The Square Peg

Just when you thing things are becoming routine, someone throws in a curve-ball like this.

This manhole was on a sloping pavement behind a warehouse, and was obviously in need of attention.

From the outset it looked to be a routine reinstatement. The pavement had cracked around the edge of the manhole, and it seemed to have sunk into the gravel. The concrete was cut with two distinct slabs, which I imagined was due to some earlier failure. Because of this we decided to cut the minimum thickness – just enough to ensure a clean finish – which would allow us more time to inspect the hole as we cleared it out.

Given the uneven surface I was not sure what to expect from the cutting, but that proved effortless. We set the blade circumference to take in all the damaged asphalt, and most of the first block of concrete. A touch with the jackhammer loosened the asphalt and in moments we had exposed the aggregate around the manhole.

This is where things started to get interesting.

Our decision to minimise the cut depth had been a good one. Our first surprise was a metal cannister about 20mm under the pavement. Had we cut for another 30 seconds we most likely would have hit it. We did not disturb it or try to discover it’s purpose, but I assume was a water valve. The next surprise was a much bigger one. As I probed around the iron lid, breaking up the gravel which was attached to the rim, the jackhammer suddenly dropped several inches into the ground. We took it out, and cleared the loose gravel, to find a square hole under the foot of the metal rim.

It turned out that this round manhole was set on a square drain

At this point our reinstatement went more from a cautious excavation to a forensic search, and most of the rest of the work was conducted with a wire brush and a garden trowel. We soon found another similar hole, then another and another. As we carefully extracted the material, it became evident that the manhole was situated very precariously over a square hole.

Clearly this reinstatement was compromised. Our advice to the client would have been to swap out the manhole cover for a square one. But the client was unavailable for consultation, and no expenditure was authorised. Clearly the manhole couldn’t be left in this condition while we waited for an answer, so the decision was made to complete the reinstatement as agreed and submit the photos to the client for later review and or subsequent action.

Our original plan had been to lift the manhole lid out, and pack a layer of Neuchatel MWR Mastic Asphalt underneath to bring the lid flush with the pavement. But with so little space underneath, and the lid finely balanced right on the lip, we decided it was judicious to leave the rim where it was, and rely on our product to hold the lid firm.

Due to the fact there was very little underneath to support it, the manhole was left directly on the concrete drain, and the surface contoured to meet the pavement

After that, the reinstatement went like clockwork. A base layer was applied stopping around 20mm from the surface of the hole, and this was allowed to ‘break’ for 30 minutes so that it created a firm base for the final layer. Given the slope of the pavement, we set the mixer to low heat, so the asphalt would cool quickly as we worked it to the contour of the pavement. And once it had set satisfactorily, we sanded the surface for skid resistance.

Because this work was done in parallel with some other asphalt repairs there was no down-time on the job. Even with the painstaking excavation work, the actual time ‘on the tools’ had been less than an hour.

The finished work ensured that the manhole was fixed firmly in place, and the seal around it was watertight. While we hope the client will take our advice and switch the cover for a square one, we are confident that we have strengthened this manhole, and lessened the likelihood of it moving or caving in.

HoleWorld is a specialist manhole reinstatement and pothole repair provider. Their systems and products and revolutionary in NZ, overcoming the multitude of problems faced by conventional methods of repair, and offering life-of-the-road fixes. For more information, call us on 027-421-5214 or email us at [email protected]