1. Introduction
The design of an engineering structure includes the use of safety factors to properly safeguard the occurrence of permanent and variable actions. For instance, safety factors should be applied to the properties of a construction material to prevent the possible occurrence of adverse deviations of those properties over time.
The use of geosynthetics in engineering structures has become a common practice in the last decades. Their high versatility and efficiency, associated to the ease of installation and relatively low cost, make them suitable construction materials to perform many different functions, for example, protection, reinforcement, separation, filtration, drainage and fluid barrier [
1,
2]. Geosynthetics can be used in a wide range of applications such as embankments, roads and railways infrastructures, retaining walls, erosion control or coastal protection, in which they may be exposed to the action of many degradation agents. Creep, abrasion, high temperatures, oxygen, atmospheric agents (e.g., solar radiation) and chemical substances (e.g., acids or alkalis) are examples of agents that may affect the durability of geosynthetics [
1,
3]. An extended exposure to these agents can promote relevant changes in the properties of geosynthetics, compromising their performance and reducing their lifetime [
1,
3].
The installation procedures may also affect the properties of geosynthetics and, consequently, have a negative impact on their performance. The handling of geosynthetics and the placement and compaction of filling materials over them may induce damage (predominantly mechanical) on their structure [
1,
3]. Indeed, in a wide range of applications, it is during the installation process that geosynthetics are submitted to the highest mechanical stresses [
3,
4]. The installation procedures may induce cuts in components, tears, holes and, consequently, a reduction in mechanical resistance of geosynthetics [
5,
6]. The survivability of these materials during installation is highly dependent on different factors, such as the physical properties of geosynthetics, the characteristics of the soils, the compaction energy and the use, or not, of adequate installation procedures [
4,
6,
7,
8]. In addition to the types of damage mentioned before, the installation procedures may also provoke abrasion on geosynthetics due to the mobilization of frictional forces in the interface between these materials and the contacting aggregates. For example, abrasion is prone to occur during the placement and compaction of filling materials in road construction or during the sand filling of geosystems (e.g., geocontainers, geobags or geotubes) for coastal protection. In some applications (such as roads or railways infrastructures), the occurrence of abrasion is not restricted to the installation phase. Indeed, due to the occurrence of cyclic loads over time (e.g., resulting from vehicle traffic), geosynthetics may also experience abrasion. The effect of the installation process on the properties of geosynthetics must be known and accounted for at the design phase. Depending on the application, the effect of abrasion (if considered a relevant degradation mechanism) has also to be taken into account.
The level of damage imposed by the installation procedures to geosynthetics can be simulated through field tests (which require the use of heavy equipment and significant human and financial resources) or by conducting laboratory tests (which try to simulate the in situ damaging actions). The standard EN ISO 10722 [
9] describes a laboratory method to induce mechanical damage under repeated loading on geosynthetics. Many authors have used this method to estimate the damage suffered by geosynthetics during installation [
8,
10,
11], while others tried to establish correlations between this method and the damage occurred under real installation conditions [
6,
12]. Pinho-Lopes and Lopes [
6] concluded that the laboratory damage (induced by the method presented in ENV ISO 10722-1 [
13], which was superseded by EN ISO 10722 [
9]) can be more severe to geosynthetics in comparison with field installation damage. The effect of abrasion on geosynthetics has not been as studied as installation damage. However, there is also a method (described in EN ISO 13427 [
14]) for evaluating the resistance of geosynthetics against this degradation mechanism. This method can induce relevant damage on those materials, such as cuts in components, splitting or disintegration, significantly affecting their properties [
15,
16].
The evaluation of the damage suffered by geosynthetics during the degradation tests is often accomplished by analysing the changes occurred in their mechanical properties [
4,
5,
6,
8,
15,
16]. In addition, the changes occurred in their hydraulic properties have also been assessed in some works [
8,
16,
17]. Mechanical characterisation (after the degradation tests) often includes the determination of the tensile [
4,
5,
6,
8,
15,
16] (especially relevant for geogrids, which are used for soil reinforcement) and puncture [
8,
15] behaviours of the geosynthetics. Hydraulic characterisation (for example, by water permeability tests) is important when considering the use of geosynthetics for performing filtration or drainage functions.
The design values of geosynthetics must take into account the level of degradation that those construction materials are going to experience over time. For that purpose, partial reduction factors are often used, each representing the effect (known or estimated) of one, or more, degradation agents [
18,
19,
20]. For example, for reinforcement applications, ISO/TR 20432 [
18] considers the use of four partial reduction factors for affecting the tensile strength of geosynthetics, accounting for the effects of installation damage, creep, weathering, and chemical and biological agents. The partial reduction factors are usually obtained in isolation, discarding the possible interactions that may occur between the degradation agents [
21]. The global reduction factor (used in the design) is traditionally obtained by multiplying the partial reduction factors. However, the multiplication of two, or more, partial reduction factors obtained in isolation does not always represent accurately the combined effect of the degradation agents [
15,
21]. Literature review has revealed the existence of interactions between the different degradation agents. When analysing the action of installation damage and creep, Allen and Bathurst [
22], Greenwood [
23] and Cho et al. [
24] concluded that the traditional method to obtain the reduction factors for the combined action of those degradation agents might be conservative. By contrast, Carneiro et al. [
21] and Carneiro et al. [
25] showed that the existence of interactions between chemical degradation agents can result in inaccurate reduction factors (by underestimation) if the traditional method is followed to obtain the reduction factors for the combined action of those agents. Interactions have also been found between mechanical damage under repeated loading and abrasion, which also led to inaccurate reduction factors when using the traditional method [
15,
16].
This work focused on the evaluation of the effect of mechanical damage under repeated loading and abrasion on geosynthetics, contributing to improve the knowledge about these two degradation mechanisms. Five geosynthetics (a woven and two nonwoven geotextiles, and two geogrids) were submitted individually and successively to mechanical damage under repeated loading and abrasion tests. The damage induced by these tests on geosynthetics was assessed by performing tensile tests and water permeability normal to the plane tests (the latter, only for the geotextiles). Based on the changes occurred in the tensile strength of the geosynthetics, reduction factors were determined. The reduction factors determined by the traditional method (multiplication of reduction factors obtained in isolation for each degradation mechanism) were compared with those obtained in the successive exposures to mechanical damage under repeated loading and abrasion. The results showed that the action of the degradation mechanisms tended to affect the mechanical and hydraulic behaviours of the geosynthetics and that the traditional method (for the determination of reduction factors) was not able to represent accurately the combined effect of mechanical damage under repeated loading and abrasion. Indeed, the predicted reduction factors tended to be lower in comparison with those found in the successive exposures to both degradation mechanisms.
4. Conclusions
This work evaluated the damage suffered by five geosynthetics after being individually and successively exposed to two degradation mechanisms: mechanical damage under repeated loading and abrasion. The results showed that the single exposures of the materials to mechanical damage under repeated loading and abrasion led, in general, to losses in tensile strength. However, the woven and nonwoven geotextiles were more affected by the mechanical damage under repeated loading tests, whereas the woven geogrids experienced higher tensile strength losses after being exposed to abrasion. The most adverse scenario to all geosynthetics (i.e., where higher tensile strength losses occurred) was the successive exposure to both degradation mechanisms.
The isolated and combined actions of mechanical damage under repeated loading and abrasion led to changes in the water permeability behaviour normal to the plane of the nonwoven geotextile with the lowest mass per unit area (325 g·m−2). By contrast, the geotextile with higher mass per unit area (476 g·m−2) had no relevant changes in its hydraulic behaviour. Besides the previous effect, mass per unit area also had a key influence in the tensile behaviour of the nonwoven geotextiles. Indeed, the geotextile with higher mass per unit area was significantly less affected by the degradation tests (higher survivability).
Finally, the reduction factors determined by the traditional method for the combined effect of mechanical damage under repeated loading and abrasion tended to be lower than those found in the successive exposure to both degradation mechanisms. This shows that the traditional method may not be representing correctly the combined effect of the degradation mechanisms, not being able to account for the interactions occurred between them. It is important to refer that the reduction factors obtained in this work resulted from particular degradation conditions (which may not correspond to field conditions) and, therefore, it is not reasonable nor advisable the use of these reduction factors for design purposes.