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Rasulov Khayat Zairovich, doctor of Technical Sciences, professor, Tashkent Institute of Architecture and Construction E-mail: [email protected]
METHOD OF "THE SEISMOSTEADY BASIS" FOR THE ESTIMATION OF SEISMICITY OF THE CONSTRUCTION PLATFORM
Abstract: The article is devoted to a statement of substantive provisions of the method developed by the author of "The seismosteady basis" for an estimation of settlement seismicity of a concrete area taking into account properties composing it soils. For this purpose is used the criterion of dynamic stability of breed - the threshold acceleration based on straight of indicators of a soil. As advantage of an offered method its simplicity and comparative accuracy is marked.
Keywords: soil, the seismosteady basis, point, seismic factor, acoustic rigidity, threshold acceleration, factor of seismic stability.
However, we see another picture on clay soils in plastic conditions durability of which strongly depends on their condition of humidity. Humidifying semi-rocky soils (plaster, stone salt, etc.) also can lower their seismo-durability.
Mentioned above leads to research continuation on revealing the true factors which influence the increment of seismicity of various soils at their fluctuation. In this plan we listed strength indicators of a soil and capacity of thickness on dependence on an increment of seismic intensity of breeds.
Noted positions have formed a basis for working out more perfect methods of an estimation of an increment of the seismicity, allowing expressing the phenomena already in a quantitative kind with the account in each special case strength indicators and a concrete thickness composing soils.
So, for definition of an increment seismic mark D. D. Bar-kan's building areas started with quantity of settlement resistance of soil R defined on the basis strengths of characteristics soil of a concrete area that has allowed already on new to approach to an estimation of its increment seismic mark [1]. However such approach to the question decision also is not deprived of some lacks, in connection with absence of the account of a thickness of a fluctuating soil.
According to a method offered by the author of the present report for an estimation of seismicity of a building area and its increment mark it is established on values of threshold acceleration ап, peculiar to the given soil conditions. The concept of threshold acceleration is entered for the first time into a science by O. A. Savinov and further by N. N. Maslov in kind "critical acceleration" [2].
Threshold acceleration а is understood as acceleration
n
of concussion below which keeps the static condition of soils, above this value stability of structure of a soil is broken also it starts to be condensed, showing thus seismic residual
Currently, the definition of the specified seismicity of a platform of building is made on the basis of cards of the seismic microdivision, which allows estimating the intensity of the phenomenon in points. Usually such estimation is accompanied by instructions of maximum quantity of seismic the accelerations necessary for an establishment of seismic factor quantity used for calculation of seismic inertial loading at designing of construction.
All existing methods of seismic micro-division have the common beginning: at division of any territory into districts it is necessary to use the analysis of engineering-geological and hydro-geological data of the studied so-called "a reference soil" area and refer to all signs which correspond to the mark on a card of seismic division into districts. Further, for increment definition mark on other concerned soils we use an indicator of acoustic rigidity (product of density of a soil for speed of distribution in it of seismic waves) and position of level of subsoil waters depending on soils (except rocky and semi-rocky) and afterwards define an identical increment mark, irrespective of physic-mechanical and strength their properties.
As the analysis of buildings and the constructions suffered from the earthquakes shows, the estimation of seismicity of a building areas on indicator of acoustic rigidity is not always justified and can show positive result only at rocky and rather dense soils, characterized by elastic deformations at fluctuation. However, as results of numerous experimental researches testify, that soils in less dense (friable) condition practically do not possess to elastic properties at concussion. Besides statement mark the territories combined not rocky soils on dependence on position of subsoil waters as it follows from the recommendation of seismic micro-division into districts, also not always justifies itself. As an example it is possible to note sandy, large granular (gravel, rubble, etc.) soils seismo-stabil-ity of which practically does not depend on their humidity.
Section 1. Architecture
deformation. For definition of H. Z. Rasulov's threshold acceleration a is offered [3]:
a =
np
g (pntg^wCw )
Y TV '
f w s
(1)
where pn - normal pressure from weight of constructions and weight of the soil which is above considered horizon; p - a corner of an internal friction of a soil; Cw - the general cohesion of a soil consisting from rigid structural and plastic cohesions [2]; Yw - density of a natural soil; T - the fluctuation period; Vs - speed of seismic waves. Indexes n and w specify in dependence of factors accordingly on density and humidity.
Figure 1. Dependence of threshold acceleration on cohesion of a soil. On the line schedule represent different soils
40 42 44 46 48
Density of a soil, %
Figure 2. Character of change of threshold acceleration depending on density various soils
According to this dependence on seismic stability of any soils' structure, first of all, they are defined by their static du-
rability expressed to resistibility to shift , Cw ) and also by their thickness of natural location (TVs) [4]. This position has received the acknowledgement on numerous experiences on various on structure and a condition soils at different dynamic modes (fig. 1-4).
As is known, the soils in each special case are characterized by those or other parameters defining their durability. So, for example, durability rocky soils is caused by the rigid structural cohesion giving to these soil known hardness (fig. 5).
Figure 3. Dependence of threshold acceleration on the general cohesion loess soils. a = 2500-2800 mm/s2
Figure 4. Dependence of threshold acceleration on external loading at a thickness of soil H=200 see
Durability of inconsistent soils in the most friable condition is defined by a corner of an internal friction and normal pressure (fig. 6). With increase in their density some forces of cohesion (fig. 6) start to show. At the same time, durability of clay soils in certain degree depends and on quantity of the cohesion having water-kolloid the nature (fig. 7). It once again testifies that durability of every version of soils is defined by
various indicators depending on a kind, conditions location, and also density-humidity conditions. At the same time, this circumstance testifies the various dynamic stability soils in the bases of constructions. With it is necessary to be considered at an estimation of an increment of seismicity of a concrete building area.
essary to define on values of factor of seismic stability к in
' ' уст
the form of [5]:
о %
о
Li">
m
о m
о
(N
0 1 2 3 4 5 6 External load, t/rrr
Figure 5. Dependence of resistibility to shift of a rocky soil from loading
20 40
External loading, t/m2
Figure 6. Dependence of resistibility to shift of a sandy soil from loading. Porosity decreases from below upwards
According to developed method of "The seismosteady basis", the increments mark a concrete building area it is nec-
fc =0Р-
уст ' )
a
(2)
where ac - the maximum seismic acceleration on a card of seismic division into districts.
Conformity condition mark a concrete area with mark, established for the given region ("The reference soil") is k =1.0.
ycm
Finally the quantity of factor of seismic stability k , revealed under the formula (2), is used in calculations as the correction factor considering soil conditions. This account in the most simple form can be carried out in a kind:
k
kp =k-, (3)
c k
ycm
where kp - settlement value of factor of seismicity; kc - the factor of seismicity defined for average soil conditions on cards of seismic zoning.
Advantage of the offered method of "The seismosteady basis" is its simplicity and comparative accuracy. Work by definition of parametres of resistibility to shift and thickness of a layer can be executed in engineering - geological expeditions and the building company without additional expenses. One of advantages of an offered method is reliability of the received results, based to the account in each concrete case strength soils characteristics.
External loading, t/m2
Figure 7. Resistibility to shift of a loamy soil the sample at various humidity. On the schedule moisture content is killed from below upwards
References:
1. Barkan D. D., etc. Influence of properties soils the bases at calculation of constructions on seismic influence // Materials of Ill-conference on dynamics of the bases and underground constructions.- Tashkent: Publishing house "Fan": - 1975.-P. 54-57.
2. Maslov N. N. Condition of stability of the water sated sand.- M: Publishing house "Energy",- 1959.- 328 p.
3. Rasulov H. Z. About critical acceleration soils at Earthquakes // J. "Building and architecture of Uzbekistan",- 1975.-No. 6.- P. 17-20.
4. Medvedev S. V. Engineering seismology - M: "Building publish",- 1962.- 284 p.
5. Rasulov H. Z. Seismic stability of the soil bases.- Tashkent: Publishing house "Uzbekistan",- 1984.- 192 p.
