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1
SHAKING TABLE TESTS OF A ROCKING BRIDGE PIER ON
IMPROVED SOIL
Angelos TSATSIS1, Ioannis ANASTASOPOULOS
2 and George GAZETAS
3
ABSTRACT
Recent studies have highlighted the beneficial role of the potential inelastic foundation response
during strong seismic shaking. According to an emerging seismic design concept, termed “rocking
isolation”, instead of over-designing the foundation to ensure that the loading stemming from the
structural inertia can be “safely” transmitted onto the soil (as with conventional capacity design), it is
intentionally under-designed to promote nonlinear response. This nonlinear behaviour manifests
through uplifting in case of relatively large FSV values, while in case of low FSV values it manifests
through mobilization of bearing capacity mechanism, being accompanied with significant settlement.
To ensure that rocking is materialized through uplifting rather than sinking, and to mitigate possible
settlement accumulation, the application of a shallow layer of improved soil is considered. To this end,
a slender idealized bridge pier is selected as an illustrative example. A series of reduced scale 1g
shaking table tests are conducted in the Laboratory of Soil Mechanics of the National Technical
University of Athens in order to explore the dynamic response of the system focusing on the behaviour
of the shallow layer of improved soil. Various soil profiles were accounted for in the experiments,
simulating the initial “poor” soil conditions, the proposed soil improved by means of a shallow soil
crust of different depths and finally the “ideal” soil profile consisting solely of the improved soil
material. The results of this experimental study showed that the concept of shallow soil improvement
is quite effective in enhancing the dynamic performance of the system and in mitigating the
accumulation of settlement and rotation. The effectiveness of the improved layer is ameliorated with
its depth, with a layer of depth equal to the footing width yielding practically identical behaviour to
that of the “ideal” soil conditions. Moreover, the performance of shallow soil improvement is found to
depend on the rotation amplitude: it was shown that with the increase of the rotation amplitude the
effectiveness of the soil increases.
INTRODUCTION
According to current seismic codes, the foundation soil is not allowed to fully mobilize its strength,
and plastic deformation is restricted to above-ground structural members. “Capacity” design is applied
to the foundation guiding failure to the superstructure, thus prohibiting mobilization of soil bearing-
capacity, uplifting and/or sliding, or any relevant combination. However, a significant body of
pragmatic evidence provides robust justification that allowing strongly nonlinear foundation response
is not only unavoidable, but may even be advantageous (e.g., Paolucci, 1997; Pecker, 1998; 2003;
Gazetas et al., 2003; Gajan et al., 2005; Kawashima et al., 2007; Anastasopoulos et al, 2010a).
1 PhD Student, National Technical University of Athens, Athens Greece, [email protected]
2 Professor in Civil Engineering, University of Dundee, Dundee Scotland, [email protected]
3 Professor of Soil Mechanics, National Technical University of Athens, Athens Greece, [email protected]
2
In this framework, recent studies have investigated the idea of exploiting inelastic foundation
response in order to limit the stresses transmitted onto the superstructure during strong shaking. In
contrast to conventional capacity design the foundation is deliberately “under-designed” to promote
rocking, limiting the inertia forces transmitted onto the superstructure. The effectiveness of such an
alternative seismic design philosophy, termed “rocking isolation” (Mergos & Kawashima, 2005), has
been explored analytically and experimentally for bridge piers (Anastasopoulos et al., 2010a; 2013a)
and frames (Gelagoti et al., 2012; Anastasopoulos et al., 2013b). Such “reversal” of capacity design
may substantially improve the performance, drastically increasing the safety margins.
Yet, this benefit comes at the expense of permanent settlement and rotation, which could
threaten the serviceability of the structure. Such undesired deformation can be maintained within
tolerable limits, provided that the safety factor against static (vertical) loads FSV is adequately large
(e.g., Gajan et al., 2005). In such a case, the response of the foundation is uplifting–dominated, and
there is no substantial accumulation of cyclic settlement. The response is markedly different for lower
values of FSV, becoming sinking-dominated: excessive soil yielding take place underneath the
foundation, leading to accumulation of substantial settlement and permanent rotation. Evidently,
ensuring an adequately large FSV in order to promote uplifting-dominated response greatly depends on
the exact soil properties, which may not always be the ones that are desired. Shallow soil improvement
(a concept that is commonly applicable in geotechnical engineering) may offer a viable solution to this
problem. This paper investigates experimentally the effectiveness of such a remediation technique.
Following a series of reduced–scale monotonic and cyclic pushover tests, a series of reduced–scale 1g
shaking table tests were conducted at the Laboratory of Soil Mechanics of the National Technical
University of Athens (NTUA) to explore the efficiency of shallow soil improvement under truly
dynamic loading.
PROBLEM DEFINITION
Based on the work presented in Anastasopoulos et al. (2012), a slender rocking–isolated bridge pier of
height h = 9 m supported on a surface square footing of width B = 3 m is used as a conceptual
prototype. Taking account of the capacity of the NTUA shaking table, a linear geometric scale of 1:20
(n = 20) was selected for the experiments, and model properties were scaled down according to
relevant scaling laws (Muir Wood, 2004). In all cases examined, to focus on foundation performance,
the superstructure is assumed rigid and elastic. Three different soil profiles were simulated in the
experiments: (a) loose (Dr = 45%) sand representing poor soil conditions; (b) soil improvement by
means of a shallow “crust” of dense sand, of varying depth z/B = 0.5 to 1; and (c) the reference case of
dense (Dr = 93%) sand, representing ideal soil conditions. The studied configurations are presented in
Fig.1. The sand used in the experiments is Longstone sand. It is characterized by a uniformity
coefficient Cu = 1.42 and mean grain sized diameter D50 = 0.15 mm and, was pluviated using an
automated sand raining system. The properties of the sand have been measured through laboratory
tests, also conducted at NTUA, and are documented in Anastasopoulos et al. (2010b).
Dense Sand : Dr = 93 %
FSV = 14
Loose Sand : Dr = 45 %
FSV = 5
Loose Sand : Dr = 45 %
Dense Sand [ Dr = 93 % ]
z/B = 0.5, 1 FSV = 7, 10
Poor soil conditions Soil improvement Ideal soil conditions
h = 9 m
B = 3 m
d =
3B
Figure 1. The experimental configurations and their properties used to model the bridge pier lying on the four
different soil profiles.
A. Tsatsis, I. Anastasopoulos and G. Gazetas 3
Before proceeding to the testing sequence, a brief discussion of the monotonic response of the
studied systems is necessary. A detailed description of the pushover tests and their key results can be
found in Anastasopoulos et al. (2012). Fig. 2 summarizes the moment–rotation (Μ–θ) and settlement–
rotation (w–θ) response of the system founded on the three different soil profiles. When founded on
poor soil conditions (i.e., loose sand) its maximum moment capacity of the system reaches Mmax = 1.8
MNm (unless otherwise stated, all results are discussed in prototype scale). Considering the dynamic
response, a critical acceleration ac can be defined as the maximum acceleration that can develop at the
mass of the oscillator (representing the bridge deck). For rocking isolated systems, such as those
examined herein, the maximum moment developed at the mass is bounded by the moment capacity of
the foundation, and therefore, ac can be defined as the acceleration for which the developed moment is
equal to the moment capacity. Based on the above, the system founded on poor soil is characterized by
a critical acceleration ac = 0.072 g. The moment capacity is substantially increased for the case of
shallow soil improvement, leading to a proportional increase of the critical acceleration to ac = 0.117 g
for z/B = 0.5 and ac = 0.130 g for a deeper z/B = 1 dense sand crust. The latter is still lower than the
one for ideal soil conditions, ac = 0.150 g, but the efficiency of the improvement is evident. Most
importantly, shallow soil improvement leads to a substantial reduction of the tendency for
accumulation of settlement. As evidenced by the w–θ response, the application of shallow soil
improvement tends to suppress the sinking behavior of the foundation. The performance of the tested
configurations is summarized in Table 1.
Table 1. Properties of the studied systems as measured from monotonic push-over tests.
FSV
Mmax ac Kinitial Tinitial
(MNm) (g) (MN/m) (sec)
loose sand (Dr = 45%) 5 1.8 0.072 7 1.21
z/B = 0.5 7 2.9 0.117 13 0.92
z/B = 1 10 3.2 0.130 20 0.75
dense sand (Dr = 93%) 14 4.0 0.162 26 0.66
INSTRUMENTATION AND TESTING SEQUENCE
The model was instrumented to allow direct recording of translational and rotational deformations, and
lateral accelerations. Wire (WDT) and laser displacement transducers (LDT) were utilized to measure
horizontal and vertical displacements, while accelerometers were installed at characteristic locations,
on the model (on the foundation and at mass level), and within the soil. The different configurations
were subjected to shaking table testing, using a variety of real seismic records and artificial motions as
base excitation.
WDT
WDTWDTWDT
WDT
WDT
accelerometers
LDTaccelerometers
Figure 2. Photos of the physical model ready for testing showing the instrumentation.
4
The selected seismic motions were scaled down in time (divided by √n) according to the relevant
scaling laws. Two different seismic shaking sequences were imposed. The first consists of artificial
motions (sinusoidal excitations), while the second of real records. In all cases examined, the PGA of
the seismic excitations was well beyond the critical acceleration of the tested systems (progressively
increasing). This was done to explore the strongly nonlinear foundation response, driving the models
well into their metaplastic regime.
-1.2
-0.7
-0.2
0.3
0.8
0 10 20 30 40 50 60
1.25g
Pacoima Dam
-1.2
-0.7
-0.2
0.3
0.8
0 10 20 30 40 50 60
0.36g
Sakarya
-1.2
-0.7
-0.2
0.3
0.8
0 10 20 30 40 50 60
0.43g
Lefkada 2003
-1.2
-0.7
-0.2
0.3
0.8
0 10 20 30 40 50 60
sin 2 Hz
0.4g
-1.2
-0.7
-0.2
0.3
0.8
0 10 20 30 40 50 60
0.39g
Aegion
-1.2
-0.7
-0.2
0.3
0.8
0 10 20 30 40 50 60
0.27g
Kalamata
-1.2
-0.7
-0.2
0.3
0.8
0 10 20 30 40 50 60
0.61g
Takatori
-1.2
-0.7
-0.2
0.3
0.8
0 10 20 30 40 50 60
JMA
0.82g -1.2
-0.7
-0.2
0.3
0.8
0 10 20 30 40 50 60
0.84g
Rinaldi
-1.2
-0.7
-0.2
0.3
0.8
0 10 20 30 40 50 60
0.4g
sin 1 Hz
5 10
t (sec)
0
0.5
1
a (g)
0
0.5
1
a (g)
0
Motion Sequence I
Motion Sequence II
-1.2
-0.7
-0.2
0.3
0.8
0 10 20 30 40 50 60
0.2g
sin 1 Hz
-1.2
-0.7
-0.2
0.3
0.8
0 10 20 30 40 50 60
sin 2 Hz
0.2g
5 10
t (sec)
0
Figure 3. Real records and artificial motions used as seismic “bedrock” excitations in the shaking table tests.
THE EFFECT OF ROTATION AMPLITUDE
In terms of monotonic and slow cyclic pushover response, shallow soil improvement was proven to be
quite effective (Anastasopoulos et al., 2012). Fig. 3 summarizes the results of slow cyclic push-over
tests, depicting the settlement per cycle wc as a function of the imposed cyclic rotation amplitude θc. A
A. Tsatsis, I. Anastasopoulos and G. Gazetas 5
z/B = 1 dense sand crust is proven enough to achieve practically the same performance with the ideal
case of dense sand. For the shallower z/B = 0.5 soil improvement layer, the cyclic settlement reduction
is quite evident, but the response differs substantially from the ideal case of dense sand. Still though, a
shallower z/B = 0.5 soil improvement layer may be adequate, depending on design requirements. The
efficiency of shallow soil improvement is ameliorated with the increase of the cyclic rotation
amplitude. For both investigated soil crusts the additional settlement per cycle decreases as the
rotation amplitude increases, tending to the respective settlement of the ideal system.
0
0.01
0.02
0.03
0.04
0 0.01 0.02 0.03 0.04 0.05
θc (rad)
Loose sand
z/B = 0.5
Dense sand
z/B = 1
wc
(m)
Figure 4. Slow-cyclic pushover tests results. Settlement per cycle wc as a function of cyclic rotation amplitude θ
To explore the role of rotation amplitude under dynamic loading, two idealized 15-cylce
sinusoidal motions with a PGA of 0.2 g are used, with the only difference being the frequency: f = 1
Hz and 2 Hz. Fig. 4 compares the performance of the two cases of soil improvement with the ideal
case of dense sand. Although the acceleration is exactly the same (Fig. 4a), the “slower” f = 1 Hz
excitation will develop larger (theoretically double) ground displacement compared to the “faster” f =
2 Hz sinus. And since the rotation of the rocking system is depends largely on the ground
displacement, the f = 1 Hz excitation should produce larger rotation. Indeed, the maximum rotation
(Fig. 4b) reaches θ = 0.0035 rad (on average) for the f = 2 Hz excitation, being almost twice as much
for f = 1 Hz: θ = 0.007 rad (on average).
Fig. 4c compares the settlement for the two excitation frequencies. In the case of dense sand, the
settlement is not particularly sensitive to the excitation frequency. The settlement w reaches 2.5 cm for
the high frequency f = 2 Hz sine, being only slightly lower (2 cm) for the lower frequency f = 1 Hz
excitation. This is in accord with the cyclic pushover test results, according to which the cyclic
settlement of the system on dense sand remains practically constant for 0.003 < θc < 0.03 rad. The
small difference is possibly related to a limited amount of dynamic compaction, which mainly affected
the f = 2 Hz sine which was the first excitation in this seismic shaking sequence. The differences are
much more pronounced for the two cases of shallow soil improvement. In both cases (i.e., z/B = 0.5
and 1.0), the accumulated settlement is much larger for the high–frequency f = 2 Hz excitation:
roughly 2 times larger than for the low–frequency f = 1 Hz sine. This very substantial difference
cannot be solely attributed to dynamic compaction of the underlying loose sand, but is also related to
the dependence of the efficiency of shallow soil improvement on cyclic rotation. In agreement with the
results of monotonic and cyclic pushover tests, the efficiency of the crust is found to increase with
rotation (the amplitude of θ for f = 1 Hz is almost twice as much for f = 2 Hz). While for smaller
rotation the foundation is in full contact with the supporting soil, generating a deeper stress bulb and
hence being affected by the underlying loose sand layer, when uplifting is initiated the effective
foundation width is drastically decreased, reducing the depth of the generated stress bulb. Hence, a
larger portion of the rocking induced stresses are obtained by the “healthy” soil material of the crust,
improving the performance of the system.
6
-0.01
0
0.01
0.02
0.03
0 5 10 15 20 25
-0.01
0
0.01
0.02
0.03
0 5 10 15 20 25
-0.1
-0.08
-0.06
-0.04
-0.02
0
0.02
0 5 10 15 20 25
-0.1
-0.08
-0.06
-0.04
-0.02
0
0.02
0 5 10 15 20 25
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0 5 10 15 20 25
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0 5 10 15 20 25
a(g)
θ(rad)
w(m)
z/B = 0.5 dense sandz/B = 1
t (sec) t (sec)
8.4 cm
5.8 cm
2.5 cm
2 cm
3.3 cm
(a)
(b)
(c)
2.7 cm
sine 2 Hz – 0.2g sine 1 Hz – 0.2g
Figure 5. The effect of rotation amplitude on the efficiency of shallow soil improvement. Comparison of shallow
soil improvement with ideal soil conditions subjected to seismic shaking using two 15-cylce sinusoidal
excitations of frequency f = 2 Hz (left) and f = 1 Hz (right). Time histories of: (a) base excitation; (b) foundation
rotation; and (c) settlement.
In order to consolidate the conclusions drawn from the response of the four systems subjected to
the artificial motions, a real record is imposed. To that end, the Pacoima Dam record from the 1971
San Fernando earthquake is imposed. The Pacoima Dam record is definitely a very strong seismic
record. Apart from its impressive PGA of 1.25 g, this record is characterized by a long duration (and
hence low frequency) directivity pulse, having an amplitude of 0.6 g (see the shaded area in Fig. 6a).
Given the previously discussed critical acceleration of the system on loose sand, merely ac = 0.072 g
collapse should have been expected, and this is exactly what happened. The same applies to the z/B =
0.5 crust, The system managed to survive such strong shaking only when founded on dense sand, or in
the case of the deeper z/B = 1 soil improvement. This alone is a very important conclusion, confirming
the efficiency of the z/B = 1 shallow soil improvement in terms of survivability. Hence, in Fig. 6 the
performance of the two systems that did not collapse is presented.
The response can roughly be divided in two phases. In the first phase, which approximately lasts
from t = 3 to 6 sec, the previously mentioned strong directivity pulse dominates the response. With a
very large period T ≈ 1.2 sec, this pulse drives both systems well within their metaplastic regime,
developing a maximum rotation θ ≈ 0.04 rad (Fig. 6b). The second phase (for t > 6 sec) is
characterized by a multitude of strong motion cycles of even larger amplitude (up to 1.25 g) but of
substantially smaller period, ranging from 0.1 to 0.4 sec. As revealed by the free field acceleration
measurements (Fig. 6a), due to soil amplification there are three acceleration peaks in excess of 1 g,
with one of them reaching a PGA of 1.8 g. Under such unrealistically extreme seismic excitation,
either on dense sand or on z/B = 1 soil improvement, the rocking system survives. Although the
A. Tsatsis, I. Anastasopoulos and G. Gazetas 7
differences in θmax are again negligible, there is a substantial difference in θres: 0.045 rad for the case of
z/B = 1 soil improvement, compared to 0.012 rad for ideal soil conditions.
In terms of settlement accumulation, the response is distinctly different during the two phases of
the response. As illustrated in Fig. 6c, during the long period directivity pulse (phase 1) the settlement
is minimal in both cases. In fact, the rocking system is mainly subjected to uplifting and the
accumulated settlement at t = 6 sec does not exceed 1 cm. During this phase of response, the z/B = 1
crust is proven very effective, exhibiting practically identical behavior to that of the ideal case of
dense sand. The performance is markedly different during the second phase, which is characterized by
a multitude of strong motion cycles of larger amplitude but of much higher frequency. The settlement
mainly takes place during this second phase, with the accumulated settlement reaching 8 cm for dense
sand and 12 cm in the case of the z/B = 1 soil crust. Evidently, the accumulation of settlement and the
efficiency of shallow soil improvement are clearly affected by the excitation frequency, and therefore
the rotation amplitude of the system.
-0.15
-0.1
-0.05
0
0.05
0 2 4 6 8 10 12 14 16 18 20
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0 2 4 6 8 10 12 14 16 18 20
-1.5
-1
-0.5
0
0.5
1
1.5
2
0 2 4 6 8 10 12 14 16 18 20
θmax≈ 0.04 rad
t (sec)
a (g)
θ(rad)
w(m)
(a)
(b)
(c)
0.04 rad
0.01 rad
wdense ≈ wz/B=1 ≈ 0.01 m
wdense ≈ 0.08 m
wz/B=1 ≈ 0.12 m
≈
bedrock excitation
free field
z/B = 1
Dense sand
z/B = 1
Dense sand
Figure 6. Seismic performance of the ideal system and of the system lying on the deeper soil crust subjected to
seismic shaking using the Pacoima dam record as excitation. Time histories of: (a) free field and base excitation;
(b) foundation rotation; and (c) settlement. An extract from Fig. 4 (bottom right) is also shown to allow direct
comparison with the cyclic pushover tests.
8
THE EFFECT OF THE NUMBER OF CYCLES
Apart from rotation, the efficiency of shallow soil improvement is also ameliorated with the number of
strong motion cycles. Figure 7 presents the dynamic settlement δw per cycle of each artificial motion
with respect to the number of cycles, and as a function of excitation frequency and amplitude. In all
cases examined, irrespective of excitation frequency or amplitude, the settlement per cycle of motion
reduces with the number of cycles, thanks to soil densification underneath the footing.
0
0.02
0.04
0.06
0.08
0.1
0 2 4 6 8 100
0.005
0.01
0.015
0.02
0 2 4 6 8 10
0
0.003
0.006
0.009
0.012
0.015
0 2 4 6 8 10 12
0
0.003
0.006
0.009
0.012
0.015
0 2 4 6 8 10
number of cycles
dense sandloose sand z/B = 1z/B = 0.5
-1.2
-0.7
-0.2
0.3
0.8
0 10 20 30 40 50 60
f = 2 Hz
0.2 g
-1.2
-0.7
-0.2
0.3
0.8
0 10 20 30 40 50 60
f = 2 Hz
0.4 g
-1.2
-0.7
-0.2
0.3
0.8
0 10 20 30 40 50 60
f = 1 Hz
0.2 g
-1.2
-0.7
-0.2
0.3
0.8
0 10 20 30 40 50 60
f = 1 Hz
0.4 g
δw(m)
δw(m)
number of cycles
Figure 7. The effect of the number of strong motion cycles on the efficiency of shallow soil improvement.
Comparison of shallow soil improvement with poor and ideal soil conditions. Foundation settlement per cycle
for four different sinusoidal excitations: (a) f = 2 Hz, a = 0.2 g; (b) f = 2 Hz, a = 0.4 g; (c) f = 1 Hz, a = 0.2 g; and
(d) f = 1 Hz, a = 0.4 g.
For the two high–frequency (f = 2 Hz) excitations the rate of settlement δw is reduced almost
linearly with the number of strong motion cycles. Quite interestingly, the decrease of δw is much more
intense when the frequency of excitation is lower (f = 1 Hz). In this case, the first two or three cycles
are enough to cause substantial dynamic compaction of the soil, and as a result the remaining strong
motion cycles are not leading to any substantial additional settlement. As previously discussed, the
oscillation of the lightly loaded system is of significantly larger amplitude for the low–frequency
sinusoidal excitation, and therefore the first two or three cycles of large rotational amplitude are
enough to compact the sand under the footing. On the contrary, the sand is continuously compacted
due to small vibrations of the footing when the system is subjected to the f = 2 Hz sinusoidal
excitation.
SYNOPSIS AND DISCUSSION
Fig. 8 and Fig. 9 summarize the performance of the four systems in terms of settlement w and residual
rotation θres respectively as a function of PGA. Although the excitations were imposed in a sequence
(i.e., one after the other), the results presented herein refer to values recorded during each excitation
(not the cumulative ones). Therefore, the results are not to be considered representative of the
A. Tsatsis, I. Anastasopoulos and G. Gazetas 9
performance of the system subjected to each excitation separately, but rather in a comparative manner
in order to assess the efficiency of shallow soil improvement.
0
0.1
0.2
0.3
0.4
0 0.5 1 1.5 2
0
0.1
0.2
0.3
0.4
0 0.5 1 1.5 2
0
0.1
0.2
0.3
0.4
0 0.5 1 1.5 2
0
0.1
0.2
0.3
0.4
0 0.5 1 1.5 2
w(m)
w(m)
PGA (g) PGA (g)
collapse collapse
sin – 2Hz sin – 1Hz Aegion Kalamata
JMA Rinaldi Pacoima TakatoriLefkada 2003
Sakarya
loose sand z/B = 0.5
dense sandz/B = 1
Figure 8. Synopsis of the performance of the four systems in terms of settlement accumulated during each
excitation as a function of PGA.
As vividly shown in Fig. 8, when the system is founded on loose sand (representative of poor soil
conditions), it can only sustain 3 out of 12 seismic excitations (considering both shaking sequences).
Even for these three excitations, the settlement is quite substantial (in excess of 10 cm). The
improvement is quite evident for shallow soil improvement of depth z/B = 0.5. The system is able to
withstand seismic excitations of PGA up to 1.1 g without toppling. Observe that the residual rotation
θres is reduced by almost 50% compared to the untreated case of loose sand (Fig. 9). However, the
settlement w is not reduced to the same extent, and the system still topples in 7 out of 12 seismic
excitations. A deeper z/B = 1 dense sand crust is required to decrease the settlement substantially. In
this case, the performance is practically the same with that of the ideal case of dense sand, confirming
the efficiency of shallow soil improvement with a z/B = 1 dense sand crust.
10
0
0.02
0.04
0.06
0.08
0.1
0 0.5 1 1.5 2 2.5
0
0.02
0.04
0.06
0.08
0.1
0 0.5 1 1.5 2 2.5
0
0.02
0.04
0.06
0.08
0.1
0 0.5 1 1.5 2 2.5
0
0.02
0.04
0.06
0.08
0.1
0 0.5 1 1.5 2 2.5
θ(rad)
θ(rad)
PGA (g) PGA (g)
collapse collapse
sin – 2Hz sin – 1Hz Aegion Kalamata
JMA Rinaldi Pacoima TakatoriLefkada 2003
Sakarya
loose sand z/B = 0.5
dense sandz/B = 1
Figure 9. Synopsis of the performance of the four systems in terms of rotation accumulated during each
excitation as a function of PGA.
CONCLUSIONS
Aiming to explore the effectiveness of shallow soil improvement as a means to increase soil strength
and to mitigate settlement accumulation attributed to nonlinear response of the footing during an
earthquake, this paper experimentally investigated the seismic response of a conceptual bridge pier
modelled by a relatively slender h/B = 3 SDOF system lying on square foundation of width B. The
physical model was subjected to reduced–scale shaking table testing at the Laboratory of Soil
Mechanics of the National Technical University of Athens. It was first tested on poor soil conditions
in order to demonstrate the necessity for soil improvement. Then, the effectiveness of shallow soil
improvement was studied by investigating the performance of the system lying on soil crusts of depth
z/B = 1 and 0.5. Finally, the performance of the system lying on the improved soil profiles was
compared to that considering ideal soil conditions.
The main conclusions of the presented research can be summarized as follows:
1. Based on the conducted reduced-scale tests, and at least for the cases examined herein, the
concept of shallow soil improvement is proven quite effective. A z/B = 1 dense sand crust is
enough to achieve practically the same performance with the ideal case of dense sand. A
shallower z/B = 0.5 soil improvement may also be considered effective, depending on design
requirements.
A. Tsatsis, I. Anastasopoulos and G. Gazetas 11
2. The results of the shaking table tests are in very good qualitative agreement with previously
published (Anastasopoulos et al., 2012) experimental results from monotonic and slow cyclic
pushover tests.
3. As with the slow cyclic pushover tests, the performance of shallow soil improvement is found
to depend on the rotation amplitude. Real records and artificial motions of different frequency
were examined, that forced the two systems to oscillate at various rotation amplitudes. It was
shown that with the increase of the rotation amplitude the effectiveness of the soil crusts
increases.
4. The performance of shallow soil improvement is ameliorated with the number of cycles of the
motion. The rate of settlement reduces with the increase of the number of cycles for all cases
examined.
REFERENCES
Anastasopoulos I., Loli M., Georgarakos T., Drosos V. (2013a), “Shaking Table Testing of Rocking−isolated
Bridge Pier”, Journal of Earthquake Engineering, Vol. 17(1), pp. 1–32.
Anastasopoulos I., Gelagoti F., Spyridaki A., Sideri Tz., Gazetas G. (2013b), “Seismic Rocking Isolation of
Asymmetric Frame on Spread Footings”, Journal of Geotechnical and Geoenvironmental Engineering,
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