Passive force-deflection behavior for densely compacted backfills must be considered in bridge design to ensure adequate resistance to both seismic and thermally induced forces. Current codes and practices do not distinguish between skewed and non-skewed bridge abutment geometries; however, in recent years, numerical models and small-scale, plane-strain laboratory tests have suggested a significant reduction in passive force for skewed bridge abutments. Also, various case studies have suggested higher soil stresses might be experienced on the acute side of the skew angle. For these reasons, three large-scale tests were performed with abutment skew angles of 0, 15 and 30 degrees using an existing pile cap [11-ft (3.35-m) wide by 15-ft (4.57-m) long by 5.5-ft (1.68-m) high] and densely compacted sand backfill confined by MSE wingwalls. These tests showed a significant reduction in passive force (approximately 38% as a result of the 15 degree skew angle and 51% as a result of the 30° skew angle. The maximum passive force was achieved at a deflection of approximately 5% of the backwall height; however, a substantial loss in the rate of strength gain was observed at a deflection of approximately 3% of the backwall height for the 15° and 30° skew tests. Additionally, the soil stiffness appears to be largely unaffected by skew angle for small displacements. These results correlate very well with data available from numerical modeling and small-scale lab tests. Maximum vertical backfill displacement and maximum soil pressure measured normal to the skewed backwall face were located on the acute side of the skew for the 15° and 30° skew test. This observation appears to be consistent with observations made in various case studies for skewed bridge abutments. Also, the maximum outward displacement of the MSE wingwalls was located on the obtuse side of the 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 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. To compare the effects of backfill geometries, three large-scale tests with dense compact sand were performed with abutment backfill heights of 3 ft (0.91 m), 5.5 ft (1.68 m), and 5.5 ft (1.68 m) confined with MSE wingwalls. Using an existing pile cap 11 ft (3.35 m) wide and 5.5 ft (1.68 m) high, width to height ratios for the abutment backfills were 3.7 for the 3ft test, and 2.0 for the 5.5ft and MSE tests. The failure surface for the unconfined backfills exhibited a 3D geometry with failure surfaces extending beyond the edge of the cap, increasing the "effective width", and producing a failure "bulb". In contrast, the constraint provided by the MSE wingwalls produced a more 2D failure geometry. The "effective width" of the failure surface increased as the width to height ratio decreased. In terms of total passive force, the unconfined 5.5ft wall provided about 6% more resistance than the 5.5ft MSE wall. However, in terms of passive force/width the MSE wall provided about 70% more resistance than the unconfined wall, which is more consistent with a plane strain, or 2D, failure geometry. In comparison with predicted forces, the MSE curve never seemed to fit, while the 3ft and 5.5ft curves were better represented with different methods. Even with optimizing between both the unconfined curves, the predicted Log Spiral peak passive forces were most accurate, within 12% of the measured peak resistances. The components of passive force between the unconfined tests suggest the passive force is influenced more by frictional resistance and less by the cohesion as the height of the backwall increases.