Alberta Transportation Central Region #401, 4902 51 Street Red Deer, Alberta T4N 6K8 June 7, 2002 Mr. Melvin Mayfield, P.Eng. Project Engineer Dear Mr. Mayfield: Central Region Landslide Assessment Site C8 SH597:02 Blackfalds Slide 2002 Annual Inspection Report Alberta Transportation has initiated a process of risk management at site-specific slope movement sites that includes a 3-ring binder document control system. This Annual Inspection report forms Section B of the document control system for the above site. The annual site inspection was undertaken on May 23, 2002 by Mr. Darren Ratcliffe, P.Eng., of Consultants Ltd. Mr. Ratcliffe was accompanied by Mr. Melvin Mayfield, P.Eng., and Mr. Roger Skirrow, P.Eng., of Alberta Transportation. This report was prepared by Consultants Ltd. for Alberta Transportation Central Region under Contract No. CE053/2000. 1. PROJECT BACKGROUND From Sta. 305+00 to 309+00 of SH597 (about 5 miles east of Blackfalds), the grade line for the highway fill is set on a side hill crossing of a bowl shaped valley. A creek is located at the toe of the fill, which flows to the Red Deer River approximately 1 km away. Shortly after construction in 1977, settlement and pavement cracking was observed between Sta. 306+35 to 307+81. Further subsidence was observed in the ditch on the north side of the road in 1982. Instrumentation measurements indicated that the highway fill was failing along the bedrock contact and moving southwards towards the creek. Remedial works installed in 1983 included a key trench, a toe berm and horizontal drains. 020601L- C8 Annual Report.doc Suite 114, 6815 8 Street NE, Calgary, Alberta, Canada T2E 7H7 T (403) 274 3424 F (403) 274 5349 www.klohn.com
ALBERTA TRANSPORTATION CENTRAL REGION June 7, 2002 From 1993 to 1996, about 100 mm to 150 mm of asphalt was added each year to maintain the road surface. Movements were typically noted following periods of heavy rain. In 1996 to the east of the main embankment, a section of pavement about 50 m long was excavated to a depth of about 8 m. The excavation was backfilled with compacted pit run gravel, with a filter cloth and perforated pipe drainage system, which was overlain with compacted clay fill. A 3 m deep french drain about 200 m long was installed below the north ditch. The highway was re-paved in 1998 and 2000. No movements were observed in the period 1996 to August 1999. In August 1999, following a heavy rainfall, significant cracking in the slope was observed and this was attributed to settlement of the fill placed in 1996. To confirm this theory, instrument installation was performed in 1999. Note that no detailed information or construction drawings are available for the reconstruction work carried out at this site. The slide location, site plan, instrument locations and cross-section are illustrated on Figures 1 and 2. The slide features are also illustrated in the attached photographs. 2. SITE OBSERVATIONS The main embankment in the valley appears to be performing well with no evidence of cracking or deformation. Some erosion is occurring on the west side due to ditch flows flowing down the slope. During the 2000 inspection, we observed on the east of the main embankment, that the highway had been patched in two locations on the south side of the road. The patches were about 3 m by 4 m and corresponded with the historical crack locations originally observed at this site. The highway was repaved in the summer of 2000 and the new pavement surface shows no sign of cracking in 2002. To the south, the slope had evidently been re-graded and geotechnical instrumentation consisting of 3 slope inclinometers and 2 standpipe piezometers had been installed. Comments on the SI data obtained on June 10, 2002 are provided below: SI 99-1 There is only a very small (<5 mm) movement occurring on the north side of SH597. SI 99-2 Minor movement has continued in a downslope direction during this reporting period. Cumulative movement at the surface has increased to about 35 mm to the southeast with an apparent shear zone at about 6 m depth that approximately 020601L- C8 Annual Report.doc Page 2
ALBERTA TRANSPORTATION CENTRAL REGION June 7, 2002 corresponds with the base of the pit run gravel zone. Another shear zone is apparent at about 2 m depth, which approximately corresponds with the base of the overlying clay fill. SI 99-3 There has been an increase in downslope movement at the surface in the order of 8 mm since the last reading in October 2001. Cumulative downslope movement is about 75 mm at the surface since November 1999. The observed shear zones are located at depths of about 2 m and 5 m below the surface of the slope with about 13 mm of total movement at the 5 m depth. The movements appear to coincide with the fill/native soil interface and drain locations. The results from the standpipe piezometers are provided in the Table below: Piezometer Ground Elevation Stick-Up Tip Depth Depth to Water (BTOP) Water Depth below ground Phreatic Surface Elevation SP 99-4 80.00 1.10 8 7.87 6.77 73.23 SP 99-5 77.50 1.05 6 5.59 4.54 72.96 The standpipe readings indicate a rise in the groundwater level of about 1.2 m in SP99-4 and 0.1 m in SP99-5 since November 2001. To the east of the slope, it would appear that new 100 mm diameter perforated pipes have been installed in the ditches. These discharge into cobble protection at the crest of the slopes. Cracks and a minor scarp have formed from the southeast drain and follow the fence line (see Figure 1). The upper crack (Crack B) has a small scarp up to 300 mm high. The lower crack (Crack A) is about 10 m long, about 50 mm wide and 200 mm to 500 mm deep. There has been no changes in these cracks since 2000. About 500 m to the east of the site, the north ditch was observed to be very soft and wet. 3. SITE ASSESSMENT There is no sign of imminent slope instability that could jeopardize the highway. It would appear that any movement that is occurring is confined to the south side of the highway. The current instrumentation readings would suggest movement in the slope has slowed 020601L- C8 Annual Report.doc Page 3
ALBERTA TRANSPORTATION CENTRAL REGION June 7, 2002 significantly or stopped. This may be due to the very dry conditions in 2001. This would be consistent with the observations that movements occur following heavy rainfall. The section shown in Figure 2 is very approximate and is based on a sketch from Thurber Consultants Ltd. The movements in SI 99-2 would appear to be originating within the pit run gravel, although it is more likely that the movements are occurring below the gravel. The water levels in the slope are consistent with the drains at the base of the gravel layer. Based on the risk level criteria provided by Alberta Transportation, a risk rating of 18 has been assigned to this site. This is based on a probability factor of 9 for a potentially active slide and a consequence factor of 2 as closure of the highway is unlikely. 4. RECOMMENDATIONS As there have been no observed cracks in the pavement for the last two years, it is recommended that the annual site inspection be discontinued. However, one set of instrument readings should be taken per year, which would also include a brief visual inspection, particularly if a period of extended wet or very heavy rainfall occurs. Please contact the undersigned if you have any questions regarding this report. Yours truly, KLOHN CRIPPEN CONSULTANTS LTD. Darren Ratcliffe, P.Eng. Senior Geotechnical Engineer 020601L- C8 Annual Report.doc Page 4
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ALBERTA TRANSPORTATION CENTRAL REGION FIGURES 020601L- C8 Annual Report.doc