Soil-reinforcement Interface Research Articles

Moisture Reduction Factors for Shear Strength of Unsaturated Reinforced Embankments

AbstractThis paper describes the construction and testing of six 1-m-high model embankments constructed at three different gravimetric water content (GWC) values to study their performance and to validate a set of moisture reduction factors (MRFs) introduced by the authors in their recent studies. The earlier MRF values were obtained from a series of pullout and interface shear tests on the same soil and reinforcement materials. The MRF in this study is defined as the ratio of soil-reinforcement interface shear strength at an increased GWC value [e.g., optimum moisture content (OMC) + 2%] to the shear strength at OMC − 2% representing construction conditions. Therefore, MRF values can be used to account for an anticipated reduction in the shear strength of the soil-geotextile reinforcement interface caused by wetting in the stability analysis and design of reinforced soil structures constructed with marginal soils. The embankment models were built using a lean clay (CL) at the GWC values ranging between O...

Behaviour of extensible reinforcement resting on non-linear Pasternak subgrade subjected to oblique pull

A generalised model is presented for evaluation of pullout capacity of an extensible reinforcement subjected to oblique pullout force. The reinforcement is assumed as a rough membrane resting on a non-linear Pasternak subgrade. The shear stress mobilised along the soil–reinforcement interface is assumed to be a non-linear hyperbolic function of the interface displacement. The analysis considers the non-linear response of subgrade in shear as well as in vertical compression. The solution is obtained by applying vertical and horizontal force equilibrium to the final deformed shape of the reinforcement. A parametric study shows that pullout is the predominant failure mechanism for high values of reinforcement stiffness J*, whereas for low values of J* the maximum reinforcement strain may exceed the allowable limit, causing tension failure before pullout. A back-analysis of published and model tests data shows that, in most cases, the present analysis predicts the experimental data reasonably well.

Unsaturated soil–woven geotextile interface strength properties from small-scale pullout and interface tests

ABSTRACT: One main concern related to the performance of unsaturated soils during the construction and service life of earthen structures is loss of matric suction due to the seasonal variations of gravimetric water content (GWC), ground water infiltration and possible development of excess pore water pressure. In addition to reducing the soil shear strength, loss of matric suction as a result of wetting could also reduce the soil–reinforcement interface shear strength in comparison with the as-built value at a lower GWC. This paper presents the results of small-scale pullout and interface tests on a woven geotextile reinforcement material in different marginal soils in order to quantify the difference in the soil–geotextile interface shear strength as a function of GWC for practical applications. A moisture reduction factor [MRF = μ(ω)] is used to account for the reduction in the soil–geotextile interface shear strength as a function of matric suction over a range of GWC values that includes the dry and wet sides of the soil optimum gravimetric water content (GWCopt) or optimum moisture content (OMC). It was observed that the interface shear strength of geotextile reinforcement in marginal soils could be significantly lower (e.g. by as much 50%) at only 2% wet of optimum (i.e. OMC+2%) in comparison with OMC−2%, which is assumed to represent the as-built condition.

Continuum Modeling and Analysis of Polymeric Sheet Reinforcement Subjected to Axial Pull

Stability of reinforced soil structures depends on the soil–reinforcement interaction. Continuum modeling of polymeric sheet reinforcement–backfill interaction when subjected to axial pull at one end of the reinforcement is presented. Analysis is carried out by FLAC, soil is modeled as linear elastic material and reinforcement as cable elements. Displacements of reinforcement are quantified for different pullout forces. Variations of displacement and tension along the reinforcement length are presented. Effect of axial stiffness of reinforcement, soil–reinforcement interface shear stiffness and length of reinforcement are quantified. For a given pullout load the displacement of reinforcement decreases with increase of interface shear stiffness, axial stiffness of reinforcement and length of reinforcement. Results of the present numerical model compare closely with other mechanical models and pullout tests.

Seismic stability of reinforced soil walls under bearing capacity failure by pseudo-dynamic method

In order to evaluate the seismic stability of reinforced soil walls against bearing capacity failure, the seismic safety factor of reinforced soil walls was determined by using pseudo-dynamic method, and calculated by considering different parameters, such as horizontal and vertical seismic acceleration coefficients, ratio of reinforcement length to wall height, back fill friction angle, foundation soil friction angle, soil-reinforcement interface friction angle and surcharge. The parametric study shows that the seismic safety factor increases by 24-fold when the foundation soil friction angle varies from 25° to 45°, and increases by 2-fold when the soil-reinforcement interface friction angle varies from 0 to 30°. That is to say, the bigger values the foundation soil and/or soil-reinforcement interface friction angles have, the safer the reinforced soil walls become in the seismic design. The results were also compared with those obtained from pseudo-static method. It is found that there is a higher value of the safety factor by the present work.

Reinforcement Pullout Capacity in Mechanically Stabilized Earth Walls with Marginal-Quality Soils

Pullout capacity of geotextile reinforcement is an important consideration in the analysis of internal stability of reinforced soil structures, especially those constructed with marginal soils. Precipitation, ground water infiltration, and seasonal variations of water content during the construction process or service life of the structure could result in significant reductions in the matric suction and lead to a reduction in the strength of the soil–geotextile interface. Consequently, the reinforced soil structure may experience unacceptable deformations or even failure during its construction or postconstruction periods. The loss of matric suction in the soil influences both the shear strength of the soil and the soil–reinforcement interface. However, the focus of this study was merely on the latter. Nine pullout tests and 18 interface shear tests were performed to measure the pullout resistance of a reinforcement geotextile in a marginal soil that was compacted at different gravimetric water contents (GWCs). The marginal soil was selected to meet the limiting requirements of the National Concrete Masonry Association guidelines for segmental retaining walls with respect to fines content, gradation, and plasticity. The range of GWC values investigated varied from the dry to the wet side of the optimum moisture content of the soil. The matric suction in the soil was measured to evaluate its influence on the soil–reinforcement interface shear strength. A moisture reduction factor is proposed to account for the reduction in the soil–geotextile interface strength as a result of the loss in matric suction.

To A Probabilistic Approach of Reinforced Slope Stability Analysis

A developed probabilistic model to analyze reinforced slopes is presented in this paper where, a log-normal distribution law and a first order of second moment method FOSM are used. Moreover, an interactive code, using C ++ language, based on a developed model, is carried out. With idea to validate the developed model, a parametric study was conducted, considering three types of current soils, taking into account the following parameters: cohesion ''C'', internal friction angle ''φ '', the unit weight ''γ'' and apparent soil-reinforcement friction angle ''α''. Expected variables were: reliability index ''  '', the probability of failure (ruin) Pr'' and the safety factor '' FS. Obtained results show that the friction angle φ and cohesion C seem most significant while, the reinforced soil unit weight ''γ'' and the soil-reinforcement interface friction angle ''α'' show a slightly sensitive influence. Besides, it is clearly showed that safety factor increases with the increase of reinforcement length.

Unsaturated soil–geotextile interface behavior

The behavior of mechanically stabilized earth (MSE) structures under seasonal climatic variations, i.e. wetting and drying, is not well understood. Stability and serviceability of MSE walls and embankments can significantly depend on the soil-reinforcement (e.g., geosynthetics) interface shearing behavior in unsaturated conditions. This is especially true for reinforced soil slopes and embankments that have significant fines contents. This paper presents results of a laboratory study on the mechanical behavior of unsaturated soil-geotextile interfaces using a specially modified direct shear apparatus. Several suction-controlled laboratory tests were conducted to investigate the effect of soil suction on the soil-geotextile interface. Results of the study indicate that the peak shear strength of the soil-geotextile interface increases nonlinearly with the soil suction. On the other hand, while inconclusive, the effect of suction on the post-peak shear strength of the interface was negligible in some cases. An elastoplastic constitutive model was used to simulate the laboratory results. This study demonstrates that the constitutive model is capable of capturing the mechanical behavior of the unsaturated soil-geotextile interface subjected to constant suction. Both shearing and volume change responses were reasonably simulated by the model.

Seismic displacement analysis of a reinforced soil model wall considering progressive development of reinforcement force

Progressive development of the reinforcement force in a geosynthetic-reinforced model soil retaining wall subjected to irregular ground accelerations was investigated in a pseudo-static-based analytical model. The progressive development of the reinforcement force was expressed as a function of maximum input ground accelerations in the proposed analytical model, which also incorporated a multi-wedge failure mechanism and Newmark's sliding-block theory. This analytical model eliminated a disadvantage inherent in the conventional pseudo-static approach, namely a possible overestimation of the seismic stability of the wall due to the assumption of full mobilization of the reinforcement force regardless of the intensity of ground excitation. It was also found that the value of the critical seismic coefficient [Formula: see text] calculated using a peak internal friction angle (ϕp) and a fully mobilized reinforcement pull-out force can be used to simulate the ultimate state of the model wall when subjected to irregular ground excitations. A softened friction angle of ϕ = 0.9ϕp simulated the wall behavior well when the horizontal displacement of the wall (Dh) was greater than 5% of the wall height (Ht). This finding is consistent with that for a conventional gravity-type soil retaining wall previously reported. A non-uniform distribution of tensile force along the reinforcement, with a peak value at the intersection of the reinforcement and slip surface and a reduced value at the facing-backfill interface was obtained, using a reduction factor of Rr–f = 0.5, based on the calibration of seismic displacements of the tested wall. A reduction factor that accounted for the mobilization of the friction angle along the soil-reinforcement interface, Rr–s, was also calibrated, based on the displacement analysis. It was found that values of Rr–s between 0.7 and 0.8 could be used to represent the pull-out behavior of reinforcement subjected to seismic loading. The proposed method is based on a single set of experimental results and therefore the applicability of the proposed method needs to be verified using more physical tests and/or numerical analyses of various wall geometries and reinforcement configurations.

Dilatant stresses at the interface of granular fills and geogrid strip reinforcements

Design guidelines for geosynthetic-reinforced soil walls recommend the use of densely compacted granular soils as select fills. The prevailing construction practices employ two reinforcement layouts. In one, geosynthetics are laid out continuously throughout the length of reinforced area (sheet reinforcement): this layout is usually associated with wrap-around and modular block-type facing units. In the other, geosynthetics are laid out discretely (strip reinforcement): this is usually associated with panel-type facing units. These two reinforcement layouts interact differently with densely compacted granular fills, which are inherently dilatant. Sheet reinforcement corresponds to a free dilatancy condition whereas strip reinforcement corresponds to a restrained dilatancy condition. The effect of restrained dilatancy results in an increase in normal stresses or mobilization of dilatant stresses at the soil–reinforcement interface during reinforcement pullout and in turn generates additional localized compressive stresses in the surrounding granular fill. This has a generally positive influence because it enhances the pullout resistance of the reinforcement and also raises the effective stresses, resulting in an increase in the shear strength of the granular fill and thus improving the internal stability of reinforced soil walls. This paper presents an extension to earlier work by Alfaro et al. on the pullout interaction mechanisms of geogrid strip reinforcements. It examines the mobilization of dilatant stresses at the soil–reinforcement interface during reinforcement pullout. The study shows that it is essential to take into account the influence of dilatant stresses in the internal stability analysis of reinforced soil walls that use geogrid strip reinforcements.

Laboratory Evaluation of Pullout Capacity of Reinforced Silty Sands in Drained and Undrained Conditions

Abstract This paper presents results of laboratory pullout tests performed to determine the drained and undrained pullout capacity of reinforced silty sands. The pullout tests were conducted for different soil types: clean sand and silty sand with 5, 10, 15, and 35 % silt, and overburden pressures: 30, 100, and 200 kPa. Pullout experiments were performed on a large pullout box with dimension 1.5 m long, 0.5 m high, and 0.4 m wide, where a single steel inclusion was placed inside a mass of homogeneous soil. Both drained and undrained pullout capacities depended on the silt content and increased with internal friction angle of the soil and with overburden pressure. Undrained conditions could significantly reduce the pullout capacity. The reduction was caused by the generation of excess pore pressures in the soil under rapid loading, which in turn decreased the effective normal stress at the soil-reinforcement interface.

Limit Loads on Reinforced Foundation Soils

Reinforced soil is a practical solution to construction of unpaved roads or placement of footings over weak soils. A stability analysis of reinforced foundation soil is presented in this paper. A method is suggested for calculating limit loads on strip footings over foundation soils reinforced with horizontal layers of geosynthetics. Separate solutions are given for a case where reinforcement layers slip within the soil, and for a case where reinforcement ruptures. In the former case, an increase in the bearing capacity is dependent on the characteristics of the soil-reinforcement interface, whereas in the latter case it depends on the strength of the reinforcement. In both cases the increase in the limit load is dependent on the internal friction angle of the soil. Expressions are developed for quantitative predictions of bearing capacity of the foundation soil with horizontal layers of reinforcement. An optimum reinforcement depth is also given.

A macroscopic constitutive law for elasto-plastic multilayered materials with imperfect interfaces: application to reinforced soils

This paper concerns the formulation of a macroscopic constitutive relation for multilayered materials with imperfect interface, which allows taking into consideration the possibility of slipping at the interface of constituents. The model is developed using the homogenization approach assuming elasto-plastic behavior for constituents. The paper is composed of three parts. The first one presents the mathematical formulation of the macroscopic constitutive relation. The second describes the numerical implementation of this relation in a finite element program. The last part concerns the use of this constitutive relation to study the influence of the interface properties on the behavior of reinforced soils. Results of numerical simulations show the necessity of the consideration of slipping at the soil-reinforcement interface in the calculation of reinforced earth structures.

A Theoretical Model for Anchored Geosynthetics in Pull-Out Tests

A theoretical model is presented for the pull-out response of strip anchor reinforcement based on a nonlinear elastic (hyperbolic) shear stress-displacement relationship for the soil-reinforcement interface. The interface pull-out mechanism has been simplified by formulating the boundary conditions for the anchor section. Additional terms for the soil-reinforcement interaction, which incorporate the anchor section, are also introduced. A new concept of an anchorage factor (anchor-to-strip reinforcement capacities), the relative stiffness, and the displacement parameters are defined to explain the pull-out behaviour. A parametric study is carried out for a range of relative stiffness values, interface shear stresses, and anchorage factors. The normalised load-displacement relationship and the variations of pull-out force and displacements with distance along the reinforcement, at different anchorage factors, are presented. Experimental pull-out test results for planar reinforcements are also examined/compared with the model using a zero anchorage factor. The model helps to evaluate the pull-out displacements, stresses, and strains along the length of the strip anchor reinforcement.

Changes in Interface Stresses During Pullout Tests on Geogrid Strip Reinforcement

Abstract For a strip geogrid reinforcement, pullout resistance per unit width is known to increase with decreasing specimen width at low applied normal stresses. This phenomenon can be explained by a conceptualized model for a pullout interaction mechanism consisting of combined 2-D and 3-D interaction mechanisms, and the results obtained from laboratory pullout tests can be extended to the field. In this paper, a series of pullout tests on stiff extruded geogrid specimens embedded in dense sandy gravel was carried out and the normal stresses at the soil-reinforcement interface were measured by small-diameter earth pressure cells. On the basis of these tests, the fundamental behavior of soil-geogrid reinforcement interaction is explained and the conceptualized pullout interaction model is corroborated. It is shown that restrained positive dilatancy, observed at low applied normal stresses, results in an increase in actual normal stresses at the soil-reinforcement interface at the edges of the geogrid, thereby increasing the pullout resistance of the reinforcement.

Constitutive modelling of a reinforced soil using hierarchical model

Drained triaxial tests are conducted on natural and reinforced sand under various stress paths. Direct shear tests and pull-out tests are conducted on soil–reinforcement interface and on reinforcement, respectively. The effects of two types of reinforcement, viz, woven and non-woven geotextile and number of layers of reinforcement are investigated. Hierarchical single surface model is used to depict the behaviour of natural and reinforced soil by treating the soil as a single composite material and by considering soil, reinforcement and interface as independent elements. It is shown that the material parameters are very much affected by the type and the number of layers of reinforcement. The hierarchical model provides satisfactory prediction for both natural and reinforced soil. Copyright © 1999 John Wiley & Sons, Ltd.

Soil-Geosynthetic Interaction - Influence of Soil Particle Size and Geosynthetic Structure

This paper reports the results of pullout tests on five different geosynthetics embedded in two different granular soils. Soil and geosynthetic properties are described and soil-geosynthetic interaction behaviour is studied. Based on the results of pullout tests, the influence of soil particle size, geosynthetic structure, and the role of geogrid bearing members are discussed. The main conclusion is that the influence of soil particle size on soil-geosynthetic interaction is important, but its significance depends on several factors. With geogrids, the relative sizes of soil particles and geogrid apertures, and the thickness of the geogrid bearing members, determine soil-geogrid interface shear resistance. A marked increase in soil-geogrid interface shear resistance was observed when the soil contained a significant percentage of particles with sizes slightly greater than the thickness of the geogrid bearing members, but smaller than the geogrid apertures. Tests, on geogrids in which the bearing members had been cut, show a significant decrease in soil-geogrid interface shear resistance. The influence of soil particle size is less important for geotextiles and geocomposites. Although the structure of geotextiles and geocomposites has an effect on soil-reinforcement interface behaviour, mobilised pullout resistance is also affected by the axial tensile stiffness of geotextiles and geocomposites.

Comparative Model Study of Geosynthetic Pull-Out Response

This technical note describes the analysis of reinforcement pull-out tests using a shear model that incorporates a hyperbolic shear stress-displacement relation for the soil-reinforcement interface. Numerical studies of pull-out tests were performed for small to large strains in inextensible and extensible reinforcements. Predictions based upon a hyperbolic model of shear mobilisation are compared with a theoretical bilinear model presented by Madhav et al. in 1998. Comparative parametric studies using both models were carried out for ranges of relative stiffness and bond resistance values. Normalised load-displacement relations and the variations of pull-out force and displacements with distance, are presented for comparative evaluation. The finite difference equations of the hyperbolic model result in a faster convergence than the bilinear model. The incorporated hyperbolic model estimates the pre- and post-yield deformations, and shear stress and tensile force variations along the length of the reinforcement. Predictions of laboratory and field pull-out results from the available theoretical and experimental tests for inextensible steel straps, and extensible geotextiles, polymers, and nylon geosynthetics are also presented.

Settlement of Reinforced Sand in Foundations

The technique of reinforcing soil for foundation improvement is well established. This paper addresses the aspect of settlement of reinforced sand foundations, where the major part of the existing work deals with the aspect of bearing capacity. A detailed analysis is made paying individual attention to soil, reinforcement, and the interface between the two. A three-dimensional, nonlinear finite-element analysis is presented that uses a three-dimensional, nonlinear soil-reinforcement interface friction element, along with other three-dimensional elements to model the system. The results of the analysis are compared with those from tests conducted in the laboratory and are found to be in good agreement. The studies lead to a better understanding of the behavior of the system at different stages of loading.

Modelling of interface problems by the finite element method with considerable displacements

The method proposed here is intended for modelling interface problems where there is considerable displacement by using the finite element method. This method is different from standard formulations (contact elements or others) in that it takes into account special conditions for ensuring contact between any of the nodes of the interface (condition of non-interpenetration of areas and condition of equilibrium of the forces on either side of the slip line). This method is of particular interest in the case of considerable displacement when the contact areas deform a great deal and where the relative displacement between these areas is great. Numerous applications are possible since the modelling can be applied to all flexible and deformable structures where there are preferential slip planes (soil-structure contact and soil-reinforcement interface in the case of reinforced structures).