2020

Permanent URI for this collectionhttps://hdl.handle.net/1807/98907

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Reliability based formulation of building vulnerability to debris flow impacts
(Canadian Science Publishing, 2020-11-21) Luo, Hongyu; Zhang, L.M.; He, Jian; Yin, Kesheng
Physical building vulnerability to debris flows is defined as the potential damage degree of buildings for a given debris-flow intensity. In this paper, the physical characteristics on both debris flow intensity and building response are considered. Uncertainties in building capacity and debris flow intensity are explicitly quantified to evaluate the damage probability of a typical reinforced concrete building subject to debris flow impact. Four damage states with clear failure mechanisms are defined using multi-source information from field observations, numerical simulation and expert experience. Two series of fragility models have been proposed based on practical debris-flow impact pressure models. Several debris flow intensity measures are investigated. A better indication can be provided using the intensity measure that represents specific failure mechanism, for example, impact force (hv2) for force-dominated failures or overturning moment (h2v2) for moment dominated failures, where h and v are debris flow depth and velocity, respectively. The corresponding fragility surfaces best express the potential building damage. The intensity thresholds in the proposed fragility curves are consistent with those in empirical vulnerability curves. The methodology presented in this paper promotes the vulnerability assessment using physics based modeling, leading to a more reliable evaluation of building damage caused by debris flows.
Large-scale model tests of a single pile and two-pile groups for an offshore platform in sand
(Canadian Science Publishing, 2020-12-06) Foglia, Aligi; Abdel-Rahman, Khalid; Wisotzki, Elmar; Quiroz, Tulio; Achmus, Martin
Estimating pile group efficiency for open-ended steel piles in small group arrangements is a challenging task for designers. This paper reports on the large-scale experimental campaign performed for the BorWin gamma offshore converter platform, which involved single piles and two-pile group systems on a scale of 1:10. The experimental works included installation, dynamic end-of-driving tests, dynamic restrike tests, and static load tests of a single pile and a pair of two-pile groups in densely compacted, artificially prepared homogeneous sand. The CPT profiles and the blow counts confirmed that the foundation systems are comparable to each other. The experimental results of the single pile system were compared with conventional design methods. Such comparison indicated that CPT-based methods and load-transfer methods are applicable at the considered model scale. The bearing capacity prediction obtained via the CAPWAP method is conservative with respect to the static capacity. A consistent setup effect can be detected by analyzing the complete dynamic loading session. The pile group efficiency for the given foundation system was found to be less than 1.0 at both very small and very large soil strains, while it equaled 1.0 at failure.
Relationship between monotonic and cyclic behavior of saturated soft clay in undrained triaxial compression tests
(Canadian Science Publishing, 2020-07-12) Wu, Tingyu; Han, Jie; Cai, Yuanqiang; Guo, Lin; Wang, Jun
Cyclic loading-induced deformation of soil is a common problem in the engineering practice. In the current practice, however, monotonic triaxial tests are more commonly used in the practice, due to the availability of apparatus and ease of operation. Thus, it will be very useful and practical if the monotonic triaxial tests can be used to evaluate the behavior of soil under cyclic loading. This study aims to find an explicit relationship between monotonic and cyclic behavior of saturated soft clay. Six monotonic and nine cyclic triaxial compression tests were conducted on undisturbed saturated soft clay under an undrained condition. The test results showed that the monotonic and cyclic tests shared the same stress-strain surface in a three-dimensional space p^'-q-ε_a. It is also found possible to evaluate the effective stress states of cyclic tests at two specific numbers of cycles, using corresponding monotonic tests. Based on these two findings, a simple procedure was then proposed to predict the peak axial strain for the saturated soft clay under different cyclic loadings based on the monotonic tests and only one cyclic test, which was further verified against more test data from the previous literature.
The impact of scour on the lateral resistance of wind turbine monopiles: an experimental study
(Canadian Science Publishing, 2020-12-02) Li, Qiang; Askarinejad, Amin; Gavin, Kenneth
The majority of offshore wind structures are supported on large-diameter, rigid monopile foundations. These piles may be subjected to scour due to the waves and currents that causes a loss of soil support and consequently decreases the pile capacity and system stiffness. The results of numerical models suggest that the shape of the scour-hole affects the magnitude of pile capacity loss, however, there is a dearth of experimental test data that quantify this effect. This paper presents a series of centrifuge model tests on an instrumented model pile that investigates the effects of scour-hole geometry on the response of a laterally loaded pile embedded in sand. The pile instrumentation allowed load-displacement and p-y (soil reaction-displacement) curves to be derived. Three scour geometries (global, local wide and local narrow) and three scour depths (1D, 1.5D and 2D; where D is pile diameter) were modelled. For all three scour types, pile moment capacity decreased almost linearly with increase of scour depth. Simple empirical relations were proposed to evaluate the detrimental influence of scour on the pile moment capacity. A new method has been developed to allow designers to quantify the effect of scour-hole shape and severity of scour on the pile response.
A non-associative macro-element model for vertical plate anchors in clay
(Canadian Science Publishing, 2020-12-02) Peccin da Silva, Anderson; Diambra, Andrea; Karamitros, Dimitrios K.; Chow, Shiao Huey
This work proposes a new plastic hardening, non-associative macro-element model to predict the behaviour of anchors in clay for floating offshore structures during keying and up to the peak load. Building on available models for anchors, a non-associated plastic potential is introduced to improve prediction of anchor trajectory and loss of embedment at peak conditions for a large range of padeye offsets and different pull-out directions. The proposed model also includes a displacement-hardening rule to simulate the force and displacement mobilisation at the early stages of the keying process. The model is challenged and validated against different sets of numerical and centrifuge data. This extensive validation process revealed that two of the four newly introduced model parameters assume a constant value for the range of simulated cases. This suggests that only two of the newly introduced parameters may need to be calibrated for the use of the proposed macro-element model in practice.
Selection of Standard Penetration Test Number for Geotechnical Investigation of a Vertical Cross-section Considering Spatial Variability and Correlation in Soil Properties
(Canadian Science Publishing, 2020-12-25) Guan, Zheng; Wang, Yu
Standard Penetration test (SPT) is a widely used in-situ test for characterizing variation of subsurface soil properties, and results of site investigation are usually simplified as a 2D vertical cross-section for subsequent geotechnical design and construction. Current geotechnical design codes and guidelines only provide general recommendations for selection of an appropriate number of in-situ tests (e.g., SPT) (e.g., the greater variability of subsurface conditions, the larger number of SPTs required to obtain sufficient underground information). No quantitative or rational method is available for selecting the appropriate number of SPTs considering spatial variability and correlation in subsurface conditions. A comprehensive parametric study is carried out in this study to investigate the influence of spatial variability in subsurface conditions on the minimum SPT number needed for satisfying an accuracy requirement of site investigation. Random field is adopted to model spatial variation and correlation in soil properties in typical site conditions, and 2D Bayesian compressive sampling is used to interpolate sparse SPT data. Based on the parametric study results, a statistical chart is developed for geotechnical engineers to conveniently select appropriate number of SPTs in a vertical cross-section. Real SPT data from New Zealand are used to illustrate and validate the proposed method.
The mechanisms underlying long-term shaft resistance enhancement of energy pile in clays
(Canadian Science Publishing, 2020-06-26) Yazdani, Saeed; Helwany, Sam; Olgun, Guney
Although there are several studies indicating that heating increases the long-term shaft resistance of energy piles, the mechanisms by which heating causes this increase have not been adequately evaluated yet. This article aims to perform comprehensive analysis and discussion to assess the important factors contributing to this increase by integrating the findings from three recently published papers studying the thermo-mechanical behavior of clay and clay-pile interface. In these three studies, reconstituted kaolin clay was used, and cyclic and monotonic heat ranging between 24 C and 34C were applied to the clay and interface. The interface was sheared under two stiffness boundary conditions; Constant Normal Stiffness (CNS) and Constant Normal Load (CNL), where the normal stresses varied between 100 kPa and 300 kPa. The analysis performed in this article reveals that the increase in strength of interface under CNL condition is primarily attributed to clay stiffening at interface. However, the increase in shaft resistance under CNS condition is primarily attributed to the heating-induced increase of effective lateral stress, although clay stiffening at interface also partially contributes to the total increase of shaft resistance.
Generalized effective stress concept for saturated active clays
(Canadian Science Publishing, 2020-12-08) Tuttolomondo, Angelica; Ferrari, Alessio; Laloui, Lyesse
Experimental evidence shows that changes in pore water chemistry can significantly affect the mechanical behavior of saturated active clays. Despite this evidence, how the chemical composition of the pore water can be considered in effective stress definition is questionable. This paper develops the concept of generalized effective stress for active clays. To this end, physicochemical studies on water–clay mineral interactions are used to clearly define the different types of ions and water present in an active clay. In particular, the presence of both movable and non-movable ions within the liquid water is highlighted. Taking this into account, thermodynamic and geochemistry principles are applied to the representative elementary volume scale for determining the pore water pressure and redefine the effective stress accordingly. The theoretical development results in the dependence of the effective stress on the pore water chemistry through the effective solute suction variable. Equations for the determination of this chemical variable are developed. The implications of the use of the proposed effective stress concept are investigated using experimental results taken from the literature. The results show advantages both in the interpretation of shear strength and volumetric data, and all support the theoretical explanation underlying the proposed concept.
Flow liquefaction potential of loose sand: stress path envelope and energy–based evaluation
(Canadian Science Publishing, 2020-12-22) Pan, Kun; Yang, Zhongxuan; Cai, Yuanqiang
Flow liquefaction, which is characterized by sudden collapse following the unstable behavior of saturated loose sand, may lead to the most catastrophic consequence of all liquefaction–related phenomena. This note presents a systematic experimental investigation into the flow liquefaction potential of sand under various initial and cyclic shear conditions. The cyclic flow liquefaction responses are compared to the monotonic shear results under an identical initial testing condition. It is found that the effective stress path of a monotonic test appears to envelop that of its corresponding cyclic test. The energy–based liquefaction potential evaluation indicates that the accumulative dissipated energy is uniquely correlated not only with the pore pressure and axial strain induced in sand, but also with the degraded stiffness during cyclic loading. Furthermore, the energy capacity for triggering the flow liquefaction appears to be intimately related to the cyclic resistance of sand; this signifies the potential applicability of energy–based liquefaction potential evaluation using strength data available in conventional analysis.
Hydrogeology of a complex Champlain Sea deposit (Quebec, Canada): Implications for slope stability
(Canadian Science Publishing, 2020-10-17) Germain, Alexandra; Young, Nathan Lee; Lemieux, Jean-Michel; Locat, A.; Delottier, Hugo; Fortier, Philippe; Leroueil, Serge; Locat, Pascal; Demers, Denis; Locat, Jacques; Cloutier, Catherine
The thick sequences of marine clayey deposits which blanket the St. Lawrence Lowlands in south-eastern Canada are highly susceptible to landslides. With 89% of the population of the Province of Quebec living in this region, improving our understanding of the mechanisms causing landslides in these sediments is a matter of public security. To accomplish this goal, instruments were deployed at a field site in Sainte-Anne-de-la-Prade, Quebec, Canada to monitor atmospheric, soil, and groundwater conditions. Field and laboratory measurements of soil geotechnical and hydraulic properties were also performed. Results indicate that the groundwater and pore pressure dynamics at the site cannot be explained using simplified site conceptual models. Further analysis indicates that groundwater dynamics and pore pressures in the massive clay deposits on-site are determined by (i) the highly-heterogeneous nature of the local geological materials (ii) the contrasting hydraulic and geotechnical properties of these materials, (iii) the presence of two unconfined aquifers at the site, one surficial and one at depth, and (iv), the presence of the Sainte-Anne River. These results were used to create a new conceptual model which illustrates the complex groundwater flow system present on site, and shows the importance of including hydrogeologic context in slope stability analysis.
Towards rational use of baffle arrays on sloped and horizontal terrain for filtering boulders
(Canadian Science Publishing, 2020-11-25) Goodwin, Saoirse Robin; Choi, Clarence E.; Yune, Chan-Young
Baffle arrays are used to filter boulders from granular flows, such that the impact load exerted on barriers is reduced. However, current guidelines provide limited recommendations on baffle design. In this study, a calibrated Discrete Element Method modelled boulders entrained in a bulk granular assembly interacting with baffles and a terminal rigid barrier. Different baffle spacings relative to the boulder diameter (1
NORMALLY LOADED INCLINED STRIP ANCHORS IN COHESIONLESS SOIL
(Canadian Science Publishing, 2020-11-18) Al Hakeem, Nabil; Aubeny, Charles
Plate anchors are among the most effective anchorage systems that are widely used to resist horizontal and inclined uplift loads in many offshore and onshore applications. Previous research on plate anchors has largely focused on the horizontal or vertical breakout problems, with limited attention directed towards obtaining a full characterization of the effects of anchor orientation angle. The present study utilizes displacement-based finite element analyses to investigate the stability and performance of strip anchor embedded in cohesionless soil for plate inclination angles ranging from 0o to 90o from horizontal, where the applied load is normal to and acts at the center of the plate. This study investigates the effects of scale and roughness, along with the geometry of the failure mechanism for various plate orientations and embedment depths. The analyses, presented in terms of a non-dimensional breakout factor Nq, show that the breakout factor increases significantly with an increase in the inclination, especially for angles greater than 45 degrees in loose sand and greater than 60 in dense sand. The analyses also show that scale effects (anchor width) can affect capacity. Finite element analyses have been used to introduce simple design charts relating the breakout factor to the embedment depth and relative density. Comparisons to experimental and numerical studies showed good agreement.
Experimental and Theoretical Investigations on the Fin Configuration Effects of Dynamically Installed Anchors in Clay
(Canadian Science Publishing, 2020-07-23) Fu, Yong; Liu, Yong
Dynamically installed anchor (DIA) for deepwater mooring is an innovative and cost-effective technology in offshore foundation engineering; however, research on the effects of fin configuration of DIAs is limited. This study therefore systematically examines the effects of fin length-width ratio, fin surface area, fin position, and fin shape on the embedment depth and time-dependent holding capacity of DIAs by experimental and theoretical approaches. First, a series of centrifuge tests was performed on four DIA models with an identical fin surface area but different fin configurations. It is found that the DIA model with short-wide-rectangular fins which are installed at the rear of the shaft achieves deeper embedment depth and higher holding capacity; whereas the rectangular- and elliptical-shaped DIA models are similar in terms of embedment and capacity. This motivates the proposal for another nine types of finned DIAs in a more systematic manner. Furthermore, the performance of embedment depth and time-dependent holding capacity for these nine DIAs was theoretically assessed using the well-established theoretical methods. In this study, two scenarios with identical impact velocity or embedment depth were examined; wherein the variation of holding capacity with reconsolidation time was explicitly accounted for. Based on the findings, practical implications of fin configurations were arrived which may benefit the design of finned DIAs in practice.
Geotechnical Centrifuge Modelling of Retrogressive Sensitive Clay Landslides
(Canadian Science Publishing, 2020-11-01) Kennedy, Ray; Take, W. Andy; Siemens, Gregory A.
Sensitive clay landslides represent a significant geohazard due to their well-known potential for extensive retrogressive failures, on the scale of multiple hectares, which could encompass surrounding communities and infrastructure. Interpretation of retrogression mechanisms is often limited since only forensic investigations are possible. This work presents the results of a physical modelling study to examine retrogressive failures, analysis of each failure episode, and interpretation of the results using published relationships. Five novel centrifuge model tests were conducted under a defined range of undrained shear strength and slope angle conditions. The models are constructed of a sensitive cement-soil mixture that allows for a consistent contractile material with bespoke shear strength. Results indicate the observed retrogression distance correlates with Taylor’s stability number. The addition of a 5-degree slope angle to invoke a static shear stress on the model provoked notably larger retrogression distances. Post-test undrained shear strength measurements quantified softening of the material along the failure surface. Stability analyses on each failure episode captured the observed failure geometry and factor of safety. Results indicate that the geometric parameters of a slope, specifically the slope angle, may be able to explain a component of the scatter for relating the Taylor’s stability number with retrogression distances.
Undrained Cylindrical and Spherical Cavity Expansion in Elastic-Viscoplastic Soils
(Canadian Science Publishing, 2020-12-01) Zhou, Hang; Wang , Zengliang; Liu, Hanlong; Shen, Hang; Ding, Xuanming
Various undrained cavity expansion solutions for elastic-plastic soil have been proposed previously. However, no solution has been presented for elastic-viscoplastic (EVP) soil until now. This paper presents a general solution method for solving the classical one-dimensional (1D) boundary value problem (BVP) for undrained cylindrical or spherical cavity expansion in EVP soil with an emphasis on the rate effect of soil. The solution method is summarized as three standard procedures: (a) obtaining the soil displacement and strain under incompressible conditions; (b) calculating the effective stress of soil through a suitable constitutive law; and (c) obtaining the pore pressure by numerically solving the stress equilibrium equation through the finite difference method (FDM) or other numerical solution techniques. The numerical algorithms for calculating the effective stress and pore pressure are very simple without any complex iteration processes, and they require little calculation time but provide high computational accuracy. In addition, some numerical results are given to investigate the influence of the cavity expansion velocity on the cavity expansion response. The proposed solution procedure is general and can be applied not only for the EVP model but also for other plasticity models, and the given EVP solution can be applied to interpret the rate effect of the CPT test and pile penetration.
Model for predicting the variation of shear stress in unsaturated soils during strain-softening
(Canadian Science Publishing, 2020-11-16) Yang, Xiuhan; Vanapalli, Sai
Several of the geotechnical structures constructed with unsaturated soils undergo a large deformation prior to reaching failure conditions (e.g. progressive failure of a soil slope). During this process, the shear stress in soils typically increases initially and then reduces with an increase in the shear strain. The prediction of the stress-strain relationship is critical for reasonable interpretation of the mechanical behavior of those geo-structures that undergo large deformation. This paper introduces a model based on the disturbed state concept (DSC) to predict the variation of shear stress in unsaturated soils during strain-softening process under consolidated drained triaxial compression condition. In this model, the apparent stress-strain relationship is formulated as a weighted average of a hyperbolic hardening response extending the pre-peak state stress-strain curve and a linear response extending the critical state stress-strain curve with an assumed disturbance function as the weight. The prediction procedure is described in detail and the proposed model is validated using several sets of published data on unsaturated soils varying from coarse- to fine-grained soils. Finally, a comprehensive error analysis is undertaken based on an index of agreement approach.
Suitability of empirical equations for estimating permanent settlement of railway foundation materials subject to cyclic loading with principal stress rotation
(Canadian Science Publishing, 2020-10-30) Mamou, Anna; Powrie, William; Clayton, C.R.I.; Priest, Jeffrey A.
This paper uses the results of a series of laboratory tests with cyclic principal stress rotation to assess the suitability of a number of empirical equations for estimating the development of plastic settlements in railway track foundations. The laboratory tests were carried out on three sand-clay mixes representative of railway track foundation materials, in both free-to-drain and undrained conditions. The results of a non-linear regression analysis demonstrate that the drainage conditions are the key factor affecting the estimation accuracy of the models, with the clay content playing a secondary role. The correlation coefficient was generally higher in free-to-drain than in undrained conditions, and reduced slightly with increasing clay content.
Experimental investigation and constitutive modeling of the behaviour of highly plastic Lower Rhine Clay under monotonic and cyclic loading
(Canadian Science Publishing, 2020-10-24) Tafili, Merita; Wichtmann, Torsten; Triantafyllidis, Theodoros
A new experimental series on the highly plastic (I_P = 34 %) Lower Rhine Clay (LRC) is presented. The study comprises tests on normally as well as over consolidated samples under monotonic and cyclic loading. The loading velocity has been varied in order to evaluate the strain rate dependency of the LRC behaviour testifying i.a. the well-known reduction of undrained shear strength with decreasing displacement rate. Isotropic consolidation followed by a cyclic loading with constant deviatoric stress amplitude leads to a failure due to large strain amplitudes with eight-shaped effective stress paths in the final phase of the tests. The inherent anisotropy has been additionally evaluated using samples cut out in either the vertical or the horizontal direction. Furthermore, the behaviour of LRC is compared with the behaviour of low plastic Kaolin silt (I_P = 12:2 %). A new visco-hypoplastic-type constitutive model with a historiotropic yield surface has been used to simulate some of the experiments with cyclic loading. Even the eight-shaped stress loops at cyclic mobility are reproduced well with this model. The data of this paper can be also used by other researchers for the examination, calibration, improvement or development of constitutive models dedicated to fine-grained soils under monotonic and cyclic loading.
A coupled CFD-DEM investigation of internal erosion considering suspension flow
(Canadian Science Publishing, 2020-10-19) Liu, Yajing; Yin, Zhen-Yu; Wang, Li-Zhong; Hong, Yi
The influence of two-phase flows containing suspension particles, which are common in nature, on internal erosion with coupling effect of clogging remains unclear. This paper presents a three-dimensional coupled discrete element method and computational fluid dynamics (CFD-DEM) analysis of internal erosion considering different concentrations of suspension C (i.e., mass of the suspended particles in unit volume of fluid) in gap-graded granular soils with different fine fraction Fc (i.e., the percentage by mass of the fine particles in the gap-graded sample). The influences of C and Fc on the erosion and clogging behavior of soils are investigated from both the macroscopic and microscopic perspectives. It is found that for gap-graded samples being under-filled with Fc=15%, the suspension flow (i.e., influent fluid with suspending particles) decreases the cumulative eroded fine particle loss and the increasing rate of soil hydraulic conductivity due to clogging at the top of the sample. The degree of clogging is found to jointly be determined by both constriction size distribution and the suspension concentration. Clogging in a local area usually occurs with the formation of the clusters which has a high resistance to the drag force applied by the fluid flow.
Upper bound analysis of laterally loaded rigid monopiles in clay with linearly increasing strength
(Canadian Science Publishing, 2020-08-28) Yu, Jian; Wang, Hongyu; Huang, Maosong; Leung, Chun Fai
Monopiles supporting offshore wind farms are often subject to severe lateral environmental loads due to wind, waves, and currents. Previous studies reported various failure patterns for such rigid monopiles in clay; hence predicting lateral pile capacity of widely different magnitudes. In this study, both single-sided wedge failure mechanism involving passive soil failure only and two-sided wedge failure mechanism with simultaneous active and passive soil failures are proposed. The single wedge mechanism is found to be applicable if the soil behind the pile does not move together with pile resulting in a gap between the soil and the pile upon loading. On the other hand, the two-sided wedge is found appropriate for the soil behind the pile moves together with the pile upon loading. Two formulations are then derived from the two upper-bound failure mechanisms. The lateral pile capacity can be determined, and the corresponding failure mechanism identified based on the formulation which yields the lower capacity magnitude. In addition, the reliability of the formulations is verified against reported finite element methods as well as existing experimental and field test results.

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