Publications
224 results found
Chen B, Wang Y, Cui S, et al., 2023, Reconstruction of granite microstructure model using simulated annealing method and Voronoi tessellation, ENGINEERING COMPUTATIONS, Vol: 40, Pages: 1289-1304, ISSN: 0264-4401
Wang H-J, Liao H-L, Wei J, et al., 2023, Stress release mechanism of deep bottom hole rock by ultra-high- pressure water jet slotting, PETROLEUM SCIENCE, Vol: 20, Pages: 1828-1842, ISSN: 1672-5107
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- Citations: 7
Chen B, Xiang J, Latham J-P, 2023, Influence of inter-grain cementation stiffness on the effective elastic properties of porous Bentheim sandstone, JOURNAL OF ROCK MECHANICS AND GEOTECHNICAL ENGINEERING, Vol: 15, Pages: 573-583, ISSN: 1674-7755
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- Citations: 3
Velmurugan N, Gerbaud L, Chambres C, et al., 2023, Influencing Factors in Rock Cutting Using High Pressure Water Jets Under Submerged Downhole Conditions
Utilizing deep geothermal energy is an important factor in successful transition to a clean, affordable and non-intermittent source of energy. However, high costs due to low drilling performance is a showstopper in scaling geothermal energy. It has been noted that there is a significant drop in the rate of penetration (ROP) in hard crystalline granite rocks averaging between 1-2m/h. These hard rocks are found at depths greater than 4 km and increasing the ROP by a factor of 4 could reduce the drilling costs by 65%. In the framework of ORCHYD (Novel Drilling Technology Combining Hydro-Jet and Percussion for ROP Improvement in Deep Geothermal Drilling) project, we investigate ways to reduce the confining stresses of the underground rock so as the energy required to break the rock will be significantly lower and thus, increasing the ROP. A high-pressure water jet (HPWJ) of up to 250 MPa is generated in the bottomhole conditions to create peripheral groove of a few millimeters to release the rock layer from the surrounding stress regimes - reducing the energy required to break the rock using percussion drilling. Nevertheless, generating the required energy for rock slotting depends on several factors such as nozzle type, output pressure and confining pressure. In this paper, we investigate experimentally the influencing factors of HPWJ for rock slotting in bottomhole drilling conditions.
Xiang J, Latham JP, 2023, Algorithms and Capabilities of Solidity to Simulate Interactions and Packing of Complex Shapes, Simulations in Bulk Solids Handling: Applications of DEM and other Methods, Pages: 231-246, ISBN: 9783527350100
A number of numerical algorithms for simulation of particle packing have been proposed and used in a wide range of industries: mining, chemical engineering, pharmaceuticals, agriculture, food handling, etc. However, most of them can only deal with simple and regular shapes due to the complex and expensive numerical algorithms needed to simulate complex shapes. In this chapter, a FDEM code, Solidity, is used to capture the influence of complex shape more accurately. It combines deformable fracturing arbitrary-shaped particle interactions modelled by FEM with discrete particulate motion modelled by DEM. This chapter will cover recent code optimisation for the contact force calculation with arbitrary body shape, parallelisation performance, and discussion of results showing both deformable and rigid body versions of the code in the application of cube packing. Solidity can easily be combined with post-processing tools to analyse the particle packing structure in terms of local porosity and orientation distributions, contact forces, coordination number, etc. Some examples of Platonic and Archimedean body packs are presented.
Wang H, Liao H, Wei J, et al., 2022, Pressure Drop Model and Jet Features of Ultra High Pressure Water Jet for Downhole Intensifier, JOURNAL OF ENERGY RESOURCES TECHNOLOGY-TRANSACTIONS OF THE ASME, Vol: 144, ISSN: 0195-0738
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- Citations: 6
Xiang J, Chen B, Latham J-P, et al., 2022, Numerical simulation of rock erosion performance of a high-speed water jet using an immersed-body method, INTERNATIONAL JOURNAL OF ROCK MECHANICS AND MINING SCIENCES, Vol: 158, ISSN: 1365-1609
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- Citations: 2
Yang J, Kane A, Didriksen T, et al., 2022, Novel environmentally friendly nano-additives for drilling fluids
The potential for environmentally friendly nano-additives based drilling fluids is extended here in the context of the ongoing EU project 'ORCHYD' where a hybrid drilling technology is developed by combining High Pressure Water Jet (HPWJ) and Percussive Drilling to improve the efficiency of deep geothermal drilling. In this work, different types of nanomaterials such as graphene oxide, hybrid nano silica and their composite nanomaterials are used as additives in a water-based drilling fluid. The influence of additives on the rheological properties and the friction of the drilling fluid has been investigated. The laboratory results showed that an increase in the concentration of additives will increase the viscosity of fluids. A significant reduction in the coefficient of friction (during wear testing of steel material) was also revealed with the addition of GO and chemical functionalization. The need for further characterizing and the perspective to upscale the enhanced nano-additives based fluids for HPWJ and percussive drilling application, in deep geothermal conditions, are discussed.
Scaravaglione G, Latham J-P, Xiang J, 2021, Numerical Model Study of Prototype Drop Tests on Cube and Cubipod(R) Concrete Armor Units Using the Combined Finite-Discrete Element Method, JOURNAL OF MARINE SCIENCE AND ENGINEERING, Vol: 9
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- Citations: 2
Farsi A, Xiang J, Latham J-P, et al., 2021, Packing simulations of complex-shaped rigid particles using FDEM: An application to catalyst pellets, Powder Technology, Vol: 380, Pages: 443-461, ISSN: 0032-5910
A new component of the combined finite-discrete element method (FDEM) is employed to estimate the effects of geometrical features, friction and energy dissipation parameters on the bulk properties of rigid pellet packs. This work constitutes the first systematic validation of the Solidity FDEM code for rigid particles. The experimental and numerical axial and radial packing density profiles and orientation distributions have been compared, confirming that the numerical simulations of packing of cylindrical catalyst supports, glass beads and trilobe pellets deposited in a cylindrical container match the corresponding emergent bulk properties obtained from X-Ray CT scans. The presented results are a first confirmation of the applicability of FDEM based methods to the simulation of this class of multi-body problems. The assessment of the accuracy of the simulated topology of the pellet pack that is established in this work is encouraging for further investigations of the multi-physical engineering systems of interest for catalyst pellets involving hydro-thermo-mechanical, fracturing and fragmentation interactions using coupled FDEM formulations.
ViaEstrem L, Salinas P, Xie Z, et al., 2020, Robust control volume finite element methods for numerical wave tanks using extreme adaptive anisotropic meshes, International Journal for Numerical Methods in Fluids, Vol: 92, Pages: 1707-1722, ISSN: 0271-2091
Multiphase inertia‐dominated flow simulations, and free surface flow models in particular, continue to this day to present many challenges in terms of accuracy and computational cost to industry and research communities. Numerical wave tanks and their use for studying wave‐structure interactions are a good example. Finite element method (FEM) with anisotropic meshes combined with dynamic mesh algorithms has already shown the potential to significantly reduce the number of elements and simulation time with no accuracy loss. However, mesh anisotropy can lead to mesh quality‐related instabilities. This article presents a very robust FEM approach based on a control volume discretization of the pressure field for inertia dominated flows, which can overcome the typically encountered mesh quality limitations associated with extremely anisotropic elements. Highly compressive methods for the water‐air interface are used here. The combination of these methods is validated with multiphase free surface flow benchmark cases, showing very good agreement with experiments even for extremely anisotropic meshes, reducing by up to two orders of magnitude the required number of elements to obtain accurate solutions.
Guo L, Latham J-P, Xiang J, et al., 2020, A generic computational model for three-dimensional fracture and fragmentation problems of quasi-brittle materials, European Journal of Mechanics A: Solids, Vol: 84, Pages: 1-20, ISSN: 0997-7538
Fracture and fragmentation in three dimensions are of great importance to understand the mechanical behaviour of quasi-brittle materials in failure stress states. In this paper, a generic computational model has been developed in an in-house C/C++ code using the combined finite-discrete element method, which is capable of modelling the entire three-dimensional fracturing process, including pre-peak hardening deformation, post-peak strain softening, transition from continuum to discontinuum, and explicit interaction between discrete fragments. The computational model is validated by Brazilian tests and polyaxial compression tests, and a realistic multi-layer rock model in an in situ stress condition is presented as an application example. The results show that the computational model can capture both continuum and discontinuum behaviour and therefore it provides an ideal numerical tool for fracture and fragmentation problems.
Farsi A, Bedi A, Latham JP, et al., 2020, Simulation of fracture propagation in fibre-reinforced concrete using FDEM: an application to tunnel linings, Computational Particle Mechanics, Vol: 7, Pages: 961-974, ISSN: 2196-4378
The application of the combined finite-discrete element method (FDEM) to simulate fracture propagation in fibre-reinforced-concrete (FRC)-lined tunnels has been investigated. This constitutes the first attempt of using FDEM for the simulation of fracture in FRC structures. The mathematical implementations of the new FDEM joint-element constitutive model are first introduced, and the numerical model is then validated comparing the results for plain and FRC beams with three-point bending experimental data. The code has also been applied to two practical tunnel design case studies, showing different behaviours depending on the type of concrete and shape of tunnel section. The FDEM simulations of the linings are also compared with results from a finite element code that is commonly used in the engineering design practise. These results show the capabilities of FDEM for better understanding of the fracture mechanics and crack propagation in FRC tunnels. A methodology for directly inferring the numerical parameters from three-point bending tests is also illustrated. The results of this research can be applied to any FRC structure.
Latham J-P, Xiang J, Farsi A, et al., 2020, A class of particulate problems suited to FDEM requiring accurate simulation of shape effects in packed granular structures, Computational Particle Mechanics, Vol: 7, Pages: 975-986, ISSN: 2196-4378
In many granular material simulation applications, DEM capability is focused on the dynamic solid particulate flow properties and on systems in which millions of particles are involved. The time of relevance is many seconds or even minutes of real time. Simplifying assumptions are made to achieve run completion in practical timescales. There are certain applications, typically involving manufactured particles, where a representative pack is of the order of a thousand particles. More accurate capturing of the influence of complex shape is then often possible. Higher accuracies are necessary to model the topology of the void space, for example, for further CFD simulation and optimisation of fluid flow properties. Alternatively, the accuracy may be critical for structural performance and the force or stress transmission through the contact points is to be controlled to avoid material damage and poor function. This paper briefly summarises methods for simulation of shape effects on packing structures in the granular community and narrows the scope to problems where shape effects are of overriding concern. Two applications of mono-sized, mono-shaped packing problems are highlighted: catalyst support pellets in gas reforming and concrete armour units in breakwater structures. The clear advantages of FDEM for complex-shaped particle interactions in packed systems with relatively few particles are discussed. A class of particulate problems, ‘FDEM-suited’ problems, ones that are ideal to be solved by FDEM rather than by DEM, is proposed for science and engineering use.
Joulin C, Xiang J, Latham J-P, et al., 2020, Capturing heat transfer for complex-shaped multibody contact problems, a new FDEM approach, Computational Particle Mechanics, Vol: 7, Pages: 919-934, ISSN: 2196-4378
This paper presents a new approach for the modelling of heat transfer in 3D discrete particle systems. Using a combined finite–discrete element (FDEM) method, the surface of contact is numerically computed when two discrete meshes of two solids experience a small overlap. Incoming heat flux and heat conduction inside and between solid bodies are linked. In traditional FEM (finite element method) or DEM (discrete element method) approaches, to model heat transfer across contacting bodies, the surface of contact is not directly reconstructed. The approach adopted here uses the number of surface elements from the penetrating boundary meshes to form a polygon of the intersection, resulting in a significant decrease in the mesh dependency of the method. Moreover, this new method is suitable for any sizes or shapes making up the particle system, and heat distribution across particles is an inherent feature of the model. This FDEM approach is validated against two models: a FEM model and a DEM pipe network model. In addition, a multi-particle heat transfer contact problem of complex-shaped particles is presented.
Joulin C, Xiang J, Latham J-P, 2020, A novel thermo-mechanical coupling approach for thermal fracturing of rocks in the three-dimensional FDEM, Computational Particle Mechanics, Vol: 7, Pages: 935-946, ISSN: 2196-4378
This paper presents a new three-dimensional thermo-mechanical (TM) coupling approach for thermal fracturing of rocks in the finite–discrete element method (FDEM). The linear thermal expansion formula is implemented in the context of FDEM according to the concept of the multiplicative split of the deformation gradient. The presented TM formulation is derived in the geo-mechanical solver, enabling thermal expansion and thermally induced fracturing. This TM approach is validated against analytical solutions of the Cauchy stress, thermal expansion and stress distribution. Additionally, the thermal load on the previously validated configurations is increased and the resulting fracture initiation and propagation are observed. Finally, simulation results of the cracking of a reinforced concrete structure under thermal stress are compared to experimental results. Results are in excellent agreement.
Latham J-P, Xiang J, Chen B, et al., 2020, Grain-scale failure mechanism of porous sandstone: an experimental and numerical FDEM study of the Brazilian tensile strength test using CT-scan microstructure, International Journal of Rock Mechanics and Mining Sciences, Vol: 132, Pages: 1-17, ISSN: 0020-7624
Many widely used numerical models of rock fracture based on mesoscale laboratory test characterisation of effective ‘intact’ strength parameters neglect microstructure effects. They therefore cannot explain grain boundary and pore effects on crack propagation and consequently are inadequate for models of rock destruction that exploit point and indentation stresses. Understanding deep drilling processes involving drill-bit buttons and/or water-jetting where rock loading is concentrated in domains with fewer mineral grains will therefore require models with microstructure. To investigate microscale failure mechanisms of granular rocks in diverse scenarios, we target a porous sandstone and introduce a novel workflow consisting of a computerized tomography (CT) based microstructure construction approach and a complementary mechanical numerical approach. The construction approach extracts the realistic rock microstructure and transforms the large voxel number CT-scan data into significantly fewer triangular elements. The finite-discrete element method (FDEM) with grain-based model (GBM) is adopted to solve the mechanics. The microscale failure mechanism of sandstone during the Brazilian test was thoroughly analysed using the numerical results together with the post failure CT-scan test data. The build-up of compressive and tensile stress chains, micro-crack nucleation, local relaxation, chain switching and final crack-path development exploiting pores was illustrated, revealing the micro-to-macro failure mechanism in time and space. Fracture paths in the specimens during Brazilian tensile test were dominated by the pores and the inter-grain boundaries. The tensile strength of the inter-grain joints was estimated to be at least 3.67 times the mesoscale specimen's intact tensile strength, while the pores account for 72.76% of the fracture path. The influence of the cementation distribution and microscale discontinuities was investigated with numerical cases.
Yang P, Lei Q, Xiang J, et al., 2020, Numerical simulation of blasting in confined fractured rocks using an immersed-body fluid-solid interaction model, Tunnelling and Underground Space Technology, Vol: 98, Pages: 1-14, ISSN: 0886-7798
We model blast-induced fracturing and fragmentation processes in fractured rocks using a fully coupled fluid-solid interaction model. This model links a finite-discrete element solid solver with a control volume-finite element fluid solver through an immersed-body method. The solid simulator can capture the deformation of intact rocks, interaction of matrix blocks, displacement of existing fractures and propagation of new cracks. The fluid simulator can simulate the highly compressible gas flow involved in the blasting and explosion process, which is assumed to follow the John-Wilkins-Lee equation of state. We design numerical experiments as follows. First, we generate a series of 1 m × 1 m discrete fracture networks associated with different fracture density and mean length values to consider various scenarios of distributed pre-existing fractures in rock. We apply isotropic/anisotropic in-situ stresses to the rock such that the system reaches an equilibrium state. Then we release the compressible gas associated with a prescribed high pressure in the borehole to simulate explosion, which engenders stress wave propagation and new crack generation in the system. We observe that the presence of natural fractures has a significant impact on the blast behaviour of fractured rocks such that new cracks tend to be arrested by pre-existing discontinuities which however accommodate wing cracks at their tips linking with other structures. Blast-driven cracks attempt to propagate along the maximum principal stress direction if an anisotropic stress condition is imposed. Our research findings have important implications for the design and assessment of blasting for underground excavation in fractured formations.
Farsi A, Xiang J, Latham JP, et al., 2020, Strength and fragmentation behaviour of complex-shaped catalyst pellets: A numerical and experimental study, Chemical Engineering Science, Vol: 213, Pages: 1-18, ISSN: 0009-2509
The effects of catalyst support shapes on their final strength and fragmentation behaviour are investigated. Uniaxial compression tests by diametrical loading of solid and four-holed discs with high-speed video recordings are employed to investigate strengths and pellet crushing behaviours. The combined finite-discrete element method (FDEM) is employed to simulate the effects of geometrical features and loading orientation on the pre- and post-failure behaviour of catalysts. A comparison with experimental results is also presented and the remarkable agreement in failure evolution and mode is discussed. A methodology to derive representative fragment size distributions from defined pellet shapes and material properties is introduced, providing a further tool to understand the strength and fragmentation behaviour of catalyst supports. The results suggest that fixed-bed reactors made with solid cylindrical catalysts will be more likely to be affected by pressure drops caused by the choking effect of a significant portion of fines than if it was made with catalyst supports with four holes. Two designs of four-hole catalyst supports sintered with different porosities have also been studied, showing different fragment size distributions and fines production. Characterisation of fines production for different catalyst support designs will improve prediction of reactor clogging and pressure drops.
Tomasicchio GR, Vicinanza D, Belloli M, et al., 2020, PHYSICAL MODEL TESTS ON SPAR BUOY FOR OFFSHORE FLOATING WIND ENERGY CONVERSION, Italian Journal of Engineering Geology and Environment, Vol: 20, Pages: 129-143, ISSN: 1825-6635
La domanda globale di energia eolica sta aumentando rapidamente e sta acquisendo sempre più importanza come risorsa energetica, dato l'interesse crescente per le energie rinnovabili. Le risorse eoliche offshore hanno attirato un'attenzione significativa e, rispetto alle risorse eoliche terrestri, sembrano essere più promettenti per lo sviluppo. I venti marini sono generalmente più forti e più affidabili e grazie agli enormi miglioramenti della tecnologia, il mare è diventato un hot spot per nuovi design e metodi di installazione per le turbine eoliche. C'è molto interesse in questo campo, poiché si ritiene che svolga un ruolo importante nel futuro dell'eolico offshore. Vari carichi dinamici vengono trasmessi dalla torre della turbina eolica alla sua piattaforma: carico del vento, carico delle onde del mare, carico dinamico dovuto al rotore, effetti di schermatura del vento della pala sulla torre che crea un carico ciclico. Per una turbina eolica offshore che opera sulla superficie del mare in continua evoluzione, è quindi fondamentale studiare il comportamento dinamico a cui è soggetta la struttura e in che modo la complessa interazione dei carichi delle onde e del vento influisca sul sistema. Un robusto processo di progettazione deve garantire che la frequenza naturale dell'intero sistema non si avvicini alle frequenze dei carichi ambientali imposti. In caso contrario, si potrebbe amplificare la risposta dinamica della struttura, portando a maggiori deflessioni della torre che possono compromettere le prestazioni della turbina eolica. Le turbine eoliche galleggianti sono supportate da strutture galleggianti e, quindi, hanno 6 gradi di libertà, che possono essere eccitate da carichi di onde, vento e correnti oceaniche. L'intero sistema è ormeggiato e stabilizzato mediante un sistema di molle e contrappesi. Sono strutture relativamente grandi che variano tra 5000 e 10.000 tonnellate per un'u
Latham J-P, Farsi A, Xiang J, et al., 2019, Numerical modelling of the influence of in-situ stress, rock strength and hole-profile geometry on the stability of Radial Water Jet Drill (RJD) boreholes, American Rock Mechanics Association
Obeysekara A, Salinas P, Xiang J, et al., 2019, Numerical Modelling of Coupled Flow and Fluid-Driven Fracturing in Fractured Porous Media using the Immersed Body Method, Interpore 2019
Chen B, Latham JP, Xiang J, et al., 2019, Experimental and numerical investigation of the microscale failure mechanism of a porous sandstone during Brazilian tensile test conditions
Rock failure is normally simulated with the mesoscale or macroscale model and the corresponding damage or fracture model based on laboratory scale intact strength parameters. The mesoscale or macroscale model predicts the rock failure plausibly in mesoscale or macro scale problems but may fail in some specific failure mechanisms where the microstructure is believed to play an important role. As an initial step of investigating the complex failure mechanism of sandstone in the context of water jet drilling, the microstructure model of the sandstone was constructed based on CT-scan data in this work and the corresponding failure mechanism during Brazilian tensile test was analyzed. A novel CT-scan based approach is proposed to mimic sandstone microstructure in the numerical model and a compromise between the accuracy of the microstructure model and the computation cost is reached. The microstructure model is solved with the FEMDEM. The capacity of the proposed model in simulating microscale failure of sandstone is well proved in numerical cases.
Xiang J, Latham JP, Pain C, 2019, Numerical simulation of rock erosion performance of a high-speed water jet using an immersed body method
The paper presents a new immersed body method (IBM) in which the combined Finite-Discrete Element Method (FEMDEM) that deals with solids interactions is coupled to other modelling technologies e.g. CFD, interface tracking, porous media etc. The CFD solver, Fluidity which is a general purpose multiphase CFD code is capable of modelling a wide range of fluid phenomena involving single and multiphase flows. The FEMDEM code, Solidity, can capture the deformation of the rocks, the initiation/propagation of new cracks. The immersed body method combined with adaptive mesh refinement has been applied to simulate the interaction between the high-speed water jet and rock mass. The paper aims to deeply understand the rock fragmentation mechanism and to explain the reasons for crack initiation, propagation and fragment removal under the impact load of a high-speed water jet. It also investigates the effect of pore water pressure on rock erosion performance. The results are in good agreement with experimental results.
Latham JP, Farsi A, Xiang J, et al., 2019, Numerical modelling of the influence of in-situ stress, rock strength and hole-profile geometry on the stability of Radial Water Jet Drill (RJD) boreholes
Radial water jet drilling (RJD) is a method of enhancing heat recovery by accessing and connecting to high permeable zones within geothermal reservoirs. The wall rock geometry behind an advancing water jet borehole under in-situ conditions is largely unknown. Water jet drilling tests were performed on 300 mm cubical blocks of weak porous sandstone under true-triaxial boundary stress conditions at the Delft Technical University (DTU) rock mechanics laboratory. Some of these tests showed distinct breakout features depending on the applied stress field. Geometries of resulting boreholes are recovered using X-Ray CT scans, and are analysed using segmentation software (Avizo). The code Solidity, using a combined finite-discrete element method with a cohesive zone fracture model, simulates stress take-up and wall shearing giving breakouts comparable to the experiments. The results lead to the suggestion that criteria based on Kirsch solutions would be suitable to provide general guidance on in-situ stress and rock strength conditions free of breakouts. FEMDEM models appear well-suited to examine geometries and dimensions that can be sustained by given strengths under deeper in-situ conditions. Wall-rock failure and a process of jet-hole enlargement together with the potential benefits of greater heat recovery arising from larger holes is also briefly discussed.
Chen B, Latham JP, Xiang J, et al., 2019, Experimental and numerical investigation of the microscale failure mechanism of a porous sandstone during Brazilian tensile test conditions
Copyright 2019 ARMA, American Rock Mechanics Association. Rock failure is normally simulated with the mesoscale or macroscale model and the corresponding damage or fracture model based on laboratory scale intact strength parameters. The mesoscale or macroscale model predicts the rock failure plausibly in mesoscale or macro scale problems but may fail in some specific failure mechanisms where the microstructure is believed to play an important role. As an initial step of investigating the complex failure mechanism of sandstone in the context of water jet drilling, the microstructure model of the sandstone was constructed based on CT-scan data in this work and the corresponding failure mechanism during Brazilian tensile test was analyzed. A novel CT-scan based approach is proposed to mimic sandstone microstructure in the numerical model and a compromise between the accuracy of the microstructure model and the computation cost is reached. The microstructure model is solved with the FEMDEM. The capacity of the proposed model in simulating microscale failure of sandstone is well proved in numerical cases.
Obeysekara A, Xiang J, Latham JP, et al., 2018, Modelling stress-dependent single and multi-phase flows in fractured porous media based on an immersed-body method with mesh adaptivity, Computers and Geotechnics, Vol: 103, Pages: 229-241, ISSN: 0266-352X
This paper presents a novel approach for hydromechanical modelling of fractured rocks by linking a finite-discrete element solid model with a control volume-finite element fluid model based on an immersed-body approach. The adaptive meshing capability permits flow within/near fractures to be accurately captured by locally-refined mesh. The model is validated against analytical solutions for single-phase flow through a smooth/rough fracture and reported numerical solutions for multi-phase flow through intersecting fractures. Examples of modelling single- and multi-phase flows through fracture networks under in situ stresses are further presented, illustrating the important geomechanical effects on the hydrological behaviour of fractured porous media.
Yang L, Lyu Z, Yang P, et al., 2018, Numerical Simulation of Attenuator Wave Energy Converter using One-Fluid Formulation, Proceedings of the 28th International Ocean and Polar Engineering Conference
Lei Q, Latham, Xiang J, et al., 2017, Role of natural fractures in damage evolution around tunnel excavation in fractured rocks, Engineering Geology, Vol: 231, Pages: 100-113, ISSN: 0013-7952
This paper studies the role of pre-existing fractures in the damage evolution around tunnel excavation in fractured rocks. The length distribution of natural fractures can be described by a power law model, whose exponent a defines the relative proportion of large and small fractures in the system. The larger a is, the higher proportion of small fractures is. A series of two-dimensional discrete fracture networks (DFNs) associated with different length exponent a and fracture intensity P21 is generated to represent various scenarios of distributed pre-existing fractures in the rock. The geomechanical behaviour of the fractured rock embedded with DFN geometry in response to isotropic/anisotropic in-situ stress conditions and excavation-induced perturbations is simulated using the hybrid finite-discrete element method (FEMDEM), which can capture the deformation of intact rocks, the interaction of matrix blocks, the displacement of natural fractures, and the propagation of new cracks. An excavation damaged zone (EDZ) develops around the man-made opening as a result of reactivation of pre-existing fractures and propagation of wing cracks. The simulation results show that when a is small, the system which is dominated by large fractures can remain stable after excavation given that P21 is not very high; however, intensive structurally-governed kinematic instability can occur if P21 is sufficiently high and the fracture spacing is much smaller than the tunnel size. With the increase of a, the system becomes more dominated by small fractures, and the EDZ is mainly created by the coalescence of small fractures near the tunnel boundary. The results of this study have important implications for designing stable underground openings for radioactive waste repositories as well as other engineering facilities that are intended to generate minimal damage in the host rock mass.
Lei Q, Wang X, Xiang J, et al., 2017, Polyaxial stress-dependent permeability of a three-dimensional fractured rock layer, Hydrogeology Journal, Vol: 25, Pages: 2251-2262, ISSN: 1435-0157
A study about the influence of polyaxial (true-triaxial) stresses on the permeability of a three-dimensional (3D) fractured rock layer is presented. The 3D fracture system is constructed by extruding a two-dimensional (2D) outcrop pattern of a limestone bed that exhibits a ladder structure consisting of a “through-going” joint set abutted by later-stage short fractures. Geomechanical behaviour of the 3D fractured rock in response to in-situ stresses is modelled by the finite-discrete element method, which can capture the deformation of matrix blocks, variation of stress fields, reactivation of pre-existing rough fractures and propagation of new cracks. A series of numerical simulations is designed to load the fractured rock using various polyaxial in-situ stresses and the stress-dependent flow properties are further calculated. The fractured layer tends to exhibit stronger flow localisation and higher equivalent permeability as the far-field stress ratio is increased and the stress field is rotated such that fractures are preferentially oriented for shearing. The shear dilation of pre-existing fractures has dominant effects on flow localisation in the system, while the propagation of new fractures has minor impacts. The role of the overburden stress suggests that the conventional 2D analysis that neglects the effect of the out-of-plane stress (perpendicular to the bedding interface) may provide indicative approximations but not fully capture the polyaxial stress-dependent fracture network behaviour. The results of this study have important implications for understanding the heterogeneous flow of geological fluids (e.g. groundwater, petroleum) in subsurface and upscaling permeability for large-scale assessments.
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