March/April 2021 - 81

213 kips (948 kN) reached the top of the
bearing layer when 388 kips (1,726 kN)
was applied at the column head,
resulting in 0.3 in (0.8 cm) of total
deflection, consisting of 0.2 in (0.5 cm)
of elastic deformation and 0.1 in
(0.25 cm) of tip deflection. Ultimately,
Menard installed 136 CMCs of 12 and
18 in (30 or 46 cm) diameter to be used
for the garage, with an average rigid
inclusion depth of 35 ft (11 m).

Typically, net settlement at ultimate
load for helical piles is taken as 10% of
the average helix diameter per ICC
AC358 (2007); thus, piles would have
been allowed a net settlement of 1.48 in
(3.8 cm) in addition to elastic shortening of the pile shaft at ultimate load.
The helical piles were planned to
support strip footings for stairwells,
elevator shafts and an interior garage
ramp, as well as providing temporary

installation criteria was revised to a
length of 24 ft (7.3 m), or an installation
torque of 12,500 ft-lbs (16,948 Nm).

Helical Pile Load Test
Helical pile responses were similar
even though the bearing layer had
different torque values and the piles
had different lengths. The 0.29 in
(0.74 cm) of net deflection is the worstcase scenario and resulted in a 1.8 in
(4.6 cm) horizontal offset at the top of the
tower, relative to the base. The helical
pile design vertical net deflection is
0.2 in (0.5 cm), which was projected by
the structural engineer to result in a
horizontal displacement at the top of
the 80 ft (24.4 m) elevator shaft equal to
1.5 in (3.8 cm). Thus, the load per pile
had to be reduced to 24 kips (107 kN),
and additional piles were added.

Micropile Design Challenges

Boring coverage (red pins) at and near the site, which includes 4-story building outline (red dashed line)
(credit: https://earth.google.com/)

The micropile layout and design
replaced the original 20 helical pile
locations for stairway D. Nicholson
designed the micropiles with 5.5 in
(14 cm) diameter 80 ksi (552 MPa) mill
secondary casing with a 5 ft (1.5 m)
bond zone in rock. The micropiles were

4-Story Design Challenges
Working in the basement of an
abandoned 4-story building, the biggest
challenge Magnum had was the low
overhead clearance and tight working
space between existing steel columns
and temporary shoring. In addition, the
interior piles had strict deflection
criteria as they were used in conjunction
with existing H-piles to support new
4-story stairwell and elevator shafts.
The H-piles were considered rigid, and
there was concern that the towers
would cause significant horizontal
offsets at the top relative to the base,
should the piles have a net vertical
settlement of more than 0.2 in (0.5 cm).
A total settlement criteria of not
more than 1 in (2.5 cm) at two times the
design load also had to be met. Total
settlement is the result of both elastic
pile shortening and soil deflection.

Helical pile load test results

construction shoring. Installing piles
within the existing building limited the
equipment to a small skid steer, which
also limited the pile size. The limited
pile size increased the elastic shortening, which affected the total pile
settlement and dictated that the pile
length be kept to a minimum. Magnum
Piering designed the helical piles to bear
in the medium-dense sand with an
anticipated installation length of 30 to
40 ft (9.1 to 12.1 m) below ground
surface. Based on load testing, the

designed to be embedded in slightly
weathered siltstone with compressive
strengths ranging from 3,000 to
5,130 psi (20,684 to 35,370 kPa) to limit
settlement. The design grout strength
was 4,000 psi (27,579 kPa) and a #9
grade 75 ksi (517 MPa) all-thread bar
was placed in the bond zone with a
minimum 5 ft (1.5 m) overlap into the
casing. Due to changes in bedrock
elevation across the Strip District, the
total length of the piles was determined
in the field by the driller when they hit
DEEP FOUNDATIONS * MAR/APR 2021 * 81
https://earth.google.com/
March/April 2021

Table of Contents for the Digital Edition of March/April 2021
