The Xcel Energy 414 Building Plaza is home to Xcel Energy’s corporate headquarters in Minneapolis, Minnesota, and a large underground transformer substation, which serves the majority of downtown Minneapolis and is located below ground, beneath the pedestrian plaza.Each day, thousands of downtown workers and visitors use the exterior plaza, which is adjacent to several bus routes and the Hiawatha Light Rail Transit line.Xcel Energy recognized the need to maintain protection of the underground transformer station, and at the same time, update the exterior plaza functionality and appearance.
A Complex and Difficult Renovation Project
The Xcel Energy Plaza was a complex, time-consuming project, requiring the owner to jump through several hurdles.The plaza area needed to remain in operation during the entire project.Four huge 110,000 volt transformers that served downtown needed to remain in constant operation. Xcel Energy wanted a state-of-the art waterproofing system, protected with decorative pavers and plantings to make a welcoming and comfortable space for the public.The project team solved several problems during the renovation, including:
- Keeping a tight and hectic schedule to accomplish the repair within one construction season.
- Removing the existing plaza using expansive chemical demolition techniques.The vibration, noise and dust generated had to be kept to a minimum, so the contractor drilled a series of holes and inserted a special expanding chemical grout to break up the existing plaza construction.Removals were found to be much more problematic than expected, as the planned-for lightweight concrete topping was instead found to be regular-weight high strength concrete!Some of the concrete was discovered to be over 3’ thick instead of the expected 1’.
- Keeping the underground transformers in constant operation during the repair.Each transformer was covered with a removable but massive concrete slab, measuring 2’ thick and weighing more than 18 tons. Only one of the four transformers could be shut down at any one time.Special handling and protection methods were needed during all construction work, to avoid electrocution and disruption to the power feeding downtown.
- Several utility and communication lines crossed the plaza and all were protected, rerouted and maintained during the repair.A water pipeline was heavily corroded and was replaced.
- Installing a two-layer, flexible waterproofing system to protect the vital transformer station below.Xcel Energy wanted a state-of-the art system to protect the electrical services, and with a long service life, one lasting 50 years or more.A multi-layer, hot-applied reinforced rubberized asphalt system was selected.The waterproofing was protected with cast stone pavers on a pedestal support system, new diversion and screen walls, and decorative planters and walkways.
- Keeping all transportation services intact during the repair.The exterior plaza is used by thousands of business commuters each day, and is served by several bus routes and Light Rail Transit.During the repair, one of the LRT poles needed to be temporarily moved, and one of the two tracks stabilized to avoid shutting down or undermining its support.An innovative temporary power pole support system was designed to keep the trains running, and soil stabilization techniques allowed a nearly vertical cut in the soil near the tracks.
The restoration engineering firm, the owner, architect, landscape architect, and contractors worked together to sequence the work to provide the necessary renovation while keeping Xcel Energy’s neighbors happy and costs minimized.
This Is an Important Building for Minneapolis
A project of this scope required protection of the existing transformer station, the one funnel for all of the electric power to downtown Minneapolis.Any disruption would have costly repercussions for businesses and workers.
A Successful Renovation for the Xcel Energy
After years of water leakage and deterioration, protecting the vital electrical transformers saved downtown Minneapolis from a possible major disruption.During the repair, several concealed and unexpected conditions were found, yet the project team charged on ahead and kept the project moving ahead.Any major work in a downtown environment impacts the building neighbors, and Xcel Energy, its consultants and contractors all worked together to successfully complete the work on time and within the approximate $3,000,000 budget.
This project demonstrates that engineers, using their design and analytical skills, can design and implement difficult repairs to save our historic past.By taking the initiative to work closely and responsively with the owner and contractor, the result will be a beautiful and economically successful renovation.