Last year saw a massive geotechnical operation at the Oregon State Capitol: the entire building had to be jacked up off the ground. Robit’s micropile casing systems played an important part in the process.
The Oregon State Capitol building has faced many misfortunes throughout its history. The first capitol burned down in 1855 – only a few months after its inauguration. The second capitol met the same fate in 1935.
However, fire is not the only force of nature that has put the state capitol in peril. Large parts of Oregon, including the capital city of Salem, sit on a tectonically active fault line called the Cascadia Subduction Zone.
In March 1993, a 5.6-magnitude earthquake – the largest in the Pacific Northwest in over a decade – cracked the cupola of the Capitol and created a massive bulge on its west end. Parts of the building remained closed for repairs for almost two years.
Over the following decades, the State of Oregon conducted several studies to determine how best to protect the historic building from further seismic events. After investigating various options, the review committee decided on seismic isolation. In other words, the foundation of the building would rest on so-called pendulum isolators and a seismic joint that allows the entire complex to move independently in any direction from the surrounding ground during an earthquake.
This would be no small task; it meant that the Capitol building would have to be lifted off the ground. This would be achieved by constructing a shoring tower structure on which the building would rest while the new isolation system was being built. As the Capitol had needed extra space for years, it was also decided that an additional basement level would be constructed.
The main contractorship for the geotechnical work was awarded to Pacific Foundation, a family business based in Vancouver, WA. The company started as a smallscale drilling operation in 2012; since then, it has grown into a large contractor capable of tackling complex geotechnical challenges.
Regarding challenges, the Oregon State Capitol project was far from a typical scenario where, slightly simplified, you arrive at the job site, drill the piles into the ground – and that’s it. Here, the first challenge was to gain access underneath the Capitol and excavate enough space for the drill rigs to fit in. This phase started in late 2022.
The work continued throughout 2023 and entailed drilling micropiles into the ground across the entire footprint of the building. At best, seven drill rigs worked simultaneously in narrow spaces with only a few inches of clearance above. Two-thirds of the micropiles were Robit casing systems, while one-third, the outside piles, were double rotary systems. The micropiles supported a total of 175 shoring towers upon which the jacked-up building would sit until the work was completed. The tower construction and the jacking operation were performed by trusted partners.
By Christmas 2023, the micropiles, totaling as many as 722, were installed. By late January 2024, the building was resting on the shoring tower structure, paving the way for the seismic isolation system and reconstruction efforts.
For Pacific Foundation, mission success has probably never been more critical. The project required massive amounts of structural engineering and careful planning and coordination by all participating parties to stay on schedule and maintain safety – not to mention that while the work was in progress, the state legislature was in session just a few feet above.
The construction work at the Capitol will continue into 2025. Once finished, the historic marble building should finally be able to stand firm against any future seismic events.
Shoring towers stretching across the entire footprint of the Capitol building.
