/ ALBERTA TRANSPORTATION LANDSLIDE RISK ASSESSMENT SECTION A: GEOTECHNICAL FILE REVIEW NORTH CENTRAL REGION (ATHABASCA AREA) SITE NC17A (BACKSLOPE FAILURE OF HIGHWAY TWINNING) HIGHWAY CONTROL SECTION Hwy 63:12, km 2 to km 10 NEAREST LANDMARK: Fort McMurray is located approx. 2 km south of the subject section LEGAL LOCATION: NW7 & SW18-90-9-W4 Date of Initial Observation: 1999 Date of Last Inspection: June 23, 2009 Last Inspected By: Thurber Engineering Ltd. (TEL) Instruments Installed: 2 Slope Inclinometers (in 1999); 4 Slope Inclinometers and 4 "Poorboy" standpipes (in 2009). Instruments Operational: 6 Slope Inclinometers and 1 UPoorboy" standpipe. PF(11) * CF(3) = Risk Assessment: 33 Last Updated: Thurber, December 2009 Previous Update: None
1. LOCATION The site consists of the SBL and backslope of Hwy 63:12 extending between km 2 and km 10, north of Fort McMurray. 2 . BACKGROUND AND GENERAL DESCRIPTION OF SLOPE INSTABILITIES Highway 63:12 extends in a north-south direction on a river terrace along the toe of the west valley slope of the Athabasca River valley. The valley slope above the highway is approximately 50 m deep along this section. Construction for twinning the highway was undertaken in 1999 to construct the new SBL along the west side of the existing highway. Highway twinning was constrained by the presence of a 760 mm diameter high pressure oil pipeline operated by Enbridge, which was located along the east side of the existing highway along the south portion of the study area, crossing over to the west side north of about km 3. 7, where the pipeline was located within the lower reaches of the valley slope, above the proposed location of the SBL. Weak marginally stable clay colluvium was present draped over the lower portion of the valley slope in the vicinity of the planned twinning. North of km 3.7,-where the pipeline was located west of the highway, the general design philosophy adopted was to locate the SBL at a higher level than the NBL to maintain backslope cuts to a maximum of about 2.5 m to reduce the risk of a possible slope failure occurring in the colluvium that could damage the pipeline. Slope inclinometers were installed at select locations between the highway backslope and the pipeline along this section to warn of potential slope movements that might affect the pipeline.· A 12 to 13 m high backslope cut centered at km 2.4 and km 2.7 could not be avoided along a section where the pipeline was located on the east side of the highway. At these locations the main/lower backslope was cut at 3H:1V with 3 m wide benches located at 6 m height intervals. The area at the top of the colluvium deposit, behind the lower backslope, was offloaded as borrow material to a create a wide upper bench extending to a depth where a global factor of safety of about 1.1 was achieved for a potential· slip surface through the lower backslope. The west side of the offloaded area (upper backslope) was inclined at 5.5H:1V. Client: Alberta Transportation D a t e : D e c e m b e r 10, 2 0 0 9 File: 15-16-216 Page 2 of6 e-file: 08\16-216 let PH 17 A Section A
Following slope offioading, 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 slope offloading approach was successful, however some localized slope failures developed in the colluvium since construction. Two slumps occurred (about 50 m wide and 30 m wide centered at km 2.67 and km 2.74, respectively) in the upper 5.5H:1V backslope of the offloaded area in November, 1999 along pre-sheared surfaces in the colluvium. Remedial measures were carried out in the spring of 2000 (see Dec. 2, 1999 reference in Section 4 - Chronology), consisting of installing French drains perpendicular to the backslope through the slumps, tied into a 1 00 mm diameter "Big 0" subdrain located 4 m deep parallel to the highway along the toe of the backslope, with a higher center and outlets at both the north and south ends of the backslope. The filter zone of the subdrain trench was protected with non-woven geotextile along the upsiope side and top, and geomembrane along the bottom and downslope side. The slump surfaces were also re-graded, and the cracks sealed. 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 near the lower backslope in June 2000, at a location 34 m south of Sl99-2. The toe roll of the slump was present in the west highway ditch. A fax (see June 27, 2000 reference in Section 4- Chronology) provided analyses and alternate recommendations. 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 through the colluvium deposit. Other options considered were placing a culvert in the highway ditch to allow a shallow toe berm to be constructed versus sub-excavation of the slump and reconstructing the slope using re-compacted clay or gravel, or clay placed overtop a gravel drainage layer. Since the slump was not affecting the highway, a decision was made to only carry out some minor re-grading to seal the scarp cracks and to continue to monitor the condition. From 2001 to 2004, additional slight movements and retrogression were observed at the above slump locations. Since 2004, the lower highway backslope slump has steadily continued to grow in size while the North and South slumps in the upper offloaded area have also indicated some additional movements in the form of more pronounced toe rolls, backscarp retrogression, ponded water, and fallen trees. Slope inclinometer Sl99-1 in the offloaded backslope area between the north and south slumps has only indicated negligible movement, except that in 2008 new movement appeared within the upper 6 m due to progressive slide retrogression. Sl99-2 at the crest of the highway backslope about 34 m north of the local slump showed that the deep colluvium layer had experienced global movement at a depth of 21 m along the limestone contact at a relatively steady rate of about 2 to 3 mm/year since construction in 1999, highlighting the marginal global stability of the colluvium deposit. Alberta Transportation Client: Date: December 10, 2009 File: 15-16-216 Page 3 of6 e-file: 08\16-2161et PH 17A Section A
I n 2006, another slide area began developing in the lower backslope at km 2.4 over a distance of about 165 m, and the toe of the slide started to heave the SBL and west highway shoulder and ditch. The asphalt had to be patched in 2007 and 2008 to smooth out the highway, along with minor re-grading to remove the humps in the west sideslope/ditch. 3 . GEOLOGICAUGEOTECHNICAL CONDITIONS The 1999 test holes and cross-sections for the highway twinning (see 4. Chronology/References) indicate that the soil stratigraphy at this site consists of clay surficial soils, overlying Clearwater Shale, overlying Wabiscaw Sandstone, overlying McMurray Oilsand, overlying Limestone, with a colluvium deposit (predominantly high plastic clay) overlying the eroded bedrock in the lower reaches of the valley. The highway location at this site is located within the colluvium, which was up to 21 m deep. The available map references indicated the following: • Physiographic Region- McMurray Lowland. • Bedrock Geology- Consists of Lower Cretaceous age deposits, (McMurray Formation - thick bedded quartzose ·sandstone and siltstone, oil impregnated grey silty shale interbeds in upper part, non marine to deltaic); o r Upper Devonian (Waterways Formation - grey and greenish grey shale and argillaceous limestone interbedded with grey to grey-brown fine- grained to course clastic limestone, marine). Bedrock elevation - 270 m. The drift thickness is less than 15m. • Surficial Geology - Highway is located on a river terrace bordering a colluvium slope. • Hydrogeology - On border of: Unconsolidated deposits (sand/silt), and where it is overlying Devonian Formation, with yields in the range of 0.1 to 3 litres/sec. 4. CHRONOLOGY/REFERENCES The following new or existing developments are noted: Nov., 1999 Report {#19-2363-13) prepared by Thurber Engineering Ltd. to EXH Engineering Services Ltd. dated Nov. 12, titled "Highway 63:12 Twinning, Backslope Cuts and Borrow Pits, Geotechnical Investigation". This report covers a total length of 12.7 km for the Client: Alberta Transportation Date: December 10,2009 File: 15-1&216 Page 4 of6 e-file: 08\1&2161et PH 17A Section A
Recommend
More recommend
