January 12, 2006 File: 15-85-13 Alberta Infrastructure and - - PDF document

January 12, 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 – HIGH LEVEL AREA) GEOHAZARD ASSESSMENT HWY 726:02 EUREKA RIVER PH 10, SITES #1 AND #2, SOUTH OF BRIDGE 2005 ANNUAL INSPECTION REPORT Dear Sir; This letter documents the 2005 annual site inspection of areas of slope instability and some erosion located along Hwy 726:02, on the south side of the Eureka River, south of the village of Worsely, Alberta (refer to Figure 1). Site #1 is at about Station 10+850 and Site #2 is at Station 10+600 (further south along the highway). 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).

• Mr. Barry Meays, P.Eng and Mr. Don Proudfoot, P.Eng. of Thurber undertook

the inspection on June 14, 2005 in the presence of Mr. Roger Skirrow, P. Eng.,

• Mr. Ed Szmata, and Ms. Amanda Russell, all 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

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

The highway crosses the approximate 30 m deep valley of the Eureka River in a north-south direction. Previously, the PH10 Eureka River area encompassed three geohazard sites extending along a 0.5 km length of this highway on both sides of the bridge, as shown on Figure 1. However, the area has now been

subdivided into 2 separate “PH” areas using the bridge as a split, and PH10 now includes only two of these sites (#1 and #2) located south of the highway. 2. SITE OBSERVATIONS The changes in condition since last year are shown on the attached site plans and cross-section. Selected photographs taken during the visit are also attached. SITE #1 At the time of our June 2005 visit, the highway surface was generally in good condition however there were some diagonal cracks in the pavement surface extending into the gravel shoulder and further towards the south of this site. Some

• f these cracks had been sealed since the last visit.

On the sloping ground east of the highway, the slide scarp observed last year about midway between the river and the highway had increased up to about 500 mm in height (compared to 300 mm last year), and an additional intermediate crack about 3 m long and 50 mm wide was observed about 3 m behind the main scarp crack. The scarp was 16 m from the edge of the highway at its closest point, and appeared to have its toe at the river. No visible seepage was observed from the exposed slot drain outlet on the east side of the highway. The riprap lying

• vertop the non-woven geotextile appeared in generally good condition, with only

slight evidence of erosion. An asphalt curb located along the east shoulder of the highway trains surface water into an 800 mm diameter manhole and then down a corrugated metal ½ pipe surface drainage channel. On the west side of the highway, the erosion gully documented last year north of the last ditch barrier was still about 50 m long, but had grown to about 0.6 m wide and 0.7 m deep. Closer to the river, the flow emanating from the outlet of the 100 mm diameter Big O subdrain located downstream of the box culvert on the south bank of the River was still about 6 litres/minute, however the erosion was more pronounced this year, with gully measurements shown on the attached figure. SITE #2 At the time of our June 2005 visit, the general indication was that conditions at the south portion of the slide was similar to last year, but the northern half to two-thirds

• f the slide has gotten noticeably worse. There were indications of fresh slide

movements consisting of enlarged or additional cracks and scarps, and additional vertical scarp height drops since the previous year in the northern portion. There was a sharp dip and noticeable settlement over the asphalt patch previously placed over the affected section of roadway, and additional cracks were observed in this area this year in addition to the north and south main slide outline cracks previously observed. Similar to last year, the outline of the slide was observed to

Client: Alberta Infrastructure and Transportation Date: January 12, 2006 File: 15-85-13 Page 2 of 6 e-file: 08\15\85-13 let PH 10 – Sites #1 and #2

extend outside of the highway, on the west side in the form of a slide scarp and settlement with water in the ditch, to the southeast in the form of a well defined crack which becomes intermittent further southeast, and to the northeast where it joins a long and more well-defined 400 to 600 mm scarp about half-way to the

• River. This scarp to the northeast was observed to contain a second parallel scarp

crack about 2 to 3 m closer to the river, which was tilted in the opposite direction (towards the highway), indicating it is likely a sunken, graben block type of failure. During our latest visit we have identified areas of new/enlarged scarps, cracks, tilted/leaning trees and wet conditions in the lower slope and toe area. The slump scarps that were observed last year on the slope east of the highway towards the north end appeared to have worsened significantly this year, as shown on the attached site plan, while to the south significant changes were not observed. Similar to last year, immediately downslope of the highway in the main slide area, the base of the fill/drainage blanket was still observed to be wet, and the backslope slumping was still observed over a length of about 95 m south of where the slide scarp crosses the highway. Observation of the exposed riverbank towards the upstream side of Site #2 indicates that the native soil just above river level is varved, highly plastic clay. 3. ASSESSMENT SITE #1 Additional movement of the slide east (downslope) of the highway indicates movements at a similar location to the slide that was remediated in 1988, extending down to the river. The remediation at that time involved unloading and flattening, but not complete excavation and rebuilding of the failed slide mass. The slide could eventually affect the course of the river, pushing it over to the east, and could affect the riprap protection at the culvert inlet, but would not affect the highway until significant retrogression takes place. However, the cracking

• bserved in the highway surface and west shoulder suggests a deep slide block

may be developing, which may be connected to the slide area at Site #2. SITE #2 The previous test holes indicated subsurface conditions consisting of discontinuous clay fill, peat and gravel overlying medium to high plastic clay. Previous assessments indicate that probable contributing factors to the slide were loading of the slope by fill, increased pore pressure of the native material in response to the fill, weak high plastic previously sheared clay, high initial pore water pressures in the clay, and possible settlement of the road surface due to deterioration of layers of peat and sawdust light-weight fill.

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The slide appears to be relatively deep seated, extending from the upslope edge

• f the highway to the river. River erosion at the toe of the slope likely keeps the

slide in motion. The spring 2005 monitoring of the two slope inclinometers (SI’s) installed downslope (east) of the highway indicates the presence of a slip zone at depths between 9 m to 16 m. SI98-1 is showing some movement at the tip of the SI at a depth of about 14 m below ground surface, but since the SI is not installed deep enough, the rate of movement cannot be properly tracked. SI02-3, which was located just outside the main slide area to the north, is registering movement at 8 to 10.4 m depth, which suggests the slide area is more extensive than previously thought and has progressed north of the previous described area of movement. SI98-2 located just above the road near the edge of the scarp crack indicated about 60 mm/year of movement in the spring within the upper 2.5 m of the casing. The three pneumatic piezometers indicated relatively unchanged water levels compared to the last previous measurement. 4. RISK LEVEL The risk level for Site #1 has been assessed as follows: PF(11) * CF(4) = 44 (same as last year) A Probability Factor of 11 is considered appropriate since the slide is active, with a perceived moderate to high, steady rate of movement. A Consequence Factor of 4 is considered appropriate since a partial closure of the road or a significant detour could be a direct result if there is a sudden movement of the larger slide between Sites #1 and #2. The risk level for Site #2 has been assessed as follows: PF(12) * CF(5) = 60 (same as last year) A Probability Factor of 12 is considered appropriate since the slide is active, fairly deep-seated, with a perceived moderate to high, steady or increasing rate of

• movement. A Consequence Factor of 5 is considered appropriate since the slide

cuts completely across the highway and a partial (possibly total) closure of the road would be a direct result of an aggressive slide movement. 5. RECOMMENDATIONS 5.1 Short Term At Site #1, the more visible slide block located downslope of the highway is currently not affecting the highway but should be closely monitored. The cracks

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that cross the highway and extend further south also warrant close monitoring to see if they are connected to the slope movement down slope of the highway at this location or if they propagate southwards towards Site #2. The slide at Site #2 is currently affecting the highway pavement and requires

• ngoing asphalt patching and crack sealing maintenance.

5.2 Long Term An intermediate option that might be considered to reduce slide movements consists of armouring, and possibly re-aligning the river at the sharp meander at Site #2, to reduce further erosion. At Site #2, potential additional long term stabilization measures consist of re-aligning the road over a length of about 400 to 500 m upslope of the existing road on higher ground, which would involve a long cut. The shallow backslope slumps could be solved by flattening the backslope. However, there is a powerline at the top of the backslope which would require re-routing. The ball park cost of this work, excluding land and engineering costs is estimated to be about $700,000. A more significant re-alignment could also be considered to address the problems at all of the sites (including PH26 north of the River) by adopting a perpendicular crossing of the river, however this would be quite expensive. Alternatively, a pile wall could be considered with a partial re-alignment or it may be possible to train the river through a large culvert to allow construction of a stabilization berm at the toe of the slope. At Site #2, options such as subexcavating and replacing the failed material in the upper slope with stronger (gravel) material or constructing a gravel shear key are not considered suitable as the slip surface is fairly deep-seated within the lacustrine clay. Subdrains do not appear feasible either as the material is predominantly low permeability clay that does not dissipate pore pressures quickly. 5.3 Investigation Prior to implementing remedial options, it is recommended that additional inclinometers (total of 3) be installed to replace the one that is too short (SI98-1) and to provide additional information between Sites #1 and #2 to assess whether the sites have now joined into one larger landslide and to determine the depth of

• movement. Piezometers should also be installed at each location in conjunction

with the inclinometers to determine the depth to the groundwater table. The proposed locations and depths are shown on the drawings. These could be installed at the same time as those recommended for PH26 north of the River to expedite overall costs (estimated cost of $20,000).

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5.4 Maintenance For the short term, regular monitoring by the MCI and the visual observations should continue at these sites. At Site #2, 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. 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 Barry Meays, P.Eng. Project Engineer

/slp

Attachments

Client: Alberta Infrastructure and Transportation Date: January 12, 2006 File: 15-85-13 Page 6 of 6 e-file: 08\15\85-13 let PH 10 – Sites #1 and #2