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Geosynthetics 2015 February 15-18, Portland, Oregon
Dynamic Reinforcement Strains from a Shake Table Test of a
Full-Scale Geogrid-Reinforced MSE Wall
Andrew C. Sander, University of California San Diego, USA,
[email protected]
Willie Liew, PE, Tensar International Corporation, USA,
[email protected]
Patrick J. Fox, PhD, PE, University of California San Diego,
USA, [email protected]
ABSTRACT Mechanically Stabilized Earth (MSE) retaining walls
have been used extensively in practice for decades. Static response
of such structures has been assessed via field instrumented sites,
but controlled dynamic tests have been largely relegated to small-
to moderate-scale 1g shake tables or centrifuge specimens. In an
effort to improve seismic design guidelines, harmonic
and recorded earthquake ground motions were applied to a
full-scale, well instrumented, 6.1 m tall modular block MSE wall
reinforced with uniaxial high density polyethylene (HDPE) geogrid.
The tests were performed using a new large soil confinement box
(LSCB) on the NEES@UCSD Large High Performance Outdoor Shake Table
(LHPOST). Local strain response of the geogrid reinforcement was
measured with bonded strain gages at the midpoint of each rib at
seven elevations along the centerline of the wall specimen, as well
as numerous redundant locations away from the centerline. This
paper summarizes some of the findings from ongoing analysis of this
testing program. 1. INTRODUCTION Characterization of the dynamic
properties and response of Mechanically Stabilized Earth (MSE)
walls, particularly those reinforced with polymeric geogrid, has
been a key focus of research over the last three decades (e.g.,
Bolton and Pang 1982; Bathurst and Hatami 1998; Ling et al. 2005).
Data from field reconnaissance, small centrifuge models, and
small-to-moderate scale 1g shake table models, as well as numerical
analyses, have demonstrated that such structures perform well under
seismic loading, typically better than traditional rigid and
semi-rigid retaining wall systems (Ling et al. 2001; Bathurst et
al. 2002; Yen et al. 2011). However, full scale controlled dynamic
tests of geogrid-reinforced MSE walls are still needed to develop
benchmark datasets against which to compare results from previous
research that may be subject to certain limitations (Iai 1989).
Such tests are needed to extrapolate results from model to
prototype scale and assess and calibrate numerical models for
design and analysis of full-scale MSE walls. In order to overcome
some of the limitations associated with small-scale tests, a full
scale dynamic test was conducted on a modular block,
geogrid-reinforced MSE wall using the Large High Performance
Outdoor Shake Table (LHPOST) at the University of California-San
Diego. With addition of a newly fabricated boundary apparatus, the
Large Soil Confinement Box (LSCB), the LHPOST is well-suited for
this type of experimental work because of its high vertical payload
and lateral thrust capacity, large table area, and outdoor setting
that has no overhead constraints (Figure 1). For the first test
using the LSCB, a 6.1 m tall MSE wall was subjected to a series of
harmonic and recorded earthquake ground motions in order to
evaluate its fundamental dynamic behavior and seismic response.
This paper provides an update to the project and previous
information published by Sander et al. (2013, 2014) and Fox et al.
(2014).
Figure 1. The LSCB at UCSD.
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2. EXPERIMENTAL DESIGN 2.1 Specimen A MESA
Retaining Wall System, manufactured by Tensar International
Corporation of Atlanta, GA, was selected for the test.
This retaining wall system is a fully integrated modular block
wall that utilizes mechanical connectors to attach HDPE geogrid
reinforcement to high strength dry cast masonry facing blocks. The
wall was designed to optimize the space available within the narrow
configuration (width = 4.62 m) of the LSCB and achieve maximum
height with minimum payload while still allowing full development
of the anticipated failure surface. The reinforcement was designed
to minimum code requirements with consideration of expected peak
ground acceleration (PGA) up to 0.7g.
The entire test specimen measures 6.55 m tall, 4.62 m wide, and
9.10 m long in the direction of motion, as shown in Figure 2. It is
composed of a clean angular sand backfill (< 3% P200) and Tensar
UX1400 geogrid, spaced uniformly at every third course of block
(0.61 m) in the vertical direction, along with the facing and
connector components. The wall facing extends to a height of 6.1 m
above an unreinforced concrete leveling pad with 2000 psi minimum
compressive strength. A layer of foundation soil (31 cm thick)
separates the table from the leveling pad (15 cm thick) in order to
allow sliding and rotation of the toe. The foundation soil was
compacted using the same sand and procedures as for the backfill.
At 95% relative density, the backfill soil yields peak and residual
friction angles of 42
and 38
, respectively, and a cohesion intercept of essentially
zero.
Transverse steel angle sections were bolted to the top of the
shake table to enforce a no-slip condition at the bottom boundary
of the specimen during shaking.
Figure 2. MSE wall specimen inside LSCB. The test specimen was
contained by the rigid walls of the LSCB. For simplicity of
numerical modeling and to avoid uncertainties associated with
response of a seismic buffer material (e.g., geofoam) under cyclic
loading, the back boundary was also rigid. On all sides, three
sheets of plastic film (dry) were used to separate the soil from
the concrete walls of the LSCB and minimize interface friction.
Figure 3 presents failure envelopes from a series of large-scale
direct shear tests that were conducted on the layers of plastic
film to measure interface friction angle for this boundary
condition. The interface friction angle was approximately 18
o for displacement rates typical of the applied motions. In
these tests, sliding occurred
between the sheets of plastic film layered between a sample of
the soil from the MSE wall experiment and the steel plate of the
direct shear machine, which was approximately as rough as the
smooth concrete walls of the LSCB.
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Figure 3. Interface friction failure envelopes (Fox et al.
2014). 2.2 Instrumentation The specimen was instrumented to monitor
facing and backfill accelerations, facing displacements,
reinforcement strain, vertical and horizontal total earth pressure,
and backfill settlement. A schematic diagram of the centerline
section (A-A) of the instrumented wall specimen is shown in Figure
4. This section was the most densely instrumented of three (B-B and
C-C not shown) with 30 uniaxial and 30 biaxial accelerometers
tracking backfill motion, as well as 5 uniaxial accelerometers
mounted directly to the wall face. Additionally, 20 string
potentiometers measured wall displacements relative to the west
wall of the LSCB, 14 pairs of vertical/horizontal total pressure
cells measured dynamic lateral earth pressure coefficients, and 130
bonded strain gages measured local strain at the midpoint of each
geogrid aperture for subsequent correlation to reinforcement load
at 7 elevations.
Figure 4. MSE wall instrumentation section A-A.
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Select redundant measurements were made on vertical planes
located 1.40 and 2.01 m (sections B-B and C-C, respectively) off
centerline to provide a check to Section A-A and to assess the
effectiveness of the boundary at modeling plane strain conditions
(Fox et al. 2014). The LSCB itself was also fitted with
accelerometers to ensure that its rigidity was preserved throughout
shaking such that it could serve as a reliable reference for
relative displacement and acceleration measurements of the wall
specimen. Backfill settlement measurements were the only values not
recorded dynamically during shaking and the location of these
markers was set to coincide with roof framing members of the LSCB
as a measurement reference. 3. CONSTRUCTION PROCEDURES Construction
of the MSE wall specimen was completed with techniques and
equipment consistent with those used for field installations.
Backfill soil was initially placed using a concrete hopper and
crane in order to spread a soil pad large enough to place a small
skid steer loader inside the LSCB without damaging the loader
tracks on the bottom angle sections. Additional backfill soil was
then placed using a conveyor belt with the skid steer loader inside
the LSCB to spread and compact the soil. The first 31 cm was placed
and compacted as a foundation layer upon which the leveling pad was
poured. Each soil lift was placed to a loose height of 19 cm and
then compacted down to 15 cm using a 7.1 kN smooth drum vibratory
roller attached to the skid steer loader. The final relative
density for each lift ranged approximately from 90 to 100%. Sand
cone tests were performed every two lifts and nuclear density gauge
tests every ten lifts. The majority of the backfill was compacted
with 6 passes of the roller. Near the wall face, a walk-behind
vibratory plate compactor was used in order to minimize local
outward deformations of the facing units. The plate compactor was
also used around the edge of the fill, where the roller could not
reach. After installation of each geogrid layer, three courses of
facing blocks were placed and leveled with the connectors oriented
to maintain a vertical wall profile. In lieu of the typical gravel
layer adjacent to the wall face, a nonwoven geotextile was placed
behind the blocks and around the side gaps (i.e. near the side
walls of the LSCB) as a filter to prevent loss of backfill soil.
Construction procedures remained consistent until the final three
lifts, at which point the elevation of the backfill relative to the
roof framing prevented further use of the skid steer. Backfill
placement resumed by crane for elevations between 6.10 and 6.55 m
and a walk-behind jumping jack plate compactor was used for
compaction over the area formerly compacted by use of the vibratory
roller. An overhead photograph of the finished specimen prior to
shaking is shown in Figure 5.
Figure 5. MSE wall specimen prior to shaking.
4. RESULTS Over a three day period, the specimen was subjected
to a series of harmonic motions of varying frequency and amplitude,
scaled ground motion records from the 1995 Kobe, 2010 Maule, and
1994 Northridge earthquakes, and a subsequent set of harmonic
motions for direct comparison to the initial series. Some initial
results for displacements and accelerations have been presented by
Sander et al. (2014) and Fox et al. (2014) and demonstrate good
agreement across all three instrumented sections, indicating that
the desired plane strain conditions were adequately approximated
for the center section.
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Maximum strains and connection strains measured for the
instrumented geogrid layers after each 1 Hz harmonic motion are
displayed in Figures 6a and 6b for the initial and final series of
harmonic motions, respectively. Connection strains were measured at
the wall facing and maximum strains are the largest value of local
strain measured within each elevation. During the initial harmonic
motions, deformations within the reinforced zone of the backfill
accumulated and the majority of the resulting strain demand was
localized in the lower one-third of the specimen. Connection
strains and maximum strains at the toe and crest of the wall
experienced small changes while a clear peak in demand among the
measured reinforcement layers developed at an elevation of
approximately 1.88 m above the bottom boundary (1.42 m above the
leveling pad). In contrast, the final set of harmonic motions
display only minor additional changes in reinforcement strain
(i.e., demand). Note that the initial motions show sequentially
increasing amplitude while the final motions do not. Throughout the
entire loading sequence, strains in the lowest (first) and highest
(tenth) layers show only small variations, while the most active
regions of the reinforced zone (third, fifth, and seventh layers)
indicate increases in residual local strain of 1% or more. For the
same two series of harmonic motions, the peak dynamic increments of
maximum and connection tensile strain (i.e., in excess of the
measurement at the beginning of each motion) are presented in
Figures 6c (initial) and 6d (final). In both cases, the small
amplitude motion (PGA = 0.1g) generated negligible dynamic strains.
The moderate amplitude motion (PGA = 0.25g) led to similar
distributions of dynamic strain increment with only the fifth and
tenth layers experiencing significantly different demand before and
after application of historical records. However the high amplitude
motion (PGA = 0.4g) produced response characteristics which are
distinct between the initial and final series. For the initial
series, dynamic strains increased linearly with depth except near
the toe. This suggests a primarily sliding deformation mechanism,
whereas for the final series (i.e., after the recorded earthquake
records), the largest strains are localized above mid-height
suggesting more of an overturning mechanism or secondary sliding
higher above the toe.
a) Residual, Initial b) Residual, Final
c) Dynamic Increment, Initial d) Dynamic Increment, Final
Figure 6. Measured strains from 1 Hz harmonic motions.
0 0.5 1 1.5 20
1
2
3
4
5
6
7
Axial Strain Amplitude (%)
Ele
va
tio
n (
m)
max
, 0.10g
con
, 0.10g
max
, 0.25g
con
, 0.25g
max
, 0.40g
con
, 0.40g
0 0.5 1 1.5 20
1
2
3
4
5
6
7
Axial Strain Amplitude (%)
Ele
va
tio
n (
m)
max
, 0.10g
con
, 0.10g
max
, 0.25g
con
, 0.25g
max
, 0.40g
con
, 0.40g
0 0.2 0.4 0.6 0.8 1 1.20
1
2
3
4
5
6
7
Axial Strain Amplitude (%)
Ele
va
tio
n (
m)
max,dyn
, 0.10g
con,dyn
, 0.10g
max,dyn
, 0.25g
con,dyn
, 0.25g
max,dyn
, 0.40g
con,dyn
, 0.40g
0 0.2 0.4 0.6 0.8 1 1.20
1
2
3
4
5
6
7
Axial Strain Amplitude (%)
Ele
va
tio
n (
m)
max,dyn
, 0.10g
con,dyn
, 0.10g
max,dyn
, 0.25g
con,dyn
, 0.25g
max,dyn
, 0.40g
con,dyn
, 0.40g
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A comparison of the strain profiles before and after all motions
shows a pronounced bulge in the lower one-third of the wall with
maximum strain accumulation of 1.1% in the third geogrid layer
(Figure 7). Maximum strains increased in all layers through the
loading sequence with minimal change (~0.1%) in the layers near the
toe and crest. Connection strains are approximately linear with
depth and in relatively close agreement for initial and final
conditions. The measurements of maximum and connection strain near
the crest display less consistent response trends than in the rest
of the specimen. This inconsistency is likely the result of higher
relative incidence of strain gage failures (delamination or
otherwise) in these layers during the late stages of the testing
program. Figure 8a shows the gap behind the top course of blocks
(roughly 0.3 m wide and deep) that resulted from shaking. This loss
of confinement, as well as that from surface cracking (more than 3
cm wide at the edge of the reinforced zone), may also contribute to
the anomalous strain measurements near the crest. Figure 8b shows
the final deformed profile of the wall face, including a bulge at
approximately one-third height.
Figure 7. Reinforcement strains before and after all shaking
tests.
a) Gap behind top course of blocks b) Overhead view of displaced
profile
Figure 8. MSE wall specimen after final motion.
0 0.5 1 1.5 20
1
2
3
4
5
6
7
Axial Strain Amplitude (%)
Ele
vati
on
(m
)
max
, Initial
con
, Initial
max
, Final
con
, Final
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5. CONCLUSIONS Full-scale dynamic tests (both sinusoidal and
seismic base excitation) of a 6.1 m tall MSE retaining wall were
conducted within a new confinement structure (the LSCB) on the
NEES@UCSD LHPOST. A dense array of instrumentation was used to
record the dynamic behavior of the specimen, including
accelerations, wall displacements, tensile strains in
reinforcement, and dynamic earth pressures. Strain measurements
presented above suggest the following:
a maximum increase in residual strain of 1.1% developed in the
third geogrid layer (out of ten) from the bottom over the course of
the entire loading sequence
small changes in residual strain were found to develop at the
toe and crest as compared to the mid-section of the wall
comparison of strains produced by similar severe harmonic
motions early and late in the sequence indicates different
deformation mechanisms for undamaged and damaged states of the
specimen
Findings from this research program are expected to provide
guidance in the continued development of design standards and
assessment of numerical models used to analyze the dynamic response
of MSE walls in seismic regions. This test was a success insofar as
the wall performed well and the capability of the LSCB for dynamic
testing of large geotechnical structures was demonstrated. Several
tests on rocking bridge foundations followed this MSE wall test and
have also demonstrated the capability of the LSCB for other
interesting applications in geotechnical earthquake engineering
research. ACKNOWLEDGEMENTS
Financial support for this investigation was provided by Grant
No. CMMI-1041656 from the George E. Brown, Jr. Network for
Earthquake Engineering Simulation (NEES) Research Program of the
U.S. National Science Foundation. Supplemental funds were provided
by Tensar International Corporation, the California Department of
Transportation, the Washington Department of Transportation, and
the Jacobs School of Engineering and Department of Structural
Engineering at UCSD. This support is gratefully acknowledged. The
authors sincerely thank the staff of the Englekirk Structural
Engineering Center, in particular Paul Greco and Dan Radulescu, and
UCSD students Stuart Thielmann, Mike Sanders, Mary Klepin, and
Sydney Sroka for their assistance with the project. In addition, we
would like to thank Tony Allen, State Geotechnical Engineer for the
Washington Department of Transportation, and Dr. Richard Bathurst,
Professor at the Royal Military College of Canada, for several
helpful conversations with regard to details of the experimental
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