Idaho's Capitol Goes Underground

Story by Stephanie Worrell | September 28, 2010

Mechanical installations proceed inside the underground wings.

In many cases, public structures are designed to be outwardly visual, even spectacular to the human eye. This is especially true for a typical state capitol, but the current Idaho State Capitol project is not typical — at least not the two-wing, $27.5-million statehouse addition project headed up by McAlvain Construction of Idaho.

The wings will have historical relevance, but essentially the project is all underground and will not be appreciated fully from the sidewalk. Visitors will just need to "dig" a little deeper to appreciate the unique addition to Idaho's State Capitol building.

"This is not your usual statehouse or even design/built project," said McAlvain spokesperson Kirk Davenport. "This is a sub-grade expansion, plus we are connecting to a 105-year-oldbuilding underground with tunnels from the wings to the basement. It has been challenging, yet successful."

The two wings are part of a larger Idaho State Capitol building facelift aimed at sprucing up and expanding the historic building. The project includes, among other things, repairing original marble, decorative plaster and wooden doors; upgrading electrical, sewer and ventilation systems; and adding 50,000 square feet in two underground wings. McAlvain is the general contractor for the shell and the core of the wing project, while Jacobsen-Hunt Construction (Utah/Arizona) is tasked with the interior of the main Capitol Building, along with the interior of the wings.

Design of the wing expansion project began in early November 2006, and the completion of the core and shell portion is scheduled for Dec. 10, 2008. All of the sandstone, concrete and steel have been secured through local Idaho sources, which was 95 percent of the materials. And, McAlvain has employed 100 percent of its work force (including subcontractors) for the project locally. Approximately 5,200 cubic yards of concrete has been poured — all secured through Idaho Concrete (Caldwell, ID).

"At the peak of the project, there were upwards of 125 people working on the site," said Davenport. "But, typically, there were 60 to 75 on site."

Two Stories to One

The two wings, measuring approximately 25,000 square feet each, are being constructed on the east and west side of the existing Idaho State Capitol building. Offices, conference rooms, a kitchen dining area and an auditorium-style large hearing room are being added, along with exterior stairs for underground access. Skylights are being used throughout to enhance light to the space. Originally, the project called for a two-story, underground wing addition, but the project hit a snag due to budget constraints.

Idaho Gov. "Butch" Otter stopped the work because of budget concerns in January 2007, less than two weeks after taking office, and then cut a deal with Republican legislative leaders to allow two underground wings as long as they were one story each (not two, as originally planned) and the space was primarily devoted to public hearing rooms. The entire project, including a renovation of the 100-year-old Capitol, was budgeted at that time to cost about $120 million and funded by bonds that will be repaid with cigarette taxes. A citizen panel (the Idaho Capital Commission) has been charged with overseeing the renovation and Capitol Mall planning.

McAlvain has served as both project designer and general contractor for the "wings expansion." The project kicked off in earnest in June 2007 with mass excavation of the pits, beginning with the east, and a few weeks later digging on the west side began. In September 2007, the project scope grew with the addition of the two tunnels, one in each wing leading into the capitol's garden level (i.e. basement). Upon completion (with proper access), people will be able to walk unimpeded from 6th Street to 8th Street.

"This was an exciting expansion to the overall scope, but it meant taking out the foundations of two giant sandstone columns on each side and some tricky engineering," said McAlvain project manager John Clark. "To do this, we (with KPFF Engineers) designed a cantilevered wall system to support steel beams supporting the foundationless columns. It worked, and we had zero subsidence — it was quite an engineering feat. Stop motion photography was done during construction."

Despite the delays, caused in part by the last-minute change from two-story to one-story underground wings, McAlvain has been able to keep the project on schedule and on budget.

Connecting tunnels to the building required partial demolition of a 2-foot wall that had been supporting the original structure for more than 100 years. This was accomplished through careful planning, along with bouncing ideas off every trade involved as well as the project manager, owner and engineers.

Although the soil was generally clean and easy to move, a few treasures were found along the way.

"We found a time capsule, a pre-Civil War cannon, the basement of a school, the old Ada County Courthouse foundation, three layers of sidewalk, and lots of wood that appeared to have served as formwork structures during the last construction period," said McAlvain general superintendent Jack Shalz. "Anything of historical value that was discovered was sent off for proper preservation and storage."

Extensive Waterproofing

Upon completion of the two pits, 10-inch concrete walls were poured and extensive waterproofing methods were employed even below the slab to ensure there will be no leaking, as the water table is only 10 to 12 feet below the project's floor. Historically, the existing Capitol building has experienced some flooding.

"The waterproofing system is very robust," said Clark. "It is designed to last 100 years." The waterproofing consists of a Laurenco three-ply waterproofing membrane with a mastic adhesive. The waterproofing application was installed below the concrete and then continued to the roof deck, almost totally enclosing the entire project.

In March of 2008, the structural concrete portion of the work was completed, and in June all of the structural steel was in place and the concrete top was formed. Inland Crane of Idaho installed all of the steel piles around the building, auguring them down 30-40 feet to support the walls and prevent collapsing under the weight of the soil that embodies the two wings.

"Kind of a neat feature was employed to both waterproof and alleviate some of the soil weight on the top of the two wing structures," said Clark. "Up to 2 feet of Geofoam was incorporated with the soil, which weighs only 2-1/2 pounds per cubic feet versus soil — a 110-pound weight savings per cubic foot. The Geofoam is engineered so it does not decompose over time."

Six specially engineered and structurally designed fritted glass skylights have been installed, which essentially helps reduce solar gain and preserve the view of the Capitol building's dome. On each side, two large skylights have been placed to allow a perfect view of the dome and the golden eagle perched on top.

Waterproofing was completed by July, and exterior concrete work began in August and continues at the time of this writing, along with all of the exterior hardscaping. Landscaping is scheduled to finish up the first week of December, just in time to meet the Dec. 10, 2008, completion date.

The interior of the wings will be finished by Jacobson-Hunt, and some of that work is under way, including interior framing and drywall. Ceilings are taller than normal to eliminate a basement feel to the wings — up to 12 feet in most areas.

The area in which the Idaho State Capitol building sits on is looped into a natural geothermal heating and cooling system where natural hot springs are tapped to provide steam and hot water to heat buildings. The system for the two wings will be installed on the same timeline with the larger restoration project. A final date for completion of the larger restoration project is not set at this time.

"McAlvain is proud to have worked on something this important, which our kids and community will enjoy for years to come," said Clark. "Knowing this, our company went the extra mile to upgrade in every situation possible to ensure the wings will become part of a long-standing legacy left behind by those who worked on this project."


Key Project Participants


Architect: Hummel Architects

Civil: H.W. Lochner

Electrical: Romar

Mechanical: Musgrove Engineering

Structural: KPFF


DeBest Plumbing (Boise)

Rule Steel Fabricators (Boise)

B&B Steel Erectors (Boise)

Inland Crane (Boise)