Combined methodology for three-dimensional slope stability analysis coupled with time effect: a case study in Germany

High-steep slopes in open pit mines are much more likely to collapse due to mining operations. Challenges such as data acquisition, precise numerical models and adaptable methodologies have impeded more reliable results of slope stability analysis based on the current methods. Within this context, this paper proposes a combined methodology using light detection and ranging technology to capture high-resolution slope geometry, three-dimensional geological and geotechnical modeling technologies for creating high-quality numerical simulation models and finite-element slope stability analyses combined with a new automatic strength reduction technique to analyze complex geotechnical problems. At the end, the methodology introduces a time series analysis to improve the reliability of the calculated factor of safety. A case study in the deepest open pit mine in Hambach, Germany, was conducted to test and demonstrate the effectiveness and applicability of the proposed methodology.     

Engineering geological evaluation of geological barrier rocks at landfills and repositories

 A site specific evaluation of geological barriers is required for landfills and waste repositories. The waste-repository-rock system has to be taken into consideration for this. Since the geotechnical barrier in conjunction with geological barriers contributes considerably to long-term isolation of the harmful substances from the biosphere, it is absolutely necessary to use engineering geology and hydrogeological methods for a quantitative assessment of the barrier effect of the host rock and the geological environment. For a waste disposal mine, the load-bearing capacity of the rock, the protective properties of the surrounding rock formations and the geological stability of the area are important factors in a safety analysis, of which the geotechnical stability analysis is an important part. Such an analysis comprises an engineering geological study of the site, laboratory and in situ experiments and model calculations, long-term monitoring, and special geological and geochemical investigations. Existing geomechanical modelling techniques provide a useful tool to perform barrier integrity calculations and to design an underground disposal mine. As an example, the barrier performance of the rock salt of the Gorleben salt dome is demonstrated using various safety indicators. Received: 3 January 1997 · Accepted: 29 January 1997     

Improving Short- and Long-term Stability of Underground Gypsum Mine Using Partial and Total Backfill

The stability of underground mines represents a key issue for active and abandoned mines. Over the last few years, several collapses of underground mines in France have affected existing buildings and infrastructures. Many factors are generally identified as the cause of failures: pillar ageing, fractures, and pillars’ height to width ratio, etc. Among the treatment techniques available to prevent instability and reduce the deterioration of pillars, backfill is the most frequently used. A research programme, supported by the French Ministry of the Environment, was developed to study the operability of partial and total backfill using waste material in the Livry-Gargan gypsum mine (near Paris, France), where pillar height is 17 m. The paper focuses on: (1) the characterisation of the gypsum and fill material (laboratory and in situ tests), (2) the in situ measurements, involving 5 pillars equipped with 19 pressure cells, since 1999, (3) and numerical modelling of fractured pillars performed in order to improve understanding of the effects of backfill on the stability of room-and-pillar mines. The study clearly shows the operability and the advantages of partial and total backfill for short-term pillar stability. The induced horizontal pressure generated by backfill can reach 200 kPa. The use of numerical modelling also shows the effect of backfill on fractures and that backfill reduces indicatively the shear displacement and the opening of fractures. Numerical modelling helps in identifying the mechanisms of backfill and in a better understanding of the behaviour of backfilled mines.     

Optimization of slope angle of a lead-zinc mine

Summary The present study represents an attempt to optimize the overall slope angle of a lead-zinc mine in Rajasthan state, which could extend to a depth of 170 m during the first phase of mining. Detailed geotechnical investigations were conducted. These included geotechnical mapping, determination of the physio-mechanical properties of intact rock, determination of the rock mass rating and estimation of rock mass properties. Information was collected from mapping of benches and borehole logs. Based on these data, limit equilibrium and numerical simulation techniques were applied in order to assess the stability of the slopes and determine an optimum slope angle. From these investigations it has been inferred that the overall footwall and hangingwall slopes should be 42° and 48° respectively.     

岩体裂隙模拟的扩展有限元法应用研究

岩体中普遍存在着断层﹑节理和裂隙等结构面,这些结构面的存在和发展对岩体的整体强度﹑变形及稳定性有极大的影响。因此,研究岩体中原生结构面的萌生﹑发展以及贯通演化过程对评估岩体工程安全性和可靠性具有非常重要的理论与现实意义。扩展有限元法（XFEM）作为一种求解不连续问题的有效数值方法,模拟裂隙时独立于网格,因此,在模拟岩体裂隙扩展﹑水力劈裂等方面具有独特优势。针对扩展有限元法的基本理论及其在岩体裂隙扩展模拟中的应用展开了研究,建立了扩展有限元法求解岩体裂隙摩擦接触、岩体裂隙破坏等问题的数值模型,并将计算模型应用于岩质边坡稳定性分析和重力坝坝基断裂破坏等工程问题。     

A Methodology for the Definition of Geotechnical Mine Sectors Based on Multivariate Cluster Analysis

This paper offers a new method for the definition of geotechnical sectors in open pit mines based on multivariate cluster analysis. A geological-geotechnical data set of a manganese open pit mine was used to demonstrate the methodology. The data set consists of a survey of geological and geotechnical parameters of the rock mass, measured directly in several points of the mine, structured initially in twenty-eight variables. After the preprocessing of the data set, the clustering technique was applied using the k-Prototype algorithm. The squared Euclidean distance was used to quantify the proximity between numerical variables, and the Jaccard's coefficient of similarity was used to quantify the proximity between the nominal variables. The different cluster results obtained were validated by the multivariate analysis of variance. The identification of cluster structures was achieved by plotting them on the mine map for spatial visualization and definition of geotechnical sectors. These sectors are spatially contiguous and relatively homogeneous regarding their geological–geotechnical properties, indicated by a high density of points of the same group. It was possible to observe a great adherence of the proposed sectors to the mine geology, demonstrating the practical representativeness of the clustering results and the proposed sectors.

     

盐岩储气库蠕变损伤分析

通过对盐岩本构关系的实验研究,对Norton Power 盐岩蠕变本构模型引入损伤变量,在损伤等效应力中引入考虑偏应力和围压影响的函数,建立了一种能反映盐岩蠕变和加速蠕变的本构模型。通过对某盐矿盐岩实验数据进行拟合,获得了本构模型的参数,理论曲线与实验曲线吻合得较好。通过三维数值模拟方法,应用该本构模型对某盐矿天然气储存库进行了数值模拟研究,探讨了该盐矿盐岩储气库最低内压工况下,腔周损伤区的扩展、变形规律及最长持续运行时间。研究结果表明：（1）盐岩本构模型在盐岩加速蠕变期具有较好的数值稳定性;（2）在最低运行压力下,腔周盐岩进入加速蠕变期后,储气库损伤区的扩展速度非常快,储气库最危险部位位于腔体顶部;（3）腔周盐岩进行加速蠕变期后,盐岩的腔体体积收敛变形主要表现为损伤蠕变体积收敛,稳态蠕变体积收敛趋于稳定;（4）该盐矿盐岩储气库在该压力下最长持续运行时间约为3个月。     

A Review on the Design,Applications and Numerical Modeling of Geocell Reinforced Soil

For improving the stability and load carrying capacity of weak subgrade, strengthening methods are to be followed in the field. Among the various approaches, geocells have been identified as an effective soil reinforcement technique for improving soft subgrade behaviour. The three-dimensional honeycomb structure of geocell offers more lateral confinement to the infill soil resulting in improved load carrying capacity. This led to the widespread use of geocells for different geotechnical applications like pavements, foundations, embankments, slope protection, erosion control etc. Many researchers in the past have confirmed the suitability of geocell reinforcement through their experimental, numerical and field studies. In this paper, a comprehensive review of the reinforcement mechanisms, design aspect and numerical modelling techniques of geocell reinforced soil is provided. In addition, this paper highlights the various field application scenarios where different types of geocells have been used and explores the research challenges and scope for further research in this field.

     

Hydromechanical modelling of the SEALEX experiments

Numerical modelling of coupled physical processes in bentonite–sand mixtures under the geological conditions is significant for designing and constructing sealing systems in deep underground repositories for highly radioactive nuclear waste. Within the framework of DECOVALEX 2015, Task A, this work presents the model validation of OpenGeoSys by numerical modelling of coupled hydromechanical (HM) processes in bentonite–sand mixtures. Parameters used in the HM model were determined by modelling the laboratory tests of the sealing experiment (SEALEX). Afterwards these parameters were applied for the modelling of a small-scale mock-up test considering the influence of technological gap and incidental fail of the seal in the sealing system. In order to investigate the availability of employing these HM parameters and numerical models directly to field predictions, the modelling results and measured data of an in situ SEALEX experiment were analysed comparatively. The modelling results reproduced well the main features in HM behaviour of the compacted bentonite–sand mixture, which denotes that the adopted HM models and parameters are adequate for describing the HM processes in the sealing system. It is necessary to take the elastoplastic behaviour and evolution of the permeability of bentonite–sand mixtures into account when using the adopted models to reproduce the HM processes of a sealing system.     

无底柱崩落采矿大断面结构参数的数值模拟研究

针对金山店铁矿亟需确定的大断面结构参数问题,采用东北大学研制的SLS崩落放矿模拟系统和ITASCA公司的岩土工程软件FLAC3D,对该矿三期的采场结构参数进行了模拟分析和研究。数值放矿试验表明,崩矿步距对矿石回收指标影响较大,崩矿步距3.5～4.5 m更有利于指标回收,兼顾爆破参数因素时推荐为3.5 m。通过进路群开挖的数值模拟,并对开挖过程围岩的位移、应力和塑性区进行分析,发现开挖后塑性区范围不大,巷道具有良好的稳定性。因此,拟采用的进路参数从矿石回收指标和巷道稳定性方面看是较合理的方案。     

基于细观试验和离散元法的盐岩力学特性

当前盐岩的宏观力学模型通常是唯象模型,不能很好地解释盐岩受力变形破坏的真正物理基础。盐岩是由于化学沉积而形成的矿物集合体,是一种主要由NaCl和少量杂质组成的多晶体,其变形机制主要由晶粒与晶界的力学特性控制。通过扫描电镜（SEM）,获得盐岩晶粒的微细观结构特征,采用分子动力学方法和纳米压痕技术,确定盐岩晶粒和晶界的微细观力学参数;将盐岩晶粒作为块体,基于Voronoi多边形技术,建立盐岩的微细观数值模型;利用离散元方法,对盐岩试件在单轴压缩和直剪条件下的宏观力学行为进行了数值模拟。数值模拟结果与宏观力学试验结果吻合度高,表明基于盐岩微细观晶粒结构特征并结合离散元数值模拟的方法能够较好地研究盐岩的宏观力学性能及其材料物理基础。     

Experimental and numerical studies of rock salt strain hardening

The work presented in this paper comes as part of a research program dealing with the thermomechanical behaviour of rock salt. It aims to study laboratory and in-situ long-term behaviour by means of creep tests with deviator and temperature changes. The laboratory results, using a triaxial multi-stages creep tests, highlighted the strain hardening character of rock salt. Furthermore, the in-situ results, using a borehole dilatometer multi-step creep test, have shown that the drilling is carried out in a weakly stressed pillar. The interpretation of the laboratory results, using the J.LEMAITRE law, did not indicate full agreement with all the test results. As a result a ‘double’ J.LEMAITRE model, which takes into account a double strain hardening variable, has been put forward. The validation of this model on the laboratory creep tests is very satisfactory. Furthermore, the activation energy seems satisfactory to represent the influence of the temperature. The in-situ behaviour modelling is clearly more complex than the modelization based on laboratory tests. In fact, it seems that if the rock salt behaviour is maintained by J.LEMAITRE law, it is necessary to vary with the stress, at least, one of the parameters assumed constant in the basic law.     

盐岩地下储气库体积收敛失效概率分析与模型试验验证

盐岩地下储气库受盐岩蠕变特性影响会产生较大的体积收缩变形,影响储气库安全稳定运行。目前对地下储气库体积稳定性分析方法和评价准则各国没有统一标准,国内主要采用数值计算的方法来评价储气库体积稳定性。以江苏金坛盐矿地下储气库体积收敛数值分析为基础,借鉴国外盐岩地下储气库稳定性评价标准,建立盐岩地下储气库体积收敛失效风险评价矩阵,采用一次二阶矩法显示功能函数分析储气库体积收敛失效概率,分析得出：储气库在长期恒定内压工况下体积收敛失效概率随内压的增大而减小;在短期调峰低压工况下体积收敛失效概率随内压的减小而增大;最小内压应保持在4.2 MPa以上。通过交变气压条件下层状盐岩地下储气库大型三维地质力学模型试验得出:储气库体积收敛变形随内压的增大而减小;短期运行最小内压应大于4.0 MPa。模型试验结果与失效概率分析结论较为相似。因此,为避免盐岩地下储气库产生体积收敛破坏,应保证调峰短期最小内压在4.0 MPa以上。     

深部盐岩蠕变特性研究

通过对湖北云应的盐岩和泥岩,以及江苏金坛盐岩的单轴、三轴蠕变试验结果的分析,研究了包括应力、围压等外在条件以及内部组成结构对盐岩蠕变特性的影响。发现盐岩无论在何种围压下,稳态蠕变率都随偏应力的增加而显著地增大;围压起到限制变形的作用,而随着围压增高,围压对稳态蠕变率的影响越来越小;晶粒越大和杂质含量越多,盐岩的蠕变属性越弱。盐岩的蠕变变形是应变硬化和回复效应等内部变形机制共同作用的结果,选用内应力作为内变量可用来描述这种内部微观结构的演化过程。对云应盐岩的多级加载蠕变试验数据进行拟合,确定了非线性蠕变本构方程中的参数,理论与试验结果的比较表明,两者吻合性较好。     

A study on geotechnical characterization and stability of downstream slope of a tailings dam to improve its storage capacity (Turkey)

Tailings, waste products of the ore-dressing processes, are usually stored in a slurry form and pumped into sedimentation pond which is surrounded by natural heights and/or artificial walls. The database of International Commission of Large Dams and United Nations Environment Programme showed that several hundreds of tailings dam have failed since 1910. One of the reasons of these failures is slope instability. Construction of mine tailings dams in Turkey has started to increase, particularly in the recent decade. This paper focuses the study undertaken to assess the geotechnical characteristics and stability of downstream slope of a tailings dam at a copper-zinc mine in the Black Sea Region of Turkey. At the mine site there are two tailings dams located at two different elevations. The upper dam was completely filled and is out of service. Increasing amount of tailings filling the pond of the lower dam resulted in a limited storage, and therefore, it has been decided that heightening of the dam about 7 m to improve its storage facility would be necessary. For the purpose, a series of investigations in the vicinity of the tailings dam and some geomechanical laboratory tests were carried in conjunction with the use of empirical methods to determine mechanical properties of the current dam material, tailings and the rockfill material that will be used for heightening of the dam. The results of the static and pseudo-static stability analyses based on the methods of limit equilibrium and numerical modelling indicated that circular and non-circular failures are the most critical modes of failure for the dam and safety factor gradually decreases when the tailings rise up to a specific elevation in the pond. However, the ranges of the calculated safety factors from both methods are considerably similar and suitable for heightening of the dam by 7 m in terms of stability. The results suggest that any instability is not expected under both static and dynamic conditions after the current height of the tailings dam is increased by 7 m, if the slope geometry considered in the analyses is not changed and the tailings in the pond do not rise up to an elevation of about 475 m. In addition, magnitude of permanent settlement of the dam due to a probable earthquake was also assessed using two empirical methods and the results from both methods showed similar amount of settlements which were in tolerable limits.     

Effect of soil variability on the bearing capacity of clay and in slope stability problems

Site-specific geotechnical data are always random and variable in space. In the present study, a procedure for quantifying the variability in geotechnical characterization and design parameters is discussed using the site-specific cone tip resistance data (q_c) obtained from static cone penetration test (SCPT). The parameters for the spatial variability modeling of geotechnical parameters i.e. (i) existing trend function in the in situ q_c data; (ii) second moment statistics i.e. analysis of mean, variance, and auto-correlation structure of the soil strength and stiffness parameters; and (iii) inputs from the spatial correlation analysis, are utilized in the numerical modeling procedures using the finite difference numerical code FLAC 5.0. The influence of consideration of spatially variable soil parameters on the reliability-based geotechnical deign is studied for the two cases i.e. (a) bearing capacity analysis of a shallow foundation resting on a clayey soil, and (b) analysis of stability and deformation pattern of a cohesive-frictional soil slope. The study highlights the procedure for conducting a site-specific study using field test data such as SCPT in geotechnical analysis and demonstrates that a few additional computations involving soil variability provide a better insight into the role of variability in designs.     

复杂矿区三维地质可视化及数值模型构建

复杂三维地质可视化和数值模型的构建是采矿、岩土及水利水电工程中常常遇到的关键问题。针对数值模拟软件在复杂矿区地质体模型构建及单元网格划分等前处理问题上存在的不直观、工作量大等缺点,结合矿业软件在复杂三维地质体建模的强大优势,在研究线框模型和块体模型差异的基础上,提出了基于线框模型的复杂矿区三维地质可视化及数值模型构建技术。根据线框模型特点不同,以地表和采空区模型为型,分别阐述了基于Surpac的表面模型和实体模型无缝转换成MIDAS/GTS数值模型的有效方法,并将该技术成功应用于冬瓜山铜矿复杂采空区群围岩稳定性研究中。结果表明,该方法是可行、有效的,从而为矿山开采设计、安全分析及复杂地质体结构的岩土工程数值模拟分析提供了一种新的建模方法。     

Numerical Modelling of Steep Slopes in Open Rock Quarries

The key question regarding steep rock slopes along rock quarries is their stability because a rock slope failure can have critical results. In this study, the aim is to investigate the areas with potential risk for jointed karstic limestones in a rock quarry. First, to determine rock mass properties, scan-line surveys were performed, and the major orientations of discontinuities were analyzed using stereographic projection. Then, the physicomechanical properties of the slope-forming rock were determined in the laboratory, and geomechanical properties of the rock mass were determined using an empirical failure criterion. Finally, the quarry slope stability was assessed in accordance with numerical modelling. According to the results obtained, the numerical modelling of steep rock slopes can be efficiently evaluated by using finite element method. Beside this, the presence of joints intersecting the main discontinuity sets, the filling materials of discontinuities resulting from weathering of limestone and surface deposits, surcharge load due to mine waste dumped on the slopes and excavation blasting during construction of quarry area play a key role when modelling the steep rock slopes by using finite element method.     

Modification of rock failure criteria considering the RMR caused by joints

The aim of this study is to present a modification of the failure criteria of rock masses, separately considering the effect that the Bieniawski’s rock mass rating (RMR) has due to joints, as well as the RMR linked to compressive strength of the mass rock. This modification can be applied to three of the already existing criteria: that of Bieniawski–Yudhbir–Kalamaras, that of Sheorey, and that of Hoek–Brown, comparatively analysing the effects of the three modifications that have been proposed. The new, modified criterion will be validated with new data obtained in a marble mine exploited by means of rooms and pillars. The values observed and measured in the mine are compared to those obtained through numerical modelling of the pillars, using the Análisis Lagrangiano de Medios Continuos, Lagrangian Analysis of Continuous Media (ALMEC) program. Finally, several conclusions related to the application of these rock failure criteria to the analysis of mining excavation of rooms and pillars will be proposed.     

Adaptive integration of constitutive rate equations

In finite element calculations the constitutive model plays a key role. The evaluation of the stress response of the constitutive relation for a given strain increment, which is a time integration in the case of models of the rate type, is a typical sub task in such calculations. Adaptive behaviour of the time integration is essential to assure numerical stability and to control the accuracy of the solution. An adaptive second order semi-implicit method is developed in this paper. Its numerical behaviour is compared with an adaptive second order explicit scheme. The two proposed methods control the local error and guarantee numerical stability of the time integration. We include several numerical geotechnical element tests using hypoplasticity with intergranular strain. The element tests simulate the behaviour of a finite element method based on the displacement formulation.