Friction Mobilization Research Articles

A Survey of Soil-Reinforcement Interface Friction

The most important aspect of reinforced earth is the sliding shear resistance between the soil and reinforcements expressed in terms of a sliding friction angle or a coefficient of friction. A number of different procedures have been followed in order to quantify this physical property and as a result there is a wide range of reported values. In the present paper, the friction coefficient values of smooth and rough brass strips as determined by modified direct shear tests and by pull out and wall rotation tests in a model box are given and interpreted. For smooth strips, it is found that direct shear and pull out tests give comparable results and the friction coefficients are not influenced by the density. For rough strips, however, pull out tests may give much higher friction values than the direct shear tests as density increases. In wall rotation tests which are an attempt to represent the friction mobilization mechanism in reinforced earth, the friction coefficient values appear to be limited by the values obtained from direct shear tests for both smooth and rough strips, irrespective of density

Effect of Parallel Gradations on Crushed Rock-Concrete Interface Behaviors

Abstract The response of a soil-structure system subjected to monotonic or cyclic loading is significantly influenced by the mechanical behavior of its interface. In this paper, the friction characteristics of crushed rock-concrete interface and the role of the crushed rock deformability were examined through a series of monotonic and cyclic direct shear tests under the framework of parallel gradation technique. Two parallel gradation curves of the crushed rocks were used. All the tests were carried out in dry condition and with two initial void ratios representing both loose and dense states. Two concrete interfaces of different roughness were used in the interface direct shear tests. The static test results show that parallel gradation technique can be used to characterize the residual shear strength but not the volume change. Moreover, under cyclic loading, it can neither yield the same volume change, nor the same shear strength. The critical state of the soil interfaces was also studied, which shed some light on the mechanism of pile skin friction mobilization and modeling of soil-concrete interface behaviors.

A numerical study of the seismic behaviour of timber shear walls with slip-friction connectors

In the event of seismic overloading, timber shear walls have normally been designed to yield by allowing inelastic distortion of the sheathing-to-framing nail connections, thereby reducing the likelihood of brittle failure of timber chords or plywood sheathing. A new concept in shear wall design is presented. It involves the use of slip-friction connectors in lieu of traditional hold-down connectors. Slip-friction connectors, originally developed for the steel framing industry, rely on the mobilisation of friction across steel plates to resist loading up to a predetermined threshold. Upon this threshold being exceeded, relative sliding between the steel plates allows the shear wall to displace in an inelastic manner. This paper discusses the results of numerical analyses of timber shear walls which utilise slip-friction connectors. The results suggest that slip-friction connectors hold the promise of being able to effectively protect sheathing, framing, and nail connections from excessive stresses and deformations during earthquake events of design level intensity or higher. Walls with appropriately adjusted slip-friction connectors are highly ductile, are efficient dissipaters of seismic energy, and have a tendency to self-centre after an earthquake.

Experimental Study of Brittle Behavior of Clay Shale in Rapid Triaxial Compression

The brittle failure behavior of an over-consolidated clay shale (Opalinus Clay) in undrained rapid triaxial compression was studied. The confining stress levels were chosen to simulate the range of confining stresses relevant for underground excavations at the Mont Terri Underground Research Laboratory, and to investigate the transition from axial splitting failure to macroscopic shear failure. Micro-crack initiation was observed throughout the confining stress range utilized in this study at a differential stress of 2.1 MPa on average, which indicates that friction was not mobilized at this stage of brittle failure. The rupture stress was dependent on confinement indicating friction mobilization during the brittle failure process. With increasing confinement net volumetric strain decreased suggesting that dilation was suppressed, which is possibly related to a change in the failure mode. At confining stress levels ≤0.5 MPa specimen rupture was associated with axial splitting. With increasing confinement, transition to a macroscopic shearing mode was observed. Multi-stage triaxial tests consistently showed lower strengths than single-stage tests, demonstrating cumulative damage in the specimens. Both the Mohr–Coulomb and Hoek–Brown failure criteria could not satisfactorily fit the data over the entire confining stress range. A bi-linear or S-shaped failure criterion was found to satisfactorily fit the test data over the entire confinement range studied.

Rheology of granular materials composed of nonconvex particles

By means of contact dynamics simulations, we investigate the shear strength and internal structure of granular materials composed of two-dimensional nonconvex aggregates. We find that the packing fraction first grows as the nonconvexity is increased but declines at higher nonconvexity. This unmonotonic dependence reflects the competing effects of pore size reduction between convex borders of aggregates and gain in porosity at the nonconvex borders that are captured in a simple model fitting nicely the simulation data both in the isotropic and sheared packings. On the other hand, the internal angle of friction increases linearly with nonconvexity and saturates to a value independent of nonconvexity. We show that fabric anisotropy, force anisotropy, and friction mobilization, all enhanced by multiple contacts between aggregates, govern the observed increase of shear strength and its saturation with increasing nonconvexity. The main effect of interlocking is to dislocate frictional dissipation from the locked double and triple contacts between aggregates to the simple contacts between clusters of aggregates. This self-organization of particle motions allows the packing to keep a constant shear strength at high nonconvexity.

A new procedure for measuring geosynthetic friction with an inclined plane

A method for the determination of the friction angle of geosynthetic interfaces (specifically those in contact with soils at very low normal stresses) using an inclined plane is described by the European Standard EN ISO 12957-2. Following this “Standard Displacement Procedure”, the friction angle of a geosynthetic interface is determined using a displacement criterion between the tested geosynthetics. However, the “Standard Displacement Procedure” seems to be poorly suited for many interfaces. Herein, a new procedure is proposed, called the “Force Procedure”, which consists of measuring the force required to restrain a box on top of the interface beyond a limiting value of sliding displacement. With the “Force Procedure”, the friction is determined from the curve of friction mobilization versus plane-inclination. The angle determined with the “Force Procedure” is not sensitive to the conditions of the test and is more representative of real-world conditions, as it takes into account displacements observed in the field. Based on the results of this study, it seems reasonable to suggest a revision of the EN ISO 12957-2 standard testing procedure.

Geotechnical Analysis of Heat Exchanger Piles

There is currently a lack of established calculation method for the geotechnical design of heat exchanger piles, although the technology is experiencing a fast expansion. Instead of quantifying the effects of temperature changes on the static behavior of heat exchanger piles, the common geotechnical practice is to apply a large overall security factor. This is done in order to be on the side of safety with respect to thermal effects. The few existing in situ experiments show that applying a thermal load induces a significant change in the stress-strain state of a pile. This paper presents a geotechnical numerical analysis method, based on the load transfer approach, which assesses the main effects of temperature changes on pile behavior. The method is validated on the basis of two in situ measurements of the loads and deformations experienced by heat exchanger test piles. The occurrence of critical design situations is further discussed. Some conclusions are formulated on concrete failure and the full mobilization of the pile shaft friction and base resistance during the operation of the heat exchange system.

Subsidence Control of Construction on Soft Soils with “Akar Foundation”

Good quality soils are always preferred in development projects, where the bearing capacity of the grounds is sufficiently high and the resulting subsidence is non-excessive. However these sites may not be as readily available with increased population and land use, making it inevitable to construct on less favourable soils, like soft organic materials. To ensure long term stability of structures erected on such soft soils, elaborate and extensive foundation systems are generally required. Alternatively, pre-treatment with ground improvement techniques is necessary. Both these methods tend to incur high costs and labour as well as require prolonged construction period, while making a rather negative impact on the environment in terms of raw material sourcing, heavy machinery mobilization and exposure to the risk of groundwater contamination. This paper describes an exploratory study on a simple shallow foundation system for construction on soft soils, which can be a potentially cheaper and more sustainable approach. The foundation system, termed the “Akar Foundation”, literally translates as “Root Foundation”, is essentially a lightweight platform supported by a group of hollow stumps (i.e. PVC pipes). The ‘root’ base served dual functions: 1. to collectively assert a stronger grip of the soft soils, hence giving higher bearing capacity; 2. to spread the imposed structural load evenly into the subsoil, thus avoiding excessive and non-uniform settlements. The effects of the end condition of the pipes (i.e. open or close) as well as the spacing between the pipes on the foundation settlement mechanism were examined, with static load tests conducted using a lab-scale simulation chamber. The findings showed that the effectiveness of the “Akar Foundation” depends on the compatibility of the pipe spacing and individual pipe lengths, highlighting the inter-relationship between the mobilisation of end resistance, skin friction and formation of soil plug in the open-end system. In a promising light, the reduced subsidence suggest the potential of the “Akar Foundation” as an economical yet effective foundation system in economically challenged soft soil areas.

FE-investigations of micro-polar boundary conditions along interface between soil and structure

The interface between granular bodies and structures is analysed with the finite element method and a micro-polar hypoplastic constitutive model. Quasi-static shearing of an infinitely long and narrow granular strip between two rigid walls of different roughness under conditions of free dilatancy and constant vertical pressure is investigated. The constitutive model can reproduce the essential features of granular bodies during shear localization. To model the different roughness of the interface, micro-polar boundary conditions are proposed taking into account the asperity of the wall roughness and grain diameter. Some emphasis is given to the influence of the wall roughness on the thickness of shear zone and the mobilization of wall friction.

Modeling Lac du Bonnet granite using a discrete element model

A discrete element method code developed by the authors is used to model Lac du Bonnet granite in triaxial compression. The paper first presents the features of the model and the algorithms used to identify the micro deformability and strength parameters. With these identification algorithms, the model was calibrated to match experimental triaxial test data on Lac du Bonnet granite. With the calibrated model, investigation on failure evolution was then performed. The monitored evolution of the number and type of contact failures (micro cracks) reveals that at micro level tensile failures occur first, followed by mobilization of residual friction. Three distinct stages of stress–strain curve can be well identified by the accumulated number of contact failures and the mode of contact failures.

Shear deformation and interface behaviour of concrete-filled CFRP box beams

Abstract A beam structure composed of CFRP box with infill concrete is studied for an application under transverse and axial loads. This paper highlights the shear deformation of box beams under empty condition and concrete-filled condition by simulating the beams tested in an experiment with finite element method. Firstly, empty CFRP box beam is studied. Shear deformation contributes with about 60% of total beam deflection. Furthermore, different analyses with perfect bond assumption and slip assumption are performed to investigate concrete–CFRP interface interaction in a filled beam. From these analyses, slip between concrete and CFRP is evident in the experiment. Consequently, the shear deformation of CFRP becomes very high and the stiffness of the beam is not improved even with concrete infill. Effect of axial load on friction mobilization and P-delta effect are also discussed in relation to the deformation behaviour of concrete-filled beams.

Internal states of model isotropic granular packings. II. Compression and pressure cycles

This is the second paper of a series of three investigating, by numerical means, the geometric and mechanical properties of spherical bead packings under isotropic stresses. We study the effects of varying the applied pressure P (from 1 or 10 kPa up to 100 MPa in the case of glass beads) on several types of configurations assembled by different procedures, as reported in the preceding paper [I. Agnolin and J.-N. Roux, Phys. Rev. E 76, 061302 (2007)]. As functions of P , we monitor changes in solid fraction Phi, coordination number z, proportion of rattlers (grains carrying no force) x_(0) , the distribution of normal forces, the level of friction mobilization, and the distribution of near neighbor distances. Assuming that the contact law does not involve material plasticity or damage, Phi is found to vary very nearly reversibly with P in an isotropic compression cycle, but all other quantities, due to the frictional hysteresis of contact forces, change irreversibly. In particular, initial low P states with high coordination numbers lose many contacts in a compression cycle and end up with values of z and x_(0) close to those of the most poorly coordinated initial configurations. Proportional load variations which do not entail notable configuration changes can therefore nevertheless significantly affect contact networks of granular packings in quasistatic conditions.

Nonlinear numerical method for earthquake site response analysis I — elastoplastic cyclic model and parameter identification strategy

This paper, along with its companion paper, presents the importance of the adequate soil behaviour model to simulate earthquake site response analysis. An elastoplastic model taking into account the elementary necessary plastic mechanisms such as progressive friction mobilization, Coulomb type failure, critical state and dilatancy/contractance flow rule, is used. However, one of the obstacles in the use of elastoplastic models in the everyday design processes for evaluation of the seismic soil response is the difficulty in identifying their parameters. In this paper, a methodology to identify a coherent set of parameters of the elastoplastic model for a given type of soil is presented. The strategy behind the decision making process proposed here is based on the use of minimum physical and easily measurable properties of the soil to directly provide or indirectly assess the required model parameters.

Behaviour of impact- and vibratory-driven piles in stiff clay during installation and static loading

This paper summarises the results of an extensive field test carried out in the framework of the "French National Research Project on Vibratory Driving" and the ProfilArbed Research Programme, to investigate and compare the behaviour and performance of eight impact- and vibratory-driven piles. The test site is located in Northern France. The subsoil consists of dense Flanders clay. The piles were driven using an ICE8 l5 vibrator and an IHC70s impact hammer to the same depth. All these piles were instrumented with accelerometers and strain gauges positioned at the top and bottom of the piles. Penetration rate, uplift load applied by the crane, vibration transmitted to the ground, operating pressure, oil flow at the vibratory power-pack, and energy per blow for the hammer were continuously recorded. The paper presents the main results obtained from double sheet piles (AUl6 type) driven to 7 m depth, a small wall of two double sheet piles (AU20 type) driven to 8 m depth, HP bearing piles driven to 10.2 m, and steel open-ended tubes of508 mm diameter driven to 9.4 m depth. After a delay of 6 to 8 weeks, the piles were statically loaded to failure. The piles were instrumented with removable extensometers, which made it possible to measure the mobilisation of shaft friction and toe resistance. The measured bearing capacity was significantly lower for the vibratory-driven piles (around 35%) and it is consistent with the results obtained by the Laboratoire des Ponts et Chaussees de Paris at other sites.

Friction versus texture at the approach of a granular avalanche

We perform an analysis of the granular texture of a granular bed close to stability limit. Our analysis is based on a unique criterion of friction mobilization in a simulated two-dimensional packing. In this way, we recover the bimodal character of granular texture and the coexistence of weak and strong phases in the sense of distinct contacts populations. Moreover, we show the existence of a well-defined subset of contacts within the weak contact network. These contacts are characterized by their important friction and form a highly coherent population in terms of fabric. They play an antagonistic role with respect to force chains. Thus, we are able to discriminate between incoherent contacts and coherent contacts in the weak phase and to specify the role that the latter plays in the destabilization process.

Intermittent flow of a collection of rigid frictional disks in a vertical pipe

We study the quasistatic flow of a collection of rigid frictional disks pushed upward (against the gravity) inside a narrow vertical pipe by a compliant mechanism. The contact dynamics method was used for the numerical simulations in combination with a friction law at disk-disk and wall-disk contacts characterized by discontinuous velocity weakening from a static threshold to a dynamic coefficient of friction. The material is sheared by the rolling of particles at the walls inducing a convective motion in the bulk. We observe a transition from constant flow to an intermittent flow when the driving velocity is reduced below a characteristic velocity that scales as k(-1/2) with the stiffness k of the pushing mechanism. The intermittent flow is composed of alternating phases of creep motion, where the pressure at the bottom of the granular column rises nonlinearly with time, and sudden slip, corresponding to a fast pressure drop. We show that the mean static pressure is correctly predicted by the Janssen model. The interplay between friction mobilization at the walls and structural changes in the bulk gives rise to a broad distribution of slip amplitudes characterized by a power law with an exponent approximately -1.7 that appears to be robust with respect to our system parameters.

Full-scale load tests on instrumented micropiles

In September 2001, following an exceptional rainfall event with an estimated return time of about 500 years, a number of ancient buildings located in a restricted area in the historical centre of Napoli underwent significant settlement and heavy structural damage. Structural collapse on saturation of the pyroclastic soils and loss of bearing capacity by submersion are believed to be the origin of the observed phenomena. After the settlements came to a standstill, a subsoil investigation was carried out and a remedial solution was designed, based on a massive underpinning of the damaged buildings by micropiles. The size of the project and the well-known dependence of the behaviour of micropiles on technological details suggested that some preliminary load tests on piles should be undertaken. The results of two load tests on instrumented piles installed with different procedures are reported and discussed. A new technique to install embedded vibrating wire gauges was tested, and was revealed to be successful. The experimental findings are in good agreement with the Bustamante and Doix method. Also, the simplified approach proposed by Randolph and Wroth to evaluate the stiffness of the soil layer was checked, providing good results. The experimental finding about the mobilisation of some negative skin friction along the pile shaft, probably due to the large grouted volumes, is confirmed by several finite element analyses.

Full-scale load tests on instrumented micropiles

In September 2001, following an exceptional rainfall event with an estimated return time of about 500 years, a number of ancient buildings located in a restricted area in the historical centre of Napoli underwent significant settlement and heavy structural damage. Structural collapse on saturation of the pyroclastic soils and loss of bearing capacity by submersion are believed to be the origin of the observed phenomena. After the settlements came to a standstill, a subsoil investigation was carried out and a remedial solution was designed, based on a massive underpinning of the damaged buildings by micropiles. The size of the project and the well-known dependence of the behaviour of micropiles on technological details suggested that some preliminary load tests on piles should be undertaken. The results of two load tests on instrumented piles installed with different procedures are reported and discussed. A new technique to install embedded vibrating wire gauges was tested, and was revealed to be successful. The experimental findings are in good agreement with the Bustamante and Doix method. Also, the simplified approach proposed by Randolph and Wroth to evaluate the stiffness of the soil layer was checked, providing good results. The experimental finding about the mobilisation of some negative skin friction along the pile shaft, probably due to the large grouted volumes, is confirmed by several finite element analyses.

Physical-chemistry of “polygomer” surfaces through AFM force measurements

AFM microscope is used here as a surface forces apparatus at the nanometer scale. We show through several examples that the curves forces (nN) vs. distance (nm) obtained are the physico-chemical signature of the interacting surfaces. From such a curve, chemical, frictional (mobility) and mechanical top surface aspects can be studied.

Comportement d'un pieu bi-fonction, fondation et échangeur de chaleur

The behaviour of a pile subjected to thermo-mechanical loads was studied in situ with the aim of quantifying the thermal influence on the bearing capacity of heat exchanger piles. To accomplish this, a pile situated in a building under construction was equipped with a pipe system to inject heat into it using a special heat pump. Load cells, deformation gauges, and thermometers were installed to evaluate the behaviour of the pile during seven tests with coupled thermo-mechanical loads. The temperature variations applied to the pile were of the order of 15°C and the mechanical load reached 1300 kN. The results permitted the quantification of three significant effects brought about by the temperature increase: (i) pile uplift, (ii) mobilization of skin friction due to the relative displacement of the pile with respect to the ground, (iii) additional load generated in the pile by constrained dilation.Key words: heat exchanger pile, in situ test, loading test, thermo-mechanical loads, mobilization of skin friction.