As part of pooled fund study TPF-5(264), led by UDOT and supported by FHWA and a few other state DOTs, large-scale passive force-deflection tests were performed on a simulated bridge abutment to investigate the effect of skew angle on passive force behavior. Tests were conducted at abutment skew angles of 0°, 15°, 30°, and 45° with a backwall that was 11 ft wide and 5.5 ft tall. Backfills included sand and sandy gravel compacted to 95% of the modified Proctor maximum dry unit weight. Test results indicate that the passive force decreases significantly as the abutment skew angle increases to 45° relative to non-skewed walls. The results also indicate that the reduced passive force for a skewed abutment, P_p(skew), can be predicted using a simple reduction factor, R_skew, multiplied by the passive force for a non-skewed abutment with the same roadway width. The skew reduction factor was relatively consistent for all soil types, wingwall styles, and backfill width-to-height ratios investigated. The Phase II part of the previous study included testing of additional backfill materials and an inclined loading (push-and-rotate) condition for a 30° skew angle. No significant effect on the passive force skew reduction factor was observed in the inclined loading testing that involved relatively small rotation.

Based on the previous study results, the skew reduction factor has already been implemented in the Caltrans Seismic Design Criteria, along with geotechnical guidelines for Oregon DOT and UDOT. However, as designers have started applying this approach, several questions have arisen. For example, in most of the field abutment tests, the simulated bridge abutment was forced to move longitudinally into the backfill soil.  In contrast, during earthquake loadings, the abutment has been observed to rotate. Although this rotation angle is quite small, it can lead to a significant difference in longitudinal displacement from the edges of the abutment from rotation. This would be expected to lead to a triangular distribution of pressure on the backwall of the abutment. Designers want to know (1) if the skew reduction factors remain the same when rotation is involved, and (2) if it is necessary to distribute the passive force non-uniformly along the backwall of the abutment.

The objective of this new study is to assist with the calibration of numerical models by conducting a series of large-scale skewed abutment, passive force-displacement tests with enough abutment offset from rotation to evaluate the skew reduction factor and backwall pressure distribution. The maximum rotation and displacement would be larger than in the previous testing. As availability allows, the tests would be conducted at the same Salt Lake Airport test site used in the previous study.

Planned tasks for this new study are as follows:

1. Analysis of the existing abutment to determine acceptable rotation and loading scheme,
2. Performance of large-scale skew abutment tests,
3. Analysis of test results including determination of passive force reduction factors,
4. Comparison with longitudinal test results and modifications,
5. Supplemental numerical analysis of parameters affecting results,
6. Preparation of interim and final reports, and
7. Dissemination of results, including presentations at AASHTO committee meetings.

In Task 2, lateral load tests will be performed on the simulated bridge abutment with skew angles of 0°, 15°, 30°, and 45° relative to the direction of loading. The backfill will consist of concrete sand compacted to 95% of the modified Proctor maximum dry unit weight to provide direct comparisons with the previous tests performed with longitudinal loading. The passive force provided by the backfill will be determined by loading the abutment before and after compacting backfill behind the abutment. As with previous tests, the applied lateral force, abutment displacement and rotation, pressure on the backwall, vertical and horizontal movement of the backfill, and location of the failure surfaces in the backfill will be measured.

The Utah Department of Transportation (UDOT) will be the lead agency for this study, with Darin Sjoblom (dsjoblom@utah.gov) as the UDOT Champion. We intend to hire a firm or university as the prime consultant through qualifications-based selection in the UDOT General Engineering Services Pool, Research Work Discipline, after sufficient funds are committed by study partner agencies.

The study is planned to begin in the spring/summer of 2024 or 2025, with study completion expected in two or three years’ time.

The minimum partner commitment expected for the study is $20,000 per year in FFY 2024 and 2025. If partner agencies would like to contribute lower amounts or shift the contributions to FFY 2025 and 2026, we are open to discussing these options. Reach out to the Lead Study Contact above as needed.

The 100% SPR approval will be requested.