Friction Mobilization Research Articles

Comparison between the measured and the numerically predicted behaviour of an instrumented piled raft

This paper presents the results of numerical predictions for an instrumented piled raft foundation of a tall building in tropical soil. The analysis was performed using 3D finite element analysis and included a comparison of foundation settlements, pile loads, skin friction mobilisation, and raft-soil contact pressure. Initially, the geotechnical model was defined based on field and laboratory test data to conduct a Class A geotechnical prediction. Subsequently, the soil parameters used in the numerical model were re-evaluated to match the measured settlements, aiming for a Class C prediction. The study discusses the main factors that affect the accuracy of the computed settlements, providing valuable insights for foundation designers. Additionally, it highlights the significance of considering the pile-soil-pile and raft-soil-pile interactions in the design of the piles beneath the raft, as these interactions impact the mobilisation of the pile skin friction at different depths.

DEM study on the effect of particle shape on the shear behaviour of granular materials

This study investigates the effects of particle convexity, sphericity and aspect ratio (AR) on the behaviour of sheared granular materials using two-dimensional discrete element method simulations. Isotropic, dense and loose assemblies with different particle shapes were prepared and subjected to drained shearing via biaxial compression until the critical state was reached. Macroscopic characteristics such as strength and dilatancy are presented. The factors underlying the macroscopic behaviour are then investigated by considering the coordination number, fabric anisotropy, particle moment, friction mobilisation at contacts and particle rotation. For the range of shapes considered here, the data indicate that the shear strength decreases as particle convexity and sphericity increases while the shear strength increases with increasing AR. The shear strength and convexity are weakly correlated, however a stronger correlation is observed between AR and strength. The volumetric strain at large strains tends to increase with increasing AR. There is a stronger correlation between the critical state strength and both the critical state coordination number and the critical state mechanical void ratio than there is between the critical state void ratio and the critical state strength. The contact fabric anisotropy, the magnitude of the moment transmitted by particles and the friction mobilised at the contacts are important factors underlying strength. The critical state strength increases as both the mean particle moment and the mean mobilised friction increased. Analysis of particle rotation provides insights into the response of the granular materials to shearing.

Axial pullout behaviour of fusible PVC pipes and fusion weld joints assessed using distributed fibre optic sensors

Pipes are subjected to axial pulling forces that lead to shear stresses at the soil–pipe interface and axial strain distributions from various circumstances, including pulled-in-place pipe installation and permanent ground deformations. This study investigates the axial pullout response of fusible polyvinyl chloride pipes in dense sand. Three tests were conducted using a large-scale soil chamber to assess the development of axial resistance and the impact of non-continuous pulling and fusion weld joints. The measured maximum load response was significantly underpredicted by guidelines typically used in practice. Additionally, non-continuous pulling and changes to the pull rate had negligible impacts on the load–displacement response. Pipes were instrumented with distributed fibre optic sensors (DFOS) to capture the nonlinear development of strains along these flexible pipes and the mobilization of soil friction along the soil–pipe interface. Measurements have provided insight into the propagation of axial strains along the pipe, the additional longitudinal restraint provided by fusion weld joints, and their impact on the axial force distribution along pipes. The “locked-in” strains due to residual soil–pipe friction remaining after the axial pulling force is removed have also been measured using DFOS.

Moving with and against the state: digital nomads and frictional mobility regimes

The mobile lifestyle of digital nomads mingles remote work, international travel, and multi-local living in ways that both submit to and resist state-based mobility regimes. In this article, we examine this apparent paradox by asking how digital nomads move both with the state and against it. Employing the metaphor of ‘friction’, the analytical lens of ‘governmobility’ and ethnographic fieldwork with digital nomads, the article illustrates how nomads leverage state-imposed constraints into creative forms of ‘border artistry’ that allow them to achieve their lifestyle goals in the shadow of the state. At the same time, however, the article suggests that states are also border artists, an argument developed through an analysis of governments’ recently established special visa programs. The findings suggest that mobility regimes do not merely determine who can or cannot move, enter, or stay, but rather exercise a kind of governmobility that encourages mobile individuals to discipline themselves according to desirable qualities such as self-sufficiency, consumer citizenship, and depoliticised mobility. In this sense, mobility regimes emerge as the mutual interface between digital nomads’ individual strategies to stay on the move and states’ institutional strategies to codify and commodify their legal status.

A set of hyper-viscoplastic critical state models with different friction mobilisation criteria

The approach of thermodynamics with internal variables, known as hyperplasticity, is used to develop a set of hyper-viscoplastic clay models that comply with the critical state soil mechanics and isotache viscosity. Different friction criteria of Drucker-Prager, Mohr-Coulomb, and Matsuoka-Nakai have been considered, and their conjugate features in terms of friction mobilisation and inelastic flow direction are explored. One of the distinctions of the proposed hyper-viscoplastic models regarding the existing models is having a unique friction envelope at the critical state (the paradigm of critical state soil mechanics) while adopting versatile dynamic yield surfaces and inelastic flow rules. To achieve this importance, an emphasis has been put on the practice of the stress ratio tensor (the deviatoric stress tensor per the spherical effective stress) as an essential state variable of frictional material. Along the way, some consistencies with the results and insights of studies with the discrete element method (DEM) in the literature are reported and interpreted.

DEM study of microstructural effects in friction of wheat on corrugated steel surface

In the reported study, discrete element method simulations of a shearing process of wheat grains on galvanized corrugated sheet have been performed. Coefficients of grain-wall friction estimated from simulations were found in reasonable agreement with experimental coefficients. Mode of deformation observed on cross section of frozen experimental samples was properly reproduced in simulations. The simulations have shown that for lower normal load and shearing velocity the deformation takes place in the whole volume of the sample. For the highest velocity, material undergoes shear deformation (plastic behavior) in the layer adjacent to motionless cover, while the layer adjacent to moving corrugated surface remains stiff. Mobilization of friction increases with the deformation, resulting in increasing number of sliding contacts.

Load transfer analysis of vertically loaded bored piles in sea reclamation areas considering the effect of the pile-gravel interface roughness

A load transfer model for vertically loaded bored piles in sea reclamation areas with gravel is proposed in this paper. The focus is on quantifying the mobilization of pile-side friction, which is closely related to the shear behaviors of the pile-gravel interface. The surface profile of the gravel particles and the concrete piles are assumed to be circle and wave curves, respectively. The shear displacement and dilatancy calculation are based on the macro and micro geometry relationship between gravel particles and pile surface. The shear stress calculation of the pile-gravel interface is dealt with by empirical elastoplastic equations. A simple slip-line field is also built to identify the critical shear displacement. The finite difference equations are used to derive the load-displacement curves for vertically loaded bored piles in sea reclamation areas with gravel. The model predictions are compared with field observations and particle flow numerical simulation to validate the performance of the proposed method, and the results prove to be reasonable. Moreover, parameter studies are conducted and the results prove that the pile can bear more load when the particle diameter is similar to the pile's rough surface, and both the increases of wave angle and the relative friction coefficient can promote the bearing capacity.

Performance of inclined-vertical framed retaining wall for excavation in clay

A new strut-free retaining wall system, referred to as an inclined-vertical framed retaining wall (IVFRW), is constructed for excavation in clay. It has been successfully used in engineering practice to limit excavation-induced ground movement and protect adjacent buildings. This paper presents the performance of an excavation constructed by using IVFRW in Tianjin, China. Field investigations indicate that the proposed retaining structures effectively limit the wall deflection. Parametric studies based on the validated three-dimensional finite element model are conducted to improve the fundamental understanding of the mechanism of IVFRW. The influence of the inclined pile and vertical pile lengths on the deformation, internal forces, and wall friction are studied. A concept of neutral plane in retaining wall is proposed, and the mobilization of the skin friction and the axial force linking with earth pressure are analysed. Finally, a design concept for IVFRW is discussed.

Full scale study of pile response to EPBS tunnelling on a Grand Paris Express site

This article presents a full-scale experiment on the effect of EPBS tunnelling on pile foundations in the Paris area. Three cast in-situ piles, instrumented with strain gauges and optical fibre cables, were installed at different distances from tunnel axis with various pile base positions to monitor pile strains as the EPBS progresses. This paper investigates pile – soil differential settlements at ground surface, axial load variations, shaft friction mobilization, base load variations, bending moments and pile structural failure possibility. The pile behaviour changed depending on the pile position with respect to the tunnelling machine, with the main changes occurring during the passage of the EPBS beneath or at pile level. Estimation of pile behaviour based on simplified modelling was confirmed by the field monitoring.

Effect of various inserts on flow behavior of Fe2O3 beads in a three-dimensional silo subjected to cyclic discharge- Part II: Exploration of internal dynamic properties

This paper investigates internal dynamical properties of Fe2O3 beads in 3D silos subjected to cyclic discharge by using discrete element modeling. The effect of inserts on the internal dynamic properties was explored. Six types of inserts, including the absence of inserts for comparison, were adopted in this study: conical insert, disk insert, BINSERT, hollow cylinder insert and hollow silo insert. Results reveal that the installation of inserts affects wall tractions, solid volume fraction and coordination number, depending on insert geometry and placement position. The gradients in the wall tractions are larger in the hopper region than in the cylindrical region. The hollow cylinder and hollow silo inserts separate the granular flows, leading to smaller solid fractions and exhibiting more uneven distributions in coordination number. The silos with conical and disk inserts generally show larger coordination numbers than the other silos. The effect of inserts on friction mobilization is markedly limited.

Analytical solution for calculation of pull out force-deformation of geosynthetics reinforcing unsaturated soils

Performing tests on geosynthetic-reinforced soils under pullout conditions gives a comprehensive understanding of soil-geosynthetic interaction. However, they may be expensive and time-consuming. Therefore, the development of theoretical solutions may be beneficial to recognize characteristics of geosynthetic-reinforced soils under pullout conditions. This paper proposes an analytical solution for the determination of pullout force-deformation variation of planar geosynthetics reinforcing unsaturated cohesive-frictional soils. The solution predictions were validated with experimental results, demonstrating the capability of the presented solution. The solution can extend limited results obtained from experiments to a wider range of conditions. It has been demonstrated that under 20 kPa overburden stress, with decreasing the soil suction from 1153 to 312 kPa, corresponding to increasing the moisture content from 16% to 20%, the peak pullout force decreases by about 28%. The results also illustrate that by increasing the value of matric suction, the amount of U* (relative soil-reinforcement displacement corresponding to the total mobilization of friction) decreases.

Evolution of granular media under constant-volume multidirectional cyclic shearing

By means of the three-dimensional discrete element method, we study the long-time evolution toward liquefaction state in granular materials composed of spherical particles under multidirectional cyclic shearing at constant volume. Extensive simulations were carried out along 1-D linear, 2-D linear, circular/oval, and 8-like shear paths, and the evolution of the system was analyzed in terms of pore pressure, shear strain, and granular texture. The macroscopic stress path and stress–strain response agree well with laboratory experiments. We find that the liquefaction resistance, i.e., the number of cycles necessary to reach the liquefaction state, is generally lower under multidirectional loading as compared to unidirectional loading. As the transient vanishing of mean stress does not occur for all stress paths, we introduce a shear strain-based liquefaction criterion that can be consistently applied to all strain paths. The granular texture is monitored through the coordination number, particle connectivity, force and fabric anisotropies, and friction mobilization. In particular, a particle-void descriptor, named centroid distance, is found to be closely related to the shear strain accumulation. We show that the force anisotropy tensors become almost proportional to the deviatoric stress tensor more quickly than the fabric anisotropy tensor, which takes most of the pre-liquefaction period to follow the external loading. The relationship between deviatoric stress ratio and the force and fabric anisotropies, known to hold in monotonic triaxial loading, also holds with high accuracy in the studied multidirectional cyclic shearing paths; the contributing weights of the anisotropies level off in the post-liquefaction period and do not depend on the shear path.

Development and application of an interface constitutive model for fully grouted rock-bolts and cable-bolts

This paper proposes a new interface constitutive model for fully grouted rock-bolts and cable-bolts based on pull-out test results. A database was created combining published experimental data with in-house tests. By means of a comprehensive framework, a Coulomb-type failure criterion accounting for friction mobilization was defined. During the elastic phase, in which the interface joint is not yet created, the proposed model provides zero radial displacement, and once the interface joint is created, interface dilatancy is modeled using a non-associated plastic potential inspired from the behavior of rock joints. The results predicted by the proposed model are in good agreement with experimental results. The model has been implemented in a finite element method (FEM) code and numerical simulations have been performed at the elementary and the structural scales. The results obtained provide confidence in the ability of the new model to assist in the design and optimization of bolting patterns. • New interface constitutive model suitable for both rock-bolts and cable-bolts. • The new model is based on laboratory pull-out test results (in-house and external). • The interface radial behavior has no elastic component. • Model has been implemented in a FEM code and successfully applied to pull-out tests.

Evolution of granular materials under isochoric cyclic simple shearing.

By means of 3D particle dynamics simulations, we analyze the microstructure of granular materials subjected to isochoric (constant volume) cyclic shearing, which drives the system towards a liquefaction state characterized by loops of jamming-unjamming transition with periodic loss of strength and irreversible accumulation of shear strain. We first show that the macroscopic response obtained by these simulations agrees well with the most salient features of the well-known cyclic behavior of granular materials both before and after liquefaction. Then we investigate the evolution of particle connectivity, force transmission, and anisotropies of contact and force networks. The onset of liquefaction is marked by partial collapse of the force-bearing network with rapid drop of the coordination number and nonrattler fraction of particles, and significant broadening of the contact force probability density function, which begins in the preliquefaction period. We find that the jamming transition in each cycle occurs for a critical value of the coordination number that can be interpreted as the percolation threshold of the contact network and appears to be independent of the initial mean stress, void ratio, and cyclic shear amplitude. We show that upon unjamming in each cycle an isotropic loss of contacts occurs and is followed by the development of high contact anisotropy and a large proportion of particles with only two or three contacts. The higher mobility of the particles also involves a lower degree of frustration of particle rotations and thus lower friction mobilization and tangential force anisotropy. These findings are relevant to both undrained cyclic deformations of saturated soils and rheology of dense non-Brownian suspensions where volume change is coupled with pore liquid drainage conditions.

Mobilisation of friction in unstationary flows down a model topography

The frictional behaviour of a series of numerical 2D granular mass flows down a model topography is analysed. Effective friction coefficients estimated from final deposits are compared with data from documented natural geophysical flows, and show a consistent behaviour as far as run-out distances are concerned. The latter is used to estimate effective friction coefficients which capture well the frictional behaviour derived from the computation of micro-mechanical stress tensors near the gravity centre. Distinguishing between the different parts of the mass while spreading, we show that the downstream part of the flow exhibits a much larger friction than the core and the tail. A dependence between friction and flow volume is however observed in each region.

Centrifuge modelling of rock-socketed drilled shafts under uplift load

Rock-socketed drilled shafts are widely used to transfer the heavy loads from the superstructure especially in mountainous area. Extensive research has been done on the behavior of rock-socketed drilled shafts under compressive load. However, little attention has been paid to uplift behavior of drilled shaft in rock, which govern the overall behavior of the foundation system. In this paper, a series of centrifuge tests have been performed to investigate the uplift response of rock-socketed drilled shafts. The pull-out tests of drilled shafts installed in layered rocks having various strengths were conducted. The load-displacement response, axial load distributions in the shaft and the unit skin friction distribution under pull-out loads were investigated. The effects of the strength of rock socket on the initial stiffness, ultimate capacity and mobilization of friction of the foundation, were also examined. The results indicated that characteristics of rock-socket has a significant influence on the uplift behavior of drilled shaft. Most of the applied uplift load were carried by socketed rock when the drilled shaft was installed in the sand over rock layer, whereas substantial load was carried by both upper and lower rock layers when the drilled shaft was completely socketed into layered rock. The pattern of mobilized shaft friction and point where the maximum unit shaft friction occurred were also found to be affected by the socket condition surrounding the drilled shaft.

Progressive Damage of a Canadian Granite in Laboratory Compression Tests and Underground Excavations

The crystalline rock formations of the Canadian Shield are currently one candidate rock type for the geological disposal of radioactive waste in Canada. This article starts with a critical review of past research results on the geomechanical behaviour of Lac du Bonnet granite, a rock type found at an Underground Research Laboratory (URL) in Pinawa, Manitoba, Canada. Based on the published data, a constitutive model was developed, based on Mohr-Coulomb plasticity, which includes the concept of asynchronous degradation of cohesion and mobilization of friction with progressive damage, as well as time-dependent degradation of strength. The constitutive model was used to simulate laboratory compression tests. It was then implemented in a coupled hydro-mechanical model to simulate the response of the rock mass induced by excavation of a test tunnel at 420 m depth at the URL.

The characterisation of geosynthetic interface friction by means of the inclined plane test

The paper focuses on the evaluation of the shear strength in conditions of low normal stress of various geosynthetic-geosynthetic interfaces, which are typical of landfill cover systems, by means of the inclined plane test, with the aim of studying the friction mobilisation in relation to various kinematic behaviours. The results of three different methods to evaluate the angle of friction were analysed, together with the sensitivity of the interfaces in relation to the wear effect and the influence of the state of hydration. The results showed very different responses of the interfaces to the shear stress, which involved three main types of sliding mechanisms, referred to as sudden, gradual and uneven sliding. Another outcome observed was that the shear strength of geosynthetic-geosynthetic interfaces cannot always be properly characterised following the procedure proposed by the European standard for soil-geosynthetic interfaces (EN ISO 12957–2), since the actual mobilised kinematic behaviour should be taken into consideration. In this regard, the paper provides some hints on the choice of the more representative parameter of friction for each type of sliding. A particular focus was given to the case of gradual sliding interfaces, for which the static friction is difficult to detect due to the very slow movements; for practical purposes, the design friction of these interfaces should be evaluated by using an adequate safety factor with respect to the friction evaluated at 1 mm of displacement.

Inter-particle friction and particle sphericity effects on isotropic compression behavior in real-shaped sand assemblies

A numerical investigation on the effects of inter-particle friction and particle sphericity on isotropic compression in real-shaped sand assemblies is carried out using the discrete element method. Real-shaped sand assemblies are generated in terms of the cumulative distribution of the shape descriptor (sphericity) of real sand particles. The mechanical behaviors of compression are quantified in terms of the volumetric deformation, coordination number, probability distribution of the contact forces, and fabric. The results show that the compression deformation becomes larger with decreasing inter-particle friction, especially for an assembly with frictionless particles. The probability distribution of the coordination number is almost described by a Gaussian distribution, which is somewhat different from that of the monodisperse particles reported in previous studies. Moreover, the inter-particle friction and sphericity have an insignificant effect on the probability distribution of the normalized contact forces, which exhibit a clear exponential decay. Assemblies with lower inter-particle friction and lower sphericity experience greater friction mobilization during compression. The results of the anisotropy of the contact normal force show that all the samples are strongly homogeneous and isotropic, and inter-particle friction and sphericity have substantial effects on the magnitude of the normal contact force.

A simplified numerical approach to the evaluation of residual shaft friction induced by concrete curing in drilled shafts on granular soils

ABSTRACT: Conventional interpretation procedures of load tests on instrumented piles rely upon measurements of strains that assume as zero for strains measured at the instant immediately before starting the test as reference configuration. However, some experimental evidence shows that concrete in drilled shafts undergoes strains induced by the curing process comparable in magnitude to the strains measured during the load tests. It is therefore expected that mobilization of shaft friction takes place before the load test. Several authors have performed experimental and numerical analyses aiming to quantify the influence of those pre-load test concrete volumetric strains on the measured bearing capacity using different approaches. The present work aimed to establish a reference framework for the existing and future works on this topic. In order to assess the influence of concrete strains induced by curing process on the shaft friction before the start of the load tests in drilled shafts, several finite element numerical simulations are performed, considering the thermal, autogenous and drying strains. The analyses consider concrete as an isotropic linear-elastic material and the soil as an elastic-plastic material using the Mohr-Coulomb constitutive model natively implemented in the software ABAQUS. The results are interpreted focusing on the relevancy on the bearing capacity and load distribution along drilled shafts considering or not the strains induced by concrete curing.