Design Considerations for Large Prestressed Water Reservoirs

When the time came to fill the Twin Oaks II Reservoir with water, there was virtually no detectable loss of water. After passing the watertighness test, the reservoir was backfilled and placed online. It is expected to serve with no regular maintenance for decades. The success of a large prestressed concrete reservoir can be summarized as follows: 1 The larger the tank is, the greater volume (water capacity) to surface area (structure) ratio. For example, one 40 million gallon tank has less material than two 20 million gallon tanks or four 10 million gallon tanks would. This translates to a significant savings to the owner by a shorter construction duration, less material and equipment usage. 2 Prestressing can be used to balance virtually any hydrostatic or hydrodynamic load. Without prestressing, water-induced hoop-stress would go into the reinforced concrete as tensile stress. Filling and draining a reservoir without prestressing will cycle the structure through different degrees of tensile stress, which are problematic in liquid-containing reinforced concrete structures. As a prestressed concrete reservoir is filled and drained the concrete will always remain in compression, which ensures long-term durability. 3 High water pressure near the base of the wall of an anchored-flexible base will neither transfer moment into the footing nor retain these stresses by a restrained connection. Large water pressures can simply be balanced by providing an appropriate quantity of prestressing. 4 The design and construction of a large prestressed concrete reservoir parallels that of a smaller one in many ways. However, special attention to certain details and the ability to manage the logistics of such projects will assure success. Having an experienced designer and contractor would be an essential part of a successful team.