Short-Term Tensile Response
The short-term tensile behavior of intact and exhumed geotextile specimens was evaluated by wide-width tensile tests performed according to the International Standard ISO 10319:2015 (
ISO 2015). These tests were carried out at a strain rate of
using a universal testing machine. The associated results were discussed in detail in a previous paper (
Vieira and Pereira 2021), but a summary of the main data is also presented herein given its relevance for this current study. Table
2 presents the results obtained for five geotextile specimens tested under intact conditions, in terms of the maximum tensile force (
) and corresponding strain (
), as well as the secant stiffness modulus at 2% strain (
) and the secant stiffness modulus at maximum load (
). In turn, the results from tests carried out on exhumed geotextile specimens after a 24-month exposure to the C&D material and the clayey sand are summarized in Table
3. The average tensile load-strain curves of the geotextile for each test condition are shown in Fig.
5.
From the comparison of the results presented in Tables
2 and
3, it can be concluded that the exposure of the geotextile to both the recycled C&D material and the clayey sand led to a considerable degradation of the short-term tensile strength of the material. The reduction in the peak tensile strength of the exhumed specimens may be attributed to material handling and construction procedures, as well as the chemical and environmental degradation induced by the exposure of the specimens to the backfill materials. Vieira and Pereira (
2021) evaluated the short-term tensile behavior of geotextile specimens exhumed right after the construction of the embankment built with recycled C&D material. They reported that the specimens that were exhumed immediately after embankment construction exhibited a loss of tensile strength of about 17%, in comparison with that of the fresh specimens. This reduction in the measured tensile strength was attributed to the less effective binding of the PET yarns to the nonwoven geotextile backing, caused by material handling and installation. According to the results presented in Tables
2 and
3, the decrease in tensile strength for the geotextile specimens exhumed from the C&D material was approximately 26% (i.e., 9% greater than that measured for the specimens exhumed right after installation). Thus, 9% of the tensile strength reduction resulted from the exposure to the C&D material during 24 months (i.e., chemical and environmental degradation), whereas the remaining reduction resulted from handling and construction procedures. On the other hand, the tensile strength loss after a 24-month exposure to the recycled C&D material (26% on average) was comparable to that induced by the clayey sand (27% on average), indicating that the use of the recycled backfill material did not cause any additional degradation.
The results in Tables
2 and
3 also indicate that the tensile stiffness at 2% strain (
) was not significantly affected by the geotextile exposure to the different backfill materials, with the mean values for the exhumed specimens falling within the confidence interval for intact specimens. This small variation of tensile stiffness under low strains can be clearly observed in Fig.
5, which also shows that the strain at maximum load was rather similar irrespective of the test conditions. As a result, the secant stiffness at maximum load (
) decreased significantly when the materials were exposed to the backfill materials for a period of 24 months (up to 25%).
The average load-strain curve of the geotextile specimens after exposure to the clayey sand was slightly different from those of the remaining specimens, particularly in terms of tensile stiffness for strains exceeding 3% (Fig.
5). This is likely related to the higher amount of fine particles of the clayey sand that remained embedded within the geotextile pores after exhumation. As indicated earlier in Fig.
1, the fines content of the clayey sand (28.8%) was considerably higher than that of the recycled C&D material (11.7%), leading to more significant accumulation of fine particles within the geotextile. This evidence is further clarified in Fig.
6, which shows the scanning electron microscopy (SEM) images of fresh and exhumed geotextile specimens obtained through a high resolution environmental scanning electron microscope with X-ray microanalysis and electron backscattered diffraction analysis, Quanta 400 FEG ESEM/EDAX Genesis X4M (Field Electron and Ion Company, Hillsboro, Oregon) from the Materials Centre of the University of Porto. Figs.
6(a, c, and e) illustrate a PET yarn connected to the nonwoven geotextile backing (magnification
) for a fresh specimen, an exhumed specimen from the C&D material, and an exhumed specimen from the clayey sand, respectively, whereas Figs.
6(b, d, and f) show the individual wires of a PET yarn (magnification
) for the different exposure conditions. Irrespective of the exposure condition, the connections of the PET yarn to the nonwoven geotextile were still visible [Figs.
6(a, c, and e)]. Furthermore, as shown in Figs.
6(c–f), a significant amount of fine particles remained attached to the geotextile after exhumation, particularly in the case of the specimen that was previously exposed to the clayey sand [Figs.
6(e and f)].
Creep Reduction Factor
According to the Technical Report ISO/TR 20432:2007 (
ISO 2007), the long-term design tensile strength (
) of geosynthetics used for soil reinforcement should be obtained by separately accounting for the detrimental effects of several influential factors on the tensile strength and can be expressed as
where
= characteristic strength (i.e., a statistic value typically derived from the mean tensile strength of the geosynthetic subtracted by two standard deviations);
= creep reduction factor (to cover the effect of sustained static load);
= reduction factor for mechanical damage (or installation damage);
= reduction factor for weathering (to account for weathering during exposure prior to installation or of permanently exposed geosynthetics);
= reduction factor to account for the strength loss resulting from chemical and biological degradation; and
= safety factor to account for the extrapolation uncertainty, particularly in extrapolation over long durations.
The creep rupture behavior of the high-strength geotextile used in this study was characterized in the previous section by measuring the time to rupture for each specimen subjected to a given sustained load. The results can be extrapolated to predict longer lifetimes at lower loads and thereby estimate the creep reduction factor, which is required to limit the load acting on the reinforcement to a level that will prevent creep rupture over the design life of the structure.
As per the ISO/TR 20432:2007 (
ISO 2007), a condition on the extrapolation of the creep rupture envelope is that there is no evidence to believe that the creep rupture behavior will change over time. This implies checking that at long durations there is no (1) abrupt change in the gradient of the creep rupture envelope; (2) abrupt change in the strain to failure; and (3) significant change in the nature of the fracture surface.
In this study, the creep reduction factors for the tensile strength of the geotextile for a given design period (
) were computed based on ISO/TR 20432:2007 (
ISO 2007) by considering the data obtained from the wide-width tensile tests and the creep rupture tests, as given by
where
= maximum tensile strength obtained from wide-width tensile tests performed on fresh geotextile specimens (mean value); and
= load leading to geosynthetic rupture (also known as the creep rupture load or creep limit) for a given design lifetime (
), predicted by extrapolating the corresponding creep rupture envelope.
Table
4 lists the values of the creep rupture load and creep reduction factor for fresh and exhumed geotextile specimens considering design lifetimes of 30 and 75 years. The design lives of geosynthetic-reinforced soil structures may range from several years, for temporary structures, to 75–120 years for permanent structures. The selected design lifetimes of 30 and 75 years were aimed at enabling the comparison of the creep reduction factors with those from databases available in the literature (
Bathurst et al. 2012;
Miyata et al. 2014;
Pinho-Lopes et al. 2018). It is noteworthy that, for design purposes, European and North American practices (
ISO 2007;
WSDOT 2009;
AASHTO 2013) recommend not to extrapolate the creep rupture envelope by more than two log cycles beyond the longest observed time to creep rupture (i.e., the test with the longest duration). While the available creep rupture data for fresh specimens and exhumed specimens from the clayey sand would not fully meet this criterion, the creep reduction factors have been estimated for the purposes of this investigation.
It can be observed from Table
4 that the computed creep rupture load for a design life of 30 years increased about 4.3% and 8.3% for exhumed specimens from the C&D material and the natural soil, respectively, in comparison to that predicted on the basis of the results for fresh specimens. Accordingly, the creep reduction factor corresponding to the geotextile tested under intact conditions (
exceeded those obtained for the exhumed specimens.
As expected, similar trends were also observed when considering a longer design life of 75 years. The creep rupture load for increased about 6.6% and 11.6% for exhumed specimens from the C&D material and the natural soil, respectively, in relation to that for the fresh specimens. The highest creep reduction factor ( was obtained for specimens tested under intact conditions. Therefore, the use of fresh specimens for creep rupture testing (i.e., the usual approach), rather than specimens that were previously exposed to the backfill materials under real environmental conditions can be considered as a conservative (i.e., safe) procedure with regard to the analysis of the long-term strength of this geosynthetic.
The creep rupture factors obtained herein are comparable to those reported by other researchers in previous related studies. Bathurst et al. (
2012) provided a summary of computed creep reduction factors for a wide range of geosynthetic products. For a design lifetime of 75 years, the creep reduction factors ranged from 1.36 to 1.67 for PET geosynthetics, from 1.76 to 2.8 for PP geotextiles, and from 2.48 to 3.12 for high-density polyethylene (HDPE) geogrids (
Bathurst et al. 2012). Miyata et al. (
2014) collected creep test data from a large database of geogrid products and found that the creep reduction factors for a design life of 75 years (computed using reference tensile strengths based on 10% strain/min tests) were in the range of 2.07–3.03 for uniaxial HDPE geogrids, 1.55–4.26 for PP geogrids, and 1.20–1.92 for woven and knitted PET geogrids. Pinho-Lopes et al. (
2018) obtained a creep reduction factor of 2.08 for a woven PP geotextile and a lower creep reduction factor of 1.68 for a PET geogrid considering a design lifetime of 30 years.
The differences in the results obtained from different studies involving distinct geosynthetic materials is not surprising. The reference tensile strength used to calculate the creep reduction factor may change from country to country and may also be influenced by the axial strain rate imposed in the tensile tests (
Miyata et al. 2014). Furthermore, as noted by Bathurst et al. (
2012), several factors such as the manufacturing process, constituent polymer type and grade, thickness of axially loaded members for integral punched and drawn polyolefins, temperature, and load level are all susceptible to affect the creep behavior of geosynthetic products to a significant extent. Therefore, the creep strain and rupture behavior of geosynthetics should ideally be evaluated using the specific geosynthetic product to be used in the project.