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1.
Concrete is a heterogeneous, multiphase, composite material, and the size and shape of the coarse aggregate used have an important influence on the rheological properties of the concrete. The aggregate is usually simulated with spherical particles in the discrete element method (DEM). However, the shape of real aggregates is uncontrolled and polytropic. Therefore, spherical particles hardly reflect the actual situation. To comprehensively analyze the rheological characteristics of self-compacting concrete (SCC), experimental and simulated tests of slump-flow and L-box tests of different performative SCC are investigated. An efficient and fast random polyhedron particle generation method is proposed to simulate the real shape of the coarse aggregate, which is close to the actual state. The slump-flow and L-box tests of SCC are simulated by using the established discrete element model and the irregular generating particle method. The slump-flow test shows that the generation method could effectively simulate the flow state of concrete, and the L-box test evaluates the passing ability of SCC. The rheological characteristics of the yield stress τ0 and plastic viscosity η are verified as Bingham model parameters, and the numerical results are perfectly consistent with the experimental results.  相似文献   

2.
The Faroe Islands in the North Atlantic Ocean are susceptible to flow-type landslides in coarse-grained highly organic colluvium. Following several hazardous debris avalanche events, research work has been initiated to quantify landslide risk. A central task in this work is to predict landslide runout behavior. From numerical simulation of four debris avalanches, this study provides a first screening of which rheology and appertaining input parameters best predict runout behavior of debris avalanches in the Faroe Islands. Three rheologies (frictional, Voellmy, and Bingham) are selected and used for individual back analysis of the events in the numerical models BING and DAN3D. A best fit rheology is selected from comparing predicted and observed landslide runout behavior. General back analysis to identify the optimal input parameters for the chosen rheology is performed by cross validation, where each debris avalanche is modeled with input parameters from the three other events. Optimal input parameters are found from the model run producing the most accurate runout length and velocity. The Bingham is selected as the best fit rheology, a result differing from similar studies of coarse-grained landslides. A reason for why particularly the frictional rheology proves unsuitable is its tendency to produce too long runout lengths of the low-weight runout material, a result showing important limitations for using the frictional rheology in DAN3D. Optimal Bingham input parameters are τ y ?=?980 Pa and μ b ?=?117 Pa/s. However, future studies performed in 2D models are needed for precise parameterization before results can be used for landslide risk assessment.  相似文献   

3.
Non-magnetized suspensions of magnetite particles with concentrations in excess of 30% by mass and particle size less than 75 μm exhibit Bingham plastic behaviour. When exposed to external magnetic fields of strengths in excess of 41 × 10−4 T, the rheological behaviour of the suspensions departs from the Bingham model and can be described by a Herschel–Bulkley model of the form τ = τ0 + kγn. The value of the index n was found to range from 0.38 to 0.9, depending on the magnetic field strength, solids concentration and particle size and correlations are proposed for the apparent viscosity of magnetized suspensions as a function of magnetic field strength and solids concentration.  相似文献   

4.
In this paper, liquefied and laterally spreading soils triggered by seismic shaking are modeled as viscoplastic Bingham media characterized by two rheological parameters: the undrained residual shear strength and the Bingham viscosity coefficient. Since the precise evaluation of these two characteristics directly by appropriate in situ or laboratory experimental tests remains a very difficult task, an identification procedure is developed to assign numerically realistic values to both rheological characteristics from back-calculation of liquefaction-induced lateral spreading using centrifuge experiments. The proposed numerical procedure is applied successfully to two series of reported centrifuge tests where lateral displacements data during shaking were available.  相似文献   

5.
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6.
This paper presents analytical solutions to the one‐dimensional consolidation problem taking into consideration the rheological properties of clayey soil under variable loadings. A four‐element rheological model is introduced, and different loading types are involved, i.e. constant loading, one‐step loading, triangular loading, rectangular loading, and isosceles–trapezoidal cyclic loading. The differential equations governing consolidation are solved by the Laplace transform. Based on the solutions obtained, the influences of the rheological parameters and loading conditions on the consolidation process are investigated. It has been shown that the consolidation behavior is mainly governed by four dimensionless parameters, a1, a2, b, and Tv0. Load shape has a great influence on the rate of consolidation. A decrease either in the modulus of the spring in the Kelvin body or in the viscosity coefficient of independent dashpot will slow down the rate of consolidation. An increase in the viscosity coefficient of the dashpot in the Kelvin body will make the rate of consolidation increase at an early stage but decrease at a later stage. For isosceles–trapezoidal cyclic loading, the consolidation rate in each cycle reaches a maximum at the end of the constant loading phase and the minimum at the end of this cycle. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

7.
Rheological model of hard rock pillar   总被引：1，自引：1，他引：0  
Conclusions The verified Burgers' model for the pillars built of hard rocks corresponds qualitatively to their behaviour under the long term compressive load. During the experiments, both elastic immediate and after-effect (delayed) strains were observed. The occurrence of creep was proven for stress levels equal to or exceeding 5 MPa (about 40% of the ultimate strength of the fissured pillar samples). Approximated numerical rheological parameters:E 1,E 2, 1, 2 are, with a simplification, the parameters which can be applied in engineering computation since they are connected with strain characteristics of in situ pillar samples and not the rocks being only a component of broken-up rock mass. A thorough recognition of the effect of time on the behaviour of pillars during the mining of ore deposits is significant because of the possibility of their rational surveying and thus the decrease of exploitation losses.The results are the first to be obtained in in situ investigations and based on the classical creep test but on large-scale samples of pillars.  相似文献   

8.
Recently, composite analysis (CA), which simultaneously analyzes all drawdown data from multiple observation wells, has been applied to determine the hydraulic parameters of an unconfined aquifer. Moench (1994) claimed that the value of specific yield (S y) determined from non-composite analysis (nonCA) is sometimes unrealistically low as compared with that obtained by water-balance calculation, and results from CA are better representative of aquifer properties than those from nonCA. To examine the validity of this assertion, the drawdown data from a pumping test conducted at Cape Cod, Massachusetts, USA, were analyzed using both nonCA and CA methods. The results show that the mean estimates of hydraulic conductivity and S y determined from CA are close to those determined from nonCA. In some cases the analysis based on CA also results in low estimates of S y as compared with those determined based on nonCA. A hypothetical case study is presented, which examines the effect of measurement errors on the estimated parameters. The results indicate that the CA method also gives poorer estimates of S y than the nonCA method if the pumping test data contain measurement errors. Moench AF (1994) Specific yield as determined by type-curve analysis of aquifer-test data. Ground Water, 32(6):949–957.  相似文献   

9.
连云港海相软土流变特性试验及双屈服面流变模型   总被引：16，自引：6，他引：10  
张军辉  缪林昌 《岩土力学》2005,26(1):145-149
为研究连云港海相软土流变特性,进行了三轴流变试验。根据对试验结果的分析,可以将连云港海相软土作为弹-粘塑性体来研究;将广义Bingham模型和椭圆-抛物线双屈服面模型相结合,建立了新的流变模型来描述其流变特性,得出了各参数,并验证了该模型的适用性。  相似文献   

10.
The specific energy (SE) is the most important parameter to estimate the energy consumption in tunnel boring machines (TBMs). It is defined as the amount of required energy to excavate a unit volume of rock mass which used to predict the performance of TBMs. Several models are used to estimate the SE based on different parameters such as the rock mass properties, disc cutter dimensions and cutting geometry. The aim of this work is to propose new relations between the SE and the strain energy of rock mass (W) using the geological mappings of rock mass and TBM operational parameters from Amir-Kabir Water Transferring Tunnel of Iran. W is an appropriate criterion to estimate SE because it is a function of different parameters such as rock mass behavior, pre and post failure properties and peak and residual strains. In this study, to increase the correlation coefficient of relation between the mentioned parameters, the rock mass is classified in two methods, in the first method according to the geological strength index (GSI) all data is classified in three classes such as weak, fair and good and in the second method using the drop to deformation modulus ratio (η) the classification of data is performed in three classes such as η < 0.05, 0.05 ≤ η < 10 and η ≥ 10. The results show that there are direct relations between both parameters. It is suggested to estimate SE in all rock mass classes using the proposed relations based on GSI classification.  相似文献   

11.

Chalk breaks easily when subjected to human action such as mechanical handling, earthworks operations or pile installation. These actions break the cemented structure of chalk, which turns into a degraded material known as putty, with lower strength and stiffness than the intact chalk. The addition of Portland cement can improve the behaviour of chalk putties. Yet, there are no studies determining the tensile strength of chalk putty–cement blends, the initial stiffness evolution during the curing time and other design parameters such as friction angle and cohesion of this material. This paper addresses this knowledge gap and provides an interpretation of new experimental results based on the dimensionless index expressed as the ratio between porosity and volumetric content of cement (η/Civ) or its exponential modification (η/Civa). This index aids the selection of the amount of cement and density for key design parameters of compacted chalk putty–cement blends required in geotechnical engineering projects such as road foundations and pavements, embankments, and also bored concrete pile foundations.

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12.
Improved, microfabric‐inspired rotational hardening rules for the plastic potential and bounding surfaces associated with the generalized bounding surface model for cohesive soils are presented. These hardening rules include 2 new functions, fη and , that improve the simulation of anisotropically consolidated cohesive soils. Three model parameters are associated with the improved hardening rules. A detailed procedure for obtaining suitable values for these parameters is presented. The first 2 parameters affect the simulation of constant stress ratio loading where, because of the presence of fη, the third parameter is inactive. The second new function, , accelerates the rotation of the plastic potential and bounding surfaces during shearing, which is particularly important for overconsolidated soils tested in extension. This paper also describes the proper manner in which to define the inherent anisotropy. This seemingly straightforward test has rarely been discussed in sufficient detail.  相似文献   

13.
Summary Five series of test blocks of Pendeli marble with artificially created discontinuities of different crack densities (simulating three mutually orthogonal joint sets) were tested in uniaxial compression in order to study the effect of discontinuities on: (a) the compressive strength and the modulus of elasticity, and (b) certain fracture energy parameters expressed by the ratio W A/W V, where W A is the surface energy and W V the volume elastic strain energy. Mathematical relationships are derived similar to those suggested by other authors relating strength parameters to crack densities. Such relationships clearly show a reduction in strength with increased crack density. The experimental results obtained permit the extension of Persson's relation (which refers to ideal intact rock) to the more realistic case of discontinuous rock mass by introducing the appropriate term that takes into consideration the effect of rock mass discontinuities on the energy ratio W A/W V. A comparison between laboratory results and field observations was subsequently carried out assuming the rock mass to behave as a linearly elastic material, obeying the Hoek and Brown failure criterion. This comparison showed that laboratory results can be extended to larger scale. Furthermore, in order to predict the in situ strength and stability of a rock mass in uniaxial compression (which is of major importance in underground excavations) certain concepts are proposed based on laboratory tests, in situ investigations and first principles of linear elastic fracture mechanics.  相似文献   

14.
东秦岭松树沟超镁铁岩侵位机制及其构造演化   总被引：3，自引：0，他引：3  
 东秦岭松树沟蛇绿岩主要由镁铁质-超镁铁质岩石组成。镁铁质岩类的Sm-Nd全岩等时年龄为1030±46(2δ)Ma,εNd(t)=+5.7±0.2,代表了蛇绿岩的形成时代。超镁铁质岩石由不同成因的橄榄质糜棱岩和中粗粒橄榄岩组成,橄榄质糜棱岩是地幔橄榄岩经历复杂变形并多次部分熔融的残余体,具LREE亏损特征,其中发育橄榄石高温位错构造和高温组构以及低温位错构造和低温组构。中粗粒橄榄岩具LREE略富集的分布特征,是地幔橄榄岩残余体再次部分熔融熔体分离结晶的产物。野外地质、地球化学、构造变形特征均表明超镁铁岩块是因洋壳俯冲而底辟侵位于上覆玄武岩中的地幔橄榄岩残余体。综合分析认为,松树沟蛇绿岩经历了古陆块裂解或洋脊扩张(1271-1440Ma)-洋壳形成(1030-1271Ma)-洋壳俯冲消减-橄榄岩块底辟侵位(983Ma)-蛇绿岩构造侵位及其后构造变形叠加改造的复杂演化过程。  相似文献   

15.
The solvus on the Mn-rich side in the system MnCO3-CaCO3 was studied experimentally at 10 kb and lower pressures. The critical point lies at 540 °C and Mn0.68Ca0.32CO3. Reaction rates of demising are extremely sluggish. X-ray calibration curves for (Mn, Ca)CO3 solid solutions are given. Depicting microprobe analyses of about 300 carbonates from manganese calc silicate marbles affected by different degrees of metamorphism confirm the experimentally derived solvus.Margules parameters were derived from the experimental solvus, giving for spinodal unmixing W a=20.1+0.032·T and W b=18.3-0.0033·T, and for binodal unmixing W a=–22.9+0.035 ·T and W b=3.36+0.0147·T (All W, in kJ/-mol)A physical interpretation of the Margules parameters based on deformations of the ionic radii in the solid solution is presented. The Margules parameters obtained by this model agree fairly well with the experimental data.  相似文献   

16.
Geomorphic features such as drifts, sediment waves and channels have been documented in the Upper Cretaceous of north‐west Europe. These features are interpreted to result from bottom currents and have been used to refine chalk depositional models and quantify palaeocirculation patterns. Chalk was first deposited as calcareous nannofossil ooze and geomorphic features are the result of sediment reworking after deposition. There is limited knowledge on the processes that govern nannofossil ooze mobility, thus forcing uncertainty onto numerical models based on sedimentological observations. This article provides an extensive view of the erosional and depositional behaviour of calcareous nannofossil ooze based on experimental work using annular flumes. A fundamental observation of this study is the significant decrease of nannofossil ooze mobility with decreasing bed porosity. Erosion characteristics, labelled as erosion types, vary with total bed porosity (φ) and applied shear stress (τ0). High‐porosity ooze (φ >80%) is characterized by constant erosion rates (Em). At φ <77%, however, erosion characteristics showed greater variance. Surface erosion was typically followed by transitional erosion (with asymptotically decreasing Em), and stages of erosion with constant, and exponential erosion rates. The estimated erosion thresholds (τc) vary from ca 0·05 to 0·08 Pa for the onset of surface erosion and up to ca 0·19 Pa for the onset of constant erosion (φ of 60 to 85%). Variability of deposition thresholds (τcd) from ca 0·04 to 0·13 Pa reflects the influence of variable suspended sediment concentration and τ0 on settling particle size due to the identified potential for chalk ooze aggregation and flocculation. Additionally, deposition thresholds seem to be affected by the size of eroded aggregates whose size correlates with bed porosity. Lastly, slow sediment transport without resuspension occurred in high‐porosity ooze as surface creep, forming low‐relief sedimentary features resembling ripples. This process represents a previously undescribed mode of fine‐grained nannofossil ooze transport.  相似文献   

17.
The spin Hamiltonian (SH) parameters (g factors g x , g y and g z and the hyperfine structure constants A x , A y and A z ) and local structure for the rhombic Rh4+ and Ir4+ centers in TiO2 (rutile) are theoretically studied from the perturbation formulas of these parameters for a low spin (S = 1/2) d 5 ion under rhombically distorted octahedra. In the calculations, the ligand orbital and spin–orbit coupling contributions as well as the influence of the local lattice distortions are taken into account using the cluster approach. The local axial elongation ratios are found to be about 1.7 and 3 times, respectively, larger for the Rh4+ and Ir4+ centers than that (≈0.0075) for the host Ti4+ site in rutile, while the perpendicular distortion angles (≈−0.28° and −0.42°, respectively) are more than one order in magnitude smaller than the host value (≈−9.12°). This means that the impurity centers exhibit further elongations of the oxygen octahedra and much smaller perpendicular rhombic distortions as compared with those of the host Ti4+ site in TiO2. The above local lattice distortions can be mainly ascribed to the substitution of the host Ti4+ by the nd 5 impurities, which may induce different physical and chemical properties for the metal–ligand clusters. In addition, the influence of the Jahn–Teller effect on the local structure may not be completely excluded. The calculated SH parameters show reasonable agreement with the observed values.  相似文献   

18.
Analytical formulae are proposed to describe the first-order temporal evolution of the head in large groundwater systems (such as those found in North Africa or eastern Australia) that are subjected to drastic modifications of their recharge conditions (such as those in Pleistocene and Holocene times). The mathematical model is based on the hydrodynamics of a mixed-aquifer system composed of a confined aquifer connected to an unconfined one with a large storage capacity. The transient behaviour of the head following a sudden change of recharge conditions is computed with Laplace transforms for linear one-dimensional and cylindrical geometries. This transient evolution closely follows an exponential trend exp(?t/τ). The time constant τ is expressed analytically as a function of the various parameters characterizing the system. In many commonly occurring situations, τ depends on only four parameters: the width a c of the main confined aquifer, its transmissivity T c, the integrated storage situated upstream in the unconfined aquifer M?=?S u a u, and a curvature parameter accounting for convergence/divergence effects. This model is applied to the natural decay of large aquifer basins of the Sahara and Australia following the end of the mid-Holocene humid period. The observed persistence of the resource is discussed on the basis of the time constant estimated with the system parameters. This comparison confirms the role of the upstream water reserve, which is modelled as an unconfined aquifer, and highlights the significant increase of the time constant in case of converging flow.  相似文献   

19.
Rosascoet al. (1975), reported the first successful application of laser-excited Raman spectroscopy for the identification and nondestructive partial analysis of individual solid, liquid, and gaseous phases in selected fluid inclusions. We report here the results of the application of a new instrument, based on back-scattering, that eliminates many of the previous stringent sample limitations and hence greatly expands the range of applicability of Raman spectroscopy to fluid inclusions.Fluid inclusions in many porphyry copper deposits contain 5–10 μm ‘daughter’ crystals thought to be anhydrite but too small for identification by the previous Raman technique. Using the new instrument, we have verified that such daughter crystals in quartz from Bingham, Utah, are anhydrite. They may form by leakage of hydrogen causing internal autooxidation of sulfide ion. Daughter crystals were also examined in apatite (Durango, Mexico) and emerald (Muzo, Colombia).Valid analyses of sulfur species in solution in small fluid inclusions from ore deposits would be valuable, but are generally impossible by conventional methods. We present a calibration procedure for analyses for SO42? in such inclusions from Bingham, Utah (12,000 ± 4000 ppm) and Creede, Colo. (probably < 500 ppm). A fetid Brazilian quartz, originally thought to contain liquid H2S, is shown to contain only HS? in major amounts.  相似文献   

20.
In recent years, debris flows have represented a severe natural hazard in South-Gargano watersheds (Puglia Region, Southern Italy). Hill slopes erosion, caused by the inadequate protection of the degraded forests, produces large amounts of soil and debris that are conveyed downstream during heavy rainstorms. The involved material is characterized by limestone fragments and blocks in a sandy-silt matrix. In this paper, the rheological properties of such debris-flow materials have been investigated. Eight specimens of particulated sediments of particle diameter of d ≤ 4 mm have been analysed using a rheometric tool for large particle suspensions, the ball measuring system (BMS). The influence of sediment concentration on the bulk rheological behaviour has been evaluated at concentrations by volume obtained in fully water-saturated conditions (volumetric sediment concentration C V ~ 0.8) and with fixed water content (C V = 0.42). The rheological data were fitted to the Bingham, Herschel–Bulkley, and O’Brien and Julien models to define viscosity and yield stress dependency on sediment concentration, to provide viscosity information for hazard mitigation as well as for comparison to other debris-flow events.  相似文献   

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