DC Water Uses 3D FEM in Assessing Century Old Trunk Sewer

摘要

In a recent construction, a new four story building was constructed over a 110 year old brick arch sewer (22' wide and 23'6" high). DC Water learned of the construction after it was completed when the developer applied for water and sewage connection. The developer submitted a structural engineering assessment in an effort to prove that the building does not compromise the brick arch sewer tunnel. Developer's engineer used a simple load-for-load comparison that computes vertical loads only to substantiate that the load of the wood framed building including excavation for basement did not exceed the previous earth load on the sewer. DC Water engineer rejected this simple arithmetic computation as the calculation failed to analyze the impact of the unbalanced load on the masonry arch sewer. Fully aware of the potential of its impact, DC Water's engineers responded by immediately organizing a series of engineering evaluations that consist of in-house FEM (finite element model) structural analysis as well as a geo-structural FEM by a consulting engineer. In this paper, we discuss the results from the two independent FEM analyses. Both these analyses confirm a similar stress/strain mode (tension) on the brick arch sewer. Armed with the results of the analysis, DC Water engineers focused on the tension zones during the inspection and identified a crown fracture under the new building. This paper will provide details and owner's lesson learned regarding the impact of the new construction on a conventional masonry tunnel with modern day engineering tools.