July/August 2023 - 90

day and night. The bridge is also located
immediately adjacent to several active
railroad tracks, which constrained any
working room in this direction, and
have very stringent movement criteria.
Considering this, tieback drilling
rigs, which would need to be elevated
above the frontage roads to install the
tiebacks, would be very difficult and
cause significant traffic impact.
Additionally, some of the tiebacks
would be very difficult to install because
they were located behind a portion of
the abutment that was to remain. In
addition, other bracing options, such as
rakers and concrete footblocks, were not
feasible because of the limited working
space and proximity of the frontage
roads. Cantilevered piles were also not
feasible because of the potential for high
lateral movements, which may impact
the active railroad tracks. GEODesign
was retained by the contractor to assist
in developing an alternate support of
excavation method that would present
less construction difficulty and allow for
the contractor to maximize the space
within the excavated area.
As various lateral support options
for the soldier piles were eliminated,
the use of " deadman " anchor piles to
support the excavation was evaluated.
The anchor piles would be driven a
specified distance away from the face of
the soldier pile and lagging wall and
would brace the excavation with high
strength steel threaded tierods connecting
the soldier piles to the anchor
piles. The main challenge was to find a
reasonable approach to estimate the
lateral capacity of the anchor piles.
Based on available subsurface information,
the soil conditions consist of
medium-dense to dense sands with
varying amounts of gravel and silt. The
groundwater was measured in one test
boring at a depth of approximately 20 ft
(6 m) below the top of the existing abutments.
Due to the subsurface conditions
being favorable from a geotechnical
design perspective, the theory of using
laterally loaded anchor piles was further
90 * DEEP FOUNDATIONS * JULY/AUG 2023
strengthened as the estimated lateral
pile capacity would be relatively high.
A design method using typical
active/passive wedge analysis and
lateral pile analyses was developed.
The following steps were taken to
determine the required properties of
the various elements of the support of
excavation system:
1. Staged excavation analyses were
performed to determine the soldier
pile size, spacing and embedment,
and to estimate the lateral load to be
resisted by the tierod on a per-foot
basis.
2. Lateral pile analyses were then
performed to estimate the anchor
pile deflection, bending capacity,
point of fixity and minimum depth.
3. A section, drawn to scale, of the
proposed soldier pile, tierod and
anchor pile elements was created.
Using Rankine's Earth Pressure
theory, the area in which the active
soil wedge intersects the passive
soil wedge was estimated.
4. Using the theory of the " arching
effect " in piles subject to a lateral
load, as described below, and a
reasonable factor of safety, the
required spacing of the anchor piles
was calculated.
Once the anchor pile spacing was
determined, a final lateral pile analysis
was performed to confirm the
estimated pile point of fixity and to
verify that the pile head deflection and
bending moment were within acceptable
limits.
Due to site constraints and a limited
amount of space at each abutment, the
distance between the anchor piles and
the face of the excavation was limited,
which resulted in the active and passive
soil wedges intersecting. This
would therefore result in reduced
Due to the subsurface
conditions being favorable
from a geotechnical
design perspective, the
theory of using laterally
loaded anchor piles was
further strengthened as
the estimated lateral pile
capacity would be
relatively high.
The soldier pile-anchor pile system
July/August 2023

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