Posts of a beam guardrail system are occasionally placed close to underground obstructions, which can include utilities and drainage features (see Figure 1). For instance, a utility line may run parallel to a beam guardrail installation, a pipe may intersect the beam guardrail run, or an inlet may be located near a post. These scenarios raise the critical question of when the placement of an underground obstruction interferes with the proper operation of the guardrail posts, potentially impacting the effectiveness and structural integrity of the guardrail. In past research, Texas A&M Transportation Institute (TTI) researchers identified the allowable offsets from the back of a W-beam guardrail steel post to a non-soil pavement. Sheikh et al. (1) found that the concrete mow-strip and back filled with low strength grout design was successfully worked with 7-inch offset from the back edge of the post to the concrete cutout without performance interruption based on the full-scale tests under the American Association of State Highway and Transportation official (AASHTO) Manual for Assessing Safety Hardware (MASH) Test Level 3 (TL-3) conditions (2). Sheikh et al (3) designed and crash tested a guardrail system that can be installed in 2-inch-thick asphalt pavement with an 8-inch offset from the edge of the pavement. In this research, the researchers conducted eight bogie tests to identify the suitable asphalt pavement thickness and the maximum offset from the back edge of the guardrail post to the edge of the pavement. The final design was successfully crash tested for MASH TL-3 criteria. Findings from the above-mentioned studies can be used as a reference, but the studies are all related to the pavement, which is only a maximum of 4 inches deep. However, the underground obstructions can be placed anywhere adjacent to a post (impact side, backside, right, or left side) at any depth.

 

The objective of this project is to determine the allowable proximity of underground obstructions to steel-post W-beam guardrail posts without compromising their operational effectiveness. This project will determine the minimum offset required, and whether the required offset varies based on the location of the obstruction relative to the guardrail post, such as on the impact side, backside, left, or right. The research will also determine if variations in the elevation of the underground obstruction affect the overall performance of the W-beam guardrail system. Additionally, the research will provide information on the forces being applied to the object at various locations. This information will be provided with force distribution profiles.

Benefits

States will have a better understanding of how close underground obstructions can be to a W-beam guardrail post. States can provide information to a utility what forces will be imparted to their equipment at a given offset.

Products

The primary product of this project will be a comprehensive set of guidelines defining acceptable offset ranges from different post faces to underground obstructions, ensuring that beam guardrail operations remain unaffected. Additionally, the study will also provide estimates of the forces imparted at various distances from the post, offering valuable data for designing and placing beam guardrail systems in proximity to underground features.

Work Plan

The work plan for this research includes five (5) tasks as described below:

Task 1: Literature Review
This task involves reviewing previous studies and test reports to identify currently used, existing, or previously tested posts placed near underground obstructions. The review will also determine the most common and critical types of obstructions and their characteristics including shapes and materials. To enhance understanding of field practices, this step may include a brief questionnaire for member states to gather relevant information. Additionally, the review will assess the critical depth range of obstructions relative to a side of post. Furthermore, the research team will review previously conducted steel post bogie tests to establish the baseline force-deflection range for steel posts. The research team will analyze the behavior of individual steel posts installed in standard soil conditions to define a force-deflection threshold. This threshold will account for variations in post-soil interaction without obstructions, serving as a reference for subsequent analyses. In this task, the research team will establish computer simulation scenarios based on the data from the literature review and will conduct a meeting with technical representatives to discuss and confirm the simulation scenarios.

Task 2: Simulation Analysis
In this task, the research team will calibrate and validate post and soil finite element (FE) models using data from TTI-conducted baseline bogie tests which were reviewed in Task 1. Note that the baseline tests conducted by other agencies will not be used for the FE model calibration because the model should be calibrated to represent the testing condition that the research team will use in the next task. However, since test conditions including soil and testing vehicles are different by test agencies, the bogie test data from the other agencies will not be used. Once the post and soil FE models are calibrated, the research team will use the models to simulate scenarios with different types of underground obstructions identified in Task 1. The simulations will explore critical cases involving combinations of posts and obstructions, including their location relative to the post (e.g. impact side, backside, left, or right) and their distance from the post. The research team will perform the simulations with the validated single post-soil model to identify the top five critical cases.

Task 3: Bogie Testing
Based on the critical cases identified through simulation, the research team will perform five bogie tests of below-grade obstructions near W-beam guardrail posts to observe the force-deflection interactions between the posts and underground obstructions. In this task, the research team will perform one baseline test without obstruction to provide the baseline data for the day of testing soil conditions. The tests will provide data to verify and validate the FE models with obstructions.

Task 4: FE Model Validation and Simulations Analysis
The research team will validate the FE models using data from Task 3. The research team will verify and validate the soil and post models with obstructions installed at the critical locations. The research team will adopt the validated soil and post FE models to a full Midwest Guardrail System (MGS) FE model to perform an impact simulation. The research team will perform impact simulations under MASH Test Level 3 conditions to determine how the obstructions will affect the entire beam guardrail system by comparing the system behavior with and without obstructions. Also with the MGS system model, the research team will identify a range of offsets from the post faces to underground obstructions that eliminate interference with beam guardrail operations.

Task 5: Final Report
The final report will include recommendations for offset ranges and estimates of forces imparted at various distances from the post, providing valuable data for designing and placing steel-post W-beam guardrail systems in proximity to underground features. This report will also include a guide for the field implementation with different guardrail system installation conditions. If needed, the general guide for future research will also be provided.

Time Schedule

Started: March 2025
Time frame: 22 months

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March 25, 2025