DC Water at Work: Tackling Fast Track CIP with In-House Design of SIPP

DC Water completed a sophisticated man entry inspection of a century old trunk sewer winding through the National Mall and other robust neighborhoods in the winter of 2015. This happened after the internal design engineer rejected a 5 million dollar capital improvement program (CIP) project that included only mortar patching and crack injection work exclusively based on a flow through video inspection. Because there was no consultant lined up to address the follow-up engineering for rehabilitation and it would take eight months to screen and engage a consultant for this CIP design project, the project delivery on the preset tight schedule was in jeopardy. In light of the in-house engineer’s experience and their in-depth involvement of the latest condition assessment, the rejection of the early “patch work CIP” and the large sewer analysis and modeling, in-house engineering by DC Water design engineers was a viable option to meet the project delivery. The in-house design engineers stepped up to the challenge and was able to transfer one million dollars (the saving in the design fee) toward the rehabilitation budget of the sewer tunnel. As the engineer of record, DC Water design engineers screened through the observed defects, weighed the criticality of the documented defects, and the consequence of failure in the high profiled streets. Approximately 4,300 feet out of 22,000 feet was selected for trenchless rehabilitation using sprayed in place pipe (SIPP) with geopolymer. SIPP geopolymer presents both an advantage and disadvantage to the design engineer. Its stellar corrosion performance, the superior bonding strength, and ASTM C-78 flexural strength facilitate a composite structural liner with less or no steel reinforcement. Its challenge is the “chaotic” design formulas that differ from vendor to vendor, resulting in an inconsistent design criteria and liner thicknesses, a big issue for open bid public work. Our in-house engineering started with finite element model (FEM) analysis to compute the design moment. This resulted in a fair practice without siding with any “preferred formulas,” none sanctioned by ASTM or AWWA. Based on post processing the design moment in a composite liner approach, the liner design provided a minimum thickness for the host tunnel ranging from 12-ft span to 18-ft span. The low bid public work project then became a much more straight forward construction of the specified SIPP work. This in-house project was completed on time and within budget.