January 17, 2008 File: 15-85-73 Alberta Infrastructure and Transportation Room 301, Provincial Building 9621-96 Avenue Peace River, Alberta T8S 1T4 Attention: Mr. Ed Szmata PEACE REGION (SWAN HILLS AREA) GEOHAZARD ASSESSMENT HWY 750:04 EMBANKMENT SLUMP (SH 14), SALT CREEK 2007 ANNUAL INSPECTION REPORT Dear Sir: This letter documents the 2007 annual site inspection of an area of slope instability located along Hwy 750:04 about 4 km north of the junction of Hwy 679, northeast of High Prairie, Alberta. Thurber Engineering Ltd. (Thurber) undertook this inspection in partial fulfillment of our Geotechnical Services for Geohazard Assessment, Instrumentation Monitoring and Related Work contract (CE047/2004) with Alberta Infrastructure and Transportation (INFTRA). Mr. Barry Meays, P.Eng. and Mr. Gurpreet Bala, M.Sc. of Thurber undertook the inspection on June 15, 2007 in the presence of Mr. Ed Szmata, Mr. Rocky Wang and Mr. Rodney Johnston of INFTRA. 1. BACKGROUND The available background for this site was summarized in June 2004 in the Geotechnical File Review in Section A of the binder. The site has a history of instability going back to 1988 when a slide occurred in the west side of the highway. At that time, the slide was about 80 m wide with a 25 m crack at the highway centreline and 0.3 m of differential settlement. Following the slide event, a 9 m wide by 3 m high toe berm was constructed and two 150 mm diameter subdrain pipes were installed. Repairs in the last few years have consisted mainly of maintenance to the highway surface to preserve a smooth surface.
2. SITE OBSERVATIONS The changes in condition since last year are shown on the attached site sketch plan and discussed below. Selected photographs taken during the visit are also attached. The current instability consists of a 40 m-long slump in the west sideslope. The asphalt surface contains arc-shaped cracks. A drop of about 30 mm was measured at the crack in the highway. A dip was observed to the south of the crack within the highway pavement. Mr. Johnston informed that the drivers of the snowploughs felt the dip during winter snow clearing activities. An additional 5 m long crack was also noted in the sideslope of the highway at about 4 m from the edge of highway embankment. The toe roll of the slide has pushed out the barbed-wire fence about 0.8 m at the contact between the upper slope and the toe berm. Two Telus cables have been routed overland along the fence line over the slide area. Erosion and shallow instability have also been observed northwest of the slide area. Both of the north and south drain pipes near the toe of the slump were dry at the time of our visit. Additional slumping was noted in the creek banks west of the highway. A beaver dam was being constructed west of the culvert inlet. Mr. Johnston noted that the culvert was running full for a week during the spring snow melt this year. The shallow slump scarps that were observed last year on the backslope east of the highway have resulted in a slump around 50 m wide and about 12 m in length from the back scarp to the toe. The back scarp of the slump was 0.9 m high and the toe roll was about 0.3 m high and was extending up to the highway ditch. Two intermittent cracks extending along the entire length of the scarp were observed. At about 150 m to the south of the main slide zone, a 40 m wide and about 10 m long backslope slump was observed. The height of the backslope from the bottom of the drainage ditch to the crest was about 3.5 m. The backscarp was 1 m in height with a 0.3 m to 0.4 m high toe roll. A portion of the slump to the north was within a treed area. The right of way (ROW) limits of the highway appeared to be along the crest of the slope and the slump extended slightly into the farmer s field. A survey marker was present within the treed area. An erosion gully was present at the south corner of the slump. File: 15-85-73 Page 2 of 5
3. ASSESSMENT The test holes drilled at the site in 2002 indicated subsurface conditions consisting of gravel and/or high plastic clay fill over high plastic clay over medium plastic clay till. The clay till has some sand layers interbedded in it. SI02-4, which was installed in the shoulder of the highway but was covered in asphalt in 2004, indicates that the slip surface of the slide was located in the clay layer about 5.2 m below ground surface. SI02-2, which was installed near the downslope crest of the berm, indicates that a slip surface also extends below the berm in the clay layer at about 4 m below ground surface at the SI location. The slide creep rate was 23.7 mm/year at the spring 2006 readings compared to 19 mm/year in 2005. None of the two slope inclinometers were operational as observed during the 2007 spring monitoring. The piezometers indicated a piezometric surface at 2.2 to 2.8 m below ground surface, and were mainly unchanged since the measurements of 2002. The standpipe piezometer SP02-4 showed a slight increase of about 0.44 m in the groundwater elevation since last year winter readings. Based on the above, the overall height and inclination of the highway embankment and toe berm fills is too steep for the existing relatively weak and wet clay foundation soils. The slide appears to be moving in two blocks, an upper block, which is spilling out onto the berm and pushing out the fence and a lower more deeply seated block that extends below the slope and the toe berm. The scarp scallops observed on the east backslope are believed to be separate, isolated slides, and likely not associated to either of the blocks on the downslope side of the highway. 4. RISK LEVEL The risk level for this site has been assessed as follows: PF(10) * CF(4) = 40 (40 in 2006) A Probability Factor of 10 is considered appropriate since the slide is active with a perceived moderate steady rate of movement. A Consequence Factor of 4 is considered appropriate since the embankment fill is fairly high and a partial closure of the road would be a direct result of an aggressive slide movement. File: 15-85-73 Page 3 of 5
5. RECOMMENDATIONS 5.1 Short Term The slide is currently affecting the highway pavement and requires ongoing asphalt patching and crack sealing maintenance. In the short term the site should be regularly inspected by the MCI to determine when further asphalt patching maintenance is required to maintain a smooth even road surface for traffic safety. The slumped soil mass at both the backslope slides on the east and west side of the highway should be cleaned from the highway ditch, in order to allow proper drainage. The slope can be regraded at the present slope with a dozer and vegetated to prevent erosion and water from entering the slumped mass of soil. 5.2 Intermediate Term An intermediate term fix for the backslopes consist of excavating the slumps to depths below the estimated slip surfaces, moisture conditioning the excavated clay and then reconstructing the slopes with well compacted horizontal lifts to establish the slope(s) to the original slope angles. The estimated cost for the repairs of the slumps can be in order of $ 50,000. 5.3 Long Term A potential long term stabilization measure for the affected highway section consists of enlarging the toe berm, digging out the lower toe roll and building a shear key that extends into the clay till, and subexcavating and replacing the failed material in the upper slope with stronger (gravel) material. In addition, consideration could be given to installing a subdrain under the upslope ditch. The road surface will also need to be repaved through the slide zone. Another long-term option consists of a pile wall to stabilize the west shoulder of the highway. The wall would need to be about 40 m long. Based on previous similar projects the cost for the construction of either of these long-term remedial measures is expected to be in the order of $500,000 to $700,000. For the two backslope slumps, the long term solution would be to flatten and regrade the slope at 4(H) to 1(V) or flatter. For this the ROW limits of INFTRA would need to be extended into the farmer s property. The cost of the long-term File: 15-85-73 Page 4 of 5
measures for the slopes is expected to be in order of $150,000, excluding the cost of extra right-of-way. 5.4 Investigation A survey of the Salt Creek slide area was carried out by EXH Engineering Services Ltd. on April 24, 2006 (refer to attached site plan). A marshland assessment (fisheries/amphibians) was completed by Spencer Environmental Management Services Ltd. and will be included with a preliminary engineering report currently being prepared by Thurber, in the near future. 5.5 Maintenance It is recommended that the site should be regularly inspected by the MCI to determine when further asphalt patching maintenance is required to maintain a smooth even road surface for traffic safety. The highway drainage ditch should be cleaned of the slumped mass to provide adequate drainage for the surface runoff. 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 conditions worsen. Yours very truly, Thurber Engineering Ltd. Don Proudfoot, P.Eng. Review Principal Barry Meays, P.Eng. Gurpreet Bala, M.Sc. Project Engineer Project Coordinator /dw Attachments cc: Mr. Roger Skirrow, P. Eng. Director of Geotechnical Services, INFTRA File: 15-85-73 Page 5 of 5