Constitutive model for gas hydrate-bearing soils considering different types of hydrate morphology and prediction of strength-band

The present study proposes a new elasto-plastic constitutive model that considers different types of hydrates in pore spaces. Many triaxial compression tests on both methane hydrate-bearing soils and carbon dioxide hydrate-bearing soils have been carried out over the last few decades. It has been revealed that methane hydrate-bearing soils and carbon dioxide hydrate-bearing soils have different strength and dilatancy properties even though they have the same hydrate contents. The reason for this might be due to the different types of hydrate morphology. In this study, therefore, the effect of the hydrate morphology on the mechanical response of gas-hydrate-bearing sediments is investigated through a model analysis by taking into account the different hardening rules corresponding to each type of hydrate morphology. In order to evaluate the capability of the proposed model, it is applied to the results of past triaxial compression tests on both methane hydrate-containing and carbon dioxide hydrate-containing sand specimens. The model is found to successfully reproduce the different stress–strain relations and dilatancy behaviors, by only giving consideration to the different morphology distributions and not changing the fitting parameters. The model is then used to predict a possible range in which the maximum deviator stress can move for various hydrate morphology ratios; the range is defined as the strength-band. The predicted curve of the maximum deviator stress obtained by the constitutive model matches the empirical equations obtained from past experiments. It supports the fact that the hydrate morphology ratio changes with the total hydrate saturation. These findings will contribute to a better understanding of the relation between the microscopic structures and macro-mechanical behaviors of gas-hydrate-bearing sediments.     

Development of a transparent triaxial cell and observation of rock deformation in compression and creep tests

A transparent triaxial cell was designed and manufactured using acrylic resin. The cell was used to conduct strength and creep tests. Photographs were taken of the specimens at constant time intervals during the constant strain rate test. Photographs were also taken at constant intervals of strain during the creep test, but this rate was changed to one image per second when the specimens first showed tertiary creep. Comparison of the axial and lateral strains during the constant strain-rate and creep tests indicated no significant differences between the two tests. It is well known that the axial creep strain rate is inversely proportional to remaining life in tertiary creep. This study showed that the lateral creep strain rate is also inversely proportional to remaining life. The constant strain-rate tests were conducted with transparent end pieces attached firmly to the upper and lower ends of the specimens. Three holes were drilled into the end pieces, and water was expelled into the holes when the specimens were compressed. It was clearly observed that the water began flowing from the holes back into the specimen during the volumetric expansion of the specimen. The transparent triaxial cell permitted easy observation of water ejection and re-absorption into the specimens.     

Generic creep behavior and creep modeling of an aged surface support liner under tension

Polymer-based materials have been motivated to be an alternative support system element in the mining/tunneling industry due to their logistic and geotechnical benefits. Thin spray-on liner (TSL), a term to define the application of the material on the rock surface with a layer ranging from 2 mm to 10 mm in thickness, shows some promising results. TSLs are mainly composed of plastic, polymer, or cement-based ingredients to a certain proportion. This study intends to reveal the time-dependent response of TSL specimens, cured throughout 500 d, under four constant stress levels for stable laboratory conditions. The results were correlated using two interrelated equations to predict the material's service life (creep-rupture envelopes). The proposed correlations offered an insight into both the effective permanent support time and the strain amount at the liner failure. The time-dependent deformation of TSL, whose performance is highly responsive to creep behavior, was obtained so that the design engineers may use the findings to avoid the severe problems of material creep. Experimental data were also used to develop a Burgers (four-element) creep model. Since the liner has a nonlinear time-dependent behavior, creep models were built for each stress level separately. Subsequently, a generic equation was obtained using the nonlinear parametric dependencies. There is a good agreement between the proposed model and the experimental results. The proposed model can be used as a basis for future numerical studies related to the support behavior of aged surface support liners.     

统一考虑加载变形与流变的粗粒土弹塑性本构模型及应用

实测资料表明,现行粗粒土本构模型明显低估高度200m以上特高土石坝的沉降量,主要原因之一是现行本构模型普遍割裂加载变形与流变,计算时忽略施工过程中坝料产生的流变。在总结分析典型粗颗粒土石料流变特性的基础上,以应力、应变和时间为基本变量,提出了一个可以统一模拟粗粒土加载变形、流变、应力松弛等性质的弹塑性本构模型。模型假定加载塑性变形和流变可同时发生,应力和时间变化均会引起屈服面扩张,从而产生塑性变形,但两者服从不同的塑性流动准则。运用某特高心墙堆石坝坝壳堆石料和砾石土心墙料的试验结果对模型的合理性进行了验证,并对该大坝填筑施工过程进行了模拟。计算结果表明,忽略坝料施工期流变可使大坝竣工期沉降量低估10%以上。因此,采用统一模拟粗粒土加载变形与流变的本构模型可有效提高特高土石坝变形的预测精度。     

土工合成材料循环受载、蠕变和应力松弛特性的统一本构模拟

应用双曲线方程、修正曼辛准则(Masing rule)和过应力型粘塑性理论,建立了一种可以同时描述土工合成材料应力应变关系的非线性、循环荷载作用下的弹塑性和滞回性、蠕变和应力松弛的一维本构模型。这个本构模型的独特之处包括功能全面、随加载路径移动的蠕变界线、非线性流变特性、以及参数确定方法简单直观等,且它可以很方便的植入已有的有限元软件包中。该模型还可以蜕化为率无关的非线性循环受载模型,可以很简单的应用于非线性动力数值分析。本文应用几种土工格栅试验结果验证了所提出的本构模型。     

Variation in strength and creep life of six Japanese rocks

In this paper, variations in strength and creep life are investigated for rocks under various conditions: dry and wet, uniaxial and triaxial, and compressional and tensile. A number of parameters are introduced for this purpose; to assess the time-dependent failure under constant and monotonic loading the following parameters are used: the parameter of time dependency δ, coefficient of creep life α and coefficient of strength β. δ explains the rate dependency of strength or stress level dependency of creep life. α and β are related to each other. Variations in β have been evaluated using data from previous experiments. It is confirmed here that δ, β and variations in β determined by creep tests are in most cases identical to those determined using strength tests. Variation in β in the wet condition is almost the same as that in the dry condition; however, variation in tension increases more than in compression. Under confining pressure, variation in β is reduced for Neogene sedimentary rocks, and does not appear to change for igneous rock and welded tuff.     

油砂的蠕变特性与本构模型研究

油砂是一种富含天然沥青的沉积砂。作为一种软化材料,油砂的蠕变性质较松砂和正常固结黏土等硬化材料更为复杂。通过开展油砂三轴蠕变试验,获得了不同应力和应变状态下油砂的蠕变特性;讨论了蠕变势与塑性势之间的关系,提出了等效变形过程的概念,分析等效变形过程的微观机制;讨论了蠕变速率与时间和不可恢复应变之间的关系,提出采用不可恢复应变作为内变量可以描述复杂条件下油砂的蠕变性质,并据此提出了油砂的内变量蠕变模型;应用临界状态理论,分析了硬化型和软化型土的蠕变性质;讨论了剪切带对密砂蠕变性质的影响。     

Tests on mechanical behavior of unsaturated decomposed granite and its modelling considering finite deformation

Generally speaking, most of the geomaterials in the surface ground are in an unsaturated state. The mechanical and hydraulic properties of unsaturated soil are much more complicated than those of saturated soil. To rationally describe these properties, it is important to couple the stress-strain relation of the unsaturated soil with its water retention characteristics using rational state variables. In this paper, oedometer and triaxial compression tests on decomposed granite under constant-suction and constant degree of saturation conditions were conducted. Based on the test results, a modified constitutive model was proposed to build an incremental relation between the degree of saturation and suction that considers the influence of finite deformation. The modified model was utilized to simulate the corresponding laboratory tests. It is found that the modified constitutive model has satisfactory accuracy to describe the mechanical and hydraulic properties of unsaturated decomposed granite, which verified the reasonability of the assumption adopted in this paper. The test results are also helpful for the understanding of the moisture characteristics of the decomposed granite under constant degree of saturation condition.     

考虑初始各向异性的砂土弹塑性本构模型及试验验证

基于复杂应力条件下的试验结果,就针对不同主应力方向对应力–应变关系的影响进行了试验研究,并在此基础上结合状态概念提出了基于应力–剪胀关系和应力–应变拟双曲线关系的弹塑性本构模型。利用应力–应变拟双曲线关系描述了主应力方向对有效应力路径、应力–应变发展模式的影响,并通过常规三轴试验测定了试验参数,同时与试验结果进行对比,说明基于状态概念,引入状态参数,利用主应力方向对塑性模量的影响,能够同时描述初始物理状态及初始各向异性对砂土应力–应变关系的影响。     

Attempt to reproduce the mechanical behavior of cement-treated soil using elasto-plastic model considering soil skeleton structure

The improvement of sand and clay using lime or cement to control solidification is common practice. Among the many constitutive models for solidification proposed for clay and sand, few can reproduce the combined behavior of cement-treated clay and cement-treated sand. Here, four typical experimental results for cement-treated soil have been chosen from the literature to consider the shear and consolidation behavior for clay and sand, especially for a low cement mixing ratio. The elasto-plastic constitutive model was used to simulate this behavior considering the soil skeleton structure.The simulation results obtained using the model agreed with the experimental test results both for the cement-treated clay and the cement-treated sand. In the case of the clays, the experimental results were reproduceable using material constants for elasto-plastic and evolution parameters and only required changes in the initial state values, regardless of whether the soil was treated or untreated. In the case of the sands, the structure decay index of the treated sand became smaller than that of the untreated sand. Moreover, the cement-treated loose sand did not exhibit softening behavior. This was attributed to the slow rate of decay of the highly structured cement-treated loose sand due to the solidification of the cement. The degree of structure and the overconsolidation ratio both increased with higher amounts of admixed cement. The model developed in this study was capable of describing the mechanical behavior of both cement-treated clay and cement-treated sand.     

Effect of geotextile reinforcement on the mechanical behavior of sand

Laboratory triaxial compression tests were carried out in order to determine the stress–strain and dilation characteristics of geotextile-reinforced dry beach sand. The mechanical behavior of the composite material was investigated through varying the number of geotextile layers, type of geotextile, confining pressure, and geotextile arrangement. In order to study the effect of sample-size on the results, tests were performed on samples with two different diameters. The results demonstrated that geotextile inclusion increases the peak strength, axial strain at failure, and ductility. However, it reduces dilation. Such improvements in the behavior of reinforced sand are more pronounced for small-size samples. Failure envelopes for reinforced sand were observed as bilinear or curved. Bulging between layers was detected in reinforced samples which failed.     

Experimental investigation and constitutive modeling of the shear creep behavior of an overconsolidated soft clay

This paper presents the results of shear creep experimental investigations carried out on Huzhou overconsolidated soft clay and subsequently proposes a constitutive model that is able to reproduce the shear creep characteristics under complex stress conditions. First, shear creep characteristics are obtained based on the analysis of results from drained triaxial shear creep tests conducted on clay samples. Then, the Yin-Graham equivalent time is extended into shear stress states; thus, the concept of shear equivalent time is formulated. Using the shear creep characteristics observed from experiments and the shear equivalent time concept, a shear stress-strain-strain rate creep model reflecting a complex loading history and loading path is proposed. Finally, the model is solved numerically with the fourth-order Runge-Kutta method, and the predictions are compared with the measured values. The results show that (1) the shear creep coefficient of Huzhou overconsolidated soft clay varies with the overconsolidation ratio levels, and there is a good correspondence between them, which is similar to the relationship of the volumetric creep coefficient and overconsolidation ratio; (2) the predicted curves of the new model are in good agreement with measured curves, which demonstrates that the model can well simulate the shear creep characteristics of soft clay under complex stress conditions.

     

Influence of acidic environment and temperature on mechanical behavior of cement-treated Masado and numerical modeling with a thermoelasto-viscoplastic model

Cement-treated Masado (CTM), as a common cement-mixed geomaterial, is usually only used in ground improvement for temporal structures in Japan. However, in recent years, it has been used for new permanent structures, such as the supporting ground of pile foundations and direct foundations. Masado, a completely decomposed granite, is distributed widely in Japan. However, the long-term stability of CMT has not been thoroughly investigated, despite that it has the risk of exposure to environmental changes in acid conditions and temperature, which are very common in volcanic areas in Japan. In this study, to investigate the influence of acidic environments and temperatures on the mechanical behavior of CMT, a systematic test program using uniaxial and triaxial tests was conducted on CMT specimens under different acidic environments, temperatures, and confining pressures. To avoid the extra influence of water on the mechanical behavior of CMT, a special double-cell was designed within the pressure chamber of the triaxial loading device so that the volumetric strain of the specimens completely sealed with rubber sleeve can be accurately measured in triaxial compression and creep tests. In addition, the chemical components of all the tested specimens are investigated by X-ray fluorescence spectrometry analysis to identify the influence of calcium leaching and hydration reactions in the curing period. Combined with the influence of the initial confining pressure, the influence of the acidic environment and temperature on the strength and dilatancy of CMT is carefully investigated. Based on the test results, a relation between the stress ratio at the critical state and the influential factors, including the initial confining pressure, acidic environment and temperature, is proposed by regression analyses. Meanwhile, an existing thermoelasto-viscoplastic model is modified to properly describe the influence of the abovementioned influential factors on the mechanical behavior CMT. The applicability of the modified model is then verified by triaxial compression and creep tests.     

土的负蠕变特性及其本构模型

蠕变是指有效应力不变的情况下,变形随时间的发展。土力学中,蠕变往往指压缩蠕变,如山区机场高填方工程中填方区的工后沉降。然而,高填方工程中有时还需要对山体进行大体积深开挖,以得到大面积的平整场地。开挖后,土体的膨胀变形不能在短时间内稳定,而是随时间继续发展。将这种随时间发展的膨胀变形定义为负蠕变,相对应地,将压缩蠕变定义为正蠕变。试验表明,正、负蠕变都有趋于稳定的趋势。因此可以假设,在e–lnp空间中,当时间足够长时,不论发生正蠕变还是负蠕变,土体都稳定在一条平行于正常压缩线的直线上,这条线为稳定线。在上述概念的基础上,提出了一个可以合理地描述土的负蠕变效应的一维计算公式,结合UH模型,建立了考虑负蠕变效应的UH模型。最后,将模型预测结果与三轴不排水剪切正负蠕变试验结果进行了对比,验证了模型的正确性。     

隐伏非贯通结构面剪切蠕变特性及本构模型研究

结构面控制岩质边坡失稳的边界,自然界中大量结构面是非完全贯通的,且隐藏在斜坡内部,非贯通结构面研究对于揭示斜坡启动破坏机理及稳定性具有重要意义。隐伏非贯通结构面的岩块取样困难且不能大量复制,通过室内制作一种隐伏非贯通结构面模型,在单轴压缩试验和剪切试验的基础上,采用YZJL-300型岩石剪切流变仪,对隐伏非贯通结构面岩体进行剪切蠕变试验,并对蠕变特性进行研究。根据蠕变试验的阶段特征,引入一个适用于结构面的非线性黏性加速元件和材料损伤变量,建立一种非贯通结构面的剪切蠕变损伤本构模型,并根据拉普拉斯变化推导三维剪切蠕变本构方程。利用L-M算法和全局优化法对所得的蠕变试验曲线进行辨识,求解模型参数。经过辨识并对比试验结果和模型拟合结果,新建立的本构模型能较好地反映非贯通结构面蠕变特性,对揭示岩质斜坡的长期稳定性和时效演化有借鉴意义。     

Rate effect on the stress-strain relations of synthetic carbon dioxide hydrate-bearing sand and dissociation tests by thermal stimulation

Methane hydrates (MHs) have been recognized as an important material for use as a new energy resource. Recently, not only MHs, but also carbon dioxide hydrates (CO₂-hydrates), have been attracting attention from the viewpoint of CO₂ storage in the form of CO₂-hydrates. It is essential, therefore, to investigate the mechanical behaviour of gas hydrate-bearing sediments in order to achieve safe MH extraction and to ensure the long-term stability of CO₂-hydrate-storaged submarine sediments. In order to gain further knowledge of CO₂-hydrate-bearing sediments, we carried out three kinds of laboratory experiments on synthetic CO₂-hydrate-bearing sand specimens: (1) undrained triaxial compression tests with a constant strain rate, (2) undrained triaxial compression tests with a step-changed strain rate, and (3) dissociation tests on CO₂-hydrate-bearing sand specimens using the thermal stimulation method.The main findings obtained from these three experiments are as follows: First, it was found that CO₂-hydrate-bearing sand shows larger strength and larger positive dilatancy than water-saturated sand without CO₂-hydrates under undrained conditions. Second, CO₂-hydrate-bearing sand clearly exhibits strain rate dependency with an increase in hydrate saturation. It is interesting to note that hydrate saturation’s dependency on strength does not appear in cases where the strain rate is quite slow. Third, a drastic increase in pore pressure and extensive tensile strain are observed simultaneously during the dissociation of CO₂-hydrates. The pore gas pressure of CO₂, produced by the hydrate dissociation, can exceed the liquefied boundary of CO₂-gas, and the increase in pore gas pressure can be limited by the liquefaction of CO₂-gas.     

Accurate time-dependent analysis of concrete bridges considering concrete creep, concrete shrinkage and cable relaxation

This paper proposes a new relaxation model for steel tendons based on the equivalent creep coefficient to enable the accurate estimation of losses of cable forces. The equivalent creep coefficient works not only in the case of intrinsic relaxation but also under various boundary conditions. Based on the proposed relaxation model, an accurate finite element analysis of the time-dependent behavior of concrete bridges considering concrete creep, concrete shrinkage and cable relaxation is devised based on the time integration method. Concrete members are modeled by beam elements while tendons are modeled by truss elements with nodes connected to the beam axis by perpendicular rigid arms. Then the individual and combined effects of concrete creep, concrete shrinkage and cable relaxation on the long-term performance of concrete structures are investigated. It is found that the proposed relaxation model and time integration method can provide a reliable method for time-dependent analysis. The numerical results obtained indicate that the interactions among these factors should be considered carefully in analyzing the long-term performance of concrete bridges.     

A constitutive model for evaluation of mechanical behavior of fiber-reinforced cemented sand

The aim of this study is to present a constitutive model for prediction of the mechanical behavior of fiber-reinforced cemented sand. For this purpose, a generalized plasticity constitutive model of sandy soil is selected and the parameters of the model are determined for three types of sandy soils using the results of triaxial tests. Next, the proposed model is developed using the existing models based on the physico-mechanical characteristics of fiber-reinforced cemented sand. The elastic parameters, flow rule and hardening law of the base model are modified for fiber-reinforced cemented sand. To verify the proposed model, the predicted results are compared with those of triaxial tests performed on fiber-reinforced cemented sand. Finally, the efficiency of the proposed model is studied at different confining pressures, and cement and fiber contents.     

考虑流变参数弱化综合影响的软岩蠕变损伤本构模型及其参数智能辨识

根据损伤的定义和软岩的蠕变破坏特点,提出反映应力水平和时间因素对弹性模量弱化综合影响的软岩蠕变损伤变量的一般表达式,推导并建立软岩蠕变损伤演化方程,探讨损伤变量随应力水平和时间的变化规律。以Burgers模型为基础,建立可考虑参数综合弱化的软岩蠕变损伤本构方程。修正后的Burgers蠕变损伤本构模型体现了参数随时间增长和应力水平增大的弱化现象,反映了岩石材料的损伤劣化过程。基于粒子群优化算法(PSO)具有迭代过程简单、能有效地收敛到全局最优解等优良特性,提出基于PSO的蠕变损伤本构模型参数智能辨识的方法和步骤。采用MATLAB软件,编制基于粒子群优化算法的参数辨识程序。以某深部软岩试样蠕变试验为例进行验证分析,结果表明采用考虑参数综合弱化的蠕变损伤本构模型具有较好的实用性。