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. . . ." URS Agenda Item: Work Session #3 MEMORANDUM Thresher Square 700 Third Street South Minneapolis, MN 55415 Phone: (612) 370-0700 Fax: (612) 370-1378 To: Ms. Michelle Wolfe! City of Arden Hills Copy: Nick LandwermRS Tom Moore/AH File: Pending From: Greg Brown Arden Hills City Engineer Date: March 14,2005 Subject: Infrastructure Assessment and Rehabilitation Discnssion Background At the February 28, 2005 Council Meeting, the Council requested that staff provide detailed infonnation regarding the City's underground infrastructure and the processes used to detennine which facilities are included in the CIP. We have completed the annual update of City watennain, sanitary sewer and stonn sewer system maps including annotations of pipe material and initial construction year (for sanitary and watennain) as requested by Council. The following discussion is a general overview of current system condition, potential problem areas and rehabilitation techniques and strategies for sanitary sewer and watermain. Sanitary Sewer Collection Svstem Overview Sanitary sewer effluent from the city is collected by a network of gravity pipes which enter the Met Council Interceptor sewer network at three metered locations. Throughout the network some sewage is routed Ihrough lift stations (several stations in some cases) prior to leaving the city network. The lift stations are necessary to pump the sewage from low points to higher sewers to continue the path to the MCES interceptor sewer. Arden Hills is blessed with rolling terrain which requires an inordinate number of lift slations relative to its geographical size. There are 14 lift stations in the city, eight of these utilize efficient submersible pumps in wet wells, six 40+ year old "can-type" stations which utilize drywells (pumps are in a separate chamber or can from the sewage). Sewage flowing tbrough collector pipe and trunk pipe is not under pressure and usually fills only a small portion of the pipe. Because the pipes are not under pressure a common problem is the infiltration of ground water into the system. Infiltration typically does not create structural problems for the pipe but is added to the city's sewage bill as it passes the meter stations along with the sewage effluent. VCP sewers are more prone to infiltration due to age and number of joints in the pipe(3-4 times as many as PVC pipe). Other typical problems involve pipes which have sagged (creating low points with high potential to clog) and pipes which have collapsed or cracked (also blocking flow).