A spring-like seepage that saturated the ground is the likely cause of Jan. 17’s mudslide at 738 South Island Highway, according to a consultant’s report prepared for the City of Campbell River.
And, not surprisingly, the report warns that there is potential for more slides adjacent to the site of the Jan. 17 incident.
“There is believed to have been long-term seepage on the slope face at the location of the landslide,” says a geotechnical report on the slope failure conducted by WSP Canada Inc. out of Nanaimo which is available from the City of Campbell River’s website.
A review of local weather stations did not identify particularly high precipitation in the days prior to the landslide, says the report, but consistent light precipitation had been occurring for several days prior to the landslide. There are signs of previous landslide activity in the area. The slope on the property to the north of the landslide site, 723 Ash St., “contains a local shallow landslide feature indicating that other portions of the slope may be prone to instability,” the report says.
“The escarpment runs parallel to the South Island Highway for several kilometres and has historically been subject to landslides of a similar nature,” the WSP report says.
In fact, WSP has assessed previous landslides in the area. The slope is also identified in the city’s Official Community Plan as a “Hazardous conditions Development Permit Area.”
The failure of the slope is considered a mudslide, WSP says, and actually occurred on two private single family residential properties located at 739 and 751 Ash St. in Campbell River on the escarpment behind the properties on the Island Highway. The properties are located 20 metres behind the crest of the slope and were not directly affected by the slide. Run out from the mudslide did inflict significant damage beyond the toe of the slope where a three-storey condominium is located at 738 Island Highway South. That building was impacted and evacuated and the rear parking garage bore the brunt of the slide.
The City of Campbell River has a statutory right of way containing a sewer line that services the residences on Ash Street at the top of the slope. The sewer line is close to the crest of the landslide but the report says it does not seem to have been directly affected by the mudslide.
It is estimated that approximately 550 cubic metres of soil was displaced in the mudslide.
The slide occurred in the upper half of the approximately 40 metre high slope. In the southern exposed face of the slide, “significant and concentrated amount of groundwater emerges from the face and flows down the slope in the same eroded trajectory as the landslide debris. The source of the groundwater appears to be natural.”
Drainage collection and conveyance measures had been built before the slide occurred and indicate a historic need for drainage measures.
“Our preliminary observations suggest that the concentrated spring-like seepage likely caused saturation of the surficial colluvial type soil cover and a reduction in the shear strength of the sandy deposits,” the report says.
The concentrated spring-like discharge is considered to central to triggering the slide. There is evidence of historical erosion at the location and the city has designated the slope as a hazardous steep slope within the context of the development permit approval process due to this history of slides in the area.
“The recent observations may raise the potential concern for future failures of over-steepened material adjacent to the failure scarp or associated with other areas of concentrated groundwater seepage,” the report says.
The parking structures at the rear of the building were impacted significantly and the report says it’s not clear if they can be salvaged.
“It is noted that the parking structures absorbed a substantial amount of energy from the mudslide that could otherwise have caused more serious damage to the rear of the building,” the report says.
The report concludes that the city consider a program of community education related to good slope management practices for residents/strata councils with properties on or adjacent to the steep slope. A geotechnical engineer could be hired to work with the city to develop a guideline that would include landscaping waste end dumping, fill and landscape retaining walls, drainage control and other pertinent items. City Public Works personnel should also be trained to have basic knowledge of such practices, the report says.