Tensile Behavior Of Geotextiles Confined In Granule

Reinforcing a soil mass with extensible inclusions, such as geotextiles, has gained increasing popularity in earth structure construction. Application of such geo-reinforcement, however, needs a proper understanding of the effects of soil confinement on geotextiles due to its non-linear deformations associated with highly extensible nature. The tensile property of geotextiles determines the quality and safety for engineering project. The purpose of this study is to interpret the interface interaction performance between geotextiles and granule and to develop a theoretical model for predicting tensile behaviour of geotextile confined in granule on the basis of its non-linear characteristics.Laboratory pull-out and confined extension tests are widely used to understand and evaluate the interface mechanisms and the impact of soil for geotextiles. A simple test device composed of a confinement box and spring loading device which designed to link to the Instron material testing system was developed with an overview of current typical testing methods. Interface friction and tensile behaviour of geotextiles with and without confining pressure is investigated through the established experimental instrument.A power function was proposed to describe the stress-strain relationship of geotextile on the basis of tensile test without confinement. Fit results reveal that the power function can well expressed different non-linear characteristics of woven and nonwoven needle-punched geotextile. The two fit parameters (a, b) have definitely physics meaning representing the non-linear characteristics and extensibility of geotextiles in quantification, respectively.The frictional characteristics between geotextiles and the granule under different normal stress were investigated by pull-out tests.According to the definition of frictional coefficients, the peak and residual pullout load of six specimens were measured to calculate the statics and kinetic frictional coefficients under different confining pressure at 200mm of embedded length. Test results reveal that the frictional coefficients differ with the specification of specimens and types of geotextiles. The frictional coefficient was seen to reduce with the increase of the normal stress applied to the geotextile-granule interfaces for the needle-punched nonwoven geotextiles. This is thought to be due to dilatancy effects and intermixing of particles with the surface of the fabrics thereby reducing the roughness of the interface. A hyperbolic function can be used to describe the relationship between the interface coefficient of friction and applied normal stress for the needle-punched nonwoven geotextiles. The pullout tests results show that the coefficient of friction of woven geotextiles does not change with the applied confining stress. This is because of higher surface density of woven geotextiles. And particicles does not set in geotextiles.The pullout processes can be divided into two stages: static friction and kinetic friction. The pullout force increased continuously during the static friction stage till the maximum. The pullout tests of specimens at different embedded length reveal that the increasing of pullout force related to the transferred length. The shear stress is well-distributed as a result of linear fit between the pullout force and the transfer length of specimens.Analysing the pattern of the pullout curves it is evident that the pullout behaviour is strongly influenced by the pullout stiffness, the applied normal stress, the embedded length and by the frictional coefficient of geotextiles. The relation between the pullout force and displacement can be described by the parameters above mentioned. The pullout stiffness determines the curvature and the non-linear characteristics of pullout curves of geotextiles. The pullout curves under different normal stresses show that granule restricted the deformation of geotextiles. As a result, the peak pullout force and the pullout stiffness of geotextiles increase with the normal stress. The embedded length of geotextiles has effects on the peak pullout force of geotextiles. The peak pullout force increase along with the increasing embedded length as a result of offering the larger interface area. But the embedded length does not influence the extensibility of geotextile and the pullout stiffness is not changed with the increasing of specimens length.The experimental results also show that the extensibility of geotextile has an influence on peak pullout strength and pullout displacement. The geotexiles possessed high modulus has higner pullout stiffness and peak pullout force but lower displacement.Analysing the pattern of the tensile curves of geotextiles confined in granule, it is evident that the influence of granule confinement on the needle-punched nonwoven and woven geotextiles is different. Two stages existed in tensile process of needle-punched nonwoven geotextiles. The first stage is the initial pullout stage, in which the stasic friction force increased with the increasing of transferred length before slippage between specimens and granule interface and the slope of the curves increasing rapidly. The following stage is commonly extension of geotextiles. And the interface friction change to kinetic friction which the slippage happened between geotextiles and granule. The shape of the curves is similar to that of unconfined extension. The confined extension test results also reveal that the initial modulus and breakage strength improved and deformation decreased of the needle-punched geotextiles as a result of granule confinement. However, the influence of granule confinement is not evident as a result of higher modulus for the woven geotextiles. Compared to the unconfined extension, the initial slope of tensile curves has no change and the boundary between static friction and kinetic friction is not clearly.Based on the analyses of interface frictional characteristics and confined extension tests, a theoretical model for predicting behaviour of geotextile confined in-granule was then proposed. For the developed model, a non-linear geotextiles for which the stress-strain is assumed to meet the relation:σ=aε~b. And the shear stress on the geotextile-granule interface along the embedded length is assumed uniform. Following the two stages in tensile process, the established model has two expressions which present the initial pullout and the follow-up extension process, respectively. And the coefficient of friction is different at the first stage and the following stage. Then the tensile behavior of geotextiles confined in granule can be predicted by the related parameters, such as the unconfined fit parameters a and b, the normal stress, static and kinetic frictional coefficients, the embedded length, the thickness of specimens and so on.To validate the confined extension model, the predicted tensile curves are compared with the laboratory test results. There is a closer fit between the numerical model and the experimental results for woven geotextiles. And so do the second stage for needle-punched geotextiles. Some discrepancies at the first stage in the predictions were explored for needle-punched geotextiles. Based on the non-linear relation between the peak pullout load and the embedded length, shear stress was assumed nonuniform distribution. And a shear stress distributed function was proposed to modify the original model. The agreement between experiment and the modified model shows that the shear stess distribution is probably nonuniform along the embedded length for geotextile which has lower modulus and higher deformation.The model developed in this study is in common use in view that the unconfined stress-strain relation of geotextiles can be described by a power function (σ=aε~b). And the tensile non-linear extent of geotextiles is different with the the simulated parameters (a, b) changed.The confined extension model of geotextiles provides a new method to evaluate the tensile behaviour of geotextile in practical use. It is benefit from analysis method in this study to predict exactly the tensile behaviour of geotextiles confined in granule according to the shear stress distribution and the stress-strain relationship of geotextiles which possessed nonlinearity.