State Department of Transportations (DOTs) often have issues with installing beam guardrail over buried structures, buried utilities, or other buried objects due to conflicts between the bottom of the post and the structure/utility/object (see Figure 1).

The conflicts between the guardrail post and underground/buried objects may lead to installation challenges such as the posts cannot be embedded at the required depth, compromising their function. Improperly installed posts reduce structural support, weaken energy absorption, and may rotate or shift upon impact, leading to inconsistent or failed performance. This can increase crash severity and the risk of guardrail failure.

Background

A box culvert guardrail system was also developed to allow the guardrail system pass over the box culverts as shown in Figure 4 [4]. There are two MGS box culvert guardrail designs that use either epoxy anchors to bolt the posts directly to the culvert lid or use a thru-bolt anchor to attach the guardrail posts to the culvert lid. These designs work for shallow fill structures with a depth of 9 inches or greater of fill over the lid. There is no length limitation to this system but there are significant long-term maintenance concerns due to difficulty of repairs (spalling of structure lid at post connection point, having to excavate down for repair, having to access culvert from below, etc.). Also, these systems require several specialty/custom parts which can be difficult for maintenance to obtain/manufacture for repairs.

Concrete barrier systems are also used when buried obstructions are present. A concrete barrier is typically embedded only a few inches into soil/asphalt/concrete, or a portable barrier is surface mounted. Therefore, concrete barrier can often be used to span over an underground structure, utility, or other object. However, often there are shoulder embankment widening needs which can be problematic and the cost to install concrete barrier are quite a bit higher than beam guardrail. Most recently, MwRSF conducted research on shortening MGS posts with a minimum depth of 28 inches [5]. They investigated reduced spacing options with reduced post embedment by performing a series of bogie tests and computer simulations. They recommended the half-spacing MGS system with 28 inches post embedded depth for both TL-3 and TL-2 tests. If the TL-3 test fails, the alternative would be TL-2 testing or use a stiffer MGS (i.e., 28-inch quarter-post spacing). MwRSF did not perform any full-scale crash testing, and no testing is planned at this moment. Although MwRSF recommended a minimum of 28 inches of embedded depth with half post spacings, this research proposes to use of soil plate along with reduced post spacing, which may allow for the use of larger (standard) post spacing and/or reduce embedded depth further (less than 28 inches). Texas A&M Transportation Institute (TTI) conducted a series of full-scale crash tests to evaluate TxDOT roadside safety systems [6]. One of the tested systems was a 31-inch height W-beam guardrail system with W6x8.5 wide flange guardrail posts installed in rocky terrain. To install the post in rocky terrain they drilled an oval shaped void measuring 12-inch wide x 22 inch long x 24 inch deep. Any excess post length is cut (cut to have 24-inch embedded depth) and the void was backfilled with ASTM C33 coarse aggregate, size no. 57, and hand-tamped with a rod in 6- inch lifts, as shown in Figure 5. This system was evaluated by performing full-scale tests taken under MASH TL-3 conditions. Although three posts during Test 3-10 and five posts during Test 3-11 that were placed in the middle of span were pulled-out from their holes and released from the rail element, the vehicles were successfully contained and redirected. During the literature review task, this study will be comprehensively reviewed to further investigate options to successfully reduce the post length for a MGS system under MASH TL-3 condition.

The primary objective of this project will develop a MASH Test Level 3 (TL-3) MGS with a short post system and to determine the minimum post length (see Figure 6). This study will also seek to determine if a short post system of indefinite length can be constructed or find the maximum length of a short post system. The standard MGS uses a 6-foot long W6x8.5 post with embedded depth of 40 inches (3 feet – 4 inches). This project will determine if a 5-foot post with 28 inches embedment (starting point), or 4.5-foot post with 22 inches embedment can be used while meeting MASH evaluation criteria. This project will first attempt to design a standard MGS system with only reduced post embedment. However, if bogie tests or computer modeling indicate that this cannot be achieved, the research team will consider reduced post spacing and/or ground anchor/soil plate modifications.

Benefits

A shorter post MGS system could avoid conflicts with underground structures, utilities, or other objects while potentially providing a long enough length of the system to go over large underground obstacles. This project will provide another MASH compliant barrier option for state DOT’s to use when encountering an underground obstruction which does not allow them to install standard MGS with 6-foot-long posts.

Products

This project will provide a MASH compliant short post MGS for use in situations where sub-surface obstructions with low soil cover are present. The final report will summarize the bogie tests, the modification concepts, the result of computer simulations, the results of the MASH TL-3 full scale crash testing for the system, provide system implementation recommendations, and recommendations for further research.

Work Plan

The work plan for this research includes the following tasks.

Task 1: Literature Review
The research team will perform literature review to search for past reduced embedment W-beam guardrail systems and installations. This includes searches for both MGS and older W-beam guardrail systems. The researchers will also review various specialty W-beam guardrail systems with posts anchored on culverts or restrained by other means.

Task 2: Bogie Testing
The research team will conduct four bogie tests to collect force-deflection curve/data to compare to the previous testing data for the standard post in soil [7]. This aids the research team to understand the effect of an embedded depth of the post and the use of a soil plate. The planned bogie tests include the tests using 2 different post lengths:
• 5 ft (28-inch embedment) with and without soil plate
• 4.5 ft (22-inch embedment) with and without soil plate

Task 3: Computer Simulation
The research team will develop and validate an FE model. First, the research team will develop and validate the FE model for the one post in standard soil by comparing it to the corresponding bogie tests performed in the past research [7]. Then research team will develop and validate the FE models with a reduced post embedment and soil plate with the bogie test conducted in Task 2. Once the one post FE models are validated, the research team will develop and validate the full-scale standard MGS system installed in the standard soil with the past full-scale MGS test [8]. With the validated full-scale MGS FE model, the research team will perform a series of impact simulations with different configuration models to:
• Evaluate short post MGS with 28-inch and 22-inch embedment depth.
• Evaluate short post MGS with soil plate only.
• Evaluate short post MGS with soil plate and reduced post spacing.
• Determine the maximum system length of the short post MGS system.

Based on the simulation results, the research team will recommend a short post MGS configuration for a full-scale crash testing. If multiple configurations are eligible, a poll would be involved for pooled fund members for the preferred short post MGS system.

 

Task 4: Full Scale Test
The research team will construct and test the recommended short post system under MASH TL-3 conditions. According to MASH 2016, two full-scale vehicle crash tests are required for evaluating a longitudinal barrier:
• Test 3-11: A 2270P vehicle weighing 5000 lb impacting the longitudinal barrier while traveling at 62 mph and 25 degrees.
• Test 3-10: An 1100C vehicle weighing 2420 lb impacting the longitudinal barrier while traveling at 62 mph and 25 degrees.

 

Time Schedule

Started: May 2025
Time frame: 20 months

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May 22, 2025