Volume 02 Issue 06-2022
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VOLUME
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Pages:
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ABSTRACT
This scientific article presents the results of studies of the nature of changes in the connectivity of moist loess soils
during vibration. Basically, the results of laboratory experimental experiments are presented to determine the
weakening of the connectivity of moistened loess soil and its transition to a dynamically unstable (liquefied) state in
the process of oscillation, which also significantly affect internal and external factors (the state of density-moisture
content of the soil, the presence of colloidal minerals, particle size distribution, the value external load, nature,
duration and intensity of dynamic impact, etc.) to reduce soil cohesion.
KEYWORDS
Loess soil, soil cohesion, moist soil, macroporous soil, fluctuation, soil density-moisture, colloidal minerals,
granulometric composition, duration and intensity of dynamic impact, magnitude of external load.
INTRODUCTION
Macroporous loess soils occupy large areas on the
globe (about 13 million km2, which is approximately 10%
of the land). Large areas occupied by loess soils are
located in the CIS republics (former republics of the
USSR), China, India, Iraq, Afghanistan, Australia, USA,
Canada, Argentina, Uruguay, Brazil, North Africa,
Ruminia, Hungary, Bulgaria, Germany, Poland, France
and many other parts of the world. Most large
industrial and high-rise civil facilities, as well as many
cities, are located in the development zone of loess
Research Article
THE NATURE OF THE CHANGE IN THE CONNECTIVITY OF MOISTENED
LOESS SOILS DURING VIBRATION
Submission Date:
May 30, 2022,
Accepted Date:
June 10, 2022,
Published Date:
June 21, 2022
Crossref doi:
https://doi.org/10.37547/ajast/Volume02Issue06-05
Gayrat Akramovich Khakimov
PhD, Associated professor at department “Design of Buildings and structures”, Tashkent Institute of
Architecture and Civi Engineering, Tashkent, Uzbekistan
Journal
Website:
https://theusajournals.
com/index.php/ajast
Copyright:
Original
content from this work
may be used under the
terms of the creative
commons
attributes
4.0 licence.
Volume 02 Issue 06-2022
27
American Journal Of Applied Science And Technology
(ISSN
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2771-2745)
VOLUME
02
I
SSUE
06
Pages:
26-41
SJIF
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(2021:
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705
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(2022:
5.
705
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–
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–
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macroporous, subsiding soils, so the issues of accident-
free construction on these soils will become
particularly relevant. Some territories of the above
states are located in seismically active zones of the
globe, i.e. in these areas, earthquakes often occur with
a magnitude of 6-9, and sometimes even 10 points on
the International MSK-64 scale. The design and
construction of buildings and structures on loess soils,
in seismic regions, ensuring their strength, stability and
reliable operation is one of the most difficult problems
of modern construction. When designing and erecting
buildings and structures on loess soils in seismically
active areas, serious difficulties arise due to insufficient
knowledge of the nature of the phenomena occurring
in such soils during their fluctuations of various
intensities [1-8]. The difficulty of using macroporous,
subsiding moist loess soils as the foundations of
buildings and structures in seismically active areas is
mainly caused by the following conditions [4,5]:
a)
Loess subsidence soils in natural conditions, being
in a moist state, are characterized by relatively low
strength, which indicates a slight stability of their
structure under dynamic influences;
b)
Buildings and structures erected on moist loess
soils, even with low-intensity seismic effects,
experience significant seismic precipitation due to
structural disturbance and additional compaction
of foundation soils.
The need to take into account these phenomena leads
to the solution of a set of issues related to the
identification of processes that cause dynamic
structural disturbance and subsequent compaction of
moist loess soils. Due to the lack of studies of the
dynamic stability of moistened loess soils, as well as the
developed calculation methods, designers are not able
to correctly take into account soil conditions when
designing buildings and structures. This often leads to
unreasonable solutions to the problems of designing
the foundations of structures and to unjustified
economic costs, and in some cases to severe
consequences during strong earthquakes.
Quantitatively, the degree of compaction of moistened
loess soils depends on many factors: bond strength,
soil porosity, and the intensity and duration of
oscillations. Consequently, the deformation of loess
soils during vibrations is the result of very complex
processes occurring in the thickness of the soil, which
cannot be assessed by individual indicators, for
example, by macroporosity or moisture content, etc.
The deformation of the loess during shaking is
associated with its unstable structure, which is
characterized for loess soils by a weak connection of
structural elements [9, 10].
Despite the long period of study of loess soils, the
origin, as well as the mechanism of their deformation
due to their internal connection, remain unclear. This is
due to the diversity of genesis, properties and
composition, as well as various natural moisture
content of the rocks.
At present, experts have proposed various hypotheses
about the structure of internal bonds of loess soils.
However, in moistened loess soils, these bonds have a
nature that is well described by modern electrokinetic
theory. Additional water saturation of the rock is
always accompanied by swelling of the soil associated
with further thickening of the water shells of the
particles. In this case, the soil particles move away from
each other, leaving the zones of molecular extension,
weakening the bonding forces between the particles.
The force of attraction of water to a particle depends,
in turn, on the thickness of the water shells, with an
increase in which the force of molecular attraction
decreases. This circumstance indicates a relatively
Volume 02 Issue 06-2022
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American Journal Of Applied Science And Technology
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VOLUME
02
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Pages:
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5.
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slight disruption of the structure of water-saturated
loess soils when exposed to a dynamic load [6, 7].
An analysis of experimental data and cases of
numerous damage to buildings and structures during
earthquakes shows that the strength of moistened
loess soils can decrease under certain dynamic
conditions, and the soils themselves can go into a
liquefied state. The strength of loess soils, as is known,
is determined by their internal cohesion. This
circumstance indicates the only possible option for
reducing connectivity due to the transition of bound
water in the water shells of soil particles into free water
during the oscillation process.
Determining the change in the connectivity of loess
soils under dynamic conditions is a known difficulty,
and it becomes more complicated as the duration of
the impact of the dynamic load on the soil decreases.
Fixing at least a few points in experiments within tens
of seconds, coinciding with the duration of the
earthquake, requires very thorough experiments using
the most accurate, continuously recording instruments
and equipment. According to the analysis, to
determine the change in soil cohesion in a few tens of
seconds, a special research technique is needed.
MATERIALS AND METHODS
In our experiments, the nature of the change in loess
connectivity was fixed by immersing a ball into the soil,
installed on the surface of the sample. The ball stamp
method proposed by Prof. N.A. Tsytovich, in order to
determine the change in the magnitude of the
adhesion forces of cohesive soils, very simply,
conveniently and quickly (without unloading the
sample from the vibrating platform) it is possible to
establish the magnitude of soil cohesion before and
after vibration, which in the conditions of our
experiments was not possible in other ways [11,12 ].
This makes it possible to determine the value of loess
soil connectivity according to the formula (when the
ratio of the stamp settlement
𝑙
𝑠
to its diameter
𝑑
𝑠
should be less than 0.1, i.e.
𝑙
𝑠
𝑑
𝑠
≤ 0.1
):
С
w
= 0.18
P
s
πd
s
l
s
where,
𝑃
𝑠
– is the weight of the ball with the load;
𝑑
𝑠
–
is the diameter of the ball;
𝑙
𝑠
– draft of the stamp; 0.18
- the coefficient was found theoretically on the basis of
the established academician. A.Yu.Ishlinsky for plastic
bodies of constancy of the ratio of hardness to yield
strength.
Cohesion Сw , determined by the ball stamp method,
should be considered as some complex characteristic
that allows estimating not only cohesion (adhesion),
but also internal friction for plastic soils to a certain
extent, which can be used, for example, when
calculating the ultimate load on clay soils according to
the formulas of ideally coupled bodies (excluding
friction, which is automatically taken into account by
the value Cw).
The experiments were carried out according to the
following methodology:
1.
Two samples were taken from a single monolith.
After preliminary compaction at a given load on
one of them, the initial value of connectivity was
determined.
2.
The second sample was subjected to dynamic
impact while maintaining the same static load.
After the shaking ceased, a new connectivity value
was determined (Table 1).
Volume 02 Issue 06-2022
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American Journal Of Applied Science And Technology
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VOLUME
02
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06
Pages:
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(2021:
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(2022:
5.
705
)
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–
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Table 1.
Change in the connectivity of loess soils during vibration
Ground
Oscillation
acceleration, mm/s
2
Connectivity, MPa
initial
ultimate
Loess loam
3000
2000
0,05
0,035
0,0025
0,010
Loess-like sandy loam
3000
2000
0,005
0,01
0,0003
0,001
RESULTS AND DISCUSSION
As a result of the experiments, two values of the
connectivity values were obtained, corresponding to
the initial Сw(н) (before vibration) and the final Сw(к)
(after vibration) states of the loess soil.
To elucidate the general nature of the change in the
value of connectivity during vibrations, we relied on
the indications of the deformation in time of the balls,
which were widely used in our experiments. As noted,
the immersion of the ball into the ground and its
velocity during shaking testified to a decrease in the
cohesiveness or viscosity of the soil under the
conditions of the experiment.
All experiments were carried out in triplicate. The
immersion of the ball into the soil and its speed during
vibrations showed a decrease in the soil cohesion value
under the experimental conditions.
However, it must be remembered that in all cases the
loess shaking is associated with the transition of soil
from one state to another (for example, from plastic to
fluid, etc.), which after some time acquires a new state
as a result of compaction under its own weight. This, in
turn, slows down the progressive decrease in soil
cohesion, and in the process of long-term shaking, we
are already faced with the opposite picture, i.e. with a
progressive increase in the value of Сw(к). Obviously,
this is explained by the fact that, with prolonged
shaking, the contacts between the
particles are restored again and free water turns into
bound water, i.e. soil bonds are restored.
On the basis of these studies, it was possible to
conclude that, unlike non-cohesive soils that can
immediately compact after a structural breakdown,
loose water-saturated loess soils with the compaction
process undergo complex internal transformations as
their cohesion is broken under vibration conditions.
Obviously, this process in water-saturated loess soils is
accompanied by a change in their connectivity, which
was confirmed by experiments with a ball.
At the same time, special studies in laboratory
conditions have established that the weakening of soil
cohesion and its transition to a liquefied state depends
on many internal and external factors, and we will
briefly dwell on these factors.
Soil density-moisture state. When conducting an
experiment with cohesive soils with preliminary
compression of samples under a shear load, the total
shear resistance under successively increasing loads
increases not only due to internal friction, but also due
to an increase in cohesive forces under conditions of
increasing soil density and decreasing soil moisture.
This circumstance gives reason to believe that the
values of normal stresses (determining the initial
density-moisture content of the soil) acting in the
thickness of the soil can play a significant role as
Volume 02 Issue 06-2022
30
American Journal Of Applied Science And Technology
(ISSN
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VOLUME
02
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Pages:
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(2022:
5.
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)
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–
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–
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indicators of the dynamic stability of the soil (due to an
increase in cohesion).
To clarify the nature of the phenomena under study,
the author carried out numerous experiments on
undisturbed structures on samples of loess soils with
varying degrees of moisture. At the same time, the
initial and final values of connectivity were recorded,
corresponding to a certain density-moisture content of
the soil after shaking. The initial state of the density-
moisture content of loess soils was created as they
were moistened and subsequently compressed under
various loads (0.01; 0.02; 0.04; 0.06; 0.08; 0.1 MPa) on
pre-compaction devices.
It can be seen from Fig. 1 that the values of the
connectivity of loess-like soil (loess-like sandy loam) at
the same intensity of dynamic impact depend on its
initial value, determined by the state of density-
humidity equivalent to the acting pressure. Moreover,
a more gentle change in connectivity under these
conditions is characteristic of specimens preliminarily
compressed by higher loads. Also, from Fig. 1, there is
a change in the connectivity of compacted loess-like
soil depending on various vibration accelerations, i.e.
with an increase in the acceleration of vibrations, the
soil cohesion decreases.
A similar pattern is also observed from the graph,
which illustrates the change in soil cohesion as the pre-
compaction pressure increases. The higher the
moisture content of the soil and the lower its density
(ceteris paribus), the more significantly the soil
cohesion decreases. This circumstance is explained by
a decrease in the forces of molecular attraction (with
an increase in soil moisture), the magnitude of which
depends mainly on the density-moisture state of the
soil.
The case under consideration is characterized by a
graph (Fig. 2) in the form of a function Сw = f(w),
compiled according to the results of the experiment on
loess-like loam. During the experiment, the tested soil
samples were given different humidity by artificial
soaking. In all cases, pre-moistened soils were
subjected to shaking with intensity α = 3000 mm/s2
while maintaining the static load. With an increase in
humidity up to 10%, even at vibration accelerations of
3000 mm/s2, which is equal to the value of seismic
vibration accelerations at a 9-point seismic impact (
according to the international scale MSK-64), no
disturbance of the soil structure occurred, no decrease
in soil cohesion was observed, and the soil did not
experience any vertical deformations. When a
moisture content of 14-15% (optimal humidity) was
reached, a sharp decrease in soil cohesion occurred.
This is due to a weakening of the cohesion of rocks with
an increase in humidity. This decrease continues to the
degree of humidity Sr = 0,8 and then the cohesion
value tends to a constant value, which is clearly seen
from the graph shown in Fig.2.
It follows that the decrease in the connectivity of the
studied soils occurs most intensively in the range of
moisture content from optimal to water saturation (up
to the degree of moisture content Sr = 0,8).
The results of the experiments once again confirmed
the previously noted conclusion about a sharp
decrease in the cohesiveness of soils with an increase
in their moisture content and a decrease in density.
Thus, the experiments showed a positive effect of
density-moisture (due to the action of pressure Po) on
the value of the initial cohesion of the soil, in addition,
a decrease in the magnitude of the seismic effect
(αcalc), which can transfer the soil into a dynamically
unstable state with an increase in the density-moisture
of the soil. However, in this case, the processes
Volume 02 Issue 06-2022
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American Journal Of Applied Science And Technology
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VOLUME
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Pages:
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5.
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observed in the experiments are distorted, since the
role of the change in the stress state during oscillation
and the duration of the shaking is not taken into
account. This issue related to the nature of
connectivity required a more detailed study.
The role of water-colloidal connectivity.
Colloidal
minerals that are part of soils affect the degree of
damage to their structure during shaking.
The process of hydration is accompanied by a
thickening of the water shells on the particles, which
leads to an increase in the distance between them, and
as a result, to a weakening of the manifestation of
intermolecular forces and the very cohesion of the
soil. The properties of soils containing clay particles are
also largely determined by the group of colloids.
The relationship between the composition of colloidal-
dispersed soil minerals and the violation of their
structure was studied by prof. B.M. Gumensky,
determining the effect of vibration on the properties
of montmorillonite, kaolinite and hydromicaceous
clays at a vibration frequency of 4000
Volume 02 Issue 06-2022
32
American Journal Of Applied Science And Technology
(ISSN
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VOLUME
02
I
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06
Pages:
26-41
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FACTOR
(2021:
5.
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)
(2022:
5.
705
)
OCLC
–
1121105677
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–
5.582
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rpm and an amplitude of 1 mm. Based on these
experiments, B.M. Gumensky made the following
conclusions [9]:
1.
The degree of compaction of clay soils during
vibration does not depend on their initial physical
state, but is determined only by the duration of the
shaking. This is explained by the fact that shaking
causes a redistribution of particles in clays and
causes their denser packing due to the appearance
of free water due to the transformation of
physically bound water during vibration. The
transformation, in turn, depends on the duration of
the shaking. Vibration creates, as it were,
lubrication and the possibility of easier movement
of particles relative to each other. In this case,
bound water, which is in the cells of the framework
and is released during its destruction, also plays a
certain role.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
2.
Vibration has the following effect on the strength
of the structure of clays of different mineralogical
composition. With the initial moisture content of
clays vibrating for 15 minutes, the highest structure
strength was noted for hydromicaceous clay
(compaction 12.5%), the lowest for kaolinite (14.6%),
and the lowest for montmorillonite clay (35.4%).
The large compaction of montmorillonite clay is
explained by the content of a large amount of diffuse
water in it, compared with hydromicaceous and
kaolinite clays.
Consequently, in minerals from the montmorillonite
group, the ability to transition into a dynamically
unstable state is more pronounced than in minerals
Volume 02 Issue 06-2022
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American Journal Of Applied Science And Technology
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VOLUME
02
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Pages:
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SJIF
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(2021:
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)
(2022:
5.
705
)
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–
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–
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from the hydromica or kaolinite groups. This suggests
that the violation of the soil structure depends on the
structure of crystal lattices and colloidal minerals. The
latter indicates an increase in the ability of the
transition to a dynamically unstable state of soils, in the
colloidal shells of which there are montmorillonite
minerals.
The role of the granulometric composition of the soil.
When considering issues related to the violation of the
structure of the soil during vibration, a certain
importance is attached to the size of its particles.
Researchers who have studied the thixotropic
transformation of clay soils set the maximum particle
size limit for the transition of soils to a dynamically
unstable state of 0.005 mm. Views of this kind were
expressed by Winkler. In his opinion, all substances
having a particle size of less than 0.005 mm, if they are
sufficiently friable, can be liquefied in water.
Of interest are the statements of Prof. M.N.
Gersevanova that the necessary condition for soil
liquefaction is colloidal fractions no larger than 0.001
mm. At the same time, there should not be fractions
larger than 0.01 mm in the soil, since they quickly settle.
However, studies by B.M. Gumensky revealed a greater
role for the mineralogical composition of colloidal
particles, rather than grain size [14,13].
To clarify the significance of the particle size in the
possibility of violating their stability, we conducted a
series of experiments with various soils. The soils taken
for testing were characterized by different physical
properties (in terms of density, granulometric
composition, etc.). The main parameter to be
measured in these experiments was the fixation of the
beginning of the immersion of the ball installed on the
surface of the test sample. As already noted, during the
vibration process, the ball sinks into the ground due to
the weakening of its connectivity.
The results of the experiments made it possible to
draw the following conclusions. The transition of
cohesive soils to a dynamically unstable (liquefied)
state does not depend on the particle size and
mineralogical composition, but is determined by the
porosity of the soil, the intensity and duration of the
shaking.
P.L. Ivanov made a similar conclusion for sandy soils.
With appropriate shaking, it was possible to break the
structures of soils having particles of any size. At the
same time, the duration of stay in a liquefied state
depends, under all other conditions, on the particle
size and the amount of heavy minerals in the
composition of the soil. With an increase in the particle
size, the duration of the presence of particles in a
disturbed state is reduced. The role of heavy minerals
in the composition of the soil during the acceleration
or deceleration of the process of breaking its structure
is quite obvious and does not require further
explanation [15, 16].
Intensity and nature of oscillation.
As is known, at
present, in the field of dynamics of non-cohesive soils,
the question of the violation of the static stability of
any non-cohesive soils under the corresponding
dynamic conditions is indisputable, only the magnitude
(intensity) of the applied dynamic load is unknown.
This generally applies to loess soils. However, along
with the intensity, the oscillation duration will also play
a certain role in the violation of dynamic stability.
The results of numerous experiments made it possible
to establish a directly proportional dependence of the
immersion of the ball into the ground on the intensity
of the oscillation. This confirmed the influence of the
Volume 02 Issue 06-2022
34
American Journal Of Applied Science And Technology
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VOLUME
02
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Pages:
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SJIF
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(2021:
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705
)
(2022:
5.
705
)
OCLC
–
1121105677
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–
5.582
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shaking intensity on the violation (weakening) of the
soil strength.
The main task of our research is to establish the
decrease in the cohesion of loess soils depending on
the intensity of the oscillation. For this purpose,
experiments were carried out with various types of
loess soils (sandy loam, loam). The sample was
subjected to dynamic impacts of various intensity.
It can be seen from Fig. 3 that the values of the
connectivity of loess soils, other things being equal,
depend on the intensity of the dynamic impact, i.e.
decrease with increasing intensity, measured by
acceleration.
The decrease in the connectivity of loess soils with
increasing accelerations is unconditional. However, it is
necessary to single out the most significant influence
on the decrease in the cohesion of soils of the
oscillation frequency. As the practice of construction
and analysis of the consequences of earthquakes
shows, for the foundations of structures, the most
dangerous (in terms of violation of dynamic stability)
are high-frequency earthquakes (Gazli, Uzbekistan,
1976, frequency 16 Hz; Tashkent, Uzbekistan, 1966,
frequency 10 Hz, etc.).
The author conducted experiments to determine the
role of vibration frequency in reducing the cohesion of
moist loess soil. The experiments were carried out with
vibration with an acceleration of α=600mm/s
2
. The
change in the vibration mode in these experiments was
achieved due to the oscillation frequency at a constant
amplitude value. Soil cohesion was determined before
and after vibration. The experiments were carried out
at vibration frequencies from 2 to 12 Hz. The frequency
changed stepwise with three-minute intervals (Table
2).
Volume 02 Issue 06-2022
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American Journal Of Applied Science And Technology
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VOLUME
02
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Pages:
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(2021:
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705
)
(2022:
5.
705
)
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–
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–
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Table 2.
Influence of oscillation frequency on the change in the connectivity of moistened loess soil
Ground
Oscillation
acceleration,
mm/s
2
Vibration
frequency, Hz
Connectivity to
vibration, MPa
Connectivity after
vibration, MPa
Loess loam
600
2
0,0041
0,0025
Loess loam
600
6
0,0045
0,0010
Loess loam
600
10
0,0050
0,0004
Loess loam
600
12
0,0046
0
It is noted that at frequencies above 10 Hz (high-
frequency earthquakes), the value of the connectivity
of moistened loess soil decreases to zero even with a
6-magnitude earthquake.
As follows from the performed experiments, the
decrease in the connectivity of moistened loess soil
also depends on the oscillation frequency. The
influence of dynamic action on the decrease in soil
cohesion is more effective if, all other things being
equal, this action is characterized by a high frequency.
With an increase in the acceleration of the oscillatory
movement, as studies have shown, the degree of
change in the strength characteristics of the soil
increases accordingly. However, in many experiments
with a ball, its gradual immersion into the ground was
noted at constant values of the shaking acceleration.
This shows that, along with the magnitude of the
calculated acceleration, the duration of the impact is of
great importance. Depending on it, the degree of
violation of the structure (connectivity) of the soil
increases.
Oscillation duration.
Back in the fifties of the last
century, Prof. N.N.Maslov and Yu.Ya.Velli, in
experiments with water-saturated clays, drew
attention to the great importance of the duration of
ground shaking in the process of violation of its
stability. In particular, Yu.Ya.Velli noted that the value
of critical acceleration for cohesive soils should be
determined taking into account the duration of the
expected shaking. In studies with loess soils, we came
across some of their specific features:
a.
Compaction of moistened loess during shaking
manifested itself after a certain period of time;
b.
The compaction intensity at the initial moment was
characterized by relatively low values.
This indicated the need to take into account the
duration of the shaking along with its intensity when
assessing the seismic resistance of cohesive soils,
which made it possible to make the amount of time
required for the destruction of the soil structure and
the manifestation of its corresponding deformation
primarily dependent on the strength of the bonds. The
instability of the structure of loess soils is explained by
the characteristic weak connectivity of their structural
elements. The bond strength depends on the
composition and water resistance of the aggregate.
The ability of softening and dissolution in water of a
natural cementing substance, which creates a
connection between loess particles, determines
completely or to a large extent the nature of the
connections. The nature of the connectivity of loess
Volume 02 Issue 06-2022
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soils is expressed by the physicochemical nature of the
bonds, their water resistance and mechanical strength.
Let us assume that the soil has a loose structure and
has cohesive forces. The loss of stability of the
structure (liquefaction) of such a soil is possible in case
of violation of the cohesion forces between its
individual particles under the action of pressure on the
contact particles in the process of shaking.
The analysis of studies shows that the following basic
conditions are necessary for the violation of the
structure of loess soils:
1)
Loose composition of soil particles, in which the
porosity of the soil n
n
before the start of the
shaking, causing a violation of its structure, would
be greater than the porosity of the soil n
k
after the
impact of the specified factor, i.e. n
n
> n
k
;
2)
The intensity of the shaking, expressed as
acceleration, must be able to break the forces of
connection between its particles;
3)
The duration of the shaking should be the time
required for the transition of bound water into free
water.
An analysis of these conditions shows that if the
cohesive forces between the soil particles are not
disturbed by the current vibration, the soil does not
deform. Soil deformation does not occur even when
the duration of the oscillation is measured in only a few
seconds (for example, the duration of an explosive
effect).
In nature, there may be cases when, in different zones
of the soil mass, the cohesion forces between particles
are due to cements of different strength. Obviously,
during vibration, the deformation of the particles will
be different in different places, the soil structure will be
preserved where the cohesive forces are the strongest
and are due to more rigid cements. Figure 4 shows the
effect of shaking duration when changing the
connectivity of moist loess-like soils. As follows from
the graphs, the magnitude of the decrease in soil
cohesion within 60-120 sec. when fluctuating with
intensity α = 2500 mm/s
2
is about 5-15 times or more.
With further shaking, the cohesiveness of the soil
begins to gradually increase. The beginning of
intensive deformation of moistened loess soils in the
process of oscillation corresponds to 5-30 sec. and
more from the moment of applying the dynamic load
on the ground. This is explained by the fact that during
the shaking of the loess soil, which has some
connection between particles, the dynamic load is
perceived primarily by these connections, for the
complete destruction of which a certain time is
required. The nature of the change in connectivity over
time obviously depends on the physical and chemical
phenomena in the soil that occur during the oscillation
process.
Volume 02 Issue 06-2022
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02
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SJIF
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5.
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OCLC
–
1121105677
METADATA
IF
–
5.582
Publisher:
Oscar Publishing Services
Servi
From these experiments, it was also possible to trace
the fact that short-term dynamic loads applied to the
soil violated only the weakest structural bonds of the
soil, and subsequent, relatively long-term vibrations
violated these bonds more effectively. Therefore, we
can conclude that the decisive role in the seismic
disruption of the connectivity of moist loess soils
belongs to the oscillation time. To substantiate this
conclusion, experiments were carried out with soils
with the same physical and mechanical properties and
vibration intensity. The duration of the shaking served
as a variable parameter. It should be noted that the soil
compacted by 5 mm at an acceleration of shaking of
1000 mm/s
2
for 3 minutes did not deform at an
acceleration of 1600 mm/s
2
for 40 seconds. So the
dynamic
The stability of loess soils is maintained by the strength
of structural bonds, and along with other factors, the
duration of oscillation plays a certain role in their
disruption.
The greater or lesser resistance of soils to the applied
dynamic load depends on their strength indicators - the
angle of friction and cohesion. The value of the angle
of friction and cohesion in loess soils depends primarily
on moisture and decreases with increasing moisture
content.
Volume 02 Issue 06-2022
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SJIF
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OCLC
–
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METADATA
IF
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5.582
Publisher:
Oscar Publishing Services
Servi
The strength of highly moistened (water-saturated)
loess soils in the plastic state is due only to the
cohesion forces Cw, which have a water-colloidal
character. In plastic loess, both internal friction forces
( φ ) and brittle bonds (Сс ) are practically reduced to
zero. According to studies, these types of soils are
capable of moving into a dynamically disturbed state at
the smallest values of seismic acceleration α
с
, since
they are characterized by small values of critical
acceleration α
cr
[where: α
с
is the maximum seismic
acceleration acting on the soil mass; α
cr
- critical
acceleration-threshold acceleration, determined by
the strength characteristics of the soil structure.
Calculation of foundations composed of weak water-
saturated loesses in seismic regions can be made using
well-known formulas of soil mechanics with the
obligatory observance of the condition decrease in the
strength (cohesion) of the soil during vibration should
be taken into account].
Low-moistened loess soils are characterized by the
presence of internal friction (φ) and cohesion (Cw) in
their strength, and sometimes in a very weak form,
brittle adhesion Cc. To break the stability of such soils,
slightly more acceleration and duration are required
due to their increased strength. However, in this case,
the decisive factor is the state of moisture (on which φ
and Cc depend) and the value of the connectivity of
these soils. As has been repeatedly noted, an increase
in the moisture content of wetted loesses reduces
their connectivity and, accordingly, the magnitude of
the critical acceleration α
cr
. The above facts allow us to
assume the relative invariability in time of the factors
of internal friction ( φ ) and structural cohesion (Сс)
under conditions of shaking of loess soils. It is believed
that the connectivity Cw retains its original state within
the current seismic acceleration up to the critical value,
and subject to the conditional α
с
> α
cr
, it decreases in
time under certain conditions to zero.
Thus, the influence of the duration of oscillations in the
weakening of the dynamic strength of soils is
characteristic only of loess soils that are in moistened
and water-saturated states. This was repeatedly
observed in our experiments, partially described.
In conclusion, it should be noted that in the considered
plan, the task of the study is reduced to establishing
the cohesion of the soil, the strength of which under all
conditions determines the necessary duration of the
shaking. For this reason, many clay soils can be
earthquake resistant if they have the strongest
cohesive forces. The duration of one or more phases of
the earthquake in these cases will be insufficient to
break these links.
CONCLUSIONS AND RECOMMENDATIONS
As we said above, loess soils (loesses, loess-like soils)
are among the most structurally unstable soils in the
group of cohesive soils. In their natural occurrence,
they are low-moisture, macroporous, subsiding, and
generally have a loose structure. Under the influence
of external forces and humidity, their structure is
sharply disturbed, strength (cohesion) decreases, and
the soil is deformed, i.e. are compacted, and buildings
and structures built on them without special measures
to prevent deformation are damaged and can even
lead to destruction. In particular, this moment is more
aggravated in difficult ground conditions, especially in
seismic regions. Therefore, special attention should be
paid to the construction of buildings and structures on
moist loess soils.
Studies of the nature of the change in the connectivity
of moist loess soils during vibration showed:
1.
Reduction of the strength characteristics of
moistened loess soil, the angle of internal friction
Volume 02 Issue 06-2022
39
American Journal Of Applied Science And Technology
(ISSN
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VOLUME
02
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06
Pages:
26-41
SJIF
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FACTOR
(2021:
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)
(2022:
5.
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)
OCLC
–
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METADATA
IF
–
5.582
Publisher:
Oscar Publishing Services
Servi
and cohesion (adhesion) during vibrations with
acceleration
exceeding
the
critical
value
unconditionally. But at the same time, a change in
the strength of connectivity is especially noted,
which is intense in the process of oscillations.
2.
The decrease in the connectivity of moistened
loess soils in the process of shaking is explained by
the transition of part (under certain conditions
completely) of bound water into free water,
increasing the volume of free water in the soil. The
results of the studies showed less than about 50%
of the amount of bound water in pre-vibrated
samples compared to samples investigated
without vibration, which indicates the transition of
bound water into free water in the process of
ground vibration. The dependence of the
transformation of bound water into free water on
the value of soil cohesion, the duration of the
vibration affecting the soil, etc. is established.
3.
The weakening of the connectivity of moistened
loess soils and its transition to a liquefied state
depends
on
the
initial
density-humidity
corresponding to the current stress state of the
soil, water-colloidal minerals, granulometric
composition, on the intensity, nature (in frequency
and amplitude) and duration of the oscillatory
movement. At the same time, in montmorillonite
minerals, the ability to transition into a dynamically
disturbed state is more pronounced than in
minerals from the hydromicas and kaolinite
groups, which is explained by the content of a large
amount of diffuse water in montmorillonite
minerals than in others. The transition of loess soils
to a liquefied state does not depend on the particle
size, but is determined by the intensity and
duration of the shaking. With an increase in particle
size, the time spent in a liquefied state is
correspondingly reduced. The role of actively
acting acceleration and duration of shaking in the
disturbance of the loess soil structure is very
significant.
4.
With an increase in the active acceleration on the
ground, the degree of destruction of the structure
(connectivity) of the moistened loess soil
increases.
5.
Analysis of the research results showed that if the
cohesion (adhesion forces) between the particles
of the soil are not violated by the current vibration,
the soil is not deformed. Soil deformation also does
not occur when the duration of the shaking is
measured in just a few seconds. Despite the fact
that the studied soils were characterized by almost
the same physical and mechanical parameters, the
effect of deformation as a result of shaking of
different duration is not the same.
6.
The compaction of moist loess soils and the
associated liquefaction occurs as the connectivity
that determines their structural strength is broken.
7.
Reducing the connectivity (seismic resistance) of
moist loess soils with an increase in vibration
accelerations is unconditional. However, it is
necessary to single out the most significant
influence on the seismic resistance of soils of
vibration
frequency.
As
the
practice
of
construction and analysis of the consequences of
earthquakes shows, for the foundations of
structures the most dangerous (in terms of
violation of dynamic stability) are high-frequency
earthquakes (Gazli, Uzbekistan, 1976, frequency 16
Hz; Tashkent, Uzbekistan, 1966, frequency 10 Hz,
etc.).
As follows from the performed experiments, the
decrease in cohesion (deformability) of the soil
subjected to research depends on the vibration
frequency. The influence of dynamic action on the
decrease in the cohesion (deformability) of the soil is
Volume 02 Issue 06-2022
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VOLUME
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SJIF
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OCLC
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METADATA
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–
5.582
Publisher:
Oscar Publishing Services
Servi
more effective if, all other things being equal, this
action is characterized by a higher frequency.
8.
Vibration methods of soil compaction give good
results in seismic areas. Our experiments have
shown that under high-frequency dynamic
impacts, the soil structure is easily disturbed and
the soil is effectively compacted. Our studies also
showed an intensive increase in time of the
strength characteristics (especially due to
cohesion) of pre-vibrated moist loess soils. A
particularly great effect is achieved under such
conditions when exposed to high-frequency
vibrations on the ground. Also, in order to achieve
a given soil density, it is necessary to take into
account its certain moisture content, i.e. soils must
be compacted with optimal moisture. This is
necessary to ensure its proper strength,
deformability within certain limits and workability,
in relation to the sealing mechanisms available to
the construction site. With vibrocompaction, the
soil of the bases perceives the dynamic impact on
itself beforehand, before the construction of
buildings, which has a positive effect on the seismic
resistance of the foundations of structures in
seismic areas. Among the soil compaction methods
that ensure the complete elimination of the
subsidence properties of the loess layer, the
method of soil compaction using vibratory
machines (vibratory rollers) is reliable and
economical.
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and
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American Journal Of Applied Science And Technology
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VOLUME
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Pages:
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SJIF
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FACTOR
(2021:
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)
(2022:
5.
705
)
OCLC
–
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METADATA
IF
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5.582
Publisher:
Oscar Publishing Services
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