Design Approach, Inspection For Manhole Rehabilitation Technologies

3nd In A 3-Part Series
By Gerhard “Gerry” P. Muenchmeyer, P.E., NASSCO Technical Director | November 2011, Vol. 66 No. 11

Carpenetti: “Manholes are installed by stacking individual components on top of one another. Therefore, no single design approach is applicable -- each existing structure and surrounding area should be evaluated on a case by case basis to determine the best way to rehabilitate a manhole. In my opinion, ASTM F1216 is applicable for sewer mains, not manholes. The forces acting on a manhole are different than the forces acting on a sewer main since a manhole is orientated perpendicular to the ground surface as opposed to parallel.”

“With our structural spray wall product, the design can be for partially deteriorated or fully deteriorated manholes per ASTM F1216-10. This standard has been the industry wide accepted protocol for buried circular geometries for conduit and manhole design for 20-plus years.”

“A manhole is a vertical structure and any rehabilitation alternative used must be designed accordingly. The design appendix in ASTM F1216 is based upon a classical elastic buckling theory for an ‘infinitely long tube’ (L > 4.9r (r/t)½) with essentially a uniform external loading acting upon it (the manhole’s loading is increasing with depth). Further, the close-fit liner can be subject to varying dynamic loadings in a vertical (end) orientation. To validly use the classical buckling theory in its F1216 embodiment, a 48-inch diameter manhole would have to be 68-feet deep for a 0.5-inch thick liner. So the design appendix of F1216 is not a viable design tool. In my engineering judgment, a close-fit liner system installed in a manhole must be designed to resist hydrostatic buckling only. For proper insight into the analysis of buried silos, I recommend looking at Reynold Watkins book, Structural Mechanics of Buried Pipes. In a reasonably circular shape, the performance limit is ring compression yield strength. Olivier Thépot in his 2001 paper, A New Design Method for Non-Circular Sewer Linings, presents a design method for evaluating the effect of external long-term hydrostatic pressure, acting on a thin lining. It takes into consideration the exact geometry of the lining (curve radii) and the beneficial interaction with the host structure that may be attenuated by an annular void space. Provided the host structure’s shape meets the criteria given therein it would be my choice for close-fit liner design analysis; otherwise, one must employ a non-linear finite element analysis.”

Should coatings, that bond, be designed the same as liners, that don’t typically bond?

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