May/June 2024 - 96

the top of the slope next to the tracks
and drainage pipes installed sometime
prior to 1992, and a slag/cement slurry
grouting program performed beneath
the tracks in 2015. The 2015 treatment
went to depths of about 25 ft (7.5 m) and
spanned about 400 ft (122 m) of track.
In 2020, the same segment of rail,
approximately 330 ft (100 m) long,
experienced increased deformations
that posed a risk to safety and operations.
Inclinometers were installed to
monitor the rate and degree of slope
movement and site reconnaissance
was performed. Key observations
illustrated instability of the northern
slope including erosion of the toe and
around the three-sided culvert, downslope
movement of trees, and settlement
at track grade. No signs of movement
were observed on the south slope
of the embankment.
Investigation
Engineers performed a geotechnical
investigation that consisted of four
borings drilled through the embankment
and into bedrock, two from the top
of the slope next to the tracks and two
further down on the slope from a lower
working bench. Measured from the top
of the embankment, soils consisted of
70 ft (21 m) of fill (micaceous silty sand)
underlain by 5-20 ft (1.5-6 m) of residual
soil (silty sand with decomposed gneiss)
and bedrock (gneiss). The fill also
contained 25 ft (7.5 m) of ballast and
cinders directly beneath the track. Slope
inclinometers were installed in three of
the four borings to record lateral
movement of soils.
Displacement data was used to
evaluate and analyze an estimated
creeping shear zone in the embankment.
In just two months, from April to June
2021, approximately 4 in (101 mm) of
horizontal movement was measured in
the inclinometer located at top of
embankment and 1.5-2 in (38-50 mm)
in the lower inclinometers. The cause of
sliding was attributed to a combination
of oversteepened slopes, erosion at the
96 * DEEP FOUNDATIONS * MAY/JUNE 2024
embankment toe
and excessive rainfall.
When comparing
the depth of
p r ev i ous l y i nstalled
mitigation
measures and the
measurements recorded
in the inclinometers,
the
sliding plane appeared
to be deeper than the existing
timber piles and cement treatments.
Project site
Slope Stabilization
The engineer and the owner worked
together to come up with a stabilization
solution that would address both global
and local track instabilities - something
more robust than past attempts -
while keeping in mind that the track
must remain act ive during any
construction activities and that the
existing slopes are steep, highly
vegetated and potentially unstable. The
selected solution was a two-tiered
system using small diameter structural
elements that allowed for
installation with small equipment
more conducive to the limited working
room at the top of the embankment. The
performance goal of the design was to
increase factor of safety from an
assumed existing creeping shear plane
condition of 1.0 (using a back calculated
The selected solution was a two-tiered system using
small diameter structural elements that allowed for
installation with small equipment more conducive to
the limited working room at the top of the embankment.
Factor of safety after installing upper and lower stabilization
May/June 2024

Table of Contents for the Digital Edition of May/June 2024
