Experimental Analysis of Heaving Phenomena in Sandy Soils

Seepage erosion phenomena can arise when particles of soil are transported underneath any hydraulic structure by the seepage flow, as a consequence of the difference in the surface elevation upstream and downstream of the hydraulic structure. To assess the stability of hydraulic structures against seepage erosion computational models are available varying from simple empirical rules to advanced models. In this paper, the safety coefficient is assessed by comparing the exit gradient with the critical gradient: the former represents the hydraulic gradient at the exit point along the downstream ground surface, whereas the latter is the gradient at which upward drag forces on the soil particles equal the submerged weight of the soil particles and a quicksand condition originates. An experimental research was carried out to analyze different thickness sheet piles embedded in sandy soils, resulting in agreement between experimental data and theoretical values if the thickness of the sheet pile and the ground surface deformation are properly taken into account.     

Contact Erosion at the Interface between Granular Coarse Soil and Various Base Soils under Tangential Flow Condition

Under embankment dams and dykes, horizontal groundwater seepage prevails. If the subsoil is layered, and if some coarse layers are not appropriate filters for finer layers, there can be contact erosion at the interface between fine and coarse soils. In order to study contact erosion threshold, some base-soil and coarse-soil combinations were submitted to a flow parallel to the interface between the coarse soil and the base soil. Critical velocities and critical hydraulic gradients were measured for various base soils. Using effective base-soil grain diameter, an empirical expression for critical velocity was proposed that is well adapted for silts or sand/clay mixtures as well as for sands. The mass of eroded soil was measured relative to the flow velocity for each base-soil/coarse-soil setup. The shear stress applied to the interface between base soil and coarse soil was derived from the hydraulic gradient. Using an empirical relationship between applied shear stress and measured eroded mass, erosion rate was estimated for each base-soil/coarse-soil setup.     

Lactate Transport in Soil by DC Fields

Electrokinetic injection of lactate, a negatively charged biodegradable organic, in homogeneous soils is evaluated. Net lactate migration rate on the order of 5?cm2/V?day is measured in sand from cathode towards the anode. The ionic injection in sand was dependent on current density; however, the increase in electric current did not result in an equivalent increase in lactate transport due to development of an appreciable electroosmotic (EO) flow from the anode to the cathode. While high EO flow (ke on the order 10?6 to 10?5?cm2/V?s) occurred in clay samples, ion migration from cathode to anode is the dominant transport process under relatively high current density (5.3?A/m2 in this study) and can be used as an effective transport mechanism for negatively charged additives. An effective lactate reactive transport rate of more than 3?cm/d (under 1?V/cm) can be achieved in clays, which is at least two orders of magnitude greater than hydraulic injection under unit hydraulic gradient. Even though lactate concentrations in the clay were below 10% of the boundary value due to biological transformation, these concentrations (few 100?s?mg/L) are high enough to maintain microbial activities capable of degrading organic contaminants. At the same time, control experiments showed that, while lactate adsorption was negligible, hydraulic injection under a unit gradient was ineffective because of the low hydraulic conductivity of clay and the biodegradation of lactate.     

Stability of I-Walls in New Orleans during Hurricane Katrina

Failures of I-walls during Hurricane Katrina were responsible for many breaches in the flood protection system in New Orleans. Six breaches were examined in detail by Task Group 7 of the Interagency Performance Evaluation Taskforce. Four of these failures and breaches, which occurred before the water levels reached the top of the wall, were not caused by overtopping erosion. The failure of the I-wall at the 17th Street Canal resulted from shear through the weak foundation clay. The south failure of the London Avenue I-wall was caused by subsurface erosion, which carried massive amounts of sand inland, and removed support for the wall, leading to catastrophic instability. At the north breach on London Avenue, the failure was caused by high pore pressures, combined with a lower friction angle in the loose sand, which resulted in gross instability of the I-wall under the water pressure load from the storm surge. Looking back, with the benefit of 20-20 hindsight, these stability and erosion failures can be explained in terms of modern soil mechanics, exploration techniques, laboratory test procedures, and analysis methods. An important factor in all of the cases investigated was development of a gap behind the wall as the water rose against the wall and caused it to deflect. Formation of the gap increased the load on the wall, because the water pressures in the gap were higher than the earth pressures that had acted on the wall before the gap formed. Where the foundation soil was clay, formation of a gap eliminated the shearing resistance of the soil on the flood side of the wall, because the slip surface stopped at the gap. Where the foundation soil was sand, formation of the gap opened a direct hydraulic connection between the water in the canal and the sand beneath the levee. This hydraulic short circuit made seepage conditions worse, and erosion due to underseepage more likely. It also increased the uplift pressures on the base of the levee and marsh layer landward of the levee, reducing stability. Because gap formation has such important effects on I-wall stability, and because gaps behind I-walls were found in many locations after the storm surge receded, the presence of the gap should always be assumed in I-wall design studies.     

离子型稀土浸矿过程中渗透性变化规律的试验研究

离子型稀土矿体的渗透性对浸矿工艺和浸矿引起边坡稳定变化具有重要影响,为此,采用室内柱浸试验研究渗流过程离子型稀土渗透性变化规律,通过对比原矿和筛分样的渗透系数变化,探讨了浸矿过程渗透系数的变化机理.研究结果表明:在清水渗流过程中,微颗粒迁移引起渗流稳定时间增长.渗流变化幅度增大,随着水力梯度的增大,流场稳定时间减小.流量变化幅度增加;浸矿过程中,离子吸附和离子交换使渗透系数减小,微颗粒迁移引起渗透系数增加,两种相反的作用同时存在,当水力梯度小于某一临界值时,引起渗透系数变化的主要原因是离子吸附和离子交换,当水力梯度大于某一临界值时,引起渗透系数变化的主要原因是微颗粒迁移,水力梯度增大,微颗粒迁移对渗透系数的变化影响增大;由于原矿级配良好,粒径大小相差悬殊,导致原矿比筛分样流场稳定时间更长.      

小间距平行顶管管道土压力计算方法研究

小间距顶管过程中,由于管?管相互作用的影响,使得管周土压力分布与单管顶进土压力分布模式产生差异,从而造成小间距顶管荷载确定、结构计算及顶力估算与控制等设计施工难题。结合数值模拟反分析,基于太沙基土压力理论和极限平衡理论,假设了土体松动线和上部既有顶管的支挡作用线,进一步构建了小间距平行顶管管道拱顶垂直土压力的计算方法。基于构建的土压力计算方法,分析了土体抗剪强度、管径、管间距等对新建顶管拱顶土压力的影响,并与不考虑既有顶管影响的土柱理论和太沙基理论计算值进行了对比。计算结果表明:土体抗剪强度越大,新建顶管拱顶垂直土压力越大,而其侧面的土压力越小;抗剪强度较大时,新构建方法计算拱顶土压力小于太沙基理论计算结果,抗剪强度较小时,新构建方法计算拱顶土压力大于太沙基理论计算结果;顶管埋深增加时,新建顶管拱顶土压力增加,相较于土柱理论和太沙基理论,新构建方法计算的新建顶管拱顶土压力增量最小;随着管间距增加,新建顶管拱顶土压力越来越大。      

离子型稀土矿浸出过程微细颗粒迁移规律及调控

离子型稀土矿在浸出过程中浸出剂与矿石表面水合机制较为复杂,颗粒间的桥式胶结因离子吸附交换过程中存在多种作用力与分散作用而容易发生断裂,从而使微细颗粒发生迁移和重新排列,并在孔喉处沉淀,产生堵塞现象,影响离子型稀土的浸出效率。为揭示离子型稀土矿在原地浸出过程中微细颗粒的迁移规律,并找到适宜的调控方法,以龙南足洞离子型稀土矿为研究对象,采用实验室柱式溶浸法,考察了浸出剂质量浓度、黏度、流速、水力梯度、矿体高度及矿体含水率对微细颗粒迁移的影响。结果表明,离子型稀土矿浸出过程微细颗粒的迁移是影响浸出效率的重要因素之一。在外力的作用下,微细颗粒在浸出过程中易随浸出剂发生迁移运动。当调控浸出剂质量浓度低于4%,浸出剂黏度不超过1.5 mPa?s,水力梯度小于0.75,浸出剂流速低于3 mL/min,原矿含水量大于11%时,矿体中微细颗粒迁移率较低,矿体渗透性保持良好,有利于浸出液的渗流和稀土离子的浸取。     

Percolation Threshold of Sand-Clay Binary Mixtures

Many poorly graded granular materials of engineering importance can be characterized as gap-graded binary mixtures. Such mixtures display a volume-change response at a threshold value of the coarse fraction that is reminiscent of systems described by percolation theory. An experimental investigation on a sand-clay mixture is presented that clearly displays threshold behavior and sheds light on the role that each soil fraction plays in transferring loads through the medium. There are two key effects. First, an analysis of void ratio of the interpore clay fraction for varying compaction energies reveals an abrupt reduction in clay density at the threshold fraction of sand, whereby it is virtually impossible to impart compaction on the clay fraction at sand contents exceeding this threshold. Second, although force chains cannot be observed directly, analysis of the sand in terms of its component void ratio, computed based on treating the clay as part of the void space, shows that the sand carries a majority of the load at component void ratios that are too high to form stable force chains. The traditional interrelationship between mean stress and void ratio based on critical state theory breaks down when the sand content nears its threshold fraction. When the sand content is near the threshold limit, increasing mean stress results in a greater dilative tendency. Results are compared with findings on consolidation of sand-bentonite mixtures, and so-called reverse behavior of sand-silt mixtures.     

Installation of Suction Caissons in Sand with Silt Layers

Suction caissons are increasingly being used for offshore anchors because of their ease of installation. However, for soil profiles that have sand overlain by layers of silt, there are reasons to believe that it may not be possible to install suction caissons. This may happen because the low permeability of the silt will create a hydraulic blockage, and thus diminish the upward hydraulic gradient required to reduce the penetration resistance in the sand to enable caisson penetration. The tendency of the silt to be sucked up, and its effect on the underlying sand, are not clearly understood. Furthermore, the blockage by the silt layer may be less than if a clay layer is present. This is because scouring of the silt may occur, allowing an upward seepage gradient to develop, and hence causing a reduction in penetration resistance. A series of suction caisson installation tests have been conducted in a geotechnical centrifuge to provide data on the penetration resistance and mechanisms for soil profiles where silt layers are present, either on top, or interbedded between layers of sand. In these tests, the thickness of the silt layer and its depth below the sand surface have also been varied. It has been found that if silt overlies the sand, a larger suction force is required for installation than when installation occurs in homogeneous sand, but that this force is still much smaller than the soil resistance when the caisson is pushed into the ground. When a silt layer is present, the suction pressures are observed to follow a similar trend that is independent of the position and thickness of the silt layer. Scouring of silt was evident in the tests, and significant upward movement of the soil plug inside the caisson was observed in all soil profiles when silt layers were present. The plug height was seen to reduce after the suction was turned off, suggesting that the resulting heave was unstable.     

Investigation of Factors Affecting Vertical Drain Behavior

Some influencing factors on vertical drain behavior were investigated by laboratory tests as well as by back-analyses of test embankments on vertical drain improved subsoil at Saga Airport, Saga, Japan. Based on the results from this study, suggestions are made on determining the design parameters for vertical drain improvement. For the discharge capacity test of a prefabricated vertical drain, confining the drain in clay is essential. Also, due to the creep of the filter and the clogging caused by the fine particles entering the drainage channel, the long-term discharge capacity is significantly smaller than the short-term one, and this should be considered in design. For smear effect, a new equation is proposed for determining the ratio of the hydraulic conductivity of the natural subsoil to that of the smear zone, which considers the fact that laboratory tests normally underestimate the hydraulic conductivity of natural deposits. Regarding the effect of the sand mat, the numerical analysis results in this study show that if the hydraulic conductivity of sand is larger than 10?4 m∕s, the assumption of a free drainage condition in the sand mat may not result in significant error. Finally, a methodology of predicting the behavior of vertical drain improved subsoil is proposed.     

Design of a Rigid Cutoff Wall

A sheet pile cutoff wall simply called cutoff wall is used in almost all types of water retaining structures to reduce the exit gradient at the downstream toe and prevent the chances of sand boiling and piping. In 1954, Terzaghi has suggested a simplified approach for the design of cutoff walls considering average seepage force. In Terzaghi’s approach, the unbalanced water pressure around the cutoff wall is coupled with the active earth pressure and the resultant thrust is determined by subtracting the passive thrust from the combined active thrust. The value of passive earth pressure coefficient has been assumed to correspond with a downstream failure wedge having an angle of (45°?1/2) with the horizontal, where 1 = effective angle of internal friction of the foundation soil. In this paper, it is shown that under steady state flow conditions for a flexible levee base, failure does not take place always at this angle. Considering exact seepage forces and applying the laws of equilibrium, active and passive earth pressures acting on a rigid sheet pile cutoff wall on an impervious levee have been determined. The weight of the levee has been considered in the computation of active thrust. The design procedure of a rigid cutoff wall is also explained. There is a difference of approximately 10% between the passive thrusts for a rigid cutoff wall of normal length computed by Terzaghi’s method and by the rigorous method presented herein.     

聚乙二醇对风化壳淋积型稀土矿浸出的影响

针对氯化铵作为浸取剂浸取风化壳淋积型稀土时,稀土浸取速率较低和浸出周期长等问题,使用聚乙二醇(PEG)为添加剂,通过柱浸试验模拟工业生产中的原地浸出工艺,探究不同聚合度的聚乙二醇对稀土矿渗透性和稀土浸出的影响。结果表明,聚乙二醇400、聚乙二醇1000和聚乙二醇4000溶液在稀土浸出过程中表现出较好的渗透效果。采用质量分数2.00%的聚乙二醇400、聚乙二醇1000、聚乙二醇4000分别与0.20mol/L的氯化铵复配,复配浸取剂溶液在稀土矿样中的渗透速率随着水力梯度的增大呈线性增大,符合达西定律。PEG400+NH_4Cl、PEG1000+NH_4Cl、PEG4000+NH_4Cl以及NH_4Cl溶液的稀土浸出率分别为82.61%、89.12%、70.67%和88.06%,且当聚乙二醇1000作为助浸剂时,渗流速率最大,渗透效果最佳。添加了聚乙二醇1000的复配浸取剂溶液,在浸取风化壳淋积型稀土矿中稀土的反应过程符合内扩散动力学模型。研究结果对提升复配浸取剂溶液在稀土矿体中的渗流速率有重要意义。     

Large Woody Debris Structures for Sand-Bed Channels

Described is a method for channel erosion control and habitat rehabilitation featuring intermittent placement of structures made of large woody debris. This method is expressly tailored to address severe problems typical of incised channels with little sediment coarser than sand. In these types of environments, buoyancy forces are typically more important factors in woody debris stability than fluid drag. Buoyant forces are counteracted by the weight of the structure, earth anchors, and sediment deposits. Design concepts were tested in a demonstration project constructed along 2 km of channel draining a 37-km2 watershed. Large woody debris structures reduced velocities in the region adjacent to the bank toe and induced sediment deposition and retention. Construction costs per unit channel length were 23–58% of costs for recent stone bank stabilization projects within the same region. During the second year following construction, 31% of the structures failed during high flows, probably due to inadequate anchoring.     

Centrifuge Permeameter for Unsaturated Soils. I: Theoretical Basis and Experimental Developments

A new centrifuge permeameter was developed with the specific objective of expediting the measurement of the hydraulic characteristics of unsaturated soils. The development, theoretical basis, and typical results associated with using the centrifuge permeameter for concurrent determination of the soil-water retention curve (SWRC) and hydraulic conductivity function (K function) of unsaturated soils are presented in this paper. Components developed for the centrifuge permeameter are described, including the centrifuge, permeameter, water flow control system, and instrumentation used to concurrently and nondestructively measure the infiltration rate (flow pump and outflow transducer), volumetric water content (time domain reflectometry), and matric suction (tensiometers) in flight during steady-state infiltration. A companion paper focuses on definition of the SWRC and K function for a clay soil using the procedures described in this paper. While conventional geotechnical centrifuges are used to reproduce the response of earth structure prototypes, the centrifuge developed in this study is used to accelerate flow processes. Accordingly, it required a comparatively small radius (0.7 m) but high angular velocity (up to 875 rpm or 600 g’s) to impart a wide range of hydraulic gradients to an unsaturated soil specimen. Analytical solutions to Richards’ equation in the centrifuge indicate that steady-state infiltration allows direct determination of the relationships between suction, volumetric water content, and hydraulic conductivity from the instrumentation results. Typical instrumentation results during a drying stage are presented to illustrate determination of data points on the SWRC and K function at steady state. These results were found to be consistent with analytical flow solutions.     

Laboratory Evaluation of Sand Underdrains

Constant-head hydraulic conductivity tests are performed on layered heterogeneous porous media to evaluate the use of underdrains to calculate the hydraulic conductivity of an overlying, less permeable medium. The layered profiles consist of a barrier layer comprising sand mixed with 10% kaolin, overlying a foundation layer comprising sand mixed with only 5% kaolin. Underdrains are evaluated by replacing excavated portions of the foundation layer with only sand. The results indicate that preferential flow of water occurs around, rather than through, the sand underdrains resulting in an underestimate of the measured hydraulic conductivity of the barrier layer assuming 1D, saturated flow in accordance with standard practice. The observed preferential flow effect is consistent with previously published numerical simulations of unsaturated flow through similarly layered heterogeneous soil profiles that indicate lateral flow around underdrains due to the contrast in unsaturated properties of the soils. The results of this study have important ramifications with respect to the use of underdrains to measure in situ hydraulic conductivity of compacted clay liners for waste containment.     

Experimental Investigation of the Hydraulic Erosion of Noncohesive Compacted Soils

This paper presents the results of a laboratory investigation whose purpose was to evaluate the effects of compaction on the erodibility of cohesionless soils. By means of a recently developed flume experiment, sediment erosion rates and incipient motion, as a function of shear stress, average velocity, and dry density, have been determined for three compacted sand and gravel mixtures. A preliminary comparison of the incipient motion values shows that granular soils compacted at the Proctor optimum have a higher resistance to free surface flow erosion than those compacted at lower and higher densities. This leads one to infer that the Proctor optimum, generally used as a standard for construction, might also be an optimum for hydraulic resistance and stability. Additional comparison of the experimental data with two commonly used incipient motion criteria also suggests that Yang’s criterion is a better predictor of soil detachment than the Shields-Yalin criterion.     

Electroseismic Wave Theory of Frenkel and More Recent Developments

Frenkel’s 1944 theory of porous media acoustics is carefully scrutinized. After some manipulation, Frenkel’s equations are seen to have nearly identical form to Biot’s 1962 equations. The only difference is that Frenkel includes an extraneous fluid-pressure gradient in his bulk force balance. Frenkel also makes a slight error in the development of his effective poroelastic moduli that prevents him from being the first to obtain the so-called “fluid-substitution” relations. Outside of these two small problems, Frenkel’s analysis can be considered correct. He limits his treatment of electroseismic phenomena to explain the electric field that accompanies a compressional seismic wave in a homogeneous material. He predicts that the electric field in a compressional wave is directly proportional to the particle acceleration and this has been verified by the recent field measurements of Garambois and Dietrich. However, such electric fields are only a small part of the total electroseismic response of the earth. Accordingly, some of the additional phenomena not discussed by Frenkel are also presented and discussed.     

Al_2O_3La_2O_3Y_2O_3/Cu复合材料的电弧侵蚀特性研究

采用喷射沉积和内氧化法制备出Al2O3La2O3Y2O3/Cu复合材料,研究该材料在直流20 V/20 A的工作条件下触点的电弧侵蚀特性,并与Al2O3/Cu材料进行了对比分析.利用电子天平、扫描电镜等方法分析电弧侵蚀后触点的质量变化和表面微观结构.结果表明,通过添加Y2O3、La2O3稀土氧化物颗粒,可有效降低触头材料的材料转移量.Al2O3La2O3Y2O3/Cu材料的抗熔焊性和抗烧损性优于Al2O3/Cu材料的性能.在直流阻性负载条件下Al2O3La2O3Y2O3/Cu阳极触头表面形成凹坑,阴极触头表面形成凸起,触点表面显示出浆糊状凝固物和喷发坑等电弧侵蚀形貌特征.