جستجوی مقالات مرتبط با کلیدواژه "سرعت موج برشی" در نشریات گروه "مهندسی زلزله"

The environment is constantly exposed to various pollutants. Petroleum products may contaminate soils located next to industrial areas and other facilities. Oil pollution is one of the pollutions that can cause irreparable damage to the environment. Every day, a large amount of petroleum products enters the environment in various ways. Oil pollution affects the mechanical, chemical and dynamic properties of the soil. Changing the geotechnical properties of the soil is an important issue for structures adjacent to or on oil contaminated soil that can cause collapsing or variation in soil resistance. Since the behavior of many structures and foundations during dynamic loads is in the range of small strains, investigating and evaluating the velocity of waves in the soil skeleton can provide researchers and engineers with useful and significant information about the small-strain behavior of the soil. The importance and value of oil industry structures in Iran, as one of the active countries among oil exporting countries and as a country with a high level of seismicity, has made the research a vital way to improve the design level and accuracy of the behavior of structures exposed to pollution. Despite the wide range of oil industry structures in Iran and the other countries, there is limited literatures on oil-contaminated soil behaviors. Heretofore, the effects of diverse kinds of hydrocarbon contaminants on majority of geotechnical properties of clay soils such as grain size, hydraulic conductivity, plasticity, compressibility, internal friction, cohesion, and shear strength have been investigated. However, there has not been a concentrated research study examining shear wave velocity of hydrocarbon-contaminated clay soils as an important geotechnical property of soil due to the fact that, in small/very small strain levels, the maximum shear modulus of soils can be determined using shear wave velocity. This study aimed to measure the shear wave velocity and consequently, identify the shear modulus of clay soils in oil-contaminated condition with different percentages of contamination, and to compare them with non-oil-contaminated clay soils on small-strain range, using a Bender Element system. In order to prepare comparable clean and contaminated samples (containing 2, 4, 6, 8, 10 and 12 weight percent (wt%) of crude oil, respectively), it was performed similar to the density test method. In this regard, all the clean and contaminated clay samples were tested with a minimum moisture content equal to the optimal moisture content. Bender element tests were conducted on the identically prepared clean and contaminated clay samples at various amounts of frequency (5–20 kHz) and under various confining pressure (50–500 kPa) to find the best method for accurately determining shear wave travel time in the Bender Elements tests. Thereafter, Bender Elements placed in triaxial cell in Razi university laboratory. Bender Elements test conducted to examine shear wave velocity in the clean and contaminated specimens. As a contribution to the literature and potential engineering application, the experimental test results indicated that whilst adding 10wt% of crude oil in clay samples with an increase in the confining pressure, it correspondingly increase the shear wave velocity and consequently increase the shear modulus among of all samples. In addition, at each level of confining pressure, the shear modulus of clean clay was lower than the other contaminated samples. Moreover, under all confining pressures and excitation frequencies, the addition of 10wt% of crude oil caused significant changes in the maximum shear modulus, which was due to the effects of hydrocarbons on the behavior of clay particles. The degree of change due to hydrocarbons is highly dependent on the amount of confining pressure, so that the more the confining pressure is increased; the shear waves velocity an then the shear modulus is increased.

Shear wave velocity, Vs, is a useful soil mechanical property for determining the low shear strain (γ ≈ 0.0001%) elastic shear modulus, which is required for both static and dynamic response analyses of earth structures. It is an important geotechnical soil property for the design and analysis of geotechnical structures. It can be employed to determine the maximum or small-strain (≤10-3%) dynamic shear modulus (Gmax or G0) of the soil mass. Vs can be easily obtained using laboratory or in situ testing techniques. Laboratory tests can be carried out on undisturbed soil samples by simulating the field stress conditions or reconstituted soil samples from a site. However, obtaining good quality undisturbed samples of granular soil deposits is very difficult and moderately expensive. In most geotechnical investigations, the seismic properties (P- or S-wave velocity and the dynamic elastic properties) of granular soil layers from in situ tests (such as the seismic cone penetration test) cannot be determined because of the high cost of undisturbed specimens and the need for highly specialized personnel and equipment. There is increasing interest in the use of Vs to study soil particle properties (e.g. shape, elastic properties, gradation) and the soil state and fabric (e.g. void ratio, boundary stress). In the current study, the bender element and resonant column tests were conducted on sand-gravel specimens. A bender element is an electro-mechanical transducer composed of two-layers of piezo-ceramic plates that are cross-sectionally polarized. This allows for straightforward wave velocity measurement of soil specimens. The BE converts electrical energy to mechanical energy (movement). The resonant column device is commonly used in the laboratory to measure the low-strain properties of soil. This includes the dynamic properties (shear modulus and damping ratio) of soil specimens at strain levels of 10-6 to 5×10-3 conducted according to ASTM standards. In the RC test, Vs can be calculated using the solution of the equation for the linear vibration of a column-mass system. The RC device is the most reliable laboratory method for measurement of Vs. The aim of this research was to explain the effects on Vs of the relative density, mean effective stress, grading characteristics, consolidation stress ratio and initial fabric anisotropy produced during specimen preparation. Five gradations of gravelly and sandy material were used to study the influence of grading characteristics, consolidation stress ratio, depositional method, relative density and mean effective stress on the dynamic properties of granular soil. The pure sand was clean, uniformly-graded fine sand with a mean grain size of 0.6 and a silt content of less than 1% that was classified as SP according to the unified soil classification system (USCS). The pure gravel was uniformly-graded soil with a maximum particle size of less than 16 mm that was classified as GP according to the USCS. The measured values from the resonant column and bender element tests also were compared. Comparison of Vs-BE and Vs-RC shows that the results obtained by both techniques were in acceptable agreement for all specimens; however, there was a slight difference between the two techniques at low values of Vs in which Vs-BE was consistently lower than the corresponding values of Vs-RC. The results of these tests were employed to develop a generalized relationship for predicting the Vs of granular soil. The Vs model was validated using experimental data from the current study and from previous studies. The results indicate that the proposed model is capable of predicting the Vs of granular soil.

Shear wave velocity is one of the most important geophysical parameters in which the seismic response of the sites is expressed. This seismic parameter gives valuable information about the project site, but since geophysical tests are usually expensive and time consuming, the use of indirect methods to reduce costs is increasing. Much research has been done in different regions of the world, in most of them have used both simple power equations and multiple power equations to derive equations. However, in this paper, a comprehensive study is conducted to evaluate the effects of standard penetration number and depth on shear wave velocity estimation as one of the most important soil dynamic parameters with Bayesian statistics approach. In summary, data from 28 boreholes drilled in three cities of Hormozgan province were collected. The collected data were subdivided into four main categories of all soils: clay soils, silty soils, sandy and gravel soils. In this way, the researchers identified 8 variables and 13 functions with the help of Bayesian statistics to determine the mathematical functions with greater reliability, taking into account standard penetration number, depths or a combination of both. The results of this analysis showed that the standard penetration number parameter alone for all soils and classifications as well as simple and multiple power equations are not the best parameter and equations in predicting shear wave velocity, respectively. Other results can be pointed out that soil clustering is not always the most effective factor in estimating shear wave velocity. Finally, it is suggested that if the correlation equation is defined on the basis of standard penetration number with higher confidence percentage, the equation will be extracted by intervals from standard penetration number. In addition, despite the results, it should be noted that these equations have been developed for a specific site and these results should be used with regard to site specific arrangements with other geotechnical conditions.

Under small strains (ε≤10−3%), the shear-wave velocity (Vs) and its resultant maximum shear modulus (Gmax) are important parameters in geotechnical engineering calculations and soil dynamics analyses. At present, the shear wave velocity of sand is typically determined using measurement and theoretical analysis methods. The measurement methods include in-situ and laboratory tests. In-situ tests are commonly conducted using a borehole method or a surface wave dispersion analysis method. Laboratory tests include bender element tests, resonant column tests, ultrasonic tests, and dynamic triaxial tests. In this regard, the evaluation of the influences of soil particle size on the dynamic behaviour of soils during wave propagation has been an important issue in geotechnical engineering. Heretofore, the effects of particle size on shear-wave velocities in soils have been examined using various experimental techniques. Most of this research was carried out over a limited range of particle sizes, and the results indicated various effects of particle size on shear-wave velocity: there has been no comprehensive and unambiguous outcome describing the influences of particle size on shear-wave velocity in soils. This research focused on the influences of particle size on shear-wave parameters in a particular type of sandy soil. A digitally controlled triaxial testing machine equipped with bender elements was used. A significant advantage of bender element test is that it can be incorporated in standard soil mechanics apparatuses such as triaxial and oedometer devices, and the approaches for data interpretation are relatively simple. This research aims to experimentally examine the effects of a wider range of particle sizes on shear-wave velocity and other shear-wave parameters, transmitted in dry sandy soils, using a bender element apparatus embedded in a triaxial testing machine under confining pressures of 50-500 kpa. In this research, the sandy soil was initially categorized into 10 different groups using ASTM standard sieves, and all triaxial samples were prepared with an identical void ratio. The void ratio plays a vital role in the determination of the maximum shear modulus of soil. For all ranges of particle size, the maximum and minimum void ratios were determined, in order to provide an acceptable level of comparison among the results, all samples were prepared with a single void ratio of 0.80. In this study, homogeneously identical samples were assumed as a prerequisite for all experiments. Therefore, it was necessary to take practical measures to ensure this crucial prerequisite in all specimens. In this regard, various experimental methods may be used to achieve a desirable void ratio, including the wet and dry tamping method, dry pouring technique, and water precipitation methods. In this study, the dry tamping method was carried out to prepare similar specimens with an identical void ratio. To measure the shear-wave travel time, the frequencies between 5 and 12 kHz were used. The significant results obtained in this study were as follows. 1) With reference to different methods of determining the shear-wave travel time, the results of this research showed that the cross-correlation and peak-to-peak methods gave the most reasonable values of the shear-wave velocity. 2) The outcomes revealed that, in a particular soil sample, as the excitation frequency increases, the received signals possess significant amounts of higher frequency components, and surprisingly, these signals are similar in shape. 3) Particle size influences the shape of the received signals, such that the frequency content of received signals in both fine and coarse grained soils are quite similar, but medium-sized soils increased with increasing confining pressure. 5) The results showed that the increasing size of soil grains leads to increased shear-wave velocity in a particular range of particle sizes, and decreased shear-wave velocity in the other range. 6) Although the effects of particle size on shear-wave velocity were the main subject of this study, it seems that this factor alone cannot dominate, and other factors must also be considered, such as the type and shape of particles and the surface roughness.

Most of the previous studies on liquefaction of soils have been concentrated on sandy soils since it was traditionally believed that only clean sandy soils are prone to liquefaction and those containing fine or coarse portions are unliquefiable. However, the evidences during past earthquakes have shown that sandy-gravel composites can also liquefy under certain circumstances. Although a number of recent laboratory studies have considered the liquefaction behavior of sand-gravel mixtures, some aspects of this behavior have not fully been understood yet. In this study, a series of undrained cyclic triaxial tests was performed to characterize the liquefaction behavior of sand-gravel mixtures with different gravel contents (GC) and relative densities (Dr). The tested soils comprised of Firoozkooh silica sand (no. 161) mixed with different amounts of a river gravel obtained from Tehran. In order to characterize the behavior of the mixtures in small strain range, shear wave velocity (Vs) of all tested specimens was measured using bender element tests. Combining all testing data, the effects of GC and Dr values on the liquefaction resistance and shear wave velocity of the samples were quantified. The tested samples had a diameter of 100 mm and a height of 200 mm and were prepared according to the wet tamping technique. The tests were performed on samples with three different Dr values of 10%, 30%, and 50% and five different GC values of 0%, 10%, 30%, 50% and 75%. In addition, for any combination of Dr and GC values, the tests were repeated with three different cyclic stress ratio (CSR) values, namely, 0.1, 0.15 and 0.2. The testing procedure included sample preparation, saturation, consolidation under a confining stress of 100 kPa, Vs measurement with bender elements and application of a cyclic deviatoric stress at the desired CSR value. The results of bender element tests show that for a constant GC value, Vs of the sample increases with its relative density. In addition, at any constant Dr, Vs increases as the GC value in the mixture increases up to 50%. In order to quantify the liquefaction behavior of the mixture, the variation of cyclic stress ratio (CSR) versus number of cyclic stress cycles (N) required to cause initial liquefaction in the samples defined by a double amplitude strain value of 5% or an excess pore pressure ratio (ru) of 1.0 were also scrutinized. The results demonstrate that N decreases with increasing CSR values. For all GC values, as the relative density of the mixture increases, the liquefaction resistance increases accordingly. The variation of liquefaction resistance of the mixture at a constant Dr value with gravel content shows that as GC increases up to 10%, the liquefaction resistance increases as well; however, with further increasing GC to 50%, the liquefaction resistance decreases and further increase in GC up to 75% increases the liquefaction resistance of the mixture. In other words, the highest liquefaction resistance is observed for a GC value of 75%. It should be added that at higher Dr values, the effect of GC on liquefaction resistance was found to be more significant. To interpret this behavior, it was further showed that the liquefaction behavior of the mixture can be well illustrated by dividing its behavior into two distinct parts, i.e. the sand-controlled and the gravel-controlled parts. For a sand-controlled part, the interfine void ratio (ef) can be used to characterize the liquefaction behavior of the mixture while within the gravel-controlled portion, the interangular void ratio (ec) is better to be adopted. These two definitions of void ratios can be substituted for Dr, which was conventionally used to quantify the liquefaction behavior of the soils. Finally, the correlation between the liquefaction resistance of the mixtures with their Vsvalues was evaluated. The observations describe that for a constant GC value, as Vs of the mixture increases, the liquefaction resistance increases as well whereas for a constant Vs value, with increasing GC up to 50%, the liquefaction resistance decreases. The reason is that in order to keep Vs constant during increasing the GC value, the sand matrix of the mixture should be kept looser and therefore, the mixture whose behavior in this case is profoundly controlled by the sand matrix, shows a lower liquefaction resistance.

Liquefaction is one of the most common ground deformation effects of earthquakes often a major cause of damage and destruction to buildings and infrastructures. The area on the study (Chalus and Nowshahr region) is located on the coastal strip of Caspian Sea, on the loose alluvial material which has been hit by numerous earthquakes throughout history. In this study, the shear wave velocity (Vs) method has been used with five experimental relationships to evaluate the soil liquefaction potential of Chalus and Nowshahr region under the assumption of non-cementation and cementation conditions. Due to the distribution of boreholes evaluated in the sediment section of the area which is less than 10,000 years old, solutions with noncemented conditions are acceptable. Analyses have shown that among 46 borehole loops, 35 boreholes are prone to liquefaction, which most of them are located in the region of Nowshahr. Considering the values of the liquefaction potential index obtained, based on the shear wave velocity method, in non-cementation conditions, 8.7% of area is PL = 0, 15.2%, in the range of 5> PL> 0, 41.3% in the range of 15> PL> 5 and 34.8% in the range of 15< PL. Due to the fact that the surface depth is an important parameter in soil lubricity, and the high level of the station, considering the high potential of seismicity of the area as well, the likelihood of liquefaction increases.

Horizontal to vertical spectral ratio technique on single station ambient noise data, is a well-known technique in study of site effect. Recently, this technique is introduced as a tool for identification of shear wave velocity profile of soil beside its normal usage.
Many studies in recent years showed that the ellipticity of the fundamental mode of Rayleigh waves can be obtained by reducing the Love and Body waves effects from the H/V spectral ratio. Based on the relation of the H/V curves and the ellipticity of Rayleigh waves and dependency of ellipticity to the shear-wave, this method can retrieve the S-velocity structure in a thick alluvial deposit.
In this paper, HVTFA and RayDEC methods are used to retrieve the ellipticity curves for more than 140 single-station ambient noise measurements. The HVTFA technique based on time-frequency analysis with Continuous Wavelet Transform tries to reduce the SH-wave influence that is possible by identifying P-SV wavelets along the signal and computing the spectral ratio from these wavelets. It is assumed that the energetic points in time-frequency representation of the vertical signal is related to a single Rayleigh wave wavelet. The average over all wavelets defines as ellipticity.
Based on random decrement technique, the Ray DEC method uses the vertical component as a master trigger and stacks a large number of horizontal and vertical signals from three-component records of seismic noise to obtain ellipticity curves.
The right flank of ellipticity curves (from the first peak of curves to the next trough) were used in inversion, because numerical studies show that the right flank is the most reliable part of ellipticity, and the energy of the Rayleigh-wave fundamental mode strongly dominates in these frequency ranges.
In the following, ellipticity curves were classified based on the f0 peaks and the right flanks in two ways; visual observation of similarities and k-means clustering statistical approach.
Inversions process performed using the Neighborhood Algorithm based on the partition of the parameter space into Voronoi cells. The Voronoi decomposition of the parameter space is the base of an approximation of the misfit function, which is progressively refined during the inversion. The method uses prior information (initial parameterizations) and try to optimize the computation at the different stages of inversion.
The results of inversion show the existence of the thick alluvial deposits in the northern and eastern parts of the city. For the southern parts, the method shows higher velocity and lower depth of bedrock. These results are in agreement with geological situation of the region, existence of mountainous area at the southern and western parts, and extensive alluvial plains at northern and eastern parts.

Nowadays, the use of zonation maps such as ground type zonation, geotechnical parameters, seismic hazard intensity as well as landslides zonation maps have been deployed. These maps are prepared with regard to special purposes and are based on widespread data banks and existing standards. Using these maps, projects costs can be decreased and decision making speed in engineering judgments will increase. In this paper, a new empirical relation is recommended between dynamic probing tests results and shear wave velocity using geotechnical data and dynamic probing tests data related to different sites and the results of down hole tests in 10 different locations in Qom plateau.