Down the Hole

An interceptor sewer rupture and resulting sinkhole required a work team of 20 contractors, innovative technology and a lot of ingenuity

A 30-foot-deep sinkhole, 65- by 235-feet, developed in Sterling Heights, Mich., when sandy soil eroded beneath 15 Mile Road after an 11-foot sewer interceptor ruptured 70 feet below ground. The line serves 600,000 people in the Detroit metro area, and its complete failure would leave all of Macomb County without service.

Victor Mercado, director of the Detroit Water and Sewerage Department (DWSD), called Dennis Oszust, vice president of Inland Waters Pollution Control Inc. (IWPC), a division of Inland Pipe Rehabilitation. The company, a Detroit-based industrial and environmental service firm, had a contract with the city for emergency repairs and pipe rehabilitation.

“Because of the project’s scope, no one company had enough resources to provide all the services,” says Oszust. “We were the prime contractor, coordinating 20 subcontractors representing 125 people. Everyone followed OSHA regulations.”

Crews faced the possibility of the interceptor’s total collapse and the challenges of rapid mobilization, coordinating materials and locating qualified divers. To eliminate the need for jetters, vacuum trucks and confined-space entries, IWPC engineers designed an award-winning, robotic sewer-cleaning tool. Work continued 24/7 for 285 days before 15 Mile Road was paved and the lives of its residents returned to normal.

Grouting

IWPC used a high-powered jetter to keep the remaining downstream interceptor open as trucks pumped sewage from the hole. Local contractors created two 12-inch emergency bypasses that diverted 30 to 60 mgd. An overflow would affect about 350 families.

“Our first concerns were replacing the emergency bypass with a stronger one, and stabilizing the ground around the interceptor to prevent the sinkhole from expanding,” says Oszust. Two houses were in jeopardy of falling into it.

Compaction and jet grouting stabilized the sandy soil, which had a high water table. For compaction grouting, crews hammered 4-inch pipe into the ground, then injected a mixture of sand, silt and cement at 800 to 1,200 psi. The expanding grout filled voids. For jet grouting, workers used drill rods to blend the soil with a slurry of injected water-cement. The homogeneous mixture solidified the sand and prevented aggregate from breaking off the ruptured interceptor, a poured-in-place concrete cylinder.

At the same time, crews installed 55-foot-long augur case piles (sheeting) behind two houses threatened by the sinkhole. To minimize noise and vibration, they augured 30 feet down, inserted the wide flange steel H-sections, drove them in using a hairpin pneumatic hammer, then filled the piles with concrete. “We scheduled most of the driving during the day,” says Walter Rozycki, senior project manager of IWPC. The piles overlapped to form walls.

Meanwhile, other crews constructed 14 dewatering wells 100 feet deep. Each 16- or 24-inch well had a 30-hp submersible pump. Diesel generators supplied power until electrical contractors could divert lines, relocate utilities and install enough three-phase drops to power all the different pumps. Water was directed to a downstream manhole.

Bypass and bulkheads

The collapse blocked sewage flowing through the interceptor, creating head pressure. Unless it was equalized, the top of the pipe could blow off. “We were still dewatering the work area,” says Oszust. “To reach the interceptor and relieve the pressure, we built 14-foot steel caissons or access shafts, then filled them with 35 feet of water.” Divers then went down to remove the steel-banded wood lagging around the pipe, saw through its 18-inch-thick wall, and place pumps to extract the sewage.

Simultaneously, crews drilled two 70-foot-deep holes upstream for the pump stations. They welded inch-thick steel casings to form caissons, 14-foot-diameter shafts that narrowed to 11 feet in diameter. Both stations had three 30-inch stacked pumps and three 24-inch stacked pumps. The three-stage axial-flow pumps discharged into two 36-inch HDPE lines.

The pumps couldn’t contain all the sewage, so divers installed prefabricated bulkheads at both ends of the damaged interceptor. “Finding divers with the necessary skills was challenging enough, but we needed 14, as the work mandated mostly two-hour shifts for the two-man teams,” says Oszust, who notified dive shops nationwide and used the Internet to spread the word.

Belly of the beast

With the site dewatered and enclosed by 250 steel-braced augur cast piles, crews excavated a 40- by 240-foot pit to expose 240 feet of the interceptor. “We removed the old pipe where we could, then poured a mud mat over the crushed invert,” says Oszust. The lean concrete mix formed a stable, waterproof work platform.

Where the interceptor had sunk below the retention system, workers drilled holes in the pipe, filling every cavity with concrete to create a solid base for the steel and concrete cradle that went on top of it. The 11-foot reinforced concrete replacement pipe was installed in the cradle and in line with the original inverts. “The profile of the failed sections was so severe that this procedure was necessary to maintain the existing grade,” says Rozycki. The 184-foot repair required 23 8-foot pipe sections.

The project’s last phase was removing the 4,000 cubic yards of sand and debris that migrated into the downstream interceptor during the formation of the sinkhole. “We needed all the other men and machinery out of our way to gain access to the pipe,” says Rozycki. “The sand in it was 5 feet deep for 8,000 feet.”

Conventional cleaning with high-pressure jetters and vacuum trucks would cost the city a fortune, so IWPC engineers enlarged an experimental design of their Hydrosled. The pneumatically controlled machine creates a bulkhead as it cleans from one access point to the next.

“A winch retains the sled’s components in the sewer,” says Rozycki. “When we raise the gate at the front of the underflow channel, the action is similar to holding the tip of your thumb over a garden hose. It increases the velocity of the flow.” Air directed through an umbilical cord lifts the gate and retracts the sides of the sled for downstream movement. The force of the flowing sewage – 45 to 150 cubic feet per second in this case – moves the sled forward, pushing sand and sewage to the extraction point.

IWPC crews clean-ed between manholes, usually 2,000 feet apart. The umbilical cord also controlled the pan-and-tilt camera mounted on the sled. A portable home air compressor provided the operating pressure. Two-stage 8-inch pumps in the downstream manhole lifted the material into a dewatering trailer. The decanted water was discharged back down the manhole, while the debris was transported to a landfill. At no time did personnel enter the interceptor.

Crews encased the replacement pipe in flowable fill (a cementitious slurry) to 18 inches above the crown, then covered it with 12-inch lifts of granular fill. Other workers removed the steel bracing as the fill was installed, while more men removed the bulkheads and dismantled the dewatering wells. Three months after the interceptor returned to service, traffic flowed on repaved 15 Mile Road.

The Hydrosled won the 2007 National Environmental Achieve-ment Award from the National Association of Clean Water Agencies, and IWPC was awarded the American Society of Civil Engineers Award for Best Project in 2007.



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