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 of 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 of 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 overtop 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 of 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 observed 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. Client: Alberta Infrastructure and Transportation Date: January 12, 2006 File: 15-85-13 Page 3 of 6 e-file: 08\15\85-13 let PH 10 – Sites #1 and #2