July/August 2024 - 17

length. For the final wall condition, a minimum factor of safety
of 1.5 was provided for the permanent wall. The permanent
walls were also analyzed with seismic loading with the
required factor of safety of 1.10. The overall design of the wall
was done in accordance with methods presented in the Federal
Highway Administration's Circular No. 7 Soil Nail Walls.
The soldier pile excavation support system installed at the
stone foundation consisted of drilled-in soldier piles with two
levels of tieback anchors for lateral bracing. The design height
of support was 23 ft (7 m). The system was designed to resist
lateral pressures due to soil and building surcharge loadings.
The lateral soil loading for the cantilever condition was a
triangular loading based on at rest earth pressures. For the
braced condition, a rectangular soil loading of 0.65*K *γ was
o
utilized. The resulting lateral loading on the system was
determined with software that uses Boussinesq equations. The
total lateral loadings on the excavation support system due to
soil and surcharge loadings was modeled using Shoring Suite
software to determine the design loads for the soldier piles and
tiebacks. From the calculated forces the soldier piles and lateral
bracing was designed based on allowable stress design.
Intended Sequence of Construction
The project consisted of a phased renovation with additions to
the existing facility, allowing the school to remain
operational. The initial phase kicked off with the demolition
of the C Wing. Mass excavation for the new addition and the
lower-level gym was intended to immediately follow the
completion of the demolition. Contract drawings indicated
that the foundation of the high school was a cast-in-place
concrete footing, which was conducive to the proposed
composite underpinning system.
As scheduled, the demolition of the C Wing began in
summer 2018. The site excavation was anticipated to start in
the late summer or early fall. The entire support of excavation
for this phase consisted of the upper level and the lower sublevel
system. The upper excavation from elevation 100 ft
(30.5 m) down to about elevation 78 ft (24 m) was intended to
be constructed in a continuous operation. In the shape of a
horseshoe, it was to start along the remaining portion of the
D Wing, span across the former location of the C Wing,
connect up with the B Wing, continue along the remaining
length of its foundation and taper out on the downhill slope.
The lower sub-level excavation from about elevation 78 ft
(24 m) down to about elevation 55 ft (17 m) was located on the
downhill side of the upper-level excavation and intended to
follow its completion.
This intended sequence of construction was completely
discarded during demolition after lengthy delays were
experienced upon the discovery of contaminated soil within
the footprint of the former C Wing and the dry-stone rubble
foundation along the 1895 portion of the B Wing, which will
be discussed in the next installment of this series.
Composite Underpinning Along D-Wing
The very dense soil, combined with the building's concrete
footing, made conditions ideal for the composite
underpinning system. The micropiles were drilled through
the existing cast-in-place concrete footing to carry the axial
load of the building. The soil nail wall provided the earth
retention component of the system. Lifts were excavated in 5
ft (1.5 m) deep increments and repeated until subgrade was
achieved. The exposed face of soil was covered with
pneumatically applied, ready-mix shotcrete. The 5,000 Psi (34
MPa ), 0.375 in (9.5 mm) shotcrete mix was reinforced using
4x4 W2.5 welded wire fabric, which was held in place with tie
wire and No. 4 rebar dowels driven into the soil. This
reinforced shotcrete face served as lagging spanning between
the soil nails. Both the micropiles and soil nails were
constructed using hollow-bar tendons. The hollow-bar
tendons were drilled into the ground using a Krupp DHR-80
rotary-percussion hydraulic drill rig retrofitted with a TEI
TE560 hydraulic drifter. A grout swivel adaptor, attached to
the drill head, allowed drilling grout to be pumped through
the tendon's core hole, exiting through angled ports within a
disposable drill bit at the tip of the tendon. Given the dense
nature of the soil, a cross-cut carbide drill bit was selected. A
key component to the success of hollow-bar installation was
the colloidal grout plant, capable of a pumping pressure of
1,740 Psi (12 MPa) and flow rate of 26.5 gal/min (100 L/min).
This high pumping pressure allowed
the grout to cut a column diameter
about 3 in (76 mm) larger than the
drill bit diameter. A weak
drilling grout was used for
drilling the tendon to its
design depth. Shortly
before reaching the soil
nail design depth, the
drilling grout was
s w i t c h e d t o a
structural grout.
Shotcreting under D Wing
DEEP FOUNDATIONS * JULY/AUG 2024 * 17
July/August 2024

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