Geoysynthetics August/September 2023 - 24

Searching to maximize stress reduction ratio with biaxial geogrids and timber piling
At depths of about 20 feet (6.1 meters)
The geometry of the
piling was designed
to support both the
wall loading for 18foot
(5.5-meter) long
wall stems, as well
as to provide global
stability for the
embankment up and
behind the wall.
and elevations of 630 to 620 feet (192 to
189 meters), the borings encountered a
layer of soft to medium stiff, gray silty
clay loam with higher strength and
noticeably lower moisture content than
the upper and lower organic deposits.
Laboratory strength testing by unconfined
compressive strength show undrained
shear strength (Su) values closer
to 4.1 psi (28.7 kPa) with corresponding
moisture content values of 35 to 48%.
Unlike the organic layers, this material
is much more discontinuous across the
site; where it was encountered it has a
maximum thickness of only 5 to 7 feet
(1.5 to 2.1 meters).
Deeper foundation soils across the site
consisted of stiff to very stiff clays with Su
values of 9.0 to 23.6 psi (62 to 163 kPa)
and dense sand with Standard Penetration
Test rates (N-values) generally greater
than 40 blows per foot (0.3 meters).
Groundwater levels along the length
of the proposed wall were monitored
in an observation well. The well is
screened within the upper organic
deposit between about 6 to 10 feet (1.8
and 3.0 meters) below the Burlington
Avenue pavement elevation. Hydrostatic
groundwater was measured at an average
elevation of 640 feet (195 meters),
or only 3 to 4 feet (0.9 to 1.2 meters)
below the existing grade. Considering
the groundwater levels in the immediate
vicinity of the site, precipitation and
surface runoff are the only sources of
groundwater recharge.
Pile and geosynthetic
support design
The proposed temporary wall, slated to
be a modular-type wall, was designed
at a bit higher than 19 feet (5.8 meters)
and would transfer an ultimate bearing
pressure of around 31.9 psi (220 kPa)
due to the required 3-track design. It
24
Geosynthetics | August September 2023
was clear the in-situ soil profile would
be incapable of providing adequate
bearing capacity, even considering only
the service dead load of 14.6 psi (100
kPa). Considering the required depth
of embedment for the wall, excavation
would take the leveling pad vertically
through the fill and essentially to the
top of the upper organic deposit; there
would be no fill or 'crust' to provide load
transfer from the soft and compressible
deposits over to more rigid elements.
In this case, the Stress Reduction Ratio
(SRR) or ratio of the load transferred
to the peat and muck versus the load
transferred to the piles would need to
be maximized. Furthermore, to provide
meaningful cost savings, the foundation
support would need to be provided via
inexpensive timber piling.
The geometry of the piling was
designed to support both the wall loading
for 18-foot (5.5-meter) long wall stems,
as well as to provide global stability for
the embankment up and behind the wall.
The final pile layout included two rows of
large-diameter timber piles immediately
beneath the wall, spaced at 6 feet (1.8
meters) on-center and capped with 3-foot
(0.91-meter) square concrete caps for an
(s-a)/a ratio, where s is defined as the
spacing a defined as the distance between
the caps, of 1. The loads in the piles were
determined based on the tributary area
and were calculated at nominal required
bearing values of 232 kips (1032 kN)
with corresponding factored loads of 151
kips (672 kN). The system then included
four additional rows of smaller-diameter
piles up into the embankment, designed
for 108 kips (480 kN), to provide global
stability support.
Previous research suggested an unreinforced
piled-embankment system
with this geometry and H/(s-a) ratio,
with s and a previously defined and H
as the height of the embankment, would
provide a SRR of around 0.4, a stress
Geoysynthetics August/September 2023

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