Geosynthetics October/November 2019 - 37

well limited how close construction and
excavation activities could encroach upon
the well in addition to limiting how much
failed soil could be removed at elevations
lower down the slope. The design was
performed assuming all fill placed below
the reinforced soil slope (RSS) would
have the properties determined for the
failed soil from the failure analysis to
allow for selective removal of the failed
soils, if necessary, to maintain stability.
The site was also in a remote location; therefore, to avoid considerable cost
associated with removal of existing soils,
the design would need to reuse as much
soil as possible. This required moisture
conditioning of the soils to get them to a
moisture content for proper compaction.
Adjacent platforms were used for laying
out and drying soils, which required handling the soil multiple times.
Retaining wall systems, such as sheet
piles and secant walls, were initially investigated but were determined to be inappropriate given the site and design constraints.
The alignment of the wall would place
the system into the existing failed soils,
requiring considerable tiebacks and excessive embedment depths to develop the
resistance necessary to restrain the soils.
Additionally, a system that could manage
subsurface water flows was desired to prevent future slope stability problems due to
pore water pressure increases. The retaining wall systems did not meet these criteria.
An RSS combined with riprap at the
base of the failure was the preferred solution to meet the site and design constraints. Some of the positive features
were that it could be designed with drain
tile to manage water, it would be flexible
to adapt to long-term consolidation of
the fine-grain soils, it could be designed
to reuse some of the site material at the
upper elevation, and it could be vegetated
to fit better into the surroundings.
An analysis was performed to determine the amount of material that could be

removed while maintaining the stability
of the wellhead. It was determined that a
3H:1V slope starting 35 feet (10.6 m) from
the wellhead could be cut down to provide
a stable working bench. At this stage, a
drain trench on the slope was installed
to capture water and reduce pore water
pressures in the soils. Also, another drain
trench was installed on the opposite side
of the platform to capture water affecting
the site from the upslope side of the butte.
From this bench, test pits would be
performed to determine the extent of the
failed mass and the necessary soil removals for the slope below the RSS.
Next, riprap was placed at the toe to
buttress the system and provide erosion
protection from the stream that the slope
abuts. To complete the platform, the soils
under the RSS needed to be removed and
compacted in select locations to bridge
over the failed slope. Global stability controlled the design of the RSS, requiring

4b
FIGURES 4a and 4b Inclinometer
measurements from a) boring S07
and b) boring S08

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Geosynthetics October/November 2019

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