Friction Mobilization Research Articles

Mobilised strength components in brittle failure of rock

In deep underground excavations in hard rocks where stresses easily exceed the micro-crack initiation stress level inside the rock mass, proper consideration of the behaviour of rockmass during the brittle fracturing process in constitutive modelling is of paramount importance. Current empirical and conventional experimental methods for obtaining the deformational behaviours of hard rocks under loading do not lead to results that can be matched with in situ failure observations. This paper demonstrates that this problem is not necessarily a matter of the general notion of size effect but rather can be related to the different circumstances under which the cohesive and frictional strength components are mobilized in laboratory compression tests and around underground openings. It is also demonstrated that the propagation of the failed or breakout zone (depth and extent) is a function of the strain-dependent brittleness index IBε introduced in this paper, which explicitly considers the relative delay in friction mobilization relative to the rate of cohesion loss as functions of plastic strain. This new brittleness index characterizes the entire stress–strain curve (pre- to post-peak stages) and represents the involved micro-mechanisms during the brittle failure process: that is, initiation, propagation, and coalescence of cracks. This study shows that brittleness of rock is the most dominant factor, in controlling breakout shape, which explains the failure of stress-based criteria adopted by many researchers in predicting the stress-induced breakout depth around openings.

Mobilised strength components in brittle failure of rock

In deep underground excavations in hard rocks where stresses easily exceed the micro-crack initiation stress level inside the rock mass, proper consideration of the behaviour of rockmass during the brittle fracturing process in constitutive modelling is of paramount importance. Current empirical and conventional experimental methods for obtaining the deformational behaviours of hard rocks under loading do not lead to results that can be matched with in situ failure observations. This paper demonstrates that this problem is not necessarily a matter of the general notion of size effect but rather can be related to the different circumstances under which the cohesive and frictional strength components are mobilized in laboratory compression tests and around underground openings. It is also demonstrated that the propagation of the failed or breakout zone (depth and extent) is a function of the strain-dependent brittleness index IBε introduced in this paper, which explicitly considers the relative delay in friction mobilization relative to the rate of cohesion loss as functions of plastic strain. This new brittleness index characterizes the entire stress–strain curve (pre- to post-peak stages) and represents the involved micro-mechanisms during the brittle failure process: that is, initiation, propagation, and coalescence of cracks. This study shows that brittleness of rock is the most dominant factor, in controlling breakout shape, which explains the failure of stress-based criteria adopted by many researchers in predicting the stress-induced breakout depth around openings.

On the Shaft Friction Modelling of Non-Displacement Piles in Sand

ABSTRACT To evaluate the capability of a centrifuge physical model to catch the main aspects of the shaft friction mobilisation of non-displacement piles in sand, a series of centrifuge tests was carried out by axially loading an instrumented model pile embedded without displacement, in homogeneous dry sand. Three model piles with different roughness were combined with two silica sands (Toyoura and FF) with different grain size. After a description of the experimental set-up, test results were analysed with reference to the current practice (β method). Moreover, the effects of the normalised pile skin roughness, Rn, on the shear stress mobilised were investigated. Finally, the shaft friction mechanisms observed from centrifuge tests, were interpreted by direct shear tests on sand/rigid plate interface under constant normal stiffness.

Evaluation of side wall friction for a buried pipe testing facility

Methods of limiting the mobilization of boundary friction during large-scale laboratory testing of buried pipes (geopipes) are investigated. A small scale investigation using a direct shear apparatus examined the effectiveness of different configurations of unlubricated and lubricated geosynthetic treatments intended to reduce the boundary friction mobilized between a steel–soil interface. Results from direct shear tests indicated that applying a double layer of thin polyethylene sheeting lubricated with silicon grease yielded an angle of friction of about 5°, provided that the interface treatment received adequate protection from the backfill material. Tests conducted in the large-scale test facility suggest that this interface treatment successfully limits boundary effects arising from interface friction.

Pressures from flowing granular solids in silos

Large displacement flows in granular solids occur whenever silos are discharged. Measurements of pressures during flow, combined with visual observations of flow patterns and control tests on the e...

Geotextile friction mobilization during field pullout test

Small-scale laboratory tests may not yield reliable values for soil–geotextile interface properties due to the effect of scaling. Field tests offer a better alternative to overcome this problem. In situ pullout tests were performed on a full-scale embankment built over a woven polyester geotextile sheet. The field tests were conducted using different fill heights of compacted clayey soil. The performance of the geotextile was examined when the base soil underneath the geotextile was stiff clay of high plasticity or compacted clean river sand. The test section was instrumented to monitor the geotextile movement and the applied pullout force. Results of the tests showed a linear increase in the geotextile frictional resistance with the increase in the fill height. Also, a successive movement response at different locations on the geotextile away from the loaded edge was detected during the early stages of loading. At the slippage load, a continuous rigid body translation occurred in the geotextile sheet without any further increase in the applied pullout force. The suitability of a widely used interface numerical model that consists of a zero-thickness joint element and a nonlinear elastic hyperbolic stress-deformation material model was examined using the finite element technique. The results showed the inadequacy of this model to deal with large deformation problems typically experienced in geosynthetic reinforced soils.

Shear bands in plane strain compression of loose sand

Experimental results are presented which characterize the behaviour of a loose, fine-grained, water-saturated sand tested under globally undrained and drained conditions in a plane strain apparatus. The objective of this investigation is to provide insight into the phenomenon of shear banding in loose sand. Together with local measurements of boundary forces and deformations, stereophotogrammetry is used to investigate the progression of strain localization in plane strain compression. Typical results and findings concerning the evolution of non-homogeneous deformation are presented in detail. Shear banding occurred in both undrained and drained experiments on loose masonry sand. In general, temporary modes of strain localization, observed during macroscopically ‘uniform’ deformations of a specimen, gave way to a single, persistent shear band. A clear pattern of onset of the formation of the persistent shear band, mobilization of the maximum effective friction and complete formation of the band was observed in all tests. The stress state when the localization begins is very close to, yet precedes that corresponding to the maximum mobilized friction. The persistent shear bands evolve with changing width and orientation. L'exposé présente les résultats d'essais de compression plane visant à caractériser le comportement de sable meuble, à grains fins et saturé d'ean dans des conditions générales non drainées et drainées. L'objectif de cette étude est de mieux faire comprendre la formation de bandes de cisaillement dans le sable meuble. En plus de mesurer les forces limites et les déformations, on a eu recours à la stéréophotogrammétrie pour étudier la progression de la localisation des tensions sous l'effect d'une compression plane. L'exposé présente en détail les résultats et constatations types concernant l'évolution de déformations non homogénes. Lors d'essais non drainés et drainés, des bandes de cisaillement se sont formées dans du sable de construction meuble. En règlegénérale, la formation d'une seule bande de cisaillement persistante s'est substituée aux modes temporaires de localisation des contraintes, observés pendant des déformations macroscopiquement «uniformes« d'une éprouvette. Dans tous les essais, le début de la formation d'une bande de cisaillement persistante, la mobillsation du frottement efficace maximal et la formation complète de la bande ont suivi un schéma bien défini. L'état de contrainte où commence la localisation est très proche de celui qui correspond au frottement mobilisé maximal, mais le précède. Les bandes de cisaillement persistantes évoluent en fonction de la largeur et de l'orientation

Strain Localization and Undrained Steady State of Sand

Experimental results are presented that characterize the behavior of a loose, fine-grained, water-saturated sand tested under globally undrained conditions is a plane strain apparatus. Together with local measurements of boundary forces and deformations, stereophotogrammetry is used to track the progressive localization of strain. The constitutive behaviors in the tests prior to localization can be characterized as either undrained local softening or undrained load softening and subsequent rehardening. In both cases, a consistent pattern of onset of the formation of the persistent shear band, mobilization of the maximum effective friction, and the complete formation of the band was observed. Conditions approximating steady state only were observed in the shear bands. The effect of strain localization on the conventionally derived steady state line was found to be minimal when undrained load softening behavior was observed, but significantly different steady-state lines were found from results of plane strain and axisymmetric tests. The differences between tests conducted under these two stress conditions assessed to evaluate the practical implications of the observed differences.

The progressive fracture of Lac du Bonnet granite

The strength of intact rock is made up of two components: the intrinsic strength, or cohesion; and the frictional strength. It is generally assumed that cohesion and friction are mobilized at the same displacements such that both components can be relied on simultaneously. Damage testing of samples of Lac du Bonnet granite has shown that as friction is mobilized in the sample, cohesion is reduced. This progressive loss of intrinsic strength and mobilization of friction is modelled using the Griffith locus based on a sliding-crack model. There appears to be a maximum cohesion that can be relied on for engineering purposes, and this strength is less than half of the unconfined compressive strength measured in the laboratory.

Evaluation of interfacial frictional resistance

Three distinct mechanisms — sliding, bonding and bearing — for the mobilisation of interfacial friction have been identified. In the light of these mechanisms, the effect of variation in reinforcement parameters, such as extensibility, flexibility and hardness on mobilisation of interfacial friction, and the mechanisms themselves has been examined. The influence of boundary effects of apparatus on the interfacial friction has been discussed and a method of estimating the same in a pull-out box has been proposed.

Comportement, en chambre triaxiale, d'un pieu à frottement dans l’argile

This study deals with the behaviour of a friction pile in a clay, in a triaxial chamber, and in particular with the generation of pore pressures during driving, and with the mobilization of lateral friction during loading tests at constant rate of penetration and static. The results show that the setting compares with that of a piezocone, and that the pore pressures generated at the point during driving, in the laboratory, are of the same order of magnitude as those obtained on the field. The conditions of reconsolidation used in the laboratory, which define the effective stresses around the pile, allow the calculation of the lateral friction on the model pile. The same conditions of reconsolidation were applied to the full-scale tests, on the Maskinongé site, and yielded values of angles of friction δ of 16° for steel and 20° for concrete. Key words: pile, clay, triaxial chamber, friction, pore pressure. [Journal translation]

Mobilization of soil-geofabric interface friction : Makiuchi, K; Miyamori, T Proc international geotechnical symposium on theory and practice of earth reinforcement, Fukuoka Kyushu, 5–7 October 1988P129–134. Publ Rotterdam: A A Balkema, 1988

Mobilization of soil-geofabric interface friction : Makiuchi, K; Miyamori, T Proc international geotechnical symposium on theory and practice of earth reinforcement, Fukuoka Kyushu, 5–7 October 1988P129–134. Publ Rotterdam: A A Balkema, 1988

886306 Friction mobilization F-y curves for laterally loaded piles from the pressuremeter : Smith, T D Proc International Symposium on Prediction and Performance in Geotechnical Engineering, Calgary, 17–19 June 1987 P89–95. Publ Rotterdam: A A Balkema, 1987

886306 Friction mobilization F-y curves for laterally loaded piles from the pressuremeter : Smith, T D Proc International Symposium on Prediction and Performance in Geotechnical Engineering, Calgary, 17–19 June 1987 P89–95. Publ Rotterdam: A A Balkema, 1987

Effective stress analysis of the stability of embankments on soft soils: Discussion

The authors are to be congratulated for their very clear and useful paper, which points out the efficiency of the effective stress analysis applied to embankment stability on soft soils, provided that the actually observed in-situ pore pressures are introduced into the analysis. We agree with the authors in their comments on the relative merits and shortcomings of the total and effective stress approaches. In cases in which significant consolidation takes place during the construction period, the effective stress analysis is the only efficient one. Regarding the improvements carried out in the last years to the total stress analysis, summarized by the authors in their introduction, a model has been presented by the writers in which the influence of the clay stress history on the undrained shear strength is taken into account (Ballester and Sagaseta 1979). The proposed yield condition is an anisotropic extension of Tresca's criterion. It is a total stress limit condition, but it is defined by the intrinsic effective strength parameters and the clay stress history. Thus, the model parameters are: -Hvorslev's parameters ( k, +,) -degree of friction mobilisation, DM (0 5 DM 5 1) -overburden effective pressure, p,' -overconsolidation pressure, p,' -lateral earth pressure coefficient, KO The variation with depth of the last three parameters produces strength gradient with depth. If KO # 1 .O, the behavior is anisotropic. The model has been implemented into a limit equilibrium analysis by the method of characteristics and applied in a general form to surface loads on soft clays (Arroyo and Sagaseta 1982). In order to check the validity of the model, it has been applied to a number of well-documented embankment failures, namely those of

The Behavior of Drammen Plastic Clay under Low Effective Stresses

Shear tests on undisturbed samples of clay at low normal effective stresses have proved to be useful in order to understand the actual nature of cohesion. Drammen plastic clay was considered to be a suitable material for this study as it is nonfissured and noncemented. Triaxial compression and extension tests without membranes were performed on this clay in the low stress range. The samples in compression failed along inclined planes, while those in extension failed nearly along a horizontal plane. It was found that the shear resistance in the case of extension tests was independent of the average effective normal stress in the low stress range. The magnitude of the cohesion was nearly the same as the cohesion observed in compression tests. The reasons for these differences are discussed in terms of a proposed model. The shear resistance in extension tests is probably a structural cohesion and no friction seems to be mobilized under these conditions. When the samples were reconsolidated to in situ stresses, the shear parameters in extension tests were the same as in compression tests. It is, therefore, suggested that mobilization of friction depends on the nature of the contacts which undergo modification when disturbance of structure on account of reconsolidation takes place.

Failure of a Test Embankment On a Sensitive Champlain Clay Deposit

The present paper reports on the failure of a test embankment built on a soft, sensitive, and cemented clay in Saint-Alban, Quebec. The embankment was built as a first stage of a research program aimed at studying the short and long term behavior of embankments on soft clay foundations.A complete description of the embankment, of the instrumentation, and of the failure is given, followed by the analysis of the failure performed on the basis of different assumptions of fill behavior and of vane strength values mobilized in the clay crust. The analyses show that the assumptions of full mobilization of friction in the fill seems to be the most representative of the fill behavior in the present case and that a suitable factor of safety is obtained only when a reduction of vane strength is assumed to act in the crust.A new approach based on the residual undrained strength is suggested and seems to offer some potential as a valuable means of analyzing the stability of embankments on clay foundations.

Passive Earth Pressure Measurements

Synopsis Apparatus for the measurement of passive earth pressure of dry sand is described which allows a control of the wall direction in space and the consequent rate of mobilization of wall friction. Boundary forces at the sides and base of a central measuring wall are eliminated. The tests demonstrate that the peak values of φ′ and δ observed on an element of the sand should not necessarily be inserted into theoretical expressions for earth pressure. For loose sand, good agreement between theory and observations is obtained after wall displacements so large as to be unacceptable in practice. For dense sand, progressive failure of elements in the mass leads to average maximum Coulomb values of φ′ which are smaller than those predicted by the plane strain compression test. Where the peak is reached at small wall displacements, as when the angle of wall friction is approximately zero, good agreement is obtained with the triaxial compression value of φ′max. For higher displacements and angles of wall friction, lower average peak φ′ values in the mass are observed. The shape of the rupture surface is also believed to be the result of progressive deformations and volume changes throughout the mass up to the peak wall load. In practice the maximum passive pressure coefficient for sands can fall well below theoretical values assuming δ = φ′, and values given in the “Code of Practice” err on the unsafe side. Alternative recommendations are made. On décrit un appareil de mesure de la pression passive du sol de sable sec qui permet un contrôle de la direction du mur dans l'espace et le coefficient de mobilisation de friction du mur qui en résulte. Les forces limitrophes sur les côtés et à la base d'un mur de mesure central sont éliminées. Les essais prouvent que les valeurs maximum de φ′ et δ observées sur un élément de sable ne doivent pas nécessairement être comprises dans les expressions théoriques de pression du sol. Pour le sable meuble, on obtient un bon accord entre la théorie et les observations avec des déplacements de mur si importants que dans la pratique ils sont inacceptables. Pour du sable dense, la rupture progressive des éléments dans la masse tend vers les valeurs Coulomb moyennes maximum de φ′ qui sont plus petites que celles prévues par un essai de compression de déformation plane. Quand on a atteint le maximum pour des petits déplacements de mur, comme lorsque l'angle de friction du mur est environ zéro, on obtient un bon accord avec la valeur de compression de φ′ maximum. Pour des déplacements et des angles de friction de mur plus grands, on remarque dans la masse des valeurs φ′ maximum de moyenne plus basse. On pense aussi que la forme de la surface de rupture est le résultat de déformations progressives et de changements de volume dans toute la masse jusqua'là la charge maximum du mur. Dans la pratique le coefficient de pression passive maximum pour les sables peut descendre au dessous des valeurs théoriques en admettant que δ = φ′ et les valeurs données dans le Manuel “Code of Practice” deviennent douteuses. On fait des recommandations alternatives.

A Stress-Strain Theory for Cohesionless Soil with Applications to Earth Pressures at Rest and Moving Walls

Synopsis Experiments with cohesionless soils in the shear-box and triaxial compression machines are described in which the influence of sample thickness, soil type, length of slip line, type of test, confining pressure, soil density, direction of loading, and strain history of the soil, has been studied. The results substantiate the hypothesis, that the degree of mobilization of internal friction depends upon the degree of granular interlocking, which is a function of the movement of the grains per unit length of the slip line. A new type of confined compression test is described, the results being used to predict the variation of the “at rest” earth pressure coefficient with depth. The variation in earth-pressure distribution on stiff walls moving into, and away from, the soil by sliding and tilting motions is calculated. General agreement is reached with observations of Terzaghi on a large retaining wall. Des essais faites avec des sols non cohérent dans des appareillages de compression à boîte de cisaillement et triaxiale font l'objet d'une description, où l'influence de l'epaisseur d'échantillon, type de sol, longueur de ligne de glissement, type d'essai, pression de resserrement, direction de charge et historique des déformations subis par le sol est étudiée. Les résultats viennent à l'appui de l'hypothèse selon laquelle le degré de mobilisation de frottement interne dépend du degré d'enchevêtrement granulaire qui est fonction du mouvement des grains par unité de longueur de la linge de glissement. Un nouveau type d'essai de compression dans une enceinte rigide est également décrit; ses résultats servant à prédire les variations du coefficient de poussée de la terre “au repos” par la profondeur. Les variations de distribution de poussée de la terre sur les murs rigides se rapprochant et s'écartant du sol par mouvements de glissement et de basculement sont calculées. Un accord général se fait avec les observations de Terzaghi sur un grand inur de soutènement.