Dharmapriya Wijewickreme

Professor

Relevant Degree Programs

 

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Mar 2019)
Discrete element modeling of direct simple shear response of granular soils and model validation using laboratory tests (2014)

The direct simple shear (DSS) device is one of the most commonly used laboratory testing tools to characterize the shear behavior of soils. In the Norwegian Geotechnical Institute (NGI) version of the DSS test, where a cylindrical soil specimen is confined by a wire-reinforced membrane, only normal and shear stresses on the horizontal planes are measured. The knowledge of these stresses alone does not provide adequate information to calculate friction angles for use in geotechnical design. Further, the absence of complementary shear stresses at the soil-membrane interface causes stress non-uniformities within DSS specimens, which makes the task of interpreting DSS testing results even more difficult. With the recent advances in computers, it is now possible to model soil in a realistic manner as a collection of particles using the discrete element method (DEM). With this background, a DEM model of a cylindrical DSS specimen was developed to provide insight on the state of stress and strain in DSS specimens. A laboratory DSS testing program was undertaken on glass beads as part of this study. The results of the glass beads tests were used for comparison with the DEM model results. Further, free-form sensors (paper-thin flexible pressure sensors mounted on the reinforced part of the DSS membrane) were used to measure lateral stresses acting on reconstituted Fraser River silt specimens. It was shown that: i) the adopted DEM modeling approach is effective in capturing the salient characteristics of the DSS behavior of the tested glass beads; ii) during the shearing phase, the distribution of shear strains across the specimen is more uniform at lower shear strain levels; iii) significant stress non-uniformities during shearing are limited to a narrow zone of about two particles diameter near the lateral boundaries, while stresses at central specimen locations are relatively more uniform (i.e. most representative of “ideal” simple shear conditions); and iv) at large shear strains, the horizontal plane becomes the plane of maximum obliquity, and the friction angle calculated using the stress state on the horizontal plane is a good approximation to the mobilized friction angle at such strain levels.

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Soil restraints on steel buried pipelines crossing active seismic faults (2013)

The quantification and prediction of soil restraint on buried pipelines are essential for the design of pipeline systems crossing seismic faults, and in turn to reduce the risk of pipeline damage due to geotechnical earthquake hazards. Full-scale soil-pipe interaction tests were undertaken to better simulate the mobilization of soil restraints under controlled conditions and to provide insight on a number of currently unresolved technical issues that so far have been investigated only based on small-scale tests. In particular, an existing full-scale testing chamber was significantly modified to simulate pipeline breakout from its soil embedment on one side of a strike-slip fault and on the footwall side of a reverse fault in an effort to characterize lateral, combined axial and lateral, and vertical oblique soil restraints. The experimental system was also used to assess the effectiveness of reducing soil loads on pipelines using geotextiles. The following was noted: (1) approaches based on limit equilibrium reasonably well predict maximum values of lateral soil restraint for shallow pipelines backfilled with sand, with mixture of crushed gravel and sand, and with crushed limestone; (2) the lateral soil restraint on pipes in geotextile-lined trenches increased with increasing relative pipe displacement and could even be higher than the restraint without the geotextile lining. A procedure was developed to capture this behaviour; (3) experimental and numerical results for geotextile-lined trenches suggest that the shear resistance is not controlled solely by the geotextile interface; as such, there is no clear benefit in using geotextile-based mitigation measures for reducing soil loads; (4) the results from tests on combined axial and lateral soil restraints provided limited clarification on whether or not these soil restraints should be considered independent for fault crossing designs. This was due to the difficulty in selecting an axial soil restraint value to anchor existing soil restraint interaction relationships. No axial soil restraint tests were conducted in this work; and (5) values for the maximum vertical oblique soil restraint diminish as the inclination of the angle of breakout of buried pipelines increases with respect to the horizontal.

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A hybrid effective stress – total stress procedure for analyzing soil embankments subjected to potential liquefaction and flow (2011)

Seismic design of major civil structures (bridges, dams and embankments) is moving increasingly towards using performance design methodologies which require determination of earthquake induced movements. Development of these numerical design tools and procedures for use in engineering practice for estimating the earthquake induced ground deformations of potentially liquefiable soil is the topic of this dissertation. Fully coupled effective stress numerical analyses procedures developed at the University of British Columbia (UBC) were used to simulate field and centrifuge test case histories. These analyses can offer considerable insight, but due to the complexity of the problem and variability of the parameters involved, there is considerable uncertainty. The author, therefore, recommends that the relatively new state-of-the-art effective stress analyses should be augmented by carrying out an additional analysis compatible with conventional design processes. This latter analysis uses published post-liquefaction “residual” soil strengths derived from back-analysis of field case histories by others. The developed design methodology uses the effective stress (UBCSAND) soil constitutive model for dynamic analyses, and empirical “residual” post-liquefaction soil strengths for a post-shaking total stress static analysis. In the proposed approach, the effective stress dynamic analysis is used to determine zones of liquefaction, to quantify earthquake induced deformations, and to provide overall insight. The post-shaking total stress static analysis, with “residual” strength parameters used in elements which liquefied, is carried out to capture the effects of complex stratigraphy and localization that may be missed by the effective stress model.Calibration and validation of the UBCSAND model was undertaken by comparing the model with field case histories and laboratory simple shear, shake table, and centrifuge tests. The measured response of some centrifuge tests being used for validation was indicative of the centrifuge model not being fully saturated. This was problematic as P-wave measurements within the centrifuge model suggested full saturation. A series of triaxial tests with P-wave measurements was carried out. These tests, and the numerical modeling of them, showed that high P-wave velocities were not always indicative of full saturation and they provided a logical explanation for the observed centrifuge response.

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Cyclic shear loading response of Fraser River delta silt (2011)

The cyclic shear response of low-plastic Fraser River silt was investigated using constant-volume direct simple shear testing. Silt specimens, initially consolidated to stress levels at or above the preconsolidation stress, displayed cyclic mobility type strain development during cyclic loading. Liquefaction in the form of strain softening accompanied by loss of shear strength did not manifest regardless of the applied cyclic stress ratio (CSR), or the level of induced excess pore water pressure (Δu). Cyclic mobility type stress-strain behaviour was observed in spite of the initial static shear stress bias. The potential for excess pore water pressure generation and associated shear strain development during cyclic loading was observed to increase with increasing level of initial static shear. Tests on specimens of undisturbed field samples and specimens reconstituted using the same silt material showed that undisturbed silt, despite having a looser density under identical consolidation stress conditions, exhibited more dilative response and larger shear resistance compared to those displayed by reconstituted specimens. In addition to consolidation stress conditions and resulting void ratios, it appears that other naturally inherited parameters such as soil fabric and aging effects would influence the shear response of natural silt. Studies were also conducted to examine the post-cyclic reconsolidation response of low-plastic silt using specimens of undisturbed and reconstituted Fraser River silt and reconstituted quartz powder initially subjected to constant volume cyclic loading at different CSR values and then reconsolidated to their initial effective stresses. The volumetric strains during post-cyclic reconsolidation (εv-ps) were noted to increase with the maximum Δu and maximum cyclic shear strain experienced during cyclic loading. The values of εv-ps and maximum excess cyclic pore water pressure ratio (ru max) were observed to form a coherent relationship regardless of overconsolidation effects, particle fabric, and initial void ratio of the soil. The specimens with high ru-max suffered significantly higher post-cyclic reconsolidation strains. The observed εv-ps versus ru-max relationship, when combined with the observed dependence of ru on CSR and number of load cycles, seems to provide a reasonable approach to estimate post-cyclic reconsolidation strains of low-plastic silt.

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Pipe-soil interaction aspects in buried extensible pipes (2011)

The performance of buried pipelines in areas subjected to permanent ground displacements is an important engineering consideration in the gas distribution industry, since the failure of such systems poses a risk to public and property safety. Although, the ground movements and its variations over time can be detected and mapped with reasonable confidence, these data are of little use due to a lack of reliable models to correlate such displacements to the condition of the buried pipe. The objective of this thesis is to develop methods to estimate the pipe performance based on the measured ground displacement. An analytical method was developed to estimate the pipe performance when the pipe is subjected to tensile loading caused by the relative ground movements occurring along the pipe axis. As a part of the derivation, a modified interface friction model was developed considering the increase in friction due to constrained dilation of the soil, and the impact of mean effective stress on soil dilation. This interface friction model was combined with a nonlinear pipe stress–strain model to derive an analytical solution to represent the performance of the pipe. Using the proposed model, axial force, strain, and mobilized frictional length along the pipe can be obtained for a measured ground displacement can be obtained. Large-scale field pipe pullout tests were performed to verify the results of the proposed analytical model, in which good agreements were observed for tests conducted at different soil/burial conditions, displacement rates and pipe properties. Considering the similarities in the axial pullout mechanism, the analytical model was extended to explain the pullout response of geotextiles buried in reinforced soil structures. In this derivation, a new interface friction model was developed for planar members by considering the changes in normal stress due to constrained soil dilation. Another analytical model was derived for the case of a pipe that is subjected to combined loading from axial tension and bending when the initial soil loading is acting perpendicular to the pipe axis. With the direct account of the axial tensile force development, more realistic pipe performance behaviors were obtained as compared to the results obtained from traditional numerical formulations.

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Mechanical response of highly gap-graded mixtures of waste rock and tailings (paste rock) (2009)

The mixing of mine tailings and waste rock to form “paste rock” prior to disposal is now receiving significant attention from the point of view of sustainable mine waste management practice. This approach has been viewed as a favourable alternative to traditional methods of mine waste disposal because paste rock has the potential to overcome deficiencies, such as acid rock drainage and mechanical instability, associated with traditional methods of mine waste disposal. In consideration of the current limited understanding of the fundamental mechanical response, a systematic laboratory triaxial testing research program was undertaken on paste rock specimens prepared such that the tailings would “just fill” the void spaces between the coarse-particle skeleton. A new “slurry displacement” method was developed for reconstitution of saturated, uniform/homogeneous specimens of highly gap-graded paste rock for triaxial testing. Undrained cyclic triaxial tests indicated that reconstituted paste rock displayed “cyclic-mobility-type” strain development. Strain-softening accompanied by loss of shear strength did not manifest regardless of the applied cyclic stress ratio (CSR). The results suggest that the material is not likely to experience flow deformation under monotonic (static) and/or cyclic loading conditions at least up to the tested initial effective confining stress conditions of up to ≤400 kPa. The behaviour of paste rock was noted to be more similar to the behaviour of rock-only material than that of tailings-only material indicating that the rock skeleton mostly controls the shear resistance in “just filled” paste rock. This finding is in accord with the behaviour of paste rock observed from one-dimensional consolidation tests. In relative terms, paste rock has a higher potential for strain development under a given cyclic stress ratio and number of load cycles in comparison to tailings-only and rock-only materials. The presence of tailings in the pore space between the rock particles appears to decrease the ability of the rock particles to engage contact and develop inter-particle stresses in comparison to the case with rock-only material.

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Master's Student Supervision (2010-2017)
Monotonic and cyclic shear response of reconstituted natural silt (2016)

A triaxial apparatus was upgraded and a specimen preparation device was developed to enable monotonic and cyclic triaxial testing of low plastic reconstituted silts. The silt reconstitution technique involves consolidating silt slurry inside a cylindrical split mold, directly on the triaxial base pedestal. The slurry is carefully poured into the split mold using a flexible hose. A vertical load is then applied to slurry using a top cap and loading ram. Loading is applied in an incremental manner and the slurry is allowed to consolidate, creating a specimen firm enough to carry on with triaxial testing. The newly developed silt reconstitution device was verified with respect to specimen uniformity, saturation and test repeatability. Using the new triaxial apparatus and silt reconstitution device, the monotonic and cyclic shear response of Kamloops silt was investigated, contributing to the understanding of the material behaviour of relatively low plastic silt. Silt specimens, initially hydrostatically consolidated to various stress levels, displayed cyclic mobility type strain development during both monotonic and cyclic loading. The specimen preparation technique was capable of producing laboratory test specimens having Skempton’s B values of greater than 0.98, indicating a high level of saturation of prepared specimens. The undrained shear strength measured in undrained monotonic triaxial extension was found to be 20% lower than the undrained shear strength measured in monotonic triaxial compression. This difference is in accord with the stress-path dependency typically found in gravity deposited sediments, and is considered to be due to the anisotropic soil fabric.Liquefaction in the form of strain softening accompanied by loss of shear strength did not manifest in the reconstituted Kamloops silt regardless of the applied cyclic stress ratio (CSR). The cyclic shear resistance of the material was found to be relatively insensitive to the applied confining stress level. The cyclic mobility type stress-strain behaviour was observed in spite of the initial static shear stress bias. The potential for excess pore water pressure generation was observed to decrease significantly with increasing level of initial static shear.

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Monotonic and cyclic shear loading response of natural silts (2015)

An experimental research program comprising constant-volume direct simple shear (DSS) tests was conducted to study the monotonic, cyclic shear and post cyclic consolidation response of natural silts. Relatively undisturbed samples of silt which were obtained from three different locations in the Lower Mainland area of British Columbia were used for this purpose. Plasticity indices of the natural silt samples which were considered for the study were 5, 7, and 34. Monotonic shear response of the natural silt was studied with the constant volume DSS test results that were conducted with different vertical effective stresses and different overconsolidation ratios (OCRs). Stress-strain response of normally consolidated silt at different consolidation stresses were found to be stress-history-normalizable where as higher OCR and higher plasticity resulted greater shear strength. Normally consolidated silt specimen, despite of their difference plasticity, exhibit gradual strain accumulation without abrupt loss of shear stiffness during cyclic loading with different cyclic stress ratios (CSRs) at different consolidation stress levels. The potential and rate of strain accumulation and development of excess pore-water pressure (Δu) were noted to be increased with higher CSRs at all tested consolidation stress levels. The cyclic shear resistances of silt, derived from cyclic direct simple shear (CDSS) tests, were not sensitive to the tested range of different consolidation stress levels, whereas higher plasticity resulted greater cyclic shear resistance. Relative undisturbed specimens exhibit comparatively higher cyclic shear resistance than the reconstituted specimens despite of comparatively denser particle arrangement in reconstituted specimens. However, during the constant-volume monotonic DSS tests, relative undisturbed specimens exhibit comparatively lesser shear resistance than the reconstituted specimens implying that soil fabric / microstructure plays a significant role in governing the shear loading response of silt. The examination of consolidation responses of silt specimens that were initially normally consolidated and subjected to constant-volume CDSS loading revealed that the post cyclic consolidation volumetric strain increases with the maximum cyclic pore-water pressure ratio developed during constant volume CDSS loading for all tested silt specimens with different plasticity.

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A field investigation to study the response of buried polyethylene natural gas pipelines subjected to ground movement (2014)

The performance of buried natural gas pipelines located in areas prone to ground movement is a major concern for utility owners since the failures of such pipeline systems during service is extremely serious due to the potential for loss of life, as well as the associated environmental and economical impacts. With plastic pipes now the industry standard for most utility distribution systems (e.g., medium density polyethylene (MDPE) pipes for natural gas distribution), understanding the response of these extensible pipes when subjected to ground movements is an important consideration and critical for their integrity.Through previous research work conducted at the University of British Columbia (UBC) on the subject of extensible natural gas pipelines subject to relative ground movements, a new analytical model was developed to account for the soil-pipe interaction mechanisms for buried MDPE pipes. The new approach can be used to estimate the relative ground surface movements needed for pipe failure, which is an important consideration for evaluating the field-performance of pipe systems in areas prone to landslide movements.In order to further validate the new analytical model, a large-scale field-testing program was implemented that consists of five MDPE pipeline alignments buried at a site which is part of a slow-moving landslide. The pipelines were instrumented with over 200 strain gauges that provide pipe strain data induced due to continuing ground movements at the research site. Along with the pipe strain data, close monitoring of the system for overall pipe and ground surface movements is ongoing, and the collected information is expected to provide a reliable database of ground movement and associated pipe strain to further validate the new analytical model.Laboratory element-level testing was conducted to investigate the effects of strain gauge stiffening on local strain readings on the MDPE pipes used in this study. The results indicate that the strain gauge installation procedures used throughout this research have minimal stiffening effects on the pipes.In addition to implementing the field experiment, a framework for using the field data to predict the axial pipe strain for the pipes in this study using the new UBC model is presented.

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An experimental investigation of thermal effects on the axial resistance to relative ground movement of buried district heating pipes (2014)

District heating (DH) systems are commonly used in urban areas to distribute thermal energy from central heat sources. Buried pipes, with a composite cross-sectional construction, are used to transport a heated medium, usually water. These pipes expand and contract radially and axially due to changing water temperatures, invoking soil-pipe interaction situations during operation, and potentially leading to significant pipeline material strains. Measures to account for these soil-pipe interactions are an important consideration and a significant cost factor when designing and installing robust and cost-effective DH pipe systems.A series of full-scale tests were undertaken to provide experimental data on the axial and lateral soil resistance of DH pipes. An existing soil chamber that is part of the Advanced Soil Pipe Interaction Research™ (ASPIRe™) facility at the The University of British Columbia (UBC) was adapted to test full-size water-filled pipes. As a part of this project, a heating system was developed specifically to apply different heating histories to the water mass before the pipe is pulled. Strain gauges were mounted on the pipe at the soil interface to contribute to understanding the mechanisms involved in soil-pipe interaction.It was shown that changes in the temperature of the water mass have a significant influence on axial pullout resistance of the DH pipe. After heating the water mass by ∆T = 50 °C, large-strain resistance increased by roughly 15 % compared to the control tests. Three full cooling and heating cycles reduced the axial soil resistance of the pipe, potentially due to an arching mechanism in the soil.Considerable strain was measured at the soil-pipe interface both in axial and radial direction during heating of the water mass. Based on the development of strain with the heating history, it was inferred that the expansions at the pipe surface result from a combination of strains from both the steel pipe at the core and the high-density polyethylene (HDPE) cover. Consequently, DH pipes have to be treated as a complete system in combination with the surrounding soil mass in order to accurately model their mechanical behaviour under thermal load.

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Monotonic and cyclic shear loading response of fine-grained gold tailings (2014)

The monotonic, cyclic and post-cyclic shear response of gold tailings was investigated using constant-volume direct simple shear test device. The reconstituted gold tailings specimens normally consolidated to vertical effective stress levels ranging from 50 kPa to 400 kPa initially exhibited contractive behaviour followed by a dilative response under monotonic loading, with their shear stiffness and strength increasing with increasing initial effective confining stress. Overconsolidated specimens developed negative excess pore pressures during monotonic shear, with increasing dilative response, shear resistance, and stiffness displayed with increasing overconsolidation ratio (OCR). Overall, the monotonic behaviour of normally consolidated reconstituted gold tailings specimens is similar to the typical monotonic behaviour of normally consolidated clays and low-plastic silts; similarly, the behaviour of overconsolidated reconstituted gold tailings specimens is similar to the typical monotonic behaviour of overconsolidated clays. During cyclic loading, the tailings exhibited cumulative decrease in effective stress (or increase in equivalent excess pore-water pressure) with increasing number of loading cycles, resulting in progressive degradation of shear stiffness. The cyclic shear resistance increased with increasing OCR. The findings on the cyclic shear response of normally consolidated reconstituted gold tailings are in general agreement with those available published data on the cyclic response of different tailings, obtained from tests carried out on cyclic triaxial (TX) and DSS devices. The CRR of the gold tailings from this study, however, was found to be higher than that observed in Fraser river sand and Quartz rock powder, but in the same range as natural Fraser river silt. The post-cyclic monotonic shearing response, obtained from DSS tests, carried out on normally consolidated and overconsolidated reconstituted gold tailings specimens was also studied as a part of the current research work. The post-cyclic shear strength of normally and overconsolidated specimens, normalized to the initial effective confining stress, were observed to increase with increasing OCR. The post-cyclic consolidation volume changes experienced by the gold tailings specimens were in agreement with previously published results suggesting that post-cyclic volumetric strains would increase with increasing maximum excess pore water pressure ratio developed during cyclic loading.

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Macro-scale direct shear device for studying the large displacement shear strength of soil-structure interfaces under very low effective normal stresses. (2013)

This thesis describes a new macro-scale test device for assessing the large-displacementsoil/solid interface shear strength at very low effective normal stresses (3 kPa to 6kPa). The testing method arises from a need to obtain the interface friction betweensoils and epoxy-coated pipes under low effective normal stress levels which is animportant consideration in the design of partly buried seabed pipelines. The testdevice is fundamentally similar to the conventional small-scale direct-shear apparatusexcept for its large footprint that provides a plan interface shear area of 1.72 m by1.75 m. The device is designed to impart displacement-controlled interface-shearingat displacement rates ranging from 0.0001 mm/s to 1 mm/s and with the ability toreach a maximum interface shear displacement of 1.2 m. The desired normal stress atthe soil/solid interface is obtained using surcharge loads externally applied by meansof bulk sand or water masses, or both in certain cases. The device is instrumentedwith pressure transducers mounted flush with the top surface of the solid test surfacefor the measurement of pore water pressure at the shear interface, in turn, allowingaccurate determination of the effective normal stress at the soil/solid interface duringshearing. The key features of this device are described, and the device capabilities aredemonstrated by testing three soil types (Fraser-River sand, non-plastic silt, kaolinite)on two test surfaces (mild steel, epoxy-coated mild steel) at effective normal stressesbetween 3 kPa and 7 kPa. Comparison of the test results with available findings fromother devices is used to further confirm the suitability of the device for the intendedpurpose.

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Effect of particle fabric on the one-dimensional compression response of Fraser River sand (2010)

One-dimensional compression of sand with lateral stress measurement allows for laboratory determination of the coefficient of lateral pressure at rest, Ko. Commonly used to define the initial state of stress in soil where no lateral strain occurs, Ko is calculated as the ratio of horizontal to vertical effective stress. The present study aims to investigate the role of initial particle fabric in one-dimensional compression and to determine the effect of fabric on the coefficient of lateral pressure at rest in Fraser River sand. One-dimensional compression with lateral stress measurement was carried out on reconstituted Fraser River sand specimens using an instrumented oedometer. Laboratory specimen reconstitution methods were developed in order to construct different particle fabrics. Three different techniques were utilized: air pluviation, tamping and vibration. In addition, the effects of initial relative density and loading history on the compression response were evaluated. Each one-dimensional compression test was executed in three distinct phases: virgin loading, unloading and reloading. The key results from the testing program were compared with current methods available for estimation of Ko. The results from the present study show that specimens resulting from different laboratory reconstitution methods (i.e., initial particle fabrics) exhibit different one-dimensional compression responses. For Fraser River sand in one-dimensional compression, air-pluviated specimens yield the highest Ko values, tamped specimens produce the lowest Ko values and vibrated specimens rank intermediate. With increasing initial relative density, regardless of the initial specimen preparation method, the measured Ko values generally decrease. Upon reloading, measured Ko values are slightly reduced from those observed during virgin loading. Furthermore, results from the present study indicate that the current methods commonly used for determination of Ko do not necessarily provide suitable estimations for variable granular particle fabrics arising from different specimen reconstitution techniques. A new method for determination of Ko is proposed, as a function of the constant-volume friction angle, initial relative density and a factor accounting for the initial particle fabric.

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