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This paper presents a numerical investigation of hydraulic fracturing in oil sands during cold water injection by considering the aspects of both geomechanics and reservoir fluid flow. According to previous studies, the low shear strengths of unconsolidated or weakly consolidated sandstone reservoirs significantly influence the hydraulic fracturing process. Therefore, classical hydraulic fracture models cannot simulate the fracturing process in weak sandstone reservoirs. In the current numerical models, the direction of a tensile fracture is predetermined based on in situ stress conditions. Additionally, the potential transformation of a shear fracture into a tensile fracture and the potential reorientation of a tensile fracture owing to shear banding at the fracture tip have not yet been addressed in the literature. In this study, a smeared fracture technique is employed to simulate tensile and shear fractures in oil sands. The model used combines many important fracture features, which include the matrix flow, poroelasticity and plasticity modeling, saturation-dependent permeability, gradual degradation of the oil sands as a result of dilative shear deformation, and the tensile fracturing and shear failure that occur with the simultaneous enhancement of permeability. Furthermore, sensitivity analyses are also performed with respect to the reservoir and geomechanical parameters, including the apparent tensile strength and cohesion of the oil sands, magnitude of the minimum and maximum principal stress, absolute permeability and elastic modulus of the oil sands and ramp-up time. All these analyses are performed to clarify the influences of these parameters on the fracturing response of the oil sands.

This study numerically investigates the effect of material micro- and macro-parameters on the failure mechanisms and geometry of a wellbore breakout. The analysis of the borehole breakout is essential in addressing wellbore stability, well completion, and sand production problems. The three-dimensional discrete element method (DEM) was used in the simulations. The numerical tool was used in numerical model simulations of drilling through sandstone in cubic samples at the laboratory scale subjected to pre-existing far-field stresses. Besides, a series of triaxial testing simulations were performed to relate the micromechanical parameters to the macromechanical material properties. The results showed that the geometry of the breakout is affected, among the material micro-parameters, by the particle contact modulus, bond normal and shear strengths, particle crushing strength, and particle size distribution. Further, it was found that the macro-parameters including Young’s modulus, friction and dilation angles, and uniaxial compression strength (UCS) also affect the type of breakout.

This paper presents the analysis of an earth dam breaching process which incorporates an analytical erosion model into CFX in ANSYS to calculate the rate of erosion in order to capture the kinematic characteristics of dam breach. Based on the water level and the storage capacity of the reservoir on the upstream side of the dam, the discharge of water over the dam can be calculated. The discharge of water is also used to validate the results from the numerical simulations. The height of the experimental dam during the breaching process is calculated using an empirical relationship based on the water level, the discharge rate, and the width of the breach. A particle-scale progressive scouring entrainment model has been incorporated into CFX in ANSYS to calculate the rate of erosion which is used to simulate the development of the breach. When comparing with field observations, the root-mean-square error (RMSE) between the calculated and measured surface velocities is equal to 0.37 m/s, indicating a discrepancy of 16%, and the RMSE of the calculated and measured discharge is equal to 1.13 m3/s, showing an average discrepancy of 19%. Additionally, the numerical results are validated by comparing the calculated and observed water levels in the breach and the calculated and observed width of the top of the breach. The RMSE between the calculated and observed water levels in the upstream of the dam (hup), the water level in the center of the breach (hc + hd), and the width of the top of the breach (BT) are equal to 0.026 m, 0.095 m, and 0.44 m, respectively, which show average discrepancies of 1%, 4%, and 13%, respectively. This dam break experiment is a unique full-scale experiment for the evaluation of numerical and analytical models. This research shows that the erosion model incorporated into CFX is able to capture the main characteristics of the dam failure process. The incorporation of the erosional model into the dam breach analysis can be used to analyze the failure process of barrier dams.

In this study, we carried out a series of physical experiments using a submersed flume model to investigate how sand/clay content influences the depositional mechanism of submarine debris flows. A three-dimensional biphasic numerical model, with a Herschel-Bulkley rheology, was used to back-analyze the physical experiments. The calibrated numerical model was then used in a back-calculation to investigate the effects of viscosity on the deposition process. The results show that as submarine debris flows mix with water during the deposition process, shear stress at the slurry-water interface generates a vortex that leads to a swirl-wedge front head. High-viscosity slurry flows with a swirl-wedge front head travel at a higher aspect ratio and with a greater radius of rotation. Hydroplaning was observed when the front head was lifted by water during flow. The lifting height increased with flow depth fluctuation. Higher viscosity slurry was found to lift more rapidly due to its larger vortex and the decrease in density at the front head over time, both of which promote fluctuation. Although a high-density slurry has a greater lifting height, the ratio of lifting height to front head height is lower, indicating a smaller lifting force influence. Lower density flows have higher kinetic energy as the transfer of potential energy into kinetic energy is more efficient. Kinetic energy dissipation comprises three stages: (1) gravity-dominated coherent flow and hydroplaning lead to a rapid increase in kinetic energy; (2) sharp reduction in kinetic energy as slurry mixes with water, coherence and hydroplaning are reduced, and the influence of the shear stress increases; (3) slurry mixed very well with water, turbidity current dominates the kinetic energy dissipation. High-density slurry dissipates quicker in the last two stages. In stage 3, which dominates the temporal evolution of the debris flow, the Froude number is lower than 1, the flow thins and elongates, and the deposition process of submarine debris flows is dominated by gravity, and the difference of morphology of the different cases become clear.

Estimating erosion rate of solid particles in a porous medium is of interest to geotechnical engineers; which use analytical or numerical models for this purpose. Constitutive law of erosio...

Background Resolution and prevention of peri‐implant mucositis are a key in preventing peri‐implantitis. This case–control study aims to assess the modifying effect of a deep mucosal tunnel (DMT) on the induction and resolution phases of experimental peri‐implant mucositis. Methods Nineteen subjects with a tissue level implant were assigned to cases (DMT, depth ≥3 mm) or controls (shallow mucosal tunnel ≤1 mm, SMT). Subjects underwent a standard experimental peri‐implant mucositis protocol characterized by an oral hygiene optimization phase, a 3‐week induction phase using an acrylic stent to prevent self‐performed oral hygiene at the experimental implant, and a 3 + 2 weeks resolution phase. Modified plaque (mPI), gingival index (mGI) and peri‐implant sulcus fluid IL‐1β concentrations were measured over time. Differences between DMT and SMT were assessed with the Mann–Whitney test. Results Modified plaque index and mGI increased in parallel during the induction phase. After resumption of oral hygiene practice, mPI and mGI resolved towards baseline values in the SMT group. In DMT, mPI and mGI values diverged: plaque resolved but resolution of inflammation was delayed and of smaller magnitude during the first 3 weeks after resumption of oral hygiene. IL‐1β concentrations were significantly higher in DMT at 21 days (end of induction) and during the resolution phase corroborating the clinical findings. Removal of the crown and submucosal professional cleaning were needed to revert mGI to baseline values in DMT implants. Conclusions The depth of the mucosal tunnel modifies the resolution of experimental peri‐implant mucositis at transmucosal implants. This observation raises important questions on the effectiveness of self‐performed oral hygiene in cases where implants are placed deeper and the ability to resolve mucositis and effectively prevent peri‐implantitis in such situations.

A smeared fracture type hydraulic fracture simulator is developed through numerical coupling between an in-house reservoir simulator and a geomechanical commercial software (FLAC2D). The new package falls within the category of partially decoupled model and is versatile, flexible and efficient. This approach can be used to couple any other advanced commercial fluid flow or geomechanical simulators for an accurate description of the initiation and propagation of hydraulic fractures. The paper contains a discussion of the partial coupling technique to link fluid flow and geomechanical calculations in modeling fracture initiation and propagation. The models use a common gridblock for the fracture and reservoir and use the deformation calculations to update the porosity and permeability. The method captures the interactive effects of the fracture on reservoir fluid flow and formation geomechanics through stress dependent permeability and porosity. The developed smeared fracture model can capture both tensile and shear fractures in the formation. Major features of this model include modeling poroelasticity and plasticity, matrix flow, shear and tensile fracturing with concomitant permeability enhancement, saturation-dependent permeability, stress-dependent stiffness and gradual degradation of oil sands due to dilatant shear deformation . The model has been applied to numerically simulate field size hydraulic fracturing in oil sands during cold-water injection to show the predictive capability of the simulator.

A progressive entrainment model has been incorporated into an energy-based runout model to calculate the kinematic characteristics of debris flow. The progressive entrainment model considers both rolling and sliding motions of erodible material. To study the effect of the controlling parameters in the entrainment model, sensitivity analyses have been carried out by varying model parameters such as the internal friction angle of the debris, the basal friction angle of the channel, the turbulent flow coefficient, and the mean value of the probabilistic density function (PDF) for the resting angle. The results are evaluated by calculating the change of the maximum entrainment rate, the maximum frontal velocity, the final total volume, the longitudinal length of deposition, the maximum debris flow height, and the runout distance of the debris. The progressive entrainment runout model is sensitive to the characteristic particle size, basal friction angle and turbulent coefficient. Measurements from the 1990 Tsingshan debris flow, Hong Kong, China, are used to validate the model. Velocities at different channel sections estimated using the super-elevation method and entrainment depth are used to evaluate the simulation results. Comparing the results between simulated and observed maximum velocity and entrainment depth indicate that the progressive entrainment model together with the runout model can capture the erosion characteristics of the granular material. The results are also compared with that using a dynamic entrainment model. The results demonstrate that the progressive entrainment model provides a more realistic calculation of entrainment for this case compared with the dynamic entrainment model.

Loose wide-grading soils are commonly found in the source areas of debris flows, and in landslides after an earthquake. During rainfall events, fine particles (fines) in the soils gradually migrate downward, and eventually the loss of fines results in an increase in the pore volume of the soil and a reduction in the stability of the soil skeleton, which can lead to subsequent slope failure. To gain more understanding of the fine migration process at the microscopic scale, a 3D discrete element-fluid flow sequentially coupled model is developed, based on Darcy’s Law, to simulate fluid flow through a porous medium and calculate the transportation of soil solids. The erosion model is verified using experimental data. Parametric studies are carried out to investigate the effects of coarse particle size. The results reveal that changes in pore structure caused by fine particle migration can change the local permeability of the material. For the case of the average pore throat diameter to fine particle ratio ( $$J$$ ) of 2.41, changes in local porosity with time from internal erosion in the sample can be divided into four stages: (1) a rapid increase with some variations in porosity, (2) a slow increase in porosity, (3) a rapid increase in porosity, and (4) a steady state with no change in porosity. Not all stages are present for all value of $$J$$ . Stages (1) (2) (4) are present for 2.48 ≤ $$J\le$$ 2.58 and stages (1) (4) are present for $$J$$ ≤ 2.24 and $$J\hspace{0.17em}$$ ≥ 2.74. A sharp increase in the fine’s erosion possibility occurs for a $$J$$ value lies between 2.58 and 2.74. The erosion possibility sensibility shows an exponential relationship with $$J$$ . The model provides an effective and efficient way to investigate the process of pore blockage and internal soil erosion.

This paper focuses on the study of proppant transport mechanisms in fractures during frac-packing operation. A multi-module, numerical proppant, reservoir and geomechanics simulator has been developed, which improves the current numerical modeling techniques for proppant transport. The modules are linked together and tailored to capture the processes and mechanisms that are significant in frac-pack operations. The proposed approach takes advantage of a robust and sophisticated numerical smeared fracture simulator and incorporates an in-house proppant transport module to calculate propped fracture dimensions and concentration distribution. In the development of software capability, the propped fracture geometry and proppant concentration, which are the output of the proppant module, are imported to the hydraulic fracture simulator through mobility modification. Complex issues of proppant transport in fractures that are addressed in the literature and captured by the current model are: hindered settling velocity (terminal velocity of proppant in the injection fluid), the effect of fracture walls, proppant concentration and inertia on settling (due to extra drag forces applied on particles, compared to single-particle motion in Stokes regime in unbounded medium), possible propped fracture porosity and also mobility change due to the presence of proppant, and fracture closure or extension during proppant injection. A sensitivity analysis is conducted using realistic parameters to provide guidelines that allow more accurate predictions of the proppant concentration and fluid flow. The main objective of this study is to link a numerical hydraulic fracture model to a proppant transport model to study the fracturing response and proppant distribution and to investigate the effect of proppant injection on fracture propagation and fracture dimensions.

This paper presents a model to simulate the monotonic and cyclic behaviours of weakly cemented sands. An elastoplastic constitutive model within the framework of bounding surface plasticity theory is adopted to predict the mechanical behaviour of soft sandstone under monotonic and cyclic loadings. In this model, the loading surface always passes through the current stress state regardless of the type of loading. Destruction of the cementation bonds by plastic deformation in the model is considered as the primary mechanism responsible for the mechanical degradation of loosely cemented sands/weak rock. To model cyclic response, the unloading plastic and elastic moduli are formulated based on the loading/reloading plastic and elastic moduli. The proposed model was implemented in FLAC2D and evaluated against laboratory triaxial tests under monotonic and cyclic loadings, and the model results agreed well with the experimental observations. For cyclic tests, hysteresis loops are captured with reasonable accuracy.

The deformation characteristics of soil obtained from landslides at the Three Gorges Dam Reservoir have been examined. Due to the annual fluctuation in the water level of the reservoir, landslides around the Three Gorges Dam Reservoir are subjected to cyclic loading. Laboratory triaxial tests have been carried out on silty clay specimens subjected to cyclic changes in pore water pressure. The test results indicate that when the soil is subjected to repeated unloading–reloading with constant deviatoric stress in an overconsolidated state, the silty clay material shows inelastic deformation with the accumulation of irrecoverable strain. To capture this behavior, a constitutive model has been developed to simulate the deformation induced by changes in pore water pressure. This model is based on a modification of Yao et al. (2009)’s model to include the effect of cyclic loading. The model has been used to calculate the shear strain during cyclic loading and for a comparison with the laboratory test results. It has been shown that the proposed model is capable of capturing the deformation response of landslide soil subjected to changes in pore water pressure caused by the fluctuating water level of the reservoir.

A central issue in hydraulic fracturing treatment in petroleum wells is the transport of proppant particles by the injection fluid. In this paper, we present an innovative proppant transport model in a fixed rectangular- and elliptic-shaped slots. The proposed model is an improvement to the current modeling of proppant transport by applying a non-oscillatory numerical scheme which has high accuracy everywhere in solution domain, even close to the steep gradients. In addition, inertia, fracture wall, and concentration effects on proppant settling along with slurry evolution as a function of proppant concentration has been considered. This paper introduces the mathematical equations that govern the proppant transport phenomenon and discusses special front capturing numerical techniques, boundary conditions, coupling between proppant and slurry mass conservation equations and time stepping restrictions required for the solution stability. We incorporated published correlations obtained from proppant transport laboratory experiments in our numerical model to better capture the physics of the problem. 5th- order WENO scheme was used to avoid oscillation and diffusion at the proppant front since traditional finite difference discretization was found to be insufficient in solving the hyperbolic transport partial differential equations. Results show that the technique used in this study can capture the proppant distribution with minimum oscillation and diffusion. A series of sensitivity analysis was conducted to explore the legitimacy of these assumptions and to provide guidelines that allow more accurate predictions of the proppant and fluid transfer. Numerical results are presented to show how proppant distribution is impacted by the injection fluid viscosity, density difference between proppant particles and injection fluid, proppant size, and fluid flow injection rate. Results of the sensitivity analysis illustrate the significance of choosing appropriate viscosity of the injection fluid as small changes in the viscosity may cause noticeable effects on the concentration distribution. In addition, we found that variation of proppant size and density within a reasonable range have a modest effect on proppant concentration distribution. Furthermore, we also investigated the amount of gravity driven vertical motion of proppant which is driven by density differences (convection) and compare it to a second gravity driven motion which is proppant settlement. Both of these two well recognized mechanisms can occur inside a fracture during proppant placement, however, the importance of each mechanism as a function of proppant injection design parameters is not fully understood.

In petroleum engineering, the transport phenomenon of proppants in a fracture caused by hydraulic fracturing is captured by hyperbolic partial differential equations (PDEs). The solution of this kind of PDEs may encounter smooth transitions, or there can be large gradients of the field variables. The numerical challenge posed in a shock situation is that high-order finite difference schemes lead to significant oscillations in the vicinity of shocks despite that such schemes result in higher accuracy in smooth regions. On the other hand, first-order methods provide monotonic solution convergences near the shocks, while giving poorer accuracy in the smooth regions. Accurate numerical simulation of such systems is a challenging task using conventional numerical methods. In this paper, we investigate several shock-capturing schemes. The competency of each scheme was tested against one-dimensional benchmark problems as well as published numerical experiments. The numerical results have shown good performance of high-resolution finite volume methods in capturing shocks by resolving discontinuities while maintaining accuracy in the smooth regions. These methods along with Godunov splitting are applied to model proppant transport in fractures. It is concluded that the proposed scheme produces non-oscillatory and accurate results in obtaining a solution for proppant transport problems.

An analytical solution was proposed for the groundwater flow through a defective pipe, which can be used to estimate the water flow rate into the pipe and predict the pore water pressure distribution in surrounding soils. This analytical solution was verified by comparing with experimental results, and the predicted pressure distribution around the defective pipe is proved to be consistent with numerical simulations using the finite element method. From the parametric analysis, the infiltration rate increases as the defect position changes from top to bottom on the pipe, and the effect of defect position is not significant if the water head above the defect is 10 times greater than pipe radius. An approximated solution for estimating the groundwater flow infiltration rate through a circular orifice on the pipe is proposed as well. From the verification and parametric studies, this proposed analytical solution is proved to be an efficient approach for the estimation of groundwater infiltration through a defective pipe.

Flowslides are a frequent type of natural disaster in loess areas and may result in the significant loss of properties and/or casualties. The Dagou flowslide is a typical event in a loess area and is accompanied by significant sediment entrainment. To analyze the mechanisms responsible for flowslides and to obtain the parameters for a runout analysis, a field investigation was conducted. Specimens were sampled on site to carry out laboratory tests, including a triaxial test, sieve analysis, and chemical component analysis. The parameters were used in the runout study employing an energy-based runout model. An analytical entrainment model was adopted to calculate the entrainment after considering physical properties and the mechanism of the entrainment process of the loess. Finally, the entrainment model was incorporated into the runout model to simulate the post-failure process of this case. Energy dissipation due to the deformation of slices was considered as it was thought to be important for a slide with a significant deformation. The simulation results were compared with the measurements, including runout distance, total volume, erosion depth, deposition height at different sections, and velocities at specific locations. The results indicate that the energy-based runout model, together with the entrainment model, can capture the kinematic characteristics of the Dagou flowslide. Therefore, it is feasible to use this model to predict the runout characteristics of flowslides in similar areas. Copyright (c) 2017 John Wiley & Sons, Ltd.

Summary This paper presents a numerical scheme for fluid-particle coupling that uses the discrete element method by taking into consideration solid deformation and pore pressure generation. A new water particle element is introduced to calculate pore water pressure due to porosity changes. The water particle element has the same size and shape as the solid element and experiences the same amount of deformation. On the basis of the effective stress principle at the element contact, the total force is equal to the sum of the force transmitted through the solid element contact and the water particle force due to pore water pressure. Analytical solutions of traditional soil mechanics problems, such as isotropic compression and consolidated triaxial undrained test, are used to quantitatively validate the proposed model. The numerical results show good agreement between the model and the analytical solutions. The model therefore provides an effective method to calculate pore pressure in a porous medium in discrete modeling.

Summary Soil erosion around defective underground pipes can cause ground collapses and sinkholes in urban areas. Most of these soil erosion events are caused by fluidization of the surrounding soil with subsequent washing into defective sewer pipes. In this study, this soil erosion process is simplified as the gradual washout of sand particles mixed with water through an orifice. The discrete element method is used to simulate the large deformation behavior of the sand particles, and the Darcy fluid model is coupled with this approach to simulate fluid flow through porous sand media. A coupled 3D discrete element model is developed and implemented based on this scheme. To simulate previous experiments using this coupled model considering the current computing capacity, we incorporated a ‘supply layer’ to study the continuous erosion process. The coupled model can predict the erosion flow rates of sand and water and the shape of erosion void. Thus, the model can be used as an effective and efficient tool to investigate the soil erosion process around defective pipes. Copyright © 2017 John Wiley & Sons, Ltd.

Sand response changes with intensity of cementation bonds between sand grains, magnitude of intermediate principal stress and with fabric anisotropy. First a critical state bounding surface plasticity model is presented in this paper. In this constitutive model, the loading surface always passes through the current stress state regardless of location or position of the stress path. Second to simulate hollow cylinder tests which represent different modes of shearing including triaxial compression and triaxial extension, the fabric anisotropy and b -parameter are incorporated in the model. Simultaneous integration of cohesion, non-associated flow rule, fabric anisotropy, kinematic hardening, critical state and state parameter makes the proposed model unique compared to previous proposed bounding surface models. Comparison of model outcomes and hollow cylinder experimental tests shows great predictive capability of the proposed model. Sensitivity analysis also suggests that triaxial compression and triaxial extension are respectively strongest and weakest modes of shearing.

This paper presents a model for the investigation of sandstone degradation and sand production mechanisms coupled with fluid flow analysis using the Discrete Element Method (DEM). The model was used to investigate the effects of in-situ stresses and flow rate on sand production. We developed a linked DEM-fluid flow model for sanding analysis. The model calculates seepage forces and applies them on solid particles in the DEM model. The model accounts for permeability and porosity changes due to sandstone deformation and sand production. The DEM model was verified against poro-elastoplastic analytical solutions. Subsequently, the model was used for sanding simulation from a block-shaped sample under different far-field stress and pressure conditions. The boundary stresses and fluid pressures were varied to study their influence on sandstone degradation and sand production. The creation of a borehole in a solid block resulted in the development of uniform or V-shape breakouts around the borehole. The failure zone around the borehole expanded after the application of fluid flow and sand grain detachments. Fluid flow was observed to influence the size and mode of failure in the breakout zone and sand production. Boundary stress dominated the sanding response at higher boundary stress conditions. However, much lower sanding occurred under higher boundary stresses but low boundary fluid pressures. High tangential stresses around the borehole caused by high confining stress resulted in strong frictional interlocking that alleviated sand production. Massive sanding was observed at lower far-field stress but higher boundary pore pressure.

This paper presents a constitutive model for describing the stress-strain response of sands under cyclic loading. The model, formulated using the critical state theory within the bounding surface plasticity framework, is an upgraded version of an existing model developed for monotonic behaviour of cohesionless sands. With modification of the hardening law, plastic volumetric strain increment and unloading plastic modulus, the original model was modified to simulate cyclic loading. The proposed model was validated against triaxial cyclic loading tests for Fuji River sand, Toyoura sand and Nigata sand. Comparison between the measured and predicted results suggests that the proposed modified model can capture the main features of cohesionless sands under drained and undrained cyclic loading.

An extremely large rock avalanche occurred on April 9, 2000 at Yigong, Tibet, China. It started with an initial volume of material of 90 × 106 m3 comprising mainly of loose material lying on the channel bed. The rock avalanche travelled around 10 km in horizontal distance and formed a 2.5-km-long by 2.5-km-wide depositional fan with a final volume of approximately 300 × 106 m3. An energy-based debris flow runout model is used to simulate the movement process with a new entrainment model. The entrainment model considers both rolling and sliding motions in calculating the volume of eroded material. Entrainment calculation is governed by a second order partial differential equation which is solved using the finite difference method. During entrainment, it is considered that the total mass is changed due to basal erosion. Also the profile of the channel bed is adjusted accordingly due to erosion at the end of each calculation time step. For Yigong, the profile used in the simulation was extracted from a digital elevation model (DEM) with a resolution of 30 m × 30 m. Measurements obtained from site investigation, including deposition depth and flow height at specific location, are used to verify the model. Ground elevation-based DEM before and after the event is also used to verify the simulation results where access was difficult. It is found that the calculated runout distance and the modified deposition height agree with the field observations. Moreover, the back-calculated flow characteristics based on field observations, such as flow velocity, are also used for model verifications. The results indicate that the new entrainment model is able to capture the entrainment volume and depth, runout distance, and deposition height for this case.

This paper presents a numerical model with a new approach for analyzing the propagation of pressure waves in porous media and investigates the dynamic response of sand in relation to the attributes of pore pressure pulses. There are various instances in which dynamic phenomena can have a significant impact on porous media in a reservoir. One notable example is the possible influence of waterhammer pressure pulsing on sand fluidization around injection wells in oil reservoirs following a hard wellbore shut-in, which can result in massive sand production. In some extreme cases, this phenomenon can even result in the loss of the wellbore. Nevertheless, the pore pressure wave propagation in porous media has often been neglected in modelling likely due to mathematical complexity. The proposed model solves the momentum balance of fluid and solid coupled with the fluid mass balance equation in the prediction of dynamic fluid flow and mechanical deformation in porous media. The model is a two-dimensional, elasto-plastic, axisymmetric, single-phase and sequentially coupled model. The numerical model was validated against experimental data for a step wave in a shock tube and good agreement between model calculations and measured data has been obtained. Two distinct waves have been observed as a result of a shock pore pressure wave. The first one is an undrained wave where fluid and solid travel at the same speed. The other one is a wave which is often damped far from the source due to the friction between fluid and solid as they no longer travel together. It is found that tortuosity plays an important role on the amplitude of the waves. The results were then compared to the predictions by Biot's theory for waves through porous media. Biot's theory is shown to be inaccurate in predicting the transient dynamic behaviour, but it is sufficient in capturing the overall trends. Finally, the model is used to predict waterhammer response in near wellbore reservoir.

Evaporation of water from a saturated soil surface with ample free water is known as potential evaporation, PE, and the associated physical processes are quite well understood. However, evaporation of water from an unsaturated soil surface is known as actual evaporation, AE, and the associated physical processes are more complex and less understood. The calculation of actual evaporation is important for many geotechnical engineering applications. Soil suction and the corresponding water content at which the AE rate begins to depart from the PE rate during a drying process are re-assessed using a series of laboratory tests (i.e., thin soil section drying tests and soil column drying tests). Laboratory results show that the suction at which the actual rate of evaporation begins to depart from the PE rate for soil columns (or thick soil layers) may be different than for thin soil layers. Suction at the “evaporation-rate reduction point” (ERRP) appears to be approximately 3000 kPa for thin soil layers, but is between the air-entry value and residual soil suction for thick soil layers or soil columns. The analyses presented in this paper have resulted in the development of a methodology for the estimation of suction corresponding to the ERRP in soil columns. Equations are also proposed to calculate the coefficient of surface moisture availability, vapour pressure, and “surface resistance” at the ground surface. The paper also presents an equation (i.e., new soil–atmosphere moisture flux equation) for predicting evaporation rate from a soil surface using “surface resistance” to vapour water diffusion from the soil to the atmosphere. The proposed soil–atmosphere model is verified using evaporation rate data collected from various drying tests on thin soil layers and soil columns. Reasonably good agreement was found between the computed and measured rates of evaporation. The findings and recommendations in this paper contribute to an improved understanding of the prediction of AE from unsaturated soil surfaces.

Fluidization of dry granular material is the transition from a solid state to a liquid state when sufficient energy is applied during vibration. This behavior is important because it is closely related to deformations of geotechnical structures during an earthquake. The scientific challenge lies in the understanding on how strain localization is related to the fluidization zone during the entire shearing process. Despite the importance of the mechanical behavior of granular material during fluidization, it cannot be easily characterized using traditional direct shear test. In this paper, 2D DEM model is firstly conduct, shear vibrational fluidization is defined for dry granular material, and the discrete element method has been used to simulate the direct shear test on granular material under vibrational loading during shearing. The peak, residual and vibro-residual shear strength envelopes have been obtained from the numerical simulations. Three distinct zones have been identified in the upper shear box based on the observed changes in volumetric strain before vibration. During vibration, fluidization occurs in the three zones with the characteristics that the shear stress, porosity, volumetric strain, and the coordination number drop to relatively lower values. During vibration, material becomes denser than the critical state, and strain localization has been relieved. Densification of the material at the shear zone leads to a strengthening of the material which increases the shearing resistance after vibration. Furthermore, a comparison of the 2D and 3D simulations is performed. Results reveal that the motion of particles in the out-of-plane direction in the 3D simulations lead to smoother shear stress and more consistent with the experimental result.

An analytical solution for groundwater flow through a circular tunnel in anisotropic soils is derived based on conformal mapping and can be used to calculate the water flow rate into the tu...

To understand the entrainment process in granular flow, numerical experiments have been conducted using a Discrete Element Method model. A flow channel of 8 m long with $$15^\circ $$ slope is setup with monitoring points located in an erodible bed. Particles, ranging from 3 to 4 mm in diameters, are used in the simulations. In the simulations, translational, rotational and average velocities, total volume, shear stresses are calculated in the measurement circles. The sizes of the measurement circles have been varied to see their effects on the results. It is found the minimum size of the measurement circles should include 20–30 particles. An new analytical model has been developed to calculate entrainment in granular flow. Results of the numerical experiment are compared with analytical model. Shear stresses at the interface between flowing particles in motion and the immobile particles in the channel bed, change of depth of erosion and entrainment rate are used to verify the analytical model. It is found that the calculated shear stresses in the PFC model agree well with the shear stresses calculated using Mohr–Coulomb frictional relationship in the analytical model. The calculated depth of erosion using the new analytical model is also compared with that from dynamic and static entrainment model. The results indicates that the analytical model is able to capture the mechanism of erosion and it can be used in granular flow analysis.

Summary In this paper, a new hydraulic-fracturing model is introduced for cohesionless sand, which is also applicable to weak sandstone formations with high permeability and low shear strength. Phenomena such as shear-band development and shear-enhanced permeability are of paramount importance during hydraulic fracturing of cohesionless sand or weak sandstones, which make the fracturing response quite different from what it is conventionally believed to be in competent rocks. The smeared approach in simulating hydraulic fracturing has been implemented in the proposed model within the continuum mechanics framework. Both matrix and fracture flow have been considered in this model. Tensile- and shear-fracture development and their fluid flow were simulated. The cubic law and Touhidi-Baghini’s shear-permeability model (Touhidi-Baghini 1998) were used to capture the permeability evolution and to model flow in tensile and shear fractures, respectively. Shear fracturing of geomaterials involves intense localization of deformation and strain softening, which is a discontinuous phenomenon, resulting in mesh dependency of the results in the continuum model. The fracture-energy-regularization method was used in this model to reduce the mesh-size dependency of the energy dissipated during fracture propagation. The smeared-fracture approach has been validated against laboratory hydraulic-fracturing experiments with reasonable agreement. Consistent with the experiments, the results of the numerical model indicate that tensile fractures are formed in a very small area around the injection point despite the application of high injection pressure compared with the minimum boundary stress. It is found that shear fracturing and shear-permeability evolution are the most important mechanisms that influence and control the fracturing response. The dominant fracturing mechanism is found to be governed by the high permeability and low shear strength of the material.

In order to investigate the effect of vibration on the critical state of dry granular soils, eighty samples of four different granular materials were tested at four normal stresses under different vibrational accelerations. The experiments were performed on a modified vibrational direct shear apparatus with vibrational loading in the horizontal (shear) direction. Strength loss versus vibration acceleration plots at different normal stresses, as well as the peak, residual and vibro-residual shear strength diagrams have been obtained for the four granular materials at different intensities of vibration. It has been determined that increasing the intensity of vibration reduces the friction angle of the granular materials. The effect of particle shape on the strength loss due to vibration has been observed as well as the effect of vibration on the normal stress.

Debris flow entrainment refers to the increase in mass by way of erosion of the channel beds or undermining the channel banks. Entrainment makes the calculation of travel distance and velocities of debris flow much more complex and difficult. Shear failure of the material in the channel bed is usually considered to be the dominant mechanism in entrainment analysis. However in granular flow, material at the surface of the channel bed can be eroded by progressive scouring under the base of the debris. This may result in more material being eroded and entrained than just considering the shear failure mechanism. A new analytical model is proposed to calculate entrainment in debris flow analysis by considering both rolling and shearing motion. Newton’s Law of Motion is used to calculate accelerations, velocities, and displacements of granular particles. To study the entrainment process inside granular flow and to verify the new entrainment model, numerical experiments have been carried out using the Discrete Element Method (DEM). Velocities, including translational velocity, rotational velocity and average velocity, total volume, shear stresses are monitored using measurement circles in the numerical experiment. Variations of the depth of erosion at specific locations along the debris flow channel are monitored and the average entrainment rates are calculated. By comparing the numerical experimental results with the analytical solutions, it is found that results from the analytical model agree well with that from the numerical experiments. In order to use the new entrainment model into debris flow runout calculation, the new entrainment model has been incorporated in a runout model based on an energy approach. Entrainment calculation governed by a second order partial differential equation is solved using the finite difference method. The total mass and profile of the channel bed are adjusted during the entrainment calculation. Sensitivity analyses have been carried out on the new model by varying the model parameters including internal friction angle, basal friction angle, turbulent coefficient and mean of Probability Density Function (PDF) etc. Back analysis of historical cases is carried out using the new model. Several case histories have been studied which include the Tsingshan debris flow, Niumian Rock Avalanche, Fjaerland debris flow, Faucon debris flow and Zymoetz River rock avalanche. An extremely large rock avalanche occurred on April 9, 2000 at Yigong is also studied in the thesis. Measurements obtained from site investigation, including flow velocity, flow height, entrainment depth at specific locations, run-out distance and total volume at deposition fan, have been used to evaluate the model. The results are encouraging based on comparisons of the run-out distance, front velocity and total volume of the debris. Improvements are required on the entrainment depth and total volume in some cases when lateral spreading of the debris is significant.

A numerical model based on computational fluid dynamics and kinetic theory of granular material is used to investigate sand-bed erosion due to an upward water jet. After verifying the numer...

A physical model study was conducted to examine the development of submerged sand erosion through a slot on a defective sewer pipe. Various parameters that affect the erosion process were s...

The engineering properties of tire derived aggregate (TDA) have been the subject of previous laboratory investigations; however, these investigations have been conducted primarily on TDA that is one-third the size of TDA used for engineering applications. In this study, the compression behavior of compacted TDA samples with a maximum particle size of 300 mm, typically used for engineering applications, has been investigated using custom-made testing apparatus designed to accommodate the large-sized TDA. Tire derived aggregate properties important to engineering applications, including compression behavior, the coefficient of lateral earth pressure at rest, and Poisson's ratio, are provided, while variation in the gradation of TDA particles and the tire type for TDA production are experiment variables. This study also proposes a method to prepare compacted TDA samples for large-scale testing.

SUMMARY Simulation of frictional contact between soils and rigid or deformable structure in the framework of smoothed particle hydrodynamics (SPH) is presented in this study. Two algorithms are implemented into the SPH code to describe contact behavior, where the contact forces are calculated using the law of conservation of momentum based on ideal plastic collision or using the criteria of partial penetrating. In both algorithms, the problem of boundary deficiency inherited from SPH is properly handled so that the particles located at contact boundary can have precise acceleration, which is critical for contact detection. And the movement and rotation of the rigid structure are taken into account so that it is easy to simulate the process of pile driving or movement of a retaining wall in geotechnical engineering analysis. Furthermore, the capability of modeling deformability of a structure during frictional contact simulations broadens the fields of SPH application. In contrast to previous work dealing with contact in SPH, which usually use particle-to-particle contact or ignoring sliding between particles and solid structure, the method proposed here is more efficient and accurate, and it is suitable to simulate interaction between soft materials and rigid or deformable structures, which are very common in geotechnical engineering. A number of numerical tests are carried out to verify the accuracy and stability of the proposed algorithms, and their results are compared with analytical solutions or results from finite element method analysis. Good agreement obtained from these comparisons suggests that the proposed algorithms are robust and can be applied to extend the capability of SPH in solving geotechnical problems. Copyright © 2013 John Wiley & Sons, Ltd.

As granular soils may be compressible or have inadequate strength, compaction is particularly useful when soils are subjected to dynamic loading or cyclic loading. A new laboratory apparatus for investigating dynamic compaction has been designed and fabricated. The basic principle of this new technique is to introduce vibrations during the expansion process in static compaction grouting. In these tests, the injection pressure, the excess pore water pressure, and the change in void ratio of the specimens are measured. The main focus is to investigate the development of the injection pressure, the void ratio, and the excess pore water pressure due to dynamic compaction and the subsequent consolidation of the soils. In addition, the relative density of the soils is used to evaluate the dynamic compaction efficiency. Scaled laboratory experiments are conducted to study the effect of this dynamic compaction frequency on compaction efficiency. The experimental results show that the change in void ratio in the dynamic compaction tests is about four times greater than that in the static compaction tests. Dynamic compaction frequency plays an important role in soil densification due to dynamic compaction.

Summary Rock failure is observed around boreholes often with certain types of failure zones, which are called breakouts. Laboratory-scale drilling tests in some high-porosity quartz-rich sandstone have shown breakouts in the form of narrow localized compacted zones in the minimum horizontal stress direction. They are called fracture-like breakouts. Such compaction bands may affect hydrocarbon extraction by forming barriers that inhibit fluid flow and may also be a source of sand production. This paper presents the results of numerical simulations of borehole breakouts using 3D discrete element method to investigate the mechanism of the fracture-like breakouts and to identify the role of far-field stresses on the breakout dimensions. The numerical tool was first verified against analytical solutions. It was then utilized to investigate the failure mechanism and breakout geometry for drilled cubic rock samples of Castlegate sandstone subjected to different pre-existing far-field stresses. Results show that failure occurs in the zones of the highest concentration of tangential stress around the borehole. It is concluded that fracture-like breakout develops as a result of a nondilatant failure mechanism consisting of localized grain debonding and repacking and grain crushing that lead to the formation of a compaction band in the minimum horizontal stress direction. In addition, it is found that the length of fracture-like breakouts depends on both the mean stress and stress anisotropy. However, the width of the breakout is not significantly changed by the far-field stresses. Copyright © 2013 John Wiley & Sons, Ltd.