November 28, 2008

Solution to land slippage problem in West Quesnel district appears imminent

JIM STERLING

correspondent

Quesnel, BC

After years of investigation, a practical solution to a serious land slippage problem in the western section of Quesnel, B.C. appears imminent.

The Uplands area of Quesnel experiences ground movements ranging from two to seven centimetres a year. That’s sufficient to have significant effects on buildings and private and city infrastructure.

A full-scale, sub-surface dewatering program is the recommended strategy to control the long-standing problem in the central British Columbia city.

A report detailing the findings of an eight-month-long trial dewatering program has been forwarded to B.C.’s Ministry of Community Development for review, said Jack Marsh, director of public works for the City of Quesnel.

The city has also asked the provincial government to help cover design, engineering and then implementation costs for a full-scale dewatering system.

Marsh said the initial cost estimate for the work was $13 million, but more data has subsequently been collected and that figure is subject to change. Quesnel has spent more than $2 million on the West Quesnel Land Stability Program since 2000.

The affected area includes about 750 houses, 2,000 residents and an elementary school. The area is under a new-construction freeze.

A horrific natural gas explosion in the slippage zone killed six people in 1997, although it was never confirmed land instability was a contributing factor.

However, ground movement in the area was conclusively proven by 2002. Geotechnical work followed to establish the boundaries of the affected area and work began on remediation strategies that resulted in the trial dewatering program launched in 2007 and completed in May, 2008.

Four vertical pumping wells and four horizontal drains were installed in different soil types to determine the effectiveness of each system. The vertical wells ranged from 31 to 61 metres in depth and intercepted varying amounts of water.

One well, for example, pumped out more than five million litres of water during a seven month period. The horizontal drains were drilled at close to right angles into slopes near the bottom of the study area. One of those removed 626,000 litres of water during the trial period, according to data collected by the City of Quesnel.

A key component in gathering insight into what’s happening and why in the affected area is the role of the city’s consulting partners. AMEC Engineering has initiated detailed programs to better understand the extent and rate that land in the study area is sliding.

“Extremely high ground water pressure is part of the problem. That coupled with weak, fine-grained soils within the bedrock is the cause of the ground movement,” stated AMEC earlier this year in a published report.

The company has used new technologies and techniques to gain insight into what’s happening and why below the ground. AMEC’s high-tech sleuthing includes the use of InSAR, a satellite that can detect minute movement in the Earth’s ground surface.

The company has also made use of LiDAR to survey the topography from the air, Electrical Resistivity Tomography to determine the geology below the surface, remote, real-time test well and ground instrumentation monitoring, computer aided slope stability analysis and a custom weather station.

The detailed scientific data provides the framework for the full scale dewatering program currently being planned.

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