DO EDMONTON, Alberta T6E 6A5 Phone [780) 438-1460 F a x (780) 4 - - PDF document

THURBER ENGINEERING LTD.

S u i t e

200, 9636- 51st Avenue

EDMONTON, Alberta T6E 6A5 Phone [780) 438-1460 F a x (780)

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January 9, 2002 Alberta Transportation Room 223, Provincial Building 4709- 44 Avenue Stony Plain, Alberta T7Z 1N4 Attention: M r . R

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Lonson, P.Eng.

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THURBER

File: 15-76-11

A L B E R T A T R A N S P O R T A T I O N

TECHNICAL STANDARDS BRANCH

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NORTH CENTRAL REGION LANDSLIDE ASSESSMENT HWY 63:12 TWINNING PROJECT KM 2 to KM 9 (NC17A) 2001 ANNUAL INSPECTION REPORT Dear Sir; This letter documents the 2001 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. This is the first inspection carried out for this site under the annual landslide inspection contract. The work was undertaken by Thurber Engineering

• Ltd. (Thurber) in partial fulfillment of our Geotechnical Services, Monitoring and

Assessment

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Instrumentation and Landslides contract with Alberta Transportation (AT). The inspection was undertaken on July 5, 2001 by Mr. Don Proudfoot, P. Eng of Thurber with Mr. Roger Skirrow, P. Eng of AT. 1. BACKGROUND 1.1 General Highway twinning over the project limits was carried out in 1999. The grading design was carried out by EXH Engineering Services Ltd. (EXH) and the highway was constructed by Thompson Bros. Contracting Ltd. Thurber were retained to provide recommendations regarding slope stability design. 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

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GEOTECHNICAL AND GEOLOGICAL ENGINEERING - CALGARY / E D M O N T O N / T O R O N T O / V A N C O U V E R / V I C T O R I A / F T . M c M U R R A Y /MEDICINE HAT

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Alberta Transportation

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January 9, 2002 colluvium was present. The main issues from a slope stability point of view were as follows:

• A 16 m deep cut was required into the colluvium slope at km 2. 710 where

the valley slope extended further out to the east through the proposed highway, and

• Backslope cuts were also required along a section where a 760 mm

diameter high pressure oil pipeline (Wildrose Pipeline) operated by Enbridge paralleled the highway partway up the colluvium slope (km 3.7 to km 1 0.3, approximately). The general layout of the project area is shown on Figures NC 17 A-1 and NC17 A-2, Section F. Further descriptions of each of the above areas is provided in the following sections. 1.2 High Cut at km 2.710 The soil conditions at the high cut section are shown on Figure NC17 A-3, Section F and consist of up to 21 m of clay colluvium overlying oilsand or limestone

• bedrock. Twinning had to be carried out on the west side of the existing highway

due to the presence of the Wildrose Pipeline on the flat lying river terrace along the east side of the highway. Our stability analyses indicated that the clay colluvium slope was only marginally stable and the proposed 3H: 1V backslope cut with 3 m wide benches at every 5 m height interval would likely not remain stable. Due to the limited options available, AT selected to adopt a design which involved offloading the top of the colluvium deposit to achieve a global factor of safety of about 1.1. The material that was removed was used for highway grade construction. Following slope offloading some small movement was observed at the base of the colluvium layer in Slope Inclinometer Sl99-2 but the creep rate diminished with

• time. As a whole, the offloading approach has appeared to be successful!.

However, there have been some smaller localized slope failures in the colluvium since construction. Two slumps occurred in the upper 5H:1V backslope of the offloading area in November 1999 along pre-sheared surfaces caused by slickensides in the clay

• colluvium. An assessment was carried out by Thurber and the results are

presented in our letter to Mr. Lee Goehring, P.Eng. of EXH dated December 2, 1999, which has been included in Section G of the binder. This document contains plans and cross-sections of the slumps, and design recommendations and sketches for implementation of slope drainage measures. The remedial measures were carried out in the spring of 2000 and consisted of installing french drains perpendicular to the backslope through the slumps, tied into a 100 mm diameter

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GEOTECHN ICAL AND GEOLOGICAL ENGINEERING · CALGARY /EDMONTON/TORONTO/VANCOUVER/VICTORIA/FT. M c MURRAY / M EDICINE HAT

THURBER ENGINEERING----------------------

Alberta Transportation

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January 9, 2002 Big "0" subdrain located at 4 m depth parallel along the toe of the backslope. The subdrain was installed with the highest point located at the middle of the backs lope and the outlets located in the bush at the north and south ends of the backslope. A change to the design consisted of the use of a combination non-woven geotextile/geomembrane around the filter gravel zone. The half consisting of non- woven geotextile was placed against the upslope wall and over the top of the drainage gravel while the geomembrane was placed under and against the downslope wall of the drainage gravel zone. The backslope was then regraded. The remedial measures appeared to perform satisfactorily initially, however the slumps re-appeared along the same limits later in the year. A 60 m wide slump occurred in the 13 m high highway backslope in June 2000, at a location 34 m south of 8199-2. The backscarp of the slump extended into Bench #2 however, there was an area of additional ground settlement extending another 21 m to the west into the flatter area of slope offloading. The toe roll of the slump was present in the west side of the highway ditch. The site conditions, our analyses and potential remedial measures are discussed in our fax dated June 27, 2000 to Mr. Goehring, which is included in Section G of the binder. The analyses indicated that offloading of the top of the slump area would locally improve the stability of the slump area but might reduce the global stability of colluvium deposit. Other options considered were placing a culvert in the highway ditch to allow a shallow toe berm to be constructed versus subexcavation of the slump and reconstructing the slope using recompacted clay, gravel or clay placed over a gravel drainage layer. Since the slump was not affecting the highway, a decision was made to only carry out some minor regarding to seal the scarp cracks and to continue to monitor the condition. 1.3 Backslope Cuts along Wildrose Pipeline Section Within the section of the alignment between km 3.586 to km 10.340 (north project limit), the Wildrose Pipeline is located on the west side of the highway within the colluvium along the lower part of the valley slope. Based on discussions with Enbridge and their geotechnical consultant AGRA Earth and Environmental Ltd., it is understood that the pipeline can tolerate broad slope creep movements but would be sensitive to abrupt shear movements due to slope failures. Given the marginal stability condition of the lower slope in which the pipeline is located, an approach was taken by the highway design team to raise the new southbound lanes and reduce the spacing between the southbound and existing northbound lanes as much as possible to eliminate or reduce the proposed backslope cuts between the highway and the pipeline to a maximum height of 3m at a 3H:1V inclination. This required replacement of the west highway ditch with a subdrain and an asphalt swale at a few locations (eg. km 6.675) to maintain the

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GEOTECHNICAL AND GEOLOGICAL ENGINEERING - CALGARY /EOMONTON/TORONTO/VAN=UVER/VICTORIA/FT. McMURRAY

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Alberta Transportation

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January 9, 2002 backslope cut height below the 3 m maximum value. The analyses to support the design are included in our geotechnical report dated November 12, 1999. As an added precaution, 3 slope inclinometers (SI99-4, Sl99-5 and Sl99-6) were installed at selected locations along the alignment where the backslope cuts were near the 3m height as a means of monitoring slope movements due to the highway construction. A fourth slope inclinometer Sl99-3 was installed in the existing colluvium slope between the highway and the pipeline at a location where no cut was required to provide a measure of potential natural slope movements. The locations of the slope inclinometers are shown on Figures NC17A-1 and NC 17 A-2 and stratigraphic cross-sections of the key slope sections are shown on Figures NC17 A-4 through NC17 A-7, Section F. The slope inclinometers located along the pipeline had not shown any significant signs of movement up to and including the Fall 2001 readings. 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 2.71 0. Given the favorable slope inclinometer

readings to date, the section along the pipeline (km 3.586 to km 1 0.340) 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 5, 2001 visit are shown on Figures NC 17 A-8 and 9 and on the selected photographs included on Figures NC17A-10 through NC17A-15, Section F. The condition of the slump in the highway backslope has not changed much since 2000, except that there is now a good grass cover over the area (Photo #1 ). The maintenance contractor had sealed some of the original scarp cracks which appears to have helped to reduce further deterioration of the slope. However, there are a few more tension cracks evident in the area of ground settlement behind the main backscarp of the slide and at the north part of the toe

• f the slump. Also, the toe of the slide has partially blocked the highway ditch

causing some ponding of water (Photo #2). The slumps in the backslope of the offload area (Photo #3) have moved. The North Slump has retrogressed further up the slope (Photos #4 and #5) with slabs

• f colluvium 1 to 3 m thick sliding down the slope and carrying trees with them.

The South Slump has performed better with much smaller observed movements. The slumps in the backslope of the offload areas are located about 150 m west of the new highway.

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GEOTECHNICAL AND GEOLOGICAL ENGINEERING - CALGARY /EDMONTON/TORONTO/VANCOUVER/VICTORIA/FT. McMURRAY

/MEDICINE HAT

THURBER ENGINEERING----------------------

Alberta Transportation

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January 9, 2002 During the Spring 2001 visit some seepage was noted from the south outlet of the subdrain located along the toe of the offload area backscarp. However, no seepage was noted at the north outlet. 3 . A S S E S S M E N T The localized slumping at k m 2.710 is considered to be due to progressive failure

• f the pre-sheared (slickensided) clay colluvium as beneficial negative pore

pressures due to offloading of the slope dissipate, reducing the effective cohesion within the slope. Based

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the observed behaviour of this material, the stable angle

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high slopes in the presheared colluvium appears to be about 11 o (5H:1V)

• r flatter.

The slump in the 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, which is the most active slump, appears to be retrogressing u p a ravine where the colluvium cover locally extends further up the slope. Based on our site observations this slump could eventually extend another 20 m up the slope, although the affected area will likely be quite narrow (10m in width) and shallow (1 to 2m 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. Sl99-1 installed in the backslope of the offload area between the North and South slumps has shown no signs of movement over the last year. This suggests a thinning of the colluvium layer in the backs lope area between these two slumps. Sl99-2 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 3 to 4 mm/year over the last year. Although this is a relatively small amount of movement it highlights the marginal global stability of the colluvium deposit. 4 . R I S K L E V E L A risk level of 18 is considered applicable to the area at k m 2 . 7 1 . This is based

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

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GEOTECHNICAL AND GEOLOGICAL ENGINEERING • CALGARY / E D M O N T O N / T O R O N T O / V A N C O U V E R / V I C T O R I A / F T . McMURRAY / M E D I C I N E HAT

THURBER E N G I N E E R I N G----------------------

Alberta Transportation

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January 9, 2002 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

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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). 5. R E C O M M E N D A T I O N S Sl99-4, Sl99-5 and Sl99-6 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

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a twice yearly basis to provide some advance warning of any potential future slope movements along this section. The local slumps in the vicinity of k m 2.710 are not currently affecting the highway however some maintenance measures are recommended to help reduce the potential for further deterioration of these slope areas. It is recommended that some ditch cleaning be carried out along the toe of the slump in the highway backslope to prevent ponding of water at this location. Care should be taken to

• nly remove the minimum amount of material required to improve the drainage

since excess removal of slumped soil could trigger additional slump movement. It is also recommended that any major tension cracks at the Highway Slump and the North and South Slumps in the backslope of the offload area be sealed to reduce infiltration from rainwater. This will involve cutting down the backscarp of the cracks with a small dozer and using the material to infill and smooth out the crack area. The infill material should be track packed to help seal the cracks. This work should be carefully carried out to avoid disturbing the vegetation in the areas between the cracks. Some of the fallen trees on the slide mass at the North Slump may need to be cut off at ground level to facilitate the crack sealing, however the root mat should be left in place. Care should also be taken to avoid pushing any significant quantity of soil back up the slope as it will likely load the slope and cause further sliding. The sealed areas should be re-seeded upon completion of the maintenance work. 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.

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GEOTECHNICAL A N D GEOLOGICAL ENGINEERING

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A L G A R Y / E D M O N T O N / T O R O N T O / V A N C O U V E R / V I C T O R I A / F T . M c M U R R A Y / MEDICINE HAT

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Alberta Transportation

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January 9, 2002 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. 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.J. Law, P.Eng. Review Principal Don Proudfoot, P.Eng. Project Engineer Attachments

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NC17A 2001 Inspection

GEOTECHNICAL AND GEOLOGICAL ENGINEERING - CALGARY /EDMONTON/TORONTO/VANCOUVER/VICTORIA/FT. McMURRAY / M E D I C I N E HAT