PART B: 2003 SITE VISIT LANDSLIDE RISK ASSESSMENT PEACE REGION (PEACE RIVER VALLEY/HIGH LEVEL) SITE PH9: SHAFTSBURY TRAIL LEGAL LOCATION: 31-83-21-W5M, 22-82-23-W5M Location along Highway: Shop Slide, along the old Hwy. 2, near ATU warehouse, west side of the Peace River Shaftsbury Trail Overpass at CNR crossing of Hwy. 2, west side of the Peace River Brick Hill Slide 20 km south of Peace River towards Grimshaw AT FILE: SH684 Date of Site Visit:: 27 May 2003 Shop Slide Significant Observations • The Shop Slide is located along the old Hwy. 2, above the railway tracks. At the Shop Slide, the highway is in cut and fill. A crack and settlement in the road surface define the northern end of the slide. Cracks extend onto the slope, where a very clearly defined scarp has developed. • The slope appears to have been regraded in the past. • The slope appears relatively dry, seepage was not observed. A subsurface drain discharges water into a small channel just to the north of the slide scarp. • A slope inclinometer is present just above the railway tracks, but it is not being read. • Slope failures have developed in the cut slope uphill of the highway. It is not clear whether the failures uphill of the road are connected with the failure of the road surface. • In the past gabion mattresses have been placed in the roadside ditch. • It appears that a slope inclinometer had been installed in the failure uphill of the road. The slope inclinometer casing lay flat on the ground. Changes from Previous Visits A narrow cracks that defines the northern end of the slide could be traced to the uphill side of the pavement (Photo 1) and a crack that defines the southern end of the slide was identified (Photo 2). The slide is approximately 200 m wide. The cracks indicate a moderate rate of movement. Discussion The slide appears to be more substantial than previously thought. The scarp extends to the roadside ditch. Thus the slide affects both lanes of the road. Based on the extent of the slide, it appears that the slide is seated in native ground, not in the fill.
The cracks that define the southern and northern limits of the slide appear to coincide with the failure in the cut slope uphill of the road (Photo 3). Potentially, the failure that affects the road surface and the failure in the cut slope form part of one slide mass. The road has been patched since 1985, but it appears that more extensive measures have not been implemented. Assessment Shop Slide: PF(9) * CF(6) = 54 , this has been increased from 18. The slide is more substantial than previously thought and appears to be more active. Recommendations It is recommended that three boreholes be drilled and slope inclinometers be installed in the Shop Slide to determine the depth and nature of the movement. A proposal for installation of instrumentation was submitted in March 2003. Shaftsbury Trail Overpass The Shaftsbury Trail Overpass was not visited in 2003. The Bridge Section was made aware of the issues with the overpass structure and it appears that remedial measures were implemented at the abutment. The Shaftsbury Trail Overpass was constructed in 1982. Movement of the western abutment was noted soon thereafter, resulting in distress of the structure. In 1986, fill was removed and additional spans were added to the overpass. Significant Observations • At the Shaftsbury Trail Overpass, movement of the fill structure appears to have occurred in the past. The initial overpass was constructed in 1982, and two spans were added at the western abutment in 1986, to unload the embankment. Construction required a very thick fill. The abutments are founded on piles. • At the eastern abutment, the bolts that hold the bridge deck in place indicate movement of the bridge deck towards the northeast relative to the abutment. At the western abutment, the bridge deck appears to move towards the west. This suggests that the western abutment is moving towards the east, pushing the bridge deck in the same direction. • Concrete slabs at both abutments have moved downward relative to the abutment piers, causing buckling around the piers. • Three slope inclinometers that had been installed in the slope in 1993 were added to the reading schedule in 2001. The slope inclinometers do not indicate significant movement. • Steel brackets have been installed at the bridge abutment, apparently to reduce the risk of further slope movement.
Discussion After removal of fill and addition of two spans in 1986, the western abutment still appears unstable. It appears that the slope has not been monitored for a number of years, such that the rate of movement is not known. However, judging from the damage to the abutment seats, it appears significant movement has occurred. Assessment The slide could impact two roadways: Shaftsbury Trail and Hwy. 2. Continued movement of the slide would probably not affect Shaftsbury Trail, because it is well removed from the bridge abutment where movement appears to be occurring. The main risk is to the bridge across Hwy. 2. Risk Assessment: The site appears to have been active between 1986 and the present, because the abutments indicate that movement has occurred. Slope inclinometer readings in the past two years have not indicated movement. Based on past experience, there is a risk that movement will occur. Whether the structure can handle such movement is not a geotechnical issue, it is to be addressed by Bridge Branch or their consultant. Recommendations It is our understanding that this site has been brought to the attention of the Bridge Section. Steel brackets have been installed at the bridge abutment, apparently to reduce the risk of further slope movement. Brick Hill Slide Remedial measures have been prepared for the Brick Hill Slide and it is intended to repair the slope in 2004. Significant Observations • The road is in cut and fill. The slope below the road is approximately 30 m high in this area. The lower part of the slope is gentle, but it is steep just below the road (approximately 2H:1V). • The settlement and cracks in the road surface were observed where the road crosses a small drainage course (Photos 4 & 5). No cracks were observed in the soil. • A culvert is not present at the ravine, instead, the water is channeled into the roadside ditch. A drainage ditch is located parallel to and downhill of the road and discharges into a buried culvert that carries the water down the hill. The culvert also receives water from the upstream ditch. • Slope inclinometer readings from J.R. Paine indicated a distinct shear zone at 8 m depth, with movement of 60 mm between December 1999 and October 2000; the slope inclinometer was sheared off in May 2001.
Changes from Previous Visits The area had been patched since last year. The cracks in the road and settlement of the road have continued (Photos 4 & 5). Discussion The slide appears to be a relatively small slide in fill. It is located where the road crosses a small ravine. Assessment Risk Assessment: PF(11) * CF(4) = 44 , the slide is active with moderate but increasing rate of movement. This is a relatively small fill, but in case of a failure, it may require closure of the road. Recommendations It is recommended that repairs be implemented once the design is completed. Brick’s Hill Washout A washout of the road has occurred at a bend in McAllister Creek at the base of Brick’s Hill. The washout was visited as part of an emergency callout on April 24 (Photo 6). Remedial measures for the washout have been designed and it is intended to implement them in the summer of 2003. Upstream of the washout, significant erosion has occurred at another bend of McAllister Creek. The creek bed had eroded to within 9 m of the fence (Photos 7 & 8). It is intended to implement erosion control measures at this location in 2004.
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