Unconventional excavation digs up award

Vancouver Regional Construction Association recognizes challenging pump station upgrade
Tuesday, December 15, 2015
By Jean Sorensen

Built in 1965, the Lynn Sewage Pump Station reached the half century mark in full need of an upgrade and expansion to meet a rising population on Vancouver’s North Shore, plus enhanced seismic and code requirements.

Owner Metro Vancouver chose Graham Construction & Engineering Inc. as the primary contractor with a condition that the work had to be performed without disrupting the North Shore portion of a Metro Vancouver wastewater collection system that handles one billion gallons of wastewater. While the Metro Vancouver collection system is largely gravity fed, pumping stations like the Lynn facility play a key role in moving wastewater over humps.

The upgrade involved an expansion, refurbishing the building equipment, electrical and mechanical and also bringing the existing building and new addition up to seismic and code provisions. Excavation would be required. Graham turned to Matcon Canada to undertake the location and excavation of the utilities and the seismic work on the extremely tight site, which left no room for error. The successful completion of the excavation and shoring work has earned Matcon the Vancouver Regional Construction Association’s 2015 Chairman’s Trade Award for a project up to $1 million in value.

“It was a deep excavation and the cut-off wall built around the perimeter ensured that our men could work safely in the hole,” says Graham’s senior project manager Ronan Deane.

Deane says there was a lot of drilling and anchoring work that had to be carried out by Matcon with large equipment on the tight site as well as large structures brought and placed on site.”The whole process was delicate and had to be well thought out. I think that is a major reason they were commended with this award. Throughout the work they carried out they protected the electrical and sewage utilities and kept the system running,” he says.

Dan Hunt, construction manager for Matcon, says the initial challenge was finding the utilities. While there were old drawings for the existing building, there was a lack of detailed information on where the utilities lay beneath the surface and at what ground depth. In total, Matcon found and unearthed 14 utilities, including two force mains measuring 600 and 450 mm plus another 1050 mm concrete pipe.

Matcon also had to devise a means of preventing ground water from flowing into the hole and stabilize the walls of the excavation as the soil consisted of sand and cobble – river-like stones that were susceptible to crumbling.

A cut-off wall was built above ground, but employed reinforced secant jet grout piles. The wall blocked ground water from the area but the jet grouting also extended below the surface holding back the wall as crews excavated at the site and around the building, which was situated at the south side of the excavation. In order to hold back the walls near the building, the crew used shotcrete walers with tie-back anchors to maintain slope integrity.

The second challenge was the congested site for the heavy equipment which had to move around in the excavation site avoiding utilities and also overhead lines. “It was a non-conventional area of about 25 metres long and 15 metres in width,” says Hunt, with the existing pumping station close to the south cut-off wall and measuring five metres in width and 18 metres in length.

Working in the area was a large Soilmec SM-30, a jet grouting rig, plus a large Hitachi 455 excavator which assisted with wall reinforcement as well as a drill rig used to place the anchors.  “In addition, we had a smaller excavator moving material around,” he says. To facilitate the jet-grouting, a batch plant was set up on two steel skids which took up an area of six by nine metres.

The final challenge on the job was supplying and installing 12 DCP soil anchors using the onsite drill rig. While there was clear access to the area where the new addition was planned, Matcon had to provide seismic anchors under the existing building. “We went through the roof,” says Hunt, with the drill going completely through the building’s structure, into the basement and the grade below to tie the structure into place. Matcon also had to test the anchors once they were placed and used a test jack and beams in the existing structure’s basement.

Hunt says it was really the small site and soil conditions, with the large amounts of cobble, that were the factors that caused Matcon to look at a different approach to excavating the area. “We couldn’t use conventional methods,” he says.

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