PEACE REGION GRANDE PRAIRIE GEOHAZARD RISK ASSESSMENT SITE INSPECTION FORM NAD 83 COORDINATES N 6,178,811 E 403,309

Transcription

1 PEACE REGION GRANDE PRAIRIE GEOHAZARD RISK ASSESSMENT SITE INSPECTION FORM SITE NUMBER GP-4a LEGAL DESCRIPTION LSD W6M SITE NAME Burnt River Bridge - West Approach Slide HIGHWAY & KM Hwy 49:06 NAD 83 COORDINATES N 6,178,811 E 403,309 PREVIOUS INSPECTION DATE June 1, 2011 INSPECTION DATE July 11, 2012 PREVIOUS RISK ASSESSMENT PF: 15 CF: 7 TOTAL: 105 CURRENT 2012 RISK ASSESSMENT PF: 15 CF: 7 TOTAL: 105 SUMMARY OF INSTRUMENTATION OPERATIONAL: Slope Indicators 2 SI s Piezometers 5 piezo s LAST READING DATE: May, 2012 For details, refer to 2012 Cycle 1 Instrumentation Report INSPECTED BY: (i)karleng: Karl Li, John Heilman (ii) AT: Ed Szmata, Roger Skirrow, Rocky Wang INSTRUMENTATION Monitoring Results/Updates: (i) Major slide plane of this translational block slide was monitored at Elevation 533m to 536m which is close to riverbed level. (ii) This major slippage plane has transgressed inland (as affirmed by SI-31 data)(2007 results) across the existing highway alignment. a) Refer to 2012 Cycle 1 Instrumentation and earlier Reports for details PRIMARY SITE ISSUES: i) DETERIORATION OF SLIDE - A shallow slide regime has developed recently. In recent two years( ), the advancement of a shallow seated escarpment front "i.e. Little Slide" has accelerated to bite into the alignment. At time of this inspection (July 2012), it has necessitated the closure of one lane to require urgent repairs. This shallow sliding regime is in addition to the deep seated sliding regime that was chronically distressing this site over the years. It was assessed that In addition to the prevailing shallow groundwater regime (as previously monitored), the annual spring wet weathers had increased high pore pressure saturation of subgrade

2 soils and invoked an acceleration of shallow ground movements. Such shallow ground movement (advance of escarpment front) is likely anticipated to recur at times of continual wet weather in the future. The advance of headscarp front will cause future steepening the existent sideslope and progressive biting into the roadway space. Thus, for prolonging service life of this highway, o o A design for drying (or dewatering) of subgrade can be advisable Appropriate road space needs to be stitched back to provide roadway serviceability. The stitching can be designed by a slope reconstruction fillet template, preferably with use of mechanically stabilized (MS) granular fill being more tolerant to ground movements. ii) iii) Despite of the above recent site deteriorations of shallow ground movements, this site is fundamentally plagued by deep seated sliding movements. Thus, it is advisable for AT to deliberate and make a determination on the repair strategy of this deep seated slide, especially on the use (or non-use) of the channel diversion and toe berm option. This will allow a long term stabilization strategy to be formulated for this site. For deep seated slide movements, the tactics of subdrain and/or shallow earthwork slope reconstruction options will only temporize and improvise to prolong life of this alignment for a while with serviceability. Historic Issues: iv) Main Slide - a translational block deep seated slide failure v) An Adjacent Slide (upstream) system was recently (2008) included because the western portion of the highway was starting to show tension cracking distress despite substantial setback distances (over 30m) to highway. Along the river channel, the erosion along the valley toe and channel bank is extreme dire and failures of lower slope is evidently causing transgression of sliding towards upslope to affect the highway. vi) For the Main Slide (similarly for Adjacent Slide), the cause (1 st causes) of sliding was primarily from downcutting and lateral erosion of the meandering Burnt River channel. The 2 nd causes being weak soils and adverse groundwater regime. Major basal slippage plane can be assessed at close to river bed elevations. vii) As indicated in earlier reports. To stabilize the slide, the options included: 1) Toe Berming and River Diversion the most preferable option and most likely be successful. For this option, the need to secure a river diversion permit from DFO cannot be overemphasized despite DFO s known resistance to granting channel diversion permit under normal circumstances. - Therefore, it is surely advisable that we should still approach DFO and inform them of our difficulties and abnormal circumstances (on the need of diverting the present river channel to allow an effective stabilizing this slide with building a large toe berm) for preservation of the highway. 2) Realignment option is medium preferable and very likely be of doubtful success. (see 2011 report) 3) Toe pile retaining wall option (least preferable) - Weak soil strength is not favorable to avail passive resistance for a cost-effective design of a Toe Pile Retaining Wall. 2

3 Refer to previous 2011 Slide Tour and earlier reports for details. APPROXIMATE DIMENSIONS: i) Main Slide measure 300m to 350m (width of headcarp) along highway and about 250m in length downslope. A tongue of headscarp front is starting to bite into the roadway (July 2011). ii) Adjacent Slide (upstream) measured about 250m in width and about 200m in length downslope. iii) About 500m length of roadway will be adversely affected by the combined Main Slide and Adjacent Slide (upstream). Recent years ( ) iv) Little Slide (30m width along roadway) has developed from advancement of an escarpment front biting into the alignment. DATE OF ANY REMEDIAL ACTION: (Updated from previous reports) i) In 1991, the present highway and the bridge was constructed as a new alignment (replacing the old highway and bridge) (pre-1991)with a shifting inland (away from the same slide). Around 1998, the slide headscarp has again transgressed to affect the present (post 1991) alignment. ii) iii) iv) To date, this 1991 alignment has not received any remedial action yet. In July, 2011, a new pavement overlay (120mm thickness mill and overlay) was constructed to smooth off pavement contortion and cracking areas. This pavement overlay is for roadway serviceability purpose only. In July 2012, a "Little Slide" of shallow seated movement required the closure of one lane. (Repair remains to be carried out.) ITEM CONDITION EXISTS DESCRIPTION AND LOCATION NOTICABLE CHANGE FROM LAST INSPECTION YES NO YES NO SLOPE MOVEMENT x i) Main Slide assessed as a translational block slide with toe of slope eroded by meander of channel. x ia) Advance of headscarp front has developed a shallow slide regime**( ) biting into the alignment at recurring wet weather times 3

4 ii) Adjacent Slide is located upstream and its retrogression towards inland has started tension cracking along top verge of valley slope and cracking of pavement. - This slide is triggered by a weakening of slope toe area (along river bank) which is being severely eroded by a sharp incision of the meander channel. PAVEMENT DISTRESS x i) Main Slide - **In July 2012, a portion of shoulder pavement subsided due to advance of escarpment front. One lane was closed. - Future frequency of pavement distress remains to be monitored. ii) Adjacent Slide- - Previous tension cracks started to manifest since CHANNEL+TOE EROSION X Meandering channel has been eroding steep banks along toe of valley slope SEEPAGE X - Seepage exit can be located along backslope ditch (SI-31 location) - * Seepage exist can be noted from pavement cracks (July 2011) ** X X * X CULVERT DISTRESS X n/a X COMMENTS: In current 2012 site visit (and an earlier callout inspection June 2012), it was reviewed that 1) For the deep seated slide regime, its effect on roadway serviceability ( ) was apparently held up by the pavement overlay which was constructed in previous year (2011). Currently, the pavement was intact and in good serviceable condition since the slide crack(s) has not reflected back yet. But, future slide movements with its development of pavement cracking and contortion 4

5 remains to be observed. 2) For the shallow slide regime (Little Slide)(i.e. advance of escarpment front), the headscarp front has bitten into the roadway to require closure of one lane. The use of earthwork "stitch and bandage" repair to reconstitute the sideslope for restoration of a travel lane is required. Such earthwork remedy (for shallow-seated slide movements) is anticipated only effective for a short duration because the major slide movements originated from deep depths. In the future, such advance of headscarp and shallow sliding(s) should be more frequently anticipated especially after events of continual wet weathers. (reiterated from previous report) In previous 2011 site visit, it was reviewed that 3) This slide site is massive and of deep seated movements. The site is anticipated to deteriorate with time and a suitable long-term remedy option should be seriously planned for adoption. The Channel Diversion and Toe Berm Option should be preferred. 4) Recent ideas (2010/2011) are for short-medium term stitch and bandage temporal strategy includes. - The alternate of constructing a fillet of slope along the steepened headscarp front with use of free drainage granular fills and horizontal drainage designs be considered to prolong the life of the alignment. - The alternate of installing horizontal drains (a trench along backslope ditch) 5) Despite the consideration of any temporal strategy (inclusive subdrain and/or shallow earthworks) to prolong life of this highway, it is advisable that a long term approach to remediation strategy of this slide be pursued. - The choice of the River Diversion and Toe Berming Option is preferred because it will remove the fundamental trigger of erosion of toe of valley slope. This Option will have a high chance of success. It is advisable that AT should approach DFO for a permit. 6) Instrumentation monitoring and site inspection should be continued. Important This form assessment is an update for current year only. Please refer to the detailed assessment provided as in the 2011 Annual Assessment and earlier Reports for background understanding of this site. END 5