Accurately understanding the creep behavior of polymer geosynthetic reinforcements is key to designing durable geosynthetic-reinforced soil (GRS) structures. Establishing the creep reduction factor (RFCR) for a specific design life has traditionally required extended conventional creep testing to produce a creep rupture curve. To expedite this process, temperature-acceleration techniques, such as the conventional time-temperature superposition (TTS) and the stepped isothermal method (SIM), have been adopted to accelerate creep deformation. Since the material's viscous properties influence both creep and stress relaxation, stress relaxation occurs at a faster rate than creep for the same irreversible strain under a given load. A framework has been empirically developed to relate the time history of stress relaxation to creep strain, allowing for effective prediction of longterm creep strain using short-term stress relaxation data. This study applies temperature-acceleration methods to short-term creep and stress relaxation tests on a high-density polyethylene (HDPE) geogrid. Results provide extended time histories for creep strain and stress relaxation, with durations extended by a factor of 250. By establishing a relationship between these time histories, a comprehensive method to predict long-term creep behavior is developed, combining the time factors of both methods to produce an approximately 1,635-fold extension. This streamlined approach enables an efficient and reliable prediction of HDPE geogrid creep behavior over long durations.

Time-dependent effects of soils have been widely recognized as a key issue, which is particularly evident for coral sand due to its high frangibility. To determine the time-dependent behaviors of coral sand under triaxial stress states for enhanced engineering applications, a series of triaxial tests was carried out on two gradations of coral sand. Besides the conventional time-dependent tests, deviator-stress rate, creep with different stress histories, drop creep/relaxation, and postpeak creep/relaxation tests, which have been rarely investigated, have also been conducted. Additionally, the particle size and shape variations in the coral sand after the tests were analyzed to elucidate the time-dependent behavior mechanism. According to the test results, different from the tests with varying axial strain rates, over- and under-shooting phenomena were not clear when the deviator stress rate changed suddenly. Creep behavior was found to be noticeably influenced by the stress history and decreased with increasing preconsolidation pressure. The stress unloading could considerably reduce the subsequent creep or relaxation response, and the response diminished with increasing magnitude of deviator stress drop. The time-dependent behavior of coral sand was mainly determined by the level of particle breakage. The coral sand particles became smaller and more regular due to particle breakage, which would increase the compressibility of specimens and weaken interlocking between the sand particles, leading to more obvious time-dependent behaviors. The influence of particle breakage on the time-dependent behaviors of coral sand could be examined from two perspectives, i.e., the particle breakage during creep or relaxation processes and that during preloading processes. The effects of the unstable broken particles that formed during preloading were larger. In addition, a unique relationship was observed between the relative breakage and input energy for the same coral sand gradation, regardless of the test conditions, which was meaningful for the time-dependent constitutive modeling considering particle breakage.

The generation of negative excess pore water pressure (u2) during cone penetration test (CPT) in a given environment represents a deviation from the actual situation, thereby affecting the accuracy of the parameter inversion. Dissipation tests have been conducted to ascertain the dissipation of the u2 over time, which in turn allows for the parameters to be corrected. However, the tip resistance (qc) and sleeve friction resistance (fs) in dissipation process also vary with time, despite its potential impact on the inversion process. In this paper, the evolution of qc and negative u2 with time is successfully obtained through the utilization of indoor CPTs on silt soils. In conjunction with a viscoelastic model, the existence of stress relaxation of qc is demonstrated and the causes of qc decay are analyzed. The detailed conclusions are as follows: (1) The CPT parameters obtained from the dissipation test can be employed to rectify the discrepancy in negative u2 that arises during soil classification. (2) The qc undergoes a gradual decrease, reaching a final equilibrium state during the dissipation process. The stress-time relationship is consistent with the Three-element viscoelasticity model, which represents a stress relaxation phenomenon. The relaxation process can be divided into three distinct phases: fast relaxation, decelerating relaxation, and residual relaxation. The residual stress is found to be correlated with the depth of the soil layer. (3) During residual phase, the loss rate of qc is observed to decrease in a linear fashion with the rate of u2, prior to which the relationship is exponential. As the penetration rate increases, the rate of u2 also increases.

This paper develops a new time-dependent hypoplastic model for normally consolidated and overconsolidated clays. A novel viscous strain rate formulation is derived from the isotach concept and incorporated into the total strain rate of the hypoplastic framework, allowing for viscous deformation at the onset of loading. The hypoplastic flow rule is defined for the direction of the viscous strain rate and its intensity directly linked to the overconsolidation ratio (OCR) and secondary compression coefficient. The Matsuoka-Nakai criterion is further introduced into the strength parameter through the transformed stress technique, enabling the model to describe the stress-strain-time behaviour of clays in general stress space. In addition, a new scalar function is proposed and implemented into the model to consider the OCR effect on the initial stiffness. The model predictive ability is finally examined by simulating laboratory tests on three different clays with various OCRs and stress paths, demonstrating that the model can capture the rate dependency, stress relaxation, and creep behaviours for both normally consolidated and overconsolidated clays under various loading conditions.

Many geotechnical failures are associated with degradation of the soil strength over time. The time-dependency behavior of unsaturated loess is often required to evaluate the long-time behavior of geotechnical engineering in loess areas. To investigate such strain rate response and stress relaxation behavior of intact loess, a series of oedometric compression and relaxation tests were conducted under different suctions and strain rates. Water retention behaviors and microstructures were also measured to characterize the tested loess. The more rapid strain rate, leading to larger yield stress at relatively low suctions (0 and 50 kPa) and roughly paralleled onedimensional normal compression lines (1D-NCL) conformed to the isotache approach. In contrast, the weakening effect of a more rapid strain rate on the clay cementation, resulted in smaller yield stress when the suction was larger than 100 kPa, which was an apparent deviation from the conception of the isotache. The reason might be that the microstructure developed during the long term (slow strain rate) under the relatively larger suction, which may increase the inter-particle bonding and structural strength. The relaxation behavior of unsaturated loess depended on suction and prerelaxation stress, which cannot be well described by the model with a soil constant viscosity I v . The results of two viscous effects (rate-dependency and relaxation) in loess demonstrated that they could not altogether be explained within the isotache concept. (c) 2024 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society. This is an open access article under the CC BY- NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

This study compares how geosynthetics behave under load, under strain, and over time when subjected to confined tensile tests in soil, employing two commonly used mechanisms in research. One test type simulates a reinforced layer, where tensile loads are indirectly applied to the geosynthetic via stresses transferred from the soil. In contrast, the other test applies tensile loads directly to the geosynthetic material using clamps while under soil confinement. The objective is to elucidate how these testing mechanisms might yield differing in-soil tensile characteristics for different geosynthetics. The study involved conducting load-strain-time tests on samples of nonwoven and woven geotextiles, as well as a geogrid, under varying sustained loads over a 120-h period within a sand clay soil providing soil confinement to geosynthetics at different surcharge levels. The results suggest that soil confinement plays a significant role in shaping the load-strain-time behavior of geosynthetics. Furthermore, it was noted that the impact of testing mechanisms on this behavior is contingent upon the type and stiffness of the geosynthetics, as well as their interaction with the confining soil. In general, in-soil tests in which tensile loads are mobilized by geosynthetics and transferred from the soil provide more confident results for better simulating operation conditions. Tests that directly apply tensile loads to the geosynthetic while maintaining stationary soil confinement may yield misleading results, especially for geosynthetics that have poor interaction with the soil.

The creep (CP) strain and stress relaxation (SR) of a clean sand, KMUTT sand, exhibiting non-Isotach viscous properties were evaluated by consolidated-drained triaxial compression (CDTC) tests on air-dried specimens. The test results are analysed based on the nonlinear three-component (NTC) model. Consistent simple empirical equations were derived to predict the elapsed time and the irreversible strain when a given irreversible strain rate takes place during CP loading from those at the same irreversible strain rate during SR loading. Noting that short-term SR tests are much simpler to perform than long-term CP tests, particularly when using an ordinary displacement-controlled axial loading device, a simple empirical method consisting of these empirical equations was formulated to predict creep strain for a relatively long period from SR behaviour for a relatively short period. The creep strains predicted by this empirical method are well comparable with the test results and also with those simulated by the NTC model. It is argued that prediction by the empirical method is relevant in case it is not practical to perform the NTC model simulation. (c) 2024 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society. This is an open access article under the CC BY- NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

This paper investigates the effect of temperature variations on the creep and stress relaxation behavior of clay samples from London Bank Station. The independent and coupled effects of strain rate and temperature on one-dimensional (1D) stress-strain and stress relaxation responses were investigated based on a series of temperature-controlled constant rate of strain (CRS) compression-relaxation tests carried out at fast, intermediate, and slow displacement rates and over 20 degrees C-55 degrees C. The temperature effect on the creep index ( C alpha) was investigated based on a series of temperature-controlled multistage loading (MSL) oedometer tests. The results of the CRS compression-relaxation tests showed that with the increase in temperature, the coefficient of stress relaxation ( R alpha) decreases for samples that were loaded at fast and intermediate prerelaxation displacement rates ( upsilon ); however, it increases for samples loaded at the slow prerelaxation displacement rate. A decrease in upsilon by a factor of 10 (i.e., from 0.010 to 0.001 mm/min) causes the R alpha values to reduce by 55%-11% with the temperature increase. The increase in temperature caused an increase in C alpha that were obtained from the MSL tests. The maximum value of C alpha increased by 18% from 35 degrees C to 45 degrees C and by 37% from 45 degrees C to 55 degrees C. The temperature effects on other conventional parameters that included the preconsolidation pressure, and the compression and swelling indexes (Cc and Cs) were comparable with the findings reported in the literature. Comparing C alpha that were obtained from the MSL tests and R alpha that were obtained from the CRS tests supports the validity of R alpha = C alpha / C c correlation for thermally influenced saturated reconstituted clays and that the time-dependent soil parameters could be obtained from relatively fast CRS compression-relaxation tests as an alternative to conventional time-consuming oedometer tests.