September 3, 2009 File: NORTH CENTRAL REGION GEOHAZARD ASSESSMENT HWY 40:28 NORTH OF CADOMIN (NC50) 2009 ANNUAL INSPECTION REPORT

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1 GEOTECHNICAL ENVIRONMENTAL MATERIALS September 3, 2009 File: Alberta Transportation 2 nd Floor, Provincial Building Street Edson, Alberta T7E 1T2 Attention: Mr. Cliff Corner NORTH CENTRAL REGION GEOHAZARD ASSESSMENT HWY 40:28 NORTH OF CADOMIN (NC50) 2009 ANNUAL INSPECTION REPORT Dear Sir: This letter documents the 2009 annual site inspection of the slope failure on Hwy 40:28 (km 11.7) north of the Hamlet of Cadomin at legal land description NW W5M (Figure NC50-1, Section F). Thurber Engineering Ltd. (Thurber) undertook this inspection in partial fulfillment of our Geotechnical Services for Geohazard Assessment, Instrumentation Monitoring and Related Work contract (CE103/2008) with Alberta Transportation (AT). Mr. Don Law, P.Eng. and Mr. Ken Froese, P.Eng., of Thurber undertook the inspection on June 16, 2009, in the presence of Mr. Roger Skirrow, P.Eng., Mr. Neil Kjelland, P.Eng., Mr. Howard Hawley, Mr. Jeff Zhang, E.I.T., and Mr. Cliff Corner, of AT. 1. BACKGROUND AND RECENT WORK A section of the highway located 200 m west of NC50 was realigned in about 2000 or 2001 due to slide activity. At that time, subdrains were installed in the south ditch to lower the groundwater table. The current distress at the NC50 location was first observed during a call-out conducted by Thurber on August 3, 2005, as summarized in a letter to AT dated August 16, 2005, included in Section E of the binder. Suite 200, Avenue, EDMONTON, AB, Canada T6E 6A5 T F EDMONTON CALGARY FORT McMURRAY VANCOUVER VICTORIA TORONTO KAMLOOPS SQUAMISH

2 A geotechnical investigation was conducted by Jacques Whitford (report dated December 19, 2006) which included the installation of two slope inclinometers and three standpipe piezometers. The report includes conceptual remedial measures for the slide. Since the 2007 site visit, several patches have been placed through this area to reduce the differential height across the crack for safety reasons. In early 2009, a conceptual design study was completed by Thurber and summarized in a report to AT dated April 30, The conceptual design study reviewed available published information regarding the site s history and geological environment. An airphoto interpretation was also undertaken for the site. Two primary alternatives were considered in the conceptual study: a pile retaining wall structure and realignment of the highway. The realignment design was undertaken by Genivar Consultants using LiDAR data for the site and considered 10 m and 20 m offset options. The pile wall alternative considered cantilever and tie-back options. The LiDAR data was also utilized for preliminary slope stability assessments of the existing slopes (downslope and upslope of the highway) to determine the improvements anticipated from and the feasibility of the options considered. Based on the cost of the alternatives and the relative risk including an assessment of the likelihood of additional slides developing in the realignment zone, it was recommended that the pile wall alternative be chosen. A geotechnical drilling program was undertaken by Thurber in May 2009 and the results of this investigation are presented under separate cover. Following the drilling program, the preliminary design was undertaken for the pile wall option. Based on the result of the preliminary design cost estimates, the chosen pile wall will be 1.8 m diameter cantilever piles spaced at 1.5 diameters (centre to centre). Detailed design and tender preparation is scheduled to be completed by the end of September with construction tentatively planned for the winter of 2009/ SITE OBSERVATIONS 2.1 Main Slide The roadway is located on a sidehill fill with the terrain sloping steeply to the north toward Gregg River. The river is located about 70 m (horizontal distance) from the edge of the highway and is approximately 25 m lower in elevation. Exposed bedrock was noted on the far (north) bank of the Gregg River below about 4 m above river elevation. The terrain between the highway and the creek was heavily treed with a thin organic cover. The distressed area of the pavement is about 45 m in length and affects the west-bound lanes. The crack pattern is arc-shaped and has a differential height File: Page 2 of 8

3 across the crack of up to 170 mm as noted on Figure NC50-1, attached for inclusion in Section F. The crack extends beyond the pavement into the sideslope of the highway and through the ditch an additional 40 m to the west (distance measured parallel to the highway alignment and corrected from previous reports) and 10 m east of where it leaves the pavement surface at the east end of the pavement distress. There was also evidence of an old slump located about 15 m north of the active scarp adjacent to the west half of the main scarp which appears to have re-activated since the 2007 site visit; however, does not appear to have changed substantially since the 2008 visit. As noted during previous visits, a toe roll exists near the toe of the slope adjacent to the Gregg River, located approximately as shown on the site plan. Water was also noted seeping from a few locations in the lower third of the sideslope as shown on the attached figures. The highway backslope is about 13 m high and inclined at a slope angle of about 3H:1V. A water collection ditch is located along the top of the backslope which discharges into a riprap-lined channel leading to a 1000 mm diameter culvert. Several shallow slides have formed in the backslope located as shown on the site plan and are similar in distribution as noted last year. There were no signs of seepage in the backslope. A small erosion gully was noted about 30 m west of the riprap channel as shown on the site plan. The 1000 mm diameter culvert is located at the edge of the eastern extent of the active scarp. At the time of the site visit, the culvert appeared to be unaffected by the current slope movement. A 150 mm diameter corrugated steel pipe subdrain outlet was observed at the base of the highway embankment near the west edge of the active scarp. No flow was observed from the pipe once the soil blocking the outlet was removed. There is no evidence in the backslope of an inlet for this pipe. Selected photographs taken during the site reconnaissance are attached. 2.2 Additional Slides The 2008 site reconnaissance was extended to include areas of potential instability observed on satellite imagery. The reconnaissance extended about 250 m west to the twin culverts and half-culvert drop structures at approximately km This are was observed again in Skin slides were observed on the sideslope on both sides of the twin half-culverts. An old slump was located about 80 m east of the culverts (160 m west of the main slide) and 18 m north of the highway. These feature locations are shown File: Page 3 of 8

4 approximately on Figure NC50-1. The scarp was about 1.5 m high and there was no evidence of recent movement. In addition, the three active slides first noted in 2008 between 70 m and 110 m west of the twin culverts (located approximately as shown on Figure NC50-1) were viewed during the 2009 site reconnaissance. These slides were located within the forested portion of the valley and about 40 m north of the highway. The slides range in width from about 4 m to 10 m with measured slope angles between 38 and 45 to the horizontal, and extend to the bottom of the slope at the river. These features do not appear to have increased size since the 2008 visit. 2.3 Other Locations with Potential to Impact the Highway The 2009 conceptual design study included a review of LiDAR imagery of the site and surrounding area. A total of five areas were identified during the study where impingement by the Gregg River on the slopes below the Hwy 40:28 embankment may result in future slope movements that may impact the highway embankment in the long term. The potential locations were ranked in order of estimated priority. During the 2009 site visit, a brief reconnaissance was made of each of the features to confirm that the ranking based on the review of LiDAR imagery was appropriate. A site plan showing the LiDAR image and the confirmed rankings is included as Figure NC50-3 for inclusion in Section F. Note that the highest rank (1) is the current instability and the lowest rank (5) is the least likely to exhibit failure in the future. In most cases, it was determined that the area of impingement had exposed bedrock well above river level and thus have a low risk for large-scale slope movement. 3. MONITORING RESULTS AND ASSESSMENT 3.1 Stratigraphy The soil conditions encountered during the instrumentation installation by Thurber in May 2009 generally consisted of surficial fill overlying clay, clay till, and siltstone bedrock. This is consistent with the results of previous investigation carried out at this site. The siltstone bedrock was encountered at depths between 14.5 m and 16.4 m below ground surface. An updated stratigraphic cross-section based on the recent drilling results is provided in Figure NC50-2 attached. 3.2 Instrumentation New instrumentation was installed about three weeks before the readings taken on June 4, As part of the pile wall investigation program, an additional set of readings was taken at this site on June 29, During the May 2009 installation File: Page 4 of 8

5 program, SP1, SI2, and SP2 were destroyed. The following summarizes the monitoring results from 2009: Slope inclinometers SI1 and SI2 were installed in July 2006 and had sheared off at the time of the Spring 2007 readings. The depths of shearing were confirmed in August 2007: SI1 at 5.1 m below ground surface and SI2 at 4.9 m. The zone of movement had been at about 5.5 m to 6 m below ground in both inclinometers. The open standpipe piezometers installed in July 2006 site had reported depths of between about 9.5 m and 10.5 m below ground surface. As of Spring 2008, SP2 had sheared off at 6.2 m below ground surface. SP1 was damaged in May 2009 and SP3 was not read. The movement depths measured in the slope inclinometers installed in the main slide (SI09-4, -5, -7, and -8) were measured between 5.6 m and 6.5 m below ground surface. The cumulative movements between May 22 (initial reading) and June 29 (most recent reading) were between 4 mm and 9 mm giving movement rates between 21 mm and 83 mm per year. The current movement rates are lower than those measured shortly after initialization on June 4, Two pneumatic piezometers (PN09-5 and -7) were installed in the main slide in May The measured water levels on June 29 were 10.5 m and 11.9 m, respectively, below ground surface. This water level is located in the lower 2 m of the clay till (just above the bedrock contact). A slope inclinometer and pneumatic piezometer pair (SI09-10 and PN09-10) were installed downslope of the old slump located about 160 m west of the centre of the main slide. The first reading (June 4, 2009) following initialization did not discern a movement zone. However, the second reading taken June 29, 2009, did note slight movement at 5.6 m below ground surface but the cumulative movement to date is only 1 mm at a rate of 10 mm per year. The water level measured in the pneumatic piezometer was 13.1 m below ground level which is in the clay till just above the bedrock contact. 3.3 Assessment Based on the observed cracking pattern at the surface, the depths of movement recorded in the slope inclinometers, and the presence of a toe-roll just above river elevation, it appears that the movement is a result of a translation slide. The slip surface appears to be located in the clay above the clay till and may be sliding along the bedrock surface near the toe of the slide. Groundwater does not appear to be a contributing factor to instability within the upper portion of the main slide File: Page 5 of 8

6 mass. Seepage was observed in the lower third of the slope which may be destabilizing the lower portion of the slide. High groundwater levels (when present) appear to be contributing to backslope instability in this area. As noted in the 2008 Geohazard report, it appears that the translational and skin slides are a frequent occurrence along this slope. The zones of movement observed to the west are expected to be similar to the main slide: potentially sliding in the clay on top of the clay till near the highway elevation and on top of the bedrock further down the slope. The presence of a slight movement in the slope inclinometer installed at the old scarp feature located near the twin culverts about 160 m west of the main slide indicates that this is a potential slide. It is expected that there is a low risk to the highway from this feature at this time as there is bedrock exposed at the toe of this slope which reduces the possibility of large-scale, deep-seated movement. It does not eliminate the possibility of a translation slide over the bedrock (such as the skin slides seen within the tree line) unloading the toe associated with the scarp and causing additional movement such as seen at the main slide. At this older scarp location, there is adequate distance from the highway such that remedial action could be taken before the embankment was threatened should the failure regress toward the highway. 4. RISK LEVEL The risk level for this site has been assessed as follows: PF(14) * CF(4) = 56 A Probability Factor of 14 is considered appropriate since the slide is active with a high rate of ongoing movement. A Consequence Factor of 4 is applicable since the embankment is relatively high and a partial closure of the road has already occurred as a result of slide movement and the east culvert could be threatened. This risk level is the same as applied in 2008 and higher than the risk level of 52 applied in RECOMMENDATIONS 5.1 Short Term It is understood that the previously recommended detour will be constructed on the south side of the highway. Additional fill material should not be placed on the north shoulder as its weight may accelerate the movements which may trigger retrogressive scarp development further into the traveling lanes of the highway. The detour fill should not impede the ditch drainage. It is also recommended that a File: Page 6 of 8

7 light-weight non-woven geotextile be placed on the ground surface prior to placing the granular fill for the detour to facilitate its removal once the roadway surface has been stabilized and repaired. This section of road should be monitored for further regression of the slide and signage or barricades should remain in place to warn motorists and to keep vehicle traffic away from the north shoulder. 5.2 Long Term Based on the results of the preliminary design and a meeting with AT, it has been determined that a large-diameter, cast-in-place concrete cantilever pile wall will be constructed to the stabilize the highway embankment. In conjunction with the installation of the pile wall, minor ditch grading will be undertaken downslope of the highway to facilitate drainage of surface away from the slip surface. It is anticipated that the pile wall will go to tender in late 2009 with all construction and roadway repair completed by mid Investigation The recommended geotechnical investigation at this site was completed by Thurber in May Further investigation is not warranted at this time. 5.4 Maintenance This section of highway should be inspected regularly by the MCI to check for significant changes in the movement which could lead to further risk to the highway and traffic. The patching and infilling that is currently being done may continue to be required; however, it is preferred that the detour recommended above be constructed rather than continuing to load the top of the slide with additional weight. It is recommended to continue monitoring the existing instrumentation on a semi-annual basis, and to undertake annual geotechnical inspections as currently programmed. File: Page 7 of 8

8 6. CLOSURE We trust this assessment and recommendations meet with your needs at this time. Please contact the undersigned should questions arise or if the slide condition worsens. Yours very truly, Thurber Engineering Ltd. Don Law, P.Eng. Review Principal Ken Froese, P.Eng. Project Engineer /dw Attachments Photographs 1 to 8 Figures NC50-1 and NC50-2 cc: Mr. Roger Skirrow, P.Eng., Director of Geotechnical Services, AT File: Page 8 of 8