Aug 19, 2002

Difficult Manhole Repair In A High-Traffic Environment

Severn Trent Services performed an innovative manhole rehabilitation for the City of Watertown, New York. While an independent contractor hired by the City's consultant had begun line cleaning and internal inspection of a 30-inch concrete line, the jet nozzle of the cleaning truck burrowed a hole through the wall of a fifteen foot deep, 90 year old brick manhole. Subsequent inspection revealed that the manhole had settled over time, shearing the connection between the 30-inch line and the manhole wall, leaving a perfect spot for the nozzle to stray. A large sink hole developed in the surrounding pavement due to bedding soils washing out into the manhole.

After City crews removed the blacktop and road base, Severn Trent crews began stabilizing the soils, driving 1-inch steel pipes into the soil surrounding the manhole to a depth two feet below the invert. These steel pipes were used as grout injection sites. The crews commenced pumping.

The crew pumped five gallons of a controlled gelatin mix of Avanti AV-100 grout fortified with a latex additive into the first injection site at its initial depth, then pulled the pipe 6 inches upward in the soil and repeated the procedure until the grout column extended four vertical feet above the failure line around the circumference of the manhole. Grout volumes at each depth varied from five gallons to twenty-four gallons. The procedure was repeated in a spiraling pattern around the circumference of the manhole. A total of over 1,000 gallons of grout was pumped during the repair.

Once the grout injection phase was completed, crews entered the manhole to confirm that all running sands had been stopped. With the surrounding ground stabilized, crews constructed a temporary flow thru bypass utilizing two sections of 27' PVC wedged inside of the 30' pipe. Crews installed plywood formwork alongside the 27' bypass, dewatered the manhole and poured a new manhole base, invert and bench, extending the bench four feet up the walls of the manhole to ensure extra stability at and beyond the location of the original manhole wall failure.

The initial repair operation took less than 48 hours with significantly less time and cost than a complete manhole replacement.