Montana’s Big Sky County Water & Sewer District (BSCWSD) recently completed a water storage project called the Hidden Village Tank, which involved the construction of a one million gallon prestressed concrete tank and pump station.
BSCWSD and Allied Engineering analyzed several factors to determine the most efficient, cost-effective, maintenance-free configuration for their needs. It was determined that prestressed concrete would offer the highest quality, longest lasting and lowest maintenance water storage structure for their project. Choosing a prestressed concrete tank gave BSCWSD the ability to completely bury their tank, reducing any environmental impact to the picturesque mountain views. Accordingly, DYK, Inc., was responsible for the complete structural design, construction and prestressing of the tank.
Many challenges were encountered in the planning and design phases of this project. When considering where the site of the project should be, options were limited given that the majority of land available was private property. Property owners were opposed to the tank construction because they would receive no benefits from the project because their water was provided through their own wells. Aesthetically, a large above ground tank would not fit the surrounding environment.
The planned site was located in the southwestern section of Big Sky, Mont., in an environmentally sensitive area tucked away in the hillside. The close proximity to residential housing and the mandate to protect the natural mountain views added a unique complexity to the project. In order to satisfy the private landowners in the area, the reservoir was designed to be completely buried while maintaining the natural slope of the hill.
Not only did the tank have to be buried, the flat top that would normally be associated with a buried tank would not maintain the natural slope of the hill and, therefore, would not be acceptable. In order to meet these requirements, the hill’s bedrock was excavated and the roof of the tank had to be able to carry the heavy load of the hill’s slope. A topping of lush grass and low-maintenance, shallow-rooted native vegetation was added later to prevent soil erosion and to blend in with the surrounding area.
Another challenge was that a large portion of the concrete work involved construction before the difficult winter months of snow and freezing weather. At an elevation of 6,700 ft, a tight schedule was maintained in order to finish the project before the area was covered with four to five feet of snow. The schedule was compressed to 31?2 months, approximately half the normal time required to construct a concrete tank of that size.
The Hidden Village project combined the use of concrete and post-tensioning to “hide” a large capacity water reservoir from the community while ensuring an essential and reliable water supply.