DFI Magazine September/October 2020 - 96
(a)
(b)
Test piles with (a) circumferential crack expansion and hole development, and
(b) cracks visible when excavated
high-strength steel anchors. The space
between the rock sockets was filled with
geofoam to facilitate easy removal for posttesting inspection of the rock-socket blocks.
Soil was placed in the pit by dry
pluviation using a self-designed and
calibrated sieve system attached at the
bottom of a concrete hopper, then leveled
upon reaching the design height. The intent
was to create a relatively loose sand deposit
so as to maximize the force effects that
reached the rock socket to exaggerate the
stiffness contrast between the rock socket
and overburden soil. In situ soil tests (soil
cone penetration and dilatometer tests)
were performed to characterize the fill
material before and after conducting the
lateral load tests. The fill sand had a friction
angle of 35 deg and cohesion of 5.0 kN/m2
(100 psf), determined through direct shear
testing (per ASTM D3080). The average in
situ density and relative density were
14.6 kN/m3 (93 pcf) and 20%, respectively.
After soil pluviation, all pile specimens
were subject to reverse cyclic loading at the
pile head, following the general guidelines
of ASCE 41-06 (2007). Loading was
applied under displacement control using a
30 in (76.2 cm) stroke, 667 kN (150 kip)
capacity hydraulic actuator. A small "pile
cap" was cast at the top of each pile to
provide a flat surface for applying load,
while a National Instruments data acquisition system monitored a total of 115
channels for each pile test.
96 * DEEP FOUNDATIONS * SEPT/OCT 2020
Selected Results
Despite the significant differences in
transverse reinforcement detailing, all
three of the piles were interpreted to have
failed in flexure without any indication of
shear failure - or for that matter, any signs
of adverse shear performance - within
the rock socket. This is a significant result
in light of the fact that pile Specimens 2
and 3 were intentionally designed to reach
shear failure prior to flexural failure on the
basis of the p-y analyses performed in
LPile. For example, recall that the nominal
shear resistance of Specimen 3 (222 kN/
50 kips) is less than half the predicted
amplified shear of 463 kN (104 kips).
This finding confirms the researchers'
hypothesis that several factors lead to
better-than-expected performance of real
rock sockets in comparison to the p-y
method. These factors include: the
confinement provided by the rock socket,
the fact that a shear failure within the rock
socket is kinematically impossible unless
accompanied by failure of the surrounding
rock mass, and that the one-dimensional
"beam shear" predicted by the p-y method
simply does not capture the actual
magnitude and spatial distribution of
stresses within the pile and the interactions between a pile and rock socket at
the concrete-rock interface.
A spray-painted grid with dimensions
of 6 x 6 in (15 x 15 cm) had been placed
around each pile specimen before testing to
provide a visual reference of soil cracking,
heaving and caving. Noticeable changes
included circumferential crack expansion
and the formation of "crater-type" holes
around the piles for lateral displacements
larger than 4 in (10 cm). Following testing,
each pile was manually excavated to
identify cracking patterns and to identify
and detect the approximate locations of
plastic hinge(s). In all three specimens, the
most significant cracking was concentrated
within 24 in (61 cm) above the rock socket,
at the expected location of maximum
moment. The cracking also extended
above this zone, with cracks at larger
spacings. Almost all cracks formed
perpendicular to the pile axis, and were
therefore indicative of a dominant flexural
response; very few diagonal cracks were
recorded. There were no signs of cracking
or damage along the socket surface or
within the rock socket itself.
The load-displacement behavior of all
three piles was analyzed along with the
analytically predicted failure load (see
"displacement with load" graph on next
page). The resulting curves represent the
"backbone" of all hysteretic loaddisplacement cycles, meaning that the
curves have been assembled by selecting
the peak load at each respective lateral
displacement level and compared to the
horizontal pile displacement. All three
piles had a similar displacement at initial
concrete cracking, yielding and load
plateau (interpreted as the ultimate lateral
capacity). When inclinometer data were
compared for Specimen 1 and 3 at selected
lateral displacement levels, it revealed that
both piles exhibited a nearly identical
bending response, which would be
expected based on the identical flexural
reinforcement. Taken together, the lateral
displacement and pile deformation results
support the interpretation that all three
piles exhibited a predominantly flexural
response and that the analytically predicted shear failure of Specimens 2 and 3
simply did not occur.
DFI Magazine September/October 2020
Table of Contents for the Digital Edition of DFI Magazine September/October 2020
CONTENTS
CONTENTS 2
DFI Magazine September/October 2020 - Intro
DFI Magazine September/October 2020 - 1
DFI Magazine September/October 2020 - 2
DFI Magazine September/October 2020 - CONTENTS
DFI Magazine September/October 2020 - 4
DFI Magazine September/October 2020 - CONTENTS 2
DFI Magazine September/October 2020 - 6
DFI Magazine September/October 2020 - 7
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