When a section of an 11-ft.-diameter, cast-in-place sanitary sewage interceptor tunnel failed approximately 60 feet below ground in August 2004, Detroit
When a section of an 11-ft.-diameter, cast-in-place sanitary sewage interceptor tunnel failed approximately 60 feet below ground in August 2004, Detroit Water and Sewerage Department officials soon realized they were facing a catastrophe. A large crater formed as surrounding soil continued to fall into the interceptor and steadily grew in size to a 235-ft.-long, 130-ft.-wide, and 30-ft.-deep chasm. At the root of the failure was a length of damaged concrete tunnel built in the late 1960s seventy feet below the surface of the roadway. An urgent need for soil stabilization and interceptor repair became apparent as the crater encroached on six residential properties on the south side of 15 Mile Road, and nearby residents in the suburb of Sterling Heights were deprived of water (though not of sewer service).
A coordinated effort was rapidly executed among Detroit Water and Sewerage Department construction and public affairs officials, while Inland Waters Pollution Control, Inc., was enlisted as general contractor. Also included in the mobilization were officials of Sterling Heights, the City of Fraser, Macomb County Road and County Public Works Commissions, the Macomb County Health Department, and Michigan Department of Environmental Quality. Additionally, participation of police, fire and emergency personnel of the affected communities facilitated effective communication, keeping the public informed throughout the emergency repair, and ensured enforcement of security precautions around the construction site.
The Romeo Arm of the Macomb Sewer Interceptor, carrying sewage to the Detroit Wastewater Treatment Plant, has a flow capacity of 30-60 million gallons a day. Initially, repair of the tunnel required intervention by various contractors to stabilize the soil and protect the area from further damage. Excavation and earthwork were performed by L. D’Agostini & Sons, Inc., while Mersino Dewatering, Inc., Thompson Pump Midwest, O’Laughlin Construction, and Rohrscheib Sons Caissons, Inc. served as major subcontractors.
The project’s lead design engineer, NTH Consultants, Ltd., was assisted by specialized engineering firms including Spalding, DeDecker, Inc.; Lakeshore Engineering Services, Inc.; Superior Engineering; Multi-Solutions, Inc.; and, Malcolm Pirnie Inc.
An emergency bypass pumping system was constructed to maintain some flow in the event of total tunnel collapse. The bypass system, comprising submersible pumps and existing manholes near the failure, was functional within three days and allowed the construction team to maintain limited flow until a more permanent bypass could be completed.
Two temporary 36-in.-diameter pressure pipelines, each stretching 2,700 feet along the edge of 15 Mile Road, were installed to relieve the interceptor during the repair; and, two shafts were sunk east and one west of the damaged section. The bypass was effected by drilling the shafts to the tunnel crown, installing steel casing, and grouting the shaft bottom and sides. After an appropriate curing period, caisson bases were cored into the tunnel.
In addition to the challenge of quickly obtaining materials for the caisson and tapping into the tunnel, divers 55 feet below ground level in a water-filled, 11-ft.-diameter access shaft had to cut and remove steel ribs and wood lagging that remained from the original structure. Concrete plugs were cast in the bottom of the caissons to maintain the tunnel’s structural integrity as holes were cut through the wall for pump access. A series of two 24-in. and two 30-in. three-stage pumps was installed to lift the sewage 70 feet through the two temporary pipes and along 1,500 feet to a discharge point downstream from the breaks. At times, as much as 80,000 gallons of sewage per minute were pumped downstream. With pumping operational, the tunnel was plugged to isolate the work area. Contractors were able to construct the permanent bypass within four weeks by working 24-hour shifts, seven days a week.
During installation of the bypass, the site was stabilized by means of sheet piling driven to 50-ft. depths at the rear of houses vulnerable to structural damage due to shifting soils. To stabilize the soil and limit sinkhole expansion, compaction grouting was completed early in the project by injecting a mixture of sand, silt and cement via 2-in.-diameter pipe driven into soil around the tunnel and on both ends of the sinkhole. A liquid grout was injected into the site to further stabilize the area. Indispensable for preventing water from seeping into openings and washing sand into the excavation, as well as providing access to the cast-in-place structure, were 12 to 14, 80- to 100-ft.-deep dewatering wells drawing down the local water table.
The urgency of site stabilization was accentuated by the nature of soils in the area, i.e., fine sand (sugar sand). In the presence of water, it turns into a weak gruel and will migrate into any crack or void on the site. The construction team was fortunate that no major rainstorms during the repair process exacerbated challenges posed by the fine, sandy soil.
To excavate to the tunnel, over 200 concrete auger piles were cast around the perimeter of the site, further preventing soil migration and protecting the work area. As excavation progressed, steel H-beam whalers were installed in addition to H-beam cross-bracing. Following soil and tunnel debris removal, the tunnel’s 11-ft. drop below original grade became evident. Once grade elevation had been achieved for the recovery shaft, sealing and stabilization of the trench floor was accomplished by placing a 2.5-ft. reinforced-concrete mud mat, which served also as a structural member to support auger piles when the lower whaler and cross-bracing were removed to install the reinforced concrete pipe (RCP). The excavation extended approximately 29 feet wide, 240 feet long, and 74 feet deep to the pipe invert.
The ends of the existing tunnel were aligned, drilled, and fitted with steel dowel pins for connection to precast pipe supplied by Northern Concrete Pipe Co. from its Bay City, Mich., plant. Steel cribbing was constructed to carry an 18-ft. _ 143-ft. steel I-beam that would act as a cradle to lay the 132-in.-diameter Class V RCP. A 150-ton Linkbelt crane was used to lower the 34-ton pipe through cross-bracing to the cradle. A Ê-yd. excavator was used to home the pipe after a rubber gasket was placed on the spigot. Steel knee braces were welded into place under the haunches of the pipe to prevent movement. A steel strap was placed around the top of the pipe and welded to the cribbing, eliminating any possibility of flotation.
As pipe laying progressed, the first closure was poured to connect the tunnel to the new precast pipe. The reinforced concrete closure overlapped the pipe and the tunnel by four feet, the width of the excavation. A structural concrete cradle was poured up to 25 percent of the outside diameter of the pipe, and then flowable fill was placed in three successive lifts to 18 inches over the top of the pipe. Finally, sand was used to bring the excavation to grade.
The last closure was poured in March 2005, following 170 days of round-the-clock construction. The sewer interceptor was back in service several weeks ahead of schedule, 204 days after the failure occurred. DWSD Director Victor Mercado attributes the speedy repair of the sanitary sewer interceptor to cooperation and communication among all parties engaged in the project. Facilitating a high level of coordination were meetings convened each morning at the jobsite office, where DWSD representatives, project engineers, the general contractor, and all subcontractors discussed and planned the next 24 hours.
While no definitive cause for the failure has been identified, evidence indicates that a crack or break in the interceptor caused surrounding sandy soil to fall into the pipe. As soil falling into the interceptor was swept away with the flow towards the wastewater treatment plant, a sinkhole resulted. Investigation by DWSD continues in order to prevent future reoccurrences.
Romeo Arm of the Macomb Sewer Interceptor 15 Mile Road Sterling Heights, Detroit, Mich.
Detroit Water and Sewage Department
Victor M. Mercado, Director
Ramesh Shukla, DWSD Project Engineer
George W. Ellenwood, DWSD Public
NTH Consultants, Ltd. (Lead Design Engineer)
Keith Swaffar, President
Harry Price, Project Engineer
Spalding, DeDecker, Inc.
Lakeshore Engineering Services, Inc.
Malcolm Pirnie Inc.
Inland Waters Pollution Control, Inc.
Walter Rozycki, Project Manager.
L. D’Agostini & Sons, Inc., (Excavation, Earthwork)
Mersino Dewatering, Inc.
Thompson Pump Midwest
Rohrscheib Sons Caissons, Inc.
230 feet of 132-inch diameter Class V
Northern Concrete Pipe, Inc.
Bay City, Mich.
NORTHERN CONCRETE PIPE, INC.
NCP has been supplying pipe in Michigan since 1958. Its Bay City and Charlotte plants produce 12-in. to 144-in.-diameter gasketed concrete pipe, as well as elliptical pipe and various other shapes. The company also produces manholes, catch basins, Tees, boxes and other precast components for bridge structures.