In January 2020, Worthington, Minnesota, was in the midst of renovating an old Campbell Soup cold storage building into an all-purpose Recreation Center for its 13,000 residents. Normally this would be a typical building renovation, however, this job came with a caveat: The building was directly over two 42-in. reinforced concrete culverts that were installed in the 1930s.
Now layer a pandemic on top of the project: COVID-19 caused the rescheduling of many fall high school sports, creating pressure on the City to provide space for indoor winter practices that usually take place outdoors in the summer. The City re-booted to adopt a phased approach to the renovation, in order to complete at least a lit indoor artificial turf field by late fall 2020.
Increasing cause for concern about the culverts’ stability is the fact that the pipeline also runs underneath the extremely active Union Pacific Railroad next to the fieldhouse. To aid in making the proper rehabilitation decision, Worthington hired Short Elliott Hendrickson Inc. (SEH). SEH is an employee-owned (800-plus employees) engineering, architectural, environmental and planning firm with its Head Office in St Paul, Minnesota.
When inspecting a culvert, typically a firm will perform a standard CCTV inspection and using the PACP report to make a rehabilitation decision. SEH decided that a video inspection would not provide the necessary quantitative data needed to make a wise rehabilitation choice and asked themselves “What are the alternatives?” LiDAR in addition to CCTV would be able to determine if the pipe had lost any interior pipe wall and check if the pipe was still in-round but how to determine what is happening outside the pipe: Are there any voids? Has there been any exterior corrosion? Is there anyone who could perform this service? After some research, SEH selected SewerVUE Technologies Corp. (SewerVUE) from Coquitlam BC and its patented pipe penetrating radar (PPR).
Pipe penetrating radar (PPR) is the underground in-pipe application of GPR, a non-destructive testing method that can identify defects and cavities within and outside mainline diameter (larger than 10 in./250 mm) non-ferrous (reinforced concrete, vitrified clay, PVC, HDPE, brick…etc.) pipes. The key advantage of PPR is the unique ability to map pipe wall thickness and deterioration including voids outside the pipe, enabling accurate predictability of needed rehabilitation or the timing of replacement. SewerVUE chose their Surveyor robot for this inspection. The surveyor is equipped with LiDAR, CCTV, PPR, inertial measurement unit (IMU), barometers and odometers.
SewerVUE and SEH arrived in Worthington on Jan. 20, to begin inspection the following day. Winter was in full-swing the next morning as the crew arrived onsite with early morning temperature of -1 F (-18 C) — even before factoring the windchill. The City of Worthington was well prepared for SewerVUE’s arrival as it diverted flow with sandbags at the upstream end on the south pipe into the north pipe to allow for optimum surveyor usage. Using an excavator, the City lowered the robot into place.
Inspection began from the downstream end of the pipe travelling 523 ft to its upstream end with the PPR sensors in the 9 and 12 o’clock positions. Once complete with these two positions, Surveyor backed up to the downstream end switching its PPR sensors the 11 and 2 o’clock positions. Surveyor then advanced again to the upstream end; providing the City with two additional continuous PPR scan lines for a total of 4 in. the south pipe. Upon completion, Worthington helped remove the Surveyor, then politely offered SewerVUE use of its heated garage to clean the tool and download the data. The results were checked before departure and all six sensors had collected the necessary information, proving the surveyor works well in cold temperature.
Upon returning the next morning all were happy to see that the weather warmed up to 31 F (1 C). The City and the SewerVUE crew, now on the north pipe, followed the same procedure from the day before. The inspection was finished before dusk and once again Worthington volunteered their garage for clean-up and data download. The City removed pipe flow diversions after SewerVUE left the site.
Upon return to SewerVUE’s head office in Coquitlam, the data analysis began. It typically takes four to six weeks for a draft report to be prepared. There was a slight delay due to the British Columbian government’s decision to shut down all offices in response to Covid-19. When the draft report was completed, the Surveyor and SEH discovered:
- North pipe has fewer defects than south pipe,
- North pipe has 2.4 times more voids outside its upper pipe walls than south pipe,
- Both pipes’ cross sections are deforming in the vertical direction (confirmed by LiDAR)
- Each pipe has undergone differential settlement. However, south pipe has almost twice as much settlement as north pipe.
- SEH and the City are confident the pipes are in satisfactory condition to install indoor lighting, insulation, heating, and artificial turf at the fieldhouse during fall 2020, to allow sports teams to practice during winter 2020/2021, and to enable continued railroad operation. This will allow for a wintertime pipe rehabilitation when Okabena Lake overflow is again minimal
- SEH is now preparing bidding documents to rehabilitate both pipes during winter 2020/2021 using trenchless grout injection into the voids followed by glass reinforced plastic cured in place pipe lining with UV-light curing method.
For this project to be finished on-time and on-budget, cooperation between the City, SEH and SewerVUE is essential and even more so under these challenging conditions. The results from this project to date have ably demonstrated working together, and all are looking forward to the successful completion of the next phase and the rehabilitation of the pipeline.