January 16, 2006 File: 15-85-13 Alberta Infrastructure and - - PDF document

January 16, 2006 File: 15-85-13 Alberta Infrastructure and Transportation Room 301, Provincial Building 9621 - 96 Avenue Peace River, Alberta T8S 1T4 Attention:

• Mr. Ed Szmata

PEACE REGION (PEACE RIVER/HIGH LEVEL) GEOHAZARD ASSESSMENT SHOP AND OVERPASS SLIDES (PH 9), Hwy 2:62 and Hwy OFF RAMP, PEACE RIVER 2005 ANNUAL INSPECTION REPORT Dear Sir; This letter documents the 2005 annual site inspection of areas of slope instability at two sites located along Hwy 2:62 and it’s south off ramp in the town of Peace River, Alberta. Thurber Engineering Ltd. (Thurber) undertook this inspection in partial fulfillment of our Geotechnical Services for Geohazard Assessment, Instrumentation Monitoring and Related Work contract (CE049/2004) with Alberta Infrastructure and Transportation (AIT).

• Messrs. Simon Cullum-Kenyon, P.Eng. and Don Proudfoot, P.Eng. of Thurber

undertook the inspection

• n

June 15, 2005 in the presence

• f
• Mr. Roger Skirrow, P. Eng., Mr. Ed Szmata and Ms. Amanda Russell of AIT.

1. BACKGROUND Thurber last visited the site in June 2004 and the site condition at that time is described in our Part B assessment letter in the site binder. Additional information

• n the site is provided in the Geotechnical File Review in Section A of the binder.

2. SITE OBSERVATIONS The changes in condition since last year are shown on the attached site sketch

• plans. Selected photographs taken during the visit are also attached.

At the Shop Slide, there is some evidence of further movement since the 2004 visit – additional cracks were spotted at the west end of the lower portion of the slide, and cracking has worsened in and around the median at the west end of the

• site. Some additional areas of minor cracking were noted at the east end of the

upper slide which indicate that it has a larger extent than first thought. Drains at the west end of the slide that were seen to be flowing in 2001, were dry this year. There are a number of wet areas at the base of the slide, along the CNR line, and there is some evidence of pumping of the sleepers, though evidence of wet conditions here was also observed last year. At the Overpass Slide, up to 10 mm of additional movement has occurred at the southern most pier on the west side of the rail line. In addition, cracking was noted in the concrete diaphragm between the piers on the east side of the rail line, suggesting relative upward movement of the second southern-most pier. The gabion mat lined ditch to the south of the bridge is sagging locally by 30 cm or more and showing severe signs of distress and loss of ground from under the gabions, immediately downstream of the drop structure. The scarp immediately south of this shows no sign of worsening. There was no apparent maintenance work completed at the Shop Slide site since the previous visit. The bridge piers were painted at the Overpass site. 3. ASSESSMENT At the Shop Slide, based on surface observations, there is complex deep-seated slide movement, consisting of a ‘lower slide bowl’ that is directly impacting the road, and an ‘upper slide bowl’ extension that appears to be less active and at present does not directly impact the road. Movement in the lower slide bowl appears to be more active at the NW corner. Other visual signs of movement are more subtle. A geotechnical investigation was conducted at this site earlier this year; the investigation results will be summarized in a separate report. A number of inclinometer casings and piezometers were installed and the estimated movement trends and depths are shown on the attached site plan. The fall 2005 SI readings are indicating a slow creep movement of 4 to 8 mm/year. Based on the preliminary information the main, lower slide appears to be seated in clay (lacustrine and till-like material) as shown on the attached Cross-section A-A’. About 6m of highway embankment fill is present overlying the clay and wet sand is present in the flat lying ground area at the base of the slope below the CN tracks. The slip surface of the slide is located at 12 to 18 m below the downslope shoulder

• f the highway.

Client: Alberta Infrastructure and Transportation Date: January 16, 2006 File: 15-85-13 Page: 2 of 5 e-file: 08/15/85-13 let PH9

We have re-drawn the estimated limits of the upper slide based on a closer review

• f the site this year. It appears that a much larger slide event occurred many years

ago to the limits of the redrawn upper slide and that the lower slide is a smaller active block within the presently dormant larger one. At the Overpass Slide, a deep-seated slide appears to be affecting the bridge

• structure. Observations indicate continued surface movement of the concrete

facing relative to the piers under the bridge, near the CNR line. The inclinometers that are being read show little or no discernable movement except at the surface in SI-4A and SI-6A, both near the CNR line. The surface movement noted in these inclinometers is minor, and may be more related to frost heave or movement of near-surface backfill rather than real slide movement. There is no apparent change in seepage occurring in the area of the slide, or the minor surface cracking noted last year to the south of the bridge. However, an area of the gabion lined- ditch on the south side of the bridge, immediately down-stream of the drop check structure, shows signs of locally severe sagging and loss of soil from under the gabion mats. 4. RISK LEVEL The risk level for these sites has been assessed as follows: Site PF CF Risk Level Comments Shop Slide 11 6 66 Overpass Slide 9 4 36 At the Shop Slide, there is evidence of continued movement. The area affected by the lower slide shows signs of continued movement. There is no change in perceived risk level from last year. At the Overpass Slide, there is evidence of continued movement at the surface. There is no change in the consequence factor, and the perceived risk remains the same as last year. 5. RECOMMENDATIONS 5.1 Short Term In the short term the Shop Slide 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. As an interim measure, consideration could be given to lining the up-slope ditch with geomembrane through the area of the slide, to prevent water seeping into the backscarp of the lower slide.

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At the Overpass Slide, some repair work is required for the gabion mat ditch lining

• n the south side of the bridge, where the soil below the mats appears to have

been eroded. Consideration should be given to using a geomembrane (perhaps Coletanche bituminous membrane) below the mats to prevent erosion. Erosion that was noted on the north side of the highway last year does not appear to have worsened – any improvement to drainage handling can probably be deferred for another year. 5.2 Long Term Long-term solutions for the Shop Slide might include:

• 1. Closure of the road – re-route traffic that currently uses this road.
• 2. Re-alignment of the railway to allow placement of a large toe berm
• 3. Construction of a tunnel along the railway to allow a toe berm to be

constructed over the current railway alignment.

• 4. Installation of a stone column shear key and smaller toe berm retained by a

mechanically stabilized earth wall to avoid relocating the railway. The above solutions are all expected to be expensive with a cost in excess of $500,000. Long term options for the shop slide will be discussed further in our forthcoming preliminary engineering report. A pile wall along the shoulder of the highway does not appear feasible due to the long length of the slide and the significant depth to the slip plane. Long-term options are not being contemplated for the Overpass Slide at this time – further monitoring is required to determine if deep-seated movement is posing a threat to the bridge abutment. 5.3 Investigation An investigation of the Shop Slide was completed during the last year, and is reported separately. Further monitoring of the slide will be required before a decision can be made on an appropriate long-term solution. Consideration should be given to the need to upgrade or replace inclinometers that are un-reliable or inoperative at the Overpass Slide. Discussions should be held with the bridge engineering section to establish whether permanent displacement monitoring points can be installed on the bridge structure, and what levels of movement are tolerable.

Client: Alberta Infrastructure and Transportation Date: January 16, 2006 File: 15-85-13 Page: 4 of 5 e-file: 08/15/85-13 let PH9

5.4 Maintenance At the Shop Slide, the following maintenance items should be addressed:

• Patch the road surface (as required) to maintain a smooth even road

surface for traffic safety – see Photo 1 At the Overpass Slide, the following maintenance items should be addressed:

• The sagging gabion mats in the ditch lining to the south of the bridge should

be removed, clayey (impermeable) soil placed and compacted in voids, and a geomembrane liner placed over the soil before replacing the gabion mats. 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 Proudfoot, P.Eng. Review Principal Simon Cullum-Kenyon, P.Eng. Project Geotechnical Engineer

/slip

Attachments

Client: Alberta Infrastructure and Transportation Date: January 16, 2006 File: 15-85-13 Page: 5 of 5 e-file: 08/15/85-13 let PH9