Evaluating Performance of Hybrid Geosynthetic-Reinforced Retaining Walls

Abstract

Geosynthetic-Reinforced Retaining (GRR) walls have been extensively used in highway projects. A novel GRR wall was proposed to mitigate the problems (e.g. high connection stress) resulting from the relatively large spacing of primary reinforcement. This new GRR wall has secondary reinforcement installed between primary reinforcement layers. This new GRR wall has a similarity in the structure of the GMSE wall and the GRS wall in terms of the vertical reinforcement spacing (i.e., it has the same vertical primary reinforcement spacing as the GMSE wall and the same vertical secondary reinforcement spacing as the GRS wall). In this study, it is named as a hybrid GRR wall. Since the hybrid GRR wall is an innovative retaining structure, few studies have been carried out on it and then its performance has not been well understood. Therefore, it is necessary to evaluate the performance of the hybrid GRR wall. Three GRR wall sections were constructed and monitored in the field: (1) a hybrid GRR wall section with uniaxial geogrid layers as primary and secondary reinforcement, (2) a hybrid GRR wall section with uniaxial geogrid layers as primary reinforcement and with biaxial geogrid layers as secondary reinforcement, and (3) a GRR wall section with uniaxial geogrid layers as primary reinforcement only (i.e., the control section). Earth pressure cells, inclinometer casings and probes, and foil-type strain gauges were used in these three test wall sections to measure vertical and lateral earth pressures, accumulated lateral wall facing deflections, and strains of primary and secondary geogrid layers during construction, respectively. The results from monitoring of these three GRR wall sections were analyzed. The test results demonstrated the effects of secondary reinforcement on the improved performance of GRR walls, including reduced accumulated wall facing deflections, a more uniform lateral earth pressure distribution, and reduced tensile strains in primary geogrid layers. In addition to the field tests, the numerical study was performed to evaluate the performance of the instrumented GRR walls in the field tests. The wall facing and the foundation soil were modelled as a linearly elastic material and the retained soil was modelled as a linearly elastic perfectly plastic material with the Mohr Coulomb (MC) failure criterion. The behavior of the backfill soil in the reinforced soil zone was modelled using the Cap Yield (CY) model and the MC model. The behavior of the reinforcement was described by a strip element with a linearly elastic perfectly plastic behavior. An 8-kPa compaction stress was considered in the numerical model. The vertical and lateral earth pressures, wall facing deflections, and strains in primary and secondary geogrid layers during construction were calculated by the numerical simulation and also compared with the measured ones in the field tests. The calculated results from the numerical simulation agreed well with the measured one in the field tests. The calculated results from the numerical simulation also demonstrated that the secondary reinforcement could reduce wall facing deflections and tensile strains in primary geogrid layers. A parametric study was also performed through varying one parameter in the baseline model to study its influence on the performance of the hybrid GRR wall, such as wall facing deflections, vertical earth pressures, lateral earth pressures, and stresses in reinforcement. The influence factors consisted of the length of primary reinforcement, the length of secondary reinforcement, the stiffness of primary reinforcement, the stiffness of secondary reinforcement, the backfill soil friction, the backfill soil dilation, the backfill soil modulus, the compaction stress, and the foundation compressibility. The parametric study shows that the maximum wall facing deflection decreased with an increase in the primary reinforcement length, the secondary reinforcement length, the primary reinforcement stiffness, the secondary reinforcement stiffness, the backfill soil friction angle, the backfill soil dilation angle, the backfill soil modulus, and the foundation compressibility while increased with an increase in the compaction stress. The lateral earth pressure increased with an increase in the primary reinforcement stiffness, the secondary reinforcement stiffness, and the backfill soil friction angle because the wall facing deflections decreased with an increase in the primary reinforcement stiffness, the secondary reinforcement stiffness, and the backfill soil friction angle. The maximum tensile stress in the primary reinforcement increased with an increase in the primary reinforcement stiffness and the compaction stress but decreased with an increase in the primary reinforcement length, the secondary reinforcement stiffness, and the backfill soil friction angle.