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DEVELOPMENT OF A CATEGORY OF SEISMOGROUND MODELS BASED ON THE
SEISMIC INDICATORS OF SOILS.
Ismailov V.A.,
Avazov Sh.B.,
Yodgorov Sh.I.,
Yadigarov E.M.,
Khusomidinov A.S.,
Aktamov B.U.,
Mansurov A.F.,
Muhammadkulov N.M.,
Saidmaxmudova O`.B.
Institute of Seismology of the AS RUz
Annotation:
This article highlights the development of seismoground models based on
seismological and geological-geomorphological research. Seismoground models play a crucial
role in assessing the variations of seismic waves in soil layers of different depths and their
impact on structures.Using computational methods, the amplitude-frequency characteristics of
soil layers and the propagation velocities of seismic waves were determined. During the study,
accelerograms of three earthquakes corresponding to real ground conditions were analyzed.
Calculations conducted in the "ProShake" software resulted in the identification of the physical-
mechanical and dynamic characteristics of soil layers.
Keywords:
seismoground models, seismic waves, soil layers, amplitude-frequency
characteristics, accelerogram, ProShake software, seismic hazard.
Seismic soil models are being developed that help to take into account seismic waves in different
layers of soils and their impact, as well as are important in assessing the seismic risk associated
with buildings and structures. Seismic ground models are primarily used to assess the dynamic
properties of the earth and their seismic impact.
Calculation methods allow determining the amplitude-frequency characteristics of the soil layer
and, accordingly, the characteristics of vibrations on the free surface of the site or at internal
points of the medium, modified by the layered medium [1].
To perform calculations using this method, it is necessary to determine the initial seismic impact,
given by an accelerogram or reaction spectrum, and construct seismogeological models of the
soil layer. Real accelerograms of three earthquakes were obtained, corresponding to the
seismological conditions of the study area in terms of their mechanism (descent and rise) and the
nature of seismic wave propagation.
In engineering-seismological and geological-geophysical studies, a seismic soil model is
compiled. This model describes how the layers of the engineering-geological floor respond to
seismic waves. This model includes the lithology, density, transverse wave velocity (Vs),
longitudinal wave velocity (Vp), moisture content, and other important properties of soils[1,2].
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In the "ProShake" program, the values of soil lithology, depth, transverse wave velocity Vs (m/s),
and dynamic modulus of elasticity y (kPa/m) are entered to obtain cross-sections of transverse
wave velocities (Vs) of soil layers.
The lithology of the soil is determined based on the results of engineering-geological data. In
seismic studies, data on the speed of seismic wave propagation in the soil are obtained. The
dynamic modulus of elasticity is determined using the soil layer density values [2,3]. In seismic
prospecting, the product of the formation density 9.81 m/s2 (gravitational acceleration, g) is used
to derive the dynamic modulus of elasticity y (kPa/m).
The dynamic modulus of elasticity has the following relationship with the average bulk density
of the layer:
U=ρ
⋅
g
Here: ρ - density of the soil layer (g/c=sm3); g - gravitational acceleration (9.81 m/s2).
From seismic exploration data, the speed of transverse wave propagation in soil layers is
determined by interpreting seismograms. The average transverse wave velocity (Vs30) is
obtained as a result of calculating the values of the transverse wave velocity obtained from each
layer in the soil layers up to a depth of 30 meters (engineering-geological layer). It is calculated
according to the following formula:
=
Si
i
S
V
h
V
30
30
Here: Vs30 - average transverse wave velocity for a soil layer up to a depth of 30 meters; hi -
thickness of each soil layer; Vasi - the transverse wave velocity of each soil layer.
The data obtained from engineering-seismological and geological-geophysical studies conducted
to study the seismic properties of the soil are used in the formation of a seismic-soil model
(Table 1).
Physico-geological parameters of point No. 197 in the city of Tashkent
Lithologi
Depth
Layer
thickness
Transverse
wave velocity
(VSi), m/s
Density (ρ),
g/cm3
Dynamic
modulus of
elasticity (U)
kPa/m
Loam
and
clay
0.00
1.16
396.6
1,77
17,36
Loam
and
clay
1.16
1.45
397.9
1,77
17,36
Loam
and
clay
2.60
1.81
221.8
1,67
16,38
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Loam
and
clay
4.41
2.26
510.7
1,82
17,85
Loam
and
clay
6.67
2.82
618.1
1,89
18,54
Crushed
stone
9.49
3.53
729.2
1,94
19,03
Crushed
stone
13.02
4.41
949.3
1,99
19,52
Crushed
stone
17.43
5.51
1106.7
2,00
19,62
Crushed
stone
22.95
6.89
1052.9
2,00
19,62
Crushed
stone
29.84
170.2
1191.2
2,20
21,58
----C-
200.00
-----
The value of Vs30 determines the properties of soils to amplify or attenuate seismic waves. The
transverse wave velocity of each layer expresses its density and strength. The value of Vs30 is
one of the main factors in determining the stability of the soil as a foundation.
A synthetic accelerogram was used as the input accelerogram (Fig. 1).
Figure 1. Synthesized accelerogram for the city of Tashkent, PGA-0.274g
The accelerogram was normalized and brought to an acceleration value corresponding to the
acceleration of soils of the first category, consisting of dense conglomerates of Neogene age and
rocky loess, distributed in the territory of the city of Tashkent at a depth of 70-250 m[4-7].
Data on the geological structure and physical properties of soils are the initial data for modeling
the soil reaction to seismic impact. Such modeling is based on the thin-layer method, as well as
the finite element method. This modeling allows taking into account the resonant properties of
the soil layer and assessing the influence of soil conditions on the amplitude, frequency spectrum,
and duration of oscillations [8-10].
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Based on this approach, 728 seismic soil models have been developed in the city of Tashkent. It
should be noted here that in the development of seismic-soil models, seismic exploration results
were used, i.e., changes in the Vs30 value of soils up to a depth of 30 meters.
For each point of the study, such an important indicator of engineering seismology - the reaction
spectrum of soils to seismic influences - was constructed [9,10].
The reaction spectra of the soil layer allow us to analyze the change in the soil's reaction to the
action in different spectral ranges, the smallest change was observed at point 197 (Fig. 2-3).
As a result of modeling, graphs of the maximum acceleration of soils and the change in the
reaction spectrum with depth were calculated.
In this geological column, based on borehole data, the lithological composition, thickness, and
depth of the rocks are shown.
In this case, the rocks are distributed at a depth of 30 meters in the following order: bulk soils -
0.0-0.9 m, sandy loam - 0.9-3.3 m, gravel-gravelly - 3.3-30.0 m.
Also, based on the results of seismic exploration research, the rates of passage of transverse
waves through soil layers at a depth of 30 meters are presented (Fig. 4).
When comparing the velocities of transverse waves through the soil with the data obtained from
drilling wells, it was revealed that the sandy loam layer has a low velocity, and the gravel-pebble
layer has a high velocity.
These indicators can be substantiated by the absorption capacity of transverse waves when
passing through soils.
Thus, the lower the soil density, the higher the absorption capacity of seismic waves, and
conversely, the lower it is in hard rock formations, which in turn determines the velocity value.
Figure 2. Peak acceleration profile of the
observation point soi
Figure 3. Reaction spectrum of the soil layer
at different depths
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Figure 4. Graph of the change in Vs30 according to the engineering geological column and depth
Seismic-soil models perform the following main functions:
Determining the dynamic properties of soils: studying soil layers and how they affect seismic
waves.
Analysis of the seismic stability of structures: assessment of the strength and stability of
buildings and structures in relation to seismic movements [11,12].
Development of anti-seismic risk measures: development of advanced technologies and
structures to increase the seismic resistance of buildings and structures and reduce seismic
vulnerability.
Modeling the propagation of seismic waves: determining the propagation of seismic waves in
different soil layers, which is the calculation of their seismic impact force [10-12].
Seismic hazard assessment: assessment of seismic hazards and identification of highly hazardous
areas due to tectonic movements of the earth and their forces.
These developed seismic ground models are mainly aimed at assessing the seismicity of the
territory of construction sites of various structures, the propagation of seismic wave movements
on the earth's surface (Fig. 5).
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Figure 5. Seismic ground model at various depths.
Based on the data obtained in the soils, a seismic soil model is developed. The seismic soil
model is used in assessing the seismic hazard of the territory, determining the dynamic properties
of soil layers, and providing engineering recommendations for construction.
Seismic-soil models are used as an important source for making reliable decisions on various
seismic conditions and construction projects, as well as ensuring the seismic safety of
construction.
This work was funded by grants from the Academy of Sciences of the Republic of Uzbekistan
“Development of scientific foundations for assessing various levels of seismic risk and reducing
earthquake losses in seismically active areas” and the Agency for Innovative Development,
#ALM202311142839 “Creation of a simulation digital model of the city of Tashkent, allowing
for assessing the level of economic damage from strong earthquakes”, #AL5822012294
“Development of technology for forecasting the risk of severe earthquakes” and
#AL5822012298 “To create an electronic database on the seismological characteristics of the
soil to replace Table 1.1 in the regulatory document. The seismological part of the KMK
“Construction of seismic zones”.
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