AWAKENING FROM THE NIGHTMARE ON 10TH STREET – TRENCHLESS REPAIR TO PREVENT DEEP EXCAVATIONS IN CLOSED REAR-YARD EASEMENTS

This paper presents a proactive methodology developed for a Southeast Florida utility to prevent collapses in deep rear-yard sewer mains. For the deeper mains, all house laterals tie into vertical stacks which enter the main at the twelve o'clock position, typically with four laterals per stack. When the mains were installed, it appears that concrete encasement was applied to the lateral stack only and not extended around the main so as to provide the intended support for the connection between the stack and main. Over time, the soils surrounding some encasements appear to have shifted to the point at which the increased point load applied by the stack has caused collapses of the main itself. Collapses have occurred near maximum pipe depths where most of a collection basin's flows converge, and resulting repairs have been extremely expensive as well as highly critical in terms of bypass pumping and sanitary sewer overflow prevention. Planning for a preventive approach was initiated after two separate emergency repairs that cost the utility over 270,000 due to the depth and difficult conditions affecting equipment access, dewatering, bypass pumping, and restoration. Alternatives were considered to reinforce the main at the point of lateral stack connection, and installation of cured-in-place mainline/lateral connection repair systems emerged as a technically feasible and cost effective option. An economic evaluation suggests that in the long run, this program of planned spending will be cheaper than the unplanned spending the utility has already experienced.