Issue
Drainage Control, at the request of the homeowner, undertook an inspection of their sewer drain which had apparently blocked. We used a CCTV camera and found that the pipe – an aged terracotta installation – had failed at a location adjacent to a lamppost which had recently been straightened.
Excavation
Due to the presence of underground utilities, it was decided to hydro-vac the excavation using a sucker truck.
The excavation revealed that the existing terracotta pipe had been crushed by the breastplates (Heavy concrete anchor blocks) that had been installed to correct the alignment of the power pole. These had been placed without the use of imported base-course (A kind of rock and soil mix that can be tightly compacted to support underground works). After placement, it appears that the existing packing was simply replaced without compaction, and it is probable that the unsupported weight of the 3 breastplates over time crushed the sewer drain.
Issues Encountered
During the excavation to access the broken section of Terracotta pipe, even before removal of the breastplates, the lamppost moved – most likely to the pre-straightening position – as a result of no compaction on the roadside of the pole. Clearly a potential hazard and it needed to be strapped to the sucker truck to secure it from further movement. A truck mounted crane was brought onto the worksite to secure the pole in its correct orientation before further work.
Remedy
With the lamppost safe, we were able to safely access the broken pipe and replace the broken section with road rated PVC100 SN16 pipe which is extremely crush resistant. This completed, basecourse was emplaced around the pipe and compacted using a heavy compactor. The breastplates were returned, supervised by a Wellington Energy contractor certified to do this work, and more basecourse – again, compacted. Finally, a layer of soil for berm reinstatement. All excess material and the broken pipe were loaded onto our truck for landfill disposal.
Comments
Sometimes, our work is far from routine, and this is a case in point. A live lamppost had not been installed correctly which caused a sewer drain failure. Drainage control had to complete this repair without taking the lamppost out of commission, and then correct the faulty emplacement of the lamppost.
Outcome
The sewer drain was successfully returned to service. We were able to confirm that the cause was a civil work that had been the cause of the failure in the first place, and thus the household that owned the sewer drain was able to be reimbursed for the cost of the repair. A successful result.