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Since MwRSF did the testing on flat ground with the low tension cable system, would you change your recommendation if the cable was placed on 1V:6H or 1V:8H slopes in front of and up to 4 ft. behind the cable before starting on a 1V:2H fill slope. It would seem the vehicle would strike the barrier higher, so it may be necessary to constrain deflection even more.
|Date||November 4, 2008|
I have reviewed you question regarding the use of the CASS adjacent to a steep slope with an 1:6H or 1:8H approach slope.
Previously we had given you guidance for using the CASS adjacent to a steep slope. We had suggested using a 4ft offset to the slope and reducing the post spacing of the CASS to 3 m. Your new question was whether or not these recommendations would hold true when the cable barrier was installed on a 1:6H or 1:8H approach slope. In order to address this issue, I looked into the performance of the CASS system, analysis of bumper trajectories for 2000P pickup trucks encroaching on approach slopes, and previous testing of cable barrier on approach slopes. Based on this analysis I have the following comments.
Ross, H.E., Smith, D.G., Sicking, D.L., and Hall, P.R., Development of Guidelines for Placement of Longitudinal Barriers on Slopes, Research Report 3659-2 (DOT-FH-11-9343), Texas Transportation Institute, May 1983.
In addition, I reviewed some analysis that we conducted on bumper trajectories of 2000P vehicles running off slopes and compared these trajectories with the cable heights of the CASS System. A chart is attached. In the chart, the green lines are the cable heights, the pink line is the slope, and the navy blue line is the truck bumper trajectory. You can see from the chart that vehicle encroaching on a 1:6H slope could have bumper heights higher than the top cable height of the CASS system which could lead to the potential for override of the system. Similar analysis performed by TTI on 1:6H slopes with the 2000P vehicle also indicated bumper heights that would exceed the top cable height of the CASS depending on the barrier offset ( http://tti.tamu.edu/documents/0-5210-3.pdf ).
Based on the existing test data and analysis of vehicles encroaching on 1:6H slopes, we are concerned about the use of cable barrier adjacent to steep slopes due to uncertainty about the effective capture and redirection of the vehicle.
|Date||February 24, 2012|
130 Whittier Research Center
2200 Vine Street
Lincoln, NE 68583-0853
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