Central Region GeoHazard Assessment Site C8 SH597:02 East of Blackfalds May 2007 Site Assessment and Instrumentation Monitoring Report

Transcription

1 Alberta Infrastructure & Transportation Central Region #401, Street Red Deer, Alberta T4N 6K8 May 18, 2007 Mr. Alain Momedi, P.Eng. Project Engineer Dear Mr. Momedi: Central Region GeoHazard Assessment Site C8 SH597:02 East of Blackfalds May 2007 Site Assessment and Instrumentation Monitoring Report Alberta Infrastructure & Transportation has initiated a process of risk management at specific geohazard sites that includes a document control system. This instrumentation report forms Section C of the document control system for the above site. This site was inspected on April 17 and May 15, 2007 by Mr. Darren Ratcliffe, P.Eng. of Klohn Crippen Berger Ltd. (KCBL). The instruments at the site were read on April 17, 2007 as part of the inspection. Significant movement of the slide occurred between the two inspection dates and it is considered that the site conditions warrant an urgent repair to prevent further deterioration and potential loss of road. Photographs from the inspections are attached for reference. This report was prepared by Klohn Crippen Berger Ltd. for Alberta Infrastructure & Transportation Central Region under Contract No. CE045/ PROJECT BACKGROUND From Sta to of H597 (about 7 km 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 to Further subsidence was observed in the ditch on the north side of the road in 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. A03038A06 Klohn Crippen Berger Ltd th Street NE Calgary AB T2E 7H7 CANADA t f

2 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 No movements were observed in the period 1996 to August 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 To confirm this theory, instrument installation was performed in A topographic survey of the site was also undertaken in September The slide location, site plan, instrument locations and cross-section are illustrated on Figures 1 and 2. Note that no detailed information or construction drawings are available for the reconstruction work carried out at this site. 2. SITE OBSERVATIONS The visual inspections of the site noted no cracking or displacement of the highway pavement. In April 2007, a small scarp about 200 mm high was apparent along the edge of the pavement extending over a distance of about 8 m. Cracks in the slope about 150 mm to 300 mm deep over a width of about 80 m were also observed. In May 2007, significant displacement of the slide had occurred. The scarp adjacent to the highway was now 1.5 m high with scarps on the slope about 0.3 m to 0.5 m high. The extent of the slide is shown in Figure 1 and measures about 80 m wide and extends about 25 m down the slope. In total, three inclinometers (SI99-1 to 3) and two standpipe piezometers (SP99-4 & 5) were installed in No installation logs are available for the instruments. SI99-3 sheared in 2006 and cannot be read. The following data plots where appropriate are provided for Section D of the document control system for SI99-1 and SI99-2: Cumulative and incremental displacement in A direction on same page. Cumulative and incremental displacement in B direction on same page. Displacement time plot showing zone of movement in A direction. Displacement time plot showing zone of movement in B direction. Resolved single movement vector plots. Comments on the SI data are provided below: Page 2

3 SI 99-1 No movement in the reporting period. November 1999 is about 8 mm. Total cumulative movement since SI 99-2 Downslope movement has continued with approximately 100 mm of movement measured at the surface since May 2006 at a rate of about 100 mm/year. Total cumulative movement at the surface is approximately 200 mm downslope. Shear zones are apparent at about 6 m and 2 m, approximately corresponding to the base of the pit run gravel zone and overlying clay fill, respectively. All of the recent movement is located in the upper material. The October 2006 and April 2007 readings show that the rate of movement has increased significantly in comparison to the rate observed in SI 99-3 This instrument has sheared at approximately 5 m below the ground surface. Approximately 50 mm of downslope movement was recorded at the surface between October 2005 and May Total cumulative movement at the surface was approximately 355 mm of resolved movement downslope as of May 2006 at a rate of 130 mm/year. Shear zones are located at approximately 2 m and 5 m below the surface of the slope with about 115 mm of total movement at a rate of 9 mm/year at the 5 m depth when last recorded. Movements appear to coincide with the fill/native soil interface and drain locations. The results from the standpipe piezometers are provided in Table 1. Table 1 Piezometer Readings Piezometer Ground Elevation Stick-Up Tip Depth Depth to Water (BTOP) Water Depth below ground Phreatic Surface Elevation SP SP The standpipe readings in the period 2000 to 2005 indicate a maximum variation in water level from m to m or about 2.2 m. The October 2006 readings indicated that the groundwater levels had risen to the highest level since installation of the standpipe piezometers, or about 1.5 m higher than the average groundwater level. The April 2007 readings indicated a fall in groundwater elevation of about 1 m; however, the groundwater level is still relatively high. Page 3

4 3. INTERPRETATION It would appear that any movement that is occurring is confined to the south side of the highway with a significant increase in the rate of movement leading to a surficial slide (about 2 m deep) that extends to the edge of the highway pavement. Annual summer precipitation data for the Red Deer area is provided in Table 2. As shown, the last three years have significantly more precipitation than the previous three years and have likely contributed to the observed rise in groundwater levels. The increase in groundwater levels could also be an indication that the drains installed in 1996 are blocked or are operating with reduced efficiency. For comparison, the recorded precipitation for September 2006 to May 2007 is about 300 mm (with 190 mm of precipitation from March 2007 to May 2007). Table 2 Precipitation in Red Deer from May to September Year Precipitation (mm) Source: The Weather Network The constructed remedial work section shown in Figure 2 is very approximate and is based on a sketch from Thurber Consultants Ltd. The movements in SI 99-2 and SI 99-3 would appear to be originating within the pit run gravel, although it is more likely that the movements are occurring at the gravel fill/clay fill interface. It is considered that the slide is surficial and extends to a depth of about 1.5 m to 2 m below the ground surface. Based on the risk level criteria provided by Alberta Infrastructure & Transportation, a risk rating of 110 has been assigned to this site. This is based on a probability factor of 11 for an active slide with increasing rate of movement, and a consequence factor of 10 due to potential partial closure of the highway. 4. RECOMMENDATIONS Although no distress in the highway surface has occurred, significant movement has occurred on the downslope side of the highway leading to a 1.5 m high scarp adjacent to the highway. It is recommended that gravel fill be placed immediately in the slide area to form a shoulder at least 3 m wide. Barricades should also be provided. It is understood that this work has been completed as shown in the photo taken on May 22, Page 4

5 Due to the potential loss of pavement, it is recommended that remedial action be taken immediately before the slide deteriorates further. A recommended stabilization method is to use launched soil nails as the observed movement at about 2 m depth occurs within the 6 m length of an individual nail. The area currently backfilled with loose gravel should be excavated and replaced with reinforced and nailed gravel. This method is significantly quicker and more economic that a conventional earthworks approach as illustrated in the cost estimate tables below. Table 1 Soil Nailing Cost Estimate Item Quantity Unit Rate Total Mob/Demob 1 LS $10,000 Traffic Accom 1 LS $15,000 Soil Nails 300 No. $600 $180,000 Reinforced Fill beside highway 1 LS $15,000 $220,000 The alternative method to repair this site would be to replace the failed material with gravel. It should be noted that there are the following disadvantages with this approach: Significant environmental disturbance due to the excavation and backfilling Site occupation period is longer with increased traffic accommodation costs Table 2 Soil Replacement Cost Estimate Item Quantity Unit Rate Total Mob/Demob 1 LS $20,000 Traffic Accom 1 LS $30,000 Common 5,000 m³ $20 $100,000 Excavation Granular fill 5,000 m³ $40 $200,000 $350,000 Based on the potential cost savings with the soil nail approach over a conventional material replacement, it is recommended that this method be considered for the repair. Only one company in Canada, Morsky Industrial Services Ltd. of Saskatchewan, presently has the technology and capability to undertake the launched soil nail repair option. We would recommend that the repair work be awarded to Morsky Industrial Page 5

6 Services Ltd. on a sole source basis based on cost, the low impact on the environment and innovation. Please contact the undersigned if you have any questions regarding this report. Yours truly, KLOHN CRIPPEN BERGER LTD. Darren W. Ratcliffe, P.Eng. Project Manager APEGGA Permit to Practice No Page 6

7 FIELD SUMMARY OF INSTRUMENTATION MONITORING SITE: C8 PROBE # NAME: EAST OF BLACKFALDS DATAMATE # FILE: SH597:02 SI NUMBER FIELD OBSERVATION GND ELEV Elevations referenced to survey STICK-UP performed in September 2001 PIPE INSTALLED PIPE INSTALLED (ft) READING DEPTH (ft) AZIMUTH OF A+ GROOVE N S S READING DATE 17-Apr Apr May and 99-2 clear FILENAME BF99-1 BF99-2 BF sheared 2006 STANDPIPE PIEZOMETER SP99-4 SP99-5 GND ELEV STICK-UP PIPE INSTALLED 8 6 DEPTH TO WATER BTOP DEPTH TO WATER BGL WATER ELEVATION