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1. 1. GGUUIIDDEELLIINNEESS FFOORR MMIINNEE HHAAUULL RROOAADD DDEESSIIGGNN Dwayne D. Tannant & Bruce Regensburg 2001
18. 18. Guidelines for Mine Haul Road Design 14 Umin = coefficient of friction at the tire-road contact area Vo = vehicle speed at time of perception The factor t is actually composed of two separate time intervals, t1 and t2. Component t1 is the elapsed time for brake reaction due to pressure build-up in the brake system after the brake pedal is depressed and the brake mechanism is actuated to effectively exert a retarding force on the wheels. A typical value of brake reaction time suggested by SAE for a haul truck (180mt GVW) is 4.5s. The brake reaction time may be higher for the much larger trucks currently being used. The second component of t, designated t2, is driver reaction time, i.e. the time between driver perception of a hazard and when his foot actually begins to depress the brake pedal. A reasonable value of t2 is 1.5s. The factor Umin was evaluated from the following expression: gS V U 2 2 min = Equation (2) Where: V = SAE test velocity of 8.94m/s (32.2km/hr) g = 9.81m/s2 S = stopping distance computed by subtracting (8.94 x t1) from the SAE recommended stopping distance Substitution of the various SAE stopping distances and t1 factors for each weight category into Equation 2 yielded an average minimum achievable coefficient of friction, Umin, of 0.3 and a corresponding vehicular deceleration of about 2.94m/s2 . Based on Equations 1 and 2, stopping distance curves can be developed for different grades and speeds. However, these formulae do not account for brake fade due to heat build-up which constant brake application may induce. Kaufman and Ault (1977) note that these equations are not based on the results of actual field tests, and can only be used as a rough guideline in the preliminary planning stage of road design. Before actual road layout begins, the haul truck manufacturer should be contacted to verify the service brake performance capabilities of their trucks without assistance from dynamic or hydraulic retardation systems. Trial service brake stopping tests for a Caterpillar 785 haul truck (GVW = 230mt) on a 9% downgrade indicated a stopping distance of 67m from an initial speed of about 60km/hr (Holman 1989). The tests were conducted in accordance with SAE J1473 procedures, which require the stopping distances set out in Table 2-1 for this vehicle weight on a 9% grade. Table 2-1 Trial service brake stopping tests for a Caterpillar 785 haul truck (Holman 1989). Speed (km/hr) Stopping distance (m) 15 10 35 50 50 100 60 150 65 170
19. 19. Guidelines for Mine Haul Road Design 15 MacMillan (1989) reports brake-stopping tests on the 830E Haulpak (GVW = 386mt) wherein a stopping distance of 84m was measured on a 10.4% downgrade. More recent brake test results for the larger haul trucks could not be found in the published literature. Haul truck brake performance curves are available for most of the Caterpillar trucks (see Figure 8-3 and Figure 8-4 for examples). These curves can be used to determine the speed that can be maintained when the truck is descending a grade with retarder applied. 2.4 Sight Distance and Vertical Curves Vertical alignment in road design requires judicious selection of grades and vertical curves that permit adequate stopping and sight distances on all segments of the haul road. The relationship between operator sight distance and vehicle stopping distance is illustrated on Figure 2-1 for safe and unsafe conditions. Figure 2-1 Sight distances for horizontal and vertical curves (after Monenco 1989).
24. 24. Guidelines for Mine Haul Road Design 20 2.7 Super-Elevation Negotiating curves can generate high lateral tire forces. These forces contribute to high tire wear and ply separation. Super-elevating the curve helps eliminate these forces. Ideally, tire wear would be reduced and steering would be effortless if road super-elevation was just equal to the vehicle weight component. There is a practical limit to which a road can be super elevated since high cross-slopes around curves can, for slow moving vehicles, cause higher loads on the inside wheels, increased tire wear, potential bending stresses in the vehicle frame and, on ice covered surfaces, vehicle sliding down the cross-slope. The amount of super-elevation depends on the curve radius and truck speed. Table 2-3 is a guide for providing the super-elevation necessary to eliminate lateral forces. Super-elevated curves present a danger when the road surface is slippery. Unless the proper speed is maintained, a vehicle may slide off of the lower edge of the roadway. For this reason, super-elevation over 10% should not be used. Super-elevated curves should be maintained in good tractive conditions. The values of super-elevations listed in Table 2-3 over-estimate the practical super-elevations that are needed in practice. Table 2-3 Curve super-elevation (in % grade) to provide no lateral tire force (Caterpillar 1999). Turn radius Vehicle speed (m) (ft) 16km/hr 10mph 24km/hr 15mph 32km/hr 20mph 40km/hr 25mph 48km/hr 30mph 56km/hr 35mph 64km/hr 40mph 72km/hr 45mph 15.2 50 13% --- --- --- --- --- --- --- 30.5 100 7% 15% --- --- --- --- --- --- 45.7 150 4% 10% --- --- --- --- --- --- 61.0 200 3% 8% 13% --- --- --- --- --- 91.5 300 2% 5% 9% 14% --- --- --- --- 152.4 500 1% 3% 5% 8% 12% 16% --- --- 213.4 700 1% 2% 4% 6% 9% 12% 15% --- 304.9 1000 1% 2% 3% 4% 6% 8% 11% 14% Another approach to designing super-elevated curves is to determine the safe speed for negotiating a turn at a certain lateral tire force. In general, a 20% lateral coefficient of traction is safe for all but slippery conditions. Table 2-4 shows the maximum speed with various super- elevations to maintain a 20% lateral coefficient of traction. A transition “zone” may be necessary at higher speeds when entering or departing from a super-elevated turn. Table 2-4 Safe speeds (km/hr) for negotiating a curve while maintaining a lateral coefficient of traction less than 0.2 (Caterpillar 1999). Radius 0% 5% 10% (m) Flat Super-elevation Super-elevation 7.6 14 16 17 15.2 20 22 24 30.5 28 31 34 45.7 34 38 42 61.0 39 44 48 91.5 48 54 59 152.5 62 70 76 213.5 74 --- ---