Hilltop Water Reservoir Challenge STS Consultants, Ltd. (STS) was retained by the City of Minneapolis Department of Water Works in 1998 to assist the City in the design and construction of their existing water reservoir expansions. The City sought to expand its water reservoir capacity with a new superstructure that would accommodate 34 million gallons of additional storage. The City’s reservoir expansion project faced the following unique design challenges: Storage capacity had to be maximized within the budget available to the City while fully utilizing the current elevation advantages. The site of the existing structure was at the highest elevation point in the City and located on a site limited in size with undulating topography. Construction was to be carried out with minimal disruption to its residential setting — including noise, dust, visual impact and traffic problems — throughout the five-year construction schedule. This was the first structure the City of Minneapolis Water Works Department had ever designed inhouse. The two-foot-thick concrete slabs and 18-inch-thick concrete walls had to be designed and built to limit cracking potential without the use of expansion joints. Solution The existing reservoir operated at the highest elevation point in the City of Minneapolis. This allowed gravity to transfer water through its distribution system. It was decided that the new structure should be built at the same elevation as the existing reservoir in order to eliminate the need for a pump station and to reduce operation and maintenance costs. Additionally, the structure had to be expanded within the existing property limits. STS designed mechanically-reinforced subgrade, which varied from two to 38 feet of ground improvement and soil correction due to pre-existing topography and soft/organic soils. Due to the confinements of the space available, the design of the reservoir called for steepened slopes greater than those typically found in construction projects. Slopes were built in lifts to the top of the structure, with geogrid reinforcement placed between each lift, reinforcing and stabilizing the embankment. STS also helped design concrete mixtures and procedures to minimize cracking. This included a curing plan using the STS maturity program. Key Benefits The City used the land adjacent to the existing reservoir and took advantage of its current elevation, thereby eliminating the cost of building a pump station. Design of custom soils mitigated costly future maintenance on the reservoir due to settlement and cracking. STS aided the City in demonstrating community responsibility by designing a system that minimized the impact to adjacent property owners. The concrete curing program provided a solid, virtually crack-free enclosure capable of holding water.