Safe conditions prevailed as Milwaukee completed first manned inspection of 19-mile-long wastewater storage and conveyance system
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The inspection vehicle is lowered into an access shaft outside of MMSD headquarters.
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300 feet below ground, crewmembers help guide the inspection vehicle into the ISS.
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After eight years of operation, the first manned inspection of Milwaukee's 405-million-gallon wastewater storage and conveyance system found a tunnel very much in solid shape.
In fact, so good that some original spray paint markings used as guides during construction in the late 1980s and early 90s are still visible on the tunnel walls, according to representatives of the Milwaukee Metropolitan Sewerage District.
The MMSD Commission awarded a $1.2 million inspection contract to Rust/Harza, a joint venture of three Milwaukee-area engineering consulting firms, in late 2001. While no known operational concerns regarding the Inline Storage System's (ISS) condition existed, the system had not been entirely inspected since being placed into operation in 1994.
Workers check safety equipment and air quality.
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Looking upstream where a section 17 feet in diameter transitions into a 30 foot diameter section.
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The contractor's safety director rappels down the transition.
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In January 2002, a team of experts ventured 300 feet below ground to begin inspecting and videotaping MMSD's 19.4-mile wastewater storage and conveyance system. The crew consisted of a safety director, foreman to operate the inspection vehicle, geotechnical expert, engineering technician and video camera operator. Contractors completed the inspection in mid February with some new insight into the challenges a project like this poses.
Original spray paint markings used as guides during the construction.
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The videotape was compared with video taken in late 1993 just prior to the opening of the tunnel system. Rust/Harza then prepared a final report on the condition and also included some recommendations for future operation and maintenance.
The inspection team used a specially outfitted 8-wheel-drive transporter capable of maneuvering through 16 inches of water to document two to three miles of tunnel conditions each day. Also designed to float if necessary, the vehicle was more than 12 feet tall and weighs 5,000 pounds (without crew weight). In addition, the transporter had its own generator/power supply, floodlights and was outfitted with an observation deck for the video camera.
55% of the ISS is unlined.
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After eight years, dowels remain intact after use during construction to support a small area of bedrock dropout.
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45% of the ISS is lined.
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One of many examples where seeping joints are healing themselves through calcification.
The crew begins work at an access shaft.
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In the event an emergency evacuation was necessary, a six-wheeled vehicle was placed at the surface of the nearest access point to where the crew was working that day. If needed, specially trained fire department emergency team members were on alert to use the second vehicle to reach inspectors in the tunnel.
Safety First
From the start, the District's main concern was safeguarding any crews venturing inside the ISS, which is a full football field's length below ground and varies in diameter between 17 and 32 feet.
Safety remained MMSD's highest priority for this important project and the contractor proposed an extensive safety plan that assured that all safety systems were backed up by secondary systems.
Besides making sure the $800 million ISS was structurally sound, the goals of the inspection were to determine if there was any debris in the tunnel as well as measure the amount of groundwater infiltration into the tunnel system during dry weather.
To accomplish that goal, the crew stopped every 2,000 feet to measure and document the volume and flow rate of water in the tunnel. Although the system was designed to handle about 6 million gallons of infiltration per day, inspection results indicated the actual infiltration was well below the designed specifications.
The inspection team measured flow every 2,000 feet to characterize infiltration.
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Workers found silty, sandy sediment on the bottom of the ISS in several areas, but nothing that was unexpected. What they did not expect, however, was difficulty maneuvering their 8-wheeled vehicle through some of the sediment. Other challenges for the project included what many would consider the simple task of communicating from the surface to the crew inside the ISS. The contractor did not feel comfortable relying on two-way radio communications. Instead, the crew used three separate modes: a hard-wired phone system; two-way radios; and air horns for backup emergency communications.
The project remained on schedule even though the original timetable did not include downtime for each day required to retrieve the hard-wired phone system. The crew would lay the phone system as it progressed through the tunnel and then take a day to retrieve the phone line before moving on to the next segment.
Three-Phased Project
The inspection project was performed in three phases, with the first measuring the amount of groundwater infiltrated on a daily basis. Phase two consisted of preliminary man-entry at two access shafts to determine tunnel conditions that could be encountered during the full inspection, and the third phase included the manned inspection with videotaping.
Overall, the inspection team was pleased with the safety conditions found inside the ISS. Ventilation and airflow were extremely positive; so much so, it was actually a little windy inside the ISS, inspectors said.
The ISS is designed to capture excess wet weather flows and prevent sewer overflows to area waterways. Before the system went on-line, there were 50 to 60 sewer overflows annually. Since 1994, that average has declined to about 2.5 overflows a year.
Deep Tunnel Inspection Videos