Use of rubber for improving the performance of domestic...
Transcript of Use of rubber for improving the performance of domestic...
Use of rubber for improving the performance of domestic buildings against seismic
liquefaction.
Georgios Nikitas1, Subhamoy Bhattacharya
1, Masayuki Hyodo
2, Atsuhi Konja
3, Stergios Mitoulis
1
1Department of Civil and Environmental Engineering, University of Surrey, Guildford, United Kingdom
2Department of Civil Engineering, Yamaguchi University, Ube, Japan
3 Hokoku Engineering Corporation, Osaka
email: [email protected], [email protected], [email protected], [email protected],
ABSTRACT: Soil liquefaction as experienced in most of the latest large earthquakes, has left behind permanent large surface
deformations on the soil that set structures unusable and very difficult to repair. Existing mitigation methods, such as deep pile
foundations, gravel drain columns and dynamic compaction can reduce the effects of liquefaction on structures, but their costs
seem prohibitive for most common residential engineering applications. This research provides experimental results from a
series of shaking-table tests carried out at 1g of a new low cost mitigation technique against liquefaction. This technique has
only recently been developed in Japan in order to find immediate application on typical domestic houses following the 2011
Tohoku earthquake. The technique is based on, shredded waste tyres packed in permeable sandbags are used for ground
improvement underneath the structure. A number of parametric scenarios concerning the thickness of the ensuing elastic base
were considered. The experiments indicate that the tyre shreds’ addition can act both as a seismic isolation (i.e. filtering effect)
as well as an efficient drainage method. This mitigation approach also proposes an innovative and sustainable way to reuse
waste tires, which otherwise set a serious environmental problem due to their large volumes produced and their recycling
complications.
KEY WORDS: Liquefaction mitigation, shaking table test, shredded tyre foundation, soil improvement
1 INTRODUCTION
Under moderate to large vibrations, loose to medium density
saturated soil deposits are prone to liquefaction; see for
example the damage to the Tokyo city following the MW 9.0
magnitude earthquake that struck the Tohoku region of Japan
on 11th
March 2011. The earthquake caused great damage to
structures and infrastructures around Tokyo Bay area, due to
liquefaction phenomena. The liquefaction phenomena were
widespread in the area causing damage to the water and
sewerage pipelines, destroying highways and pavements and
making many structures useless due to permanent surface
deformations. Figure 1 shows typical settlement pattern
observed in Urayasu city in Tokyo bay area following the
earthquake. However, the excellent performance of buildings,
such as the International Airport in Tokyo (also known as
Haneda Airport) which was built on reclaimed land, using
ground improvement techniques, proved that these techniques
can actually mitigate liquefaction hazards. In fact, the
earthquake had no effects on the airport, so it was operational
from the very next day of the earthquake. [1]
Many methods for liquefaction mitigation have been
developed throughout the years, but they tend to be used only
in big projects and their cost is prohibitive for domestic
houses (typical one to two storey houses). The current
research considers such an economic mitigation technique
against liquefaction. In the proposed method, shredded scrap
tires are used to minimize the earthquake and liquefaction
impacts on a domestic building, such as a common 1 story
house. The scrap tires are shredded into small pieces (around
2.5 mm wide) and then, after being put inside sandbags, are
placed underneath the foundations of the structure. The layout
of the proposed mitigation technique can be seen in Figure 2.
Figure 1. Differential settlement of a building in Urayasu, in
Tokyo bay area.
The perceived benefits of the proposed technique are:
a) Due to the increased permeability of this resulting sub-
base (because of the larger than sand void ratio), it is
expected to effectively act against the pore water pressure
built up that is a pre-requisite for liquefaction.
b) With the added damping and elasticity of the new
foundation type is supposed to reduce the transmission of
the shear waves, so actually to reduce the acceleration
transmitted to the structure.
For verifying the above hypothesis, a series of 1g shaking-
table experiments were conducted in order to provide
experimental data that could validate and scrutinize the
Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014Porto, Portugal, 30 June - 2 July 2014
A. Cunha, E. Caetano, P. Ribeiro, G. Müller (eds.)ISSN: 2311-9020; ISBN: 978-972-752-165-4
259
performance characteristics of the developed technique. Also
another important aspect of this technique is that it provides
an innovative way to reuse waste tires. The disposal of waste
tires has been a big environmental issue, because of the large
amount of tires produced worldwide and the durability of their
remainder as a waste material.
Figure 2. Layout of the proposed ground improvement
technique.
2 EXPERIMENTAL PROCEDURE
2.1 Test setup
The 1g shaking-table tests were conducted in the
Earthquake and Large Structures Laboratory (EQUALS),
which is part of the Bristol Laboratories for Advanced
Dynamics Engineering (BLADE) in the Faculty of
Engineering at the University of Bristol (U.K.). The actual
shaking table consists of a 3m by 3m cast aluminium platform
weighing 3.8 tonnes, with a 15 tonne capacity and allows
simulating motion in all possible 6 degrees of freedom. The
platform can accelerate horizontally up to 3.7g with no
payload, 1.6g with a 10 tonne payload and vertically up to
5.6g and 1.2g respectively. The shaking table is surrounded by
a strong floor and adjacent strong walls up to 15m high. [2]
For the test arrangement, a rigid plastic container
(1120mm x 920mm x 600mm) was rigidly mounted centrally
on the top of the table. In this design, the shear stiffness of the
end walls is much higher than the stiffness of the soil layers
contained. The end walls and the base of the containers are
designed to be rough, so that the development of shear
stresses is in vertical plane at the interface between container
and soil. [3] The container structure was separated in 2
partitions by a wooden separator (920mm x 550mm x 20mm).
One partition had the model, with the cushion of tire chips
underneath and the other had the model without the cushion.
An identical scaled slab foundation was put on the soil surface
in both cases. The main objective is to assess the qualitative
response differences that emerge when the tire-based
foundation enhancement is in place. Such a technique, of a
single box and single shaking for both alternatives, is
expected to minimize artifacts that may be induced due to
testing differences between two independent testing
arrangements.
Figure 3 illustrates in detail the top and side views of the
model apparatus that was tested. The locations of the
transducers used along with every relevant dimension for
reproducing the setup are also shown. A thorough description
of all the parameters included in the design is given below.
Figure 3. Top and side views of the test arrangement.
2.1 Materials and instrumentation used
The tire chips in Figure 3, were made by shredding ordinary
scrap tires in particle sizes between 1mm and 5mm. The chips
were packed later in small uniformly perforated bags made
out of cellophane with dimensions 89mm x 67mm x 67mm
and total weight of 150 grams. These plastic tire bags, were
evenly arranged underneath the model slab foundation with a
thin layer of soil (67mm) covering the gap between tires and
slab. Some of the properties of the tire chips are listed in
Table 1 and are similar to Hyodo et al [4]
Table 1. Physical properties of Tire Chips.
Material D50
(mm) emin emax
γdmin
(kN/m3)
γdmax
(kN/m3)
Tire
Chips 2.45 1.600 2.320 35.00 45.00
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The soil was intended to have a low density to ensure the
liquefaction occurrence. The loosely poured liquefiable soil
base consisted of Redhill Sand 110, which is sieved silica
sand with a total quartz content of 98.8%. The typical physical
properties of the sand used are summarized in Table 2. [5]
Table 2. Physical properties of Redhill Sand 110.
Material D50
(mm) emin emax
γdmin
(kN/m3)
γdmax
(kN/m3)
Φ
(o)
Redhill
110 0.12 0.547 1.037 12.76 16.80 36
For the slab foundation models, pieces of MDF wood
(200mm x 200mm x 20mm) were used to form a rigid
uniform base. On each piece of wood, 2 steel weights of 0.6kg
each were mounted centrally and later where covered with the
carton house models. Each model was scaled to weight a total
of approximately 2kg.
Identifying the liquefaction phenomenon necessitates
keeping track of the water pressure and its subsequent rise
inside the soil body. For measuring the water pressure along
the different depths indicated (in Figure 3), six nos Druck
PDCR 811 Pore Pressure Transducers (PPTs) were employed.
Acceleration is the other piece of vital information to be
recorded during dynamic testing. For such measurements 6
ADXL335 MEM accelerometers (3-axis, ±3g), which were
specially modified for underwater use, were employed. Such
instruments were identical to those devised by Bhattacharya et
al. 2012 [6], and were found as previously to produce quality
measurements, comparable to much more expensive uniaxial
instruments for a wide band of frequencies. These
accelerometers were calibrated using SETRA accelerometers
as reference.
2.3 Steps followed
An analytical break down of the testing process includes the
following steps
a) The container is separated in two partitions and marked on
each side with height indications. No mechanical measure
was taken for securing the divider plate (cut to
dimensions) to the box structure. The sand poured inside
the box was the only means of holding the plate in place.
No precautions were taken for addressing wave reflection
from the boundaries and the model is considered to be
representing an infinite space foundation. There is the
provision to use in a follow up improved test foam
coverings on all sides, as in Bhattacharya et al., 2012, but
for the current work no specific study of boundary effects
is pursued.
b) After the container is rigidly mounted on the shaking table,
the Redhill 110 sand is pluviated inside, similarly to dry
deposition method by Isihara[7]. The sand is placed in 4
layers with thicknesses of 150mm, 66.67mm, 83.33mm
and 50mm respectively from bottom to top. During the
process the instrumentation was installed at the locations
shown in Figure 3. An relative density of 45% is achieved.
c) For the saturation process water is poured locally from the
top until full saturation is achieved. This is considered to
be done when the top water level remains unchanged.
Subsequently, the excess water above the sand layer is
soaked so that only a surface of wet sand is visible. Even
minute disturbances of the container could cause a thin
film of water to resurface.
d) The foundation models are placed on the center of each
partition as seen in Figure 3. The bases are leveled to
assure that in both cases a zero initial tilt is the starting
point. An initial recording without any shaking is taken so
that detrending of the data can be later performed.
e) The following shakings were imported to the model:
i. The apparatus is shaken first with low amplitude
white-noise (0 to 80Hz), independently in each of the
x, y and z axes, in order to retrieve all relevant modal
characteristics of the scaled models.
ii. The model ground is shaken with the 40% scaled (in
terms of Peak Ground Acceleration) Christchurch
earthquake of 22 February 2011. A true earthquake is
selected since initially it was thought that a simple
harmonic input motion would clearly favour one over
the other model in terms of resonant response. This
biasing cannot be waived by the use of a true
earthquake recording but in its case this is more natural
and also representative of the full-scale analogue.
iii. Once the above procedure is completed, a second
shaking with the actual non-scaled Christchurch
earthquake takes place. This is an extreme unrealistic
event scenario which only serves to assess the added
magnitude of damage to be incurred.
3 Test results
3.1 Response of the models under white noise test
Before getting into the earthquake response outcomes, it
is interesting to see how the apparatus natural frequencies in
the two cases with and without tire chips under the foundation
would change. Intuitively the less rigid elastic foundation is
expected to respond at a lower fundamental frequency,
because the building supported on tire base will have lower
stiffness and hence lower natural frequency. Additionally this
softening effect is probably expected to be more pronounced
along the z axis where the spring action of the tire chips is
more evident. Yet it is interesting to also uncover whether the
initial white-noise shaking to extract the frequency
characteristics has an impact on them. Figure 4 shows the
acceleration spectra for all 3 axes during their relevant white
noise shaking. Odd number accelerometer names refer to the
tire foundation while their increasing value indicates an
instrument closer to the surface (see Figure 4). Interestingly,
on the x and y axes, the dynamic characteristics seem almost
identical regardless of the foundation type, but in the case of z
axis they are different when comparing their surface (i.e.
above the elastic tire base accelerometers Acc5z, Acc6z)
values. Evidently the stiffness drop appears as expected. Thus
interestingly on x and y axes the two models behave the same
for any input acceleration and the tire inclusion affects only
the vertical movement. Logically such an image is expected to
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pertain also after the main shaking. It should be noted that all
spectra were calculated based on uncalibrated data.
Figure 4. Acceleration Spectra for white noise dynamic
loading on X (top), Y (middle) and Z axis (bottom).
3.2 Response of the models under Christchurch earthquake
Further the application of the scaled and the actual
Christchurch earthquake accelerations (reproduced along all
directions) is expected to show some changes resulting in the
dynamic characteristics. Identifying these qualitative changes
is the main aspect of the current research work. It should be
mentioned that no additional white noise tests were run with
the completion of the main shaking. Thus the comparison will
not take the form of a simple contrast of spectra alike to the
ones in Figure 4. Rather a more elaborate approach is
pursued, which will not be altered by any extra possible
modifications from additional white noise tests. Figure 5
shows the time histories of excess pore water pressure ratios
ru (PPTs located at 0cm, 15cm and 30cm from the bottom of
the box as illustrated on the inset) and the synchronous input
accelerations on the x and y axes respectively during the 40%
scaled Christchurch earthquake. The instruments correspond
to the case without the ground improvement technique. It is
expected during liquefaction that the pressures start rising
when the dynamic load is applied. It is noted that the pressure
increase varies with different depth, giving an extreme change
closer to the surface. This is a clear indication of the
inhomogeneous liquefaction observed in these experiments.
The relative pressure magnitude has already been scaled with
depth and was expected to rise evenly over all depths. As a
matter of fact different phases in the increasing pressure
evolution can be distinguished as seen also in the literature
even for real-scale field data [8]. The initial shaking seems to
bring a smooth almost linear pressure rise while the
subsequent larger accelerations impose a largely fluctuating
behavior. After the 16s the pressures in all location has
stabilized until the 41s, when their decay which matches the
stop of the ground shaking begins. Indicatively it can be seen
that this decay occurs with different rate again depending on
the depth of the PPT.
Figure 5. Pore pressure rise during the 40% scaled
Christchurch shaking and its acceleration time histories
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Having observed the pressure effect that the earthquake
has on the ordinary foundation it is time to examine it aside
the tire foundation. In Figure 6 the closer to surface PPTs
from the two models (i.e. PPT 1C and 2C) are plotted
together. As observed in Figure 5, PPT 2C was the instrument
that produced the most extreme pore pressure rise. It can be
seen that when an elastic inclusion is in place the pore
pressure does not rise to high values. This is the best means of
proving the effectiveness of such a foundation solution, which
can effectively eliminate the liquefaction phenomenon, which
is synonymous to pore water pressure rise. Possible
explanations for the attained result could be the higher
permeability of the rubber cushions that do not strongly resist
the water movement. To be more precise Figure 6 is only a
local snapshot of the overall ground condition that centers on
a specific position, the one above the tire base. One may argue
that it could probably be some other change that brings up the
radical differences of Figure 6. Such should well extend
beyond the ground improvement region.
Figure 6. Surface pore pressure rise during Christchurch
shaking.
Figure 7 attempts to address any such implications.
Comparing the next set of PPTs, situated 150mm above the
bottom level, it can be concluded that the liquefaction
phenomenon in the rest of the soil body progresses in a very
similar manner. The correlations plotted for PPTs 1B and 2B
show a very close picture. Namely their autocorrelations
against lag time (nominally a plot analogous to the free decay
response in vibrating systems) decay almost identically.
Further their cross correlation has a very high initial value for
zero lag, close to 0.9, indicating the similarity of conditions in
the two positions. Thus it is natural to state that the local
ground improvement enhances under identical conditions the
impact of the liquefaction phenomenon simply where it is
needed; and this is the actual structure level. Although
liquefaction is better viewed and identified in terms of pore
pressure data, it is equally important to assess the
accelerometer data. After all, these will provide all
information about permanent deformations and the actual
details of the paths for reaching them. As it was earlier noted
the z-axis dynamic characteristics were the ones differing the
most, and as a matter of fact for the real earthquake selected
the motion is expected to be primarily a rocking strongly
influenced by the vertical axis details.
Figure 7. PPT 1B and 2B correlation function during
Christchurch shaking.
Figure 8 shows the autocorrelation of surface
accelerometers, in the z direction against lag time. It can be
witnessed that the two decay functions almost overlap
meaning that the forced response is similar for both cases,.
The harmonic component that can easily be distinguished has
to do with the predominant frequency of the input signal.
When switching the actual constants reached in the
acceleration time signals to tilt values it is concluded that the
ordinary slab foundation reaches 6˚ in contrast to less than 1˚
for the model on the tire cushions. These values were
reconfirmed also by hand height measurements. An
illustration of the vast magnitude of the difference between
remaining deformations can be viewed in all Figures 11-13,
proving the efficacy of the new foundation solution. Most
importantly in the vertical direction the vibration that reaches
the structure is the same for both models implying similar
wave propagation.
Figure 8. Correlation between surface accelerometers in the
two cases x
During testing it was seen that the model on top of the
rubber base was acting like if it was floating, which is
conceptually rather distinct from the normal foundation
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behavior. Thus differences in the actual motion that reaches
the structure were still sought in other details. Figure 9
correlates the x components of the surface accelerometers in
the two cases (Acc5x and Acc6x) and Figure 10 correlates the
surface accelerometers with the Setra accelerometer, which is
positioned on top of the shake table outside the container (see
Figure 4). Taking into account what was found earlier in
Figures 4 & 8 any changes should be probably unexpected.
Yet the figure actually shows appreciable differences in the
dynamic behavior. Specifically according to the
autocorrelation function of Acc5x and Acc6x when tires are in
place the x surface motion is much more damped and the
harmonic forced character of the earthquake is much more
difficult to make out. The same view is strengthened when the
cross correlation of the two instruments with the Setra
accelerometer is considered. The zero lag values and the
reduced periodicity in the behavior is an indication that on the
x axis movement for the tire base case, the structure behaves
like being more isolated from the imposed ground movement.
The same is also valid for the y axis.
Figure 9 Autocorrelations from surface accelerometers z in
the two cases
Figure 10 Correlations from surface accelerometers z in the
two cases with the Setra accelerometer
Figures 11-13 show actual photos after the Christchurch
shaking test, that can justify the results from the above data
analysis. It is clear from these pictures, that the model with the
tire base underneath was not affected by the shaking as much
as the one without.
Figure 11. View of the models after the shaking of
Christchurch Earthquake (non-scaled).
Figure 12. View of the model with the tire chips underneath
after the shaking of Christchurch Earthquake (non-scaled).
Figure 13. View of the model without the tire chips after the
shaking of Christchurch Earthquake (non-scaled).
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4 CONCLUSION
The current research work attempts to address and
understand the seismic mitigation measure of using an elastic
sub-base consisting of scrap tire chips just below the slab
foundation. Such a countermeasure resisting permanent
earthquake induced displacements was previously found very
effective for other types of structure e.g. manholes, Yoshida et
al., 2008 [9].
Here it was clearly shown that the tire addition alters the
liquefaction process reducing the close to surface pore
pressure rise and allows the structure to “float” on top of the
water surface that emerges for prohibiting permanent tilts.
More importantly it was shown that the tire action cannot be
realized as a single 1D action modeled e.g. by spring addition
but it rather has a more elaborate 3-dimensional character. It
was found that dynamic characteristics change during the
shaking largely in the horizontal plane isolating the structure
from excessive ground motion.
REFERENCES
[1] Bhattacharya S, Hyodo M, Goda K, Tazoh T, Taylor
CA. Liquefaction of soil in the Tokyo Bay area from the
2011 Tohoku (Japan) earthquake. Soil Dynamics and
Earthquake Engineering 31, 2011, p. 1618-1628.
[2] Dietz M, Dihoru L, Oddbjornsson O, Bocian M, Kashani
M, Norman J, Crewe AJ. Earthquake and Large
Structures testing at the University of Bristol laboratory
for advanced Dynamics Engineering. Geotechnical,
Geological and Earthquake Engineering 22, 2012, p. 21-
41.
[3] Bhattacharya S, Lombardi D, Dihoru L, Dietz M, Crewe
AJ, Taylor CA. Model container design for soil structure
interaction studies. Geotechnical, Geological and
Earthquake Engineering 22, 2012, p. 135-158.
[4] Hyodo M, Yamada S, Orense RP, Okamoto M, Hazarika
H. Undrained cyclic shear properties of tire chip-sand
mixtures, Scrap tires derived geomaterial. Oportunities
and Challenges, Taylor and Francis, UK, 2007;
[5] Kelly, R.B., Byrne, B.W., Houlsby, G.T. and Martin,
C.M. Tensile loading of Model Caisson Foundations for
Structures on Sand, Proceedings ISOPE. Conference,
Toulon, 2007;
[6] Bhattacharya S, Murali K, Lombardi D, Crewe AJ &
Alexander NA. Economic MEMS based 3-axis water
proof accelerometer for dynamic geo-engineering
applications. Soil Dynamics and Earthquake
Engineering 36, 2012, p. 111-118.
[7] Isihara, K. Soil Behaviour in Earthquake Geotechnics.
Oxford Science Publications, 1996
[8] Youd L, Steidl J, Nigbor R. Lessons learned and need
for instrumented liquefaction sites, Physics and
Mechanics of Soil. Soil Dynamics and Earthquake
Engineering 24, 2004, p. 639-646.
[9] Yoshida, M, Miyajima, M, & Kitaura, M. Experimental
Study on Mitigation of liquefaction-induced flotation of
sewrage manhole by using permeable recycled materials
packed in sandbags The 14th
World Conference on
Earthquake Engineering (Beijing, China), 2008
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265