December 11, 2006 File:

Transcription

1 December 11, 2006 File: Alberta Infrastructure and Transportation Room 301, Provincial Building Avenue Peace River, Alberta T8S 1T4 Attention: Mr. Ed Szmata PEACE REGION (SWAN HILLS AREA) GEOHAZARD ASSESSMENT Hwy 33:14, KM 6 (SH 18 - DEER MOUNTAIN) 2006 ANNUAL INSPECTION REPORT Dear Sir: This letter documents the 2006 annual site inspection of an area of erosion located at about Km 6 along Hwy 33:14 in the southeast quadrant of the intersection of Highway 33:14 and the Deer Mountain turnoff road. Thurber Engineering Ltd. (Thurber) undertook this inspection in partial fulfillment of our Geotechnical Services for Geohazard Assessment, Instrumentation Monitoring and Related Work contract (CE047/2004) with Alberta Infrastructure and Transportation (AIT). Mr. Barry Meays, P.Eng and Mr. Gurpreet Bala, M.Sc. of Thurber undertook the inspection on May 23, 2006 in the presence of Mr. Roger Skirrow, P. Eng., Mr. Ed Szmata, and Mr. Rodney Johnston, all of AIT. 1. BACKGROUND This was a new site in 2004, which had been affected by creek erosion. In the past, AIT had lined the streambed with pitrun over a non-woven geotextile. This has helped to slow the erosion, but not enough to curtail its effects. 2. SITE OBSERVATIONS The changes in condition since last year are shown on the attached updated site sketch plan and cross-section. Selected photographs taken during the visit are also attached.

2 The erosion was located at the base of a north facing 3 m high stream valley bank, upstream of the inlet of a 700 mm diameter smooth steel liner culvert crossing Highway 33:14. The culvert crossed the highway about 20 m south of the edge of the Deer Mountain turnoff road. The stream bank was quite steep, measured to be about 60 0, and a scarp had developed adjacent to the top of this south bank. The exposed bank was noted to consist predominantly of silty sand, and slow seepage was observed from near the base of this bank in a few spots and from about 1 m below the top of the eroded slope crest. Gravel or small riprap was scattered across the stream channel in front of the culvert inlet. Directly in front of the inlet for an approximate distance of 10 m, the stream channel was wider with arced depressions filled with water. The erosion extended about 42 m east from the east edge of the highway, where a small scarp was observed in the Deer Mountain road embankment fill. Trees and bush existed further east of the erosion area. A very narrow stream channel with only minor erosion continued into the bush further east. Since the 2005 inspection, slightly more surface erosion had occurred on the exposed slope face. No cracks were observed just above the slope crest. The amount of slope retrogression was 0.9 m since last year, based on the distance from the survey marker on the crest of the south slope, which was 2.3 m, compared to 3.2 m in ASSESSMENT The erosion area is anticipated to be the result of the stream eroding into the south bank during periods of high flow. The bank appears to consist predominantly of sandy silt, which is a highly erodible material. This is causing slumping and transport of sediment downstream. Slow seepage emanating from the base and about 1 m below the crest of the bank is contributing to further erosion and possible bank slumping. If this erosion is left unchecked, during subsequent high flow conditions the erosion could cause further stream bank retrogression, loss of property, and sediment transport into the Swan River which is located some distance to the west. The erosion is not expected to affect the highway embankment stability anytime in the near future based on present observations. 4. RISK LEVEL The risk level for this site has been assessed as follows: PF(13) * CF(2) = 26 (10 * 2 = 20 last year) File: Page 2 of 5

3 A Probability Factor of 13 is considered appropriate since this is an active erosion area with a moderate rate of erosion. A Consequence Factor of 2 is considered appropriate since the embankment fill is shallow and the erosion would likely not affect the road during the next occurrence, however there could be some silt transported downstream and some additional loss of the adjacent land to the south. 5. RECOMMENDATIONS 5.1 Short Term See 5.3 Maintenance. 5.2 Long Term This site could be considered as a candidate for optional bioengineering remediation solutions. The following alternate measures could be considered for remediation of the erosion and creek bank slumping: 1. The outside toe of the south creek bank should be reinforced with biologs, flattening the slope to a more stable angle (3H:1V), and planting live willows along the zones of seepage. The lower portion of the channel and sideslopes above the biologs should be lined with washed rock, gabion stone or small riprap over a non-woven geotextile, combined with Live Siltation (installing a layer of brush consisting of live willow cuttings just above the biologs along the lower channel). For upper, localized parts of the slope where it is drier, plantings could consist of Live Willow Poles (planting live willows on the bank face along the zones of seepage), possibly combined with soil covering and wild grasses or shrubs. 2. Install a vegetated gabion wall along the base of the steep stream bank, about 2 m high, based about 0.5 m into the ground. During construction live willow cuttings should be inserted through the gabion baskets that penetrate the native soil below. The void between the vertical gabion wall and the intersection of the backslope should be filled with a pitrun gravel material, after placing a non-woven geotextile against the bottom and sides of the native silt material. The failed bank should be cut/filled down to a stable slope angle of about 3H:1V, then covered with topsoil, low flow (straw) soil matting, and finally seeded. The channel bottom and sideslopes should then be lined with additional gabion stone or small riprap over a nonwoven geotextile. Live Siltation and Live Willow plantings could also be combined with this alternative as described in 1. above. File: Page 3 of 5

4 3. Place a vegetated pitrun gravel wedge having an approximate 2H:1V channel face between the stream channel and the base of the slope after trimming the slope bank to 3H:1V. The stabilized slope bank should then be covered with topsoil, low flow (straw) soil matting, and finally seeded. The gravel wedge should also be separated from the native silt with a non-woven geotextile, the toe region of the slope should be covered with riprap. Live Siltation and Live Willow plantings can be combined with this alternative as described in 1. above. 4. Assemble a gabion basket at the base of the slope, having a minimum embedded depth of about 0.5 m along the complete length of eroded slope. Re-grade the slope to a 3H:1V and cover it with low flow soil covering. Eco-blanket, specifically sized compost mulch with recommended seed mix should then be sprayed on the stabilized slope bank. This product requires specialized placement equipment and is marketed by TOPSPRAY. For each of the above alternatives, riprap, over a non-woven geotextile should be placed around the inlet and upstream of the culvert. The natural bends in the creek should be maintained to the extent practicable for all options. Silt fence should be placed along the base of the slope along the south edge of the stream to reduce silt migration until after construction and until vegetation establishes itself on the slope. In order to reduce the project costs, the slope flattening for any of the alternatives could be reduced to 2H:1V from the 3H:1V, however there would be a higher risk of further erosion and additional future remedial measures required in this case. It is recommended that a detailed site survey and streambed profile 200 m upstream and downstream of the culvert be carried out to provide sufficient information about the site topography for design of the remedial measures and drainage improvements recommended. A fisheries/marshland assessment should also be carried out, along with notification to Alberta Environment in the event of unforeseen restrictions. A brief design would be required, and it is anticipated the maintenance contractor could do the work on an hourly basis under the full time supervision of Thurber personnel. Based on previous similar projects the above repairs are expected to cost in the order of $50,000 to $100,000 excluding engineering and construction administration. 5.3 Maintenance It is recommended that silt fence be placed along the base of the slope along the south edge of the stream to reduce the volume of sand/silt migration entering the stream until remedial measures can be carried out. File: Page 4 of 5

5 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 Gurpreet Bala, M.Sc. Project Coordinator /dw Attachments cc: Mr. Roger Skirrow, P. Eng. Director of Geotechnical Services, AIT File: Page 5 of 5