Development and management of geomembrane liner hippos

The paper describes the authors experience with the formation of geomembrane liner hippos, which is the spherical deformation of the liner within a liquid or viscous medium. The paper presents a summary of six case histories, describing the key features of the hippo and how it formed. The paper also describes the management measures introduced to address the hippo feature.     

Numerical study of strain development in high-density polyethylene geomembrane liner system in landfills using a new constitutive model for municipal solid waste

Tensile strain development in high-density polyethylene (HDPE) geomembrane (GMB) liner systems in landfills was numerically investigated. A new constitutive model for municipal solid waste (MSW) that incorporates both mechanical creep and biodegradation was employed in the analyses. The MSW constitutive model is a Cam-Clay type of plasticity model and was implemented in the finite difference computer program FLAC?. The influence of the friction angle of the liner system interfaces, the biodegradation of MSW, and the MSW filling rate on tensile strains were investigated. Several design alternatives to reduce the maximum tensile strain under both short- and long-term waste settlement were evaluated. Results of the analyses indicate that landfill geometry, interface friction angles, and short- and long-term waste settlement are key factors in the development of tensile strains. The results show that long-term waste settlement can induce additional tensile strains after waste placement is complete. Using a HDPE GMB with a friction angle on its upper interface that is lower than the friction angle on the underlying interface, increasing the number of benches, and reducing the slope inclination are shown to mitigate the maximum tensile strain caused by waste placement and waste settlement.     

Failure mechanisms of geosynthetic clay liner and textured geomembrane composite systems

The objective of this study was to evaluate shear behavior and failure mechanisms of composite systems comprised of a geosynthetic clay liner (GCL) and textured geomembrane (GMX). Internal and interface direct shear tests were performed at normal stresses ranging from 100 kPa to 2000 kPa on eight different GCL/GMX composite systems. These composite systems were selected to assess the effects of (i) GCL peel strength, (ii) geotextile type, (iii) geotextile mass per area, and (iv) GMX spike density. Three failure modes were observed for the composite systems: complete interface failure, partial interface/internal failure, and complete internal failure. Increasing normal stress transitioned the failure mode from complete interface to partial interface/internal to complete internal failure. The peak critical shear strength of GCL/GMX composite systems increased with an increase in GMX spike density. However, the effect of geotextile type and mass per area more profoundly influenced peak critical shear strength at normal stress > 500 kPa, whereby an increase in geotextile mass per area enhanced interlocking between a non-woven geotextile and GMX. Peel strength of a GCL only influenced the GCL/GMX critical shear strength when the failure mode was complete internal failure.     

Shear strength of geosynthetic composite systems for design of landfill liner and cover slopes

Torsional ring shear tests were performed on composite specimens that simulate the field alignment of municipal solid waste (MSW) landfill liner and cover system components. Simultaneous shearing was provided to each test specimen without forcing failure to occur through a pre-determined plane. Composite liner specimens consisted of a textured geomembrane (GM) underlain by a needle-punched geosynthetic clay liner (GCL) which in turn underlain by a compacted silty clay. Hydrated specimens were sheared at eleven different normal stress levels. Test results revealed that shear strength of the composite liner system can be controlled by different failure modes depending on the magnitude of normal stress and the comparative values of the GCL interface and internal shear strength. Failure following these modes may result in a bilinear or trilinear peak strength envelope and a corresponding stepped residual strength envelope. Composite cover specimens that comprised textured GM placed on unreinforced smooth GM-backed GCL resting on compacted sand were sheared at five different GCL hydration conditions and a normal stress that is usually imposed on MSW landfill cover geosynthetic components. Test results showed that increasing the GCL hydration moves the shearing plane from the GCL smooth GM backing/sand interface to that of the textured GM/hydrated bentonite. Effects of these interactive shear strength behaviors of composite liner and cover system components on the possibility of developing progressive failure in landfill slopes were discussed. Recommendations for designing landfill geosynthetic-lined slopes were subsequently given. Three-dimensional stability analysis of well-documented case history of failed composite system slope was presented to support the introduced results and recommendations.     

Contaminant mitigating performance of Chinese standard municipal solid waste landfill liner systems

In 2004, Chinese Government prescribed standard municipal solid waste (MSW) landfill bottom liners. However, very limited research has been conducted to evaluate the performance of the standard MSW landfill bottom liners prescribed by the Chinese Government. In this paper, the performance of the two types of Chinese MSW landfill bottom liner systems was evaluated based on: (1) the maximum leachate head; (2) leakage rate; (3) peak concentration of the target contaminant in an aquifer which was underlain the assumed landfill, and (4) total mass per unit area of the target contaminant discharged into the aquifer. The performance of the German standard MSW landfill bottom liner system was evaluated and compared with that of Chinese ones. It is found that the calculated maximum leachate head for the Chinese landfill liner systems was much higher than that for the German one. The calculated leakage rate, peak concentration and the maximum total mass per unit area in the aquifer of the target contaminant show that the performance of the Chinese standard landfill liner Type 2 is practically the same as that of the German standard landfill liner with holed geomembrane wrinkles, while the Chinese standard liner Type 1 is less effective, with regarding the mitigation of the impact of landfills to the groundwater quality. It is concluded that the overall performance of the Chinese standard landfill liner systems is less strict than that of the German standard landfill liner system.     

Water retention curves of a geosynthetic clay liner under non-uniform temperature-stress paths

This paper presents a novel suction-controlled chamber that permits the determination of the full water retention curves of geosynthetic clay liners (GCLs) under non-uniform temperature-stress paths. It investigates field conditions encountered in brine ponds (low confining stress settings) and heap leach pads (high confining stress settings) during construction and operation stages. Consequently, the analysis of the moisture dynamics in a GCL was defined under the wetting path (construction) and drying path (operation). High vertical stresses were found to facilitate a more rapid water uptake as capillarity is established faster than at low, confined stresses. In general, the drying curves increase the water desorption over the suction range investigated due to the low water viscosity caused by high temperatures. The wetting of the GCL at 20 °C and drying at 70 °C under either low, confined stress (2 kPa) or high confining stress (130 kPa) shows a reduction in the volumetric water contents. Furthermore, on the drying path, the coupled effect of elevated temperature and high confining stress accelerates water desorption leading possibly to potential desiccation.     

Earth pressure coefficients for design of geosynthetic reinforced soil structures

There are several methods proposed in the last two decades that can be used to design geosynthetic reinforced soil retaining walls and slopes. The majority of them are based on limit equilibrium considerations, assuming bi-linear or logarithmic spiral failure surfaces. Based on these failure mechanisms, design charts have been presented by several authors. However, the use of design charts is less and less frequent. The paper presents results from a computer program, based on limit equilibrium analyses, able to quantify earth pressure coefficients for the internal design of geosynthetic reinforced soil structures under static and seismic loading conditions. Failure mechanisms are briefly presented. Earth pressure coefficients calculated by the developed program are compared with values published in the bibliography. The effect of seismic loading on the reinforcement required force is also presented. To avoid the use of design charts and based on the obtained results, approximate equations for earth pressure coefficients estimation are proposed. The performed analyses show that the failure mechanism and the assumptions made have influence on the reinforcement required strength. The increase of reinforcement required strength induced by the seismic loading, when compared to the required strength in static conditions, grows with the backfill internal friction angle. The effects of the vertical component of seismic loading are not very significant.     

Applied bearing pressure beneath a reinforced soil foundation used in a geosynthetic reinforced soil integrated bridge system

Geosynthetic reinforced soil integrated bridge system (GRS-IBS) design guidelines recommend the use of a reinforced soil foundation (RSF) to support the dead loads that are applied by the reinforced soil abutment and bridge superstructure, as well as any live loads that are applied by traffic on the bridge or abutment. The RSF is composed of high-quality granular fill material that is compacted and encapsulated within a geotextile fabric. Current GRS-IBS interim implementation design guidelines recommend the use of design methodologies for bearing capacity that are based around rigid foundation behavior, which yield a trapezoidal applied pressure distribution that is converted to a uniform applied pressure that acts over a reduced footing width for purposes of analysis. Recommended methods for determining the applied pressure distribution beneath the RSF for settlement analyses follow conventional methodologies for assessing the settlement of spread footings, which typically assume uniformly applied pressures beneath the base of the foundation that are distributed to the underlying soil layers in a fashion that can reasonably be modeled with an elastic-theory approach. Field data collected from an instrumented GRS-IBS that was constructed over a fine-grained soil foundation indicates that the RSF actually behaves in a fairly flexible way under load, yielding an applied pressure distribution that is not uniform or trapezoidal, and which is significantly different than what conventional GRS-IBS design methodologies assume. This paper consequently presents an empirical approach to determining the applied pressure distribution beneath the RSF in GRS-IBS construction. This empirical approach is a useful first step for researchers, as it draws important attention to this issue, and provides a framework for collecting meaningful field data on future projects which accurately capture real GRS-IBS foundation behavior.     

Barrier properties of a geosynthetic clay liner using polymerized sodium bentonite

The hydraulic and swelling properties of a polymerized bentonite (PB), and the self-healing capacity of a geosynthetic clay liner (GCL) using the PB as the core material (PB-GCL) were investigated experimentally. Five different test liquids included of deionized water, NaCl solutions (0.1 M and 0.6 M) and CaCl₂ solutions (0.1 M and 0.6 M) were used in this study. The PB exhibited a higher free swelling index (FSI) than that of the untreated bentonite (UB) for all test liquids. For permeability test, under a given void ratio (e), the value of k of the PB is much lower than that of the UB in NaCl and CaCl₂ solutions. The PB-GCL specimens demonstrated a higher self-healing capacity than that of the corresponding GCL specimens using the UB (UB-GCL). Specifically, when using a 0.6 M CaCl₂ solution for a 20-mm-diameter damage hole, the UB-GCL specimen provided a zero healing ratio (healed damage area/total damage area), but the PB-GCL specimen demonstrated an approximately 76% healing ratio. The results from this study indicate that the PB-GCL provided better barrier performance against cationic liquids with higher cation valence and concentrations compared to that of the UB-GCL.     

Reliability and performance-based design: a computational approach and applications

Performance-based design involves the calculation of design parameters to meet one or more performance criteria, with corresponding specified reliabilities over the service life. This work presents an approach to reliability calculations in performance-based design, using an importance sampling simulation, with performance functions evaluated by localized interpolation of a response database. Four examples are shown, including two applications to performance-based design in earthquake engineering: design parameters are obtained to optimally match target reliabilities for three limit states associated with different deformation levels.     

Landfill side slope lining system performance: A comparison of field measurements and numerical modelling analyses

Low permeability engineered landfill barriers often consist of a combination of geosynthetics and mineral layers. Even though numerical modelling software is applied during the landfill design process, a lack of data about mechanical performance of landfill barriers is available to validate and calibrate those models. Instrumentation has been installed on a landfill site to monitor multilayer landfill lining system physical performance. The lining system comprises of a compacted clay layer overlaid by high density polyethylene geomembrane, geotextile and sand. Data recorded on the site includes: geosynthetic displacements (extensometers), strains (fibre optics, Demec strain gauges, extensometers) and stresses imposed on the liner (pressure cells). In addition, temperature readings were collected by a logger installed at the surface of the geomembrane, at the clay surface using pressure cell thermistors and air temperature using a thermometer. This paper presents readings collected throughout a period of three years and compares this measured performance with the corresponding numerical modelling of the lining system for stages during construction. Numerical modelling predictions of lining system behaviour during construction are comparable with the measurements when the geosynthetics are covered soon after placement, however, where the geosynthetics are left exposed to the sun for an extended period of time, in situ behaviour of the geosynthetics cannot be replicated by the numerical analysis. This study highlights the significant influence of the effect of temperature on geosynthetics displacements. A simple thermal analysis of the exposed geosynthetics is used to support the explanation for observed behaviour.     

下卧土体局部沉陷条件下复合衬垫系统的受力变形性能及设计

考虑上覆土体的土拱效应及衬垫的大变形,将下卧土体局部沉陷条件下的复合衬垫系统分为滑动区和沉陷区。建立了其受力变形分析模型,并以衬垫系统的最大拉应变作为控制标准,建立了工程上衬垫系统的抗沉陷设计方法。利用该分析模型研究了滑动区衬垫界面强度、衬垫上覆土体的厚度、重度、有效内摩擦角等参数和衬垫结构形式的影响,发现上覆土体厚度、重度及有效内摩擦角对衬垫受力变形影响很大,而衬垫的抗拉刚度直接决定了其拉应变大小。结合工程实例进行了衬垫系统的抗沉陷设计,并建议了衬垫的抗沉陷结构形式。     

Effect of added polymer on the desiccation and healing of a geosynthetic clay liner subject to thermal gradients

The desiccation and subsequent hydraulic conductivity of both a standard (GCL_A) and polymer-enhanced (GCL_B) Na-bentonite GCL hydrated from a well-graded sandy subsoil under 20 kPa, then subjected to a thermal gradient, and finally rehydrated and permeated with distilled water or 0.325 mol/L Na⁺ synthetic brine are reported.With moderate temperature of 40 °C applied to the top of the liner, GCL_B experienced less cracking than GCL_A, but this advantage disappeared when temperatures increased. Both desiccated specimens of GCL_A and B showed significant self-healing when permeated with distilled water and their hydraulic conductivities quickly reduced to around 10⁻¹¹ m/s at 20 kPa upon rehydration. However, when GCL_B desiccated specimens were permeated with the synthetic brine, their hydraulic conductivities were found to be one to two orders of magnitude higher than corresponding values obtained with distilled water. On the other hand, GCL_A (with no polymer treatment) maintained its hydraulic conductivities at the same level obtained with distilled water. It is concluded that caution should be exercised in using polymer-bentonite in applications in which GCLs are subjected to significant thermal gradients unless there is data to show they are resistant to thermal effects.     

Cyclic loading tests on a non-compression brace considering performance-based seismic design

This paper proposes a brace whose strength, plastic deformation capacity remaining after yield and energy absorption capacity under seismic loading can be easily evaluated for performance-based seismic design. The proposed brace consists of a slender rod with tensile-connected ends incorporating a beveled washer and wedge. There is no buckling in the brace. The wedge slides between a washer and stand so as to prevent brace looseness due to axial plastic deformation under repeated loading.To verify the brace’s mechanism and performance, repeated horizontal loading tests were performed on a one-story bay frame with a tensile-connected brace having a wedge device.Results and conclusions obtained from the loading tests are summarized as follows.(1) Perfect elasto-plastic hysteresis was observed until the wedge’s sliding displacement reached its maximum. (2) Story shear stiffness, strength and energy absorption capacity were easily evaluated. (3) Plastic deformation capacity remaining after an earthquake can be evaluated by checking the wedge’s total sliding displacement. (4) Performance-based design can be easily achieved with this brace.     

Laboratory investigation of the behavior of a geosynthetic encased stone column in sand under cyclic loading

This paper presents the results of a laboratory investigation into the behavior of a geosynthetic encased stone column (GESC) installed in sand under cyclic loading using a reduced-scale model. A number of test variables were considered, such as the geosynthetic encasement stiffness and the cyclic loading characteristics, including loading frequency and amplitude. The results indicate among other things that the overall benefit of the encasement to the performance of the stone column is greater under cyclic loading than under static loading. It is shown that the degree of load transfer to the column becomes smaller when subjected to cyclic loading than under static loading, leading to a 25% decreased stress concentration ratio. The encasement is found to be more effective in improving the stone column performance when subjected to lower frequency and/or smaller amplitude loading. The lateral bulging zone of the GESC under cyclic loading tends to extend beyond the reported critical encasement length for an isolated static loading case, and therefore full encasement is recommended. Practical implications of the findings are discussed in detail.     

我国开展建筑性能化防火设计技术研究的思路

建筑物性能化防火设计方法是目前国外消防科学研究中的一个前沿的热门研究领域。性能化设计思想是以危险性和经济性评估为手段,利用现代科技成果来指导建筑防火设计,根据建筑物的火灾发展特性、建筑及其使用人员的特征来确定建设防火设计,从而实现消防安全和投资等的统一。我国也正积极开展这方面的技术研究并向实用化方向努力,目前正在编制“建筑物性能化设计导则”及开展相关评估模型的研究。在总结国内外有关建筑物性能化研究阶段性成果的基础上,提出了我国开展建筑物消防安全工程设计技术研究的一些基本思路。     

State of the practice review of heap leach pad design issues

The authors present a summary of the state of the practice of containment design in copper and gold heap leaching, focusing on recent advancements and how these applications differ from the more conventional landfill design practices. Advancements both within the Americas and worldwide are presented, including consideration of increasing heap depths, which are now approaching 150 m (with ore densities generally in the range of 1500–1800 kg/m³). Liner system performance under these pressures will be reviewed, including the latest developments in drainage pipe performance testing. The authors will also explore the recently emerging technology of using concentrated sulfuric acid pre-curing for copper ores and the related compatibility issues with conventional geomembrane materials.     

沿海高层建筑抗震设计与耐久性设计的使用年限和性能要求

从沿海高层建筑的抗震与耐久性设计出发,探讨了抗震设计与耐久性设计的设计使用年限、性能要求。设计使用年限的增加意味着地震重现期要延长,对发震断裂带上的建筑地震设防烈度会增大,远离发震断裂带的建筑地震出现的次数增加。耐久性和抗震性危害留的余地不同,耐久性小,抗震性大。但如果耐久性下降会引起抗震性降低。所以保证耐久性是保证抗震性的前提,重视突发性抗震性损坏的同时,不应该忽略长期耐久性损坏的预防。耐久性不足,既增加了结构使用过程中的修理与加固费用,也会降低结构的抗震能力。抗震的性能要求主要体现在保证结构的足够强度、刚度、整体性和稳定性,而耐久性设计要对混凝土材料、结构构造、施工要求等提出要求。     

尽快发展和建立我国的性能化防火设计规范体系

在研究国内外消防规范的基础上，提出了关于我国编制性能化防火规范体系和性能化防火规范应考虑的主要因素、遵循的原则和大体框架的一些建议。     

浅谈数值模拟在建筑防火性能化设计中的应用

运用FDS软件针对某一具体建筑进行防火性能化设计,并得出该建筑达到危险状态时刻的各个参数,对同类建筑的防火设计有一定的参考价值。