Ebook: Characterization and Behavior of Interfaces

Interfaces exist in every geotechnical system in many forms and at multiple scales. Examples include interfaces between two particles, interfaces between pile or tunnel shafts and the surrounding soil and interfaces between synthetic polymeric sheets and the underlying natural soils in engineered landfills. Irrespective of whether the interfaces are between two natural materials, a natural and man-made material, or two man-made materials, they each have their own unique set of characteristics and behavior that are governed by many factors including relative roughness, relative hardness, stress conditions, load rate effects, drainage conditions, degree of lubrication and temperature conditions. While historically, interfaces were often considered to be the “weak link” in a system, particularly as the result of a number of unexpected catastrophic failures, and thus accounted for in many subsequent design procedures as such, new insight gained over the past twenty years by researchers around the world has shown that it is possible to select combinations of materials and design an engineered interface so that it is “at least as strong” as the surrounding materials. These new insights have been gained as a result of experimental evidence, numerical modeling and analytical study of successful and failed systems.

While individual technical papers have been presented and/or published in various forums and proceedings over the years, no technical event has ever been convened for the sole purpose of allowing for the exchange of information and ideas pertaining to geotechnical interfaces. The research symposium held in September 2008 in Atlanta Georgia, USA on The Characterization and Behavior of Interfaces addressed this deficiency and the papers presented at that event are contained in these proceedings. Topics presented and discussed included:

• Characterization of interface materials

• Laboratory evaluation of interface behavior

• Field characterization of interface behavior

• Physical modeling of interface systems

• Numerical modeling of interface systems

• Analysis of systems failing at interfaces

• Field monitoring of systems containing interfaces

J. David Frost, Savannah, Georgia 2009

A series of constant load (CNL) and constant stiffness (CNS) interface tests have been performed using sand in a large 1m diameter ring shear apparatus. CNS tests were performed using silica and carbonate sands to explore effects that volume changes have on the stresses developed on rough and smooth interfaces. Volume changes are shown to be partly caused by grain crushing, which also affects the interface friction angle. Volume changes due to grain crushing can be correlated with work done on the interface. Cyclic tests were performed and show that volume changes are dependent on the cyclic displacement amplitude. These results indicate that friction fatigue is more likely to be observed in dense fine grained sands.

The mechanical response of sand-steel interfaces evolves with shear displacement. Whereas the peak and immediate post-peak stages have often been the main focus, the response at substantially larger displacements has received much less attention. Here it is demonstrated that sand-steel interfaces are subject to wear processes and that interfacial wear can have a profound effect on ultimate response. Multi-reverse interface tests conducted within a modified direct shear apparatus demonstrate the occurrence of both sand particle attrition and steel surface abrasion even at low stress levels. Fractal geometry is used to show that surface abrasion can increase or decrease surface roughness. At quite modest shear displacements a ‘run-in’ angle of friction is mobilized that is both constant and virtually independent of the initial surface roughness. It may represent the most appropriate operational angle of interface friction in many field situations.

Modern methods for estimating the axial capacity of piles driven in granular media rely on accurate interface shear failure models. While earlier studies have focused on determining shaft friction failure parameters from small displacement laboratory shear box experiments, large displacement ring-shear interface tests provide a better representation of conditions adjacent to the shafts of driven piles. This paper describes systematic studies in which granular quartzitic media, ranging from angular rock flour to sub-rounded coarse sand, were sheared against concrete and steel interfaces in ring shear experiments that involved several metres of shear slippage. The study included an examination of how the large displacement processes involve grain crushing and modify the texture of the interfaces. Conclusions are drawn regarding the constant volume angle of interface shearing resistance that may be applied in pile design, the soils' particle size distributions and the roughness of the interfaces tested, before and after the ring-shear tests.

Traditional investigation of soil-structure interfaces has been limited to global measurements of force and displacement. As a result, interpretation of soil behavior at the particle scale has focused primarily on global response and the assessment of local soil behavior in terms of strain has not been possible. This paper presents a series of monotonic and cyclic tests carried out using a modified direct interface shear device that enables digital imaging of soil behavior within the test specimen throughout shear. Particle Image Velocimetry (PIV) is implemented to analyze the image sequence and quantify the development of volumetric and shear strains within the interface shear zone. Results indicate that initial void ratio and confinement conditions (normal stress and normal stiffness) influence the development of interface shear zone characteristics. Although PIV analysis compared well with global displacement measurements, complex interfacial soil behavior cannot be fully characterized solely with global measurements. The implications of the lack of local response characterization inherent in global measurements are illustrated in a critical state soil mechanics framework modified for soil-structure interfaces.

Pipe-jacking and microtunneling technologies are being more widely used over the past decade and there is significant interest to predict the jacking forces and jacking distances achievable in order to achieve more efficient design and construction. This study focuses on the evaluation of the frictional characteristics and factors affecting the shear strength of pipe-soil interfaces. Six different pipes made from fiber reinforced polymer (FRP), polycrete, steel, concrete, and vitrified clay were tested using a new apparatus designed to conduct conventional interface shear testing on pipes of different curvature. In addition, roughness tests were performed using a stylus profilometer to quantify the surface characteristics of the individual pipes and relate these to the interface shear behavior. To extend the range of roughness values tested, two artificial pipe surfaces were created using rough sandpapers. Interface shear tests were performed using the new apparatus with air-pluviated dense specimens of Ottawa 20/30 sand.

This paper presents results from ongoing research on friction–sliding deformation behavior between planar, solid construction material inclusions and particulates. Data from unlubricated pin on disk friction experiments conducted at high contact stresses using glass beads and subrounded sand in contact with high-density polyethylene (HDPE) polymer sheet, polyvinylchloride polymer sheet and stainless steel are presented. An incremental wear approach is used to model friction–displacement behavior. Predictions of friction coefficients as a function of sliding distance are in good agreement with the experimental data and offer insight into mechanisms potentially responsible for the observed behavior. It is estimated that for the HDPE sheet systems, plowing contributes 40 percent to 60 percent to the peak friction coefficient during initial shear displacement. However the contribution of plowing rapidly decreases to approximately 10 percent to 15 percent as displacement continues. Plowing is only about 8 percent to 10 percent of the total measured resistance after steady state is reached with the balance consisting of adhesion. Particle interaction with wear debris is found to be a significant source of friction with metal surfaces.

Composite systems comprised of particulate and continuum materials in contact at an interface are frequently encountered in geotechnical engineering. An understanding of the shear behavior occurring at this interface is of considerable importance as it governs the behavior of the entire composite system. Previous research in this area has focused on the behavior involving uniformly sized particulate materials. This is despite the fact that particulate mixtures comprised of differing particle sizes and shapes are most frequently encountered in practice. There is a need for a framework whereby the macroscale response of soil mixtures in contact with an interface can be quantitatively predicted from knowledge of the particulate and interface properties. Such a framework would prove useful in selecting the optimal materials for the design of engineered soils and interfaces. This research explores the case of binary mixtures of quartz sands in contact with smooth geomembrane surfaces. Index properties of the binary sand mixtures have been determined for different particle size ratio combinations. The particle size ratio of the binary mixtures ranges from 2.1 to 6.1 and the size of the particles ranges from 0.13mm to 0.78mm. A series of interface shear tests were conducted to determine the macroscale interface shear behavior and resulting geomembrane wear. The effects of mean particle size and particle size ratio were investigated. Changes to the surface topography of the counterface material are quantified using stylus profilometry and related to the characteristics of the sand mixtures.

We explore the effect of vibration on interfacial friction by applying normal and parallel base vibration to a block that rests on an inclined plane. Results show that the block can displace at significant lower angles than the limiting static angle, and that the acceleration level required to cause sliding increases with frequency. Limiting equilibrium analysis is insufficient to explain the observed behavior. Instead, we compute the displacement per cycle and show that (1) the apparent quasi-continuous motion of the block under vibration is the accumulation of successive slip-rest events, and (2) the displacement in every cycle must exceed a threshold displacement in order to cause sliding. The threshold displacement is 0.1 μm for polished granite surfaces; a relation between the threshold displacement and the length scale of surface features is anticipated.

Geomaterials can exhibit dramatic changes in mechanical behavior upon chemical and physical weathering. Specifically, chemical weathering of iron Fe(II) minerals yields products, including iron Fe(III) oxides, hydroxides and oxyhydroxides, which precipitate as coatings on rock and soil surfaces due to their low aqueous solubility. The general objective of this research is to readily isolate, for controlled study in the laboratory, the effect of such chemical weathering on the stress-strain relationships and strength of coarse-grained soils. Particles of fine silica sand were coated by chemical adsorption of the iron Fe(III) oxyhydroxide akaganéite onto the sand surface under controlled pH and ionic strength conditions, and at standard pressure and room temperature. Akaganéite is a chemical weathering product formed by precipitation after oxidation of iron Fe(II) minerals, and is a precursor of the chemical weathering products goethite and hematite. To identify the effect of the oxyhydroxide coating on soil behavior, both the coated sand and uncoated control samples were tested in direct shear. Chemical-weathering effects were thus evaluated independently from stress history, parent rock discontinuities, and biological influence. The results presented herein show that the oxyhydroxide coating increased the residual strength, dilatancy and friction angle of the sand, with no strain delay exhibited in the peak shear strength.

Frictional discontinuities in rock masses can be viewed as fractures and their behavior can be approximated using Linear Elastic Fracture Mechanics (LEFM) theory. Slip along a frictional discontinuity can be approached as initiation and propagation of a mode II fracture along its own plane. Fracture mechanics theories predict that under pure mode II loading initiation will occur when the energy release rate of the fracture attains a critical value (G_IIC), which is generally taken as a material property. The research conducted shows that this may not be always the case. Identification and quantification of the mechanisms for the onset and propagation of slip along frictional surfaces have been carried out by testing in biaxial compression two different sets of brittle specimens: acrylic and gypsum. The specimens consist of two or three blocks with perfectly mated contact surfaces. The contact surface between blocks is equally divided in two areas, one with a lower frictional strength (weak area) and the other one with a higher frictional strength (strong area). Experiments show that slip starts first in the weak area and progresses towards the strong area with increasing load. Once slip has reached the strong area, a sharp contact is created between the area that has slipped (weak) and the area that has not (strong). Results from the tests show that the critical energy release rate, G_IIC, depends on the frictional characteristics of the surface and on the critical displacement required to decrease the frictional strength from peak to residual. Furthermore, experiments conducted on surfaces with and without cohesion indicate that cohesive debonding and frictional mobilization may not occur simultaneously. The experimental results together with an analytical formulation within the framework of fracture mechanics explain some of the inconsistencies found in the literature and provide a clear picture of how slip initiates. This paper presents the experiments, analyses and formulations carried out in support of the conclusions.

Experimental studies of the behavior of dry sand-steel interfaces have been performed in a modified direct shear apparatus. The shear stress-displacement behavior is influenced by the normal stress, the density of the sand and by the relative roughness of the interface. The patterns of response – peak, softening, dilation – are similar to those seen in the shearing of sand and a model which introduces stress level and density through a state parameter is found to be successful in matching the observed shear stress and volume change behavior at both small and large shear displacements.

This paper presents a numerical study that uses two dimensional DEM simulations to determine how strain localizes inside an idealized interphase system composed of densely-packed spherical particles in contact with rough manufactured surfaces. The manufactured surface is made up of regular or irregular triangular asperities with varying slopes. Discrete data at the micro–scale have been homogenized and transformed into stress and strain using a new discrete–continuum analysis approach. Evolution of fabric, contact force anisotropies are recorded and used to generate stress tensors for the granular media at the continuum level. The strain field is generated using a new simple method directly based on motions of individual particles. Theoretical analysis of the stress-strain behavior occurring inside the interphase zone is made possible by combining the above approach with numerical simulations. Results show that uniform pure shear deformation occurs inside the interphase zone before strain localization initiates and nonlinear stress-strain behavior begins. The computed strain field shows a distinct but discontinuous shear band above the surface early before the peak state is reached. Anisotropies of fabric, contact normal force and contact shear force increase rapidly after shearing, leading to the increase of shear stress inside the interphase zone. The principal direction of contact total force anisotropy has exclusive control over the peak interface strength behavior. It is found that the thickness of the most intense shear zone is about 8 to 10 median particle diameters above the surface.

A main objective in all geotechnical site investigations is to determine the type, extent, and properties of the geologic materials in as much detail as possible within the constraints of the given site conditions and project budget. Cone penetrometers (CPTs), specifically piezocones (CPTUs), offer a site investigation tool that can effectively identify the behavioral type and extent of tested stratigraphy, and provide unparalleled profiling ability, with CPTU response times typically sufficiently fast to identify very thin layers (< 5 mm) (Lunne et al., 1997). Accurate geostratification and classification are paramount to successful geotechnical engineering practice, as the soil layering and classification often serve as the basis for all subsequent analyses and calculations. The values of CPT data are functions of a number of fundamental soil characteristics, and as such, the variations in measured response can be used to identify both the layering and properties of tested soils. The interface behavior of soils is known to vary as both a function of soil type and the contacting interface properties. Most notably soil – continuum interface response is known to be primarily affected by the angle of internal friction of the soil and the surface roughness of the counterface material. The multi friction attachment devices recently developed at Georgia Tech have the ability to provide in situ measurements of interface behavior across the full range of typical surface roughness properties encountered in geotechnical engineering and for all soil conditions amenable to penetrometer investigations. This paper analyzes the friction data obtained for the various geologic conditions tested to date, to investigate whether the use of the multi friction attachment (MFA) and multi piezo friction attachment (MPFA) can provide data allowing for improved in situ soil classification through the use of fundamental soil-interface behavior concepts.

The use of geosynthetic above subgrade or within base course has demonstrated their success on reducing rut depth and prolonging pavement life. Experimental studies showed that geosynthetics can minimize the movement of particles in the base course. A new performance-based method was developed by the authors to modify an Asphalt Pavement Analyzer (APA) that measures the rut depth of the geosynthetic-reinforced base for a desired number of wheel passes to evaluate geosynthetic-soil interaction. In this study, numerical software-Particle Flow Code (PFC) 2D, which is based on discrete element modeling (DEM) of micromechanics, was used to simulate this performance-based test to investigate the geotextile-soil interaction. The analysis was focused on the effect of geotextile location and packing density of particles on the rut depth of geotextile-reinforced bases. A similar approach can be used to evaluate geogrid-soil interaction. The results and phenomena from the numerical analysis are discussed and compared with experimental data. The behavior of unreinforced and geotextile-reinforced bases is captured qualitatively by DEM for medium dense particle assembly as compared with the experimental data. It is observed that the geotextile helped distribute contact forces to a wider area.

An investigation of shear mechanisms at interfaces between needle punched nonwoven geotextiles and HDPE geomembranes revealed that strain within the geotextile significantly reduces the interface resistance against both smooth and textured geomembrane surfaces. A combination of sliding and plowing mechanisms was found to yield the peak interface response of geotextiles sheared against smooth geomembrane surfaces. The primary contributing factors to shear evolution for unconstrained and constrained geotextiles sheared against moderately textured geomembranes are the confined tensile behavior of the geotextiles and the strength of the geomembrane texture elements, respectively. The shear-induced micromechanical interaction of the filament-texture system was observed using digital image analysis techniques and the resulting change in void structure was quantified in terms of local void ratio distribution and largest inscribing void size distribution.

The dry friction properties at the interface between EPS geofoam blocks were investigated by means of an advanced direct shear (DS) apparatus. A pair of EPS blocks having densities equal to 20 or 30 kg/m³ were used in respective DS tests performed at different constant normal stresses and at different constant driving velocities (i.e., different lateral displacement rates). It was found that the interface friction properties depend on the normal stress, the EPS density and the driving velocity in a specific way that is different from the classical trends usually observed with solid-solid interfaces: i.e., the pre-peak lateral displacement and post-peak strain-softening are noticeable, while the friction coefficient increases with an increase in the normal stress and with a decrease in both the density and the driving velocity.

Bentonite slurry is used to lubricate the pipe-soil interface to reduce frictional resistance during pipe jacking. Many types of bentonite slurry exist, most of which are specifically engineered to optimize performance in tunneling. The characteristics of commonly used bentonite slurry were evaluated in the laboratory and results indicate large differences in the viscosity between various products due to the additives. Interface shear tests were performed to determine the magnitude of friction reduction in sand/slurry mixtures. The presence of slurry has been shown to lubricate the interface and reduce the residual friction depending on the surface roughness of the pipe.

Molecular dynamics (MD) simulation is performed to model interfaces between asphalt and aggregate. An asphalt-quartz structure model of the interface is proposed in terms of density, position, thickness, and molecular orientations. CVFF_aug (Augmented Consistent Valence Force Field) force field is adopted to characterize the inter-atom interactions. The present simulations are among the first attempts to conduct full atomistic molecular simulations on a multi-component based asphalt-aggregate interface interaction. The chemical composition is chosen to represent compounds found in real asphalts. Different molecules are chosen to reflect saturate, naphthene aromatic, asphaltene components and quartz atom structure. The interfacial atom trajectories displayed properties in simulations which are qualitatively similar to those of real asphalt-aggregate interface shearing. It is found that the CVFF_aug force field provides qualitatively acceptable results for predicting the interface atom interaction between asphalt and quartz. The simulations were performed using the computer program LAMMPS.

In the current design practice the evaluation of the skin friction developed on piles shaft in sand is obtained using the “β method”, which accounts, implicitly, for a number of factors, controlling the magnitude of the ultimate shaft friction, τ_su, specifically: the initial state of the sand, its modification caused by pile installation, the soil shear strength and the roughness of the pile-soil interface. To analyse the friction interface behaviour, avoiding the effect of the base resistance, a series of 32 pull-out centrifuge tests have been performed, from which the β coefficients were determined and compared to those estimated from back analysis of loading tests on full scale piles. The β vs. depth profile gathered from centrifuge tests in dense sand was explained by the increment of the radial stresses on the pile shaft occurred during loading.

Interface between construction materials and soils plays an important role in many geotechnical systems, including retaining walls, shallow and deep foundations. Interface behavior is studied using split samples consisting of concrete block at the bottom half and granular soil at the top half of the direct shear test. The objective of this research is to assess the effect of aggregate crushing at the concrete-soil interface. To be able to control the large number of parameters affecting the interface behavior, granular soil was manufactured using fly ash by cold-bonding pelletization technique with predetermined shape, size, grain size distribution, surface roughness, water absorption, unit weight and crushing strength. The physical and mechanical properties of these aggregates were investigated by using conventional soil mechanics tests. Interface test results between lightweight aggregates and concrete showed that grain crushing, particle shape, level of normal stress and the surface roughness of the grains play an important role in the interface behavior between granular soils and concrete. At low normal stress values, the crushing strength of aggregates did not affect the interface behavior. For the synthetic aggregates studied, the ratio of interface to internal friction angle decreased with increasing crushing strength.

Jacking forces are of concern on microtunneling projects to ensure that pipe installation is completed without damage to the pipeline, failure of the shaft, or limitation of the tunnel drive length. Extensive field measurements have revealed a relationship showing that interface friction between the jacking pipe and the soil are directly related to the jacking force required to propel the microtunneling machine and pipeline. Jacking force measurements are evaluated in sections of the tunnel where no lubrication is applied to the outside of the pipeline and in subsequent sections where lubrication is introduced. Results of the evaluation for lubricated and non-lubricated segments are presented.