Prefabricated and electrical vertical drains for consolidation of soft clay

The use of prefabricated vertical drains to consolidate soft clay is a common ground improvement method. In large projects laboratory testing of PVDs for selection and quality assurance is considered important. This paper presents a review of PVD laboratory testing. The need to provide simulated site conditions in the test is emphasized. In addition instrumented PVDs show that installation stresses in deep soft clay deposits could cause filter rupture under tensile failure. It is also shown that the maximum required discharge capacity of a PVD is obtained by equating the flow rate of the PVD under the installation and consolidation states to the maximum rate of volume reduction of the influential clay cylinder of the PVD. Consolidation can be enhanced much faster in clay soils if vertical drains manufactured with conducting polymer are used. Some laboratory tests, field tests and field applications of such electric vertical drains (EVD) are presented. A minimum current density at appropriate applied voltage is required to benefit from the electric osmosis (EO) application. EVD in dewatering clay soils, extracting heavy metals in clay soils and few other geotechnical applications are also presented.     

Electro-dewatering of wastewater sludge: influence of the operating conditions and their interactions effects

Electric field-assisted dewatering, also called electro-dewatering (EDW), is a technology in which a conventional dewatering mechanism such a pressure dewatering is combined with electrokinetic effects to realize an improved liquid/solids separation, to increase the final dry solids content and to accelerate the dewatering process with low energy consumption compared to thermal drying. The application of these additional fields can be applied to either or both dewatering stages (filtration and/or compression), or as a pre-or post-treatment of the dewatering process. In this study, the performance of the EDW on wastewater sludge was investigated. Experiments were carried out on a laboratory filtration/compression cell, provided with electrodes, in order to apply an electrical field. The chosen operating conditions pressure (200-1200 kPa) and voltage (10-50 V) are sufficient to remove a significant proportion of the water that cannot be removed using mechanical dewatering technologies alone. A response surface methodology (RSM) was used to evaluate the effects of the processing parameters of EDW on (i) the final dry solids content, which is a fundamental dewatering parameter and an excellent indicator of the extent of EDW and (ii) the energy consumption calculated for each additional mass of water removed. A two-factor central composite design was used to establish the optimum conditions for the EDW of wastewater sludge. Experiments showed that the use of an electric field combined with mechanical compression requires less than 10 and 25% of the theoretical thermal drying energy for the low and moderate voltages cases, respectively.     

细粒尾矿砂加筋增强效果实验研究

以四川某矿山企业在选矿过程中产生的细粒尾矿为材料，通过室内实验对其加筋增强效果进行了研究，并根据实验结果提出了影响细粒尾矿力学特性的最优加筋密度，研究成果对细粒尾矿加筋力学性能分析具有重要的参考价值。     

Synthesis and characterization of fly ash modified mine tailings-based geopolymers

Each year, the mining industry generates a significant amount of mine tailings. Storage of these tailings occupies large areas of land and leads to high monetary, environmental and ecological costs. In this research, a feasibility study is performed on geopolymerization of mine tailings so that they can be recycled and utilized as construction material. Considering the extremely high silicon to aluminum (Si/Al) ratio for the mine tailings, class F fly ash is used to adjust the Si/Al ratio. Sodium hydroxide (NaOH) solution is used as the alkaline reaction agent. The research consists of unconfined compression tests to evaluate the mechanical properties, scanning electron microscopy (SEM) imaging to investigate the microstructure, and the X-ray diffraction (XRD) analysis to study the phase compositions. The effects of fly ash content (which affects the Si/Al ratio), alkalinity (NaOH concentration), and curing time on the geopolymerization of mine tailings are studied in a systematic way. The results show that the Si/Al ratio and the alkalinity have profound effects on the mechanical and micro-structural properties of the mine tailings-based geopolymers. The curing time affects the mechanical and micro-structural properties of the mine tailings-based geopolymers mainly during the first 7 days. Based on the research, it can be concluded that mine tailings are a viable and promising construction material if the geopolymerization technology is utilized.     

Electro-osmosis treatment techniques and their effect on dewatering of soils,sediments, and sludge: A review

Electro-osmosis is an established method of dewatering fine soils, sediments, and sludge (SSS). The efficiency of electro-osmotic treatment is controlled by the electrical resistance of the system. Due to an increase in SSS resistance during treatment, its cost efficiency is reduced, limiting the widespread use of this technique. The aim of this paper is to discuss the main reasons for the increase in SSS resistance during treatment and then to analyze the most recent and widely spread modifications to classical electro-osmotic treatment that attempt to combat these issues and improve the efficiency of the technique. These modifications to electro-osmotic treatment are polarity reversal, an intermittent current, the injection of chemical solutions at the electrodes, and the use of geo-synthetics. The paper discusses the relevant research on the above adaptations, and the advantages and disadvantages of each are evaluated and compared using the available laboratory and field tests in the literature on electro-osmotic dewatering research. All four methods of modification are shown to provide significant improvements and can be successfully translated to the field for greater use. However, the improvements they bring about may not be sufficient to warrant their general use in geotechnical applications.     

Tailings-nonwoven geotextile filter compatibility in mining applications

Nonwoven geotextiles have been commonly used in filtration and drainage of geotechnical engineering works. This paper presents a study on the use of such materials in drainage and filtration systems of tailings dams. Different combinations of tailings and geotextiles were submitted to gradient ratio (GR) tests under confinement in the laboratory with varying values of stress levels and hydraulic gradients. The results of GR tests under confining stresses up to 2000 kPa are presented and discussed. The dimensions of the tailings particles entrapped in the geotextile specimens and those that piped through the geotextile were also assessed. Geotextile specimens from the drainage system of a tailings dam were exhumed for analyses, as part of the research programme. The results obtained showed that stress levels and the hydraulic gradients used in the tests influenced the behaviour of the system. Physical and microscopic analyses of the specimens tested showed greater geotextile impregnation by tailings particles in the field than in the laboratory. The overall performance of the geotextiles tested under laboratory conditions was satisfactory. However, in the field segregation of tailings particles and transport of fines in suspension can subject the filter to more complex and severe clogging mechanisms, not properly simulated in current standard testing procedures.     

梯级粉磨制备铁尾矿矿渣基胶凝材料

运用梯级粉磨技术制备铁尾矿矿渣基胶凝材料(TSBC）,研究了梯级粉磨时间组合、材料配比和助磨剂对TSBC粉料粒度分布与浆体强度的影响,通过压汞、X射线衍射(XRD）和扫描电镜(SEM）研究了TSBC浆体的亚微观结构及水化产物,探讨了利用TSBC和铁尾矿砂制备活性粉末混凝土的可行性.结果表明：合理的梯级粉磨时间组合有利于TSBC粉料的颗粒分布宽化和亚微米级铁尾矿颗粒含量的增加,可改善TSBC浆体的孔结构和提高强度;材料配比主要通过影响TSBC的水化活性而影响浆体早期强度;掺入适量助磨剂使TSBC粉料粒径细化,增强组分的胶凝活性和填充效应,利于改善TSBC浆体的孔结构和提高强度;TSBC和原状铁尾矿砂可用于制备活性粉末混凝土.     

The reliability of CPTu and DMT for the mechanical characterisation of soft tailings

Nowadays, the stability and the safe cover and closure of soft tailings has become one of the most challenging topics in geotechnical and environmental engineering. For appropriate analysis and modelling, geotechnical properties of tailings should be well defined. Due to low strength, viscous nature and poor trafficability, as well as due to the specific geometrical properties of fine particles and pore water chemistry in tailings, different test methods and their modification and improvements must be used for valid and reliable characterisation of tailings. This paper analyses and compares the results of different field test methods, piezocone penetration test (CPTu), flat dilatometer test (DMT) and field vane test (FVT), performed on the red gypsum sludge and complementary laboratory tests. CPTu and DMT measurements were evaluated using procedures developed for soils and the obtained mechanical properties were compared with those measured in the laboratory. The significant distinctions between different test methods and different empirical correlations were analysed taking into account differences in index properties of soils and the red gypsum sludge. Based on analysis, some modifications of conventional empirical correlations were proposed for field tests results interpretation and sludge characterisation. The obtained data also show the advantages and limitations of some conventional methods and prove the need for further development in that field of geotechnical investigation.

     

上海地铁九号线宜山路站降水引起的沉降分析

随着城市地下空间和轨道交通的开发利用，深基坑工程所伴随的降水沉降问题越来越受到人们的关注。通过对上海地铁九号线宜山路站的室内模型试验、现场监测数据的分析以及数值模拟方法对宜山路站降水引起的沉降问题作了研究。结果表明：①通过对上海轨道交通九号线宜山路地铁站的室内模型试验，在整理和分析试验数据的基础上，得出了不同的地下连续墙深度下降水井水位与地表沉降之间的规律。②借助于精确的测量分层沉降多点位移计测出的沉降曲线，分析出分层沉降曲线有着“分叉性”和“不协同性”规律性，即有些土层发生了向下的膨胀，导致位于较深的土层比较浅的土层产生较大的变形，首次发现了“逆回弹”现象。③运用数值计算软件MODFLOW对“逆回弹”现象的产生机理作了研究。     

Electrical field: A historical review of its application and contributions in wastewater sludge dewatering

Electric field-assisted dewatering, also called electro-dewatering, is a technology in which a conventional dewatering mechanism such a pressure dewatering is combined with electrokinetic effects to realize an improved liquid/solids separation, to increase the final dry solids content and to accelerate the dewatering process with low energy consumption compared to thermal drying. Electro-dewatering is not a new idea, but the practical industrial applications have been limited to niche areas in soil mechanics, civil engineering, and the ceramics industry. Recently, it has received great attention, specially, in the fields of fine-particle sludge, gelatinous sludge, sewage sludge, pharmaceutical industries, food waste and bull kelp, which could not be successfully dewatered with conventional mechanical methods. This review focuses on the scientific and practical aspects of the application of an electrical field in laboratory/industrial dewatering, and discusses this in relation to conventional dewatering techniques. A comprehensive bibliography of research in the electro-dewatering of wastewater sludges is included.As the fine-particle suspensions possess a surface charge, usually negative, they are surrounded by a layer with a higher density of positive charges, the electric double layer. When an electric field is applied, the usually negative charged particles move towards the electrode of the opposite charge. The water, commonly with cations, is driven towards the negative electrode. Electro-dewatering thus involves the well-known phenomena of electrophoresis, electro-osmosis, and electromigration. Following a detailed outline of the role of the electric double layer and electrokinetic phenomena, an analysis of the components of applied voltage and their significance is presented from an electrochemical viewpoint. The aim of this elementary analysis is to provide a fundamental understanding of the different process variables and configurations in order to identify potential improvements. Also discussed herein is the investigation of the electrical behaviour of a porous medium, with particular emphasis on porous medium conductivity determination.     

某铅锌矿尾矿坝工程勘察与稳定性分析

本文以某铅锌矿尾矿坝工程勘察与稳定性分析为实例,系统地介绍了以探明尾矿坝坝体及坝体周围工程地质、水文地质条件为目的的工程勘探手段和分析研究方法。在揭示了尾矿坝坝体材料空间分布状态及其力学特性的基础上,利用自动搜索的电算程序,分析计算了尾矿坝浸润线的情况和尾矿坝的整体与局部稳定性,分析计算结果与现场勘测结果基本吻合。     

Electrically conductive geosynthetics for consolidation and reinforced soil

The concept of electrically conductive geosynthetics (EKG) materials has recently been introduced. These materials extend the traditional functions of geosynthetic materials by incorporating electro-kinetic phenomena. Electro-kinetic geosynthetics offer technical benefits over conventional electrodes in that they can be formed as strips, sheets, blankets or three-dimensional structures. They are light and easy to install and can be structured so as not to be susceptible to electro-chemical corrosion, whilst continuing to provide conventional functions of filtration, drainage, separation, reinforcement or to act as impervious membranes. This paper describes initial laboratory tests on different types of EKG materials which can be used as combined electrodes/drains in electro-osmotic consolidation and as conductive geosynthetic reinforcement used to improve and reinforced weak cohesive soil. Results of the consolidation tests showed that the EKG electrodes were as efficient as a copper electrode and that the filtration and drainage characteristics did not deteriorate under electro-osmotic conditions. Results of the reinforced soil tests showed that EKG reinforcement can be used to increase the undrained shear strength of cohesive fill and that reinforcement/soil bond increases in proportion to the increase in shear strength.     

Thermo-hydro-mechanical characterisation of a bentonite-based buffer material by laboratory tests and numerical back analyses

This paper presents some laboratory tests performed on the bentonite used as buffer material in the engineered barrier experiment in Kamaishi mine in Japan and a collective effort of four research groups to characterise the coupled thermo-hydro-mechanical behaviour of the bentonite by comparing numerical calculations with the laboratory test results. Each research group used finite element programs with constitutive models capable to simulate both liquid and vapour flux of water, heat transfer, volume change, swelling pressure and mechanical deformation. Numerical calibrations were performed against results obtained from three types of laboratory tests: water infiltration tests, thermal gradient tests and swelling pressure tests. Parameter values, which could not be directly measured in laboratory tests, were obtained with these calculations.     

尾矿砂石混凝土的干缩性能研究

分别以铁尾矿砂和铁尾矿石代替天然砂和普通碎石配制了C30和C60两个强度等级的混凝土。测定了试件在自然养护条件下的干燥收缩率。结果表明,不论是C30等级,还是C60等级,尾矿砂石混凝土的干燥收缩率均小于天然砂石混凝土的收缩率。不论是天然砂石混凝土,还是尾矿砂石混凝土,C60强度等级的混凝土的干燥收缩率均大于C30强度等级的混凝土。     

流动化处理河道淤泥的填充性试验研究

河道淤泥的流动化处理技术就是利用淤泥的流动化性质及固化处理后淤泥的特性,考虑淤泥综合利用的目的将其作为一般土工材料加以利用,尤其在狭窄区域的施工、地下管沟或地下空洞的回灌等工程领域,将流动化处理的河道淤泥作为填充材料利用时,显示出良好的效果。在考虑将流动化处理的河道淤泥作为填充材料使用时,填充区域的填充效果是衡量其施工性能及工程质量的主要因素。本文主要以室内试验为基础,探讨影响处理土的填充性能的主要因素及其规律。     

Mechanical response of a mine composite material to extreme heat

The use of cemented paste backfills (CPB, a mine composite tailings material) in underground mining is currently increasing in importance globally. However, despite the tremendous progress made in understanding the factors affecting the performance of CPB, little attention has been devoted to the impact of high temperatures on its mechanical properties. This article presents the results of experimental investigations into the mechanical responses of CPB when subjected to high thermal loads. Samples made with silicate and pure cement or a mix of 50/50 cement and slag or fly ash were heated at 100, 200, 400 and 600°C, and the unconfined compressive strength, tensile strength, modulus of elasticity, stress–strain behaviour and microstructure evaluated by laboratory tests. The results show that high temperatures have a significant influence on the mechanical properties of CPB, which should be taken into account when determining an appropriate mix for the binder.      

Design method for geotextile tubes considering strain - Formulation and verification by laboratory tests using photogrammetry

Geotextile tubes are usually made of at least one layer of high-strength woven geotextiles. However, if only highly elastic materials are used, experience shows -comprehensibly - that the designed geometry and stresses, derived from standard formulations or special computer software, both using small strain membrane theory, essentially differ from reality. Therefore, an extended formulation, considering large strains, was developed within the scope of a study on the structural and hydraulic design of geosynthetic dewatering tubes at the University of Rostock. The solution was evaluated using a sensitivity analysis and verified in laboratory experiments, where small-scale tubes were surveyed using a low-budget, high-precision photogrammetric measurement technique. The new design method proved to produce good results. It should be used if the construction is indeed a long tube and if the limit strain of the geotextile tubes exceeds 10% strain. Finally, the results were compared with the existing methods for the structural design of geosynthetic applications, adding to the discussion about attenuation factors in geotextile tube dewatering projects. For short-term applications without refilling of the partly dewatered tubes, the product of attenuation and safety factors will usually not exceed a value of two.     

Pressurised electro-osmotic dewatering of activated and anaerobically digested sludges: electrical variables analysis

Pressurised electro-osmotic dewatering (PEOD) of two sewage sludges (activated and anaerobically digested) was studied under constant electric current (C.C.) and constant voltage (C.V.) with a laboratory chamber simulating closely an industrial filter. The influence of sludge characteristics, process parameters, and electrode/filter cloth position was investigated. The next parameters were tested: 40 and 80 A/m², 20, 30, and 50 V—for digested sludge dewatering; and 20, 40 and 80 A/m², 20, 30, and 50 V—for activated sludge dewatering. Effects of filter cloth electric resistance and initial cake thickness were also investigated.The application of PEOD provides a gain of 12 points of dry solids content for the digested sludge (47.0% w/w) and for the activated sludge (31.7% w/w). In PEOD processed at C.C. or at C.V., the dewatering flow rate was similar for the same electric field intensity. In C.C. mode, both the electric resistance of cake and voltage increase, causing a temperature rise by ohmic effect. In C.V. mode, a current intensity peak was observed in the earlier dewatering period. Applying at first a constant current and later on a constant voltage, permitted to have better control of ohmic heating effect. The dewatering rate was not significantly affected by the presence of filter cloth on electrodes, but the use of a thin filter cloth reduced remarkably the energy consumption compared to a thicker one: 69% of reduction energy input at 45% w/w of dry solids content. The reduction of the initial cake thickness is advantageous to increase the final dry solids content.     

Electrokinetic dewatering of Turkish glass sand plant tailings

In this study, the dewatering of glass sand plant tailings from Mersin, Turkey was investigated using an electrokinetic technique. The particle size (d(80)) of the solid waste material tested was less than 0.020 mm and consisted mainly of silica, orthoclase, alumina, potassium and iron oxides. In current plant practice, Larox high-pressure filters are used to produce a filter cake containing 22-25% moisture. As an alternative, a laboratory-scale dewatering pond using an electrokinetic technique was designed and a final product (cake) containing 34% moisture was obtained after 24h treatment using 14.6 V direct current applied for 40 min. When the treatment continued up to 48 h, a firm-to-hard cake was produced.     

Experimental investigation on electro-osmotic treatment combined with vacuum preloading for marine clay

An electro-osmotic consolidation (EO) combined with vacuum preloading (VP) was investigated on marine clay using laboratory tests. To improve consolidation efficiency and reduce the settlement difference, a new prefabricated device was designed to combine EO and VP for the tests. The results indicated that the vacuum preloading with intermittent electro-osmotic consolidation (VP–I-ECM) provided more water discharge with higher discharge rate and produced larger soil settlement compared to traditional vacuum preloading and electro-osmotic consolidation. For the combined method, the VP effectively removed water from the soil for the first 12 h, and its efficiency decreased with the time. After 12 h, the intermittent EO was used to further consolidate the soil and maintain a high level of drainage rate. Test results also showed that the combined method of VP-I-ECM significantly improved the shear strength and bearing capacity of the marine clay to satisfy the construction requirements with a significant reduction in the anode erosion and the energy consumption. This research study provides useful information for the design guide and practical application of the combined technique for improving marine clay.