At times, limitations imposed by terrain and other site conditions on projects presents challenges for designers to follow the AASHTO Roadside Design Guide (RDG) recommendation to install guardrail with a minimum 2-feet distance from the back of post to the slope break point of a steeper side slope. At many locations, such as mountainous terrain or areas with tight environmental controls, this 2-foot width is difficult to provide. As a result, designers often must decide between providing a reduced shoulder width or less than optimal guardrail placement on a slope or directly in front of a slope.

Also, there is a desire by transportation agencies to place guardrail on slopes where practicable. Guardrail on slopes is located further away from the traveled roadway than guardrail placed on shoulders. Having more distance between the road and the guardrail reduces the number of vehicle strikes to the barrier by providing more room for an errant vehicle to correct itself, reduces potential for injuries during a strike on the barrier, reduces resulting repair costs, and reduces initial installation costs by requiring a shorter barrier length of need needed to shield the fixed object or feature.

There has been previous research focused on evaluating the performance of guardrail systems placed on slopes. A few configurations of guardrail systems with sloped terrain have been crash tested and evaluated according to MASH TL-3. This research is summarized below.

• Thrie beam with 1:1 slope – 34-inch tall thrie beam with face of rail at slope breakpoint using 9-foot-long posts. Satisfactory for MASH TL-3 (1).
• MGS with 2:1 slope – 31-inch tall MGS with face of rail at slope breakpoint using 8-foot-long posts. Satisfactory for MASH TL-3 (2).
• MGS with 10:1 or flatter slopes – Standard 31” tall MGS using 6-foot-long-posts or thrie beam can be placed anywhere on 10H:1V or flatter slopes. Satisfactory for MASH TL-3 (3).
• MGS with 1:1 slope – 31-inch tall MGS with face of rail at slope break point of a 1:1 slope using 9-foot-long-posts. Failed MASH crash testing due to vehicle rollover (4).
• MGS with 6:1 slope – MGS with 8-foot-long posts placed 6-feet down from the slope breakpoint on a 6:1 slope. The top of rail was 31” tall in reference to the slope break point. Failed MASH crash testing due to vehicle penetration of the guardrail system (5).
• MGS with 8:1 slope – MGS placed 5-feet down from the slope break point on an 8:1 slope. The top of rail was 31” tall in reference to slope location where it was installed. Satisfactory for NCHRP 350 TL-3 (6)

Objective

The objective of this project is to begin evaluation of MASH TL-3 compliant guardrail placement on various slopes such as 4:1, 6:1, and 8:1. Bogie testing and computer simulations will be used to evaluate the performance of the guardrail on slope.

Benefits

The activities of this research would provide transportation agencies more barrier placement options on a greater range of roadway side slope grades. If successful, the Phase I project will provide conceptual guardrail designs on 4:1, 6:1, and 8:1 slopes likely to meet MASH crash testing requirements as indicated through computer simulations and bogie testing.

Products

The TTI research team will provide a final report that documents the evaluation of guardrail placement on slopes and provides initial guidance for the placement of guardrail on slopes. The report will summarize the bogie testing and computer simulations. Recommendations for future research in a possible Phase II project will be included in the report.

Work Plan

The TTI research team will review the previous studies that focused on evaluation of guardrail crashworthy performance on slopes. This will include review of NCHRP Report 230/350 and MASH guardrail performance.

The TTI research team will develop a list of guardrail configurations for 4:1, 6:1, and 8:1 slopes warranting further investigation. The configurations will include details such as embankment slopes, post lengths, and offset distances (if applicable) from the slope break point. The list will be developed based on past guardrail performance on slopes and engineering judgment.

The TTI research team will distribute a survey to state members of the Roadside Safety Pooled Fund with the list of potential configurations to identify preferred configuration options for each slope grade. The results of the survey will be used to select two configurations for each slope grade to be further evaluated using bogie testing and computer simulations.

TTI Proving Grounds will perform bogie testing to evaluate the performance of a single guardrail post installed in 4:1, 6:1, and 8:1 slopes. TTI Proving Grounds testing of six (6) bogie tests will be performed with two post configurations for each slope. The bogie vehicle will impact the posts at a low speed (15 – 20 mi/h). Accelerations from the bogie vehicle will be recorded along with observed deformation/displacement of the guardrail post.

The TTI research team will conduct computer simulations to replicate the bogie experimental tests performed in Task 2. The impact speed, bogie weight and impactor nose, and guardrail post configuration in the computer simulation will match the bogie tests. The TTI research team will compare the performance of the computer model to the bogie tests. This information will be used to validate the finite element models of the guardrail post and soil.

Once the finite element models of the guardrail posts and soil have been validated, the TTI research team will develop full-scale configurations of the guardrail systems on slope for computer simulations. These will include sufficient length and appropriate end anchorage for the guardrail system to properly retain the vehicle. Per MASH Section 3.4.2.1, this will result in a total length of 175 ft for the finite element guardrail model.

The TTI research team will evaluate each of the six guardrail systems with computer simulations according to MASH TL-3 evaluation criteria. This will consist of impacting each finite element guardrail system model to be tested with the 1100C FE model and 2270P FE model. The impact speed will be 62 mi/h. The impact angle will be 25 degrees.

If the guardrail system configuration does not meet the MASH TL-3 evaluation criteria, then a modification to the configuration details will be made and re-evaluated. Prior to making any modifications, refinements to the finite element model of the guardrail system may be made to improve and verify accuracy. This will be an iterative process to evaluate and confirm the accuracy of the finite element model. If the guardrail system is still indicating unsatisfactory crashworthy performance for MASH TL-3, then modifications to the guardrail system will be considered. These will generally consist of the following, where applicable: longer posts and lateral offset adjustment. The research team will coordinate with the state technical representative to determine a plan for possible modifications and their evaluation.

The TTI research team will generate a final report of the findings from the research. The results will include the literature review and state survey results, bogie testing results, and computer simulation results. A MASH evaluation of each guardrail configuration on 4:1, 6:1, and 8:1 will be made based on the computer simulation results. Recommendations will be made for a future research phase to perform full-scale crash testing.

Time Schedule