Self-weight Consolidation Research Articles

Shear Strength of a Cemented Paste Backfill Submitted to High Confining Pressure

Deep mining operations require special measures in order to keep safe and economic aspects. After mine ore is extracted, voids are created and need to be filled with high-strength, low-cost materials. Cemented Paste Backfill (CPB) has recently become one of the main alternatives in filling stopes. Although numerous papers have mentioned the magnitudes of the strength of this material, its behavior under high confining pressures is still not well understood. Therefore, the purpose of this study is to increase the knowledge regarding the CPB behavior. Triaxial compression tests were performed using a Hoek Cell and Load Frame System under high confining pressures. Samples with two different binder contents were used in order to obtain the CPB strength improvement. Besides the self-weight consolidation curing method, samples were subjected to a different curing method that simulated a zero gravity condition (rotating wheel) in the first curing day to compare their mixture properties. The results suggested that both curing method and binder content have influenced the geomechanical properties of Cemented Paste Backfill. By increasing the curing time, the CPB shear strength has increased slightly, whereas specimens with higher binder content presented a significant increase in shear strength values.

Gravity-driven 1-D consolidation of cemented paste backfill in 3-m-high columns

Abstract This paper investigates and simulates the effect of underground placement conditions of cemented paste backfill (CPB) on the evolution of its physical and mechanical properties. Experimental setups that consist of PVC+Makrolon® GP polycarbonate sheet columns, each 3 m high, were built and filled with CPB at two different backfill plants. These setups allow simulating undrained (UD), partial lateral drainage (PLD) and full lateral drainage (FLD) conditions and the measurement of resulting self-weight consolidation settlement of CPB. The results show that maximum drainage water percentages of 15 and 8 % of the CPB total initial water were observed for the FLD and the PLD columns. The results also suggest that in situ backfilled stopes behave in a similar way to that of FLD or PLD conditions.

(Invited) Electrokinetic Separations to Increase the Percent Solids of the Effluent from a Phosphate Mine

Phosphate mining operations create a waste product, consisting of a dilute clay/sand/phosphate suspension that, due to the properties of the suspended solids (primarily clay), requires 25 to 50 years to densify via hindered settling and self-weight consolidation. The objective of this research was to develop and apply an effective electrokinetic separation process to phosphatic clay suspensions, generated during the beneficiation of phosphate ore, a major component of fertilizer. The use of an electric field to separate the water from the solids is attractive because the inherent stability of the clay suspension is due, in part, to the surface charges residing on the platelets.Previous work by others has demonstrated the feasibility of batch-wise electrokinetic separation of phosphatic clay suspensions, but the process is widely regarded as uneconomical. The present research is predicated on the assumption that an economically feasible separation requires a continuous, electrokinetic separation procedure.Solid-liquid separation represents a significant problem for the phosphate mining industry The waste suspensions, initially containing 3-5 wt% solids, are pumped to large impoundment areas termed clay settling ponds. When incorporated, the addition of flocculating agents produce a rapid, but partial, separation, to approximately 10 wt% solids. A further increase in solids content proceeds very slowly. Hindered settling, followed by self-weight consolidation, requires as much as 25 years to reach a solids content of 40 wt%. This is an important target value, because its shear strength will then allow the addition of a surcharge (typically a sand cap) to increase the consolidation rate and magnitude. Through a systematic development of prototypes, from batch to semi-continuous with emphasis on water clarification to semi-continuous with emphasis on solids extraction, our team has developed a fully continuous electrokinetic dewatering prototype. The system takes advantage of the charged nature of the suspended solids. When subjected to an applied electric field, the solids are attracted to the anode and clarified water is collected at the cathode. The process is capable of generating a 40 wt% clay cake and clarified water with turbidity levels below 1 NTU within hours rather than years. Optimization of the design was possible by developing a constitutive relationship between the final solids content, the applied electric field, and the residence time. Successful implementation of a continuous electrokinetic separation process will reduce the vast amount of water consumed by industry. While the process was developed for phosphatic clay suspensions, it is anticipated that electrokinetic dewatering may be applicable to other mining operations such as those associated with the oil sand tar tailings in Canada. This presentation will describe the current state of development, including energy and power requirements.

Nonlinear Finite-Strain Self-Weight Consolidation of Dredged Material with Radial Drainage Using Carrillo’s Formula

AbstractEstimation of the time-rate consolidation of dredged deposits is considerably complicated when vertical drains are installed to enhance the consolidation process of the soft-soil stratum, because the vertical drains are commonly installed before self-weight consolidation is complete. In this paper, two new methods are proposed to take into account both vertical and radial drainage conditions during the nonlinear finite-strain self-weight consolidation of dredged soil deposits (i.e., precise prediction of the self-weight consolidation behavior considering radial flow to the vertical drain). For one-dimensional nonlinear finite-strain consolidation in the vertical direction, the simplified Morris’s analytical solution and the primary consolidation, secondary compression, and desiccation of dredged fill (PSDDF) analysis are adopted. In addition, to consider the radial drainage, Barron’s vertical drain theory is used. The overall average degree of self-weight consolidation of the dredged soil is estim...

Physical modelling of oil sands tailings consolidation

The self-weight consolidation of oil sands tailings and kaolinite slurry is modelled using a new geotechnical beam centrifuge located at the University of Alberta (Geomechanical Reservoir Engineering Facility (GeoREF) centrifuge). The centrifuge is used to create a prototype-effective stress regime in the model, and centrifuge tests are conducted at different initial compositions at acceleration levels of 40 to 100g. The objectives of the centrifuge tests are to study the long-term consolidation behaviour, derive consolidation parameters, verify modelling procedures and to evaluate time-domain reflectometry application for solids content profile monitoring during centrifuge testing. The interface settlement, pore pressure and solids content profiles of the consolidating materials are monitored in-flight. Large strain consolidation parameters are derived from centrifuge tests and compared with parameters determined from a large strain consolidometer. The paper describes the modelling aspect as well as the instrumentation and monitoring of centrifuge tests. The centrifuge results are presented, discussed and compared using results from large strain numerical models.

Influence of Sediment Consolidation on Hydrosuction Performance

AbstractTo alleviate problems caused by reservoir sedimentation, the hydrosuction removal of sediment deposits is an attractive option. In this technique, pumping or siphoning is used to draw water and sediment into the inlet of a pipe and convey the slurry to settling ponds. A key performance target is then to maximize the delivered sediment concentration. For fine sediment (silt and clay), deposit behavior largely depends on its degree of consolidation. To investigate how this parameter affects hydrosuction performance, a series of laboratory experiments were conducted using a small-scale suction pipe equipped with an online densimeter to measure the time evolution of the outflow sediment concentration. To help interpret the resulting signals, the evolving shape of the deposit surface near the pipe inlet was recorded. Measurements were then compared with predictions from inviscid selective withdrawal theory. Depending on the duration of self-weight consolidation prior to hydrosuction, a transition was o...

Deposition and self-weight consolidation of a shrinking fill

Rapid filling of low-permeability cemented paste backfill into underground stopes results in the generation of significant excess pore pressures that are modified by the volume shrinkage accompanying the process of cement hydration. Gibson produced results of an analysis of the excess pore pressures in a simultaneously accreting and consolidating sediment. The dimensionless governing equation lends itself to finite-difference solution: results are presented for a typical rate of volumetric shrinkage and compared with Gibson's original isochrones of excess pore pressure to account for simultaneous conventional consolidation and chemical-induced volume changes in accreting sediment. The effect of time-dependent volume shrinkage can be directly added to Gibson's results for linear problems. The solution technique can be adapted to increase realism by allowing the stiffness of the backfill to increase or permeability to fall as a consequence of the shrinkage. Making a few idealisations, an apparently complex problem can be analysed with rather simple computational resources. Benchmark results can be provided against which more complex methods of analysis (such as finite-element analysis) can be validated.

Modelling the role of self-weight consolidation on the morphodynamics of accretional mudflats

We develop a consolidation module and merge it into a morphodynamic model to assess the role of consolidation on estuarine morphodynamics. We test the model using two settings: point models without...

인천 송도지역 준설토의 침강 및 압밀특성에 대한 실험 및 해석적 연구

연약지반의 특성에 부합하는 이론을 이용하여 침하량을 합리적으로 예측하는 것은 건설방재적 관점에서도 대단히 중요하다. 특히, 준설매립지반과 같은 초연약지반의 압밀거동 모사를 위해서는 비선형 유한변형 압밀이론을 적용할 필요성이 있다. 본 연구에서는 인천 송도지역 준설토의 침강 및 압밀특성을 파악하기 위하여 침강압밀시험, 자중압밀시험 및 CRS 압밀시험을 수행하였으며, 그 결과를 PSDDF를 이용한 수치해석의 입력치로 사용하여 준설매립지반의 거동을 해석하였다. 본 지역의 준설토는 통일 분류법에 의해 저압축성 실트(ML)로 분류되었으며, Yano법 및 수치해석을 통하여 얻어진 최종 체적변화비는 각각 1.56, 1.17로 나타났다. 이러한 결과의 차이는 계면고가 상대적으로 높고 투수성이 큰 해당 구간 준설매립토의 토질 특성에 기인한 것으로 사료된다. Accurate settlement estimation of dredged soft soil deposits is significantly important to prevent potential disasters during land reclamation. An application of the non-linear finite strain consolidation theory is inevitable in dealing with a very soft ground formation such as dredged fill. In this paper, a series of the sedimentation-consolidation test, self-weight consolidation test and CRS test were conducted to clarify sedimentation and consolidation characteristics of dredged fill in the Songdo area, Incheon. In addition, the settlement of dredged fill was numerically simulated using the PSDDF program. The dredged soil obtained from the Songdo area was classified as low-compressible silt (ML) based on USCS (Unified Soil Classification System), and the final bulking factors were estimated to be 1.56 and 1.17 by Yano's method and the numerical simulation, respectively. This difference is attributable to relatively high reclaimed height and large permeability of dredged soil in this region.

Influence of Pore Water Chemistry on Hydraulic Conductivity of Kaolinite Suspensions

The role of pore water chemistry on particle aggregation and soil formation during sedimentation process has been investigated for a kaolinite clay suspensions. The modification of surfaces charge properties of clay minerals induced by different pore water chemistry rules the particles arrangement. As a consequence, at volume scale the sedimentation behaviour and in turn the physical, mechanical and hydraulic behaviour of the formed soil skeleton are changed. Sedimentation tests on several suspensions showed the influence of different aggregation on settling and self-weight consolidation stages. Hydraulic conductivity of kaolin suspensions has been estimated from the settling velocity recorded in sedimentation tests. Test results showed the relevant dependency of the hydraulic conductivity of the clay suspensions on the particle arrangement induced by the pore water chemistry.

Bottom Ash 혼합토의 자중압밀 특성 연구

In order to meet the new requirements for landfill materials, this study planned a study to apply mixed soil of mixing bottom ash and coastal dredged soil to the dredged ground. Coal ash generated from thermal power plant is divided into fly ash and bottom ash. In the case of fly ash, many studies have been conducted because small particles causes permeability coefficient to be small during recycling so no problem has been raised in the environmental area but the utilization of bottom ash has been limited because environmental problems have been raised during recycling due to its larger particle size and greater permeability coefficient. According to recently published studies, however, the results of the study that conducted the water analysis of leachate generated in the ground improvement section using bottom ash showed that heavy metal contamination levels were found to be within the reference value and no significant environmental problems were found so utilization of bottom ash is evaluated to increase significantly in the future. This bottom ash has the particle size of sand and only transportation costs need to be considered when providing materials because the majority has been disposed and it is judged as the most suitable material in dredging landfill in the economic aspect because most thermal power plants are located in the coast and transportation costs can be reduced by ship. Also, research on mixed soil that can maximize the effect of the construction period and construction cost savings than dredged soil is determined as needed because the demand for coastal dredging reclamation is increasing such as Saemangeum project etc. Therefore, we studied self-weight consolidation characteristics depending on sample processing and mixing method of mixed soil by carrying out interior self-weight consolidation experiments on mixed soil of mixing bottom ash and Kaolinite according to the new development needs of recent coastal reclaimed ground and these result findings are expected to be used as basic data when applying the large coastal dredged ground in the future.

Modelling the role of self-weight consolidation on the morphodynamics of accretional mudflats

We develop a consolidation module and merge it into a morphodynamic model to assess the role of consolidation on estuarine morphodynamics. We test the model using two settings: point models without hydrodynamic forcing to validate against two benchmark experimental datasets; and a profile model to simulate a mudflat restoration. The modelled self-weight consolidation influences the simulations by gradually reducing the bed level and decreasing the bed erodibility (i.e., increasing the critical bed shear stress). Both effects modify sediment transport processes on mudflats, leading to long-term morphodynamic effects. Depending on the initial bathymetry, the hydrodynamic forcing and the soil properties, the simulated morphological change of the restored mudflat may differ considerably with and without considering consolidation. The consolidation model developed can be utilised to assess the medium to long term effects related to estuarine development (e.g., wetland restoration) and aims to be a publicly available tool.

Innovation in dewatering process of flotation tailings by study of particle interaction in colloidal environment

Summary Many mining operations produce tailings that dewater very slowly under self-weight consolidation. On way of reducing the water content of such tailings is by coagulation/flocculation processes (C/F processes). This paper describes a study using aluminium chloride and pre-hydrated aluminium salts as aluminium chlorohydrate – ACH (F1.2A, F1AM, FD15, FDM17H, F1S) and polyaluminium chloride – PACl (PAX-18) in dewatering of flotation tailing slurries, resulting from coal flotation in the coal preparation at the Paskov Mine (Czech Republic). Experiments were conducted at different temperature and mixing conditions to simulate full-scale conditions. The results point to a potentially technique for reducing the water content of tailings streams, thus increasing storage space, improving economic and environmental aspects.

Deposited sediment settlement and consolidation mechanisms

In order to study deposited sediment settlement and consolidation mechanisms, sediment settlement experiments were conducted using a settlement column. Based on the experimental results, sediment settlement stage definition, excessive pore pressure (EPP) dissipation, and consolidation constitutive equations are discussed. Three stages, including the free settlement, hindered settlement, and self-weight consolidation settlement stages, are defined. The results of this study show that sediment settlement is mainly affected by the initial sediment concentration and initial settlement height, and the interface settlement rate is linearly attenuated with time on bilogarithmic scales during the hindered settlement and self-weight consolidation settlement stages. Moreover, the deposited sediment layer in the self-weight consolidation settlement stage experiences large strains, and the settlement amount in this stage is about 32% to 59% of the initial height of deposited sediment. EPP is nonlinearly distributed in the settlement direction, and consolidation settlement is faster than EPP dissipation in the self-weight consolidation settlement stage. Consolidation constitutive equations for the hydraulic conductivity and effective stress, applicable to large-strain consolidation calculation, were also determined and fitted in the power function form.

Underground Mine Backfilling in Australia Using Paste Fills and Hydraulic Fills

Underground mine backfilling is a form of ground improvement that has to be carried out in the mine sites. The backfilling provides ground support and regional stability, thus facilitating ore removal from nearby regions. The large underground voids created by the ore removal are backfilled with the waste tailings in the form of paste fills, hydraulic fills, and others. The tailings are placed in the form of slurry that undergoes self-weight consolidation. A small dosage of binder is added to paste fill and cemented hydraulic fill to enhance strength. Considering the high cement cost, mines are using fly ash and slag to partially replace cement with blended cements. This paper gives a practical overview of underground mine backfilling in Australia using paste fills and hydraulic fills. The mining methods and different types of backfills are briefly discussed, with major focus on paste fills and hydraulic fills.

Study of the Shear Strength of Sediments in Main Sedimentation Stages

The shear strength properties of sediments are relevant to many practical problems, including those related to predicting the bearing capacity of the man-made crust lying over dredged disposal sites and those associated with estimating the erosion resistance and the bearing capacity of sediments. In this study, an experimental apparatus and method is developed for sedimentation. This apparatus consists of a settling column, pore measurement apparatus, shear vane apparatus, and multilayer extraction sampling apparatus. The change regulation of interface height, density, excess pore pressure, peak undrained shear strength, residual undrained shear strength, and sensitivity varies before and after the excess pore pressure dissipates to zero in the self-weight consolidation stage. The higher the water content, the greater the particle segregation degree. Particles are mainly segregated in the settling stage, and they are not segregated further in the self-weight consolidation stage. Before excess pore pressure dissipates to zero in the self-weight consolidation stage, shear strength is related to water content, effective stress, and the formed structure of sediments. After excess pore pressure dissipates to zero, peak undrained shear strength is mainly associated with the structure (thixotropy) of sediments. Residual undrained shear strength increases because of the slight decrease in water content. The mechanisms of thixotropy can be expressed as the increase in the original and curing cohesions of sediments with time as determined from microscopic aspects.

An integrated model for three-dimensional cohesive sediment transport in storm event and its application on Lianyungang Harbor, China

Prediction of cohesive sediment transport in storm process is important for both navigation safety and environment of the coastal zone. The difficulties to simulate cohesive sediment transport for a small-scale area such as around a harbor during storm events mainly include the low spatial resolution of the present reanalysis atmosphere forcing, the complex hydrodynamic and sediment transport processes, and their interactions. In this paper, an integrated atmosphere-wave-3D hydrodynamic and cohesive sediment transport model with unstructured grid, which is comprised of the Weather Research and Forecasting (WRF) model, Simulating WAves Nearshore (SWAN) model, and Finite-Volume Coastal Ocean Model (FVCOM), was developed to solve the abovementioned problems. For cohesive sediment, the flocculation and hindered settling were included, and a self-weight consolidation processes was introduced to the existing FVCOM. Interactions between components were considered by providing data fields to each other in an offline manner. The integrated model was applied to simulate cohesive sediment transport around Lianyungang Harbor, China, during Typhoon Wipha in 2007. Results identify that the atmosphere model WRF performed better in the simulation of wind field during typhoon process compared with QuikSCAT/National Centers for Environmental Prediction (QSCAT/NCEP) data. Simulation of wave model was directly affected by wind results as wave vector field driven by WRF wind field showed anticlockwise vortex while waves driven by QSCAT/NCEP wind field did not. The influence of water elevation and flow field on waves was great at the nearshore area. However, the effect of wave on current was not apparent, while the wind field played a more important role, especially on the current velocity. The cohesive sediment transport was greatly affected by wave due to the combined wave-current-induced shear stress. In general, simulation results of wind, wave, current, and sediment showed reasonable agreement with measured data. It is demonstrated that the integrated model developed in the paper is capable of providing high-resolution atmosphere data for other components, reproducing the complex hydrodynamics and cohesive sediment transport processes and taking account of the interaction between components. The integrated model is necessary for simulating the cohesive sediment transport in a small-scale area during storm events.

Comparison of calculation on self-weight consolidation of dreged clay using consolidation properties expressed by “logarithmic volume ratio” and “arithmetic volume ratio”

Comparison of calculation on self-weight consolidation of dreged clay using consolidation properties expressed by “logarithmic volume ratio” and “arithmetic volume ratio”

Maintenance Dredging at Port of Townsville

The Port of Townsville conducts regular annual maintenance dredging to maintain depths of its harbor basin and approach channels for the navigational safety of the vessels against the natural accumulation of marine sediments. In addition to the regular maintenance dredging, the port undertakes emergency dredging in cases where large quantities of sediments are mobilized and deposited in port waters by cyclone or major flood events. The maintenance dredging material derived from the port may be disposed at sea or on land in accordance with relevant state and commonwealth regulations. For the land disposal, the dredged mud slurry is hydraulically placed into containment ponds and left to undergo sedimentation and self-weight consolidation to form fill material for land reclamation. This paper provides an overview of the maintenance dredging at the Port of Townsville and emphasis on maintenance dredging requirements, sediment quality, bathymetry, dredging methods used, and dredged material disposal options.

준설토의 간극비 변화 관찰을 위한 물리탐사기법의 적용

해성점토를 활용한 준설매립공사에서는 준설토의 거동분석이 선행되어야 한다. 본 연구에서는 침강압밀하는 준설토의 밀도와 간극비 변화를 관찰하기 위해 물리탐사기법을 적용하였다. 서해안에서 채취한 흙에 대한 기본물성시험을 실시하였고, 침강압밀시험기에 시료를 단계적으로 투기하여 시간에 따른 계면고와 탄성파의 변화를 관찰하였다. 흙의 침강압밀이 완료된 후 하방향의 투수시험을 실시하여 조간대의 영향이 준설토의 물리적 특성에 미치는 영향을 관찰하였다. 또한 초소형 전기저항탐침을 관입하여 깊이에 따른 시료의 전기비저항을 측정하였다. 모든 실험이 완료된 후, 시료를 채취하여 간극비를 계산하였다. 실험결과, ML 시료의 특성상 급격한 침강을 보여 침강압밀특성을 육안으로 관찰하기 어려웠으나, 단계투기가 계속될수록 탄성파의 속도는 증가하는 경향을 보였다. 하방향 투수에 따른 조간대 영향은 매우 적어 관찰할 수 없었다. 한편 탄성파 속도로 추정한 시료의 간극비는 단계투기에 따라 선형적으로 증가하는 경향을 나타내었다. 전기비저항으로부터 계산된 간극비는 깊이에 따라 반복적으로 증가, 감소하는 형태를 나타내었으며 이는 단계투기에 따라 층을 이루고 있는 시료의 영향으로 파악된다. 시료채취, 탄성파 속도, 그리고 전기비저항으로부터 구한 간극비를 비교한 결과, 서로 유사한 관계를 나타내었다. 본 연구는 물리탐사기법이 준설토의 간극비 관찰에 효과적으로 활용될 수 있음을 보여준다. It is necessary to understand the behavior of the soils for the dredging constructions. The objective of this study is to estimate void ratio and density changes of the dredged soils by using the geophysical testing methods. A series of laboratory tests is performed to obtain geotechnical index properties of the specimen, retrieved from the west coastal of Korea. The sedimentation and self-weight consolidation tests are carried out with observing changes of the interfacial height and the elastic wave velocities. The same amounts of the soils are poured into the testing column at intervals of 12 hours until the interheight reaches to a certain level. After the completion of the sedimentatation and self-weight consolidation tests, downward permeability test is performed to assess a tidal influence in the nearshore. The mini resistance cone is penetrated into the specimen to measure the electrical resistivity with depth. All tests are completely finished, the weight of specimens are measured to calculate the void ratio with the depth. Experimental results show that the aspects of the self-weight consolidation are invisible during dredging process because of rapid sedimentation characteristics of ML. However, the elastic wave velocities increase with increasing in the effective stresses. During permeability test, measured permeability and the elastic wave velocities maintain almost identical values. Void ratio based on the elastic wave velocities changes linearly with time during the step dumpings. Void ratio estimated by the electrical resistivity represents the repeatedly layered depositions according to the step-by-step dumpings. Void ratio determined by soil sampling is similar to those of elastic waves and electrical resistivity profiles. This experimental study demonstrates that the geophysical testing methods may be an effective method for evaluating the behavior of dredged soils.