Bridge abutments are designed to withstand lateral pressures from thermal expansion and seismic forces. Current design curves have been seen to dangerously over- and under-estimate the peak passive resistance and corresponding deflection of abutment backfills. Similar studies on passive pressure have shown that passive resistance changes with different types of constructed backfills. The effects of changing the length to width ratio, or including MSE wingwalls determine passive force-deflection relationships. The purpose of this study is to determine the effects of the wall heights and of the MSE support on passive pressure and backfill failure, and to compare the field results with various predictive methods.
Accounting for seismic forces and thermal expansion in bridge design requires an accurate passive force versus backwall deflection relationship. Current design codes make no allowances for skew effects on the development of the passive force. However, small-scale experimental results and available numerical models indicate that there is a significant reduction in peak passive force as skew angle increases for plane-strain cases. To further explore this issue large-scale field tests were conducted at skew angles of 0°, 15°, and 30° with unconfined backfill geometry. The abutment backwall was 11 feet (3.35-m) wide by 5.5 feet (1.68-m) high, and backfill material consisted of dense compacted sand. The peak passive force for the 15° and 30° tests was found to be 73% and 58%, respectively, of the peak passive force for the 0° test which is in good agreement with the small-scale laboratory tests and numerical model results. However, the small differences may suggest that backfill properties (e.g. geometry and density) may have some slight effect on the reduction in peak passive force with respect to skew angle. Longitudinal displacement of the backfill at the peak passive force was found to be approximately 3% of the backfill height for all field tests and is consistent with previously reported values for large-scale passive force-deflection tests, though skew angle may slightly reduce the deflection necessary to reach backfill failure. The backfill failure mechanism appears to transition from a log spiral type failure mechanism where Prandtl and Rankine failure zones develop at low skew angles, to a failure mechanism where a Prandtl failure zone does not develop as skew angle increases.
Test results in both sets of backfills confirmed previous findings that there is significant reduction in passive force with skewed abutment configurations. The reduction factor was 0.58 for the gravel backfill and 0.63 for the GRS backfill, compared to the predicted reduction factor of 0.53 for a 30° skew. These results are within the scatter of previous skewed testing, but could indicate that slightly higher reduction factors may be applicable for gravel backfills.
Approach fills behind bridge abutments are commonly supported by wrap-around mechanically stabilized earth (MSE) walls; however the effect of this geometry on passive force development is unknown. This report describes the first large-scale tests to evaluate passive force-deflection curves for abutments with MSE wingwalls. A test was also performed with fill extending beyond the edge of the abutment wall for comparison. The abutment wall was simulated with a pile supported cap 5.5 ft high, 11 ft wide, and 15 ft long in the direction of loading. The backfill behind the pile cap consisted of clean sand compacted to 96% of the modified Proctor maximum density. As the pile cap was loaded laterally, pressure on the MSE wall led to pull-out of the steel reinforcing grids and the MSE wall panels moved outward about 2% of the wall height when the ultimate passive force developed. Despite pullout, the passive force per effective width was 28 kips/ft for the pile cap with MSE wingwalls compared to 22.5 kips/ft for the cap without wingwalls. Nevertheless, the passive force with the MSE wingwalls was still only 76% of the resistance provided by the cap with fill extending beyond the edges. The pile cap with MSE walls required greater movement to reach the ultimate passive force (deflection of 4.2% of wall height vs. 3%). The Caltrans method provided good agreement with the measured passive resistance while the log spiral method required the use of a higher plane strain friction angle to provide reasonable agreement.
The effects of seismic forces and thermal expansion on bridge performance necessitate an accurate understanding of the relationship between passive force and backwall deflection. In past case studies, skewed bridges exhibited significantly more damage than non-skewed bridges. These findings prompted studies involving numerical modeling, lab-scales tests, and large-scale tests that each showed a dramatic reduction in passive force with increased skew. Using these results, a correlation was developed between peak passive force and backwall skew angle. The majority of these tests had length to height ratios of 2.0; however, for several abutments in the field, the length to height ratio might be considerably higher than 2.0. This change in geometry could potentially affect the validity of the previously found passive force reduction correlation.
To determine the relationship of passive force versus backwall displacement for a CLSM backfilled bridge abutment, two laboratory large-scale lateral load tests were conducted at skew angles of 0 and 30°. The model backwall was a 4.13 ft (1.26 m) wide and 2 ft (0.61 m) tall reinforced concrete block skewed to either 0 or 30°. The passive force-displacement curves for the two tests were hyperbolic in shape, and the displacement required to reach the peak passive resistance was approximately 0.75-2% of the wall height. The effect of skew angle on the magnitude of passive resistance in the CLSM backfill was much less significant than for conventional backfill materials. However, within displacements of 4-5% of the backwall height, the passive force-displacement curve reached a relatively constant residual or ultimate strength. The residual strength ranged from 20-40% of the measured peak passive resistance. The failure plane did not follow the logarithmic spiral pattern as the conventional backfill materials did. Instead, the failure plane was nearly linear and the failed wedge was displaced more like a block with very low compressive strains.
A comparison of passive force per unit width suggests that MSE wall abutments provide 60% more passive resistance per unit width compared to reinforced concrete wingwall and unconfined abutment geometries at zero skew. These findings suggest that changes should be made to current codes and practices to properly account for skew angle in bridge design.
Bridge abutments provide resistance to deformation and earthquake induced inertial forces from the bridge superstructure. In order to limit the inertial forces transmitted into the abutment walls and piles, the abutment walls are designed to be sheared off in major seismic events. Therefore, the force-resistance mechanism of bridge abutments in the longitudinal direction is mainly provided by backwall-soil interaction. Current design practice in California makes use of bi-linear load-deformation curve and does not account for the structure backfill properties. An experimental and an analytical research program were conducted at UCSD to further investigate such structure backfill interaction characteristics. In order to meet the objectives of this research project, a field investigation was conducted to develop a proper characterization of the soil types used for abutment structure backfills. The experimental program included five large-scale tests. In the first phase of the experiment, an abutment wall (without a foundation) was built at 50% scale of a prototype diaphragm abutment. The second phase of this research program was performed on a backwall sheared off from wingwalls and stem in seat-type abutments. The specific aims of the experimental program were to examine the effect of structure backfill soil type, backfill height, vertical movement of the wall, and pre-existing cut slope in backfilling on stiffness and capacity of the abutments in the longitudinal direction. An analytical model was developed for evaluating the response of bridge abutments loaded longitudinally. The approach involves calculating maximum passive resistance of the structure backfill material, and creating p-y curves to predict the force-displacement relationship of longitudinally loaded bridge abutments. The finite element program Plaxis 2D was used to model the abutment wall experiments. The procedure was validated by comparing the finite element results with the experimental results. In conclusion, the study indicates that the response of bridge abutments in the longitudinal direction is nonlinear and a function of several factors which need to be considered. The passive resistance of the structure backfill is controlled by the soil shear strength and the interface friction angle. Finally, the vertical movement of the wall has a significant effect on post-peak behavior of abutments.
Engineered fills, including compacted granular fill and reinforced soil, are a cost-effective alternative to conventional bridge foundation systems. The Geosynthetic Reinforced Soil Integrated Bridge System (GRS-IBS) is a fast, sustainable and cost-effective method for bridge support. The in-service performance of this innovative bridge support system is largely evaluated through the vertical and lateral deformations of the GRS abutments and the settlements of reinforced soil foundations (RSF) during their service life. While it is a common assumption that granular or engineered fills do not exhibit secondary deformation, it has been observed in in-service bridge abutment applications and large-scale laboratory tests. Evaluation of the secondary, or post-construction, deformation of engineered fills is therefore also needed. The aim of this study is to analyze and quantify the maximum deformations of GRS abutment and RSF under service loads, evaluate the stress distributions within the engineered fills of the GRS abutment and RSF, and investigate the time-dependent behavior of engineered fills for bridge support. The ultimate goal is to provide accurate yet easy-to-use analysis-based design tools that can be used in the performance assessment of GRS abutments and RSF under service loads. It is anticipated that the research performed within the scope of this dissertation will eventually help promote sustainable and efficient design practice of these structures.The research objective was achieved through development of numerical models that employed finite difference solution scheme and simulated the performance of granular backfill and reinforcement material. The backfill soil was simulated using three different constitutive models. Comparison of the simulation results with case studies showed that the behavior of GRS structures under service loads is accurately predicted by the Plastic Hardening model. The developed models were validated through comparison of model predictions with laboratory and field test data reported in the literature. A comprehensive parametric study was conducted to evaluate the effects of backfill soils properties (friction angle and cohesion), reinforcement characteristics (stiffness, spacing, and length), and structure geometry (abutment height and facing batter and foundation width) on the deformations of GRS abutments and RSF. The results of the parametric study were used to conduct a nonlinear regression analysis to develop equations for predicting the maximum lateral deformation and settlement of GRS abutments and maximum settlement of RSF under service loads. The accuracy of the proposed prediction equations was evaluated based on the results of experimental case studies. The developed prediction equations may contribute to better understanding and enable simple calculations in designing these structures. To investigate the time-dependent deformations of GRS abutment and RSF, a numerical model was developed. The time-dependent deformations are also known as secondary deformations and creep. To model the creep behavior of the backfill material, the Burgers creep viscoplastic model that combines the Burgers model and the Mohr-Coulomb model was used in the simulations. To model the creep behavior of geosynthetics, the model proposed by Karpurapu and Bathurst (1995) was used; this model uses a hyperbolic load-strain function to calculate the stiffness of the reinforcement. Results indicated that engineered fills can exhibit noticeable secondary deformation.
