1. PROJECT BACKGROUND. July 18, Alberta Infrastructure & Transportation Central Region #401, Street Red Deer, Alberta T4N 6K8

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1 July 18, 2005 Alberta Infrastructure & Transportation Central Region #401, Street Red Deer, Alberta T4N 6K8 Mr. Alain Momedi, P.Eng. Project Engineer Dear Mr. Momedi: H11:04 km 11.1 Erosion along Abraham Lake Geotechnical Callout Report - DRAFT This draft geotechnical callout report was prepared by Klohn Crippen Consultants Ltd. (Klohn Crippen) for Alberta Infrastructure & Transportation Central Region under the Geohazard Assessment Agreement CE 045/2004. Site inspections were undertaken on May 26 and June 29, 2005 by Mr. Darren Ratcliffe, P.Eng., of Klohn Crippen Consultants Ltd. Mr. Ratcliffe was accompanied by Mr. Alain Momedi, Mr. Roger Skirrow, and Mr Fred Cheng of Alberta Infrastructure & Transportation. A survey of the site was conducted by Challenger Geomatics Ltd. on July 11, 2005 at a cost of about $3,000. The site is illustrated on Figure 1 and in the attached photographs. 1. PROJECT BACKGROUND The project site is located on Highway 11:04 at km 11.1, about 40 km southwest of Nordegg, Alberta. Highway 11:04 runs alongside Abraham Lake, a reservoir along the North Saskatchewan River created by the Bighorn Dam as shown on Figure A. The reservoir is operated by TransAlta. Just south of the viewing area known as Windy Point, a combination of wave erosion and highway runoff erosion has created a steep slope from the highway edge down to the reservoir. In one location, the erosion is located right up to the road edge. It is understood that this erosion has been on-going for a significant length of time; however, the loss of soil support is now causing distress and cracks in the pavement H734 Construction Report.doc File: A3038A01 AT GeoHazards Suite 114, Street NE, Calgary, Alberta., Canada T2E 7H7 T (403) F (403)

2 To prevent further erosion at the critical point where the erosion scarp has reached the road edge, sandbags were placed between the May 26 and June 29 inspections to divert runoff. As discussed in the following sections, this approach was successful and no significant further erosion was caused in the June 2005 floods. 2. SITE INSPECTIONS A summary of site observations is provided below: A combination of wave erosion and runoff from the highway surface has created a steep slope about 15 m high down to the Abraham Lake reservoir over a length of about 100 m. The erosion slope is about 1.5H:1V and varies from 0 m to about 5 m from the highway edge. A survey of the site was performed in July 2005 and is shown on Figure 1. Typical sections are shown on Figures 2 and 3. The erosion has uncovered a Telus telephone cable and the cable is hanging unsupported across the slope over a length of about 10 m. Cracks are appearing in the pavement adjacent to the eroded slope related to the loss of soil support. The natural slope flattens to the south and is vegetated with trees. A trail in this area provides access to the reservoir floor. A culvert is located about 125 m north of the site carrying stream flows into the reservoir. The culvert is located at approximately the high point in the highway profile. The cross-fall of the road is about 3% towards the reservoir. Typical elevations are listed below: Highway Centreline Highway Edge Top of Slope Toe of Slope Reservoir FSL m m m m 1321 m File: A3038A01 AT GeoHazards Page 2

3 3. SITE ASSESSMENT The erosion at this site has been on-going for a significant length of time and is now presenting a hazard to the highway. The site was not significantly affected by the June 2005 flooding. Considering the observed distress in the pavement, it is considered that remedial action is warranted. Based on the Full Supply Elevation (FSL) of 1321 m (about 2 m above the toe of the slope), it is considered that runoff due to the cross-grade of the road is the primary cause of the erosion at this site. Simply preventing the runoff from the road flowing down the slope would stop further erosion at this site. However, the observed distress in the pavement indicates that some form of bank replacement and stabilization is required. Based on the risk level criteria provided by Alberta Infrastructure & Transportation relating to safety, a risk rating of 54 was assigned to this site. This is based on a probability factor of 9 for an active erosion feature, and a consequence factor of 6 due to the impact on the highway. 4. PROPOSED REMEDIATION The following remediation approaches are presented for discussion purposes. The final design of the remediation will be developed following a site investigation program and discussions with stakeholders such as TransAlta and Alberta Environment. The proposed site investigation will be described in a separate letter and is estimated to cost about $20,000, including drilling and excavation contractors and traffic accommodation. In the short term, runoff from the highway should not be allowed to flow down the eroded slope. In a similar fashion to the present sandbag approach, an asphalt curb should be constructed along the edge of the road to direct water into the treed area to the south. A curb about 200 m long and about 150 mm high should be considered. The actual discharge point should be selected on site. This work is estimated to be in the order of $5,000. To reinstate the crest of the slope and to prevent further long term erosion, the following approaches could be adopted: Mass Slope Fill The slope could be reinstated with compacted fill at a slope of about 2H:1V provided rip rap protection was included. A volume of fill of about 15,000 m³ would be required. A source for the fill would need to be determined and the File: A3038A01 AT GeoHazards Page 3

4 proximity to the site will determine the cost. The type of fill would need to be selected for stability requirements. Assuming about $25/m³, the estimated cost for the compacted fill is about $375,000. Rip rap materials could be gathered from the reservoir floor. Full Height Retaining Wall A full height retaining wall could be constructed from concrete or gabion baskets. The cost of concrete delivered to the site is estimated to be approximately $250/m³. For a concrete secant-pile wall about 100 m long by 20 m deep, the material cost is estimated to be about $500,000. Suitably sized rock for gabion baskets is distributed over the reservoir floor, and would provide an obvious material supply provided the volume is sufficient and approval can be obtained to excavate and use. A gabion wall about 10 m high requires about 34 m³ of gabion wall per m length. For a 100 m long wall at a unit rate of about $150/m³, indicates a cost of about $510,000. Partial Height Retaining wall with Slope Fill To reduce the volume of mass fill or retaining wall, a low height retaining wall could be constructed to above the reservoir full supply level with fill placed behind. A gabion wall about 6 m high requires about 16 m³ per m length of wall. For a 100 m long wall at a unit rate of about $150/m³, indicates a cost of about $240,000. The volume of fill required is about 4000 m³ or about $100,000. As the fill is restrained by the wall, the type of fill becomes less critical and could be obtained from the reservoir area. Alternative Solutions Other potential solutions will be considered following the site investigation to determine the subsurface stratigraphy (including depth to bedrock etc.). These solutions could include soil-nailing or other forms of slope reinforcement to avoid the need for walls and fills. At this stage, the partial height gabion wall with fill behind is the recommended approach as shown on Figure 4. To control runoff flows from the highway surface, a channel would be provided from the road, across the fill and discharging into the reservoir via the gabion face. Allowing for mobilization, traffic accommodation and drainage features, a total project cost of about $400,000 should be considered. File: A3038A01 AT GeoHazards Page 4

5 5. CLOSURE Please contact the undersigned at (403) if you have any questions regarding this report. Yours truly, CONSULTANTS LTD. Darren Ratcliffe, P.Eng. Project Manager APEGGA Permit to Practice No. 433 cc. Mr. Roger Skirrow, Alberta Transportation Attachments File: A3038A01 AT GeoHazards Page 5

6 ALBERTA INFRASTRUCTURE & TRANSPORTATION FIGURES