R Jonathan Fannin

Professor

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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
Numerical modeling and analysis of pullout tests of sheet and geogrid inclusions in sand (2019)

One way of studying the soil-inclusion interaction in the pullout test is by numerical modeling. Several of the numerical studies available in the literature lack the integration of consistent material characterization as input for the numerical model, resulting in little phenomenological description of the soil-inclusion interface behavior. There is, therefore, a need for an improved evidence-based understanding of the factors influencing the pullout resistance of different inclusions. Accordingly, the main objective of this study was to capture the pullout response of different inclusions, for which extensive laboratory pullout test data existed, through a phenomenological numerical model that uses physically-based parameters. This numerical model is henceforth used in a parametric study to assess the adequacy of the laboratory test data in the literature and ASTM D6706-01 recommendations.The finite difference software FLAC was used to simulate the laboratory response of three sheet inclusions and three geogrids, embedded in a pullout box filled with a uniformly graded sand (Badger sand) and subjected to vertical stresses up to 17 kPa. In the numerical model, the inclusions were represented by an elastic continuum at the center of the pullout box. The sand was modeled using NorSand, a constitutive model that is able to capture the dilative behavior of dense sands. An alternative approach to the usual spring interface is proposed to model the soil-inclusion interaction, where a thin continuum layer following a NorSand behavior is used, and the friction angle changed according to the interface strength of each inclusion. The soil and interface parameters were obtained from a laboratory testing program on Badger sand including triaxial, direct shear and direct simple shear tests.The results of this dissertation yield three principal contributions: 1) plane strain conditions and a stress-dependency of the critical state friction angle prevail in the pullout box; 2) the use of a constitutive model that can simulate dilation to represent the soil-inclusion interface behavior is able to capture the complete pullout response of the different inclusions; and 3) different aspects of ASTM D6706-01 pullout recommendations deserve improvement for a correct interpretation of the soil-inclusion interaction factor.

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On the influence of effective stress and micro-structure on suffusion and suffosion (2015)

The research presented within this thesis covers the development of a flexible wall permeameter, and a parametric laboratory investigation of the factors influencing seepage-induced internal instability in gap-graded granular materials. A flexible wall permeameter comprising a double-walled triaxial cell, a seepage control system, and instrumentation, with a novel measurement of volume change, has been designed and built. The apparatus was successfully commissioned and the test procedure demonstrated to yield repeatable results. Two commissioning tests, 23 tests on eight glass beads gradations and 16 tests on ten soil gradations were conducted. All gap-graded gradations were reconstituted using the modified slurry deposition method, isotropically consolidated to a cell pressure between 50 and 150 kPa, and subsequently subject to upward multi-stage seepage flow. Analysis of the test results identifies two distinct seepage-induced internal instability phenomena. First, migration of fine particles from a soil, termed suffusion, is characterised by a mass loss without change in volume, or with a small non-progressive change in volume, accompanied by an increase of hydraulic conductivity. Second, local or overall collapse of the soil structure, termed suffosion, is characterised by a seepage-induced mass loss, accompanied by a reduction in volume and a change in hydraulic conductivity. It is demonstrated that measurement of total volume change is necessary to avoid any mis-interpretation of the phenomenological response to seepage flow. It was found that the differential pore water pressure at the onset of suffosion increases with increasing mean effective stress. The micro-structure of the specimen was found to influence the susceptibility to seepage-induced internal instability: the portion of non-load bearing fine particles appears a useful parameter to quantify the potential for suffusion, whereas the proposed state parameters are predictors of the relative susceptibility to suffosion. Although particle shape does not affect the suffusive response in a transitional clast-supported micro-structure, sub-angular particles are found to yield a transitional micro-structure that is more resistant to suffosion than a similar micro-structure of spherical particles. A unified approach is presented to characterise suffosion.

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An improved hydromechanical understanding of seepage-induced instability phenomena in soil (2014)

Internal instability describes phenomena that occur when a soil cannot prevent the loss of its own small particles in the presence of forces induced by seepage flow. A significant proportion of incidents and failures in water-retaining structures and their foundations are attributed to the consequences of internal instability. There is a concern that existing dam and canal infrastructure may be vulnerable to internal instability as a consequence of deficiencies attributed to the state-of-practice at the time of design. In order to manage infrastructure risk, there is a need for an improved science-based understanding of the state-of-art 'hydromechanical framework' that describes the interacting material, stress and hydraulic factors understood to govern internal instability.A series of seepage tests were undertaken on five gradations in a large permeameter. The objectives were to observe critical seepage-induced phenomena at the 'upper bound', and to verify the presence of an extreme 'lower bound' to the hydromechanical space. A modified slurry deposition technique was used to reconstitute saturated and homogeneous specimens, and a multi-stage seepage regime was found satisfactory to identify the critical hydromechanical condition. Phenomena of fluidization and hydraulic uplift were found to characterize internally stable behaviour at the 'upper bound' of hydromechanical instability. A stress-independent 'lower bound' was experimentally defined by tests on two very unstable suffusive soils. Existing geometric methods were evaluated and found to inadequately characterize material behaviour in widely-graded till materials: rather, the presence of plasticity was found to inhibit internal instability. The present study quantified three necessary conditions for internal instability in gap-gradations: (1) a theoretical porosity-based microstructure framework was adapted to identify the 'α ≈ 0' 'particle detachment' condition, (2) the critical seepage condition at the hydromechanical 'lower bound' was verified in terms of a critical seepage velocity, and (3) a novel constriction size criterion was proposed to assess the 'transportation potential' for a particle in a porous medium. It is concluded that the hydromechanical space is not fully characterized by the stress-reduction factor 'α' alone: the present study characterizes two distinct and necessary components of material susceptibility for gap-gradations: (1) particle detachment, and (2) transportation potential.

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A laboratory permeameter study of geotextile-soil retention in cyclic flow (2010)

In the absence of an extensive body of laboratory and field data, empirical criteria for soil retention in dynamic or cyclic flow are not yet well-defined with reference to a margin of safety. A performance-based approach is taken in this study: the method of investigation involves laboratory tests on a total of seven geotextiles (needle-punched nonwoven and woven materials) and a total of four uniformly-graded soils (non-plastic fine sand and coarse silt). Filtration compatibility in unidirectional and cyclic flow reversal is evaluated using two rigid-wall permeameters: a small bench-mounted device, and a large floor-mounted device. Analysis of the results addresses the effects of specimen size (small and large), sidewall friction and stress distribution, and examines the influence of filter ratio (AOS/Dn), hydraulic gradient (i) and confining stress (σʹ) over a range of cyclic flow reversal times or wave period (T).A novel analytical framework is proposed from the permeameter test results, to unify AOS/Dn and a hydromechanical index that accounts for the combined effect of hydraulic gradient and confining stress. The framework provides a distinction between the benign actions of mass loss through the geotextile by washout, in contrast to the more problematic action of piping. A filter ratio AOS/D₈₅ appears better-suited to interpretation of the data than AOS/D₅₀. The framework is used to examine the margin of safety inherent in current design guidance. Independent verification of the framework through comparison with other laboratory studies, and a consideration of field observations reported by others, leads to a recommendation that AOS/D₈₅ ≤ 1 to address undue conservatism in design guidance.

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Seepage induced instability in widely graded soils (2008)

Internal instability of a widely graded cohesionless soil refers to a phenomenon in which its finer particles migrate within the void network of its coarser particles, as a result of seepage flow. Onset of internal instability of a soil is governed by a combination of geometric and hydromechanical constraints. Much concern exists for embankment dams and levees built using soils with a potential for internal instability. Migration of finer particles to a boundary where they can exit, by washing out, may cause erosion or piping failure and, occasionally, induce collapse of these soil structures. There is a need, in professional practice, to better understand the phenomenon and to develop improved methods to evaluate the susceptibility of a soil. A series of permeameter tests was performed on six widely-graded cohesionless materials. The objectives are to assess the geometric indices proposed for evaluation of susceptibility, and examine hydromechanical factors influence the onset of internal instability. A modified slurry mixing technique, with discrete deposition, was found satisfactory for reconstitution of the homogeneous saturated test specimens. The onset of internal instability was founded to be triggered by a combination of effective stress and hydraulic gradient. The finding yields a hydromechanical envelope, unique for a particular gradation shape, at which internal instability initiated. Three commonly used geometric criteria were comprehensively evaluated with reference to these experimental data and also a database compiled from the literature. The relative conservatism of each criterion was examined and a modified semi-empirical geometric rule then proposed based on the capillary tube model. A theoretical framework for plotting the hydromechanical envelope was established based on an extension of the α concept of Skempton and Brogan, and subsequently verified by test data. Finally, a novel unified approach was proposed to assess the onset of internal instability, based on combining geometric and hydromechanical indices of a soil.

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Master's Student Supervision (2010 - 2018)
On the feasibility of mounting bender elements in a flexible wall permeameter (2018)

Zoned embankment dams comprise some of Canada’s largest and most important infrastructure. A phenomenon that has emerged in recent times as a matter of concern for dam safety risk management is seepage-induced internal erosion in embankment dams and their foundations. A candidate method of monitoring the progression of internal erosion in the field is that of crosshole shear wave velocity measurement. To inform the interpretation of shear wave velocity data, the relation between change in shear wave velocity and occurrence of internal erosion must be characterized.This study examines the feasibility of equipping a flexible wall permeameter (FWP) with bender elements in order to measure shear wave velocity. Bender elements were designed and fabricated with a custom mounting system and a FWP was upgraded to accommodate the bender elements. The study was devised to determine if the changes made to the FWP device, including a 6 % reduction in the cross-sectional area available for flow, affect the seepage regime through the test specimen or internal erosion of its finer fraction, from tests conducted on three different gradations of glass beads. Comparisons of tests performed with and without bender elements indicate no systematic change to the seepage regime or the internal erosional response of the specimens. Thus, the inclusion of bender elements in the flexible wall permeameter, for purposes of shear wave velocity measurements, appears feasible.

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A laboratory study of filter compatibility with implications for coursier dam (2016)

Modern filter criteria are routinely used in engineering practice for the design of filters in embankment dams. Although somewhat well-developed, these criteria are based on results from a variety of non-standardised test devices and methods, and are rarely validated by means of field data or full-scale testing. Furthermore, very little work has been done towards understanding how filters built before the advent of modern filter design may be assessed. To address this knowledge gap, the Continuing Erosion Filter (CEF) test and empirical criteria for the assessment of existing filters had been developed. Decommissioned in 2003, following a long history of sinkholes, piping and seepage-related incidents, Coursier Dam presents an excellent opportunity for study. CEF tests have been conducted on soils sampled at the dam site, to determine the material susceptibility to filter incompatibility. It is concluded that the lower core from Coursier Dam is susceptible to filter incompatibility where it is in contact with a stratum of the foundation, and that this filter incompatibility may explain the occurrence of sinkholes. The finding is supported by the results of a parametric study on soil from another dam site. Furthermore, it is found that CEF testing, in conjunction with the empirical criteria for filter assessment, provides useful insights into the phenomenon of base-filter compatibility.

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Observations on the mobilization of strength in reinforced soil structures (2014)

Confidence in the design of reinforced retaining structures is based on a limited body of experimental field data from performance monitoring of a relatively small number of case studies. In order to improve upon that confidence in design, this research addresses the back-analysis of the only field study involving a reinforced steep slope with independent measurements of tensile force and strain, which was first described by Fannin & Hermann (1990). Knowing the mobilized strength in the reinforcement, a back-analysis was performed using widely accepted design practice, with the purpose of establishing the mobilized angle of friction within the backfill soil of the structure. The mobilized friction angle was compared with the findings of laboratory shear strength tests in direct shear, triaxial and plane-strain conditions. The comparison provides further evidence in support of the expectation that plane-strain conditions prevail within the reinforced steep slope, and the recommendation in the British code of practice to use the peak friction angle for design. Additionally, visual inspection and index testing on exhumed geogrid samples from the structure described by Fannin & Hermann (1990) established that the geogrid has experienced no major physical damage, nor any significant degradation associated with durability of the polymer material. Moreover, rapid loading creep tests data show excellent agreement between exhumed and typical values, implying no significant durability degradation in the geogrid of the Skedsmo structure. Accordingly, isochronous load-strain-time data can be used with confidence for predicting the long-term strain of geogrid reinforced soil structures.

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An evaluation of base soil-filter compatibility using a triaxial permeameter (2013)

Filtration compatibility of base soil and granular filter materials must be addressed in the design of zoned engineered fill structures. The evolution of design practice governing filter compatibility is reviewed, with emphasis placed on the experimental studies that have made the greatest contribution to current guidelines. Design practice governing filter compatibility for cohesionless uniform materials has remained relatively unchanged for the last 70 years. A novel and improved triaxial permeameter is used to test base soil-filter grain size ratios (D₁₅/d₈₅) close to the limit of filter incompatibility. The configuration and operation of the test device are described. Thereafter, data are reported for select combinations of base soil-filter specimens of glass beads that are reconstituted, consolidated and subject to unidirectional seepage flow. Interpretation of the test results addresses the onset of filter incompatibility with reference to independent measurements of change in permeability of the two-layer system and mass loss of the base soil through the filter. A unified framework is presented for interpretation of filter incompatibility, taking into account the influence of stress and hydraulic gradient. The implications of the test results are analyzed and discussed with reference to a confident understanding of base-soil filter compatibility.

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Cement-treated soil : a comparison of laboratory and field data from Fountain slide remediation deep mixing project (2012)

In order to better understand the influence of laboratory reconstitution methods on the strength of cement-treated soil, a laboratory program was undertaken to investigate the unconfined compressive strength of cement-treated specimens reconstituted from low plasticity soils. The laboratory program examines two soil types and two reconstitution methods. The soil samples were taken from a Cutter Soil Mixer [CSM] field improvement site in British Columbia. Two reconstitution methods were used: a saturated wet-mixing method and an unsaturated dry-mixing method. To assess the relevance of using laboratory results to guide design, a subsequent field component of this research compares the strength of test specimens cast from field-mixed cement-treated soil, with the strength obtained from laboratory-reconstituted specimens.The strength of laboratory-reconstituted soil specimens is largely independent of the soil type and reconstitution method used. A standardized approach for determining cement content in uncured mixed soil-cement is evaluated. Results from the method allow for direct comparison between the strength of field-mixed versus laboratory-reconstituted specimens as a function of the cement content, and/or the water-cement ratio. Based on the simplicity of use and accuracy of results, it is recommended that the Heat of Neutralization method (ASTM 5982-07) be incorporated into the quality assurance program of deep mixing projects.

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A modified hole erosion test (HET-P) to study erosion characteristics of soil (2011)

Today’s increasing demand for energy and natural resources requires safe and reliable infrastructure. This includes hydraulic earth structures like dikes, levees, or dams. Such structures are susceptible to piping, a fundamental type of internal soil erosion. Piping is one of the principal causes of failures and accidents affecting embankment dams. The Hole Erosion Test (HET) is based on soil piping, and is used to determine the erodibility and critical shear stress of a soil. A soil specimen with a preformed axial hole is subjected to a constant-head pressure flow, and the rate of enlargement of the soil pipe is determined indirectly from flow rate and hydraulic gradient. This study presents a Modified Hole Erosion Test (HET-P) that introduces a conventional Pitot-static tube to measure total energy head and flow velocity of the exiting jet, which is correlated to a mean velocity within the axial hole. A series of Modified Hole Erosion Tests (HET-P) was performed on non-erodible PVC specimens with axial holes of constant, but different diameter, followed by HET-P tests on two types of soil, namely glacial till material of a dam core and natural clay deposits from Ontario river banks. Results confirmed that sidewall hydraulic head measurements to determine hydraulic gradients in the standard HET overestimate the resulting axial wall shear stress by as much as an order of magnitude. Furthermore, velocity measurements increase the confidence in test results as they allow for a more direct estimate of the axial hole diameter at any time during a test. A Pitot-static tube used in the HET-P for velocity and pressure measurement can easily be incorporated, and yields more transparent and reliable results by eliminating or amending some of the limiting assumptions of the standard test. It is an easy, fast, and economical approach that can be applied to soils in both constructed earth structures including dams and embankments, and to natural river banks to determine their susceptibility to internal and surface erosion.

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Empirical rules for debris flow travel distance : a comparison of field data (2010)

Assessment of debris flow travel distance is an essential part of landslide risk management. Confidence in applying empirical rules in conditions different from their original study area is generally small. This research provides a systematic, quantitative approach to assess the utility of an empirical-statistical tool (UBCDFLOW), originally developed for the Queen Charlotte Islands, in the Kootenay area with different geo-bio-climatic conditions.High quality ground-based debris flow volume measurements are essential for this research. A systematic traversing method is described and illustrated for two example events. It is particularly useful to analyse volume change processes (entrainment, deposition), quantify volume magnitudes and examine travel distances. Data gathered can be used to compare debris flow inventories and run a variety of empirical or dynamic analysis tools.Compared to the Queen Charlotte Islands (QCI) the Kootenay study area has different geo-bio-climatic conditions, with larger event magnitudes and travel distances. Similarities were found for dominant volume change processes as a function of slope angles. Average yield rates in the Kootenays (2 to 3m³/m) are significantly smaller than on the QCI (12 and 23 m³/m). Applicability of UBCDFLOW in the Kootenay area is evaluated based on three quantitative measures for simulation success regarding volume change process, magnitude and travel distance. The empirical rules capture the volume change process (entrainment, deposition) correctly for 80% of the lengths in the Kootenay inventory and therefore appear to be portable from the QCI location. However the regression equations overestimate the magnitude of volume change. In total, 15 out of 22 simulations terminate within 89 and 110% of the observed travel distance; the remaining 7 simulations exceed it by more than 110%.Concerns that UBCDFLOW is sensitive to variations in slope angle input (±2°) are addressed in a Monte Carlo type analysis. For 13 (of 22) events, simulation of process is found sensitive, and 11 (of 22) experience considerable uncertainty in volume estimation, which ultimately yields uncertainty of travel distance estimate. Based on confidence limits for volume estimates and travel distance exceedance probability, the Monte Carlo simulation output allows more informed decision making.

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