SU1753433A1 - Method of the dispersed soil density analysis under seismic effects - Google Patents

Abstract

Usage: in engineering seismology and soil mechanics, as well as when calculating the bases of structures in seismically active areas and seismic micro-zoning. Summary of the invention: when choosing the excitation mode, the vibroplatforms simultaneously with the maximum accelerations take into account the maximum speeds of ground vibrations from the expected seismic effects.

Description

The invention relates to engineering seismology and soil mechanics and, in particular, to methods for laboratory investigation of the density of dispersed soils under seismic effects and can be used to calculate the bases of structures in seismically active areas and seismic microzonation.

A known method for studying the change in the density and strength of dispersed soils under laboratory conditions from seismic effects by the magnitude of the dynamic loads 1.

However, this method does not allow oscillatory movements corresponding to ground vibrations during real earthquakes to be transmitted to the entire volume of the studied samples, which leads to a distortion of the obtained results.

The closest in technical essence and the achieved positive effect to the proposed is a method for studying the density of the soil during seismic effects on vibrating platforms (vibrating tables) by the maximum value of the oscillation acceleration. According to this method, according to engineering seismology, the values of the maximum accelerations of the ground oscillations for a given site from the expected earthquakes are taken. Then, soil samples taken from the site are placed on a vibrating platform and examined with fluctuations with accelerations equal to the accelerations of ground vibrations from the expected earthquakes 2.

However, this does not take into account the oscillation speeds and to investigate the change in density from seismic effects, including strong ones, methods and devices are used that transmit vibrations with much smaller amplitudes and velocities to the studied samples than with actual fluctuations of dispersed soils from real earth Sseny, especially strong. This leads to the fact that with vibrations with small amplitudes and

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The oscillation rates of the studied soil samples are transmitted less energy than with earthquakes, since the density of kinetic energy transmitted through any point of the soil array is proportional to the square of the oscillation velocity. As a result, for equal oscillation cycles transmitted to a soil sample under laboratory conditions, the kinetic energy is less than the energy allocated to this sample under real conditions by an amount proportional to the difference of the squares of the vibration velocities during earthquake and when they are studied on vibrating platforms under laboratory conditions. Such a discrepancy between the oscillation velocities and the energy transferred to the sample in laboratory studies leads to overestimated estimates of the seismic stability of the foundations of structures, slopes and slopes of dispersed soils. In addition, when taking into account only the maximum accelerations of oscillations, the high-frequency components of the oscillations, which quickly decay with distance and earthquakes, are revalued, as a factor affecting the dispersed soils of the earth's surface, where the structures are located, are of secondary importance.

The aim of the invention is to increase the reliability of the method for studying dispersed soils.

The goal is achieved by the fact that, when choosing the excitation mode, the vibroplatforms simultaneously with maximum accelerations a take into account the maximum velocities of ground vibrations V during seismic effects according to the ratios f al (2n v); A v / (2л: f), where A is the amplitude; f is the oscillation frequency.

Energy E, transferred to each elementary investigated volume of soil for an equal number of cycles of oscillations by the proposed method, exceeds the energy transferred by the prototype, in EP. (EPRO). (Advert.): (Vnpor.) Times. Since the rate of ground oscillations with increasing intensity (intensity) of earthquakes increases according to the law of geometric progression, the ratio of speeds and energies between the prototype and the proposed method increases dramatically with increasing intensity of possible earthquakes or, for sinusoidal oscillations, with increasing vibration amplitude.

PRI me R. On a VEDS-10A vibratory electrodynamic test bench, vibrational compaction of various sands and their mixtures was carried out at various ratios of frequencies and oscillation amplitudes. By

the control unit and the measurement of vibration parameters were set to constant oscillation frequencies and set certain values of maximum accelerations. but

at low frequencies, the instrument readings by accelerations may not correspond to their actual values and it is recommended to calculate the accelerations by measured values of the amplitudes of the oscillations. therefore

oscillation amplitudes were measured with an accuracy of 0.02 mm by level H-3 using an elevation screw, and then values of maximum velocities and accelerations were calculated according to the amplitudes and oscillation frequencies. After vibrating the sand in a glass with an area of 28.3 cm and a height of initially loose (Id 0-5%) sand of 100 mm for a certain time, the values of its relative

density (Id). Each test was performed with two to three repetitions.

Studies have shown that relative density is associated with both maximum accelerations and maximum

speed fluctuations. Moreover, for the time of vibration of 10 min, the relative density is much more closely related to the speed than to the acceleration of the oscillation, which was confirmed and calculated by the pair correlation coefficients, which were, respectively, 0.93 and 0.35. Thus, with vibration for 10 minutes with an acceleration of 7.8 m / s2 and a speed of oscillation of 13.9 cm / s, Id was 80%, while at almost the same acceleration of 7.9 m / s2 and speed

a fluctuation of 8.3 cm / s Id was only 10%. This dependence was obtained with a relatively small increase in the oscillation velocities. For high velocities characteristic of strong earthquakes, this dependence manifests itself even brighter. In addition, for equal accelerations, with decreasing amplitude and speeds of oscillation, frequency increases, and therefore, with equal time of vibration, the number of oscillation cycles. Despite the smaller number of oscillation cycles, the effect of high speeds on the compaction of sands is more pronounced with a lower frequency and approximately equal or smaller acceleration values,

the final relative density is much higher than at higher frequency, but at lower values of the speeds (amplitudes) of the oscillation. This proves the crucial role of oscillation velocities in

vibration compaction of sands and the need to study changes in the density of sands from seismic effects, taking into account both the maximum accelerations and the maximum oscillation velocities.

Claims (1)

Claim Method A method for studying the density of dispersed soils under seismic effects, including placing a soil sample on a vibrating platform, exciting vibrating platforms with an amplitude A and frequency f based on the maximum accelerations a seismic effects and determining the change in soil density, characterized in that increase the reliability of the method for studying dispersed soils; when choosing the excitation mode of the vibroplatform simultaneously with the maximum accelerations a, take into account the maximum velocities v of seismic effects according to the ratios f or / (v A v / (27rf).