Project Overview

• Existing digester change in use
• Upgraded to meet seismic design requirements and AWWA D115 Standards
• Over 50 miles of 0.6” post-tensioning strand applied to the four digesters

Four prestressed digesters located in San Jose, CA were a part of the city’s first phase of upgrades to their biosolid processing facilities.  The project involved converting the digestion process from mesophilic to temperature-phased anaerobic digestion (TPAD).

With a conventionally reinforced core wall and an external bar-wrapped prestressing system, the original digesters were covered with three inches of shotcrete.  The phase one project included structural wall upgrades as well as roof and floor replacements of the original digesters.

STRUCTURAL TECHNOLOGIES’ crews performed the demolition and removal of the external wall shotcrete.  Each digester was de-tensioned and the pre-existing bar-wrapped systems removed.  For the repairs, crews installed a new external post-tensioning system as well as new seismic restraint connections at the floors and roofs.  An external layer of shotcrete was then applied over the post-tensioning systems.  The walls were upgraded to withstand seismic loads as well as provide the 200 psi residual compression required by the AWWA D115 Standard.

The structural design initially required three separate layers of external post-tensioning and shotcrete to wrap each tank.  STRUCTURAL TECHNOLOGIES was able to value engineer a solution that only required 1.5 layers of post-tensioning by spacing strands tighter at two inches on center, thereby maximizing spacing efficiency.  This value engineering approach provided a more cost efficient solution as well as a shorter construction schedule for the city without jeopardizing tank performance.