Repairs to 35-year-old sagging sewer lines are underway in Amado’s Lakewood subdivision. The work started about 10 days ago and is expected to be completed Friday.

A spokesman for KE&G Construction, contracted by Pima County for the project, said sections of the main sewer line along Tumacácori Drive were slowly collapsing and needed replacement.

Four long segments of the eight-inch white clay conduit, installed more than 35 years ago, were developing “bellies” – a sagging caused by settling of the sand and gravel backfill supporting the pipe.

Over time, solids settle in the belly, or depression, reducing the diameter of the pipe and restricting its drainage capacity. Left unattended, the sewer main will clog, resulting in back flow into the homes it services.

Diagnosis and repair

According to KE&G, about a year and a half ago, Pima County conducted video surveillance of Amado’s sewer system. A robotic “crawler” was deployed to inspect the eight-inch mains as well as the four-inch feeder lines connecting from individual homes.

Debris and intruding roots were removed remotely in routine maintenance. It was during this inspection that the faults in the larger pipes were discovered. The sinking identified in the four sections of main line was evaluated for severity and rate of failure. The 18-month window for repair was then assigned.

The “rehabilitation” of the sewer lines is no simple undertaking.

The location of the lines, nearly eight feet underground, was mapped out using a sophisticated instrument about the size of a car battery, called the nuclear moisture/density gauge. And yes, it’s as high-tech as it sounds: A device containing radioactive americium and cesium, by-products of atomic weapons-making material.

The penetrating radiation returns a wealth of analytical information to the gauge’s computer sensors detailing soil composition, density and the exact depth of the pipe.

Once the trenches are excavated and the main line is found, the old clay pipe is severed at each end, drained and plugged. As the old pipe is removed, the tanker monitors the manhole behind the plug, siphoning wastewater and sewage as it accumulates to prevent interruption of household service.

Clay sewer lines are now obsolete for residential applications; the new mains are green, thick-wall 8-inch PVC. During installation, it is critical that the contractors maintain the proper pitch of the main line as the sewage system is driven by gravity.

Following the splicing of the new PVC line into the system, an additional safeguard is added. A resin-bearing liner resembling a rolled-up sock is installed along the interior length of the new pipe using air pressure to invert it, making the resin contact the pipe’s

interior walls. Steam is then introduced to the liner and kept at a steady pressure for about two hours, curing and hardening the resin.

Upon completion the of the main line’s four repairs in increments as long as 77 feet, the 7-foot, 6-inch trenches are backfilled under the supervision of CMT Engineering Laboratories, also hired by Pima County Wastewater Management.

The fill is added along the length of the trench in six- to eight-inch layers then mechanically rolled. After each layer is applied, the on-site soil engineer uses the nuclear density gauge to measure compaction and ensure the uniform settling of the fill.

When asked about the complexity of the project and the precision involved, the CMT engineer said, “If we do it right, we only have to do it once.”

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