INVERT EROSION OF LARGE DIAMETER CONCRETE SEWERS UNDER THE INFLUENCE OF SEDIMENT TRANSPORT

摘要

The ever-increasing cost of sewer construction and replacement has prompted manyrnutilities to focus on protection of their existing wastewater infrastructure. More attentionrnis being directed toward protecting the original sewer capital investment as a means ofrnreducing cost. For concrete sewers, the primary focus has been on preventingrnbiogenic sulfide corrosion and protecting sewers from the ravages of the sulfuric acidrnthat is produced. This form of sewer corrosion occurs above the water line and isrnclearly visible with CCTV inspection. The consequences of corroded and deterioratedrnpipe are collapse, wastewater release, interrupted service and public relationsrnnightmares. Most utilities are aware of biogenic sulfide corrosion and many arernactively pursuing the rehabilitation and protection of susceptible pipes. The City ofrnPhoenix and other local municipalities have formed the Subregional Operating Grouprn(SROG), a wastewater collection and treatment partnership group, to providernwastewater collection and treatment for common good of the membershiprncommunities. The SROG is among the forefront of utilities which are aggressivelyrnpursuing sewer infrastructure protection.rnWhile biogenic corrosion has received the most attention as a sewer infrastructurerndestruction mechanism, another type of corrosion which is hidden from view belowrnthe water line may be slowly and systematically destroying your concrete pipes.rnInspectors have discovered the widespread occurrence of a form of sewerrndestruction that physically removes the invert of the pipe. This form of destructionrncalled "invert erosion" occurs in pipes that carry a normal bed load of sediments,rnsand, pebbles and stones at high velocity. The City of Phoenix and SROG discoveredrnthe presence of invert erosion in the large diameter (90-inch diameter) Salt RiverrnOutfall (SRO) trunk sewer during an inspection of biogenic corrosion impacts. Duringrnan unusually low flow, the inspector noticed a groove in the invert of the pipe with hisrnboot.rnThe City of Phoenix and their wastewater operating partners (SROG), immediatelyrnstarted a program to determine the presence, extent and cause of the invert erosion.rnBoth sonar and physical manned entry inspection of the sewers was performed tornidentify the severity and extent of the erosion. The manned entry was performed torn"map" a profile of the erosion artifact using a specially constructed profile tool.rnFollowing the inspection phase, an engineering study was performed to identify therncause of the pipe destruction. The study concluded that the velocity of the wastewaterrnis the critical element in the sediment erosion process. The erosion process was alsornfound to be more dependent upon wastewater velocity than the volume or mass ofrnsediment load. Depending upon the velocity of the wastewater, the sediments in thernsewer (bed load) respond differently. High wastewater velocities make the bed loadrntravel down the pipe in "single file" fashion so that the tumbling impacts from thernpebbles and stones occur in a line along the invert of the pipe. The result is creationrnof a groove in the invert of the pipe that can cut through reinforcing steel andrneventually breach the pipe.rnThe paper presents the identification, study, analysis and recommendations for thisrnunique form of sewer "corrosion" in the City of Phoenix. Calculations of the shearrnforces applied to various sediment load particles (stones, pebbles, sand) and therncalculation of wastewater velocities of concern are presented. The paper alsornpresents a profile of sewers which may be subject to this particular erosionrnphenomenon. Diameter, wastewater velocity, sediment load type, sediment loadrnsource and final recommendations for the correction of invert erosion are presented.rnThe application of the lessons learned in Phoenix are directly related to any normalrntrunk sewer flowing at velocities greater than 5 feet-per-second (fps).