Numerical comparison of performance and stability of concrete and gabion retaining walls in riverbanks protection

Introduction
A retaining wall is a structure that maintains the pressure due to the situation in the level difference caused by an embankment, excavation, or natural factors. Since comparing the performance of two different types of walls from retaining walls is the subject of this article, in this research, from rigid walls, weight retaining wall (concrete) and flexible walls, gabion retaining wall (in weight form) with almost the same conditions, It is evaluated in terms of stability and performance. The geometry of the concrete weight retaining wall is chosen so that the result of the forces acting on it (including weight and lateral forces) is in the core of the base or its horizontal sections. The expected results of this study include the study of retaining wall performance in terms of soil material change behind the wall, river water depth, and retaining wall slope, and finally, comparison and selection of suitable retaining wall to protect the river wall between two types of concrete and gabion walls.
Methodology
Software SLOPE/W is the most advanced slope stability software for soil and rock slopes from the GEO-STUDIO 2012 software suite. SLOPE/W effectively analyzes various types of slip surface shapes and determines factors of safety, pore water pressure conditions, soil properties, and loading conditions. In this study, the Morgenstern-Price method has been used. In this method, the balance of forces and anchors for sliding sections is determined, and by Entry and Exit method, the factor of safety is determined.
In this study, different factors such as type of wall and river bed materials, water depth in the river, type of retaining wall materials, the slope of retaining wall, and soil adhesion were investigated. Based on the critical conditions in the evaluation of the stability of retaining walls, the saturation state (the most critical state possible) was considered for the wall and river bed materials. To apply the soil properties behind the retaining wall in modeling, a homogeneous porous medium and isotropy (Kx/Ky = 1) were considered, and for the slip criterion of the Mohr-Coulomb resistance model, which is the most common method for expressing shear strength in geotechnical materials, used. To compare the results, the retaining wall was designed and modeled in mirror conditions, i.e. the vertical part of the wall on the riverside and the stair part of the wall under the soil.
Results and Discussion
Since the permeability and hydraulic conductivity of medium gravel are higher than that of average sand, it has a greater capacity to drain excess water. It was observed that the active force and torque of the average gravel are 7% and 11% lower than the average sand, respectively. This is also true for the gabion retaining wall as well as the state of the river with a water depth of 2 meters and 4 meters. In a river, without water, the amount of active force and torque is about 20% more than in a river with a water depth of 2 meters and about 40% more than in a river with a water depth of 4 meters. In the retaining wall in the mirror state, in the river without water, the active force and torque have both increased by about 16%, and in the river water depth of 2 meters, it has increased by about 17% and in the depth of 4 meters, it has increased by 23%. The results of stability and factor of safety for two wall slopes in the conditions of water level drop showed that the factor of safety at a depth of 2 meters decreases by about 20% compared to a depth of 4 meters and Also, the factor of safety of the river without water is reduced by about 30% compared to a depth of 4 meters. The results showed that the retaining wall in mirror conditions has a better performance than other models in terms of factor of safety. Also, the index wall in the vertical position, at a depth of 4 meters, which is the same depth equal to the total height of the wall from the riverbed, has the lowest factor of safety.
Conclusion
In this research, the stability and performance of two types of concrete and gabion retaining walls were examined with SLOPE/W software under similar river conditions. A total of 24 models were designed using two types of bank material (gravel and sand), two slopes of the retaining walls, and three river water depths. Also, 6 models of vertical walls were considered for comparison. The results indicated that the coarser-grained bank material produces a lower level of pore pressure, which in turn results in lower values of active force and torque by 11% and 7%, respectively, compared with the sand material. The results of this applied research can be used as a guideline for selection between two types of retaining walls (i.e. Concrete or masonry walls, and gabion walls) in different river conditions.