SITE S7: EMBANKMENT FAILURE WEST OF MILLARVILLE

Transcription

1 LANDSLIDE RISK ASSESSMENT SOUTHERN REGION SITE S7: EMBANKMENT FAILURE WEST OF MILLARVILLE LEGAL LOCATION: LSD W5M and W5M REFERENCE LOCATION ALONG HIGHWAY The slide area is located between Sta and on SH 549, approximately 1.5 km west of the junction with SH 762 UTM COORDINATES: N E (NAD27) NTS Map Sheet 82 J/9 (Turner Valley) AI FILE: AI PLAN & PROFILE: SH549:02 Sta (Metric)/Sta (Imperial) Date of Initial Observation: Spring 1987 Date of Last Inspection: July 2003 Instruments read May 2003 Instruments Installed: Instruments Operational: March slope inclinometers, 2 pneumatic piezometers, 4 standpipes 2 pneumatic piezometers 2 standpipes Risk Assessment: PF(13) * CF(5) = 65 (increased from 11*5 = 55 in June 2002) Last updated by: AMEC Earth & Environmental Limited, August 2003 Comments:

2 Location The slide area is located between Sta and on SH 549, approximately 1.5 km west of the junction with SH 762. This site is approximately 12 km west of Millarville, AB. General Description Of Instability The road in the failure area is aligned east-west and located on a sidehill with the south side upslope. The embankment in the failure area is 5 to 6 m in height and is constructed at the cut-to-fill transition between a cut section immediately west of the failure area and lower areas to the east. In the spring of 1987 a landslide occurred in the road embankment in this area. The slide consisted of a rotational earth slide encompassing the entire north slope of the embankment between approximately Sta and The slide was characterized by downdropping of the upper half of the slope face and bulging outwards of fill in the lower half of the slope face. In addition, sinkholes and depressions in the fill material were noted above a culvert located at Sta and upslope of the outlets of a pair of horizontal drain pipes between approximately Sta and No cracking or other signs of distress were noted in the pavement adjacent to the slide. After a geotechnical investigation by AI personnel, the failure was attributed to a combination of the following factors: Saturation of the embankment/elevated groundwater table within the embankment due to slope runoff from the south and discharge from the horizontal drains onto the embankment slope in the vicinity of Sta The presence of a weak organic layer on the natural slope face beneath the embankment fill. Poor weathering characteristics of the sandy shale rock fill used to construct the embankment. Geological Setting/Geotechnical Conditions A series of boreholes drilled by AMEC during the geotechnical investigation in March 2001 encountered clay fill in the road embankment, which was underlain by high plastic clay (likely glaciolacustrine), which was in turn underlain by high plastic clay till that extended beyond the completion depth of the boreholes. A 0.5 m thick layer of organic material was encountered at the base of the road fill in one of the boreholes. Chronology Table A1 provides the Chronological Background of the slide.

3 Past Investigations AI personnel performed a site reconnaissance and excavated a series of test pits in the slide area in June, Test pit logs were not available in the AI files. No records of any other geotechnical investigations were found. AMEC performed a geotechnical investigation in March The results of this investigation, including recommendations for further remedial measures, were submitted to AT during the summer of Remedial Measures A number of remedial measures were recommended by AI personnel in order to reconstruct the embankment with improved foundation and drainage conditions. These measures were as follows: Excavate the slide mass and any underlying weak organic material to expose a competent base. Bench the exposed base and place a 0.3 m thick gravel drainage blanket. Reconstruct the embankment using suitable material. Improve the surface drainage in the area adjacent to the north toe of the embankment so that water discharging from the drainage blanket and surface runoff is not able to accumulate adjacent to the toe of the embankment. Extend the outlets of the horizontal drain pipes between approximately Sta and so that they would not discharge onto the north embankment slope. Install a subsurface drain beneath the upslope (south) ditch to intercept groundwater flow and prevent it from saturating the embankment. Repair culverts as necessary. The remedial measures were implemented by AI personnel from August to November, It was reported that the job went well and there were no significant problems. The road surface was repaved on several occasions between the fall of 2000 and the fall of 2001 in order to repair significant cracking and settlement of the road surface. Monitoring Results No records of any follow-up site inspections or other monitoring prior to the fall of 2000 were found. The results of the ongoing monitoring by AMEC since the fall of 2000 are as follows: Active shear zones have been noted in both of the slope inclinometers installed during the March 2001 geotechnical investigation. The slope inclinometer in BH-

4 1, on the slope face below the road, measured approximately 100 mm of displacement between March and October 2001 along a shear zone approximately 5.7 m below ground surface. Additional movement at this depth sheared off the slope inclinometer in BH-1 between October 2001 and May The slope inclinometer in BH-2, in the westbound lane of the road, measured approximately 12 mm of movement between March and July 2001 along a shear plane approximately 3 m below ground surface before being paved over in late summer Tension cracking has been noted on the slope face below the road and to the east of BH-1. Significant cracking and settlement of the road surface has continued, with horizontal and vertical displacements of up to approximately 100 mm noted to have occurred between the September 2001 repaving and the last instrumentation readings on October 16, 2001.

5 Table A1 S7: Embankment Failure West Of Millarville - Chronological Background YEAR MONTH DESCRIPTION 1986 Fall Overlay construction in area. Spring Slide first observed June Initial inspection by AI personnel on June 9 th. Backhoe test pit investigation on June 23 rd. August Report on slide issued with recommended remedial measures August to November Remedial measures implemented September Site inspection by AMEC and AT personnel. October Road repaved after continued settlement and cracking. March Geotechnical site investigation by AMEC, including the installation of 2 slope inclinometers and 2 pneumatic piezometers. May Site inspection by AMEC and AT personnel. Summer Instrument readings taken by AMEC Repaving after continued settlement and cracking of road surface. One August slope inclinometer paved over. September Repaving after continued settlement and cracking of road surface. October Cracking/settlement area repaved on October 12 th. Instruments read by AMEC. Instruments read by AMEC. Slope inclinometer in BH-1 sheared off at 5.7 m depth below ground surface. May Piezometer appears to be blocked by debris/ice. Annual inspection by AMEC and AT personnel New cracking noted near cattle pass culvert. Increased Probability Factor from 11 to 13. Piezometers in BH-1 read by AMEC. October Road appears to have been repaved at least once since the site inspection in May. November AT reports that this site has been repaved twice during May Instruments read by AMEC Annual inspection by AMEC and AT personnel. July Site was seen to be recently repaved.

6 Alberta Transportation Southern Region Landslide Monitoring Program Summer 2003 Assessment Report August 2003 S7 West of Millarville The site west of Millarville site was visited on July 8, Photographs from this site visit are included in Appendix S7, along with a site plan, air photograph, and a detailed discussion of the visit. This discussion has also been submitted in separate unbound sheets for inclusion in Appendix B of the West of Millarville binder. No significant changes in the stability conditions at this site since the previous inspections were noted. The road surface at this site had been repaved recently prior to the site inspection, therefore it was not possible to directly observe the magnitude of additional cracking and settlement that had occurred since the previous inspection. It is evident that at least the eastern portion of the site is experiencing ongoing movements. AMEC has investigated this site and submitted a design report for remedial measures under separate cover. Without repairs, this area will undoubtedly require ongoing maintenance and patching. The Risk Level at this site was kept at 65. AMEC recommends that the annual assessments and semi-annual monitoring at this site be continued and that the proposed remedial measures be implemented if possible. Please refer to Appendix S7 for further discussion.

7 APPENDIX S7 West of Millarville

8 Alberta Transportation Southern Region Landslide Monitoring Program Summer 2003 Assessment Report August Site Visit The Annual Inspection site visit was conducted on July 8, At the time of the visit, the weather was clear with a light breeze. Please note that this site is approximately 1 km to the east of the site identified as S7 in the original contract. As noted in the Spring 2000 assessment report, the original S7 site was renamed Shale Fill Embankment, assigned a low priority and dropped from the list of sites requiring annual inspection in favor of the more extensive failure to the east which is now referred to as S7 West of Millarville. 2.0 Significant Observations The following observations, considered to be relevant to the stability of the slope were made. No significant changes in the slope stability conditions since the previous site inspections. The road surface through the slide area had been recently repaved (Photos1 to 4), apparently only hours before the site visit. Therefore, no observations of cracking and settlement of the road surface since the previous inspections were possible. 3.0 Changes from Previous Visits No significant changes since the previous visits. 4.0 Discussion The recent repaving had covered any new cracking or continued road surface settlement since the previous inspection. However, given the fact that this site apparently required repaving and the observations and assessment from previous work, it is considered certain that continued slope movement had caused additional settlement and cracking of the road surface. Without the construction of the recommended remedial measures, this area will undoubtedly require ongoing maintenance/patching. 5.0 Assessment The eastern portion of the slide area continues to move at a relatively high rate and will require ongoing maintenance in the future. Therefore, the Probability Factor with respect to this slide should be maintained at 13. It is likely that in the short term, continuing movements will result in repairs and patching being required to the north half of the road (westbound lane). However, larger movements are P:\Specproj\CG25132\Annual Assessment Reports\Spring 2003\files for CD\S7\S7 - Text and Photos.doc S7 - Millarville

9 Alberta Transportation Southern Region Landslide Monitoring Program Summer 2003 Assessment Report August 2003 possible and likely in the longer term, which would effect a significant portion of the highway. On this basis, the Consequence Factor for this slide should be maintained at 5. Based on the above, the Risk Level at this site is calculated as Recommendations The instrumentation monitoring program currently in place should be discontinued. The only remaining, functioning instruments at this site are the two pneumatic piezometers installed in BH1 on the slope face below the road (one of which has been giving erratic readings over the past year). The two slope inclinometers installed at this site are no longer readable (the SI on the road was paved over in the fall of 2001 and the SI on the slope face has sheared off at the slide plane). Future data from the piezometers will not provide any basis for modifying or further deferring the recommended remedial measures for this site. Therefore, future readings of these instruments will not provide significant value and should not be performed. Annual Assessments at this site should be continued. The surface conditions of the road at this location should be carefully monitored by maintenance personnel. Additional movement and damage to the road surface will occur this summer and maintenance work/patching will be required at some point. Mitigative works be considered for this site. The recommended shear key and slope flattening measures for this site should be implemented. Without remedial measures, ongoing maintenance will continue to be required. The likelihood of more significant failures will likely increase as time passes. P:\Specproj\CG25132\Annual Assessment Reports\Spring 2003\files for CD\S7\S7 - Text and Photos.doc S7 - Millarville