ABSTRACT
Gautier, Kocher, and Drean [39] have analysed the tensile behaviour of nonwoven geotextiles using optical strain-mapping techniques along with infrared thermography. They have tried to establish that needlepunched nonwovens are more anisotropic in nature in comparison to thermal or heat-bonded nonwovens. However, no information about the structure of nonwovens was provided which can help in drawing such conclusions. The tensile properties of a geotextile are generally determined through strip or grab test methods [10], but these should be measured in a manner which simulates the operational conditions of geotextiles, i.e. under confined stresses. In the past, many researchers have suggested that the tensile characteristics of geotextiles should be determined under confined conditions [40^15]. Andrawes, McGown, and Kabir [46] have reported the uniaxial tests on woven and nonwoven geotextiles by investigating the effect of width-to-length ratio of the specimens, test temperature, pressure and strain rate. It has been demonstrated that woven geotextiles are more susceptible to changes in the strain rate and temperature than nonwoven geotextiles. Nonwovens undergo structural realignment under uniaxial loading, whereas wovens have straight yams, which are highly sensitive to temperature and strain rate. Here, the intrinsic properties of polymer play a key role in determining the deformation behaviour of woven geotextiles. Furthermore, it was also found that the shapes of stressstrain curves are significantly affected at higher confined stresses and cannot be simulated
with a wider specimen size [47]. Therefore, an apparatus (shown in Figure 5) consisting of metal box with two bellows for applying confined pressure through a soil layer was built but thin clamps were used, which limited its usage for general testing. Subsequently, a stand-alone pressure chamber using ordinary grips was developed for a confined tensile test [10]. Here a composite specimen (see Figure 6) consisting of a glued section at the ends and unglued region in the middle portion were prepared and tested with and without confining pressure. Hence, the stress-strain response of the glued region under confined pressure can be determined by testing a fully glued sample without confined stresses. However, the properties of glue are required to be known in order to obtain accurate results. Nevertheless, one of the most significant findings obtained from these studies of tensile tests under confined conditions is that there is a significant increase in the stiffness and strength of the geotextiles in comparison to the unconfined conditions. However, it is quite contentious as these in-soil tests do not simulate the real conditions of a geotextile in a reinforced structure as it overcomes the frictional resistance against the stationary soil before the tensile load
within the structure is mobilised and this results in load corresponding to the combined effects of the frictional force and the stress confinement [41]. In geotextile-reinforced structures, slippage at the soil-geotextile interface does not occur until a failure state is approached, which tends to overestimate the strength and stiffness of a geotextile.
