Alberta Transportation Room 223, Provincial Building 4709 - 44 - - PDF document

08\D:\Word\15\76-11 let NC 17A 2002 Inspection Report

Continued… January 14, 2003 File: 15-76-11 Alberta Transportation Room 223, Provincial Building 4709 - 44 Avenue Stony Plain, AB T7Z 1N4 Attention:

• Mr. Rob Lonson, P.Eng.

NORTH CENTRAL REGION LANDSLIDE ASSESSMENT HWY 63:12 TWINNING PROJECT KM 2 to KM 9 (NC17A) 2002 ANNUAL INSPECTION REPORT Dear Sir; This letter documents the 2002 annual site inspection of key slope sections located along the west side of the southbound lanes of Hwy 63:12 between km 2 and km 9. The work was undertaken by Thurber Engineering Ltd. (Thurber) in partial fulfillment of our Geotechnical Services, Monitoring and Assessment of Instrumentation and Landslides contract with Alberta Transportation (AT). The inspection was undertaken on July 8, 2002 by Mr. Don Proudfoot, P. Eng of Thurber with Mr. Roger Skirrow, P. Eng of AT. 1. BACKGROUND Highway twinning over the project limits was carried out in 1999. The twinning was carried out on the west side of the existing highway along the toe of the Athabasca River valley slope where weak marginally stable clay colluvium was present. One

• f the main issues from a slope stability point of view was that a 16 m deep

backslope cut was required into the colluvium slope at km 2.710. Based on stability analyses, the top of the colluvium deposit was offloaded to reduce the potential for movement of the colluvium into the new highway alignment. A Stratigraphic cross-section, showing the slope configuration before and after

• ffloading is provided on Figure NC17A-3 in Section F of the binder.

During construction two shallow slumps (North Slump and South Slump) occurred in the 5.5H:1V upper backslope located above the offloaded area. The installation

• f French drains and regrading appeared to mitigate the slumps in the short term

but the slumps moved again at a later date. A slump also occurred after construction in the lower part of the backslope between the offloading area and the

• highway. No remedial measures have been carried out at the lower slump to date.

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Continued… Backslope cuts were also required at other locations along the new highway alignment where a 760 mm diameter high pressure oil pipeline (Wildrose Pipeline)

• perated by Enbridge paralleled the highway partway up the colluvium slope

(km 3.7 to km 10.3, approximately).The strategy used along these sections was to raise the highway profile and reduce the median width in order to limit the backslope cuts below the pipeline to a maximum height of 3m at a 3H:1V

• inclination. Slope inclinometers were installed at selected locations between the

pipeline and the new highway to provide early warning of any potential ground movements. The general layout of the project area and instrumentation locations are shown on Figures NC17A-1 and NC17A-2, Section F. Further descriptions of each of the above areas were provided in our 2001 inspection report. 2. SITE OBSERVATIONS The focus of the site visit was the high backslope cut section and area of slope

• ffloading in the vicinity of km 2.710. Given the favorable slope inclinometer

readings to date, the section along the pipeline (km 3.586 to km 10.340) was given

• nly a cursory inspection and no significant deficiencies were noted.

The updated condition of the slumps at km 2.710 at the time of the July 8, 2002 visit are shown on Figures NC17A-16 and NC17A-17 and on the selected photographs included in Section F. The condition of the slump in the lower highway backslope (Figure NC17A-16) has not changed much since 2001, except that there is now a thicker grass cover over the area and the slide cracks are more subdued. Some water still appears to be ponding in the highway ditch at the north end of the lower slump. The slumps in the upper backslope (Figure NC17A-17) of the offload area appear somewhat better than last year. Although the slumps are still evident, they now have more grass cover and the slump features (tension cracks, hummocky surface and toe rolls) are more subdued than last year. The North Slump does not appear to have retrogressed any further up the slope since the last inspection. However, tilting trees on the slump have begun to die and dry up. 3. ASSESSMENT The localized slumping at km 2.710 is considered to have been caused by progressive failure of the pre-sheared (slickensided) clay colluvium after the negative pore pressures due to offloading of the slope dissipated, reducing the effective cohesion within the slope. Based on the observed behaviour of this material, the stable angle of high slopes in the presheared colluvium appears to be about 11 ° (5H:1V) or flatter.

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Continued… The slump in the lower highway backslope is located in a thick portion of the colluvium layer. Under the current conditions, the slump may continue to creep and grow in size both back into the slope and along the slope parallel to the

• highway. However, the new grass cover and annual sealing of the main tension

cracks should help to reduce infiltration of surface water and slow down the rate of slope deterioration. The colluvium layer thins considerably in the vicinity of the upper backslope cut above the offloaded area. The North Slump is located at the mouth of a ravine where the colluvium cover locally extends further up the slope. Based on our site

• bservations this slump could eventually extend another 20 m up the slope,

although the affected area will likely be quite narrow (10 m in width) and shallow (1 to 2 m deep). The South Slump is expected to grow at a slower rate and may eventually reach a stable angle, provided cracks are routinely sealed to limit ingress of surface water. SI99-1 (refer to Figure NC17A-17) installed in the backslope of the offload area cut between the North and South slumps has shown no signs of movement over the last 2 years. This lack of movement confirms that there is little to no colluvium in the backslope area between these two slumps. SI99-2 (refer to Figure NC17A-16) installed at the crest of the highway backslope at a location 34 m north of the local slump shows that the deep colluvium layer has experienced global movement at a depth of 21 m along the contact with the limestone at a relatively steady rate of 2 to 4 mm/year over the last two years. Although this is a relatively small amount of movement it highlights the marginal global stability of the colluvium deposit. 4. RISK LEVEL A risk level of 18 is considered applicable to the area at km 2.710. This is based

• n a Probability Factor of 9 (active with moderate rate of movement) and a

Consequence Factor of 2 (site where slide movement in the backslope could result in some slide material moving onto the highway). The risk level for the pipeline section (km 3.586 to km 10.340) is assessed as 20. This is based on a Probability Factor of 2 (Inactive, low probability of slide

• ccurrence) and a Consequence Factor of 10 (site where a rupture of the pipeline

would shut off the supply from Suncor and cause a major oil spill resulting in a large financial and environmental loss).

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5. RECOMMENDATIONS SI99-4, SI99-5 and SI99-6 have indicated favorable performance of the valley slope along the pipeline section and hence no action is currently required in this

• section. Due to the high consequence of a potential slope failure involving the

pipeline, it is recommended that instrument readings be continued on a twice yearly basis to provide some advance warning of any potential future slope movements along this section. SI99-5 was damaged last year and attempts to repair the casing have not been successful. We recommend that this casing be replaced and protected with a heavier duty lockable surface casing due to the high consequence factor for this site. The local slumps in the vicinity of km 2.710 are not currently affecting the highway and appear to be in better condition than last year due to the good grass cover that has developed in this area. No maintenance measures are recommended at this time however the slumps should be monitored again next spring (2003). Effective longer term remedial measures are expected to be expensive and may not be warranted at this time. These could consist of the subexcavation of the failed soil mass, and replacement of the excavated soil in thin well compacted layers to re-establish some cohesion in the slope. Further consideration should also be given to installing a culvert/subdrain and backfilling the west highway ditch to flatten and reduce the height of that slope, in a similar manner to what was done at km 6.675. There may also be some benefit in staking the remaining thin colluvium layer at the North Slump down to the firmer base materials below the slip plane using driven timber or steel piles installed on a relatively close spacing. In the meantime, the dead trees should be cut off at ground surface leaving the roots intact, so they don’t topple over at some later date and further disturb the slump area. 6. CLOSURE We trust this assessment and recommendations meet with your needs at this time. Please contact the undersigned should questions or concerns arise. Yours very truly, Thurber Engineering Ltd.

• D. Papanicolas, P.Eng.

Review Principal

• riginal signed by:

Don Proudfoot, P.Eng. Project Engineer

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