Implementation period: 24 October – 10 November 2023
Decommissioning of branch drainpipes, either through clogging them with bentonite or through cutting them off in the lower part of the system.
The first step was to uncover the upper part of the drainage system. The excavations were performed at the locations of geodetic points that had been previously established to mark the supposed termination of the drainpipes, based on a survey of orthophoto images.
On the day of decommissioning, the throughput of the uncovered drainpipes was tested by pouring water into the pipes. It passed freely through two drainpipes, but the third pipe did not allow the water to pass (the inside was visibly clogged). Therefore, a different, unclogged drainpipe was selected for the decommissioning.
The bentonite was injected using a mobile concrete pump. The bentonite preparation was performed outside of the pump, in a concrete mixing tub using an electric paddle mixer. The material was prepared by a gradual adding of bentonite to water and continuous mixing, running the mixer in high revolutions. Otherwise, hard lumps would be formed in the water and the material would not be homogenous and smooth.
Application procedure:
- Fitting the pump hose onto the drainpipe
- Sealing the connection using the clayish soil from the excavation
- Backfilling and tamping down the excavation
- Slowly injecting the bentonite mixture into the drainpipe
- Stopping the process once the mixture starts to pour out of the lower end of the drainpipe
- Clogging the lower end of the drainpipe
- Backfilling the lower-end excavation (this was performed later, after an inspection of the new overall condition of the pipe)
Lessons learned:
- It is necessary to mix the bentonite mixture slowly, gradually, and using high revolutions of the paddle mixer
- The main drainpipe (the collector drainpipe) does not always run in the supposed route, i.e., along the line of lowest elevation in the thalweg, especially if the local drainage system is more complex
- Even if water runs on the surface, it does not necessarily mean that the drainage system is defunct
- It is necessary to seal the connection between the pipe and the hose very well before injecting the mixture into the pipe
- The drainpipe’s condition (is it clogged or not?) should be checked beforehand
