Browsing by Author "Wong, Ron Chik Kwong"

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    Flat Arch Masonry Retaining Wall
    (2020-08-21) Kurukulasuriya, Maithree Chiranthya; Shrive, Nigel; Wong, Ron Chik Kwong; Dann, Markus R.; Federico, Salvatore
    The inherent strength and durability of masonry arches is eminent through many historical structures still existing to this day. Amidst a wide range of retaining wall types and applications available, the concept of utilizing masonry arches as earth retaining structures is surprisingly unprecedented in the recent past. In this research, a novel masonry low-rise arch retaining wall is introduced. An experimental programme was implemented to explore the feasibility of the proposed earth retaining system, where a full-scale, concrete block arch wall was constructed between two rigid abutments and backfilled with soil. Surcharge loading was also applied to the backfill soil to explore the stability of the wall under additional dead loads. The concept is that the blockwork wall will resist the lateral pressure through arch action, avoiding “snap-through” of the wall. The response of the wall to the soil pressure, compaction and surcharge loading was monitored by observing deflections and strains in the wall. Furthermore, one half of the wall was fully grouted, while the other half was left hollow to compare the behaviour of the grouted and un-grouted masonry. Retaining walls are typically constructed using concrete, steel, masonry or timber. The use of concrete blocks in this instance was desirable due to its strength, ease of construction, cost effectiveness and aesthetic appearance. This makes such a wall ideal for low-rise retention, opening a new market in which masonry can compete. From the results of the experimental study, the efficacy of the proposed masonry arch retaining wall was substantiated with deflections less than 1.3% of the least dimension of the wall, compressive stresses well within the elastic range of masonry and no cracks observed in the visible region of the wall.
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    Fracture Height Propagation in Tight Reservoirs Using the Finite Element Method
    (2021-01-26) Cai, Jiujie; Chen, Shengnan; Chen, Zhangxin; Gates, Ian Donald; Wong, Ron Chik Kwong; Zhang, Yin
    In recent years, multi-stage hydraulic fracturing technology is widely applied in oil/gas industry all over the world as a successful treatment, especially in tight and shale reservoirs. The induced fracture geometries directly affect the post-stimulation production and economic profitability of the project and accurately predicting the fracture properties is quite important. In addition to fracture length and conductivity, fracture height is another critical parameter of the hydraulic fracturing treatments in the unconventional tight/shale formations. Multiple transverse fractures are usually created along the horizontal wells, where the mechanisms of fracture-height-containment can be complicated under conditions such as interactions with the natural fractures, as well as adjacent hydraulic fractures. In addition, the directions of the bounding layers may not be parallel with that of horizontal wells. Traditional fracture propagation models applied in industry do not include all the aforementioned factors comprehensively. This research targets to study the mechanisms of hydraulic fracture propagation, focusing on the fracture-height-containment in the scenarios of multiple fractures along the horizontal wells. Firstly, a two-dimensional numerical model is proposed to analyze the methodology of single fracture height propagation via the finite element method. Then, an analytical model is built to understand the mechanisms of the fracture height containment considering inclined bounding layers. Modeling results suggested that for the closely spaced multiple fractures which are growing simultaneously, the critical fluid pressure becomes larger, implying that the fracture height propagation is more difficult under such scenario. Fracture height propagates more easily when bounding layer inclination angle increases. Thirdly, a three-dimensional numerical model with cohesive method on the fracture height propagation is used to analyze the multiple fracture interactions and the effective fracture height and width in tight reservoirs. The influence of stress shadow and stress difference on effective fracture height has been investigated and results show that the interaction from adjacent fracture becomes more significant when fracture spacing is small. Fluid injection rate is also an important influencing factor on the hydraulic fracture width especially when flow rate is low. When stress shadow effect is strong, the interior fractures can hardly propagate, and the majority of the fluid volume goes into the exterior fractures.
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    Impact of hydraulic retention time and organic matter concentration on performance of side-stream aerobic granular membrane bioreactor
    (2019-07-16) Tavana, Arezoo; Wong, Ron Chik Kwong; Zhou, Qiangwei; Khoshnazar, Rahil
    Widespread membrane bioreactors (MBRs) application has been limited due to an undesirable yet inherent phenomenon named membrane fouling. Membrane fouling can be defined as unfavorable attachment of organic and inorganic matter inside (pore clogging) or onto (cake layer) membrane pores. Aerobic granulation biotechnology has become a promising substitute for activated sludge process (ASP) by offering advantages including high biomass retention, good settleability, high resiliency to high strength wastewater and shock loading and strong and round shape structure. Aerobic granulation technology is attributed to cell-to-cell interaction between microorganism consisting physical, chemical and biological phenomena. An integration of aerobic granulation technology (AG) and membrane bioreactors (MBRs) leading to the advent of aerobic granulation membrane bioreactor (AGMBR) method, has been able to suppress fouling rate. It was believed that large particle size, high density and more compact and dense structure of granules can significantly control and reduce membrane fouling compared to conventional MBRs method which are operated by activated sludge. However, granules instability in long term operation is a detrimental obstacle which is followed by delayed irreversible (i.e., irrecoverable) fouling in AGMBR application. Operational parameters majorly contributed to granule formation and membrane fouling. Hence, optimum selection of operational factors plays significant role in granules stability leading to fouling control in AGMBR application. This study investigated the effect of hydraulic retention time (HRT) and chemical oxygen demand (COD) concentration on membrane fouling in aerobic granular membrane bioreactor (AGMBR) in a systematic approach. Changes in HRT (7, 10, and 15 h) and COD (500, 1000 and 1500 mg/L) were applied in five operational phases, corresponding to different organic loading rates (OLRs), to determine the most significant parameters to control membrane fouling for enhanced AGMBR performance. Membrane fouling was associated with two critical points: initial flux reduction (primary fouling) and maximum transmembrane pressure (TMP) (secondary fouling). Membrane permeability loss was significantly intensified with increase in HRT from 7.5 to 15 h and COD from 500 to 1000 mg/L (OLR of 1.6 kg COD/m3.d). The highest polysaccharide content of loosely bound EPS (0.41 mg PS/mg VSS) and soluble microbial products (SMPs) (27 mg PS/L) occurred alongside poor AGMBR performance at this phase. Variations in membrane fouling was accompanied with considerable changes in Flavobacterium, Thauera and Paracoccus populations. Membrane performance deteriorated with reduction in Flavobacterium and Thauera relative abundances, while system recovery coincided with Paracoccus proliferation. Generally, OLR diminution from 3.6 kg COD/m3.d (phase I) to 1.6 kg COD/m3.d (phase IV) resulted in severe granules breakage alongside intensified membrane fouling. HRT and HRT and COD interaction were identified as the most significant parameters in controlling membrane fouling.
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    The impact of THF hydrate veins on the consolidation behavior of fine-grained soils
    (2021-01-26) Ma, Boning; Hayley, Jocelyn L. H.; Priest, Jeffrey A.; Wong, Ron Chik Kwong; Clarke, Matthew A.; Siemens, Greg A.
    Naturally occurring marine gas hydrates are ubiquitously found within sediments on continental slopes, where ongoing climate change or anthropogenic activities, such as hydrocarbon drilling and production, may dissociate the hydrates triggering slope instabilities. Large volumes of natural gas hydrate are contained within fine-grained marine sediments, observed as fracture filling veins, where these veins may hinder the sediment consolidation process resulting in weak, under-consolidated layers and possibly sea floor instability upon hydrate dissociation. Despite their potential impact, the behavior and properties of hydrates in fine-grained sediments remains poorly understood. To better understand the interaction between hydrate veins and consolidation, an experimental testing program was performed using cylindrical THF hydrate veins. Uni-axial compression tests, initially conducted on stand-alone THF hydrate specimens, indicated increasing specimen aspect ratio and reducing strain rate reduced compressive strength. Constant stress tests show the hydrate exhibits extensive plastic deformation as stresses approach failure conditions, with high aspect ratio specimens experiencing large out-of-plane deformations. Subsequent K0 consolidation tests on vein-bearing soil specimens, show considerable reduction in compressibility compared to those of the hydrate-free soils. Significant under consolidation of the soil occurs at high hydrate content and low to moderate confining stresses, although such effects are mitigated at higher confining stresses. These results indicate hydrate dissociation could lead to soil failure at low stress (i.e. shallow deltaic environments) and high hydrate content, and moderate to considerable volumetric deformation at high stresses (i.e. deeply buried sediments) and/or low hydrate content. Through this research, a better understanding of the impact of hydrate vein on the behavior of fine-grained soils has been achieved. This enables a more robust hypothesis to be developed for assessing the potential for submarine sea floor instabilities induced by hydrate dissociation through climate change. This may help evaluate the geohazard risk in marine environments and on coastal communities in these sensitive regions.
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    Modelling Hydraulic Fracturing in Tight Reservoirs Using Equivalent Continuum Approach
    (2021-01-29) Atdayev, Eziz; Wong, Ron Chik Kwong; Eaton, David William S.; Davidsen, Jörn; Duncan, Neil A.
    Hydraulic fracturing has transitioned into widespread use over the last few decades. There are a variety of numerical methods available to simulate hydraulic fracturing. However, most current methods require a large number of input parameters, of which the values of some parameters are poorly constrained. This research proposes a new method of modelling the hydraulically fractured region using void-ratio and strain dependent relation to define the permeability of the fractured region. This approach is computationally efficient and reduces the number of input parameters. By implementing this method with an equivalent continuum representation, uncertainties are reduced arising from heterogeneity and anisotropy of the earth materials. The computational efficiency improves modelling performance in stress-sensitive zones such as in the vicinity of the injection well or near faults.
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    Numerical Modelling using Finite Element Analysis with Advanced Soil Constitutive Models: Static and Quasi-static Approaches
    (2019-07-12) Bach, Thang Dinh; Wan, Richard; Priest, Jeffrey A.; Wong, Ron Chik Kwong
    The analysis of geostructures with failure as a central topic has been traditionally pursued through limit equilibrium methods in geotechnical engineering. In this thesis, the finite element approach together with advanced constitutive models encompassing micromechanics-based ingredients are used to examine discontinuous failure modes such as strain localization in a boundary value problem setting like in a plane-strain (biaxial) test on sand. The constitutive models used in the numerical simulations are: (1) a non-associated plasticity and fabric-based model, and (2) a micromechanically enriched model via multiscaling. These are implemented into a finite element solver following a time integration scheme that is either implicit or explicit. Under static conditions, an implicit scheme is typically adopted where a stiffness matrix needs to be inverted. Unfortunately, when a limit state, e.g. failure, is encountered, solution uniqueness is lost, thus leading to a bifurcation problem. The static problem can be alternatively formulated as a dynamic one involving velocity, acceleration and a properly chosen loading rate. This allows for an explicit time integration scheme in which the displacement field can be obtained without solving any system of equations -- so obviating the inversion of a stiffness matrix. This gives the method great potential in resolving difficulties encountered in a static simulation or in any numerical approach using the implicit scheme. To mimic the static problem in dynamic simulations, the loading rate is kept sufficiently small so that the response of the system can be quasi-static. Also, damping might be introduced to aid these simulations to converge properly toward the static equilibrium. Localized modes of failure are examined within bifurcation theory whereby there is a transition from an initially homogeneous deformation mode into one which involves localized deformations as a shear band. The anatomy of failure is examined and analyzed using the two constitutive models. It is shown that the micromechanically-enriched model gives important microstructural information such as coordination number and anisotropy at the particle level inside and outside the shear band to help understand the genesis of failure in soils.
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    Numerical Simulation of Interbedded Shale Failure in Oilsand Reservoirs by Electromagnetic Wave Excitation
    (2019-09-13) Shi, Longyang; Wan, Richard; Wan, Richard; Wong, Ron Chik Kwong; Zhou, Qi
    This thesis work offers an exploratory study of selective heating and fracturing of a shale layer embedded into an oilsand material. A three phase (oil-water-steam) thermal-fluid flow formulation enriched with electromagnetic physics is developed to serve as a basic model to investigate whether selective heating is possible, and thereafter verify the mechanism of shale fracturing by solving a reservoir geomechanics problem. The finite element method is employed to solve numerically this highly nonlinear multiphysics (thermal/fluid/wave propagation) problem in porous media following a staggered scheme. This is coined as the EMTH (Electromagnetic-Thermo-Hydro) modeling framework. It has been found that the configuration of EM sources such as density in the form of interval distances between point-dipoles, and most importantly phase angle, control the electromagnetic field pattern and intensity that determine the efficacy of directing heating towards a specified target. The proper characterization of electrical properties of multiphasic-capillary-porous media is another outstanding issue to address for fully understanding the radiation and heating pattern of electromagnetic wave excitation. A synthetic reservoir geomechanics model for verifying the potential of fracturing is constructed and interfaced with the EMTH framework in a loosely coupled fashion. As such, the evolutions of temperature and pore pressure fields under electromagnetic excitation are treated as parametric inputs into the geomechanics model. Tensile failure within the interbedded shale as expected for fracturing is achieved at a specific combination of initial water/oil saturation setup. Other cases are investigated to help reveal a full image of correlations between electromagnetic excitation, temperature/pore pressure escalation, geomechanical constraints, and natural properties of reservoir.