September/October 2022 - 88

surrounding soils. This
provides necessary skin
friction for the pile to resist
both compress ive and
tensile forces.
Design
The original helical pile
design consisted of 80
locations spanning the
length of the platform,
requiring a working load
capacity of 45 kip (200 kN).
The general contractor
encountered a concrete
obstruction, at approximately
5 ft (1.5 m) of depth
adjacent to the rail tracks,
underneath the ballast rock.
The concrete slab was not
shown on any of the original
construction plans for the
rail depot nor was it shown
in the soils report. The
initial solution was to core a
hole through the concrete
Installation work underway near
active rail operations
slab at each of the 80 helical
pile locations. However, this
solution was going to be a
significant added cost to the
project. The foundation
contractor began looking for
alternative options that
would be cost effective
while still offering the performance
and load carrying
characteristics required by
the specification.
A piece of the concrete
obstruction was exposed
and broken up to check for
the type of reinforcement.
The slab contained a wire
mesh reinforcing system.
This aided the scenario for a
successful hol low bar
micropile design, as the
drill bits used are capable of
drilling through wire mesh.
The next step was to
de t e rmi ne s t r uc t u r a l
capacity and geotechnical
capacity of a hollow bar system to
ensure proper hollow bar selection and
micropile length.
Utilizing 45kip (200 kN) as the design
load, the calculations for selecting a steel
reinforcement yielded a 40 mm (1.6 in)
outer diameter, 20 mm (0.79 in) inner
diameter hollow bar, which exceeded
both the working load (1.0 times the
design load) and testing load (2.0 times
the design load) requirements.
The geotechnical report indicated
loose to very loose mixed soils with single
digit Standard Penetration Test blows/
foot. At approximately 25 ft (7.62 m)
depth, a harder layer of sandy clay was
found. For the geotechnical capacity of
the micropile, the first 25 ft (7.62 m) were
treated as an unbonded zone that had no
contribution to the pile capacity.
Because the boring log was terminated
at 31.5 ft (10.5 m), the assumption
was that the soil strata below would
be representative of the sandy clay material
for preliminary design purposes. A
pile length of 79 ft (24 m) was estimated.
88 * DEEP FOUNDATIONS * SEPT/OCT 2022

September/October 2022

Table of Contents for the Digital Edition of September/October 2022

TOC
September/October 2022 - Intro
September/October 2022 - 1
September/October 2022 - 2
September/October 2022 - TOC
September/October 2022 - 4
September/October 2022 - 5
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