Award of Merit: Seine Riverbank Stabilization

Category: Water Resources

UMA ENGINEERING

In the summer of 2000 continuing movements on the bank of the Seine River in Winnipeg posed an immediate risk to the conduit that supplies 40 per cent of the city’s drinking water. The Branch 1 Shoal Lake Aqueduct is a 1.7 metre diameter precast reinforced concrete pipe, manufactured in 2.4 metre lengths.

Built in 1918, the inverted siphon was installed beneath the river channel in an open trench. Experience over the years has shown that even minor deflections of the pipe can result in leakage at the joints, so when instabilities affecting about 100 metres of the river bank were detected in 2000 it was decided that the 11-metre high west bank needed to be stabilized without delay. Ground movements had damaged a sidewalk at the top of the bank, a drainage sewer outfall pipe and a footbridge upstream.

The team faced the dilemma of how to stabilize the riverbank without triggering additional earth movements that could rupture the 82-year old pipe. Aside from the technical challenges, it was necessary to address concerns of local groups like Save our Seine and the Old St. Boniface Residents’ Association who wished to preserve the site’s natural and historical setting. The site is adjacent to the Lagimodiere-Gaboury Historic Park, which commemorates the first European homestead on the Prairies.

The design objective was to achieve a minimum factor of safety of 1.5 against bank failure, recognizing that the consequences of a failure during the remaining 50-year service life of the pipe would be catastrophic.

The team recommended that compacted rock columns be installed along 100 metres of the river edge. The 10-metre deep columns are sunk entirely below the existing grade, and landscaping masks any visual evidence of the stabilization measures. While rock columns are an accepted method for stabilizing Winnipeg river banks, the granular material is typically dumped into the caissons without compaction. In this case the design team recognized that higher shear strengths and design economies are possible by compacting the crushed limestone.

Laboratory and field tests were done to determine what vibratory construction methods could be used. Subterranean Manitoba fabricated a 15-metre long steel beam with steel fins extending outward at the bottom. Vibration was imparted to the beam from a vibratory pile-driving hammer, the beam was placed into the drill hole and this was backfilled with a known weight of crushed limestone. The beam was vibrated as it was extracted from the hole, compacting the crushed limestone in the process. The Manitoba Technology Centre monitored the process and found that vibration levels were well below the level to cause structural damage to the nearby aqueduct. The vibration techniques developed on this project for compaction of rock columns are now routinely employed on bank stabilization projects in Winnipeg.

Landscape architects designed bio-engineering techniques for protecting the bank against erosion. Late in the winter of 2001, live fascines were laid on exposed clay fills along the edge of the river. The live-fascines consist of bundled willow branches, some of which will eventually sprout and provide vegetative cover. This erosion protection option was less expensive than riprap and more in keeping with the natural setting. Sedge grass was also successfully salvaged and replanted.

The site was restored and enhanced with an access trail, canoe dock and a lookout structure. The emergency project was completed by November 2001. It was on budget at a cost of $1 million.

Name of project: Seine Riverbank Stabilization at the Shoal Lake Aqueduct crossing, Winnipeg