INVESTIGATION OF GEOSYNTHETIC REINFORCED SOIL BRIDGE ABUTMENTS

Abstract

The development of Geosynthetic Reinforced Soil (GRS) technology has significantly advanced bridge abutment construction by incorporating geosynthetic reinforcements to enhance the mechanical properties of soil. GRS technology is employed to improve the stability, load-bearing capacity and durability of bridge abutments by integrating layers of geosynthetic materials within the soil, resulting in a composite structure with superior strength. A typical GRS abutment comprises several key components: geosynthetic reinforcement layers, engineered backfill material, and a facing system designed to contain the soil and provide structural integrity. Despite these advancements, literature reviews have identified persistent issues with conventional GRS abutment configurations including excessive settlement of the backfill material, the development of substantial lateral earth pressures on the facing and unserviceable horizontal deformations. These problems can compromise the overall stability and serviceability of the abutment structures. To address these challenges, GRS technology has evolved towards the development of GRS integral abutments and GRS Integrated Bridge System (IBS) abutments. GRS integral abutments integrates the bridge deck with the abutment facing into a monolithic structure, aiming to reduce differential settlement and improve load distribution. The important structural characteristics of GRS integral abutments include their ability to accommodate movements due to thermal expansion and contraction, their resilience to seismic activities, and their capacity to distribute loads evenly across the structure. The IBS approach on the other hand integrates the approach road with the bridge deck and placing it directly over the reinforced backfill using a beam seat supported by modular facing. This offers additional advantages such as improved structural continuity, enhanced resistance to vetical stresses and greater overall stability of the bridge system. However, these advanced configurations present their own set of challenges. Issues such as cyclic thermal movements of the bridge deck, transient vehicular loads, and long-term performance factors like material degradation and creep still pose significant problems. These problems underscore the necessity for ongoing research to optimize design parameters, improve reinforcement layouts and develop reliable analytical models. The research in this thesis attempts to conduct comparative analysis of stability of five types of conventional GRS bridge abutments through 1g model tests with special emphasis given to the influence of type of facing and configuration of reinforcements. A total of 75 model tests on 1/5th (N = 5) scaled down abutments have been performed to evaluate the effects of footing width (B), footing offset (x), facing type and abutment type on serviceable bearing capacity, ultimate bearing capacity, footing settlement (s), horizontal displacement of facing (d), tilting of footing (θf ), rotation (θr ) and buckling of facing. The serviceability criteria based on FHWA recommendation has been chosen to ascertain field suitability of the model. An interesting mechanism correlating footing tilt with movement of facing has been hypothesized and validated by the failure mode observed at the facing of the abutments. The results indicated that abutments with modular facing are more stable as compared to continuous facing. The footing width ratio (B/H) > 0.2 with footing offset ratio (x/H) > 0.3 (where, H is height of abutment) was found to be more suitable for field application. The degree of tilt, facing rotation, buckling behavior and differential settlement of footing were minimum for abutment configuration having modular facing with secondary reinforcements in the bearing bed. The study also addressd the challenges faced by GRS integral abutments during seasonal or diurnal thermal expansion/contraction of deck slab in conjunction with peak traffic congestions. To address these challenges, researchers advocate for various facing types capable of withstanding lateral pressure, combined with backfill reinforcement to minimize surface settlement. This study investigates the mitigation effects of different abutment facing and reinforced backfill on backfill settlement and lateral earth pressure under lateral movement of the facing due to cyclic thermal expansion/contraction of the bridge deck in addition to the action of transient vehicular loads. A systematic similitude mechanism is elaborated to scale down prototype abutment characteristics in the present 1g physical modeling. An optimized facing - reinforcement configuration is proposed after a series of model analysis under varying loading rates (r) and different loading offsets (x) for three displacement modes up to 100 cycles of excitation (N). Observations reveal that the optimized model, featuring a monolithic connection between the reinforcement and facing, along with secondary reinforcements across the bearing zone, demonstrates rapid dissipation of accumulated stresses, resulting in a 48% reduction in surface settlement under cyclic thermal stresses. Based on the results of the optimization study, the optimum facing and reinforcement configurations were modified to propose a novel hybrid abutment configuration (named C1 model). This study investigates the performance of the hybrid integral abutment under lateral movement of the facing due to cyclic thermal expansion/contraction of the bridge deck through scaled down 1g physical model tests. The abutment was analyzed under varying rate of loading (r) and different loading offsets (x) for three displacement modes i.e. cyclic active (CA), cyclic passive (CP) and cyclic active – passive (CAP) for 100 cycles of excitation. The assessment included the development of lateral pressure on facing (expressed in terms of lateral earth pressure coefficient, K), surface settlement (s), magnitude and location of peak reinforcement forces (Tmax), followed by evaluating long term performance in terms of permanent strains, stiffness degradation, and strain energy dissipation (E). The analysis indicated that within the constraints of the model testing, the C1 integral abutment configuration is capable of withstanding cyclic stresses arising from prolonged lateral excitation due to seasonal thermal variations as well as vertical stresses developed during peak traffic congestions. Lastly, present research focussed on developing design charts for determining geometrical and reinforcement parameters for construction of a Geosynthetic Reinforced Soil Integrated Bridge System (GRS IBS) abutment in accordance with Federal Highway Authority (FHWA) guidelines. A numerical model validated with field observations of instrumented Virginia GRS IBS has been formulated to perform optimization simulations to determine bearing width (b), setback distance (a), width of reinforced soil foundation (B) and base reinforcement length (RL) for a range of abutment height (H), allowable bearing pressure (p) and soil conditions (φ). Additionally, correlations between tensile strength (T) and vertical spacing (Sv) of primary reinforcements for four reinforcement layouts were developed. This study further illustrates a design scheme to calculate remaining design parameters namely clear space distance (de), width of RSF on toe side (Btoe), thickness of RSF (DRSF) and total width of RSF (Btotal), primary reinforcement length (Rp) and bearing zone reinforcement length (Rb) based on FHWA recommendations for a serviceable GRS IBS. These design solutions are verified by performing stability analysis using MIDAS GTS NX and compared with theoretical solutions. These design charts offer designers streamlined solutions for constructing GRS IBS abutments under a variety of site conditions, eliminating the need for extensive calculations.