making sure the air temperature is warm enough to make a pour.

"If the air temperature is below 20 degrees — which often was this winter — things get difficult," said Johnson. "If the temperature is below zero, you can't pour."

To enable projects to move forward during cold weather, three general solutions exist. They include adding heat, retaining heat and using concrete additives or special mixes. Heat can be generated at the concrete mixing site, the pouring site or both to help create a successful pour. Concrete producers add heat at the mixing site by using hot water when mixing the concrete or they also can heat the aggregate prior to adding it to the mix.

Onsite, forms are heated and heat is retained by creating enclosures. Accelerator additives can be utilized to help the concrete set up more quickly, along with plasticizers that help it flow better at low temperature.

A Nice Focal Point

On any given day, 90 to 100 employees of contractors and subcontractors are working on the site. At press time, the exterior brick veneer shell was in place and most of the concrete work was complete. Interior LEED-certified finishes were being put in place inside the building. Johnson noted that 80 percent of waste is currently being diverted to the landfill — including steel, rebar, plastics, and wood materials. Approximately 20,000 bricks were saved from the old creamery structure for the skin of the building.

Lombard-Conrad Architects (Boise, Idaho) is serving as the project architect.

Barring contamination, water and weather, the project is still on target to be turned over to the city by Oct. 15, 2008.

"We have had an outstanding group of trades on this project," said Vaughan. "They have worked hard and helped find solutions to the many issues we've encountered on this project. This is going to be a nice focal point for the city of Meridian."

Upon completion, the Meridian City Hall will serve as a unique epicenter for the citizens of that community. The project includes many spaces where the community can gather beyond city government business. An outside amphitheatre, foundations and open gathering areas make the building multi-user friendly. And community leaders are excited that the project has equated to economic growth and development for the city.

"We are already seeing increased interest from developers proposing projects in the surrounding blocks to capitalize on the dynamics a 100,000-square-foot City Hall will create in the downtown core," said Meridian Development Corp. President Shaun Wardle.

So, in the end, the surprise of a successful project — even though it was tough getting past the construction hurdles — will mean a long-standing legacy for an Idaho community.