Open pit road
OPEN PIT ROAD (EN: open pit road, opencast automobile road; DE: Tagebaukraftverkehrsstraße; FR: autoroute de mine а ciel ouvert; ES: pista de mina а cieloabierto; RU: автодорога карьерная) is used for the safe movement of the automobiles with calculated speeds and loads during the transportation of the minerals and the waste rock from the stopes to the unloading points, as well as during the transportation of the utility and auxiliary goods.
According to the location of the roads on the way, they may be superficial (laid on the surface), in the capital trenches, on the fixed or sliding ramps, at the work sites, in the stopes and waste dumps.
According to the conditions of the exploitation, the roads are divided into the fixed, which are laid for the long duration of the exploitation, and temporary, which are moving with the working area.
They distinguish 3 categories of the roads, for each of which they identified the fastest speeds of the movement of the automobile transport, and other technical characteristics, the width of the roadway, the largest inclination, the least radius of the curvature of the curved parts, the type of the pavement. The main elements, defining the transverse profile of the road (Figure): the distance between the edges of the road, which is called the width of the roadbed, the roadway, the shoulders, the ditches. The dimensions of the elements of the transverse profile are determined in accordance with the type of the rolling stock, the quantity of the lanes, and the design of the water drainage structures.
The roadway is constructed with the one-sided or two-sided slope for the drainage of the water. They construct the surface with the one-sided slope on the loop and spiral ramps in the open pit, as well as on the waste dump and stope roads, and with the two-sided slope on the straight sections of the permanent roads, on the surface, and in the capital trenches. The width of the roadway for the temporary roads is 10.5-13.5 metres, and up to 14-20 metres for the permanent roads with the two-way traffic of the automobiles with the load carrying capacity of 27-75 tonnes.
The roadway of the permanent roads has the pavement, ensuring the movement with the high speeds, and protecting the road from the destruction by the actions of the rolling stock and the natural climatic factors. They construct the road surface of the permanent roads with one or several structural layers. They usually fill the lower, carrying layer, or the road foundation, using the hard rocks or the rocks with the medium hardness, which are obtained during the performance of the mining development, as well as using the loose soils.
The choice of the material for the top layer of the pavement depends on the intended duration of the exploitation of the road and on the load intensity of the traffic. The flexible gravel pavement (the size of the stones is up to 70 millimetres) has got the wide usage in the USSR and abroad, often with the processing by the binding materials on the base of the bitumina (the depth of the imbuement is down to 10-12 centimetres). The thickness of the gravel pavement reaches 30-45 centimetres for the foundation, which is using the rock of the medium hardness, and 10-15 centimetres while using the hard rock. With the increase in the load carrying capacity of automobiles, they use the cement concrete pavement for the permanent roads, which is distinguished by the high strength and longevity. With the loose soils and the rocks of the medium hardness in the foundation, the thickness of such pavement is 25-45 centimetres on the roads, which are destined for the movement of the automobiles with 27-75 tonnes of the load carrying capacity.
They use the overburden rocks for the foundations of the temporary roads. In these cases, they limit themselves during the construction of the roads with the profiling of these roads using the bulldozers and rolling them using the road rollers. The longitudinal profile of the roads is designed in the form of the smooth line, consisting of the platforms, ramps, and vertical curves, connecting the sections with different inclinations. During the choice of the calculated (so-called ruling) gradient, there are taken into account there the depth of the open pit, the intensity of the traffic, and the traction properties of the automobiles. The amount of the stripping decreases with the increase of the inclinations, but the speed of the movement decreases, as well as the passage capacity of the road, and the safety of the traffic. The ruling gradient on the road, which is destined for the modern automobile dump trucks, is limited to 70-90 ‰ in the direction of the movement of the loaded automobiles. The inclinations in the direction of the movement of the empty vehicles do not exceed 100-120 ‰ according to the safety conditions. On the long, lingering slopes, for every 400-500 metres of the slope, they include the inserts with the inclination of up to 20 ‰ and the least length of 50-60 metres. The radii are 200-500 metres for the convex vertical curves, and 150-200 metres for the concave vertical curves. They perform the tracing of the roads in the open pits on the basis of the least amount of the earth works.
The road in the horizontal (plan) projection comprises the straight sections, which are joined with the help of the arcs of 50-90 metres in length, or serpentines of 130-170 metres in length. They construct the artificial structures, like bridges, pipes, overpasses, interchanges, etc, at the intersections of the roads with the streams, ravines, or other means of communication.
One of the characteristics of the road is its throughput capacity, that is the largest quantity of the automobiles, which are passing on the given section of the road at the unit of time. The throughput of the lane of the road for the traffic in one direction is determined according to the formula,
N = 1000 • v / a • Lb, where
v is the estimated speed, kilometres per hour;
k = 1.75-2 is the coefficient of the non-uniformity of the movement;
Lb is the safety distance between the vehicles.
The value of the safety distance is composed from the distance, which is traveled by the automobile dump truck during the reaction time of the driver, the braking distance of the automobile, and its length, that is it may be represented as
Lb = v • tp / 3.6 + v 2/254 (p + a0 ± i) + Ia, where
tp = 0.6-1.0 s is the reaction time of the driver;
p = 0.15-0.2 is the coefficient of friction of the wheels with the road surface for the adverse conditions;
a0 = 0.02-0.025 is the specific main resistance to the movement;
i is the inclination at which the braking is performed (in the units of one per thousand);
Ia is the length of the automobile.
The throughput capacity of the two-lane roads is determined by the same formula for each direction. The road service (maintenance of the road and road structures, keeping them clean, etc.) is created in order to ensure the safe continuous movement of the automobile transport in the environments of the open pits. They build the underground roads in the mines in order to decrease the operating costs, increase the productivity of the self-propelled machines, etc. The main types of the pavement, which are widely used at the domestic and foreign mines, are the concrete, asphalt, and gravel filling. The promising direction for the mechanization of the underground construction and for the improvement of the roads is the machining of the hard rocky soil of the mine workings and the usage of it as the roadway.
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