July/August 2023 - 15

slide was moving continuously at
an average rate of 10 mm (0.4 in)
per day, accelerating to 50 mm
(2 in) per day following precipitation
events. Sliding movement
is translational, occurring along a
2 m (7 ft) thick roughly planar
basal shear zone. The mobilized
body of Ten Mile Slide is mainly
dry, has higher strength than the
basal shear surface, and behaves
as a relatively stiff and cohesive
mass. Previous efforts to stabilize
the highway had only mitigated landslide movement temporarily.
The stratigraphy at the site includes, in order of increasing
and largely determined by the
life of the structures. The designs
for all structural components of
the stabilization measures were
compliant with the Canadian
Highway Bridge Code and a 475year
return-period earthquake
event, and met the retaining wall
performance requirements for a
75-year design life.
Climate change projections
Overview of the Tunnel Earthflow and Ten Mile Slide
indicate a small increase in
annual precipitation over the
depth: Tunnel Earthflow, undifferentiated glacial sediment
and Jackass Mountain Group Bedrock. The movement of the
slide has resulted in a shear zone near the base of the Tunnel
Earthflow unit. This shear zone has been characterized
spatially and with depth by several boreholes from different
investigation efforts over many years. The Tunnel Earthflow
is approximately 20 m (65 ft) thick - the texture is
heterogeneous and unsorted, and typically comprises plastic,
well graded clayey silt, sand, and gravel with cobbles and
boulders. Except in a few instances, water seepage or wet
materials were not encountered. The undifferentiated glacial
sediment directly below comprises permeable, dense,
drained, coarse granular soil. Cobbles and boulders occur at
any depth in this soil, as do seams and lenses of low
permeability, clayey sediments. Bedrock is located at depth
and is not relevant to the stabilization measures.
Stabilization Concept Design Criteria
An important step in the design process was determining an
appropriate factor of safety (FoS) for slope stabilization.
Ongoing movement of the landslide suggested the limit
equilibrium FoS prior to stabilization was near 1.0. After
completing a rigorous site investigation, design sensitivity
analysis, geostructural numerical modelling, two test soil
anchors and consulting a technical panel comprising
independent experts, the criteria adopted by the ministry
were to design the stabilization measures to increase the FoS
on each cross-section analyzed to a minimum of 1.2. To
manage uncertainties and risks associated with the work it
was further decided to adopt an observational method,
comprising significant geotechnical and structural
monitoring before, during and after construction for the
implementation of the stabilization program.
The minimum design life of the stabilization measures is 30
years based on the rate of slide retrogression. If the combined
effect of the stabilization measures constructed by the ministry
and CN prevents future retrogression of the landslide, the
design life of the stabilization measures is expected to be longer
DEEP FOUNDATIONS * JULY/AUG 2023 * 15
design life of the stabilization measures. The impact of these
wetter conditions was considered with regard to slope
stability by checking the sensitivity of the FoS of the
stabilized slide to a permanent and sustained increase in
groundwater pressure to confirm the resulting FoS remained
sufficiently high.
Selected Solution for the Stabilization Concept
Typical solutions such as rerouting the highway, unloading
the crest or buttressing the toe of the slide were concluded not
to be feasible due to the steep slope and proximity of the
Fraser River at the toe of the slide and the railway at the crest.
Groundwater drainage was expected to be ineffective because
groundwater pressure is near-zero in both the coarse-grained
undifferentiated glacial sediments below the slide and in the
overlying Tunnel Earthflow soil, except for occasional
perched aquifers. A structural solution that increased the
shear resistance along the failure plane was therefore
pursued. The design solution needed to consider ongoing
slide movement during construction so that components
installed in early construction phases could tolerate daily
slide movement until enough additional structural support
was installed to stop movement.
The solution adopted included 276 post-tensioned double
corrosion protected soil anchors with 2.5x2.5x0.6 m
Interpreted geological profile and selected stabilization solution: Basal shear
zone (grey), undifferentiated glacial sediments (yellow)

July/August 2023

Table of Contents for the Digital Edition of July/August 2023

TOC
July/August 2023 - Intro
July/August 2023 - 1
July/August 2023 - 2
July/August 2023 - TOC
July/August 2023 - 4
July/August 2023 - 5
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