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

THURBER ENGINEERING LTD. I \ S u i t e 200, 9636- 51st Avenue DO EDMONTON, Alberta T6E 6A5 Phone [780) 438-1460 F a x (780) 4 3 7 - 7 1 2 5 THURBER January 9, 2002 File: 15-76-11 Alberta Transportation Room 223, Provincial Building A L B E R T A T R A N S P O R T A T I O N 4709- 44 Avenue TECHNICAL STANDARDS BRANCH Stony Plain, Alberta T7Z 1N4 J A N 1 5 2 0 0 2 Received Attention: M r . R o b Lonson, P.Eng. 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 of 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 Continued ... 0 8 \ 0 : \ W o r d \ 15\76-11 . 1et NC17 A 2001 Inspection 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

THURBER ENGINEERING ----------- - -- -------- Alberta Transportation - 2 - 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 Continued ... 08\D:\Word\15\76-11 . 1et NC17A 2001 Inspection GEOTECHN I CAL AND GEOLOGICAL ENGINEER I NG · CALGARY /EDMONTON/TORONTO/VANCOUVER/V I CTOR I A/FT . M c MURRA Y / M E DI CINE HA T

THURBER ENGINEERING --------------------- - - 3 - January 9, 2002 Alberta Transportation 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 Continued .. . 08\D:\Word\15\76-11 . 1et NC17A 2001 Inspection GEOTECHNICAL AND GEOLOGICAL ENGINEERING - CALGARY /EOMONTON/TORONTO/VAN=UVER/V ICTORIA/FT . McMURRAY /MED ICINE HAT