Doffing the Cap
Each new hole we fill is a learning experience. This one was no exception.
The manhole was located in a local school ground, and was on a sloping driveway. The hole had failed several times, and had been filled with asphalt and concrete on different occasions. Still, the surrounds were broken, and you could certainly catch a shoe in the resulting hole.
This hole had a number of issues – some ours, some theirs. The slope was one problem, but I’ll get to that. First, lets talk about the cutter…
A week ago we’d been working on a manhole when the cutter suddenly stopped rotating. It would still go up and down, but not round and round. This was only a minor problem, as we were able to spin the cutter by hand – not easy, but certainly possible. The ground was stable, and the job was smooth, so we were not too perturbed.
We were worried though, when we were unable to fix the rotation. What we thought was a blown fuse turned out to be a bit more serious, as the control board had failed, and needed replacing. Our supplier was very obliging, and whisked one out to us by courier, but that was coming from China, and we still had work to do.
No problem!
We took the motor apart, and determined that it was working fine, so we simply by-passed the control board and wired the motor directly to the battery (via a couple of isolators, a fuse, a switch and a variable speed unit (to give us a perfect rotation speed).
So this was the first test of our jury-rig, and I’ve got to say, we’re pretty clever. It worked like a charm.
But the hole did present us with another problem. The lid was level with the manhole underneath, while the road surface had a distinct slope. Just wheeling the cutter in to place we managed to wipe off part of the protective rubber strip under the protective cover of the cutter. Clearly we could not (efficiently) cut a hole with the cutter at an angle to the road surface…
No problem again!
The cutter came with a special frame, which I thought was redundant – the machine worked perfectly well without it. But in this situation the frame bridged the road surface, and allowed us to seat the cutter flush with the surface, not the lid. End result was a perfect cut!
Once we lifted off the manhole, it was time to test out our new cap. We placed it over the hole, and jumped on it, to see if it moved. But no. It was firm in place. The cap would allow a paving crew to mill right over the hole without stopping, and pave over it without needing to deviate the paver from it’s course or manually level the road. But for us it simply allowed us to clean up the hole without any rubble dropping down into the pit below.
We were not about to remount the iron lid directly on the manhole, so we built a spacer layer using the same Neuchatel MWR mastic asphalt that we would use for the entire job. The cap did come in handy again, as we used this to gauge the level of the lid relative tho the road surface.
Once we were happy, and the layer had set sufficiently (ideally around 15 minutes) we began to fill the bulk of the hole with MWR. We aimed to get the layer across the entire surface, and bond it with the primed edge of the asphalt. Due to the depth of the hole, this layer took a little longer to set, and we spent a lot of time massaging the MWR to keep it relatively level with (but slightly below) the road surface. Initially this took quite a lot of effort, but once the top started to set, we got on with other tasks, only occasionally returning to knead it back uphill, until it had set.
Now all that remained was to apply a final coat, and sand the finish. By this stage my camera crew had left for football practice, and I didn’t capture the finished hole. I’ll go back tomorrow to check, and get a photo then.
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