DFI Magazine March/April 2020 - 96

* A 3,000 kg (6,614 lb) coordinate
measuring machine set on five points,
which needed complete isolation from
vibrations of all other equipment
The general surface soils in the Montrose,
Colorado, area of the TEI factory consist of
adobe clay: an expansive soil overlying a
good bonding zone comprising alluvial soils
mixed with cobbles and gravel. The cobbles
vary in size, from fist-sized to boulders as
large as a car. A shale layer occurs at an
average depth of 6 m (19.7 ft). To support
the machining centers, it was necessary to
access this deeper layer as the load-bearing
stratum for some of the micropiles.

Multus B400 in operation

A hollow 40 mm/16 mm (1.6 in/0.6 in)
bar was used for the design piles of 100
kips (445kN). This bar can support the
equipment used in the manufacturing area
and has good load characteristics for the
pile design. For more lightly loaded piles,
the same equipment can be used, but with a
lighter design, hollow bar.
A carbide cross-cut style bit efficiently
cut through the cobble and gravel layer
when needed. This style of bit was also
capable of driving a rock socket into the
shale layer to create a load-bearing base for
the deeper micropiles.

Horizontal Mill Micropiles

Okuma horizontal mill set on three
micropiles

Micropile Installation Decisions
Different micropile designs were used to
build the foundations for each machine. The
hollow bar injection method was used for
the micropiles, and involves injecting grout
through a hollow core steel bar with a sacrificial bit that doubles as the drilling string.
This method afforded quick set ups,
allowing the factory to remain operational
during each installation process. The
hollow bar system also allowed installations of micropiles to be carried out using
smaller equipment that enabled staff to
keep a clean work area and to create
minimal disruption over roughly a week.

96 * DEEP FOUNDATIONS * MAR/APR 2020

The Okuma MA-500H horizontal machining center was among the heavier machinery to receive new micropile supports. The
soil beneath this factory area is mostly a
gravel fill to a depth of 2 m (6.6 ft) left from
the initial building construction. The gravel
overlays the expanding adobe clays and
mixed cobbles common to this Colorado
region, which occur between a depth of 9 m
and 11 m (29.5 ft and 36 ft). In 2010, three
micropiles were drilled using a 125 mm
(4.9 in) bit into the competent shale to produce a design working capacity of 100 kips
(445 kN) each. All three piles consisted of
the 40 mm/16 mm (1.6 in/0.6 in) hollow
bar and reached a depth of 12 m (39.4 ft).
The micropiles were grouted through the
hollow bars using a neat cement grout.
Specific gravity of the finished grout
was measured using a mud balance, with
testing done at the grout plant and at the top
of the pile itself. Grout was allowed to cure

for 7 days before pile load testing was conducted on each pile to 120% of the design
load. One pile was also pull tested (for
tension) to 110% of its maximum design
load, with all results meeting the performance criteria. After testing, the micropiles
were each capped with a 250 mm wide by
16 mm thick (9.8 in by 0.63 in) round
plate, which formed the bases for the feet of
the milling machine.

Five-Axis Machining Micropiles
The Multus B400 5-Axis machining center
from Okuma has 17 mounting pads to
stabilize the shifting weight and the torque
of machine spindles during operation. The
area of the shop where this machining
center was added in 2012 has a thin
existing pad, underlain by very loose fill. As
a result, the micropiles and foundation
could not be connected to the existing
concrete pad. The challenge was creating a
stable base so that the shifting weight and
torque of the machine spindles would not
twist the machine.
The new foundation had a simple design
with 10 micropiles over a 4 m by 1.2 m
(13.1 ft by 3.9 ft) span. In discussion with
the engineers who manufactured the
machine, the micropiles were designed to
each carry 50 kip (222 kN) loads. The
design used 38 mm (1.5 in) hollow bars,
installed with a 76 mm (3 in) bit drilled to
the prescribed termination depths of
between 6 m and 12 m (19.7 ft and 39.4 ft).
Varying amounts of grout were injected for
each micropile beneath the concrete pad to
consolidate the loose underlying fill.

DFI Magazine March/April 2020

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