January/February 2021 - 88
major cost overrun and project delays, a
lawsuit ensued.
Phase 3
A new design-build team was selected
for Phase 3, and ECS Southeast provided
geotechnical services as a
subcontractor for that team. A major
concern of the new team was the karst
geology and support of one Phase 3
mixing tower added immediately
adjacent to the troubled mixing tower
that had previously been constructed
fo r o u r c l i e n t . T h e n e w t e a m ' s
geotechnical subcontractor had
experience with sinkholes and karst,
and was selected with the goal of
devising a plan that avoided micropiles
under this Phase 3 mixing tower to
reduce overall foundation costs.
Extensive subsurface exploration
was conducted using standard
penetration test (SPT) borings and airtrack soundings to define the upper rock
surface for the new mixing tower.
Exploration in the mixing tower
footprint was performed at most
planned column locations. Widely
spaced SPT borings also were
completed in low-rise warehouse areas.
Several intermediate foundation
options were discussed with a specialty
foundation contractor. Micropiles, or at
least some version of cased pile into
rock, were recommended by that
contractor. In the experience of ECS,
though, auger cast piles (ACP) drilled to
refusal would best adjust to highly
variable conditions and pinnacled rock.
Excess grout from ACP would provide an
indirect benefit of grouting karst under
the building footprint. ACP would also
allow for quality assurance (QA) testing
and provide feedback on subsurface
variability during installation.
While ACP were more cost-effective,
with a potential savings of about $2 million dollars over micropiles, this option
was not immediately accepted because
the Japanese client had had a previous
poor experience with ACP in the Pacific
Rim. Additionally, our client had Japanese engineers reviewing the planned
design. As a matter of practice, engineers
in Japan are trained that ACP are friction
piles with no end bearing resistance.
To build our client's confidence, we
suggested installing five test piles in
one of the relatively small areas where
a mixing tower was being built and
Soil cross section showing areas of raveled soils related to karst
Typical karst conditions were found
that consisted of soft internally eroded
soils overlaying the upper rock surface,
and a highly variable depth to rock.
88 * DEEP FOUNDATIONS * JAN/FEB 2021
loading the piles to three times the
design load. Ultimately, 18 in (45.7 cm)
diameter ACP with a design axial load
capacity of 150 tons (136.1 m tons) were
selected as the preferred option.
Pile load testing - A load test was
performed on five ACP using the ASTM
D1143, Quick Load Test Method (see
table on the next page).
During static load testing of test pile
6049, signs of instability were observed
and the testing was terminated early. To
confirm that test pile 6049 did not start
to fail into the underlying rock, one SPT
boring with 10 ft (3.05 m) of rock coring
was drilled. The boring determined that
the pile was sitting on a 6 ft (1.83 m)
thick rock seam overlaying a 2 ft
(0.61 m) thick void, a common condition
in karst geology. In addition, to check
the interface between the pile tip and
pinnacled/inclined rock, our client
requested full-depth coring of 6049.
Lessons learned - During Phase 3
installation of the probe, reaction and
test piles, the following lessons were
learned:
* Subsurface exploration into karst can
reveal difficult drilling conditions
due to highly inclined/pinnacled
rock. Be prepared to lose some
exploratory drilling equipment such
as augers and rods.
*
By using innovation, you may be
able to turn a foundation system's
inherent weakness into a benefit,
such as the need for additional
grouting while drilling ACP in karst.
*
Expect high grout takes initially to
" treat " the underlying karst conditions, and to redrill and regrout
individual piles several times until
grout stops flowing from the hole
into the karst. Some piles were
redrilled three to six times before
grout losses stopped.
*
Supporting an ACP on thin rock
seams of 2 to 3 ft (0.61 to 0.91 m)
thick is practical.
*
Any piles that require redrilling will
take significant grout. For this
project, actual grout takes of redrilled
piles were typically 300% to 1,200%
of the theoretical volume. As production progressed, grout takes
lessened because early piles tended
January/February 2021
Table of Contents for the Digital Edition of January/February 2021
Contents
January/February 2021 - Intro
January/February 2021 - 1
January/February 2021 - 2
January/February 2021 - Contents
January/February 2021 - 4
January/February 2021 - 5
January/February 2021 - 6
January/February 2021 - 7
January/February 2021 - 8
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