Mount Carmel Municipal Authority Wastewater Treatment Plant SBR Tank#1 Repair
Location: Mount Carmel, PA
Engineer: Thornton Thomasetti, Inc.
Project Duration: ???
The Mount Carmel (Pa.) Municipal Authority (MCMA) Wastewater Treatment Plant serves Mount Carmel Borough, parts of Mount Carmel Township, and Cunningham Township in Columbia County. The plant was built in 1975 and consisted of a secondary contact stabilization designed for 1.5 mgd average daily flow and organic capacity of 2,500 pounds per day.
Critical plant upgrades and construction of the new SBR tanks were performed using stimulus funds and were completed in August 2010. In October 2012, the devastation of Hurricane Sandy took its toll on the newly constructed SBR tanks. The hurricane severely damaged the treatment plant SBR #1 tank. Overall resulting damage included: slab buckling, foundation movements, foundation wall displacement, walkway displacement, and column structural damage.
The tank structure consisted of 3 SBR tanks, 3 Pre-react tanks, 3 digester tanks and a gravity thickener tank. The tank was constructed with pre-cast concrete wall panels and bridge sections installed on a cast-in-place and post tensioned foundation slab/footing. The precast sections were set in place into receiving keyways and temporary bracing was installed. The precast sections have 1″ conduit through each panel and 0.60 post tensioning strands are threaded through the wall and bridge components. Partial tension is applied to the tendons, followed by grouting of the joints and full tensioning of the tendons. When performing repairs to this type of structure the stress on the tendons has to be maintained or temporary restraint measures have to be provided. The contractor elected to perform repairs to the damaged bridge structure using a phased repair approach that maintained the stress on the tendons at all times.
Heavy structural steel bracing was installed to prevent further displacement of the tank wall and consequential damage that could render the wall “un-repairable”. The shoring was constructed of vertical W18x86 compression struts spanning the width of the tank and horizontal W24x117# whalers that transferred the load from the “intact” interior wall to the “displaced” exterior wall. This temporary shoring structure was utilized to return the displaced wall and footing to its original position as follows:
The extensive temporary shoring was then removed from the tank so that the post tensioned slab could be reconstructed and re-tensioned.
The re-positioning of the displaced wall required the removal and temporary stock piling of the soil adjacent to the wall. This task was complicated by the close proximity of the wall to the property line which dictated that the soil would have to excavated, hauled off-site and brought back after the repairs were completed. A further complication was presented by two large aeration lines that had to remain in service and were at an unknown location adjacent to the wall that was to be repaired. The contractor located the lines and designed a unique method to temporarily support the aeration lines, restrain the soil and provide a safe working condition for the repairs.
Other highlighted challenges during construction include the following:
Through communication, innovation and strategic planning the project was a success from the beginning to the end. It was the commitment of the Contracting, Manufacturing, and Engineering team to provide accurate solutions to complex restoration problems by providing the highest technical support offered by a repair team. Dynamic design details and innovation demanded that CPR deliver a level of service and technical assistance that far exceeds industry standards.