Single Slope Barrier Study

The cost of constructing and installing the embedded single slope barrier is approximately $75 per linear foot, based on test article construction in this research – a cost saving of nearly 80%.

What's the problem?

Traditional concrete median barriers, often called New Jersey CMBs, have been in use on the nation’s highways since the 1950s. While the design of these barriers has evolved to reflect changes in vehicle design – chiefly, the smaller size of modern automobiles — one weakness has remained: these barriers decrease in effectiveness each time an adjacent roadway is resurfaced. That’s because adding a new layer of asphalt to the roadway effectively reduces the height of the barrier — and the traditional barrier’s effectiveness is directly related to its height.

What’s the solution?

Researchers developed a single-slope CMB designed to meet federal standards even after repeated overlays on adjacent pavement. The objective of TTI’s research was to restrict lateral deflection of a concrete barrier when placed adjacent to slopes as steep as 1.5H:1V or on top of [glossary slug=”mse”]Mechanically Stabilized Earth (MSE) walls[/glossary], without using a concrete moment slab. The design was required to meet Test Level 3 requirements of [glossary slug=”MASH”]The AASHTO Manual for Assessing Safety Hardware (MASH) is the new state of the practice for the crash testing of safety hardware divides for use on the National Highway System (NHS). It updates and replaces NCHRP Report 350. It was adopted in 2009.[/glossary].

What are the benefits?

The primary advantage of the new single-slope CMB is that the pavement adjacent to the new barrier can be overlaid several times without changing the performance of the barrier, reducing maintenance costs associated with the use of permanent CMBs. A second benefit is the potential for significant cost savings when compared to the cost of constructing and installing a traditional barrier with a moment slab – savings on the order of 80%, according to preliminary figures (more below.)

How was it tested?

Results of four crash tests show that the performance of the new single-slope CMB is roughly equivalent to the performance of the New Jersey CMB. These tests were conducted with the new single-slope CMB deployed in both the permanent and temporary configurations.Researchers first evaluated the performance of the barrier in a free standing condition using a combination of a smaller scale [glossary slug=”bogie”]bogie impact test[/glossary] and simulation analysis. It was determined that the free-standing single slope barrier with the [glossary slug=”grouted”]grouted rebar-grid slot connection[/glossary] cannot provide adequate lateral resistance to allow its use adjacent to 1.5H:1V slopes or on MSE walls. Simulation results indicated that the free standing barrier is likely to result in a lateral deflection of greater than 30 inches, which was significantly more than the desired deflection for this research.The researchers then evaluated restraining the single slope barrier by embedding it 10 inches in soil. Another series of bogie tests and simulation analysis was performed and it was determined that the lateral deflection of the barrier was significantly reduced. Based on the results of the analysis, it was recommended that a crash test be performed with the barrier embedded 10 inches in soil.

Testing details

A 100-foot installation of the 42-inch tall single slope barrier was embedded 10 inches in soil, effectively reducing the height of the barrier to 32 inches. The barrier was placed at a 2-foot lateral offset from the 1.5H:1V slope break point of the soil embankment. The concrete barrier was comprised of five 20-foot long barrier segments that were connected using the grouted rebar-grid connection. The embedded barrier was crash-tested using MASH [glossary slug=”tl3″]TL-3 criteria[/glossary].A 2,270-pound vehicle, traveling at an impact speed of 63.1 mph, impacted the single-slope barrier at an angle of 24.2 degrees. The barrier successfully contained and redirected the vehicle. The vehicle did not penetrate, underride, or override the installation.Maximum dynamic and static deflections of the barrier during the test were 5.6 inches and 5.5 inches, respectively. No detached elements, fragments, or other debris were present to penetrate or show potential to penetrate the occupant compartment, or to present undue hazard to others in the area.Maximum [glossary slug=”deformation”]occupant compartment deformation[/glossary] of the vehicle was 5.5 inches. The vehicle remained upright during and after the collision vent. Maximum roll angle was 44 degrees. [glossary slug=”risk”]Occupant risk factors[/glossary] were within the limits specified in MASH. The vehicle also remained within the [glossary slug=”exit”]exit box[/glossary].The embedded single-slope barrier in front of 1.5H:1V slope performed acceptably according to MASH requirements.

Conclusions

The embedded barrier application developed in this research is expected to result in significant cost savings for the user transportation agencies. The benefit of this application comes from the elimination of the use of a moment slab to restrict lateral barrier deflection. The cost of constructing and installing the single slope barrier with a moment slab is typically $375 per linear foot (based on recent bids received by Washington State Department of Transportation.) The cost of constructing and installing the embedded single slope barrier on the other hand is approximately $75 per linear foot, based on test article construction in this research – a cost saving of nearly 80%. While there are certain metal guardrail designs approved for use adjacent to steep slopes, a significant advantage of using the embedded concrete barrier comes from the low maintenance required in the event of a vehicle impact. Due to a small lateral barrier deflection of 5.5 inches, the barrier is expected to require little to no maintenance following most impacts. This can be very advantageous to user agencies with mountainous areas, where steep slopes and narrow shoulder widths make repairs difficult, costly, and increase crew exposure to traffic. Due to the small lateral deflection upon vehicle impact, the embedded concrete barrier application developed in this research can also be used on top of MSE walls.