Critical appraisal of piping phenomena in earth dams

This paper presents a comprehensive review of published literature on soil piping phenomena. The first tools to design earth dams to resist piping were developed during 1910–1935. Filter criteria for dispersive soils was refined in the 1970’s. Piping phenomena are generally defined as: (1) heave, (2) internal erosion, (3) backwards erosion, although other modes are possible. Recent work on piping highlights the limitations of the occurrence of piping and the role that design and construction may play in a large percentage of piping failures. Standardized laboratory procedures are available to assess piping potential in cohesive materials, but no such methods exist for non-cohesive soils. However, methods are available for evaluation of self-filtration potential. Recent advances in computer technology have facilitated the evaluation of seepage and deformation in embankments but computational methods for evaluation of piping potential are currently limited.      

Assessment of the safety of earth dams

The assessment of safety of earth dams based on the results of preliminary inspections is analyzed by fuzzy calculus. Fuzzy sets are used to represent the engineer's judgement on the seriousness (gravity) of observed defects and their relation to dam dafety. Two analytical models are proposed and applied to three earth dams exhibiting different types of defects. The results of the calculations are compared with the engineer's recommendations.     

Full scale investigation of GCL damage mechanisms in small earth dam retrofit applications under earthquake loading

This paper reports results of full scale testing to further explore potential GCL damage mechanisms in earth dam retrofit applications in seismically active areas; in particular, to a) investigate whether shear displacements could reduce the magnitude of GCL panel overlap during earthquake shaking; b) explore the influence of gravel particles on GCL thickness at localised point of contact; and c) observe the consequences of an accidental exposure of an uncovered GCL to short duration rainfall in terms of moisture content and effects during subsequent compaction. The results of these experiments indicate that even under severe shaking no movements were detected at the GCL panel overlap. Whereas gravel particles were observed to locally reduce the thickness of the GCL to 2.2 mm, no plowing of the particle into the GCL occurred due to a lack of shear displacement at the interface, resulting in no localised internal erosion through the barrier. Furthermore, hydration of GCL panels during construction due to surface wetting was observed to result in a state of hydration less than its post-construction state. These results indicate that although each of the three GCL damage mechanisms cannot be ruled out to ever be relevant in practice, the performance of the GCL retrofitted earth dam tested was satisfactory under even severe Level 2 earthquake shaking, and suggests that the retrofitting of small earth dams with GCLs is a promising strategy to improve their static and seismic resistance.     

土石坝竖向排水的渗流控制(英文)

水流通过坝体和坝基时存在压力和梯度,这在大坝工程设计中已视为重要因素。但是,在土石坝设计时往往会遇到抵御不利大坝安全因素的排水系统的选型和设计问题。目前,很多工程中虽存在一些效果不错的排水系统,但其缺乏渗流和浸润线位置的控制,可能会对工程安全产生威胁,或使渗流控制措施达不到经济有效的目的。为此,本文基于均质土坝实验研究取得的成果,研究出在竖向排水体中的渗流量的控制方法,采用这种方法可以对土石坝进行更好的管理。     

沟埋式涵洞非线性土压力试验研究与数值模拟

现行公路桥涵设计通用规范中线性土压力理论未能准确反映沟埋式涵洞涵顶垂直土压力变化规律,土压力计算结果与实际情况存在差异,导致涵洞在施工或使用过程中出现不同程度的病害。结合现场试验和数值模拟研究了沟埋式涵洞垂直土压力的变化规律,以及涵顶填土内部的土拱效应,分析了涵顶平面土压力及不均匀沉降的分布规律。研究结果表明,并非所有沟埋式涵洞涵顶垂直土压力都小于按线性土压力理论计算结果,它与填土高度、沟谷宽度、沟谷坡角、涵洞几何尺寸、填料性质及地基刚度等因素有关,涵顶垂直土压力随各影响因素呈非线性变化。填土达到一定高度后,涵顶填土内部产生土拱效应,该效应能够缓解涵顶应力集中现象,但其具有不稳定性。沟埋式涵洞的设计与施工应综合考虑各因素对涵洞受力状态的影响。     

Experimental study on interfacial heat-transfer coefficients at the casting/die interface in squeeze casting under transient condition

ABSTRACT

By using two-dimensional transient state inverse problems method, the interfacial heat-transfer coefficient (IHTC) was evaluated, based on experimentally measured temperature distributions of solidification of an Al alloy under various pressures (40, 50 and 60?MPa). The value of IHTC obtained from the mould and the casting varies with time subsequent to pouring the molten metal into a mould cavity. The impact of different applied pressure with different positions of mould insulation on IHTC was experimentally measured and calculated for the analysis. The calculated heat-transfer coefficients were used for solidification simulation in the compress casting process.     

Experimental study on stiffness degradation and monotonic response of reconstituted volcanic ash induced by internal erosion

Volcanic ash soils are a common geological body in earthquake prone regions, and they are widely used as construction materials. The present study discusses the contribution of initial density, stress state and seepage time on seepage-induced internal instability, stiffness degradation, and monotonic response of volcanic ash collected from Satozuka, Japan. Reconstituted specimens are tested using an erosion triaxial apparatus. The results exhibit that the rate of erosion is influenced by initial density, stress state and hydraulic gradient. The mercury intrusion porosimeter tests are conducted to explore the distribution of constriction sizes after erosion. Post-erosion stress-strain responses are also affected by the stress state during erosion, initial density and seepage time. Particularly, internal erosion affects the dilatancy response of relatively loose specimens and has an impact on the critical state line, brittleness, and peak strength. Further, the maximum shear modulus of eroded soil is found to be greater than that of non-eroded soil, mainly due to the particle rearrangement and removal of fines. However, a sharp reduction in stiffness during monotonic shearing is maybe evidence that temporary reinforced soil packing collapses at a large strain. This indicates that the elastic yield surface has expanded for eroded soils.     

Prediction of maximum surface settlement caused by earth pressure balance (EPB) shield tunneling with ANN methods

In order to determine the appropriate model for predicting the maximum surface settlement caused by EPB shield tunneling, three artificial neural network (ANN) methods, back-propagation (BP) neural network, the radial basis function (RBF) neural network, and the general regression neural network (GRNN), were employed and the results were compared. The nonlinear relationship between maximum ground surface settlements and geometry, geological conditions, and shield operation parameters were considered in the ANN models. A total number of 200 data sets obtained from the Changsha metro line 4 project were used to train and validate the ANN models. A modified index that defines the physical significance of the input parameters was proposed to quantify the geological parameters, which improves the prediction accuracy of ANN models. Based on the analysis, the GRNN model was found to outperform the BP and RBF neural networks in terms of accuracy and computational time. Analysis results also indicated that strong correlations were established between the predicted and measured settlements in GRNN model with MAE = 1.10, and RMSE = 1.35, respectively. Error analysis revealed that it is necessary to update datasets during EPB shield tunneling, though the database is huge.     

Erosional behavior of gravel-sand mixtures stabilized by microbially induced calcite precipitation (MICP)

The objective of this study is to evaluate the effectiveness of Microbially Induced Calcite Precipitation (MICP) for improving internal erosion resistance of gravel-sand mixtures. Two gravel-sand mixtures with 25% sand/75% gravel and 50% sand/50% gravel were used; the former was susceptible to suffusion whereas the latter was internally stable. The MICP treatment was conducted by either mixing a urea-calcium solution with the tested soils prior to bacteria injection (the pre-mixing method) or injecting the bacteria prior to the urea-calcium solution injection (the injection method). A series of pressure-controlled erosion tests was performed on specimens placed inside a column erosion test apparatus under different levels of axial stress. During the erosion test, the erosion rate, axial deformation, and hydraulic conductivity were measured. Without the MICP treatment, the specimens with 25% sand/75% gravel exhibited much faster backward erosion and suffusion. In contrast, the specimens with 50% sand/50% gravel showed slow backward erosion only. Within the tested conditions, MICP was very effective in mitigating internal erosion for the soil with 25% sand/75% gravel. However, for the soil with 50% sand/50% gravel, the MICP treatment was only successful when the injection method was applied and the erosion test was performed at a low axial stress.     

Effect of geosynthetic component characteristics on the potential for GCL internal erosion

Experiments quantifying GCL permittivity and the ultimate water head the GCLs can sustain before the initiation of internal erosion when underlain by a 50 mm angular to subangular gravel subgrade are conducted. The influence of different geotextiles over the subgrade, water heads, hydration periods before testing, masses per unit area of bentonite within the GCL, and ionic strengths of the solution (cation exchange) are considered. Test results show that GCL with the scrim-reinforced nonwoven geotextile over the subgrade has the best hydraulic performance against internal erosion, followed by the woven geotextile coated with a 110 g/m² polypropylene film. A woven or nonwoven is the least useful for preventing internal erosion, with the corresponding threshold water head initiating internal erosion >39 m for scrim-reinforced nonwoven, 21 m for lightly coated woven, 4–5 m for woven and nonwoven alone, respectively. Cation exchange, length of hydration, and mass per unit area of bentonite do not notably affect the threshold water head for the subgrade examined. Once internal erosion occurs, there is a 3-order of magnitude increase in permittivity. The practical implications are discussed.     

Responses of a cylindrical lined tunnel due to internal dynamic load in two distinct mediums: Ideal elastic and saturated porous medium

The dynamic responses of a lined tunnel subjected to dynamic loading is one of the key issues that needs to be addressed prior to the design and construction of tunnels. While the tunnel lining and surrounding soil are commonly designed in ideal explosion-proof engineering as ideal elastic media to simplify the problem, in reality; soils are porous geo-materials. Therefore, the concern is whether this practice is more conservative or close to the reality, in contrast to the scenario where the surrounding soil is assumed as a saturated porous medium. This study investigates the differences and relationships between the dynamic responses of the lining structures in two immensely disparate media: ideal elastic medium and porous saturated medium. Firstly, to avoid the complexity of 3D numerical studies, 3D analytical solutions for the responses of the lined tunnel in both the ideal elastic medium and porous medium due to internal dynamic loading are derived using Fourier and Laplace transforms. Also, the differences between the dynamic responses (e.g., the radial displacement, radial effective stress, and hoop effective stress) of the lining structures in two media are determined to assess the rationality of assuming that the soil around the lined tunnel is an infinite elastic compressible medium. Finally, the influence of the porosity on the dynamic response of a cylindrical lined tunnel subjected to dynamic loads is examined.     

工程勘察地下水中侵蚀性CO2的试验误差分析

针对侵蚀性CO2是工程勘察中地下水检测的重要项目之一，通过试验，分析了引起侵蚀性CO2测试误差的因素，并提出了相关建议，为准确测定侵蚀性CO2提供了一定的依据。     

高土石坝坝体填料的宏观抗剪强度研究

采用随机场理论和现代物理学逾渗 (percolation)临界理论相结合的方法 ,推导了用有限个小尺度材料试样的抗剪试验结果确定高土石坝填料的宏观抗剪强度的上、下限理论计算公式。研究结果表明 ,高土石坝坝体土石填料的宏观抗剪强度在数值上介于小尺度试样抗剪强度的小值平均值和算术平均值之间。     

Laboratory testing for evaluation of the influence of a small degree of internal erosion on deformation and stiffness

Suffusion is a type of internal erosion, namely, the transportation of soil driven by the seepage force of finer fractions within coarser fractions. The present work uses triaxial confining compression tests to study how a small degree of suffusion affects soil stiffness and deformation. This approach allows the suffusion inside a specimen to impose a downward seepage flow. Although a small degree of suffusion is not detectable, it can lead to the deterioration of earth structures. Linear displacement transducers and clip gages were attached to specimens to accurately measure the localized strain rates. In addition, the turbidity of the discharged water was evaluated. Following the downward seepage, each drained specimen was monotonically compressed. The experimental results show that cohesive soil undergoes suffusion and that most of the clay fractions bonding the sand particles erode. At intermediate strain, the deviator stress decreases in proportion to the degree of suffusion, but this decrease ceases at the critical state. Anisotropic behavior is observed and is tentatively attributed to the disruption of the soil texture, which is qualitatively monitored by scanning electron microscopy. Finally, a simple formula is proposed for evaluating the decrease in stiffness due to suffusion.     

Mechanical role of reinforcement in seismic behavior of steel-strip reinforced earth wall

In the current design practices of steel-strip reinforced earth walls (SSREWs), the length of the reinforcing material is determined based on the equilibrium between the reinforcement tension and the earth pressure acting on the wall. Here, the resistance of the reinforcing material laid in the active failure zone (AFZ) is not considered. Moreover, the mechanical role of the reinforcing material against the integrity of the SSREW has not been sufficiently verified. Regarding the seismic stability of SSREW, although it is investigated by treating the entire reinforced earth wall as a rigid body, this inspection method is for gravity-retaining walls, and the difference in the seismic behavior between the SSREW and the rigid body is not clear. In this study, therefore, dynamic centrifuge model tests on 6 types of SSREWs were conducted to clarify the following items: (1) the basic earthquake behavior of a SSREW, (2) the mechanical role of the reinforcing material laid in the AFZ and (3) the mechanical role of the reinforcing material against the integrity of the SSREW. The results indicated that the reinforcing material laid in the AFZ can restrain the amount of deformation of the wall during earthquakes. Furthermore, the more stable the AFZ is, the smaller the maximum wall displacement will be.     

Potential of zero-valent iron in remediation of Cd(II) contaminated soil: From laboratory experiment,mechanism study to field application

This study investigates the potential of ZVI on Cd(II) contaminated soil remediation through laboratory experiment, mechanism study and field application. The results show that the dosage of ZVI, the initial concentration level and the reaction time have significant impacts on Cd(II) adsorption and about 88% of aqueous Cd(II) can be removed from soils. The ZVI is observed to promote the increase of Cd(II) residual fraction in Cd(II) contaminated soils according to the sequential extraction procedure (SEP) results. Regression of the laboratory experimental data based on the Langmuir isotherm equation shows the adsorption capacity can reach 34.7 mg/g. Such a high value is attributed to physical and chemical adsorption, which is proved by X-ray diffraction patterns (XRD), scanning electron microscope images (SEM), and Brunner-Emmet-Teller & Barret–Joyner–Halenda (BET-BJH) analysis. The field application shows that the ZVI can reduce the Cd(II) content in soils and brown rice by 51% and alleviate soil acidification, resulting in a 9.4% increase in rice yields.     

A computational and experimental investigation of three-dimensional micro-wedge indentation-induced interfacial delamination in a soft-film-on-hard-substrate system

A three-dimensional finite element (FEM) simulation was performed to study the mechanics of micro-wedge indentation-induced interfacial delamination of a soft film from a hard substrate. In this simulation, a traction–separation law, with two major parameters: interfacial strength and interfacial energy, was used to characterize the failure behaviors of the interface. Cracking of film and residual stresses were not included. The initiation and growth of the interfacial delamination were investigated for a wide range of interfacial properties. It was found that the interfacial strength influences the initiation of delamination more than does interfacial energy, while the interfacial energy is more effective to affect the propagation of the delamination crack. The effects of the length of wedge indenter tip and the thickness of film on the onset and growth of interfacial delamination were also analyzed. Furthermore, the interfacial delamination process by micro-wedge indentation was conducted experimentally, and the delamination crack fronts as well as the P–h curves in experiments and computations were compared thoroughly. Comparisons between the computational and experimental results yield quantitative good agreement.     

Experimental study on suffusion under multiple seepages and its impact on undrained mechanical responses of gap-graded soil

An experimental investigation of multiple seepage-induced suffusion and its impact on the mechanical responses of internally unstable gap-graded soil, with a fines content of 25%, is presented in this paper. Using a modified triaxial erosion apparatus, with a redesigned seepage control system, erosion tests under multiple seepage conditions, as well as undrained monotonic and cyclic compression tests, are performed. It is found that multiple seepages cause an eroded mass without a marked change in volume and with a change in hydraulic conductivity as the number of seepage cycles increases. The monotonic compression tests show that eroded soil presents a smaller peak strength, residual strength, and a greater contractive response than non-eroded soil. The peak strength and stiffness of eroded soil are seen to decrease considerably as the number of seepage cycles increases. Multiple seepage-induced suffusion may create a collapsible soil structure in eroded soil, as shown by sudden decreases in deviator stress and stiffness, coupled by sharp increases in pore water pressure at small strain levels. As eroded soil might already be unstable, cyclic loading causes it to collapse, revealing a sudden increase in the generation of cyclic pore water pressure and a decrease in liquefaction resistance. The results highlight the importance of conducting laboratory tests to determine the impact of internal erosion on the strength and liquefaction resistance.     

香港充填土风化花岗岩场址勘探中的界面识别研究

针对香港充填土风化花岗岩场址勘探,在液压旋转钻机上安装钻孔过程监测系统(DPM),对有效轴压力、钻具转速、冲洗压力、钻头位移及穿孔速率进行了实时监测。采用变斜率作为显著性指数,对地层中的主、次界面进行了识别。t-检验表明:DPM系统对岩土界面识别的置信度为99%。此外,对穿孔参数在界面处的变化分析表明:这些参数随孔深的变化曲线在界面处存在不同涨落,轴压力和穿孔速率对界面上岩石强度变化的响应度为81.82%。     

基于老化-海蚀循环的高阻尼橡胶支座剪切性能相关性试验研究

对高阻尼橡胶支座(HDR)开展了长达150 d的老化-海蚀循环试验,定期测试其剪切性能,分析总结了老化-海蚀循环作用时间及剪应变对HDR剪切性能的影响规律。试验结果表明,在老化-海蚀循环作用时间相同时,水平等效刚度比、等效阻尼比的比值和屈服后刚度比均随剪应变的增大而减小,屈服力比则随剪应变增大而增大;在剪应变相同时,随老化-海蚀循环作用时间的增长,其水平等效刚度比和屈服后刚度比总体呈增大趋势,且剪应变越大,水平等效刚度比增长幅度越大。等效阻尼比随试验时间的增长整体呈减小趋势,而屈服力比则随试验时间先增大后减小。拟合试验数据得到HDR水平等效刚度比、等效阻尼比的比值、屈服后刚度比和屈服力比随老化-海蚀循环时间及剪应变双参数的变化规律公式,可为隔震设计人员和深入开展海洋环境下HDR性能的变化规律研究提供理论依据。