Model Test on Backward Erosion Piping under a K0 Stress State

Backward erosion piping is an important failure mechanism for water-retaining structures. It results in the formation of shallow pipes at the interface of a sandy or silty foundation and a cohesive cover layer. This paper analyzes the depth of these erosion pipes through small-scale experiments. The development of the pipe depth reveals a lot of information on the backward erosion process, but it had never been measured systematically during the erosion process. Our analysis shows that the pipes are extremely shallow (in the order of mm) and that the pipe depth increases slightly, as piping progresses. Furthermore, a relation is found between pipe depth, grain size, soil permeability, and pipe length. The experimentally obtained depths are in good agreement with those obtained with theoretically determined pipe depths based on pipe hydraulics. Finally, the experiments are compared to 2D numerical simulations using Sellmeijer’s mathematical model and 3D numerical simulations with the correct pipe dimensions.

With the utilization of underground space, backward erosion piping (BEP) has been observed in many underground structures (e.g., shield tunnels) founded on sandy aquifers. However, due to invisibility, the geometry of the eroded pipe and its spatial evolution with time during the piping process was still not clear. In this study, we developed a Hele-Shaw cell to visualize the dynamic progression of BEP. With imaging process technology, we obtained a typical process of BEP (the erosion process can be divided into a piping progression phase and a piping stabilization phase), quantitatively characterized the formation of erosion pipes, and compared the patterns of erosion (e.g., the erosion area A and the maximum erosion radius Rmax) that spontaneously develop under different fluxes of water. The most interesting finding is that the sand grains in a thicker Hele-Shaw model are easier to dislodge, which is possibly due to the granular system in a thicker model having more degrees of freedom, reducing the stability of the sand grains.

It is generally known that high strength soil is indicative of well-graded particle size distribution. However, there are some special cases of firm ground despite poor grade distribution, especially a specific gap-graded soil. Based on these discoveries, this study investigated the development of an additive of gap-graded soils designed to increase soil strength. This theoretical concept was used to calculate the mixed ratio required for optimal soil strength of the ground sample. The gap-graded aggregate was added according to Plato\'s polyhedral theory and subsequently calculated ratio and soil strength characteristics were then compared to characteristics of the original soil sample through various test results. In addition, the underground stress transfer rate was measured according to the test conditions. The test results showed that the ground settlement and stress limit thickness were reduced with the incorporation of gap-graded soil. Further field tests would confirm the reproducibility and reliability of the technology by using gap-graded soil to reinforce soft ground of a new construction site. Gap-graded soil has the potential to reduce the construction cost and time of construction compared to other reinforcing methods.

The particle size distribution (PSD) is a crucial parameter used to characterize the material composition of debris-flow deposits which determines their hydraulic permeability, affecting the mobility of debris flows and, hence, the sustainable development of debris-flow fans. Three types of graded bedding structures—normal, reverse, and mixed graded bedding structures—are characterized by discontinuous gradation within a specific deposit thickness. A series of permeability tests were conducted to study the effects of bed sediment composition, particularly coarse grain sizes and fine particle contents, on the permeability and migration of fine particles in discontinuous debris-flow deposits. An increase in fine particles within the discontinuously graded bed sediment led to a power-law decrease in the average permeability coefficient. With fine particle contents of 10% and 15% in the bed sediments, the final permeability coefficient consistently exceeded the initial value. However, this trend reversed when the fine particle contents were increased to 20%, 25%, and 30%. Lower fine particle contents indicated enhanced permeability efficiency due to more interconnected voids within the coarse particle skeleton. Conversely, an increase in fine particle content reduced the permeability efficiency, as fine particles tended to aggregate at the lower section of the seepage channel. An increase in coarse particle size decreased the formation of flow channels at the coarse–fine particle interface, causing fine particles to move slowly along adjacent or clustered slow flow channels formed by fine particles, resulting in decreased permeability efficiency. Three formulae are proposed to calculate the permeability coefficients of discontinuously graded bed sediments, which may aid in understanding the initiation mechanism of channel deposits. Based on experimental studies and field investigations, it is proposed that achieving sustainable development of debris-flow fans requires a practical approach that integrates three key components: spatial land-use planning, in situ monitoring of debris flows and the environment, and land-use adjustment and management. This comprehensive and integrated approach is essential for effectively managing and mitigating the risks associated with debris flows, ensuring sustainable development in vulnerable areas.

This study presents the results from a series of centrifuge modeling tests conducted to investigate the backward erosion piping across a sandy foundation underlying an impervious cohesive layer with a defined exit hole. Different levels of centrifuge gravitational acceleration and various exit hole sizes were tested. The behavior at different phases of erosion was analyzed based on in-flight video recordings and posttest observations, along with local and global pressure loss measurements. Results showed that the overall mechanism that was modeled was similar to the mechanism described in previous small-scale experimental studies. The results showed that the exit hole size had minimal impact on the critical hydraulic gradient but affected the characteristics of the piping path and the amount of eroded material. The critical hydraulic gradient that initiated the erosion decreased slightly as the centrifuge gravitational acceleration increased. The values of the critical hydraulic gradient, which was studied locally and globally, ranged between 0.15 and 0.40, and fell within a range of estimates from typical analytical methods. This study provides a means to improve the testing protocol and analysis of centrifuge modeling of internal erosion mechanisms, which currently is limited in the literature.

Leakage failure analysis on water wall pipes of an ultra-supercritical boiler

A rational procedure for the dimensioning of subsea pipelines is described. It consists of a computer program which determines pipe diameter and wall thickness for given quantities of flow rate, depth of water, bottom current velocity and towing capacity of the lay barge. Steel quality, out-of-roundnees of pipe and characteristics of the concrete coating are also taken into account. The paper explains in detail the bending/ buckling criteria used in determining the steel pipe wall thickness. An analytical expression is derived for the maximum combined stress of a pipe subjected to constant external pressure and an external bending moment, allowing for initial cut-of-roundness. The concrete coating dimensions are determined in such a way that sufficient negative buoyancy of the air-filled pipe in water is maintained to counteract the lift originating from bottom currents. The bending moment is calculated with the help of large-deflection beam theory, solving the non-linear differential equation by means of finite-beam-element techniques. Sample applications of the design technique are given and checked against model tests and full scale data.

Several binary and ternary Al alloys containing from 2.8 to 5.5 wt pct Mg were tested in tension at elevated temperatures (200°C to 500°C) over a range of strain rates (10−4 to 2.0 s−1). Tensile ductilities of up to 325 pct were obtained in binary Al−Mg alloys with coarse grains deformed in the solute-drag creep regime. Under test conditions in which solute-drag creep controls deformation, Mg in concentrations from 2.8 to 5.5 wt pct neither affects tensile ductility nor influences strain-rate sensitivity or flow stress significantly. Strength is shown to increase with increasing Mg concentration, however, in the power-law-break down regime. The solute-drag creep process, which leads to superplastic-like elongations, is shown to have no observable grain-size dependence in a binary Al−Mg material. Ternary alloying additions of Mn and Zr are shown to decrease the strain-rate sensitivity during solute-drag creep, negatively influencing ductility. An important cause of reduced ductility in the ternary alloys during creep deformation is found to be a transition from necking-controlled failure in the binary alloys to cavitation-controlled failure in the ternary alloys investigated. An increase in ternary element concentration, which can increase the relative volume percentage of proeutectic products, increases cavitation.

Suffusion is a process by which fine particles move through the voids between coarse particles (intergranular voids) by seepage flow. Therefore, identifying the arrangement of coarse particles (coarse matrix), intergranular voids, and the spatial distribution of fine particles is a key issue for studying suffusion. Transparent soil is being increasingly used to replace real soil in laboratory model tests for studying the internal particle movements and pore fluid flow in soils. This study established an experimental setup for scanning a transparent soil specimen during seepage by a moving laser and made an attempt to construct a 3D digital mesostructural model of the specimen based on the images of the sequentially illuminated cross-sections. An example test on a gap-graded soil was conducted to show the performance of the presented method. It demonstrated that the coarse matrix and intergranular voids can be visualized and quantified at satisfactory precisions; moreover, the loss and redistribution of fine particles as well as important phenomena such as deposition at pore bodies and clogging of pore throats can also be visualized and identified qualitatively.

Effects of wall curvature on the turbulent structure in curved channel flow are studied by means of direct numerical simulation (DNS). Four different radii of curvature, δ/R c =0.013, 0.05, 0.1 and 0.2, are examined. The DNS results show reduction of turbulence intensities on the convex side and enhancement of radial turbulence intensity in the centre region of channel with the increase in curvature. Analyses on the flux Richardson number and the Reynolds shear stress clarify that such turbulence modulation is attributed to the extra production of the Reynolds stress caused by the centrifugal force. In addition, a spatio-temporal linear stochastic estimation (LSE) is applied to the computed velocity field to extract the development of turbulence-producing eddies affected by centrifugal force. The development of turbulence-producing motion on the convex side is associated with a large-scale strong roll-cell working againstthe centrifugal stabilizing effect. The spatio-temporal LSE also reveals that on the convex side strong outward and inward motions are induced by the roll-cell before the second- and fourth-quadrant motions are developed. This paper was chosen from Selected Proceedings of the Third International Symposium on Turbulence and Shear Flow Phenomena (Sendai, Japan, 24-27 June 2003). Present address: Toyota Central Research and Development Laboratories Inc., 41-1 Yokomichi, Nagakute, Nagakute-cho, Aichi 480-1192, Japan.

Recycling portable alkaline/ZnC batteries for a circular economy: An assessment of natural resource consumption from a life cycle and criticality perspective

Usually well preserved fluidization pillars and sand filled fluidization pipes occur within submarine channel sands of the basal Uratanna Formation (Lower Cambrian) in the Adelaide Geosyncline of South Australia. The morphology of these structures reflects complex lateral and vertical movement of fluids during liquefaction and dewatering. Fluidization pipes acted as conduits for highly concentrated, upward directed fluid flow. The formation and maintenance of these pipes was dependent upon the development of a pipe wall composed of clay plugged fine sand. Formed during initial fluidization, this lining acted as a permeability barrier, confining and concentrating fluidized flow within the pipe. Each of the pipes is surrounded by a cylindrical fluidization halo in which leakage through the pipe lining produced partial fluidization of the surrounding sediment. Fine scale structures within these haloes indicate that fluids flowed radially and upward out of the fluidization pipes at an acute angle. These fluids merged with and influenced the orientation and size of adjacent fluidization pillars. The fluidization pipes of the Uratanna Formation may represent unusual preservation of the unstable fluid flow conditions that occur during incipient fluidization of sand beds.

The presence of weak interlayers and groundwater are common adverse geological conditions in tunnels. To investigate the modes of failure of rock masses surrounding tunnels owing to weak interlayers and groundwater, model tests and numerical simulations were conducted in this study based on two cases, and a model that considers only the weak interlayer was conducted for comparison. Based on the tests, differences between two models in terms of rock pressure, displacement, cracks and strain were analysed. The results reveal that the presence of groundwater has a significant effect on the space–time distribution of stress, displacement and cracks in the surrounding rock. Furthermore, based on the numerical model, the seepage field was analysed in terms of pore water pressure, permeability and the seepage process to understand the joint action of groundwater and weak interlayer on the failure mechanism of tunnels. The results show that the groundwater and interlayer complement each other to induce the failure mode of the surrounding rock. The water accelerates slip in the interlayer and the development of cracks. Conversely, low strength, muddy weak interlayers serve as the channels of water flow, resulting in deformations and cracks at different locations and different failure modes.

In order to clarify the microscopic flow structure, the ultrasonic Doppler method was applied to the measurement of two-phase bubbly flow in vertical pipe (i.d.50mm). Liquid flow structure might strongly be influenced by the characteristic of the injected bubbles, i.e. bubbles’ size and void fraction. In this study, a bubble generator was newly designed with the purpose to control the bubble size and void fraction, independent of liquid main-flow rate. The experiment was performed at z/d = 66 from the bubble generator. Liquid flow rates were of the Reynolds numbers ranging from Rem = 3700 to 6200. The gas flow rate was constant at JG = 0.00348(m/s) at the measurement position. By analyzing the bubbles’ picture, it was confirmed that bubble size distribution and average bubble size were almost constant if the liquid flow rate were changed. The ultrasonic Doppler method has the capability of measuring the instantaneous velocity profiles of both phases at the same time. By processing the data based on pattern recognition, the recorded data can be classified to several groups. Using this method, the authors have tried to measure the bubbly flow in rectangular channel. In the present study, the application of this method to bubbly flow in circular pipe was satisfactory to obtain the liquid velocity distribution in bubbly flow and surrounding bubbles. From these results, it was clarified that velocity profile in bubbly flow in circular pipe has a maximum value near the pipe wall. Furthermore, velocity profiles around the bubble are influenced by leading bubbles.

This paper investigates asphaltene deposition in an oil well produces from a deep Jurassic reservoir located in Kuwait by developing a three-phase asphaltene deposition integrated model. The objective of this study is to develop and validate an integrated well model that incorporates asphaltene thermodynamic, aggregation, deposition, and transportation models coupled with a three-phase thermal-hydrodynamic mechanistic model. The proposed model is numerically solved to predict temporal and spatial asphaltene deposition and concurrently tracks oil composition changes due to asphaltene deposition. The elemental models of the integrated model are tuned against field data, and the deposition result is validated with a caliper log data. The field validation study revealed a very good match between the measured deposition behavior and model predictions. In addition, sensitivity analysis showed that although asphaltene precipitation increases as water cut increases due to higher pressure drop, asphaltene deposition decreases due to increasing shear stress and decreasing asphaltene sticking tendency on the pipe wall. Furthermore, sensitivity analysis revealed that as GOR increases, asphaltene precipitation increases; however, asphaltene deposition decreases. The proposed model shows significant sensitivity to asphaltene particle diameter, revealing that as particle diameter increases, deposition thickness decreases.