W-beam guardrail is often used to protect motorists from steep roadside slopes adjacent to high-speed roadways. A roadside slope placed immediately behind a guardrail system greatly reduces the soil resistance associated with lateral deflection of the barrier. This reduction in the post-soil forces greatly reduces a systemâ€™s energy-absorption capability, significantly increases dynamic rail deflections, and can potentially produce issues with vehicle capture or vehicle override. Further, when the guardrail extends over the embankment, the gap between the bottom of the rail and the ground will be greatly magnified and thereby increase the risk of severe wheel snag.
The MGS guardrail system has greatly improved the safety performance and stability of guardrail installed at the slope breakpoint of slopes as steep as 2H:1V. However, current MGS installations adjacent to 2H:1V slopes utilize increased length posts in order to provide sufficient embedment to generate the proper soil resistive forces. This requirement creates issues with state DOT hardware inventories and maintenance due to the need to stock and maintain non-standard length posts. In order to reduce hardware inventories, states have chosen in some cases to install the standard MGS system at an offset from the slope. Current guidance requires a minimum offset of 1 ft to 2 ft from the back of the post to the the slope breakpoint for the standard MGS system with 6-ft long posts depending on the slope grade. This large offset maintains the safety performance of the system but creates a great deal of additional expense in terms of earthwork. Thus, a need exists to evaluate a minimum offset for the standard MGS guardrail system adjacent to a 2H:1V fill slope in order to reduce current issues with state hardware inventories and earthwork costs.
The use of guardrail adjacent to slopes has been a common concern for state DOTâ€™s. In the past, several states have requested guidance regarding safe guardrail offset distances or modification to guardrail post spacing and/or embedment when placed directly adjacent to steep fill slopes. With respect to the MGS system, MwRSF has made the following recommendations regarding placement of the MGS adjacent to slope. These recommendations were made using on the conservative engineering judgment and based on the best available information at this time regarding the use of w-beam guardrails adjacent to slopes.
1. Standard MGS guardrail placed adjacent to any slope with 2 ft of level soil behind the posts is acceptable.
2. For MGS guardrail placed 1 ft to 2 ft adjacent to a 6:1 or flatter slope, standard 6'-ft long, W6x9 posts at standard spacing are recommended.
3. For MGS guardrail placed 1 ft to 2 ft adjacent to a 3:1 to 6:1 slope, 7-ft long, W6x9 posts at standard spacing are recommended.
4. For MGS guardrail placed less than 1 ft adjacent to a 3:1 or steeper slope, 8-ft or 9-ft long, W6x9 posts or 7.5-ft long 6-in. x 8-in. SYP wood posts at standard spacing are recommended.
While these recommendations have been used by states in the past, there is a desire to use the standard MGS post types and spacings with minimal offsets adjacent to steep slopes to reduce state inventories and the required earthwork for these types of installations.
Several variations of the MGS installed adjacent to steep slopes have been developed and/or full-scale crash tested in recent years. A modified version of the Midwest Guardrail System (MGS) was developed for use adjacent to steep roadside slopes. This design incorporated 9-ft long, W6x9 steel posts spaced on 75-in. centers and installed at the slope breakpoint of a 2H:1V slope. The top mounting height of the system was 31 in. This system was successfully crash tested according to the safety performance evaluation criteria found in the Manual for Assessing Safety Hardware (MASH). For this variation, either wood or steel post options are acceptable when installed at the slope break point. The wood post version utilized 7.5-ft long 6-in. x 8-in. wood posts installed at the standard 75 in. post spacing. It should be noted that during the development of the steel post version of this system, dynamic bogie testing of 8-ft and 9-ft long W6x9 steel posts at the breakpoint of a 2H:1V slope demonstrated very similar behavior and the 9-ft long post was chosen for the system in order to be somewhat conservative and to account for variations in soil conditions.
A MGS system without blockouts was also evaluated with 6-ft long W6x9 steel posts at standard 75-in. spacing installed on a wire-faced, rock gabion or MSE wall at the slope breakpoint of a 3H:1V fill slope. This system was successfully tested with both the 1100C and 2270P vehicles under the MASH safety requirements. While this MGS system did preform acceptably using standard post lengths and spacing adjacent to a slope, it should be noted that the slope was not as severe as the 2H:1V slope tested previously and the system used a high quality and very strong fill material and the base of the posts were actually embedded in the rock layer of the wire-faced, rock gabion or MSE wall. Thus, the installation method used for these posts produced higher soil resistive forces and limited post rotation as compared to a more typical MGS installation adjacent to a steep fill slope.
Recently, Texas A&M Transportation Institute (TTI) tested a 31-in. high W-beam with posts spaced on 75-in. centers on a 2H:1V slope. This system used 8-ft long, W6x9 posts with 8-in. deep blockouts and placed the face of the rail at the slope breakpoint. TTI conducted a successful full-scale crash test of this system according to the TL-3 requirements for MASH test designation no. 3-11. Because this system used shorter posts that were installed further down the slope, it seems reasonable assume that 8â€™ posts would work at the slope breakpoint as well. In addition, there is potential for the use of even shorter posts at the post breakpoint. However, further research is required to determine if that approach is viable.
Installation of the standard MGS guardrail system with 6'-ft long, W6x9 posts at 75-in. post spacing at or very close to the slope break point of a 2H:1V fill slope creates several potential issues with respect to the performance of the guardrail. Reduced post-soil interaction forces are expected as the adjacent slope reduces the effective embedment of the post. This reduction in the post-soil resistive forces can reduce the redirective forces and energy absorption of the system which can potentially increase barrier dynamic deflections decrease the potential for vehicle capture, and increase the potential for vehicle instability.
Because the effect of the slope on the post-soil restive forces is critical to determining the minimum offset of the MGS placed adjacent to a 2H:1V slope, the soil strength chosen for the evaluation of the minimum offset should be considered. For typical longitudinal barrier designs, it has generally been assumed that the use of strong soils is more critical for full-scale crash testing and evaluation as strong soils tend to produce higher post-soil resistive forces which tend to create higher rail forces, increased snag on barrier support posts, and higher occupant risk values. However, in the case of the MGS installed at a minimum offset from a 2H:1V slope, the soil resistive forces of the standard system are being reduced by a combination of shallow post embedment and slope effects. Insufficient soil support can lead to excessive guardrail post movements and guardrail lateral deflection during vehicle collision and result in a lower system capacity to contain and redirect errant vehicles. Thus, the use of a strong soil in this situation may not be critical, but may actually artificially improve the capacity of the system.
MASH does provide for the use of weak or reduced strength soils. MASH provides the following guidance with respect to the use of alternative soils.
â€œImpact performance of some soil-mounted features depends on dynamic soil structure interaction. Longitudinal barriers with soil embedded posts and soil-embedded support structures for signs and luminaires are such features. When feasible, these features should be tested with soil conditions that replicate typical in-service conditions. Soil conditions are known to vary with time, location, and environmental factors, even within relatively small geographical areas. Therefore, except for special test conditions, it is necessary to standardize soil conditions for testing. In the absence of a specific soil, it is recommended that all features whose impact performance is sensitive to soil-structure interaction be tested in a soil that conforms to the performance specification as described in Section 3.3.1. However, product developers and user agencies should assess the potential sensitivity of a feature to foundation conditions. If the feature is likely to be installed in a soil that could be expected to degrade its performance, testing in one or more of the special soils described in Section 3.3.3 may be appropriate.â€
A3.3.1 Standard Soil
â€œUnless the test article is limited to areas of weak soils, the standard soil should be used with any feature whose impact performance is sensitive to soil-foundation or soil-structure interaction. A large percentage of previous testing has been performed in similar soil and a historical tie is needed. Although it is probably stronger than the average condition found along the roadside, it is still representative of a considerable amount of existing installations.â€
A3.3.3 Special Soils
â€œThe weak soil should be used, in addition to the standard soil, for any feature whose impact performance is sensitive to soil-foundation or soil-structure interaction if: (a) identifiable areas of the state or local jurisdiction in which the feature will be installed contain soil with similar properties, and (b) there is a reasonable uncertainty regarding performance of the feature in the weak soil. Tests have shown that some base-bending or yielding small sign supports readily pull out of the weak soil upon impact. For features of this type, the strong soil is generally more critical and tests in the weak soil may not be necessary.
MASH would recommend that the system be tested in the standard soil unless the hardware installations are expected to be placed in generally weak soils and weak soil is expected to degrade performance. Otherwise, it is recommended that the standard soil be used as it is believed to be representative of typical soil foundation conditions and provides a historical tie to previous testing. While there is an argument that weak soils may be more critical with respect to the MGS placed at a minimum offset to a 2H:1V slope, it is believed that evaluation of such a system should follow the guidance provided in MASH and evaluate the system with standard soil based on the fact the general soil condition for a given installation would not be assumed to be weak and it provides a link to previous testing of guardrail on slope.
The research effort to evaluate the minimum offset for installation of the standard MGS guardrail system with 6-ft long W6x9 posts spaced at 75 in. on centers adjacent to a 2H:1V fill slope will begin preparation of CAD details and construction of the system for full-scale testing at the MwRSF outdoor test facility. This effort will also include the construction of a 2H:1V: slope in the test pit at the outdoor test facility.
Following construction of the system, the MGS minimum slope offset configuration will be evaluated according to the MASH guidelines for test designation no. 3-11 with the 2270P pickup truck vehicle. The offset from the slope will be set such that the center of the line post is located at the slope break point of the 2H:1V: slope. It is believed that the 1100C vehicle test can be waived for this system based on previous small car testing of the MGS system. The full-scale vehicle crash test will be conducted, documented, and evaluated by MwRSF personnel and in accordance with the MASH guidelines.
After completion of the full-scale crash testing, a summary report of the research project will be completed detailing the tested barrier system, full-scale crash test results, evaluation of barrier performance, and recommendations for implementation and barrier system installation. The MGS minimum slope offset design would also be submitted for approval to FHWA.