December 5, 2005
File: 15-85-11
Alberta Infrastructure and Transportation
Room 223, Provincial Building
4709-44 Avenue
Stony Plain, Alberta
T7Z 1N4
Attention:
Mr. Mike Baik
NORTH CENTRAL REGION GEOHAZARD ASSESSMENTS
NC 17A - HWY 63:12 TWINNING SECTION
2005 ANNUAL INSPECTION REPORT
Dear Sir;
This letter documents the 2005 annual site inspection of an area of slope instability
located along Hwy 63:12 north of Ft. McMurray, Alberta. Thurber Engineering Ltd.
(Thurber) undertook this inspection in partial fulfillment of our Geotechnical
Services for Geohazard Assessment, Instrumentation Monitoring and Related
Work contract (CE046/2004) with Alberta Infrastructure and Transportation (AIT).
Mr. Don Proudfoot, P.Eng of Thurber undertook the inspection on June 23, 2005 in
the presence of Mr. Roger Skirrow, P. Eng. and Mr. Ron Behr, AIT’s Maintenance
Contract Inspector (MCI).
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.
2.
SITE OBSERVATIONS
2.1
Km 2.7 – Slumps
The changes in condition since last year are shown on the attached site sketch
plans. A slope cross-section of the subject area was provided previously in Section
F of the binder. Selected photographs taken during the visit are attached.
The highway backslope slump (photos 1, 2 and 3 and Figure NC17A-16)
continues to grow in size. There has been some slide extension to the west and
south. Although there is good grass cover, the ground surface is cracked and
hummocky which allows surface water to infiltrate the slide mass. At the time of
the visit it was raining and some runoff was ponding in the highway ditch upstream
of the toe roll of the slide.
There has also been some retrogression of the North slump (photo 4 and Figure
NC17A-17) located above the offloaded area. The backscarp has receded about
4 m further upslope and new cracks are starting to appear and trees are starting to
tilt along the south edge of the slump (photo 7). There is still an area of ponded
water (photo 6), which now has bullrushes growing along a dip in the slide mass
just above the toe roll.
There has also been some minor retrogression of the backscarp of the South
slump (photo 4 and 5) but to a much lesser degree than the north slump.
Slow steady seepage was noted emanating from the south outlet of the subdrain
(photo 8). It appears that the seepage has been ongoing for some time as
indicated by the rust staining and salt buildup on the pipe.
3.
3.1
ASSESSMENT
Km 2.7 – Slumps
The localized slumping at km 2.7 is considered to have been caused by
progressive failure of the pre-sheared (slickensided) clay colluvium after the
negative pore pressures due to offloading of the slope dissipated, reducing the
effective cohesion within the slope. Based on the observed behaviour of this
material, the stable angle of high slopes in the pre-sheared colluvium appears to
be about 11 ° (5H:1V) or flatter.
The slump in the lower highway backslope is located in a thick portion of
the colluvium layer. Under the current conditions, the slump may continue to
creep and grow in size both back into the slope and along the slope parallel to
the highway.
The colluvium layer thins considerably in the vicinity of the upper backslope cut
above the offloaded area. The North Slump is located at the mouth of a ravine
where the colluvium cover locally extends further up the slope. This slump could
eventually extend another 20 m up the slope, although the affected area will likely
be quite narrow (10 m in width) and shallow (1 to 2 m deep).
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Date: December 5, 2005
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The South Slump is expected to grow at a slower rate and may eventually reach
a stable angle, provided cracks are periodically sealed to limit ingress of
surface water.
SI99-1 (refer to Figure NC17A-17) installed in the backslope of the offload area cut
between the North and South slumps has shown minimal movement over the last
three years. This lack of movement confirms that there is little to no colluvium in
the backslope area between these two slumps.
SI99-2 (refer to Figure NC17A-16) installed at the crest of the highway backslope
at a location 34 m north of the local slump shows that the deep colluvium layer has
experienced global movement at a depth of 21 m along the contact with the
limestone at a relatively steady rate of 2 to 5 mm/year over the last four years.
Although this is a relatively small amount of movement it highlights the marginal
global stability of the colluvium deposit.
3.2
Backslopes at Other Locations Adjacent to Enbridge Pipeline
At the time of highway twinning, slope inclinometers were installed at
representative locations between the backslope of the highway and the Enbridge
pipeline to provide advance warning of potential slope movements that could affect
the pipeline. The instrument locations and readings are shown on the site plan and
plots included in Section D of the binder.
The Spring 2005 instrumentation readings indicated creep movement of
10 mm/year at 3 m depth in SI03-5, located at about Km 6.7. SI99-3 and SI99-4,
located at Km 4.3 and Km 6.0, respectively have sheared off or have become
blocked at depths of 2.6 and 2.1 m respectively and can no longer be read. In light
of the recent slumping observed at Km 11.45 and 11.15 (refer to separate report
for NC17C), the observed creep movements at SI03-5 and SI99-4, where the
backslope is about 2.5 m high, could be a precursor to future slumping at these
locations. SI99-3 was installed adjacent to the pipeline ROW at a location where
the adjacent highway was constructed in a fill to provide a baseline measure of
natural slope creep.
4.
RISK LEVEL
4.1
Km 2.7 – Slumps
The risk level to the highway for this site has been assessed as follows:
PF(9) * CF(2) = 18
We have assessed a Probability Factor of 9 since the slide is active with moderate
rate of movement and a Consequence Factor of 2 since this is a slide where
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Date: December 5, 2005
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movement in the backslope could result in some slide material moving onto
the highway.
4.2
Backslopes at Other Locations Adjacent to Enbridge Pipeline
The risk level for the highway has been assessed as follows:
PF(5) * CF(1) = 5
A Probability Factor of 5 is considered appropriate since the slides are active
with a very slow rate of movement. A Consequence Factor of 1 is considered
appropriate since the failure of the shallow backslopes might block the
highway ditch and require some maintenance but would likely not affect
the highway surface.
However, the risk for the pipeline is higher and has been assessed as:
PF(5) * CF(10) = 50
A probability factor of 5 has been assessed due to slow creep movements and
shearing/blockage of the casings, which leaves some uncertainty about the
movement. In our spring 2005 instrumentation report it was recommended that
SI99-4 be replaced and that SI99-3 be repaired so that monitoring could be
continued. This would reduce the uncertainty considered in the above risk
assessment value. A consequence factor of 10 is considered appropriate since a
failure of the pipeline has very high environmental and economical implications.
5.
RECOMMENDATIONS
5.1
Km 2.7 – Slumps
5.1.1 Short Term / Maintenance
The local slumps in the vicinity of km 2.7 are not currently affecting the highway.
The upper North and South slumps have not changed much from last year.
However, it is recommended that the area of ponded water above the toe roll of
the North slump be drained to reduce potential softening of the slide mass. As
suggested last year, the dead trees in the slide area should be cut off at ground
surface leaving the roots intact, so they don’t topple over at some later date and
further disturb the slump area.
This work is expected to cost less than $10,000.
The lower (backslope) slump has experienced additional cracking and settlement.
Crack sealing and regrading would help reduce water infiltration into the slide
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Date: December 5, 2005
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mass, however it would not be possible to do this without removing most of the
vegetative cover. Hence, it is recommended that the lower slump be left as is until
the longer term measures described below can be carried out. In the meantime
due to the roughness of the slide area, grass mowing will need to be completed
using hand operated whipper snippers.
5.1.2 Long Term
Longer term remedial measures could consist of the subexcavation of the failed
soil mass, and replacement of the excavated soil in thin well compacted layers to
re-establish some cohesion in the slope. This work will need to be carried out in
sections to reduce the time frame that the slope is left at a temporary steep angle.
Further consideration should also be given to installing a culvert/subdrain and
backfilling the west highway ditch to flatten and reduce the height of that slope, in
a similar manner to what was done at km 6.675. This work will also improve
surface drainage in the slide area.
The ballpark cost for these longer term measures is in the order of $200,000. The
work would need to be done during warm weather conditions.
5.2
Backslopes at Other Locations Adjacent to Enbridge Pipeline
5.2.1 Short Term / Monitoring
In the short term it is recommended SI99-4 and SI99-3 be replaced to allow
continued monitoring of the slope at these locations. The ballpark cost to replace
and initialize the two SI’s is $11,000. In addition, a careful review of this highway
backlslope section should be carried out in the early spring by the MCI and again
during our annual visit.
The results of the further monitoring should be reviewed to assess whether any
longer term measures are warranted.
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File:
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Alberta Infrastructure and Transportation
15-85-11
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Date: December 5, 2005
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THURBER ENGINEERING LTD.
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.
Dimitri Papanicolas, P.Eng.
Review Principal
Don Proudfoot, P.Eng.
Principal
/mes
Attachments
Client:
File:
e-file:
Alberta Infrastructure and Transportation
15-85-11
H:\85-11 NC # 17A
Date: December 5, 2005
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