Loose Bed Research Articles

Effect of the ski jump bucket angle on the scour hole downstream of a converging stepped spillway

ABSTRACT The present study is devoted to the analysis of scour downstream of a converging stepped spillway equipped with a ski jump bucket. The energy dissipated by jets leaving the spillway can be divided in two parts: one is the kinetic energy dissipated along the jet trajectory and another one is the energy dissipated on the impact with the tailwater. Due to the jet impact, the riverbed is subjected to turbulent pressure fluctuations resulting in the formation of a scour hole. Different experimental conditions allowed to analyse the influence of different variables on the configuration of the loose bed scour, namely: the tailwater depth, the flow rate and the exit angle of the ski jump bucket. The results were compared to those presented by other authors. For some specific flow conditions, the 3D configuration of the scour hole was also determined.

Behavior of sand columns reinforced by vertical geotextile encasement and horizontal geotextile layers

In this paper, the effect of a group of sand columns in the loose soil bed using triaxial tests was studied. To investigate the effect of geotextile reinforcement type on the bearing capacity of these sand columns, Vertical encased sand columns (VESCs) and horizontally reinforced sand columns (HRSCs) were used. Number of sixteen independent triaxial tests and finite element simulation were performed on specimens with a diameter of 100 mm and a height of 200 mm. Specimens were reinforced by either a single sand column or three sand columns with the same area replacement ratio (16%) to evaluate the Influence of the column arrangement. Effect the number of sand columns, the length of vertical encasement and the number of horizontal reinforcing layers were investigated, in terms of bearing capacity improvement and economy. The results indicated that the ultimate bearing capacity of the samples with three ordinary sand columns (OSCs) is eventually about 11% more than samples with an OSC. Also, comparison of the column reinforcing modes showed that four horizontal layers of geotextile achieved similar performance to a vertical encasement geotextile at the 50% of the column height, from the viewpoint of strength improvement, while from the viewpoint of economy, the geotextile needed for encasing the single column is around 2.5 times of the geotextile required for four layers.

Determination of the bearing capacity of model ring footings: Experimental and numerical investigations

In this paper, it was presented an investigation on the load-settlement and vertical stress analysis of the ring footings on the loose sand bed by conducting both laboratory model tests and numerical analyses. A total of twenty tests were conducted in geotechnical laboratory and numerical analyses of the test models were carried out using the finite element package Plaxis 3D to find the ultimate capacities of the ring footings. Moreover, the results obtained from both foregoing methods were compared with theoretical results given in the literature. The effects of the ring width on bearing capacity of the footings and vertical stresses along the depth were investigated. Consequently, the experimental observations are in a very good agreement with the numerical and theoretical results. The variation in the bearing capacity is little when ri/Ro<0.3. That means, when the ring width ratio, ri/Ro, is equal to 0.3, this option can provide more economic solutions in the applications of the ring footings. Since, this corresponds to less concrete consumption in the ring footing design.

厚松散层薄基岩上提工作面压架原因数值模拟研究

厚松散层薄基岩上提工作面压架原因数值模拟研究

Local dilation and compaction of granular materials induced by plate drag

The response of granular materials to plate drag is numerically studied using a large-scale discrete element method (DEM) simulation. The effect of the initial volume fraction of the materials on the drag force acting on the plate is examined. The results show that a volume-fraction-dependent bifurcation occurs in the force; in an initially loose granular bed, the force reaches an approximately constant value as the plate advances, while in an initially dense bed, the force oscillates with a large amplitude. The force oscillation is attributed to the periodic evolution of a shear band formed only in the dense bed. The behaviors of the drag force and shear band, which depend on the initial volume fraction in the DEM simulation, are in close agreement with those obtained experimentally in previous studies [N. Gravish et al., Phys. Rev. Lett. 105, 128301 (2010); Phys. Rev. E 89, 042202 (2014)]. Further analysis using the DEM simulation shows that the formation of the shear band is explained by the local dilation and compaction of the granular materials induced by the plate drag. Independent of the volume fraction, materials dilate in a wedge-shaped flow region that formed in front of the plate. In the loose bed, a compacted front builds up ahead of the flow region. Because the compacted front advances into a weaker undisturbed region, the flow region behind the front can constantly advance. On the other hand, in the dense bed, the materials largely dilate in a disturbed flow region formed in front of the plate. Because a denser undisturbed region is more stable compared to the flow region, the flow region is strongly confined. As a result, the shear strain is localized along a flow boundary between these regions, and the shear band develops.

Experimental and numerical investigations on interference of closely spaced square footings on sand

Current study illustrates the effect of interference on closely spaced square footings in loose, medium and dense sand beds using experimental and numerical studies. Experimental results suggest that with increase in the clear spacing interference decreases and after 2B spacing between the footings, the interference effects are almost zero, where B is width of the footing. The interference effect is quantified as Interference Factor, IF and is defined as ratio of ultimate bearing pressure of an interfering footing to that of an isolated footing. Maximum Interference Factor of about 1.25 is observed in dense sand. Interference is attributed to increase in confinement pressure due to the interaction between the failure zones of interfering footings. To this end, FLAC 3D (Fast Lagrangian Analysis of Continua in 3 Dimensions) models were developed and numerical simulations have been carried out. The effects of frictional angle and dilation on interference factor are presented.

Model test on the entrainment phenomenon and energy conversion mechanism of flow-like landslides

A reduced-scale physical model test with a scale of 1:2000 is designed based on the Wenjia Gully landslides. By changing the volume and kinetic energy of the sliding mass, we find that the material grading and bed-sediment thickness exert an important effect on the movement process of the landslide. To investigate the dependence of the entrainment on the grade of the sliding material and bed-sediment thickness, a series of comparison experiments with sliding masses of different grades and loose sediment bed materials of different thickness values are conducted. Furthermore, a high-speed camera is used to record the runout process. The experimental results indicate that because of the entrainment effect, the erosion depth of the substrate increases when the grade of the sliding mass increases. In addition, different characters appear in the erosion depth, volume of the accumulated area and entrainment length with the increase in bed-sediment thickness. Furthermore, the entrainment depth may increase when the bed-sediment thickness increases. The velocity of the sliding mass is reduced with the increase in basement thickness. Finally, based on the analysis of landslides and the previous experimental results, the energy conversion can be calculated to analyze the process of substrate entrainment for each sliding phase.

Transport of moderately sorted gravel at low bed shear stresses: The role of fine sediment infiltration

AbstractA reliable estimation of sediment transport in gravel‐bed streams is important for various practical engineering and biological studies (e.g., channel stability design, bed degradation/aggradation, restoration of spawning habitat). In the present work, we report original laboratory experiments investigating the transport of gravel particles at low bed shear stresses. The laboratory tests were conducted under unsteady flow conditions inducing low bed shear stresses, with detailed monitoring of the bed topography using a laser scanner. Effects of bed surface arrangements were documented by testing loose and packed bed configurations. Effects of fine sediments were examined by testing beds with sand, artificial fine sand or cohesive silt infiltrated in the gravel matrix. Analysis of the experimental data revealed that the transport of gravel particles depends upon the bed arrangement, the bed material properties (e.g., size and shape, consolidation index, permeability) and the concentration of fine sediments within the surface layer of moving grains. This concentration is directly related to the distribution of fine particles within the gravel matrix (i.e., bottom‐up infiltration or bridging) and their transport mode (i.e., bedload or suspended load). Compared to loose beds, the mobility of gravel is reduced for packed beds and for beds clogged from the bottom up with cohesive fine sediments; in both cases, the bed shear stress for gravel entrainment increases by about 12%. On the other hand, the mobility of gravel increases significantly (bed shear stress for particle motion decreasing up to 40%) for beds clogged at the surface by non‐cohesive sand particles. Copyright © 2017 John Wiley &amp; Sons, Ltd.

Turbulent channel flow perturbed by triangular ripples

This paper presents an experimental investigation of the perturbation of a turbulent closed-conduit flow by two-dimensional triangular ripples. Two ripple configurations were employed: one single asymmetric triangular ripple, and two consecutive asymmetric triangular ripples, all of them with the same geometry. Different water flows were imposed over either one or two ripples fixed to the bottom wall of a closed conduit, and the flow field was measured by PIV (particle image velocimetry). Reynolds numbers based on the channel height were moderate, varying between 27500 and 34700. The regime was hydraulically smooth, and the blockage ratio was significant. Experimental data for this specific case remain scarce, and the physics involved has yet to be fully elucidated. Using the instantaneous flow fields, the mean velocities and fluctuations were computed, and the shear stresses over the ripples were determined. The velocities and stresses obtained in this way for the single ripple and for the pair of ripples are compared, and the surface shear stress is discussed in terms of bed stability. Our results show that for the single and upstream ripples the shear stress increases at the leading edge and decreases toward the crest, while for the downstream ripple it decreases monotonically between the reattachment point and the crest. The stress distribution over the downstream ripple, which is different from both the upstream and single ripples, is shown to sustain existing ripples over loose beds.

Gravity-driven, dry granular flows over a loose bed in stationary and homogeneous conditions

Vertical and transversal structures clearly change as interparticle interactions become more collisional. Bottom and lateral confinement prove to be decisive for friction and energy dissipation as experimental evidence shows.

Impact of bed surface arrangement on bedload rate: comparisons between loose, armored and water-worked beds

Existing formulas for predicting bedload rate may be not adapted for mountain rivers with poorly sorted sediments, partly because they were often established using laboratory data with conditions far from those found in such rivers. Natural bed arrangement is particularly difficult to reproduce in flumes, although recent studies highlighted its importance on bedload dynamics. This study aims to quantify bed arrangement impact on bedload rate using original laboratory tests and to improve existing bedload formulas. Three types of bed composed with the same material but having different bed arrangements were studied: loose beds were installed manually in the flume and the others, packed and water-worked beds, were created using water power. Packed beds were assimilated to flat beds composed of a static armor layer whereas water-worked beds exhibited stronger bed organization, including large-scale bed forms. Laser-scanner surveys were used to characterize differences in bed morphology. Similar unsteady hydraulic conditions were applied over these beds. Results showed that bedload dynamics varies significantly depending on the initial arrangement. Compared to loose bed, bedload was enhanced over water-worked bed and reduced over packed bed. Bed surface indicators are thus important parameters to take into account when predicting gravel transport.

Application of HySpex hyperspectral images for verification of a two-dimensional hydrodynamic model

ABSTRACTThis research focuses on the use of HySpex hyperspectral images for verification of two-dimensional hydrodynamic modelling of open-channel flow over loose bed (CCHE2D) and assessment of water quality in the Zegrze Reservoir. The CCHE2D hydrodynamic model results show the distribution of hydraulic parameters of water flow and the sediment concentrations in the reservoir. HySpex images were used to obtain remote sensing indices of water quality. The images were compared to the hydrodynamic model results and field measurements. The analysis of hydrodynamic model results and hyperspectral image indices show the spatial distribution of the water’s physico-chemical properties in the reservoir, and poor mixing of the Bug River and the Narew River at their confluence. This study shows that there is synergy potential in using hydrodynamic modelling results and remote sensing indices of water quality for analysis of the reservoir’s water quality.

Feasibility of using sodium silicate as grouting in loose coal bed sections for methane drainage

Incidents such as borehole collapse, suction drill, and drill pipe drop resulting from borehole instability may occur in the process of drilling in loose coal bed sections for coal mine methane (CMM) drainage before mining. Grouting while drilling (GWD) is a technique used to reduce such incidents. Sodium Silicate used in GWD has the advantages of controllable consolidation-time, non-toxic material, non-pungent odor and relatively inexpensive. Although it will shrink after solidification, this could be used to connect gas channels. This paper presents the feasibility of using Sodium Silicate to make GWD slurry through results of laboratory experiments.The principal ingredients of the two Sodium Silicate grouting slurry formulae, namely 4% Glycerol Triacetate with 20–40% Sodium Silicate and 4% 1,4-Butyrolactone with 20–40% Sodium Silicate, were selected based on the requirements of GWD. The experimental results from a traditional test device revealed that coal cement stress is still low after 45 min thus limiting its field application. However, after 72 h, multiple cracks appear along the edge of the coal grains suggesting that the formulae can be used to connect gas channels naturally after hardening. If a PH stabilizer is not added, the optimal grouting time occurs within 20 min. In the process of GWD, the grouting slurry will not consolidate until it is injected into the borehole wall, which is enables the drill pipe to remain clean after grouting. Seepage grouting simulation experiments based on a self-developed device indicate that injecting certain slurry into pre-set positions of a drilling hole by circulation wind pressure is feasible to some extent when drilling in fractured, strongly fractured, and pulverized coal beds. In powder coal, however, it will close the gas channels, and thus cannot be applied.

Blinded T-bends flow patterns in pneumatic conveying systems

Blinded T bends are commonly used in pneumatic conveying systems. The main benefit of this design is that a loose bed of particles accumulates in the blinded pocket of the fitting, allowing particle-particle collision while experiencing a change in the direction of the flow. Such collisions are intended to reduce erosion of the construction material and attrition of the conveyed particles. Since prior research showed that particles will not necessarily accumulate in a blinded T, a series of experiments were conducted to evaluate the effect of the operating parameters on the critical point in which the blinded T becomes effective. In this study we investigate the effect of gas velocity, solid loading ratio, orientation and bend geometry on the critical point using 1.5″ and 2″ pneumatic conveying systems. The experiments were conducted with NaCl with mean particle diameter of 0.85mm, glass spheres with mean particle diameter of 1mm, plastic beads with mean particle diameter of 4mm, polystyrene beads with mean particle diameter of 2mm and cement powder. The blinded T arrangement also varies in depths of the blinded leg. To investigate these phenomena we conducted attrition experiments with NaCl for practical implications. It was found that both the conveying air velocity and the depth of the blinded pocket affect the filling of the blinded T bend. In addition, correlation for the filling of the bend as well as one for complete particle deposition is suggested with respect to the geometry of the bend and the operating conditions.

Flow and coherent structures around circular cylinders in shallow water

Eddy-resolving numerical simulations are conducted to investigate the dynamics of the large-scale coherent structures around a circular cylinder in an open channel under very shallow flow conditions where the bed friction significantly affects the wake structure. Results are reported for three test cases, for which the ratio between the cylinder diameter, D, and the channel depth, H, is D/H = 10, 25, and 50, respectively. Simulation results show that a horseshoe vortex system forms in all test cases and the dynamics of the necklace vortices is similar to that during the breakaway sub-regime observed for cases when a laminar horseshoe vortex forms around the base of the cylinder. Given the shallow conditions and turbulence in the incoming channel flow, the necklace vortices occupy a large fraction of the flow depth (they penetrate until the free surface in the shallower cases with D/H = 25 and 50). The oscillations of the necklace vortices become less regular with increasing polar angle magnitude and can induce strong amplification of the bed shear stress beneath their cores. Strong interactions are observed between the legs of the necklace vortices and the eddies shed in the separated shear layers in the cases with D/H = 25 and 50. In these two cases, a vortex-street type wake is formed and strong three-dimensional effects are observed in the near-wake flow. A secondary instability in the form of arrays of co-rotating parallel horizontal vortices develops. Once the roller vortices get away from the cylinder, the horizontal vortices in the array orient themselves along the streamwise direction. This instability is not present for moderately shallow conditions (e.g., D/H ≈ 1) nor for very shallow cases when the wake changes to an unsteady bubble type (e.g., D/H = 50). For cases when this secondary instability is present, the horizontal vortices extend vertically over a large fraction of the flow depth and play an important role in the vertical mixing of fluid situated at the wake edges (e.g., by transporting the near-bed, lower-velocity fluid toward the free surface and vice versa). The largest amplification of the bed shear stress in the near-wake region is observed beneath these horizontal vortices, which means that they would play an important role in promoting bed erosion behind the cylinder in the case of a loose bed. Simulation results suggest that these co-rotating vortices form as a result of the interactions between the legs of the main necklace vortices and the vortical eddies contained into the newly forming roller at the back of the cylinder. The paper also analyzes how D/H affects the separation angle on the cylinder, the size of the recirculation bubble, the bed friction velocity distributions, and turbulence statistics.

Estimation of bedload discharge in sewer pipes with different boundary conditions using an evolutionary algorithm

Sediment transport is a complex phenomenon due to the nonlinearity and uncertainties of the process. The present study applies Gene Expression Programming (GEP) to develop bedload transport models in sewer pipes. In this regard, two types of bedload were considered: loose bed (deposition state) and rigid bed (limit of deposition state). In order to develop the models, two scenarios with different input combinations were considered: Scenario 1 considers only hydraulic characteristics and Scenario 2 considers both hydraulic and sediment characteristics as inputs for modeling bedload discharge. The results proved the capability of GEP in prediction of sediment transport and it was found that for prediction of bedload transport in sewer pipes Scenario 2 performed more successfully than Scenario 1. According to the outcome of sensitivity analysis, Frm (Modified Froude number) and d50/y (relative particle size) for rigid boundary and Frm for loose boundary had key roles in the modeling. The outcome of the GEP models also was compared with existing empirical equations and it was found the GEP models yielded better results. It was also found that pipe diameter affected the transport capacity of the sewer pipe. Increasing pipe diameter caused an increase in model efficiency. A pipe with a diameter of 305mm yielded to the best results.

An analysis of the entrainment effect of dry debris avalanches on loose bed materials.

Substrate entrainment can greatly influence the mass movement process of a debris avalanche because it can enlarge the landslide volume and change the motion characteristics of the sliding masses. To study the interaction between debris avalanches and erodible substrate, physical modeling experiments varying in the mass of granular flow and substrate thickness were performed. The experimental results show that both the entrained materials and the maximum erosion depth are increased with increasing mass of the debris avalanche and decreasing substrate thickness. During the experiment, several tests were recorded using a high-speed digital camera with a frequency of 500 frames per second, so that the process of entrainment could be clearly observed. Combined with the experiment result and results of previous studies from predecessors, the entrainment mechanism during debris avalanches are analyzed and discussed. The entrainment effect of the sliding masses on the loose bed materials include basal abrasion and impact erosion of the avalanche front, the latter of which can contribute to the former by failing or yielding the erodible bed.

Measurement and augmentation of effective thermal conductivity of La0.8Ce0.2Ni5 hydride bed

An experimental set up for measuring Effective Thermal Conductivity (ETC) of Metal Hydride (MH) beds was fabricated using one-dimensional steady-state radial heat transfer absolute method. La0.8Ce0.2Ni5 alloy powder and its hydride were considered in the present study. Two types of compacts were fabricated for ETC augmentation of MH bed. First type was made from the mixture of 6 wt% graphite flakes with the La0.8Ce0.2Ni5 alloy powder. A three dimensional structure, having six radial fins, uniformly distributed around the outer periphery of a hollow cylinder, made with copper wire mesh, was integrated with the mixture of La0.8Ce0.2Ni5 powder and 6 wt% graphite flakes to make the second type of compacts. The ETC of the three types of beds namely loose metal powder bed (LMP), bed of compacts of metal alloy powder mixed with graphite flakes (CMPGF) and bed of compacts of metal alloy powder mixed with graphite flakes and embedded with copper wire mesh structure (CMPGFCu) were measured and compared at different argon and hydrogen pressures, hydrogen concentrations and average bed temperatures. The maximum values of effective thermal conductivity for the bed configurations of LMP/CMPGF/CMPGFCu were 1.3/4.7/6.8 W m−1 K−1. These values of effective thermal conductivity were obtained at 70 bar hydrogen gas pressure and 60 °C average bed temperature. The decrease in mass fraction of hydrogen absorbed by the La0.8Ce0.2Ni5 hydride, as a result of implementation of ETC augmentation techniques, was nominal (around 5.7% in case of CMPGF and 4.5% in case of CMPGFCu) when compared to the respective powder bed configurations.

Bearing capacity of batter piles embedded in sandy soil

This paper introduces an attempt to examine the effect of batter angle of a single pile model on its ultimate vertical load capacity. A 32-steel solid pile models are used in this research. Three lengths of piles are selected (300, 400, and 500 mm) for embedment ratios (depth to width, L/D) = 15, 20 and 25, respectively. Batter pile is embedded in three different relative densities of sand (loose, medium and dense sand bed). The constant rate penetration test (CRPT) method was selected using a manufactured driving hammer to install the pile models in a steel box with dimensions 1000, 750 and 700 mm. The vertical load is conducted monotonically by a special hydraulic apparatus. The results indicate there is an optimum batter angle 20° that gave a higher load capacity. Pile load capacity is highly affected by relative density and less affected by L/D ratio.

Mechanical behavior of embankments overlying on loose subgrade stabilized by deep mixed columns

Deep mixed column (DMC) is known as one of the effective methods for stabilizing the natural earth beneath road or railway embankments to control stability and settlements under traffic loads. The load distribution mechanism of embankment overlying on loose subgrades stabilized with DMCs considerably depends on the columns' mechanical and geometrical specifications. The present study uses the laboratory investigation to understand the behavior of embankments lying on loose sandy subgrade in three different conditions: (1) subgrade without reinforcement, (2) subgrade reinforced with DMCs in a triangular pattern and horizontal plan, and (3) subgrade reinforced with DMCs in a square pattern and horizontal plan. For this purpose, by adopting the scale factor of 1:10, a reference embankment with 20 cm height, 250 cm length, and 93% maximum dry density achieved in standard Proctor compaction test was constructed over a 70 cm thick loose sandy bed with the relative density of 50% in a loading chamber, and its load-displacement behavior was evaluated until the failure occurred. In the next two tests, DMCs (with 10 cm diameter, 40 cm length, and 25 cm center-to-center spacing) were placed in groups in two different patterns (square and triangular) in the same sandy bed beneath the embankment and, consequently, the embankments were constructed over the reinforced subgrades and gradually loaded until the failure happened. In all the three tests, the load-displacement behaviors of the embankment and the selected DMCs were instrumented for monitoring purpose. The obtained results implied 64% increase in failure load and 40% decrease in embankment crest settlement when using the square pattern of DMCs compared with those of the reference embankment, while these values were 63% and 12%, respectively, for DMCs in triangular pattern. This confirmed generally better performance of DMCs with a triangular pattern.