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~;-~' concrete and earth, and (2) an encased barrier such as barrels, membranes, or shotcrete-encased earth. The design ob- jective of barriers is to eliminate [he possibility of a vehicle vaulting the berm, and to either redirect or arrest [he motion of the vehicle. This is ac- complished by providing a near-vertical berm face, with the energy of the vehicle impact being dissipated by displacement of the berm and the berm reaction with the road surface. The advantages of the rigid-type bar- riers are (1) they can be designed to withstand severe impact without penetra- tion, (2) they can be designed to cause negligible vehicle damage for impacts of low severity, and (3) they can be reused at other locations. The disadvantages are (1) that rigid barriers are relative- ly unyielding and tend to aggravate the deceleration environment of the vehicle occupant and (2) their material and in- stallation cost is higher than that of conventional berms. 93 The advantages of the encased barriers are (1) reduced material cost compared to rigid barriers and (2) the yield- ing action of the encased berm will provide a less severe deceleration en- vironment for the vehicle occupant. The disadvantages are (1) corrosion of the metallic elements resulting in main- tenance problems and need for peri- odic inspection, if plastic barrels are not used, {2) labor requirements to con- struct the system, and (3) the encased barrier system would not be salvable for reuse. Computer analysis shows that barriers can provide effective vehicle restraint for the range of approach conditions and vehicle sizes studied during this proj- ect. Barriers should be used only in areas where maintenance of the restraint system will cause a great economic bur- den, such as heavily traveled, permanent haul roads. BOULDERS Large rocks (18 to 24 in) and large boulders (3 to 4 ft in diameter) are used as edge-of-road berms in quarries and are placed 4 to 8 ft from the edge of the road. A minesite visited had two inci- dents of vehicles being restrained by a boulder-type berm. In both cases, the vehicle was unloaded and appeared to have been stopped by high-centering on a boul- der. Boulders would be used more often in eastern coal mines, but they are nor- mally used as the rock core in the valley fill for improved drainage. The ability of a berm constructed from a continuous line of large boulders to restrain or redirect a runaway vehicle was evaluated. Typically, and also for- tunately, a vehicle impacting a row of boulders or a berm constructed from sev- eral large boulders is not stopped in- stantaneously. The vehicle's kinetic, rotational, and potential energy is dis- sipated in the work generated as the boulders slide along the road surface. The primary area of concern is the distance associated with stopping various size vehicles by using typical-size boul- ders. It was assumed that boulders are not capable of developing a force suffi- cient to redirect a vehicle. Analysis shows that if boulders are sized to stop a vehicle in a short distance, they will probably cause considerable damage. If they are sized to reduce the deceleration forces, the distance that they must be placed from the edge of the road must increase. For example, if an empty 85- ton vehicle impacted a boulder berm at 20 mph and 20°, it would push the impacted boulders 80 ft along the impact angle. This distance would necessitate position- ing the boulders about 25 ft from the edge of the toad. Therefore, boulders are not considered to be a very effective means of providing vehicle restraint, the main reason being the damage that may be incurred by the vehicle. A viable alternative to the independent use of boulders is the burial of the boulders in an earthen berm. In