Micro-tunnelling used for new Bolton trunk sewer

By Bob Chisholm, Neville Morrison and Tim Campbell

The Albion-Vaughan Sanitary Trunk Sewer is a 2.1 kilometre length of 900 mm diameter sewer, constructed through a combination of micro-tunnelling and open-cut excavation. It was commissioned by the Region of Peel in Ontario to accommodate future development in the Bolton area of the Town of Caledon and allow the Albion-Vaughan Pumping Station to be taken out of service. The sewer, which runs along Albion-Vaughan Road from Mayfield Road to the pumping station, diverts flows from the trunk system in the south part of the Bolton South Hill Subdivision.

Chisholm, Fleming and Associates completed preliminary and detailed design, contract administration and resident inspection, post-construction services, and a SUE Level “A” investigation for the $9,500,000 project. Construction was completed in only 14 months and under tight time constraints in order to provide the necessary outlet and sewer capacity for land development. This development was already undergoing approval within the area.

Native soil conditions consisted of silty clay till with deposits of cobbles and boulders. The groundwater table lay below the base of the sewer, except in some areas where perched water was identified. Several alignment alternatives and construction methods were evaluated to address the challenges of cost, scheduling, traffic, and property and environmental impacts.

A combination of micro-tunnelling and open-trench construction was the preferred construction solution. Micro-tunnelling was used in deep sewer sections, to mitigate traffic impacts, and where underground and overhead utilities limited the use of construction equipment. High-voltage power lines run along both sides of Albion-Vaughan Road, so, for much of the sewer alignment, micro-tunnelling was the fastest, most cost-effective and safest construction method.

Approximately one kilometre of 900 mm inside-diameter sanitary sewer was constructed in five separate micro-tunnelling drives, along with eight deep shafts up to 12 m in depth. The drives were 100 m to 260 m long and were completed with a Herrenknecht AVN 800 micro-tunnel boring machine (MTBM). Sufficient surface work area had to be designated at the drive/entry shaft locations for set-up and operation of the micro-tunneling equipment. This included the MTBM control centre, spoil management equipment, crane, loader, excavator, etc.

Circular concrete sections, three metres in diameter, were used for retrieval/exit shafts. These were later converted into maintenance holes. Intermediate pre-cast concrete maintenance holes were installed as per the design prior to micro-tunnelling. As work progressed, the MTBM bored directly through these maintenance holes with remarkable precision, achieving the design sewer inverts elevations well within acceptable tolerances.

Micro-tunnelling was carried out in winter, with continuous 24-hour operation. This prevented lubricating slurry/fluids from freezing and the ground from casing around the pipeline during non-active time. This would have caused high jacking forces and halted progress of the MTBM.

The tunnel sections crossed under several sub-surface utilities, Highway 50, and Canadian Pacific railway tracks. An extensive settlement monitoring program was, therefore, put in place during the project.

Virtually no settlement was observed during the tunneling operations. As well, traffic management strategies and construction staging were employed to minimize disruption of traffic along Albion-Vaughan Road. This is travelled by approximately 11,300 commercial and commuter vehicles each day.

A by-pass strategy of temporary pumping, piping and isolation chambers maintained flows in the existing sewer. This kept the pumping station in continuous operation during construction and before the new sewer was connected at both the downstream and upstream ends. Once the new sewer was operational, flows were redirected at upstream sewer chambers, the pumping station was decommissioned, and reusable equipment was salvaged for the Region.

Bob Chisholm, Neville Morrison and Tim Campbell are with Chisholm, Fleming and Associates. This article appears in ES&E Magazine’s April issue.