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94 • this way, the boulders act to make the berm more rigid while eliminating the required boulder push distance. The severity of impact is also significantly reduced. MEDIAN BERMS Median berms are unconsolidated mate- rial placed in the center of a haul road and are constructed in a manner that allows a runaway vehicle to straddle them, shearing off the portion of the berm above the vehicle's undercarriage, and eventually allowing the vehicle to come to a rest. The force required to shear the berm is dependent upon soil properties. Median berms can be constructed narrow or wide. A wide berm allows the vehicle to straddle the berm, and its tires to roll on the lower berm material. In this way, increased rolling resistance of the soft berm material allows the vehicle to stop quicker. However, model teats have shown that straddling a wide berm can re- sult in a vehicle overturning. Additional testa were conducted employ- ing a narrow berm that allowed the vehi- cle's rear track (axle) to completely straddle the berm. This requires some compaction of the-berm to retain its de- sired height. The narrow berm stopped the vehicle in a shorter distance, par- ticularly at higher impact speeds (fig. 4). The increased shear area and, to a lesser extent, the shear strength of the compacted material were responsible for the reduction in travel. Turning and misalignment of the vehicle were mini- mized by the restraining action of the narrow berm. Based on the results of these model tests, a narrow compacted median berm is recommended because of the decreased po- tential fcr rolling the vehicle over. 40 L C. E 3a I- U O ~ 20 W KEY 85-ton vehicle -6-pct grade W ~ Compacted narrow berm-120-pct load ~ v Compacted narrow berm-0-pct load U t0 • Unconsolidated wide berm-120-pct load 2 ~ Unconsolidated wide berm-0-pct load W Vehicle carrying load ~ load= Vehicle base carrying load 0 15 30 45 60 75 90 105 120 VEHICLE STOPPING DISTANCE, ft FIGURE 4. - Comparison of stopping distances on median berms for an 85-ton haulage vehicle on an 8-pct downgrade.