Constitutive Behavior of Geosynthetic Interfaces

New displacement-softening and work-softening models were developed to describe the sliding of geosynthetic interfaces, such as those in landfill liners. The displacement-softening formulation is based on the assumption that strength reduction at the interface can be related to nonrecoverable (plastic) shear displacement. The model uses three relationships: (1) the peak strength envelope; (2) the residual strength envelope; and (3) the residual factor versus displacement ratio relationship, which is a nondimensional expression of the rate at which displacement-softening occurs. The displacement-softening model is accurate for shearing when the normal stress stays constant. When normal stress increases during shearing, the displacement-softening formulation overpredicts damage to geosynthetic interfaces. The work-softening model was developed to compute interface softening during conditions of increasing normal stress. This formulation is based on the assumption that the postpeak reduction in shear strength can be attributed to plastic shear work rather than plastic shear displacement. By calculating an equivalent plastic shear displacement for a given amount of plastic shear work, the work-softening model can be formulated using the same basic relationships as the displacement-softening model. The work-softening model significantly outperformed the displacement-softening model when simulating laboratory tests under conditions of increasing normal stress.     

Shear Strength of HDPE Geomembrane∕Geosynthetic Clay Liner Interfaces

A study of interface shear strengths between smooth and textured high density polyethylene (HDPE) geomembranes (GMs) and a woven∕nonwoven needle-punched geosynthetic clay liner (GCL) is presented. Tests were performed using a large direct shear machine capable of measuring peak and large displacement (200 mm) shear strengths. The failure surface was located at the GM∕GCL interface for all tests conducted, corresponding to a normal stress range of 1–486 kPa. Small positive pore pressures were measured for all interfaces at peak shear strength. Thus, the practice of preparing failure envelopes using total normal stress, instead of effective normal stress, appears to be conservative. Interface shear strengths for textured GMs placed against the nonwoven side of the GCL were higher than those corresponding to the woven side. By comparison, differences in peak shear strength for laminated and coextruded GM interfaces were relatively less. Limited tests showed that peak and large displacement shear strengths were independent of displacement rate and dependent on the shear direction of the GM. The quantity of extruded bentonite at the interfaces generally increased with normal stress and was less for nonwoven geotextile interfaces than for woven geotextile interfaces. Implications of the findings to the testing of GM∕GCL interfaces and the characterization of GM∕GCL interface shear strength are discussed.     

Simple Model for the Cyclic Behavior of Smooth Sand-Steel Interfaces

The object of this note is the formulation of a simple elastoplastic model for the behavior of smooth sand-steel interfaces. The model is derived from a series of constant normal stiffness direct shear tests between a siliceous sand and a smooth steel plate. These tests highlight the importance of the shear stress degradation on the final value of the shear resistance and can be seen as the elementary mechanism that models in the laboratory the pile shaft-soil interaction. The goal of the presented model is to represent the observed behavior in a very simple way by using a reduced number of constitutive parameters.     

Relationship between NP GCL Internal and HDPE GMX/NP GCL Interface Shear Strengths

An investigation was conducted on the relationship between the internal shear strength of hydrated needle-punched (NP) geosynthetic clay liners (GCLs) and the interface shear strength between hydrated NP GCLs (nonwoven side) and high-density polyethylene (HDPE) textured geomembranes (GMXs). New large-scale direct shear data are presented and compared to previous results obtained using similar materials and procedures. The data indicate that both GCLs and GMX/GCL interfaces display large postpeak strength reduction, even at high normal stress. Peak and large-displacement failure envelopes are nonlinear; except for the GCL internal residual strength envelope, which passes through the origin and has a friction angle of 4.8°. GMX/GCL interfaces can be expected to have lower peak strengths and higher large-displacement strengths than GCL internal shear specimens. However, the failure mode for GMX/GCL specimens can change from interface shear to GCL internal shear as normal stress increases. Design for peak strength conditions should be based on the lowest peak strength interface in a liner system, and design for large displacement conditions should be on the basis of the residual strength of the same interface.     

Behavior of Geogrid-Sand Interface in Direct Shear Mode

The contribution of transverse ribs to the soil-geogrids interaction under pullout mode has been well documented. However, the contribution of transverse ribs to the soil-geogrid interaction under direct shear mode is, at best, unclear. Consequently, this paper presents the results of a comprehensive direct shear testing program aimed at evaluating the contribution of transverse ribs to the interface shear. The direct shear tests involved Ottawa sand and several polyester geogrids with a variety of material tensile strength, percent open area, and aperture pattern. The test results show that the shear strength of sand-geogrid interfaces under direct shear mode is significantly higher than that of sand-geotextile interfaces. Analysis of shear displacement-strength response of the interfaces indicates that, in addition to interface shear components due to sand-rib friction and sand-sand shear at the location of the openings, the transverse ribs provide additional contribution to the overall sand-geogrid interface resistance. Specifically, analysis of the results reveals that the transverse ribs of the geogrid used in this study provide approximately 10% of interface shear resistance. This contribution is positively correlated with the tensile strength and the stiffness of geogrid ribs, but is negatively correlated with the percent open area of the geogrid. A simple model is proposed to quantify the contribution of transverse ribs to the interface shear strength under direct shear mode.     

Role of large-scale slip in mode II fracture of bimaterial interface produced by diffusion bonding

Bimaterial interfaces present in diffusion-bonded (and in-situ) composites are often not flat interfaces. The unevenness of the interface can result not only from interface reaction products but also from long-range waviness associated with the surfaces of the component phases bonded together. Experimental studies aimed at determining interface mechanical properties generally ignore the departure in the local stress due to waviness and assume a theoretically flat interface. Furthermore, the commonly used testing methods involving superimposed tension often renders the interface so extremely brittle that if microplastic effects were present it becomes impossible to perceive them. This article examines the role of waviness of the interface and microplastic effects on crack initiation. To do this, a test was selected that provides significant stability against crack growth by superimposing compressive stresses. Mode II interface fracture was studied for NiAl/Mo model laminates using a recently developed asymmetrically loaded shear (ALS) interface shear test. The ALS test may be viewed as opposite of the laminate bend test. In the bend test, shear at the interface is created via tension on one surface of the bend, while in the ALS test, shear is created by compression on one side of the interface relative to the other. Normal to the interface, near the crack tip, an initially compressive state is replaced by slight tension due to Poisson’s expansion of the unbonded part of the compressed beam.     

Analysis of shear stress distribution in pushout process of fiber-reinforced ceramics

The interfacial shear stress distribution of a thin specimen of SiC fiber-reinforced glass matrix composite (fiber volume fraction of 0.1, 0.5 and 0.7) during a fiber pushout process was subjected to finite element analysis using a three concentric axisymmetrical model which consisted of fiber, matrix, and composite. A stress criterion was used to determine interface debonding. Effects of thermally-induced stress and a post debond sliding process at the interface were also included in the analysis. The analytical result showed that shear stress near the specimen surface was introduced during the specimen preparation process. Before the interfacial debonding, the distribution of shear stress during the pushout test was affected by the existence of thermally-induced stress in the specimen. The interfacial shear debonding initiated ≈ 30 μm below the pushing surface and the sliding at the debonded interface proceeded in the direction of both the pushing surface and back surface from the peak shear position; the debonding from the back surface initiated just before the complete debonding of the interface. The pushout load-displacement curve near the origin was straight, however, after the existence of interface sliding at the debonded interface, the curve exhibited non-linearity with the increase in applied load up to the complete debonding at the interface. This debonding process was essentially independent of the fiber volume fraction. The results indicate that the total of thermally-induced stress in the specimen and shear stress distribution generated by applied load are important for the initiation of debonding and the frictional sliding process of the thin specimen pushout test.     

Fiber Reinforced Polymer Composite–Wood Pile Interface Characterization by Push-Out Tests

Structural restoration of spliced or damaged wood piles with fiber reinforced polymer (FRP) composite shells requires that shear forces be transferred between the wood core and the encasing composite shells. When a repaired wood pile is loaded, shear stresses develop between the wood pile and the FRP composite shell through the grouting material. Alternatively, shear force transfer can be developed through mechanical connectors. The objective of this study was to characterize the interfaces in wood piles repaired with FRP composite shells and grout materials. Two interfaces were studied: wood pile/grout material and a grout material/innermost FRP composite shell. A set of design parameters that control the response of both interfaces was identified: (1) extent of reduction of cross section of wood pile due to deterioration (necking); (2) type of grout material (cement-based or polyurethane); (3) use of mechanical connectors; and (4) addition of frictional coating on the innermost shell. Push-out tests by compression loading were performed to characterize the interfaces and discriminate the effect of the design parameters. The outcome of the push-out tests was evaluation of the shear stress and force versus slip response and characterization of the failure mechanism. A set of repair systems that represent different combinations of the design parameters was fabricated and the interfaces evaluated. It was found that the combination of cement-based grout and polymer concrete overlay on the innermost shell provided the most efficient shear force-slip response. A simplified piecewise linear model of shear stress versus slip at the wood/grout and grout/FRP composite interfaces with and without mechanical connectors is proposed to synthesize the experimental response.     

Normal Transmission of S-Wave Across Parallel Fractures with Coulomb Slip Behavior

When an elastic wave propagates through a rock mass, its amplitude is attenuated and velocity is slowed due to the presence of fractures. During wave propagation, if the shear stress at a fracture interface reaches the fracture shear strength, the fracture will experience a large shear displacement. This paper presents a study of the normal transmission of S-waves across parallel fractures with Coulomb slip behavior. In our theoretical formulation, the method of characteristics combined with the Coulomb slip model is used to develop a set of recurrence equations with respect to particle velocities and shear stress. These equations are then solved numerically. In a comparison with the theoretical study, numerical modeling using the universal distinct element code (UDEC) has been conducted. A general agreement between UDEC modeling and theoretical analysis is achieved. The magnitude of the transmission coefficient is calculated as a function of shear stress ratio, nondimensional fracture spacing, normalized shear stiffness, and number of fractures. The study shows that the shear stress ratio is the most important factor influencing wave transmission, and the influence of other factors becomes more apparent when the shear stress ratio is small.     

Analysis of a Large Database of GCL-Geomembrane Interface Shear Strength Results

A database of 534 large-scale direct shear test results was assembled in this study to evaluate the interface shear strength between geosynthetic clay liners (GCLs) and geomembranes (GMs). The tests were conducted between 1992 and 2003 by a single independent laboratory using procedures consistent with current testing standards. The number of results in the database allowed quantification of the impact of GCL type, GM type, normal stress, and procedures for specimen hydration and consolidation on the shear strength of GCL-GM interfaces, as well as identification of sources of shear strength variability. The interface shear strength was found to be sensitive to the type of GCL internal reinforcement, GM polymer, and GM texturing, but not to the GM thickness or manufacturer. On average, the GCL internal shear strength was observed to be higher than the GCL-GM interface shear strength when tested using the same procedures. GCLs sheared internally show similar stress-displacement responses and friction angles to GCL-GM interfaces that incorporate a GCL with the same reinforcement type. Hydration under normal stresses below those used during shearing (followed by a consolidation period) led to higher GCL internal shear strength, but lower GCL-GM interface shear strength, than when hydration was conducted under the shearing normal stress. Such different responses are attributed to bentonite extrusion from the GCL into the interface. Good repeatability of test results was obtained using GCL and GM specimens from the same manufacturing lot, while high variability was obtained using specimens from different lots. GCL-GM interface peak shear strength variability was found to increase linearly with normal stress.     

Contraction, Dilation, and Failure of Sand in Triaxial, Torsional, and Rotational Shear Tests

The response of a saturated fine sand (Nevada sand No. 120) with relative density Dr ≈ 70% in drained and undrained conventional triaxial compression and extension tests and undrained cyclic shear tests in a hollow cylinder apparatus with rotation of the stress directions was studied. It was observed that the peak mobilized friction angle for this dilatant material was different in undrained and drained tests; the difference is attributed to the fact that the rate of dilation is smaller in an undrained test than it is in a drained test. Consistent with the findings of others, the material is more resistant to undrained cyclic loading for triaxial compression than for triaxial extension. In rotational shear tests in which the second invariant of the deviatoric stress tensor is held constant, the shear stress path (after being normalized by the mean normal effective stress) approached an envelope that is comparable but not identical in shape to a Mohr-Coulomb failure surface. As the stress path approached the envelope, the shear end deviatoric strains continued to increase in an unsymmetrical smooth spiral path. During the rotational shear tests, the direction of the deviatoric strain-rate vector (deviatoric strain increment divided by the magnitude of change in Lode angle) was observed to be about midway between the deviatoric stress increment vector and the normal to a Mohr-Coulomb failure surface in the deviatoric plane. The stress ratio at the transition from contractive to dilative behavior (i.e., “phase transformation”) was also observed to depend on the direction of the stress path; therefore this stress ratio is not a fundamental property. Results from torsional hollow cylinder tests with rotation of stress directions are presented in new graphical formats to help understand and interpret the fundamental soil behavior.     

Finite-Element Analysis of Time Domain Reflectometry Cable-Grout-Soil Interaction

Three-dimensional finite element analysis is combined with field and laboratory measurement of time domain reflectometry (TDR) cable-grout response to analyze the interaction between the cable, grout, and surrounding soil mass during localized shearing. Finite element (FE) model parameters for the cable and cable-grout interface elements are back-calculated by matching results from laboratory shearing tests to FE calculated response. These parameters are employed in subsequent FE model geometries to model the behavior of TDR cable-grout composites in soft soils. Optimal grout and cable design is determined by analyzing the relationship between grout strength and stiffness and calculated cable shear stress.     

Interface Constraints on Shear Band Patterns in Bonded Metallic Glass Films Under Microindentation

When using the bonded interface technique for indentation tests, the semicircular and radial shear bands can be observed on the top surfaces and bonded interfaces in bulk metallic glasses (BMGs). In addition to the stress relaxation effects at the bonded interface, indentation tests on bonded BMG films on the steel platen further demonstrate the effects of the film/substrate interface on shear band patterns. The understanding of these shear band patterns will help design internal constraints to confine shear bands and thus to prevent brittle failure of BMGs. In contrast to previous studies, which connect shear band directions to principal shear stress or effective stress, as in the Mohr?CCoulomb model, this article adopts the Rudnick?CRice instability theory??shear bands are a result of loss of material stability but are not a yield phenomenon. Shear band directions depend on material constitutive parameters (including Poisson??s ratio, coefficient of internal friction, and dilatancy factor) and principal stresses. Consequently, internal constraints such as the bonded interface and film/substrate interface may redistribute the stress fields and thus affect the shear band propagation directions. Finite element simulations were performed to determine the contact stress fields using continuum plasticity model. It is found that semicircular shear bands on the bonded interface follow the direction of the second principal stress, while radial shear band patterns depend on the two in-plane principal stresses. With the presence of film/substrate interfaces, the radial shear bands will be ??reflected?? at the interface, and the semicircular shear bands change directions and end at the interface. It should be noted that the actual stress field differs from the continuum plasticity simulations because of the strain localizations associated with shear bands. To this end, an explicit history of shear band nucleation and propagation is simulated by the free volume model, which reproduces the change from radial to semicircular shear bands when interface relaxation is introduced. These predictions agree well with our experimental observations of microindentation tests on two Zr-based BMG films laterally bonded and placed on a steel platen.     

Postpeak Strength of Interfaces in a Stress-Dilatancy Framework

Laboratory sand-steel interface tests, using a range of sand sizes on a wide range of surface roughnesses, have been conducted using a direct shear apparatus modified to enable reliable measurements of both friction and dilation. The paper looks at the minimum interface strength after peak, termed here the postpeak strength, and assesses its dependence on roughness, density, and stress level. Its upper limit is the large displacement direct shear friction angle, related to but not equal to the critical state friction angle. When data are normalized by this value, they show linear dependence on the logarithm of relative roughness in the intermediate zone between smooth and rough. Once the roughness dependence of the postpeak strength has been allowed for, dilatant interfaces are found to follow classical stress–dilatancy relationships. It appears that there is no fundamental difference in the responses of sand-on-steel or sand-on-sand interfaces.     

Stress-Strain Responses of Block Samples of Compressible Chicago Glacial Clays

This paper presents the results and analysis of a laboratory investigation of the behavior of lightly overconsolidated compressible Chicago glacial clays over a wide strain range. Each specimen was trimmed from high quality block samples taken from an excavation in Evanston, Illinois. Specimens were instrumented with three sets of bender elements and local LVDTs. After K0 consolidation to the in situ vertical effective stress of the block, drained stress probe tests were conducted. Results of bender elements tests obtained prior to stress probing show that compressible Chicago glacial clay initially is cross anisotropic. Propagation velocities measured by bender elements in axial direction after K0 reconsolidation and drained creep agrees well with the in situ shear wave velocity measured by seismic cone penetration tests. Results of drained stress probe tests are analyzed in terms of shear, volumetric and coupled stiffness, stiffness degradation, and direction of loading. The significant variability of shear, bulk and cross-coupling response depending on stress path direction and strain level provide experimental evidence that the Chicago clays are incrementally nonlinear at the strain levels investigated.     

Behavior of Interfaces between Fiber-Reinforced Polymers and Sands

Conventional construction materials used in foundations can encounter serious durability problems in contaminated subsurface or marine environments. Fiber-reinforced polymer (FRP) composites are potentially suitable for these harsh environments due to their chemical and corrosion resistant properties. Quantification of the interface behavior between FRP composites and soils is a necessary precursor to the adoption of these new materials in geotechnical engineering practice. This paper describes the results of an experimental study that was conducted to investigate the behavior of sand-FRP interfaces. Tests showed that the interface shear behavior between FRP composites and granular materials depended on the relative roughness (surface roughness∕particle mean size), the normal stress level, the initial density of the soil mass, and the angularity of the particles. The soil specimen preparation method, the rate of shearing, and the thickness of the soil specimen had little influence on the measured interface friction coefficients. The characteristics of FRP-sand and steel-sand interfaces were compared.     

Role of Initial State, Material Properties, and Confinement Condition on Local and Global Soil-Structure Interface Behavior

Difficulty in predicting the transfer of load from a structural element to the surrounding soil has limited the reliability of geotechnical design and performance. The remaining uncertainty in load transfer mechanics is primarily due to the localized nature of the mechanism. This study examines localized soil-structure interaction through a series of monotonic direct interface shear tests. Parameters investigated include relative density, particle angularity, particle hardness, surface roughness, normal stress, and normal stiffness. The soil-structure interface behavior is quantified in terms of the local two-dimensional displacement and strain distributions within the test specimens using particle image velocimetry. In addition, the localized zone of soil adjacent to the structural surface within which shear and volumetric strains occur is quantified. The relative density of the soil, and the relationship between particle characteristics (angularity and hardness) and surface roughness are shown to have the greatest effect on local interface behavior, followed by confining stress and stiffness conditions.     

Residual Shear Strength for Interfaces between Pipelines and Clays at Low Effective Normal Stresses

The residual shear strength mobilized between pipelines and supporting soils at low effective normal stresses is needed for designing stable pipelines in offshore environments. A tilt table device is used to study the effect that effective normal stress, type of pipeline coating, composition of soil, stress history, and rate of loading have on the drained residual shear strength mobilized at the interface between a variety of clays and polymeric pipe coatings. The drained residual friction angles for both the interfaces and the clays decrease substantially as the effective normal stress increases. Empirical correlations published for predicting the residual strength of clays cannot be readily extrapolated to the pipeline problem because the correlations do not cover the relatively small effective normal stresses acting on pipelines. Residual shear strengths for the interfaces range from 60 to 90% of the residual shear strength for the clay. The residual shear strength for the interface depends both on the composition of the clay and the type of pipeline coating.     

When is a velar an alveolar? Evidence supporting a revised psycholinguistic model of speech production in children

Compressive stiffness (CS) of different supporting materials used in prosthetics and orthotics and their static coefficients of friction (COF) with skin and socks were characterized. Materials tested included Spenco, Poron, nylon-reinforced silicone, Soft Pelite, Medium Pelite, Firm Plastazote, Regular Plastazote, and Nickelplast. A displacement-controlled testing device was constructed to assess the CS of 11.1 mm diameter material specimens under cyclic loading (1 Hz) to 220 kPa over 10- and 60-min periods. Results demonstrated local CS ranging from 687 kPa (Poron) to 3,990 kPa (Soft Pelite). To fit the cyclic stress-strain (S-S) data within an error of 4.0 percent full-scale output, the minimum order of fit required for Spenco, Poron, and nylon-reinforced silicone was a third-order polynomial; for Soft Pelite, Medium Pelite, Firm Plastazote, and Regular Plastazote, a second-order polynomial; and for Nickelplast, a linear fit. For all materials, the nonrecovered strains were related to loading time using an exponential fit. A biaxial force-controlled load applicator device was used to assess COF at skin-material, sock-material, and skin-sock interfaces for shear forces of 1 to 4 N applied to a 10.2 x 7.8 mm loading pad. COFs ranged from 0.48 (+/- 0.05) to 0.89 (+/- 0.09). COFs at skin-material interfaces were significantly (p < 0.05) higher than those at skin-sock interfaces. There was a trend of a higher COF at sock-material interfaces than at skin-sock interfaces. These data are of potential utility in finite element modeling sensitivity analysis of residual limb-prosthetic socket systems or body-orthosis systems to characterize effects of material features on interface pressure and shear stress distributions.     

Interface Shear Connection Analysis of Ultrahigh-Performance Fiber-Reinforced Concrete Composite Girders

The purpose is to analyze the interface shear connection behavior for ultrahigh-performance fiber-reinforced concrete (UHPFRC) and normal concrete (NC) composite girders. The shape and dimension of the shear stud in the conducted tests are referenced from the traditional interface connection design and engineering experiences. The interface shear connection parameters, i.e., initial stiffness and slippage capacity of a single shear stud, are measured from three groups of lateral direct push test specimens with different numbers of studs. Based on the UHPFRC tensile failure characteristics and cracked section rotational mechanisms of the UHPFRC-NC composite structures with flexural, or flexural and shear failure, the limit state is defined as a full pullout from the bottom fiber of the UHPFRC girders. Pseudostrain hardening behavior of the UHPFRC is simplified as an equivalent rectangular stress block. From this mechanism, the interface equilibrium equations are constituted and the interface shear connection degree of the UHPFRC-NC composite girders is derived. It is recommended that the interface shear connection degree may be used as minimum design standard for UHPFRC-NC composite interface shear connection design.