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1.
This paper proposes a yield and failure criterion for cohesive and frictional materials. The function is given by the combination of a Lode dependence for the behaviour in the deviatoric plane and a meridian function for the pressure‐dependent behaviour. A variety of shapes can be achieved with the proposed criterion including Lode dependences which are able to reproduce the behaviour of isotropic and cross‐anisotropic materials in the deviatoric plane. The criterion is validated through the comparison with experimental data based on multiaxial experimental tests on clays, sands, rocks and concrete. Finally, the convexity of the criterion is analysed and discussed. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

2.
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3.
Analogue models often require that materials with specific physical properties be engineered to satisfy scaling conditions. To achieve this goal we investigate the rheology of aqueous solutions of Natrosol 250 HH, a rheology modifier employed in various industries to thicken viscous solutions. We report the rheological properties as functions of the concentration and temperature and discuss the advantages and limitations of these materials in view of their use in analogue modelling experiments. The solutions are linear visco-elastic for low stresses (or strain-rates), becoming shear-thinning for larger stresses. For the typically slow analogue experiments of tectonics, the solutions can be considered linear visco-elastic with a Maxwell relaxation time much smaller than the characteristic observation time. This simplification is even more appropriate when the solutions are employed at temperatures higher than 20 °C, since the solutions then display a behaviour that is more viscous, less elastic at the same shear-rate, while the Newtonian viscosity reduces and the shear-rate limit between Newtonian and shear-thinning behaviours increases. The Newtonian viscosity is shown to increase non-linearly with concentration and decrease non-linearly with temperature. With concentrations between 0 and 3% and temperature between 20 and 40 °C, the viscosity varied between 10−1 and 4000 Pa s, while the density remained close to the density of water. Natrosol 250 HH thus offers the possibility to control the viscosity of a solution without significantly affecting the density, thereby facilitating the design and setup of analogue experiments.  相似文献   

4.
Magma-induced strain localization in centrifuge models of transfer zones   总被引：1，自引：0，他引：1  
Scaled centrifuge experiments have been used to investigate the dynamic relations between deformation and magma distribution in rift-related transfer zones. The physical models were built using suitable analogue materials, such as sand to represent the brittle upper crust, various kinds of silicone mixtures to simulate the lower crust and upper mantle and glycerol to reproduce magma. Models simulated the development of transfer zones across pre-existing glycerol reservoirs placed at the base of the analogue continental crust. In plan view, different geometries, dimensions and positions of subcrustal reservoirs were reproduced in three different sets of experiments; to compare results, models were also performed without magma-simulating glycerol.Set 1 experiments, incorporating a narrow rectangular glycerol reservoir, show that the low-viscosity material is able to localise deformation into the overlying crust, giving rise to discrete transfer zones. This concentrated surface deformation corresponds at depth to major magma accumulation. Set 2 experiments, with an initial wide squared glycerol reservoir, show instead that deformation is distributed across the whole model surface, corresponding at depth to relatively minor magma accumulation. Set 3 experiments explored various positions of a small squared reservoir that invariably localised faulting in the overlying analogue brittle crust at the onset of model deformation.The overall model behaviour suggests that magma distribution at depth can effectively control the strain distribution in the overlying crust and the deformative pattern of transfer zones. Strain distribution, in turn, may control magma emplacement as localized deformation would favour major accumulation of magma at transfer zones. Coupled to a strong thermal weakening of the country rocks, this process may ultimately lead to a positive feedback interaction between magma and deformation.  相似文献   

5.
《Applied Geochemistry》2001,16(2):183-195
Geochemical and microbiological evidence indicates that viable microorganisms produce and consume volatile organic acids (VOA) in the Yegua formation. Acetic and propionic acid concentrations in mudstones range from 200 to 1270 and 20 to 38 nmol·gdw−1 respectively, whereas concentrations in sands are 50–200 and less than 20 nmol·gdw−1. VOA concentrations in sediments and in laboratory incubations suggest net production of VOAs by microorganisms in mudstones, and net consumption of VOAs by SO4 reducing bacteria (SRB) in sands. Notably, SRB activity is mostly confined to aquifer sands.Vertical diffusion and advection were modeled to estimate acetic acid transport from aquitard to aquifer. Assuming that SRB completely respire the acetic acid transported into the aquifer (3.2 μmol·l−1·m·a−1), the CO2 production rate in the aquifer sands is 5.3 μmol·l−1·a−1. This slow mineralization rate of in situ organic matter is within the range for deep aquifers, and probably accounts for the long-term survival of microorganisms in oligotrophic environments. Finally, the microbial communities in Yegua sediments appear to exhibit a loose commensalism, with microorganisms in aquitards providing VOAs for respiratory processes (i.e., SO4 reduction) in aquifers.  相似文献   

6.
Measuring surface deformation is critical in analogue modelling of Earth science phenomena. Here, we present a novel application of the Microsoft Kinect sensor to measure vertical deformation in a scaled analogue model of Nisyros volcano (Greece), simulating two magmatic sources and related surface deformation. The Kinect permits capture of real time, 640 × 480 pixel, true‐colour images (RGB) and a grid of distances to the modelled surface with a horizontal and vertical resolution of ±1 mm. Using recorded distances permits quantification of vertical deformation in terms of maximum and average down‐ and uplift during deflation and inflation phases respectively, which is of crucial importance for defining the kinematics of faults formed during the modelling, determined from interpretation of the RGB images. Although other techniques have demonstrated similar or higher accuracy, our study demonstrates the cost‐effectiveness of the Kinect in analogue modelling of volcanoes.  相似文献   

7.
Landslides are serious geohazards that occur under a variety of climatic conditions and can cause many casualties and significant economic losses. Centrifuge modelling, as a representative type of physical modelling, provides a realistic simulation of the stress level in a small-scale model and has been applied over the last 50 years to develop a better understanding of landslides. With recent developments in this technology, the application of centrifuge modelling in landslide science has significantly increased. Here, we present an overview of physical models that can capture landslide processes during centrifuge modelling. This review focuses on (i) the experimental principles and considerations, (ii) landslide models subjected to various triggering factors, including centrifugal acceleration, rainfall, earthquakes, water level changes, thawing permafrost, excavation, external loading and miscellaneous conditions, and (iii) different methods for mitigating landslides modelled in centrifuge, such as the application of nails, piles, geotextiles, vegetation, etc. The behaviors of all the centrifuge models are discussed, with emphasis on the deformation and failure mechanisms and experimental techniques. Based on this review, we provide a best-practice methodology for preparing a centrifuge landslide test and propose further efforts in terms of the seven aspects of model materials, testing design and equipment, measurement methods, scaling laws, full-scale test applications, landslide early warning, and 3D modelling to better understand the complex behaviour of landslides.  相似文献   

8.
The Experimental Tectonics Laboratory at Queen's University is equipped with a large-capacity centrifuge that is capable of subjecting tectonic models measuring 127 × 76 mm in plan and up to 51 mm in depth to accelerations as high as 20,000 g. This high capacity greatly extends the range of potential model materials and permits the use of relatively stiff and/or brittle substances.A number of new techniques of model construction have been devised, that permit internal and surface strain patterns and kinematic evolution to be monitored in detail. One particularly useful technique, which will find application in non-centrifuged experiments as well, allows the preparation of highly uniform anisotropic multilayers composed of alternating layers of Plasticine and silicone putty, with individual layer thicknesses as low as 20 μm and with controllable ratio between thicknesses of the relatively competent and incompetent units. Examples of models constructed using these new techniques are illustrated.One particular type of the commonly used model material, silicone putty, has been subjected to a series of rheological test. The results indicate that at strain rates in the range 10?6-10?3s?1 (applicable to the centrifuge experiments) the silicone putty exhibits power-law rheology with n = 7 ± 2. At higher strain rates the material appears to tend towards linear behaviour.Available rheological data and dimensional analysis using standard scaling laws and appropriate model ratios suggest that the microlaminated Plasticine-silicone putty multilayer is a suitable analogue, in centrifuged experiments, for interbedded sequences of indurate limestone and incompetent shale. The excellent degree of dynamic similitude attained is demonstrated by the realistic form of fold and fault structures developed in models constructed of this material.  相似文献   

9.
Deformation models used to explain the triggering mechanism often assume pure elastic behaviour for the crust and upper mantle. In reality however, the mantle and possibly the lower crust behave viscoelastically, particularly over longer time scales. Consequently, the stress field of an earthquake is in general time-dependent. In addition, if the elastic stress increase were enough to trigger a later earthquake, this triggered event should occur instantaneously and not many years after the triggering event. Hence, it is appropriate to include inelastic behaviour when analysing stress transfer and earthquake interaction.In this work, we analyse a sequence of 10 magnitude Ms > 6.5 events along the North Anatolian Fault between 1939 and 1999 to study the evolution of the regional Coulomb stress field. We investigate the triggering of these events by stress transfer, taking viscoelastic relaxation into account. We evaluate the contribution of elastic stress changes, of post-seismic viscoelastic relaxation in the lower crust and mantle, and of steady tectonic loading to the total Coulomb stress field. We analyse the evolution of stress in the region under study, as well as on the rupture surfaces of the considered events and their epicentres. We study the state of the Coulomb stress field before the 1999 İzmit and Düzce earthquakes, as well as in the Marmara Sea region.In general, the Coulomb stress failure criterion offers a plausible explanation for the location of these events. However, we show that using a purely elastic model disregards an important part of the actual stress increase/decrease. In several cases, post-seismic relaxation effects are important and greater in magnitude than the stress changes due to steady tectonic loading. Consequently, viscoelastic relaxation should be considered in any study dealing with Coulomb stress changes.According to our study, and assuming that an important part of the rupture surface must be stressed for an earthquake to occur, the most likely value for the viscosity of the lower crust or mantle in this region is 5 · 1017–1018 Pa · s. Our results cannot rule out the possibility of other time-dependent processes involved in the triggering of the 1999 Düzce event. However, the stress increase due to viscoelastic relaxation brought 22% of the 1999 Düzce rupture area over the threshold value of Δσc ≥ 0.01 MPa (0.1 bar), and took the whole surface closer to failure by an average of 0.2 MPa. Finally, we argue that the Marmara Sea region is currently being loaded with positive Coulomb stresses at a much faster rate than would arise exclusively from steady tectonic loading on the North Anatolian Fault.  相似文献   

10.
We present centrifuge experiments to study the surface deformation induced by shallow pluton emplacement in a rheologically stratified crust. Sand simulates the topmost brittle crust; plastilina and denser silicone represent more and less competent crustal portions, respectively; lighter silicone simulates a buoyant intrusion. In the models, density differences affect the rate of intrusion but not their evolution or shape, whereas viscosity and strength stratifications control both the shape and rate of the intrusions. With a higher viscosity contrast (102–4) between the intrusion and the embedding media, the rise of the lighter silicone induces a laccolith-like intrusion, responsible for doming and thinning of the overburden; an apical depression may form, inducing silicone extrusion. Conversely, with a lower (101) viscosity contrast, the overburden and the intrusion exhibit a lens-shaped form, with a broad central depression bordered by an upward flexure towards the periphery. A sag in the floor of the intrusion is commonly observed; no silicone extrusion occurred. The intrusion is a hybrid between a laccolith and a lopolith. The comparison with nature (1) confirms roof uplift as an important means of accommodating space during pluton emplacement and (2) suggests that, where roof uplift plays a major role, pluton emplacement can induce a well-correlated sequence of events at surface: doming, the development of a depression and extrusion.  相似文献   

11.
Geochemical study of the Holocene sediments of the Meghna River Delta, Chandpur, Bangladesh was conducted to investigate the distribution of arsenic and related trace and major elements. The work carried out includes analyses of core sediments and provenance study by rare earth element (REE) analysis. Results showed that the cores pass downward from silty clays and clays into fine to medium sands. The uppermost 3 m of the core sediments are oxidized [average oxidation reduction potential (ORP) + 230 mV], and the ORP values gradually become negative with depths (−45 to −170 mV), indicating anoxic conditions prevail in the Meghna sediments. The REE patterns of all lithotypes in the study areas are similar and are comparable to the average upper continental crust. Arsenic and other trace elements (Pb, Zn, Cu, Ni, and Cr) have greater concentrations in the silts and clays compared to those in the sands. Positive correlation between As and Fe was found in the sediments, indicating As may be adsorbed on Fe oxides in aquifer sediments.  相似文献   

12.
Modelling the extension of heterogeneous hot lithosphere   总被引：2，自引：0，他引：2  
The consequences of weak heterogeneities in the extension of soft and hot lithosphere without significant previous crustal thickening has been analysed in a series of centrifuge models. The experiments examined the effects of i) the location of heterogeneities in the ductile crust and/or in the lithospheric mantle, and ii) their orientation, perpendicular or oblique to the direction of bulk extension. The observed deformation patterns are all relevant to the so-called “wide rifting” mode of extension. Weak zones located in the ductile crust exert a more pronounced influence on localisation of deformation in the brittle layer than those located in the lithospheric mantle: the former localise faulting in the brittle crust whereas the latter tend to distribute faulting over a wider area. This latter behaviour depends in turn upon the decoupling provided by the ductile crust. Localised thinning in the brittle crust is accompanied by ductile doming of both crust and mantle. Domains of maximum thinning in the brittle crust and ductile crust and mantle are in opposition. Lateral differences in brittle crust thinning are accommodated by lateral flow in the ductile crust and mantle. This contrasts with “cold and strong” lithospheres whose high strength sub-Moho mantle triggers a necking instability at the lithosphere-scale. This also differs from the extension of thickened hot and soft lithospheres whose ductile crust is thick enough to give birth to metamorphic core complexes. Thus, for the given lithospheric rheology, the models have relevance to backarc type extensional systems, such as the Aegean and the Tyrrhenian domains.  相似文献   

13.
This paper presents a simple concept which can be used for simulating a range of soil mechanics problems. The study is motivated by the observation that many experimental results are commonly described in terms of lines or curves according to a phenomenological approach. Frequently, these relations are based on rather different formulations from one application to another, and in complex forms for some cases. This leads to complications for the calibration of parameters as well as constitutive modelling. Thus, a general framework referred to as “reference curves” has been developed. This framework provides a unique treatment of the macroscopically observed behaviour of clays, sands, and structured materials under isotropic compression, as well as the water retention characteristics of granular materials and geotextiles. Several examples are provided illustrating the good accuracy of models developed with this concept. The proposed framework may be equally applied to any other behaviour where reference lines are easily identifiable from a macroscopic scope, such as some non-linear failure envelopes for granular materials. In addition, we show that the incorporation of the proposed equations into constitutive models is quite straightforward.  相似文献   

14.
This paper presents a new generalized effective stress model, referred to as MIT-S1, which is capable of predicting the rate independent, effective stress–strain–strength behaviour of uncemented soils over a wide range of confining pressures and densities. Freshly deposited sand specimens compressed from different initial formation densities approach a unique condition at high stress levels, referred to as the limiting compression curve (LCC), which is linear in a double logarithmic void ratio, e, mean effective stress space, p′. The model describes irrecoverable, plastic strains which develop throughout first loading using a simple four-parameter elasto-plastic model. The shear stiffness and strength properties of sands in the LCC regime can be normalized by the effective confining pressure and hence can be unified qualitatively, with the well-known behaviour of clays that are normally consolidated from a slurry condition along the virgin consolidation line (VCL). At lower confining pressures, the model characterizes the effects of formation density and fabric on the shear behaviour of sands through a number of key features: (a) void ratio is treated as a separate state variable in the incrementally linearized elasto-plastic formulation: (b) kinematic hardening describing the evolution of anisotropic stress–strain properties: (c) an aperture hardening function controls dilation as a function of ‘formation density’; and (d) the use of a single lemniscate-shaped yield surface with non-associated flow. These features enable the model to describe characteristic transitions from dilative to contractive shear response of sands as the confining pressure increases. This paper summarizes the procedures used to select input parameters for clays and sands, while a companion paper compares model predictions with measured data to illustrate the model capability for describing the shear behaviour of clays and sands. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

15.
An elastoplastic constitutive model is proposed for saturated sands in general stress space using the middle surface concept (MSC). In MSC, different features of stress–strain response of a material are divided into different pseudo‐yield surfaces. The true‐yield surface representing the true response is established by using various links between the yield surfaces. In this MSC sand model, several well‐known features of sand response are represented by three different pseudo‐yield surfaces, which are developed in a simple and straightforward way. These features include the critical state behaviour, the effects of state parameter, unloading and reloading plastic deformation, the influence of fabric anisotropy, and phase transformation line related behaviour. Finally, the model predictions and test results are compared for two different types of sands under a variety of loading conditions and good comparisons are obtained. The application of MSC to saturated sand modelling shows the versatility of MSC as a general concept for modelling stress–strain response of materials. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

16.
A finite element model for pile‐driving analysis is developed and used to investigate the behaviour of pre‐bored piles, which are then driven the last 1.25 or 2.25 m to their final design depth. The study was conducted for the case of saturated clays. The model traces the penetration of the pile into the soil and accommodates for large deformations. The non‐linear behaviour of the clay in this study is predicted using the bounding‐surface‐plasticity model, as applied to isotropic cohesive soils. The details of the 3‐D numerical modelling and computational schemes are presented. A significant difference was observed in the pile displacement during driving, and in the computed soil resistance at the pile tip, particularly at the earliest driving stages. No difference in soil resistance at the soil pile interface along the pile shaft was detected between the pre‐bored piles whether driven 1.25 or 2.25 m. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

17.
The Xiaobaishitou W (–Mo) deposit is located in the eastern segment of the Central Tianshan, northwestern China. The deposit represents a skarn system distributed in the contact zones of biotite granite and crystalline limestone of the Mesoproterozoic Kawabulag Group. The Xiaobaishitou deposit is characterized by a typical calc-silicate mineralogy dominated by garnet, diopside and wollastonite, with minor epidote, tremolite, actinolite, chlorite, quartz, fluorite and calcite. The prograde and retrograde skarns are characterized by garnet–clinopyroxene–wollastonite and epidote–tremolite–actinolite–chlorite, respectively, intruded and replaced by mineral assemblages of scheelite–cassiterite–magnetite, quartz–sulfides and calcite–quartz–fluorite in younger order.Six molybdenite samples from the deposit yielded Re − Os isotope model ages ranging from 239.7 ± 3.6 Ma to 251.4 ± 3.6 Ma. The zircon crystals from biotite granite and Mo-mineralized granite yield weighted 206Pb/238U age of 242 ± 1.7 and 240.5 ± 2.1 Ma, respectively. Both the zircon U − Pb and the molybdenite Re − Os ages obtained in this study fall in a narrow span of 242–240 Ma, which suggest that the Xiaobaishitou W (–Mo) system was formed in the Triassic. The Re contents of the molybdenites range from 40.33 to 64.67 ppm, suggesting that the ore-forming materials were derived mainly from continental crust together with the involvement of minor mantle components. Combined with the 87Sr/86Sr ratios of tungsten-bearing quartz veins from other studies, which scatter between 0.707153 and 0.709877, demonstrating mixing between two end-member isotopic compositions of crust and mantle. It can be concluded that the Indosinian Xiaobaishitou deposit was formed in a tectonic transition from collisional crust shortening and thickening to post-collisional extension and thinning.  相似文献   

18.
Rock interface strengths have often been assumed to be zero in numerical and analogue models of fracture propagation and magma intrusion in the crust. Rock strength tests were performed to explore the role that rock interfaces have on the geometry and propagation dynamics of fluid-filled fractures in the crust. We used a 1 kN test machine to study 5 mm thick cuboidal specimens cut from a sandstone-siltstone rock core, where the strata were known to host magma intrusions and the rock interface between the units was intact. By measuring the load required to grow a crack running along the lithological contact between the layers we calculate its fracture toughness Kc. The siltstone had an average Kc of 0.56 ± 0.03 MPa m1/2 compared to the sandstone at 0.42 ± 0.02 MPa m1/2. The rock interface had intermediate average fracture toughness to the parent units at 0.45 ± 0.03 MPa m1/2. These results have important implications on fracture propagation pathways through rocks, as well as for the geometry and propagation dynamics of magma intrusions in the crust.  相似文献   

19.
The Parashi granitoid of northeasternmost Colombia intrudes the Upper Cretaceous to Lower Paleocene accretionary complex formed by the collision of the Caribbean arc and the continental margin of South America. This granitoid presently separated of the continental margin includes a major quartzdiorite body with andesite to dacite dikes and mafic enclaves. Zircon U–Pb LA-MC-ICP-MS and K–Ar geochronology on the quartzdiorite and the dikes suggest that crystallization extended from ca. 47 to 51 Ma. Major and trace elements are characterized by a medium-K, immature continental arc signature and high Al2O3, Na2O and Ba–Sr contents. Initial 87Sr/86Sr isotopic values range between 0.7050 and 0.7054, with 143Nd/144Nd = 0.51235–0.51253, εNd and εHf values from −0.81 to −4.40 and −4.4 and −5.2. Major and trace element ratios and isotopic modeling suggest that sedimentary and/or quartzofeldspathic crustal sources were mixed with a mafic melt input. The petrotectonic and geological constraints derived from this granitoid suggest that Parashi plutonism records an immature, oblique subduction-zone setting in which the presence of a high-temperature mantle realm and strong plate coupling associated to upper crust subduction caused the partial fusion of a previously tectonically underplated mafic crust and associated metasediments exposed in the continental margin. The limited temporal expression of this magmatism and the transition to a regional magmatic hiatus are related to a subsequent change to strongly and slow oblique tectonics in the Caribbean–South America plate interactions and the underflow of a relatively thick slab of Caribbean oceanic crust.  相似文献   

20.
Yao  YangPing  He  Guan  Liu  Lin  Zhang  Jianmin  Luo  Ting 《Acta Geotechnica》2022,17(5):2021-2027
Acta Geotechnica - The modified Cam-clay model has been widely adopted as the theoretical framework for constitutive modelling of clays. However, there is so far no such model for sands on the...  相似文献   

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