Predictive potential of Perzyna viscoplastic modelling for granular geomaterials

This paper reappraises Perzyna-type viscoplasticity for the constitutive modelling of granular geomaterials, with emphasis on the simulation of rate/time effects of different magnitude. An existing elasto-plastic model for sands is first recast into a Perzyna viscoplastic formulation and then calibrated/validated against laboratory test results on Hostun sand from the literature. Notable model features include (1) enhanced definition of the viscous nucleus function and (2) void ratio dependence of stiffness and viscous parameters, to model the pycnotropic behaviour of granular materials with a single set of parameters, uniquely identified against standard creep and triaxial test results. The comparison between experimental data and numerical simulations points out the predicative capability of the developed model and the complexity of defining a unique viscous nucleus function to capture sand behaviour under different loading/initial/boundary and drainage conditions. It is concluded that the unified viscoplastic simulation of both drained and undrained response is particularly challenging within Perzyna's framework and opens to future research in the area. The discussion presented is relevant, for instance, to the simulation of multiphase strain localisation phenomena, such as those associated to slope stability problems in variably saturated soils.     

岩质边坡稳定性评价的尖点突变理论模型

考虑岩质边坡后缘张裂隙中的孔隙水压力及地下水对软弱结构面的物理化学作用,将软弱结构面分为应变硬化区和应变软化区,建立单平面滑动破坏的岩质边坡力学模型,引入尖点突变理论,建立尖点突变模型,推导岩质边坡突变失稳的充要力学条件判据,并重新推导极限平衡法的临界稳定系数。结果表明,分叉集方程等于0为岩质边坡突变失稳充要力学条件判据;由于滑面处含水量不同,稳定系数小于1,边坡不一定会发生失稳;稳定系数大于1,边坡也不一定稳定。     

Parametric study using FEM for the stability of the RCC Tannur dam

Parametric study was carried out in this paper using the Finite Element Method (FEM) to study the effects of variations of the two parameters, i.e., variations in batter slope and foundation properties under static and dynamic loading conditions on the distribution of the peak stresses within the dam and foundation soils. The response spectra for the operating basis earthquake (OBE) and the maximum credible earthquake (MCE) were used in the dynamic analysis. It is shown that the distribution of peak stresses across base of dam and the extent of tensile zone on the foundation soils at the foundation level vary with the variations in the batter slope. Generally, as the batter slope increases the extent of the tensile zone at the foundation level decreases but not necessarily the maximum tensile stresses. However, an optimum batter slope could be achieved based on the results of this parametric study. It should be noted that the greatest tension is developed in the rock adjacent to the toe of the batter. For the foundation mechanical properties, it has been shown that the assessment of accurate soil mechanical properties has a great effect on the computed design stresses from finite element analysis. However, the greatest effect of the foundation properties was on the stresses at the base of dam while it has a negligible effect within the dam body.     

Nonstationary flow surface theory for modeling the viscoplastic behaviors of soils

This paper presents a three-dimensional elastic viscoplastic model that can describe the time-dependent behaviors of soft clays. The constitutive model is formulated based on the nonstationary flow surface theory and incorporates new developments, including (i) an improved definition of the nonstationary flow surface that is capable of capturing the stress–strain behaviors under different loading paths, (ii) a unique stress–strain—viscoplastic-strain-rate equation that is able to explicitly describe the nonstationary flow surface, and (iii) a final stable state concept that identifies the final equilibrium state at the end of creep and stress relaxation, which is also used to simplify the loading criteria. The consistency condition is validated for the proposed model, and the viscoplastic multipliers are calculated by solving the consistency equations. The model performance is investigated and validated via simulation of both oedometer and triaxial tests. The numerical results demonstrate that the proposed model is able to reproduce the main viscoplastic behaviors of soils, including creep, undrained creep rupture, stress relaxation, rate effect and accumulated effect.     

加载速度对断层-围岩系统变形及快速回跳的影响

在平面应变状态下,采用拉格朗日元法模拟了加载速度对断层-围岩系统形成时的应力水平、塑性区尺寸及剪切带图案、系统的最大承载能力以及快速回跳发生时的应力水平的影响。在数值计算中,采用了莫尔-库仑与拉破坏复合的破坏准则。峰后岩石的本构关系为线性应变软化。通常断层带-围岩系统形成之后系统的承载能力达到最大,之后系统的承载能力开始下降处于应变软化状态。当位于试样加载端上的单元的压缩应力-压缩位移曲线的峰后刚度足够大时,系统就会发生弹性回跳现象,即失稳破坏。随着加载速度的增加,断层带-围岩系统形成时的应力水平、断层带-围岩系统的最大承载能力、快速回跳发生时的应力水平及上述三者所对应的加载端部位移都增加,屈服单元数目增多,塑性区域不再保持平直,这都将大大增加系统的变形阻力。当加载速度较大时,较高的剪切应变率集中在断层带位置及断层带之外的弹性体的某些区域都是可能的。     

深基坑土钉支护的有限元数值模拟及稳定性分析

在平面应变条件下,对土钉支护的基坑进行了有限元数值模拟,在此基础上采用有限元稳定分析方法评价基坑的稳定性。研究了土钉拉力及基坑变形的变化规律,土钉长度及布置方式变化对基坑变形和稳定性的影响。结果表明,当土钉足够长时,基坑的潜在滑动面与最大拉力作用点连线位置通常是一致的     

A Physical and Numerical Investigation on the Stability of Shallow Tunnels in Strain Softening Media

Summary The behaviour up to failure of shallow underground openings is discussed on the basis of some laboratory, small-scale model tests and of finite element simulation. The experimental results are first illustrated. They were obtained from two-dimensional (plane strain) and three-dimensional tunnel models tested under standard gravity conditions. Then, the phenomenon of strain localisation that characterizes the medium surrounding the model tunnels is discussed, recalling two alternative approaches for its numerical interpretation. On this basis, a finite element procedure for strain softening analyses is outlined and applied to the simulation of the tests in both two- and three-dimensional conditions. The comparison between experimental and numerical results leads to some conclusions on the influence of strain localisation on the overall behaviour of shallow tunnels and on the stability of their headings.     

A multisurface kinematic hardening model for the behavior of clays under combined static and undrained cyclic loading

In dynamic geotechnical problems, soils are often subjected to a combination of sustained static and fast cyclic loading. Under such loading conditions, saturated and normally consolidated clays generally experience a build-up of excess pore water pressure along with a degradation of stiffness and strength. If the strength of the soil falls below the static stress demand, a self-driven failure is triggered. In this paper, a constitutive model is presented for the analysis of such problems, based on a general multisurface plasticity framework. The hardening behavior, the initial arrangement of the surfaces, and the nonassociated volumetric flow rule are defined to capture important aspects of cyclic clay behavior. This includes nonlinear hysteretic stress-strain behavior, the effect of anisotropic consolidation, and the generation of excess pore water pressure during undrained cyclic loading along with a degradation of stiffness and strength. The model requires nine independent parameters, which can be derived from standard laboratory tests. A customized experimental program has been performed to validate the model performance. The model predictions show a good agreement with test results from monotonic and cyclic undrained triaxial tests, in particular with respect to the strain-softening response and the number of loading cycles to failure. A procedure for a general stress-space implicit numerical implementation for undrained, total stress-based finite element analyses is presented, including the derivation of the consistent tangent operator. Finally, a simulation of the seismic response of a submarine slope is shown to illustrate a possible application of the presented model.     

液化后砂土流体特性测试装置的研发及试验研究

为研究液化砂土的流动变形特征,开展了液化砂土的流体特性试验研究。基于边界层理论研发一套液化砂土表观黏度测试装置,主要由调速电机、电机调速器、圆柱转子、扭矩传感器等部分组成,该装置能有效测试液化砂土的表观黏度;开展了不同孔压比、转速下饱和砂土表观黏度及圆柱转子所受摩擦力矩特性研究,着重分析了液化后砂土的表观黏度特性。结果表明,液化前孔压发展及液化后孔压消散阶段摩擦力矩均受到孔压比及转速的影响;液化后砂土孔压逐渐消散,强度恢复,流动能力衰减,表观黏度升高,表观黏度与孔压比呈现出一定的线性相关性;表观黏度随着剪应变率的增大而降低,且两者呈现出幂函数关系,表明液化后砂土具有典型幂律型剪切稀化非牛顿流体特征。     

模拟岩石应变软化变形全过程的统计损伤本构模型研究

建立可以模拟岩石应力-应变全过程的本构模型一直是岩土工程中研究的热点之一.首先,在深入探讨应变软化类岩石受力变形全过程每个阶段特征的基础上,并针对Lemaitre应变等价性理论的不足之处,基于更合理的假定建立了能够反映应变软化类岩石体积变化以及峰后出现残余强度的新型损伤模型;然后,通过引入统计损伤理论并考虑损伤阈值对岩石变形过程的影响,建立了能够模拟应变软化类岩石受力变形全过程的统计损伤本构模型,该模型还可以反映岩石的体积变化以及初始孔隙率对其变形过程的影响;最后,通过理论计算与试验结果及他人模型的对比分析,表明了该模型的合理性与可行性.     

深隧道围岩稳定性的数值模拟与模型试验研究

在大型真三轴模型试验机上进行隧道的超载试验,试验过程中采用内窥摄影实时监测试件的破坏演化全过程,确定了高应力条件下隧道的变形特征和裂纹演化特征;采用应变测量技术对隧道关键部位的应变值进行了测量,据此分析了围岩体的应力变化规律。针对相似材料的力学特性,建立了应变软化模型,采用与模型试验相同的边界条件进行了数值模拟分析,对围岩体的塑性应变变化过程和应力场进行了研究。结果表明,两种手段反映的破坏演化过程是基本吻合的,所反映的围岩体应力变化规律也是一致的。     

昆明白泥井3号隧道围岩稳定性FLAC3D模拟

应用FLAC3D数值模拟软件对昆明白泥井3号隧道围岩稳定性进行了分析和评价.结果表明隧道支护后围岩的最大变形出现在隧道右侧拱顶,为1.00cm;隧道支护后左侧拱和右侧拱脚处剪应变明显增大,为应力集中区;围岩可以通过自身应力、应变和能量的调整,使整个隧道围岩处于稳定状态;同时也表明FLAC3D用于隧道围岩变形分析是可行的.     

A multi-method approach to study the stability of natural slopes and landslide susceptibility mapping

In this paper, a multi-method approach for the assessment of the stability of natural slopes and landslide hazard mapping applied to the Dakar coastal region is presented. This approach is based on the effective combination of geotechnical field and laboratory works, of GIS, and of mechanical (deterministic and numerical) stability analysis. By using this approach, valuable results were gained regarding instability factors, landslide kinematics, simulation of slope failure and coastal erosion. This led to a thorough assessment and strong reduction in the subjectivity of the slope stability and hazard assessment and to the development of an objective landslide danger map of the SW coast of Dakar. Analysis of the results shows that the slides were influenced by the geotechnical properties of the soil, the weathering, the hydrogeological situation, and the erosion by waves. The landslide susceptibility assessment based on this methodological approach has allowed for an appropriate and adequate consideration of the multiple factors affecting the stability and the optimization of planning and investment for land development in the city.     

三峡库区巴东复杂斜坡系统变形机理数值模拟与稳定性研究

三峡库区巴东斜坡系统复杂,其变形机理与稳定性研究对移民迁建和地质灾害防治具有重要意义.本文采用FLAC3D数值计算方法,对巴东复杂斜坡系统变形机理进行研究,为"重力成因论"提供了支持和佐证.从整个斜坡系统的角度,联合使用数值计算方法(FLAC3D)和极限平衡法(Sarma法),系统研究了巴东复杂斜坡系统在不同蓄水状态下不同层次的稳定性问题,结果表明斜坡系统不存在整体深层滑动的可能性;斜坡系统沿岩层界面或软弱层发生失稳的可能性较小,但赵树岭滑体和黄土坡滑体在表层滑体发生较大变形或失稳的条件下,可能导致滑坡整体复活;在水库调蓄过程中,部分滑坡表层堆积体产生变形破坏的可能性较大.另外,斜坡的表层改造可能会引起斜坡中层稳定性的恶化.     

Evaluation of water quality parameters for the Mamasin dam in Aksaray City in the central Anatolian part of Turkey by means of artificial neural networks

Sustaining the human ecological benefits of surface water requires carefully planned strategies for reducing the cumulative risks posed by diverse human activities. The municipality of Aksaray city plays a key role in developing solutions to surface water management and protection in the central Anatolian part of Turkey. The responsibility to provide drinking water and sewage works, regulate the use of private land and protect public health provides the mandate and authority to take action. The present approach discusses the main sources of contamination and the result of direct wastewater discharges into the Melendiz and Karasu rivers, which recharge the Mamasın dam sites by the use of artificial neural network (ANN) modeling techniques. The present study illustrates the ability to predict and/or approve the output values of previously measured water quality parameters of the recharge and discharge areas at the Mamasin dam site by means of ANN techniques. Using the ANN model is appreciated in such environmental research. Here, the ANN is used for estimating if the field parameters are agreeable to the results of this model or not. The present study simulates a situation in the past by means of ANN. But in case any field measurements of some relative parameters at the outlet point “discharge area” have been missed, it could be possible to predict the approximate output values from the detailed periodical water quality parameters. Because of the high variance and the inherent non-linear relationship of the water quality parameters in time series, it is difficult to produce a reliable model with conventional modeling approaches. In this paper, the ANN modeling technique is used to establish a model for evaluating the change in electrical conductivity (EC) and dissolved oxygen (DO) values in recharge (input) and discharge (output) areas of the dam water under pollution risks. A general ANN modeling scheme is also recommended for the water parameters. The modeling process includes four main stages: (1) source data analysis, (2) system priming, (3) system fine-tuning and (4) model evaluation. Results of the ANN modeling scheme indicate that the output values are agreeable to the water quality parameters, which were measured at the field in the static water mass of the Mamasın dam lake. Water contamination at the dam site is caused by the continuous increase of nutrient contents and decrease of the O₂ level in water causing an anaerobic condition. It may stimulate algae growth flow in such water bodies, consequently reducing water quality.     

宾汉钻井液圆管轴向层流压降的数值计算

在计算宾汉钻井液在钻具中的层流压降时,需要求解一个非线性方程,以往通常使用近似公式进行计算。对于求解这个非线性方程,提出了一个数值迭代算法,并对该算法的收敛性进行了证明,给出了最大迭代步数的上限值。理论分析和大量实际算例表明：本文算法具有非常稳定的收敛性和非常快的收敛速度,并且能够给出压降的精确计算值。     

A comparative study of different model families for the constitutive simulation of viscous clays

The simulation of the viscous behavior of some clays is of high importance in many geotechnical problems. The literature offers a vast amount of constitutive models able to simulate the rate dependence observed on these materials. Although most of these models are calibrated to very similar experimental observations and share similar definitions of material parameters, some discrepancies of their response have been detected, which are related to their mathematical formulations. In this work, the causes of these discrepancies are carefully studied. To that end, four different model families are analyzed, namely, nonstationary flow surface (NSFS) models, viscoplasticity with overstress function (OVP), viscoplasticity with Norton's power law (NVP), and visco-hypoplasticity (VHP). For the sake of a fair comparison, single constitutive models using the same set of material parameters, and following other requirements, are developed for each model family. Numerical implementations of the four resulting models are performed. Their response at different tests are carefully analyzed through simulation examples and direct examination of their constitutive equations. The set includes some basic tests at isotropic stress states and others as responses envelopes, undrained creep rupture, and an oedometer test with loading, unloading-reloading, creep, and relaxation. The article is concluded with some remarks about the observed discrepancies of these model families.     

河北省滦平县张家湾隧道断裂破碎带特征与围岩失稳研究

根据对张家湾隧道开挖过程中多次发生塌方地段的工程地质调查资料,采用现代地质工程的理论,系统分析了引起张家湾隧道塌方的叠瓦状断裂带的构造特征、力学性质、以及控制该地区主要工程地质特征。研究主要内容包括断裂带及其周围地区的岩石组合特征、地下水特征、断裂构造带应力场及伴生的小构造特征,重点分析了断层带活动对于隧道围岩体的力学性质所产生的影响,通过对比断层破碎带中及两侧岩石风化前后的力学参数,发现了该隧道可能产生塌方的主要地段的识别标志。结合现场实际情况提出在隧道施工中,应对危险岩体地段加强长、短期地质超前预报工作以及通过合理的喷浆、结合钢支撑的组合支护方式,控制隧道变形的防治措施。     

A classification system for the assessment of slope stability of terrains along highway routes in Jordan

 The establishment of comprehensive development plans, in general, and the proper selection of highway routes, in particular, require an assessment of landslides or instability hazards in the project sites. The frequent landslides that occurred along the routes of major highways in Jordan, and particularly along the Amman-Na'ur-Dead Sea highway and Irbid-Jerash-Amman highway, have substantially increased the cost of construction and caused a considerable delay in the completion of work. The study of many landslides that occurred in the last 25 years along the highway routes and in the sites of some major civil engineering projects in Jordan has led to the recognition of major factors that affect the stability of slopes, and thus the safety and economics of these projects. The geological formation, structural features, topographic characteristics, geometry, and climatic conditions were adopted as the basis for the classification of terrains in terms of their stability. Each factor has been assigned a rating to indicate its relative contribution to the overall stability according to engineering judgment and past experience. The areas have been classified into 5 groups according to their total stability rate. The simplicity, comprehensiveness, and accuracy are the main characteristics of the proposed classification. Its significance stems from its helpfulness as a guide to the geotechnical and highway engineers in assessing the overall stability of the alternative routes of proposed highway projects. Received: 3 December 1996 · Accepted: 29 April 1997