In August 2016, a U.S. Postal Service truck was making its rounds in Albuquerque, New Mexico, near the intersection of Barelas Road and Marquez Lane when the driver abruptly slammed on the brakes. Ahead, a sinkhole had opened in the narrow street.

Authorities at Albuquerque Bernalillo County Water Utility Authority were called and found that a breech had developed in the crown of a 60-inch interceptor pipe. Wet, silty soil had dribbled into the pipe through the hole, to be carried away by effluent. Over time, much of the surrounding alluvial soil followed, disappearing into the pipe and creating a huge void under the roadway, which finally collapsed.

This was not a total surprise. David Laughlin, principal engineer, and his colleagues at the authority had seen it coming.

They hadn’t foreseen a collapse at that particular intersection at that particular time, but the failed interceptor line had been identified as a high-risk asset. In fact, a consulting engineer’s plan to rehab the line was 70 percent complete when he learned on that August morning that time had run out. “The sinkhole just accelerated our planning,” Laughlin says. Two days after the hole was discovered, a contracted crew began the fix.

Decisions

The 2,500-foot segment of the Valley Interceptor line carries sewage to the city’s Southside Water Reclamation Plant. The interceptor line bisects a historic district of Albuquerque that parallels the Rio Grande. Consequently, the soil is sedimentary and the water table is relatively close to the surface, both conditions contributing to the sinkhole. The city sits in a swale between a high desert plateau and the Sandia Mountains and much of the city is on higher elevations. By the river, though, groundwater is encountered 6 to 8 feet below the surface.

The decision was made to slipline the existing pipe, sliding 20-foot sections of 54-inch fiberglass-reinforced polymer mortar pipe inside the interceptor pipe. The primary consideration was that the trenchless method would require excavation of five access pits versus ripping up a half-mile of street. “We attempt to do as much trenchless work as we possibly can,” Laughlin says. “We use trenchless probably 90 percent of the time. All of the lines being rehabilitated are in inhabited areas. It’s not like we are open-cutting across the desert.”

The authority contacted AUI, an Albuquerque heavy civil construction firm and one of two emergency on-call contractors the authority has on speed dial. An AUI crew, working with the authority and design consultant Carollo Engineers, firmed up the sinkhole and inserted a 12-foot-wide 24-foot-long trench box astride the exposed interceptor pipe.

The project, which ultimately cost $2 million, was handicapped by three factors. First, the shallow level of groundwater meant that the work had to be done in unstable conditions. Think of laying pipe in soil that is firm, but diggable and of lowering pipe into clean, dry holes — and then visualize just the opposite. The pipe lay buried within the soupy groundwater area.

Project partners decided to avoid constructing dewatering wells along the project route to control the water. Instead, trench boxes in access holes were edged by steel shoring plates to limit groundwater inflow. Inside the box, the sides of exposed pipe were lined with sandbags and gravel to filter out sediment while letting groundwater seep through and into the open sewer pipe. The pipe already was flowing, after all. Throughout the project, the interceptor pipe continued to function as a primary carrier of wastewater, its daily flow of 15-20 mgd only slightly interrupted by the lining work.

Odor control

In the five access holes where the AUI crew inserted the lining pipe, the existing pipe was sawed open and the crown removed. Into this opening, each 20-foot section of the 54-inch pipe was fitted, joined with another section, and slid into the old pipe. Though the flowing sewage helped lubricate the insertion process, the smell was overpowering.

“One of the things we struggled with throughout the project was the odor,” Laughlin says of the project’s second handicap. “When you open a sewage pipe — and it’s fairly warm in Albuquerque — there’s not a lot we can do to contain the odor.”

In off-hours, fences with plastic sheeting were erected around a hole to somewhat contain the odor, but breathing the ambient air still was unpleasant for residents whose homes on the narrow street were relatively close to the excavated pipe.

One consequence of this project is that authority officials are investing in a pair of so-called odor trailers. In future sewer projects, the mobile units will be parked next to opened lines when crews aren’t working. The trailers are attached to a vacuum unit and air is sucked into the machines and scrubbed free of the hydrogen sulfide “rotten egg” smell, with the clean air vented into the neighborhood. “It will create a lot more pleasant environment for residents,” Laughlin says.

To help control sewage odor — which tends to flow back through collector lines and service lines into older homes along the street — the relining crew also installed gas trap structures inside each manhole. They consist of two collector line openings. One dumps sewage from the higher collector line down into the interceptor line with the transfer made below the surface of the wastewater flow. This below-surface intersection doesn’t allow odors to flow back up through the collector and, eventually, reach the homes. The second opening in the manhole is for periodic cleaning of the line.

Laughlin recalls a situation where a maintenance crew had taken the cap off the straight clean-out line and neglected to cap it again. “We started getting massive complaints about odor. The odor was coming up in floor drains and was pretty horrific. We went out and found the plug was gone from the line. We reinstalled it and asked that residents call the next day and report if there was any difference. We learned that as soon as we plugged that line, the odor went away. That’s evidence that the odor trap is a pretty effective deterrent.”

Grout solution

Grouting the annular space between the host pipe and new pipe proved to be the third challenge on the interceptor project. The grout is intended to seal the inner pipe from groundwater entering the old pipe. The groundwater in the old pipe diluted and otherwise impaired the grout from producing a tight seal. The contractor finally hit upon the solution of sealing off a segment of the pipe and pumping compressed air into the annular space, the air being vented at the far end of the segment.

A calculated volume of the liquid cellular grout — a slurry — was then pumped into the space, and when it eventually started flowing out of the 2-inch vent tubes on the other end, the intervening space was presumed to be full of grout, which would harden in place over the following 24 to 48 hours.

“The one question is: How do you know that the grout went into every nook and cranny along the way?” Laughlin rhetorically asks. “The answer is, we don’t. We did the best we could. It may not have filled the entire annular space. But we know we have a solid piece of pipe that is pretty corrosion-resistant. If we have a few voids in there filled with groundwater, no problem.”

Laughlin notes that the initial attempt to grout without first blowing out the water was not satisfactory. He says trial and error is part of the equation when working underground. In this case, the crew was “grouting blind” and learned a better way. “That’s one of the things with trenchless technology — sometimes you have to have faith. You aren’t working in an open trench and seeing and touching what you’re doing. In terms of what is happening down there, you have to rely on your faith in the technology.”

Slippery slope

Because this interceptor is on the lower end of the valley’s elevation, engineers who laid the original pipe were limited in establishing its slope. “We can’t go deeper and deeper with our slope, so some of our pipe has a shallower slope than normally is preferred. In certain areas of the city, pipes were laid flatter than what is ideal,” Laughlin says.

Consequently, solids settle out more than they would if they were resting on a steeper slope. The solids on the bottom of the pipe were troublesome for the repair crew. “We had to clean the host pipe before we could slip the new pipe into it,” says the principal engineer. “We had to clean it and, remember, we were doing it under live flow. We would work for a week or a week and a half to take out the accumulated solids before we could slip in the pipe.”

The new interceptor line was connected to the roadway above it through seven new manholes. The contractor slipped 7-foot fiberglass manhole inserts into the original 8-foot-diameter brick-lined manholes. Seating the inserts so they didn’t leak was the challenge. This was accomplished by grouting the seating area and much of the old manhole’s brick walls. The prefab fiberglass manhole was dropped into place and made watertight at the bottom using a RAM-NEK seal. The 6-inch annular space between new and old manholes was filled with Xypex-infused lean fill material.

“Sliplining of manholes is fairly new to us,” Laughlin says. “The design came about from Carollo Engineers and the contractor. We had used it in one or two other projects and had experienced some leakage at the base. We drew on that previous failure and came up with a better mousetrap.”

Moving forward

Albuquerque and Bernalillo County have “quite a few” pipes that are nearing the end of their designed life of 40 to 60 years. Most of the interceptor pipes, the largest of which is 72-inch, are reinforced concrete. However, the system also includes some corrugated metal pipes with an asphaltic interior lining. “We are still dealing with some of that pipe,” Laughlin says, including a section that recently collapsed.

The authority is monitoring old pipe and systematically replacing it through an asset management program. Engineers and city officials are tracking the inevitable decline of infrastructure, including wastewater plants, small-diameter waterlines, wells, and reservoirs. “We are trying to see how best to spend the dollars we have and to allocate to all the different needs in the system,” Laughlin says.

The interceptor relining project took two months longer than anticipated to complete. Laughlin blames groundwater for most of the delay. Preparing the access pits and sliplining the pipe also required more time than expected, as did cleaning the lines of sediment. Still, it was a confidence-building exercise and a successful project, and the authority was generally pleased by the outcome. So were residents living along the line.

“The people who were living next door and dealing with the situation wanted it done as quickly as we could, and it never is quick enough,” Laughlin says. “They’re always happy when they see our taillights for the last time.”

Dealing with unexpected challenges

The best laid plans of mice, men and engineers sometimes are for naught. Something unexpected always crops up. One hitch in the completion of the relining project along the Rio Grande in Albuquerque, New Mexico, was the discovery of hardened concrete someone had poured into the old pipe.

When the Albuquerque Bernalillo County Water Utility Authority conducted a pre-construction CCTV inspection of the interceptor line, the puddle of concrete didn’t show up, presumably because it was beneath the level of sewage floating the camera through the pipe. But when the new pipe was inserted, it bumped up against the solid mass and blocked further penetration.

David Laughlin, principal engineer, concludes the pour probably was a “fugitive discharge.” Perhaps 30 or 40 years ago, a contractor found himself with an extra yard or so of concrete and simply opened up a manhole and emptied the extra material into the interceptor line. It wasn’t the first nor probably the last furtive dumping by contractors and other individuals. Laughlin has also found two-by-fours and couches in manholes.

The solution for the repair crew was to send a worker with a breathing mask into the pipe and manually dislodge and remove the concrete. Nasty business.

Another unanticipated change of plans on this relining project occurred after a faulty connection was made between two segments of the new pipe. “It has to be flush,” Laughlin says of the two pipe ends. “If you don’t get them straight, if they are cattywampus a little, you won’t have a full seal. You have to be super careful.”

A post-construction inspection revealed a poor seal on one of the coupled joints. As a result, an excavator had to be hauled back to the site to re-dig the slip pit. The problem was quickly unearthed: The coupler on the joint hadn’t been tightened down enough. It was securely fastened and covered up again.