Spoil pile instabilities with reference to a strip coal mine in Turkey: mechanisms and assessment of deformations

With the increasing adoption of the surface mining of coal, problems associated with spoil pile instability, which affects resource recovery, mining cost, and safety and presents environmental hazards, have become a matter of prime concern to mine planners and operators. The study of geotechnical aspects is thus very important in the rational planning for the disposal, reclamation, treatment and utilization of spoil material. A strip coal mine, one of the largest open pit mines in Turkey, is located in Central Anatolia and provides coal to a thermal power station. Coal production is carried out in two adjacent open pits, the Central Pit and South Pit. A large-scale spoil pile instability over an area of 0.3 km² occurred within the dumping area of the Central pit. In addition, small-scale movement occurred in the outside dumping area. This paper outlines the results of field and laboratory investigations to describe the mechanisms of the spoil pile instabilities and to assess deformations monitored over a long period following the failure. Shear test results indicate that the interface between the floor and spoil material dumped by dragline has a negligible cohesion and is the most critical plane of weakness for spoil pile instability. Back analyses based on the method of limit equilibrium and the numerical modelling technique, and observations in the pit revealed that failure occurred along a combined sliding surface consisting of a circular surface through the spoil material itself and a planar surface passing along the interface between the spoil piles and floor. The analyses also indicated that pore water pressure ratios of about 0.25 satisfy limiting equilibrium condition and that rainfall about one month before the failure may be a contributing factor to the instability. Movement monitoring data obtained following the failure over a 1.5-year period suggested that the ongoing deformations were mainly due to compaction of the spoil material. Based on the monitoring data and the results of the analyses, the failure mode of the local instability occurring at the outside dumping area was considerably similar to that of the large instability.     

液化地基侧向流动引起的桩基础破坏分析

20世纪60年代以来,流动地基中的桩基础的震害现象和抗震设计受到了工程师和研究者的广泛关注。对侧向流动地基中桩基础的一些典型震害现象和其可能震害原因的归纳和分析,显示目前研究仍不能完全解释侧向流动地基中桩基础的震害现象。选取新泻地震中昭和大桥桩基础破坏案例采用p-y方法进行计算,分析地基侧向流动引起的桩基础破坏的影响因素。计算结果显示,合理地描述液化砂土的p-y曲线模型在侧向流动地基桩基础分析中起到关键作用。对于侧向流动地基中桩基础的震害机制的进一步理解和抗震设计,有赖于更为合理和有效地液化砂土中的p-y模型的发展。     

Spoil disposal from the planned Northfield to Quabbin tunnel,western Massachusetts

Excavation of a 16-km water diversion tunnel from Northfield Mountain to Quabbin Reservoir in western Massachusetts may be undertaken from at least three shafts in addition to the outlet adit. This account is concerned with the disposal of spoil from the aqueduct. The rock formations are mainly gneiss and schist, with minor quartzite and amphibolite. Structurally the area is part of the Pelham dome and folds on its east side. The greatest impact on the environment would be dumping of the rock spoil in piles. The total volume would be about 215,000 m³. An underground power house, if decided upon, would add another 43,000 m³ of spoil. Some of the material could be used in the project itself, for instance as riprap along the spillway. None of the piles would be deeper than 6 m, their combined extent not exceeding 14 h. The smallest impact would result if off-site contractors were to take the material as fill or aggregate. The broken rock out of one shaft might be utilized in the realignment of a nearby highway. The sites are in hilly, rural woodland. Two are close to water and one overlooks an upland bog. Adverse effects of spoil disposal can be minimized by selecting areas that avoid slope instability and interference with natural drainage, as well as undue ecologic change. Special problems involve a site that is underlain by clay and a shaft likely to yield bedrock with sulfide minerals.     

Offshore pile system model biases and reliability

ABSTRACT

Recent hurricanes between 2004 and 2008 in the Gulf of Mexico provide valuable new information about design model biases and uncertainties because multiple offshore platforms were loaded beyond the predicted capacity of their pile systems and because there was a failure of a pile system. A Bayesian calibration of model bias factors based on predicted versus observed performance of pile systems in hurricanes indicates that the conventional American Petroleum Institute design method for pile capacity is slightly conservative by about 10% for base shear (i.e. lateral) failures of pile systems in clay, unbiased for overturning (i.e. axial) failures of pile systems in clay, and conservative by more than 50% for overturning failures of piles systems in sand. The epistemic uncertainty in the updated bias factors is represented by coefficient of variation values of about 0.25 for base shear and overturning failures of pile systems in clay and 0.35 for overturning failures of pile systems in sand. A reliability assessment with the calibrated model bias factors shows that the current design practice produces lower reliability for a pile system with three piles versus one with eight piles and lower reliability for a pile system failing in overturning versus one failing in base shear. Therefore, the current design practice could potentially be improved by taking into account the mode of failure and the redundancy in the pile system to provide for a more uniform level of reliability.     

Pyrite oxidation related to pyritic minesite spoils and its controls: A review

Pyrite oxidation is considered to be a main contribution to the acidification of minesite spoils and the generation of the Acid Mine Drainage (AMD) which has become the greatest threat to the ecological environment. In this paper, pyrite oxidation and its controls are reviewed with respect to the latest literature. Conceptual model and empirical rate law model with reference to indoor experiments are classified and presented to describe pyrite oxidation in heterogeneous minesite spoil piles. The influences of Thiobacillus (T) ferrooxidans on pyrite oxidation are simply summarized. In order to prevent the generation of the AMD, three approaches including the addition of alkali to minesite spoil, use of dry covers, and coating on the minesite spoil surface, are discussed.     

Causes, mechanism and environmental impacts of instabilities at Himmetoğlu coal mine and possible remedial measures

 Since the commencement of mining at the Himmetoğlu coal mine, northwest Turkey, serious stability problems have led to interruptions in mining and some environmental effects. A geotechnical investigation was initiated in 1997 and the significant factors that influence the stability have been defined. This paper outlines the results of the field and laboratory studies associated with the causes and mechanism of the slope instabilities and their environmental impacts. The possible remedial measures to improve the stability and to minimize the environmental problems are also described. Back-analyses and data from long-term monitoring indicate that the failures occur along two or three planar surfaces by combination of faults and localized strata steepening adjacent to the faults. The stability is sensitive to changes in length of the lower part of the basal sliding surface and shear strength of the bedding surfaces in the overburden. Suitable remedial measures include slope flattening (i.e. staged bench stripping), proper drainage and spreading of a rock blanket on the pit floor to increase spoil pile stability. Received: 18 April 2000 · Accepted: 15 August 2000     

切方边坡悬臂桩桩间土稳定性研究

在调查分析切方边坡悬臂桩桩间土破坏模式的基础上,指出在桩间净距不大的情况下,桩间土多表现为局部滑塌现象。通过分析斜坡切方前的初始应力状态及桩间土滑塌失稳的临界判据,结合岩土强度理论建立了考虑三维土拱效应及切方卸荷的桩间土稳定性分析模型。以襄－渝二线施工过程中某路堑边坡为研究对象,通过计算分析,结合现场施工的观测情况,指出桩间土滑塌曲面的后缘线一般位于桩顶土拱作用的拱圈内侧区域,桩间土多呈拉-剪破坏模式。工程实践表明,采用土钉对桩间土进行加固取得了良好的效果。数值模拟发现,土钉的钻进应穿越桩间土拱有效作用的区域,土颗粒的楔紧挤密效应在一定程度上对土钉起到等效锚固的作用。     

Transport dynamics of mass failures along weakly cohesive clinoform foresets

This study addresses the initiation mechanisms of mass failures on clinoform foresets. Previous studies have created mass flows by releasing dense water–sediment mixtures into standing water, thus imposing the initial conditions for the mass failures rather than allowing them to form on their own. Para‐meters such as the density, composition and initial momentum of the failures are pre‐determined, precluding observation of the factors that set them initially. This study uses a new experimental method that allows a range of mass failures to self‐generate. Building a clinoform using a cohesive mixture of walnut‐shell sand and kaolinite allows the foreset to build up and fail episodically, generating mass failures. Slopes undergo a series of morphological changes prior to failure, creating a concave shape that becomes exaggerated as deposition continues. This morphology leaves the slope in a metastable state. Once the slope is destabilised, failure is initiated. This study investigates the effect of clinoform progradation rates on failure size and frequency by conducting experiments over a range of water and sediment discharge rates. Neither failure size nor failure frequency changes with discharge rate; instead, increases in sediment supply are taken up by changes in the partitioning of sediment between the steep upper foreset and the more gradual delta‐front apron (toeset) below. Sediment is delivered to the delta‐front apron by a form of semi‐continuous slow creep along the foreset. This slow creep is a failure mode that has not received sufficient attention in the submarine mass‐flow literature. The independence of failure size and frequency from sediment supply rate suggests that the presence of mass‐failure deposits does not provide information on the rate of sediment delivery. If these relations hold at field scales, this would imply that individual mass failures are relatively insensitive to changes in water and sediment supply.     

Geotechnical assessments of the stability of slopes at the Cakmakkaya and Damar open pit mines (Turkey): a case study

The on-site observations, monitoring data, and results of back analyses of failures showed that large-scale failures occurred along both the interconnected sliding surfaces, consisting of (a) discontinuities in the dacites and the contact zone and (b) the circular surfaces through the weathered soil-like dacites at the Cakmakkaya and Damar open pit mines. Surface water infiltration through the weathered soil-like material after a short duration of rainfall contributed to the circular-shaped failures. After a heavy rainy period, an increase in the groundwater table above the contact zone played a major role in the initiation of bi-planar wedge failures. In addition, the stability of the slopes is likely to have been controlled by the orientation of this zone. The results of back analyses indicated that the shear strengths of the soil-like materials in the weathered dacites and the contact zone had reduced to their residual values at the time of initial sliding. The flattening of the slope angles with an effective surface drainage and long-term monitoring of the groundwater level is proposed as the most suitable remedial measure.     

Field monitoring and deformation characteristics of a landslide with piles in the Three Gorges Reservoir area

Landslides often occur within the reservoir area behind dams. In China, a common strategy for stabilizing these landslides is to install large piles through the landslide and into the stable ground below. The piles interact with the landslide and constitute a landslide-stabilizing pile system. The deformation of this system under the reservoir operation is more complicated than the deformation of the landslide itself. Understanding the behaviour of this system is very important to the long-term safety of landslides stabilized with piles in reservoirs. The Majiagou landslide, which was selected as a case study, was triggered by the first impoundment of the reservoir behind the Three Gorges dam. A row of anti-slide piles was installed in the landslide in 2007, but monitoring results found these were ineffective at stabilizing the landslide. Subsequently, in 2011, two longer test piles and an integrated monitoring system were installed in the landslide to better understand the failure mode of the landslide and to measure the deformation characteristics of the landslide-stabilizing pile system. Monitoring results show that the Majiagou landslide is a translational landslide with three slip surfaces. The test piles provided local resistance and partially slowed down the sliding mass behind the piles, and the landslide deformation response to external factors decreased for a time. However, after 2 years, the deformation of the landslide-stabilizing pile system reverted to seasonal stepwise cumulative displacements influenced by cycles of reservoir drawdown and rainfall. The monitoring results provide fundamental data for evaluating the long-term performance of anti-slide piles and for assessing long-term stability of the stabilized landslide under the reservoir operation.     

Simulation of landslide-induced surges and analysis of impact on dam based on stability evaluation of reservoir bank slope

Instability of bank slopes and their failures in reservoirs have become an increasing concern in evaluating potential hazards of hydraulic projects in China as larger hydropower engineering projects have been in operation in the past decade. To provide a quantitative evaluation of such hazards, an evaluation framework has been presented to analyze bank slope stability, identify potential slope failure bed surfaces, and evaluate wave impacts of landslide-induced surges in reservoirs. In this joint analysis framework, reservoir bank slope instability and potential failure surfaces are determined with TFINE model, a nonlinear Finite Element Method (FEM) program using Deformation Reinforcement Theory; the motion of slope failures and resulting surge waves in reservoirs and potential flooding are simulated with COMCOT tsunami simulation package. The work flow and feasibility of this analysis framework are demonstrated through a case study of Guopu slope in Laxiwa reservoir in Southwest China. Three potential failures of Guopu slope are identified through stability analysis and the modeling results of indicate that the potential landslide failures would generate waves with maximum amplitude of 20.9 m in the reservoir, but unlikely causing significant damage to the dam.     

考虑挤土和群桩效应的预制桩安全监控模型

在深入分析密集型静力压入式预制桩挤土效应和群桩效应的机理和不利影响的基础上，建立了确保地基土不因挤土效应而破坏的孔隙水压力监控模型，优化压桩线路和次序的超静孔压监控模型，确保基桩不因桩周土隆起而被拔断的基桩轴力监控模型，确保基桩不因挤土效应而产生弯折破坏以及桩间土不因群桩效应而产生剪切破坏的桩侧土压力监控模型。实测数据表明，在没有采取有效的降排水和卸压预防措施时，沉桩施工容易导致地基土强度失效和基桩拔断破坏。采取袋装砂井排水和碎石桩式降水井降水后。即使是透水性差的软土地基。也可使较大的超静孔压在1d时间内消散70％。     

钻孔桩基础的一个失败实例

列举了一个显然是按照正常施工过程步骤成桩的败例。以对整个成桩过程全面调查和详细试桩结果为基础,对该桩质量所存在的问题作了深入细致分析。提出了改进挖斗体结构、改善无循环钻孔灌注桩成孔工艺,对混凝土各个环节质量控制手段也提出建议。     

The geotechnical characteristics and stability of a spoil heap at a southwestern pennsylvania coal mine, U.S.A.

The mechanical and geotechnical properties of bulk samples and samples extracted from boreholes through a sliding spoil heap at a southwestern Pennsylvania coal mine are examined. Size gradation, moisture content, density and strength are found to vary considerably within the spoil. Gradation results suggest the material type to be more important than the age of spoil in degradation. There is also evidence from analysis that the heterogeneity of spoil and its associated density and strength do not significantly affect stability results. In other words, spoils can be sufficiently heterogeneous for purposes of stability analysis; they can be considered statistically homogeneous with regard to shear strength.     

嵌岩桩静载试验结果的研究与讨论

根据１９个工程７１根嵌岩桩静载试验的实测资料，论述了嵌岩桩静载试验结果的特点，对Ｐ－Ｓ曲线进行了分区；提出了嵌岩桩质量分类体系表；研究了嵌岩桩的变形与破坏特点，并对破坏类型作了划分；同时，还讨论了嵌岩深度和嵌固力的测定。     

Seismic stability analysis of piled embankments: A multiphase approach

The seismic stability analysis of an embankment lying over a soft foundation soil reinforced by a group of vertical piles is performed within the framework of the upper bound kinematic approach of yield design. The analysis is based on a previously developed ‘multiphase’ model of the reinforced ground, which explicitly accounts for the shear and bending resistances of the piles. Making use of appropriate failure mechanisms involving shear zones across which the reinforcements are continuously deforming, along with ‘plastic hinge’ surfaces, upper bound estimates to the critical seismic coefficient of the structure are derived. The results, which are confirmed by the simulations obtained from a finite element elastoplastic code, give clear evidence of the key role played by the bending strength capacities of the piles in ensuring the stability of the pile reinforced embankment. Copyright © 2009 John Wiley & Sons, Ltd.     

Modeling of pile installation using contact mechanics and quadratic elements

In geotechnical engineering, numerical analysis of pile capacity is often performed in such a way that piles are modeled using only the geometry of their final position in the ground and simply loaded to failure. In these analyses, the stress changes caused by the pile installation are neglected, irrespective of the installation method. For displacement piles, which are either pushed or hammered into the ground, such an approach is a very crude simplification. To model the entire installation process of displacement piles a number of additional nonlinear effects need to be considered. As the soil adjacent to the pile is displaced significantly, small deformation theory is no longer applicable and a large deformation finite element formulation is required. In addition, the continuously changing interface between the pile and the soil has to be considered. Recently, large deformation frictional contact has been used to model the pile installation and cone penetration processes. However, one significant limitation of the analysis was the use of linear elements, which have proven to be less accurate than higher order elements for nonlinear materials such as soils.

This paper presents a large deformation frictional contact formulation which can be coupled consistently with quadratic solid elements. The formulation uses the so-called mortar-type discretisation of the contact surfaces. The performance of this contact discretisation technique is demonstrated by accurately predicting the stress transfer between the pile and the soil surfaces.     

The importance of breaching as a mechanism of subaqueous slope failure in fine sand

Large bank failures, comprising up to 10⁶ m³ of sediment, are common features along steep channel banks in estuaries and large rivers that consist of clean, fine sands, and are mostly assumed to be generated by sudden liquefaction of large masses of very loosely packed sand. Another less commonly recognized type of failure is manifested by the gradual retrogression of a very steep wall, steeper than the angle-of-repose. Instead of the voluminous surging plastic sediment-water flow, or hyperconcentrated density flow (sensu Mulder & Alexander, 2001 ) generated by liquefaction, this type of failure, known as breaching by dredging companies and hydraulic engineers, produces a sustained quasi-steady, turbidity current. To date, sedimentologists have not recognized the process of breaching as such. In this paper, it is suggested that breaching may be the origin of many thick, massive sand layers known from ancient deposits from various environments, notably in some turbidite successions. Possible differences in the sedimentary structure of the deposits produced by breach failures vs. liquefaction slope failures (=liquefaction flow slides) can be deduced from a knowledge of the sediment transport processes initiated by the failure. A field study is presented on some poorly structured beds in the Eocene Vlierzele Sands in Belgium, which are supposed to have originated from liquefaction failures, but are reinterpreted to be the products of breaching. It is postulated that the local steep slope disturbance required to initiate an active breach can be produced by a small liquefaction slope failure (=liquefaction flow slide failure) or local erosion by river or tidal channel flow at the initial stage of the failure event.     

深基坑PCMW工法开挖过程离散元数值模拟分析

深基坑稳定性问题是重要的工程问题,其中,有效支护结构是保证基坑工程顺利实施的基础。本文引入离散元法,提出了基坑及支护结构建模和数值模拟方法。采用紧密堆积离散单元构建模型初始地层,引入了离散单元团簇模型来构建表面较为光滑的管桩。以南京某深基坑工程为例,采用三维离散元模拟软件MatDEM3D构建几何模型,并根据土体的力学性质设定相应单元的力学参数。数值模拟中,通过预平衡操作实现土层的压实,进而模拟了基坑开挖过程中土体和管桩的变形。将数值模拟结果与实测监测数据进行了对比分析,评价了基坑支护结构的有效性。这种基坑离散元数值模拟方法在基坑开挖过程的稳定性预判和支护结构的有效性评价上具有潜在的应用前景。     

Modelling rock fracturing and blast-induced rock mass failure via advanced discretisation within the discontinuous deformation analysis framework

Rock mass failure is a particularly complex process that involves the opening and sliding of existing discontinuities and the fracturing of the intact rock. This paper adopts an advanced discretisation approach to simulate rock failure problems within the discontinuous deformation analysis (DDA) framework. The accuracy of this approach in continuum analysis is verified first. Then, the advanced discretisation approach for fracturing modelling is presented, and the discretisation strategy is discussed. Sample rock static failures are simulated and the results are compared with experimental results. Thereafter, with a generalised definition of the artificial joints, this approach is further extended and applied in the simulation of blast-induced rock mass failures in which the instant explosion gas pressure obtained by the detonation pressure equation of state is loaded on the main blast chamber walls and the induced surrounding connected fracture surfaces. In the simulation instance of rock mass cast blasting, the whole process, including the blast chamber expansion, explosion gas penetration, rock mass failure and cast, and the formation of the final blasting pile, is wholly reproduced.