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CHAPTER 3
A CRITICAL REVIEW ON THE DYNAMIC BEHAVIOUR OF OCR OF
SOFT CLAY UNDER CYCLIC LOADING
3.1 INTRODUCTION
The soils are often vulnerable to undrained cyclic loading produced either by human activity
or natural geologic activity. These cyclic loadings have a wide range of amplitude,
frequency, and duration which its behaviour varies depends on specific factors. Therefore,
cyclic behaviour of soft clay is a critical interest in geotechnical engineering practice and has
been studied progressively over the years, [1]. At present, development of city has become
a major critical impact in Batu Pahat because of the growing economy, industry and
community. The problem that has been found after construction and during the working
period of a building is the very high settlement rate, which shortens the design life of
structure. In addition, Soft Clay are well known for their low strength and high
compressibility. Usually, due to sedimentary process on different environment, both physical
and engineering properties of the clays (namely void ratio, water content, grain size
distribution, compressibility, permeability and strength) show a significant variation.
Furthermore, they exhibit high compressibility (including an important secondary
consolidation), reduced strength, low permeability and compactness, and consequently low
quality for construction [2]. This paper reviews the research done on the effects of damping
ratio, D and shear modulus, G under dynamic cyclic loading with different frequencies, the
effects of effective vertical consolidation stress and the effect of over – consolidation ratio,
OCR.
3.2 FACTORS AFFECTING CLAY BEHAVIOUR UNDER CYCLIC LOADING
The soil deposits in many geotechnical engineering projects undergo dynamic cyclic
loadings during their design lifetime. These loadings may be due to environmental factors,
such as seismic activity and ocean storms, or human activities, such as passing traffic and
vibrating machinery installed on a structure or site. Importantly, the soil response generated
by these dynamic cyclic loadings is typically more complex than that considered when
conducting static analyses, requiring engineers to investigate the dynamic behaviour of soils
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in the laboratory, as well as in the field. A typical soft clay soil properties can be found in the
grounds of Universiti Tun Hussein Onn, UTHM which have low shear strength and bearing
capacity, and suffer large settlements when subjected to loading. The typical physical
properties of Batu Pahat Soft Clay at Research Centre of Soft Soil, RECESS UTHM have
been experimentally investigated by many researchers as shown in Table 3.1 are compared
with soft clay from other places. A study carried by Chan et. al. [2], found that clay soil at
RECESS, UTHM contained 10.8 % clay, 79.5 % silt and 10.7 % sand.
The dynamic behaviour of cyclic strength of the soft clay during cyclic loading depend on
various factors. It depends on parameter such as listed below:
i. Cyclic stress/strain amplitude
ii. Number of loading cycles
iii. Frequency/loading rate
iv. Effective vertical stress
v. Consolidation path
vi. Over consolidation ratio (OCR)
vii. Sensitivity
viii. Initial static/average shear etc.
Table 3.1: Some of soft soil geotechnical properties
Clay Batu Pahat Soft Clay, (BPSC)
Soft Bangkok Clay
Itsukaichi Japan Marine Clay
Cloverdale Clay
Researchers Chan and Ibrahim [2]
Thammatiwat.A, et.al, [3]
- -
Bulk Density (Mg/m3)
1.36 - - -
Specific Gravity 2.66 2.57 – 2.73 2.532 2.79
Plastic Limit (%) 31 30 – 42 51.4 24
Liquid Limit (%) 77 75 – 99 124.26 51
Plasticity Index (%) 46 45 - 57 72.86 27
Moisture Content (%) - 78 - 99 - -
Table 3.2 shows some type of clay tested by researchers on past studies in terms of
geotechnical properties, sample condition, mode of loading, loading period and number of
cycles.
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Table 3.2: Clay tested in undrained cyclic triaxial by previous researchers
3.3 SOURCE OF DYNAMIC CYCLIC LOADING
The dynamic cyclic loadings is the occurrence of repeated loading under specific
circumstances which built up of cycle, frequency, amplitude and time which consists of
stress – strain relationship under specific cyclic loading factors. They exist may be due to
the environmental factors, such as earthquake, wave, wind activity and etc. or human
activities, such as passing traffic, vibrating machinery installed on a structure or site,
building foundation and etc. Importantly, the soil response generated by these dynamic
cyclic loadings is typically more complex than that considered when conducting static
analyses, requiring engineers to investigate the dynamic behaviour of soils in the laboratory,
as well as in the field. The dynamic loading from wind, waves, equipment vibrations, or
earthquakes will lead to an accumulation of shear strains, and the shear modulus will
degrade once the threshold shear strain has been exceeded [4]. Large cyclic strains and
accompanied strength loss is a matter of concern for engineers especially in earthquake
response analysis. As a result, cyclic strength is usually defined in literature as the cyclic
shear required inducing a significant shear strain under triaxial or simple shear conditions.
3.4 OVER-CONSOLIDATION RATIO (OCR) OF SOIL
The over consolidation ratio (OCR) is a geotechnical parameter, related to historical
changes in the state of stress in the subsoil. The concept of over consolidation ratio was
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proposed by Cassagrande. The idea of recording the over consolidation ratio resulted from
the observation of changes in strength parameters of non-lithified deposits, depending on
the state of stress found in the subsoil. An example illustrating this dependence is an
increase in shear strength of a deposit homogenous in terms of its physic-chemical
properties with an increase in depth. Under such conditions determined strength parameters
of a given deposit, higher than expected, were thus considered to be connected with a
different state of stress in the subsoil than that generated as a result of normal consolidation
of the deposit. It needs to be emphasized that consolidation is considered to be normal if it
results from a successively increasing load, affecting the deposited sediment at a specific
depth. The over - consolidation ratio OCR was defined by Casagrande as a ratio of
maximum effective value of the vertical component of geostatic stress, found at any time in
a given subsoil point, to the present effective value of the vertical component of geostatic
stress.
3.4.1 EFFECTS OF OVER CONSOLIDATION RATIO, OCR ON SETTLEMENT
According to related studies, the over - consolidation state is an important effect for soil
liquefaction potential. If a soil mass has experienced stresses higher than its current state, it
is an over - consolidated soil (OCR > 1). The over - consolidated soils have fewer
settlements due to external loadings as compared with normally consolidated soils. Seed
and Idriss, showed that the liquefaction resistance increases as the over consolidation ratio,
OCR increases [5]. Sarsby [5] stated that the relationship between shear strength, pore
pressure and soil modulus depend on the stress history which is reflected by the over -
consolidation ratio (OCR). The over - consolidated soils, OC usually have lower porosities
than their normally consolidated, NC counterpart. This leads to their having a stiffer
behavior in respect of deformation under applied load, more dilatant behavior under shear
because of denser packing of the particles. The over - consolidation ratio of soil can affect
the settlement analysis for structures constructed on top of saturated cohesive soil.
Considerable settlement due to continued consolidation by the soil’s own weight and
applied structural load are expected if the cohesive soil is under consolidated. On the other
hand, if the cohesive soil is over - consolidated, there would be no significant settlement
when a load is applied to the soil [6].
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3.4.2 EFFECTS OF OCR ON STRESS – STRAIN RELATIONSHIPS
The over - consolidation ratio (OCR) seems to have similar effects on cyclic stress strain
behaviour of clays as it does in monotonic behaviour. For example, it is reported that NC
clays produced positive pore water pressure and shifted the stress path gradually towards
origin during cyclic shearing. On the other hand, OC clays developed negative pore water
pressure at the initial stages. Subsequently they generated positive pore pressure with
increasing loading cycles until failure. Also, magnitude of initial negative pore pressure
increases with OCR as in monotonic loading. Different observations have been reported in
literature regarding the effect of OCR on cyclic strength. The OC clays showed equal or
stronger behaviour than that of NC clays under equal cyclic stress amplitudes. However, it
is also found that number of cycles to failure is little bit lower in OC clays than that of NC
clays [1].
3.4.3 EFFECTS OF OCR ON STRENGTH
The influence of OCR and fines content on the variation of the initial target modulus is
shown in Figure 3.1. As OCR increases, the initial tangent modulus of clean sand
decreases due to the lower current confining stress and consequently, the soil specimens
become weaker. However, the OCR seems to have negligible effect on the initial tangent
modulus of soil specimens with clay fines content [6].
Figure 3.2 shows that the reduction in strength due to the increase of clay fines with
followed by specimens with 10%, 20%, 30% and 40% clay fines. According to Simons and
Menzies (2000), the strength depends on the tendency of soil in dilation, and it decreases
as the normal effective stress decreases [6].
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Figure 3.1: Initial tangent modulus vs OCR [6]
Figure 3.2: Strength vs OCR [6]
3.5 EFFECTIVE VERTICLE CONSOLIDATION OF DYNAMIC TRIAXIAL TEST
Table 3.3 shows the variance of effective vertical consolidation stress applied of the soft
clay under dynamic cyclic triaxial testing from past research within range of 40 to 800 kpa
depends on soil characteristic and geotechnical properties. Effective vertical stress (σ’vc) is
another factor that influences the cyclic behaviour of soils. For example cyclic stress ratio or
liquefaction resistance of sand has a correction factor for effective overburden stress (e.g.
Seed, 1983). However, reports show that the effect of (σ’vc) is not as important in the cyclic
behaviour of clays, as they have already been considered in their monotonic undrained
strength (su), as cyclic strength of clays is usually expressed in terms of su. It is observed
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that pore pressure generation during cyclic loading increased with effective confining stress,
and number of load cycles necessary for failure reached a minimum between effective
confining stresses of 50 and 100 kPa for Champlain Sea clay in Eastern Canada [1]
Table 3.3: Some of soft soil geotechnical properties
Researchers Type of Soil Effective vertical consolidation stress, σv’
(kpa)
J.Ni et.al, [7] Soft Clay 40
- Bangkok Soft Clay
50, 100
Theenathayarl, [1] Leda Clay 50, 100, 200, 400, 800
H. Soltani – Jigheh, [8] Mix Clay 100, 200, 350
- Kolkata Clay 50, 100, 150, 200
Table 3.3 shows the typical of test frequency that can be applied under dynamic
cyclic triaxial testing. Similar to the strain rate effects on the undrained monotonic strength
of clays, loading frequency or rate also influences cyclic strength of, even though this effect
is not as significant in granular soils. A number of investigations on the effect of loading
frequency on cyclic strength of clays show cyclic strength of clays increases significantly
with loading frequency. It has been reported that 30% cyclic strength increase when loading
frequencies have been raised through two log cycles (i.e. increased by about 100 times).
The cyclic strength increases with increasing frequency, the effect of frequency on strength
diminishes when number of load cycles are higher. It noted that shear stresses in excess of
about 40% of their undrained strength have been resisted for more than a cycle during
undrained cyclic simple shear tests due to effects of very high loading frequency. They also
found that very high loading rates in cyclic loading partially compensate the strength
degradation with number of loading cycles.
The loading frequency alters the strength envelope in structured or OC clays, but
not influences the pore water pressure generation much. As a result failure occurs below
the peak strength envelope in OC clays under slow loading rate. On the other hand in
structured or NC clays, changes in loading frequency affects pore pressure generation. As a
result, stress path is altered, but stress path meets a unique strength envelope at failure [1].
It is recognized that actual earthquake loading contains a range of frequency content (0.5 –
5 Hz range is probably the strongest modes in this region), but the effect of cyclic loading
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frequency on liquefaction characteristics of clays is rather. Further, the lower frequency
used would result in a conservative estimate of the cyclic resistance of the material.
Generally, even the most energetic earthquakes do not result in more than about 30
equivalent load cycles of shaking.
The shear modulus and damping of soil are expressed as a function of cyclic strain
amplitude and depend on various parameters such as effective confining stress, Plasticity
index (PI), OCR/Stress history, number of load cycles, loading rate etc. Various researchers
investigated modulus reduction and damping in various soils and the influence of the above
mentioned factors. It was originally provided modulus and damping curves as a function of
cyclic strain amplitude for coarse and fine grained soils such as sands and gravels. Some
analyses the available modulus data for sand and concluded that these curves mainly
depend on effective confining pressure and relative density or void ratio.
Furthermore they found modulus reduction and damping curves for sands falls
within a narrow band and no other factors significantly influence these curves. As a result,
an average modulus reduction curve and limiting curves based on relative density that can
be used in practice for cohesion less sands. Modulus and damping are a functions of shear
strain and can be obtained from stress or strain controlled cyclic loading [1].
The specimens were isotropically consolidated under confining stress of 0.5 kgf/cm2
and 1.0 kgf/cm2 were subjected to cyclic loading with a frequency of 0.1 Hz. Table 3.4
shows the relation between the double amplitude axial strain on the logarithmic scale and
the shear modulus during cyclic loading. It is seen in the table that shear modulus
corresponding to each effective confining stress increases as the effective confining stress
becomes higher. This behavior is in good agreement with the findings table 3.4 and suggest
that the dynamic shear modulus increases with increasing confining pressure. Their
relationship can be plotted as a straight line on a log-log scale at all levels of treatment and
strain amplitude. The relationship between the double amplitude axial strain on the
logarithmic scale and the damping ratio are presented in figure 3.3. It can also be seen that
the damping ratio decreases with increasing double amplitude axial strain. However, the
results presented in figure 3.3 show that at given loading frequency, the damping ratio
increases with increasing axial strain, and again when given axial strain, the damping ratio
increases with increasing loading frequency. When loading frequency increases, the axial
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strain decreases, damping energies decrease together, or damping ratios increase. The
results confirm the general findings by others for undisturbed cohesive soil.
Table 3.4: Some of soft soil geotechnical properties
The shear modulus decreases with the
increasing of confining stress
Th
e shear modulus decreases with the
increasing of frequencies
The damping ratio increases with the
increasing of confining stress
The damping ratio increases with the
increasing of frequencies
The hysteresis loops can be considered in order to calculate the secant shear
modulus, G and damping ratio, D from the slope and the total area of the loop respectively.
It has been observed an increase in shear modulus values with loading frequency from tests
on Leda clay, but they are not significant. It has been investigated that the influence of
loading frequency on modulus and damping ratio at very small strains using cyclic torsional
shear tests. They concluded that shear modulus values of soil are not sensitive to frequency
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variations, however, damping increases with decreasing frequency. However, it was found
damping is independent of changes in loading frequency from tests conducted at very small
strain amplitudes. As OCR increases, the strength of both clean sand and sand with
different clay fines decreases. The addition of clay fines in sand in over – consolidated, OC
state changes the excess pore pressure response significantly. Normally consolidated soil,
NC specimens yield highest peak strength. The peak strength of specimens decreases with
an increase in OCRs. The soil specimens generate positive maximum excess pore pressure
as clay fines increases. Skempton pore pressure parameter, A, values for soil specimens
decrease as OCR increases, but they increase with increasing clay fines content in soil
specimens. The initial tangent modulus of soil specimens increases as clay fines content
decreases. Heavily over - consolidated sand specimens yield lower initial tangent modulus,
but the effects of OCR on initial tangent modulus of sand with clay content is negligible [9].
Figure 3.3: Damping ratio vs axial strain under different OCR
3.6 CONCLUSIONS
Commonly, on past research, shows that normally consolidated, NC specimen were less
deformable than that over consolidated, OC ones but it depends on specific dynamic cyclic
parameter. The cyclic loading can produce a substantial decrease in the shear modulus, G
of the soil. The amplitudes of τcy degrade with the accumulation of pore pressure in clay
specimens as the number of cycle’s increases. The degradation of τcy and pore pressure
accumulation become more significant as strain amplitude increases in both normally and
over consolidated specimens. However, negative pore pressure is developed in over
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consolidated specimens particularly when strain amplitude is small and OCR is high which
can be attributed to dilatant tendency of clay. The hysteresis loops gradually tilt towards the
horizontal axis and gradual softening of soil is noted [9].
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REFERENCES
[1] Theenathayarl, T. (2015).
Behaviour of Sensitive Leda Clay
under Simple Shear Loading.
University of Peradeniya Sri
Lanka. Master of Applied Science
in Civil Engineering.
[2] Chan, C. et.al (2011). Some
Mechanical Properties of Cement,
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[3] Thammathiwat, A., et.al (2004).
Behavior of Strength and Pore
Pressure of Soft Bangkok Clay
under Cyclic Loading. Stress: The
International Journal on the
Biology of Stress, 9(4).
[4] Rees, S. (2014). Part Three:
Dynamic triaxial testing. White
Paper, 5. Retrieved from
http://www.gdsinstruments.com/wh
ite-paper-dynamic-triaxial-testing
[5] Moradi, G., et.al (2015). The
Influence of Overburden Pressure
on Liquefaction Potential, 1–15.
[6] Thian. S. Y, et.al (2011). Stress
History Effect on Mining Sand
with Fines Contents, 2(1), 1–10.
[7] Jiang, M. (2012). Stiffness
Degradation of Soft Marine Clay
under Uniaxial Cyclic Loading.
Vol 17.
[8] Soltani-Jigheh, et.al (2010). Cyclic
behavior of Mixed Clayey Soils.
International Journal of Civil
Engineering, 8(2), 99–106.
[9] Thian. S. Y, et.al (2016). Effect of
OCR on Cyclic Shear Strength
Degradation of Marine Clay.
(2016), 4(1), 280–285.