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1.
Intergrain contact density indices for granular mixes I： Framework   总被引：2，自引：0，他引：2  
Mechanical behavior such as stress-strain response, shear strength, resistance to liquefaction, modulus, and shear wave velocity of granular mixes containing coarse and fine grains is dependent on intergrain contact density of the soil. The global void ratio e is a poor index of contact density for such soils. The contact density depends on void ratio, fine grain content （Cv）, size disparity between particles, and gradation among other factors. A simple analysis of a two-sized particle system with large size disparity is used to develop an understanding of the effects of Cv, e, and gradation of coarse and fine grained soils in the soil mix on intergrain contact density. An equivalent intergranular void ratio （ec）oq is introduced as a useful intergrain contact density for soils at fines content of less than a threshold value Crth. Beyond this value, an equivalent interfine void ratio （ef）eq is introduced as a primary intergrain contact density index. At higher values of Cv beyond a limiting value of fine grains content CVL, an interfine void ratio ef is introduced as the primary contact density index. Relevant equivalent relative density indices （Drc）eq and （Drf）eq are also presented. Experimental data show that these new indices correlate well with steady state strength, liquefaction resistance, and shear wave velocities of sands, silty sands, sandy silts, and gravelly sand mixes.  相似文献   

2.
Whether the presence of non-plastic silt in a granular mix soil impact its liquefaction potential and how to evaluate liquefaction resistance of sand containing different amounts of silt contents are both controversial issues. This paper presents the results of an experimental evaluation to address these issues. Two parameters, namely, equivalent intergranular void ratio (ec)eq and equivalent interfine void ratio (ef)eq, proposed in a companion paper (Thevanayagam, 2007) as indices of active grain contacts in a granular mix, are used to characterize liquefaction resistance of sands and silty sands. Results indicate that, at the same global void ratio (e), liquefaction resistance of silty sand decreases with an increase in fines content (CF) up to a threshold value (CFth). This is due to a reduction in intergrain contact density between the coarse grains. Beyond CFth, with further addition of fines, the interfine contacts become significant while the inter-coarse grain contacts diminish and coarse grains become dispersed. At the same e, the liquefaction resistance increases and the soil becomes stronger with a further increase in silt content. Beyond a limiting fines content (CFL), the liquefaction resistance is controlled by interfine contacts only. When CFCFth, at the same (ef)eq, the cyclic strength of a sandy silt is comparable to the host silt at a void ratio equal to (ef)eq.  相似文献   

3.
Whether the presence of non-plastic silt in a granular mix soil impact its liquefaction potential and how to evaluate liquefaction resistance of sand containing different amounts of silt contents are both controversial issues. This paper presents the results of an experimental evaluation to address these issues. Two parameters, namely, equivalent intergranular void ratio （ec）eq and equivalent interfine void ratio （ef）eq, proposed in a companion paper （Thevanayagam, 2007） as indices of active grain contacts in a granular mix, are used to characterize liquefaction resistance of sands and silty sands. Results indicate that, at the same global void ratio （e）, liquefaction resistance of silty sand decreases with an increase in fines content （Cv） up to a threshold value （Crth）. This is due to a reduction in intergrain contact density between the coarse grains. Beyond Crth, with further addition of fines, the interfine contacts become significant while the inter-coarse grain contacts diminish and coarse grains become dispersed. At the same e, the liquefaction resistance increases and the soil becomes stronger with a fttrther increase in silt content. Beyond a limiting fines content （CrL）, the liquefaction resistance is controlled by interfine contacts only. When Cr〈Crth, at the same （e）eq, the liquefaction resistance of silty sand is comparable to that of the host clean sand at a void ratio equal to （ec）eq. When CF〉CFth, at the same （ef）eq, the cyclic strength of a sandy silt is comparable to the host silt at a void ratio equal to （ef）eq.  相似文献   

4.
The objective of this study is to develop an analytical methodology to evaluate the effectiveness ofvibro stone column (S. C.) and dynamic compaction (D.C.) techniques supplemented with wick drains to densify and mitigate liquethctionin saturated sands and non-plastic silty soils. It includes the following: (i) develop numerical models to simulate and analyze soil densification during S.C. installation and D.C. process, and (ii) identify parameters controlling post-improvement soil density in both cases, and (iii) develop design guidelines for densification of silty soils using the above techniques. An analytical procedure was developed and used to simulate soil response during S.C. and D.C. installations, and the results were compared with available case history data. Important construction design parameters and soil properties that affect the effectiveness of these techniques, and construction design choices suitable for sands and non-plastic silty soils were identified. The methodology is expected to advance the use of S.C. and DC. in silty soils reducing the reliance on expensive field trials as a design tool. The ultimate outcome of this research will be design charts and design guidelines for using composite stone columns and composite dynamic compaction techniques in liquefaction mitigation of saturated silty soils.  相似文献   

5.
For sites susceptible to liquefaction induced lateral spreading during a probable earthquake, geotechnical engineers often need to know the undrained residual shear strength of the liquefied soil deposit to estimate lateral spreading displacements, and the forces acting on the piles from the liquefied soils in order to perform post liquefaction stability analyses. The most commonly used methods to estimate the undrained residual shear strength (Sur) of liquefied sand deposits are based on the correlations determined from liquefaction induced flow failures with SPT and CPT data. In this study, 44 lateral spread case histories are analyzed and a new relationship based on only lateral spread case histories is recommended, which estimates the residual shear strength ratio of the liquefiable soil layer from normalized shear wave velocity. The new proposed method is also utilized to estimate the residual lateral displacement of an example bridge problem in an area susceptible to lateral spreading in order to provide insight into how the proposed relationship can be used in geotechnical engineering practice.  相似文献   

6.
The mechanisms of seismically-induced liquefaction of granular soils under high confining stresses are still not fully understood. Evaluation of these mechanisms is generally based on extrapolation of observed behavior at shallow depths. Three centrifuge model tests were conducted at RPI‘s experimental facility to investigate the effects of confining stresses on the dynamic response of a deep horizontal deposit of saturated sand. Liquefaction was observed at high confining stresses in each of the tests. A system identification procedure was used to estimate the associated shear strain and stress time histories. These histories revealed a response marked by shear strength degradation and dilative patterns. The recorded accelerations and pore pressures were employed to generate visual animations of the models. These visualizations revealed a liquefaction front traveling downward and leading to large shear strains and isolation of upper soil layers.  相似文献   

7.
Short-leg shear wall structures are a new form of building structure that combine the merits of both frame and shear wall structures. Its architectural features, structure bearing and engineering cost are reasonable. To analyze the elastic-plastic response of a short-leg shear wall structure during an earthquake, this study modified the multiple-vertical-rod element model of the shear wall, considered the shear lag effect and proposed a multiple-vertical-rod element coupling beam model with a new local stiffness domain. Based on the principle of minimum potential energy and the variational principle, the stiffness matrixes of a short-leg shear wall and a coupling beam are derived in this study. Furthermore, the bending shear correlation for the analysis of different parameters to describe the structure, such as the beam height to span ratio, short-leg shear wall height to thickness ratio, and steel ratio are introduced. The results show that the height to span ratio directly affects the structural integrity; and the short-leg shear wall height to thickness ratio should be limited to a range of approximately 6.0 to 7.0. The design of short-leg shear walls should be in accordance with the "strong wall and weak beam" principle.  相似文献   

8.
Based on the empirical Gardner equation describing the relationship between density and compressional wave velocity, the converted wave reflection coefficient extrema attributes for AVO analysis are proposed and the relations between the extrema position and amplitude, average velocity ratio across the interface, and shear wave reflection coefficient are derived. The extrema position is a monotonically decreasing function of average velocity ratio, and the extrema amplitude is a function of average velocity ratio and shear wave reflection coefficient. For theoretical models, the average velocity ratio and shear wave reflection coefficient are inverted from the extrema position and amplitude obtained from fitting a power function to converted wave AVO curves. Shear wave reflection coefficient sections have clearer physical meaning than conventional converted wave stacked sections and establish the theoretical foundation for geological structural interpretation and event correlation. ＂The method of inverting average velocity ratio and shear wave reflection coefficient from the extrema position and amplitude obtained from fitting a power function is applied to real CCP gathers. The inverted average velocity ratios are consistent with those computed from compressional and shear wave well logs.  相似文献   

9.
《国际泥沙研究》2016,(2):120-130
Siltation,a phenomenon resulted from the presence of fine particles in an aqueous environment,dominated by silt and clay,is a known and common environmental issue worldwide.The accumulation of fine sediments engenders murky water with low oxygen levels,which leads to the death of aquatic life.Thus,investigating the physical and mechanical properties of fine sediment by rheological methods has expanded.Rheology is the science of deformation and flow of matter in stress.This survey investigates the rheological behavior of six samples of soil as the fine particles structure(D 63 μm) from different regions of Malaysia by using a rotational rheometer with a parallel-plate measuring(using two sizes:25 mm and 50 mm) device to explore the flow and viscoelastic properties of fine particles.The samples were examined in two rheological curve and amplitude sweep test methods to investigate the effect of water content ratio,texture,and structure of patticles on rheological properties.It was found that the content of fine sand,clay,and silt had an effect on the stiffness,structural stability,and shear behavior.Thus,the pseudoplastic and viscoelastic behaviot are respectively shown.Moreover,the amount of fine sediments present in water i.e.the concentration of these particles,has a direct effect on the rheological curve.A reduction in viscosity of samples with higher concentrations of water has been observed.As a consequence,a considerable quantity of fine sediments are distributed within the water body and remain suspended over the time.As a result,the sedimentation rate slows down.It needs to be asserted that the stotage modulus G′,loss modulus G″,and yield point can vary depending on particle type.The G′ and G″were instigated for samples(70%and 45%concentrations) that demonstrated viscoelastic characteristics using the same rotational rheometer with a parallel-plate measuring device.  相似文献   

10.
Based on the dynamic triaxial liquefaction test of the loess samples which are taken from Shibei tableland, Guyuan City, Ningxia, China, the characteristics of dynamic strain, dynamic stress and pore water pressure are studied under cyclic loading. Triaxial shear test is conducted immediately after the sample reaches liquefaction point. During the test, the property of the liquefied soil is analyzed through fluid mechanics method, whereby the fluidity of the liquefied soil is represented by apparent viscosity.The results show that the fluidity of liquefied loess changes from "shear thickening" to "shear thinning" as the shear force continues, and the fluidity of liquefied loess is closely related to its structure. In addition, in the process of forming a new stable state, the apparent viscosity and deviant stress change with axial strain in a similar approach. When the sample reaches its stable state, it meanwhile shows a relatively stable apparent viscosity. According to the fluid mechanics and the law of conservation of energy, the slip distance of the liquefied soil is estimated, and the results are in good agreement with the field investigation results.  相似文献   

11.
This paper establishes the kinetic equations in atmospheric chemistry that describe the macroscopic mechanisms of secondary fine particle pollution generated by precursors during atmospheric self-purification.The dynamic and static solutions of these equations can be applied to calculate quantitative relationships between the concentration ratio of precursors and secondary fine particles as well as the physical clearance power of the atmosphere,chemical reaction rate,and the scale of a contaminated area.The dynamic solution presented here therefore corresponds with a theoretical formula for calculating the overall rate constant for the oxidation reaction of reducing pollutants in the actual atmosphere based on their local concentrations and meteorological monitoring data.In addition,the static solution presented in this paper reveals the functional relationship between the concentration of secondary fine particles and precursor emission rate as well as atmospheric self-purification capacity.This result can be applied to determine the atmospheric environmental capacity of a precursor.Hourly records collected over the last 40 years from 378 weather stations in mainland China as well as the spatiotemporal distribution sequence of overall oxidation reaction rates from precursors show that when the reference concentration limit of secondary fine particles is100μmol m~(-3),the atmospheric environmental capacity of total precursors canbe calculated as 24890×10~(10) mol yr~(-1).Thus,when the annual average concentration limit of given fine particles is 35μg m~(-3) and the ratio of sulfate and nitrate to 30% and 20% of the total amount of fine particles,the capacities of SO_2,NO_x and NH_3 are 1255,1344,and 832(10~(10)g yr~(-1)),respectively.The clearance density of precursors for different return periods across mainland China under above conditions are also provided in this study.  相似文献   

12.
Characterization of Tunisian marine sediments in Rades and Gabes harbors   总被引：1，自引：0，他引：1  
  相似文献   

13.
The erodibility of intertidal sediments is an important factor affecting coastal erosion.In July and October 2008,in situ measurement of erodibility of the surficial sediment were conducted using a recirculating flume at 20 tidal flat experiment sites along the seashore of the Yellow River delta.At the same time,the characteristics of sand ripples and biogenic features on the tidal flat were observed and the physical-mechanical sediment properties such as bulk density,water content,grain size distribution,plasticity,penetration resistance,shear strength and compressibility,were measured.By field measurement,it is obtained that the critical erosion shear stress of the surficial sediment on the tidal flat varies between 0.088 Pa and 0.254 Pa.The factors influencing sediment erodibility are complicated because of physical and biological reworking after the sediment deposited.There’s a positive correlation between shear strength and critical erosion shear stress.The burrowing crabs’ activities changed the sediment microtopography and made the sediment have greater roughness,and that is one possible reason for the higher erodibility.The formation of scour pits on the tidal flat correlates with the heterogeneous erodibility of the surficial sediment.  相似文献   

14.
The dynamic shear modulus for three types of undisturbed soil under different consolidation ratios is presented by using the resonant column test method. Its effects on surface ground motion is illustrated by calculation. The test results indicate that the power function is a suitable form for describing the relationship between the ratio of the maximum dynamic shear modulus due to anisotropic and isotropic consolidations and the increment of the consolidation ratio. When compared to sand, the increment of the maximum dynamic shear modulus for undisturbed soil due to anisotropic consolidation is much larger. Using a one-dimensional equivalent linearization method, the earthquake influence factor and the characteristic period of the surface acceleration are calculated for two soil layers subjected to several typical earthquake waves. The calculated results show that the difference in nonlinear properties due to different consolidation ratios is generally not very notable, but the degree of its influence on the surface acceleration spectrum is remarkable for the occurrence of strong earthquakes. When compared to isotropic consolidation, the consideration of actual anisotropic consolidation causes the characteristic period to decrease and the earthquake influence factor to increase.  相似文献   

15.
As one of the vital tectonic units of the Wulian Metamorphic Core Complex(MCC), the Wulian detachment fault zone(WDFZ), which developed in the Jiaodong peninsula, separates the lower plate of the ultrahigh pressure(UHP) metamorphic rocks in the Sulu orogenic belt from the upper plate of the early Cretaceous Zhucheng basin and the basin basement. The fault zone generally strikes NNE with a dip in the west along the southern portion of the MCC and strikes NE with a dip in the WNWalong the northern portion. The fault zone displays a wavy-tile-shaped extension on the plane, principally composed of the fault breccias and mylonite and transits downward to the mylonitic gneiss. As a whole, the detachment fault zone shows a top-towest or a WNW extension. By calculating the harmonic mean, we obtain a Flynn index K of 0.98–2.0, and the mean value is approximately 1.35 in the fault zone. According to the polar Mohr construction, the extensional crenulation cleavage, the RS/θ,and the quartz C-axial fabric methods, we acquire mean kinematic vorticity values of 0.64–0.97, 0.76–0.93, 0.6–0.92, and 0.63–0.98 with mean values of 0.83, 0.80, 0.78 and 0.86, respectively, for mylonite and promylonite. The strain measurement results and the kinematic vorticity values indicate that the WDFZ is a normal ductile shear zone developed in the extensional setting.The kinematic track shows that the kinematic vorticity value decreases gradually from the NW to the SE as a whole. A simple shear dominates in the middle and upper parts of the shear zone, which is reflected by a higher vorticity value(0.75, up to 0.98),a low thinning rate and a lower K value. In contrast, toward the footwall, the pure shear is increased significantly, showing a lower vorticity value(0.70, low to 0.64), a relatively high thinning rate and a higher K value. Combined with the geotectonic background, the development and evolution of the WDFZ should respond to lithospheric thinning and the destruction of the North China Craton(NCC). As a result, the WDFZ can be defined as a thinning normal shear zone developed in the extension tectonic setting and the combined result of the simple shear caused by the crust extension and pure shear led by the rapid uplift of the footwall and magmatic upwelling.  相似文献   

16.
Bridges are a part of vital infrastructure,which should operate even after a disaster to keep emergency services running.There have been numerous bridge failures during major past earthquakes due to liquefaction.Among other categories of failures,mid span collapse(without the failure of abutments)of pile supported bridges founded in liquefiable deposits are still observed even in most recent earthquakes.This mechanism of collapse is attributed to the effects related to the differential elongation of natural period of the individual piers during liquefaction.A shake table investigation has been carried out in this study to verify mechanisms behind midspan collapse of pile supported bridges in liquefiable deposits.In this investigation,a typical pile supported bridge is scaled down,and its foundations pass through the liquefiable loose sandy soil and rest in a dense gravel layer.White noise motions of increasing acceleration magnitude have been applied to initiate progressive liquefaction and to characterize the dynamic features of the bridge.It has been found that as the liquefaction of the soil sets in,the natural frequency of individual bridge support is reduced,with the highest reduction occurring near the central spans.As a result,there is differential lateral displacement and bending moment demand on the piles.It has also been observed that for the central pile,the maximum bending moment in the pile will occur at a higher elevation,as compared to that of the interface of soils of varied stiffness,unlike the abutment piles.The practical implications of this research are also highlighted.  相似文献   

17.
A coupled routing for the transport capacity and the energy slope is introduced through the definition of the control factor m whose value is linked to the bed form configuration.The coupling aims to further incorporate the interactions occurring in alluvial rivers and thus enhance the prediction of the fine sediment fluxes,especially during high stream power events.Based on a predictive rule for the control factor m that only involves water depth,velocity and bedform constitutive texture,the novel method is confronted to observations collected in one of the most strongly dynamic alluvial river namely the Lower Yellow River.Comparisons between time series of measured and computed concentrations illustrate that during high velocity events the main dynamics of the sediment transport is correctly reproduced.The main advantage of the present approach is to supply consistent time evolutions of sediment concentrations without making use of any detailed shear information.  相似文献   

18.
In this study, the effectiveness of a tuned liquid column-gas damper, TLCGD, on the suppression of seismic-induced vibrations of steel jacket platforms is evaluated. TLCGD is an interesting choice in the case of jacket platforms because it is possible to use the structural elements as the horizontal column of the TLCGD. The objective here is to find the optimum geometric parameters, namely orientation and configuration of vertical columns, length ratio, and area ratio of the TLCGD, considering nonlinear damping of the TLCGD and water-structure interaction between the jacket platform and sea water. The effects of different characteristics of ground motion such as PGA and frequency content on the optimum geometry are also investigated and it is observed that these features have some influence on the optimum area ratio. Finally it is observed that pulse arrangement of ground acceleration is one of the most important parameters affecting the efficiency of a TLCGD. In other words, it is found that the TLCGD’s capability to reduce the RMS responses depends only on the frequency content of the ground acceleration, but its capability to reduce the maximum responses depends on both the frequency content and the pulse arrangement of the ground acceleration.  相似文献   

19.
Field investigations following the 2008 Ms8.0 Wenchuan earthquake identified 118 liquefaction sites, most of which are underlain by gravelly sediment in the Chengdu Plain and adjacent Mianyang area, in the Sichuan Province. Gravel sediment in the Sichuan province is widely distributed; hence it is necessary to develop a method for prediction and evaluation of gravel liquefaction behavior. Based on liquefaction investigation data and in-situ testing, and with reference to existing procedures for sandy soil liquefaction evaluation, a fundamental procedure for gravel liquefaction evaluation using dynamic penetration tests (DPT) is proposed along with a corresponding model and calculation formula. The procedure contains two stages, i.e., pre-determination and re-determination. Pre-determination excludes impossible liquefiable or non-liquefiable soils, and re-determination explores a DPT-based critical N120 blows calculation model. Pre-determination includes three criteria, i.e., geological age, gravel contents, gravel sediment depths and water tables. The re-determination model consists of five parameters, i.e., DPT reference values, gravel contents, gravel sediment depths, water tables and seismic intensities. A normalization method is used for DPT reference values and an optimization method is used for the gravel sediment depth coefficient and water table coefficient. The gravel liquefaction evaluation method proposed herein is simple and takes most influencing factors on gravel sediment liquefaction into account.  相似文献   

20.
By using GDS dynamic hollow cylinder torsional apparatus,a series of cyclic torsional triaxial tests under complex initial consolidation condition are performed on Nanjing saturated fine sand. The effects of the initial principal stress direction α0,the initial ratio of deviatoric stress η0,the initial average effective principal stress p0 and the initial intermediate principal stress parameter b0 on the threshold shear strain γt of Nanjing saturated fine sand are then systematically investigated. The results show that γt increases as η0,p0 and b0 increase respectively,while the other three parameters remain constant. α0 has a great influence on γt,which is reduced when α0 increases from 0° to 45° and increased when α0 increases from 45° to 90°. The effect of α0 on γt plays a leading role and the effect of η0 will weaken when α0 is approximately 45°.  相似文献   

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