EDITOR’S NOTE: This is the second installment of a two-part series. Part 1 was published in the October edition of Underground Construction.
Following a major collapse of a section of the Oakland Macomb Interceptor System in 2004, the Detroit Water and Sewerage Department performed an extensive inspection of the 42-mile system, and began a major program of rehabilitation of various distressed areas. The inspection revealed significant and widespread deterioration, hydrogen sulfide attack and loss of ground through cracks in the monolithic concrete sewer liner. Much of the sewer had deteriorated to a PACP rating of four and 5, necessitating immediate attention.
By 2009, most of the eastern half of the system (consisting of the Romeo and Macomb arm) had been completed. At that time, some remedial repairs had been performed in the western half of the system (designated as the Oakland and Avon Arms), although there were significant repairs yet to be made when it was transferred to the newly formed “Oakland Macomb Interceptor Drain Drainage District” (OMIDDD) in fall 2009.
The OMIDDD is currently in the process of making these major repairs. The repairs to this are being conducted to minimize risk of future collapses and prepare the interceptor for many more years of service to the community. Flow control structure construction is nearly finished and the first of several sewer repair contracts were bid in July 2011. Total repair costs were estimated to be in the range of $160 million and will require approximately five years to complete.
Built in the 1970s, the Oakland Macomb Interceptor Drain (OMID) is composed of approximately 20 miles of interceptor sewer ranging up to 12.75 feet in diameter, serving over 880,000 people residing in southern Macomb and Oakland Counties, MI. The OMID begins from near 23 Mile Road and Dequindre Road in eastern Oakland County and extends south to the Detroit Water and Sewerage Department’s Northeast Sewage Pumping Station located near 8 Mile Road and Hoover in Detroit. The interceptor carries mostly sanitary flow from local sewers, as well as from the 11-foot diameter Romeo Arm Interceptor that feeds flow from the Macomb County Interceptor System into the OMID. Dry weather flows are typically about 120 cfs, with wet weather flows over 400 cfs at the downstream terminus. The interceptor is non-redundant; therefore flow cannot be diverted and service to the communities must be maintained at all times.
The interceptor ranges in depth from approximately 30 feet to over 100 feet and is constructed through various soil conditions including clay, silt and sand. The northern portion of the alignment is within a nature area which is difficult to access due to flooding, wetlands and areas of environmental concerns. The middle portion of the sewer is within road rights-of-way, and the southern portion is within the International Transmission Company (ITC) high voltage electrical corridor and series of private easements.
Inspections conducted by NTH in 2006 through 2009 included extensive CCTV surveys, man-entry inspections and Multiple Analysis of Surface Waves (MASW) studies to identify loosened soil areas and potential voids. The inspections revealed significant and widespread deterioration, microbially induced corrosion and loss of ground through cracks in the monolithic concrete sewer liner. The conditions resulted in a range of recommended repair options including chemical grout injection to stop active leaks, epoxy repair of fractures, void grouting with cementitious grout, shotcrete lining repair and Xypex treatment of minor deteriorated areas.
Due to high flows in the interceptor, an extensive flow control plan was developed to allow entry to the sewer to conduct the repairs. The developed flow control program involves installation of four new flow control gates and a new dewatering pump station, as well as utilization of three existing gates and an existing pumping station. The flow control plan was designed to accomplish in-system storage of flow to allow access of all of the deteriorated sections of the sewer, for repair periods ranging from six to 15 hours. Depending on the area of the sewer to be accessed and the repair period, the system will store up to about 45 million gallons which will be released between subsequent storage/repair periods.
To solve the difficulty of operating and monitoring gates located miles apart, the owner and design team have designed an innovative system of remotely operated sluice gates that can be monitored and controlled through a SCADA system. Design for installation of the flow control gates was complicated by a number of factors, including limited right-of-way for some of the structures, poor soil conditions, high groundwater and the fact that three of the structures were to be installed directly below high-voltage power lines supplying most of the population in the area and to the north. In addition, live-tapping and fluming of the existing flow during the shaft installation was a major challenge. These flow control structures were constructed under Segment 1 of the OMID reconstruction.
Once the Segment 1 flow control was completed and tested, work was started on the design to prepare the Edison Corridor Interceptor (ECI) and a portion of the Oakland Arm Interceptor (OAI) for future repairs. Based on the Segment 1 hydraulic testing and additional modeling, the need to modify the pump station at the downstream end of the ECI was identified. The existing pumps at the pump station were not suitable to fully dewater the lower portion of the ECI. Accordingly, part of the Segment 2 work included the installation of an additional large pump capable of dewatering the downstream portion of the ECI and upgrading a small pump to maintain the lowered sewage level within the pump station wet well.
There have been three major collapses on the former Oakland Macomb Interceptor System, two on the Romeo Arm Interceptor, now part of the Macomb Interceptor System and one at the upper end of the ECI. These failures all resulted from soil fines being washed into the interceptor through cracks as small as 0.01 inch. The purpose of Segment 2 of the restoration program is to seal running and gushing leaks and to stabilize loose soils adjacent to the interceptor that were identified through the use of surface geophysical testing and confirmed by a geotechnical investigation.
As a part of the contract documents for Segment 2, the specifications included detailed information on how to use the new flow control devices to control or store sewage flows to accomplish the grouting work from within the interceptor. The work under Segment 2 was issued as OMID Contract 3.
Engineering work is presently in process for Segment 3 of the OMID repairs. Work under this segment will involve the relining of selected portions of the ECI and part of the downstream portion of the OAI
ABOUT THE AUTHORS:
Harry R. Price, P.E., F. SAME, Principal Engineer, NTH Consultants
Fritz Klingler, P.E. is the principal engineer with FK Engineering Associates
Mike McMahon is a senior engineering manager for the office of the office of John McCullough, Oakland County Water Resources Commissioner, Waterford, MI
FOR MORE INFO:
NTH Consultants Ltd., Detroit, MI, (800) 736- 6842, http://nthconsultants.com
FK Engineering Associates, (313) 218-9961, http://www.fkengineering.com