Simulation of pipe progression in a levee foundation with coupled seepage and pipe flow domains

A very large percentage of piping cases have been brought about by internal erosion, which is the primary cause of dam failures. This study developed a numerical model to simulate the pipe progression in a levee foundation by analyzing the inception and transportation of erodible particles from the soil fabric. An approach that considers the turbulent flow in an erodible pipe and the seepage flow in the remaining area of a levee foundation is employed to capture the main hydraulic characteristics of piping. The mechanical analysis of individual erodible particles is considered to quantify the critical condition for particle inception in an erodible pipe. In addition, physical piping model tests are numerically simulated to examine the proposed approach. The simulation demonstrates that the flow in a pipe can progress backward from downstream to upstream when the upstream water head reaches a critical value. Furthermore, the function mechanism of a cut-off wall can be explained by this model. The results have revealed that this model can reproduce the experimental data, such as the critical water head and the progression time, which are obtained from the physical model. The relationship between the depth of a suspended cut-off wall and the critical water head is obtained; this relationship facilitates the practical design of the critical depth of a cut-off wall for a given water head.     

单层堤基管涌侵蚀过程的模型试验及数值分析

利用砂槽模型试验研究单层堤基的管涌侵蚀破坏过程,并建立单层堤基管涌侵蚀破坏发展过程的概化数学模型和数值模拟方法。结合模型试验结果和数值模拟结果,分析单层堤基管涌侵蚀破坏的机理及侵蚀破坏过程。研究管涌局部破坏的临界比降及不同渗径长度对管涌侵蚀破坏的影响。计算结果表明:①模型试验和数值模拟结果较好地揭示单层堤基管涌侵蚀破坏过程,即当上游水头低于临界水头时,管涌存在发生、发展和停止的过程。当上游水头大于临界水头时,管涌将持续发展并最终导致溃堤破坏,计算结果和试验结果吻合较好;②得到砂样内部的渗流场分布和一些难以观测到的数据,较好地解释模型试验的一些现象,提高对管涌溃堤机理和过程的认识;③单层堤基管涌破坏的临界水头和临界水平平均比降与砂层局部破坏的临界水平比降近似呈线性关系;④单层堤基管涌破坏的临界水平平均比降随着渗径长度的增大而增大,并且大体上呈线性关系。     

江河大堤均匀砂基渗透破坏机理试验研究

为了摸清大堤堤基出现渗透破坏甚至溃决的原因,专门进行了模型试验。根据对堤基地质及水文地质的分析,将复杂的堤基归纳并概化为三种类型,并分别进行了各类模拟试验。阐述了均匀砂层堤基的渗透破坏特性及影响因素。均匀砂基的渗透破坏主要发生在堤基上部,特别是大堤与堤基的接触带,破坏水力比降决定于砂层土料本身的破坏水力比降,当大堤与堤基之间存在脱开问题,则破坏水力比降为砂层表面的冲刷水力比降。     

土层结构对管涌发展影响的试验研究

不同的土层结构对管涌的发生发展有着很大影响,对3种典型的土层结构进行了管涌发展的砂槽模型试验,观察并分析了管涌发生、发展的机理和过程。试验结果表明,双层堤基的砂砾石表面夹一层很薄的无黏性粉细砂时,管涌破坏前出水口的流量很小,临界水力梯度也较小,管涌破坏发生后通道发展速度很快,较短的时间内就会贯通,管涌破坏的机理与双层堤基不同;而在砂砾石层一定深度内夹有一层粉细砂将使堤基管涌破坏的临界水力梯度大大提高,然而一旦管涌破坏发生后涌砂量和侵蚀速率将很大,形成的通道深度较大,若不及时采取有效措施,当通道规模发展到一定程度后,通道上部会发生塌落而使堤坝产生溃口,进而使堤坝溃决。     

多层堤基中土层结构变化对管涌影响的试验研究

不同土层结构的堤基,管涌的发生和发展情形不同。利用室内试验,通过改变下伏砂层内夹砂层的级配组成,对3种不同夹砂层的多层堤基进行了管涌破坏过程的模拟,研究了不同颗粒级配组成的夹砂层对管涌发生及发展过程和机理的影响。试验结果表明,多层堤基夹砂层均为细砂时,承受的水压力较大,临界水力梯度较高,一旦发生管涌破坏后其渗透流量、涌砂量以及破坏范围都比较大,所以此类堤基发生管涌破坏时具有一定的突然性和剧烈性,应及早采取防治措施;夹砂层均为粗砂时,管涌破坏时的情形与双层堤基类似,管涌破坏的范围局限于砂砾层顶部,破坏深度有限;夹砂层为细砾时,发生管涌破坏的临界水力梯度较小,管涌破坏程度逐步增加且破坏速度较快,由于涌砂量较大容易使堤基产生明显的渗透变形。     

双层堤基管涌发生发展的试验模拟与分析

利用室内试验模拟了双层堤基管涌的发生及发展过程。通过分析水头分布、流量、以及出砂量等的变化,将破坏的发生及发展归结为4个阶段：上覆层破坏前的稳定阶段、上覆层破坏阶段、上覆层破坏后的稳定阶段、整体破坏阶段。研究发现,在上覆层破坏前,上覆层承担了大部分的水头差,当上覆层破坏后,这部分水头差转由下伏砂层承担,造成下伏层水力梯度的升高,如果水力梯度超过了下伏层的临界水力梯度,则下伏层发生渗透破坏。一次渗透破坏发生后,降下上游水头,重复增加水头的过程,模拟地层在多次渗透破坏作用下,地层抵抗渗透破坏能力的变化。发现一旦渗透破坏发生后,地层再次抵御渗透破坏的能力急剧下降,多次破坏后,试样内形成贯穿上、下游的集中渗漏通道,且通道规模随着试验次数的增加而增长,若不及时采取有效措施,当通道规模发展到一定程度后,通道上部会发生塌落而使堤坝产生溃口,进而使堤坝溃决。     

Improving piping resistance using randomly distributed fibers

Piping is a problem that commonly occurs downstream of hydraulic structures under the influence of upward seepage. Piping is considered as the main mechanism of hydraulic structures failures. In this work an experimental program was set for determining the seepage velocity and piping resistance for unreinforced and randomly reinforced silty sand samples. Two types of fiber were used for preparing the reinforced samples. The experimental tests were carried out for different fiber contents (0.5, 0.75, 1.0 and 1.25%) and fiber lengths (5, 25 and 35 mm) under different hydraulic heads. Discharge velocity and seepage velocity of water flow through unreinforced and reinforced samples were calculated and compared with unreinforced sample. The results show that the inclusion of fibers reduced the seepage velocity, increased the piping resistance and increased the critical hydraulic gradient hence, considerably delaying the occurrence of piping. Furthermore, the amounts of increase in the piping resistance and hydraulic gradient are functions of percent and length of fibers.     

Investigations on secondary compression behaviours of artificial soft sand-clay mixtures

Special soft soils with notable coarse and fine fractions are not only naturally deposited in ravines or interactive marine-terrestrial sedimentation areas but also artificially formed in land reclamation projects via the hydraulic filling method and excavation cuts of different soil layers in open pit mines. The behaviour of these soils is not like that of similar normal sands or clays but rather exhibits the characteristics of transitional mixtures. A sand-clay mixture was prepared in the laboratory to simulate these special soil types, and an incremental loading oedometer test was performed. Thereafter, a secondary compression coefficient (C_α) was determined to investigate the long-term rheological properties of the mixture. The results in the C_α-log σ_v′, C_α-e, C_α-e/e_L and C_α-C_c planes revealed that a threshold e/e_L value (i.e. approximately 0.45) in the case of sands. Note that in a traditional C_α-C_c analysis system, the soils with sand exhibited the same secondary compression behaviour (SCB) as that without sand before reaching the transformation threshold, whereas after this threshold, they appeared to behave much like sandy soils. Hence, a four-phase frame of sand-clay mixture was further introduced to clarify the mechanism of the SCB of soils with and without sand. An updated C_αc/C_cc reflecting the creep mechanism of the clay phase can be effectively adopted to exclude the influence of sand particles. Because the quantitative relationship between C_αc and C_cc for soils with sand was consistent with that without sand, it can be assumed that the SCB of clay-sand mixtures was dominated by the creep of fine fractions. It is porposed that the existence pattern of the sand particles in the mixtures undergoes three stages with an increasing vertical stress, i.e. non-skeleton, partial skeleton and complete skeleton. A further discussion on the mutual relationship between phases on the behaviour of mixtures with sand revealed that isolated sand had no effect on the compressibility and SCB during the initial stage. When a partial skeleton of the sand was established in the second stage, characterized by less stress on the clay matrix, the compressibility and SCB were suppressed. When the vertical stress continuously increased or additional sand content was supplied, a complete skeleton formed. Hence, the behaviour of mixtures (regardless of compressibility and secondary compression) was completely governed by the sand fractions. Generally, the same relationship was observed between C_αc and C_cc during the first and second stages with sand, indicating that the influence of sand fractions on the compressibility and SCB can be counterweighted.     

大口径夹砂玻璃钢管安装的质量控制措施

针对玻璃纤维缠绕增强热固性树脂夹砂压力管(简称夹砂玻璃钢管)的性能特点,以山西运城引黄济运供水管线工程为例,介绍了大口径夹砂玻璃钢管安装过程质量控制。主要关键环节有材料控制、测量放线、沟槽开挖、管道敷设、管沟回填和系统水压试验,逐一介绍了控制措施。     

含浅层强透水层堤基的上覆砂层管涌破坏试验研究

堤基中往往存在局部浅层强透水层并形成渗流优先通道,该通道不能大幅度削减流体的水头势能,易引起堤基管涌破坏,此类堤基管涌破坏机理的研究尚不明朗,仍需进一步研究。采用砂槽试验模拟堤基渗流,试验中通过抬升水箱水位,观察砂土中细颗粒流失现象,并分析渗流量、渗透坡降、测压管水头、砂土颗粒级配、锥头阻力、沉降量等关键参数。试验结果表明,水箱水位增大至48cm,浅层强透水层上覆砂层被"击穿"发生管涌破坏,管涌破坏分为稳定渗流阶段、细颗粒流失阶段(0.05d≤0.075粒级砂土流失)、较细颗粒流失阶段(0.075d≤0.1粒级砂土流失)、管涌破坏扩大阶段(0.1d≤0.25粒级砂土流失)。管涌破坏过程中,细颗粒砂土流失,锥头阻力降低,砂土层发生沉降,且较细颗粒流失阶段的沉降较为突出。细颗粒砂土流失导致砂土层孔隙率和渗透系数上升,渗流量和渗透坡降随之增大。     

Optimization of dewatering schemes for a deep foundation pit near the Yangtze River,China

A deep foundation pit constructed for an underground transportation hub was excavated near the Yangtze River. Among the strata, there are two confined aquifers, between which lies an aquiclude that is partially missing. To guarantee the safety of pit excavation, the piezometric head of the upper confined aquifer, where the pit bottom is located, should be 1 m below the pit bottom, while that of the lower confined aquifer should be dewatered down to a safe water level to avoid uplift problem. The Yangtze River levee is notably close to the pit, and its deformation caused by dewatering should be controlled. A pumping test was performed to obtain the hydraulic conductivity of the upper confined aquifer. The average value of the hydraulic conductivity obtained from analytical calculation is 20.45 m/d, which is larger than the values from numerical simulation(horizontal hydraulic conductivity K_H = 16 m/d and vertical hydraulic conductivity K_V = S m/d). The difference between K_H and K_V indicates the anisotropy of the aquifer. Two dewatering schemes were designed for the construction and simulated by the numerical models for comparison purposes. The results show that though the first scheme could meet the dewatering requirements, the largest accumulated settlement and differential settlement would be94.64 mm and 3.3‰, respectively, greatly exceeding the limited values. Meanwhile, the second scheme,in which the bottoms of the waterproof curtains in ramp B and the river side of ramp A are installed at a deeper elevation of-28 m above sea level, and 27 recharge wells are set along the levee, can control the deformation of the levee significantly.     

Relationships between biological solids retention time and settling characteristics of activated sludge

It was the objective of this study to delineate the relationships between the settling characteristics of a non-filamentous activated sludge grown on a soluble waste, and a rational kinetic based parameter used to characterize the growth or loading rate of the system. The parameters used to characterize settling were Sludge Volume Index (SVI), zone settling velocity, and percentage dispersion. Biological Solids Retention Time (θ_c), the reciprocal of mean cell growth rate, was used to characterize the metabolic activity level of the activated sludge. The chosen parameter (θ_c) can be related to food to microorganism ratio and other widely used organic loading expressions.     

3D finite element method (FEM) simulation of groundwater flow during backward erosion piping

Backward erosion piping is an important failure mechanism for cohesive water retaining structures which are founded on a sandy aquifer. At present, the prediction models for safety assessment are often based on 2D assumptions. In this work, a 3D numerical approach of the groundwater flow leading to the erosion mechanism of backward erosion piping is presented and discussed. Comparison of the 2D and 3D numerical results explicitly demonstrates the inherent 3D nature of the piping phenomenon. In addition, the influence of the seepage length is investigated and discussed for both piping initiation and piping progression. The results clearly indicate the superiority of the presented 3D numerical model compared to the established 2D approach. Moreover, the 3D numerical results enable a better understanding of the complex physical mechanism involved in backward erosion piping and thus can lead to a significant improvement in the safety assessment of water retaining structures.     

Effects of flow and water chemistry on lead release rates from pipe scales

Lead release from pipe scales was investigated under different water compositions, stagnation times, and flow regimes. Pipe scales containing PbO₂ and hydrocerussite (Pb₃(OH)₂(CO₃)₂) were developed on lead pipes by conditioning the pipes with water containing free chlorine for eight months. Water chemistry and the composition of the pipe scales are two key factors affecting lead release from pipe scales. The water rarely reached equilibrium with pipe scales within one day, which makes solid-water contact time and corrosion product dissolution rates the controlling factors of lead concentrations for the conditions tested. Among five water compositions studied, a solution with orthophosphate had the lowest dissolved lead release rate and highest particulate lead release rate. Free chlorine also decreased the dissolved lead release rate at stagnant conditions. Water flow increased rates of release of both dissolved and particulate lead by accelerating the mass transfer of lead out of the porous pipe scales and by physically destabilizing pipe scales. Dissolved lead comprised the majority of the lead released at both stagnant and laminar flow conditions.     

双层堤基管涌通道扩展机制和计算方法研究

双层堤基管涌通道是否扩展,取决于砂层颗粒条件、通道内冲刷水流和边壁渗流力等因素。通过分析通道边壁颗粒的受力平衡条件,引用河流动力学的相关公式,考虑砂粒相对暴露度、脉动流速、起动标准和管涌通道水流特性等因素提出通道扩展的判定条件。用2个砂槽模型试验研究冲刷水流和边壁渗流力的不同组合对通道扩展的影响,并用一维渗透试验证实渗流溢出面附近实际坡降远小于平均坡降的现象,说明提出渗流力有效比参数的合理性。在上述研究的基础上,用通道边壁稳定条件控制,建立有限元计算迭代流程,对管涌通道的扩展进行了数值模拟,并与试验结果进行对比,验证该方法的可行性。     

Fluid shear influences on the performance of hydraulic flocculation systems

Gravity driven hydraulic flocculators that operate in the absence of reliable electric power are better suited to meet the water treatment needs of green communities, resource-poor communities, and developing countries than conventional mechanical flocculators. However, current understanding regarding the proper design and operation of hydraulic flocculation systems is insufficient. Of particular interest is the optimal fluid shear level needed to produce low turbidity water. A hydraulic tube flocculator was used to study how fluid shear levels affect the settling properties of a flocculated alum-kaolin suspension. A Flocculation Residual Turbidity Analyzer (FReTA) was used to quantitatively compare the sedimentation velocity distributions and the post-sedimentation residual turbidities of the flocculated suspensions to see how they were affected by varying fluid shear, G, and hydraulic residence time, θ, while holding collision potential, Gθ, constant. Results show that floc breakup occurred at all velocity gradients evaluated. High floc settling velocities were correlated with low residual turbidities, both of which were optimized at low fluid shear levels and long fluid residence times. This study shows that, for hydraulic flocculation systems under the conditions described in this paper, low turbidity water is produced when fluid shear is kept at a minimum. Use of the product Gθ for design of laminar flow tube flocculators is insufficient if residual turbidity is used as the metric for performance. At any Gθ within the range tested in this study, best performance is obtained when G is small and θ is long.     

Inverse analysis of laboratory data and observations for evaluation of backward erosion piping process

An inverse analysis procedure has been developed to interpret collected pore pressure data and observations during backward erosion piping (BEP) initiation and progression in sandy soils. The procedure has been applied to laboratory models designed to mimic the initiation and progression of BEP through a constricted vertical outlet. The inverse analysis uses three-dimensional (3D) finite element method (FEM) to successively produce models of the hydraulic head regime surrounding progressive stages of BEP based on observations at the sample surface and pore pressure measurements obtained from the laboratory models. The inverse analysis results in a series of 3D contour plots that represent the hydraulic-head regime at each stage of the BEP development, allowing for assessing the development of BEP mechanism as well as calculating the critical hydraulic conditions required for various BEP stages to initiate and progress. Interpretation of the results identified four significant stages of the piping process: (1) loosened zone initiation, (2) channel initiation and progression, (3) riser sand fluidization, and (4) loosened zone progression. Interpretation of the hydraulic head contour plots allows assessment of the critical hydraulic gradients needed to initiate and progress various components of the BEP development.     

双层堤基管涌破坏过程中上覆层渗透破坏发生发展的 试验与分析

利用室内试验模拟了刚性盖板下双层堤基渗透破坏的过程,研究了堤基发生管涌破坏后对上覆黏土层的影响。通过肉眼观察,照相机辅助拍摄及试验过程中测压管水位,流量和出砂量等的变化,分析了管涌过程中刚性盖板下黏土层破坏的发生及发展过程。堤基发生管涌破坏后,砂层上部细颗粒逐渐流失,形成强渗流通道并逐步向上游发展。渗流通道与上层黏土之间形成脱空,黏土层发生轻微沉降变形,在水流作用下产生不均匀变形导致黏土层中裂纹的产生并在水力劈裂作用下裂纹逐渐变成裂缝并越来越大,水流通过裂缝在塌陷黏土与刚性盖板之间急速流动,产生冲刷,破坏范围随管涌的发生向上游发展。黏土层的破坏促进了管涌的发生,而管涌通道不断向上游发展也加速了黏土层的破坏,两种过程相互作用,相互影响。     

Air entrainment and turbulent fluctuations in hydraulic jumps

A hydraulic jump is the sudden transition from a high-velocity impinging flow into a turbulent roller in an open channel. Substantial amounts of air are entrapped at the impingement point, and significant free-surface fluctuations take place above the roller. In the present study, some physical modelling was conducted in a relatively large sized facility. The flow conditions included a wide ranges of inflow Froude numbers and Reynolds numbers (3.8 < Fr₁ < 10.0, 2.1 × 10⁴ < Re < 1.6 × 10⁵). The fluctuating features of free-surface and roller position were investigated non-intrusively with a series of acoustic displacement meters. The characteristic frequencies of the fluctuating motions were documented, and some major roller surface deformation patterns were revealed. The air-water flow properties were investigated with an intrusive phase-detection probe. The void fraction and bubble count rate data were documented in the jump roller, together with the interfacial velocity distributions. The rate of air entrainment was estimated based upon the void fraction and interfacial velocity distribution data. Some simultaneous measurements of instantaneous void fraction and free-surface fluctuations as well as longitudinal jump front oscillations were conducted. The relationship between the rate of air entrainment and turbulent fluctuations is discussed. Both the turbulent fluctuation and aeration properties are basic design parameters in urban water systems in which a hydraulic jump may take place. The present work provides relevant information for water systems including covered channels and partially-filled pipes.