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Selection of least cost paths for extraction of Forest Produce using Remote Sensing and GIS - A case study of Andhra Pradesh Forest Dept. India
Road alignment in the Hills
The main considerations for hillroad alignments are 1) an extraction road should follow the valleys and should be as near as possible to the lower boundary of the areas being worked. This will facilitate the carriage of timber or other forest produce down to the road head. 2) A main communication road, should as far as possible, follow the ridges. 3) The work of road alignment may be started from the higher obligatory points to the lower obligatory points on either side. 4) Unnecessary ascents and descents are avoided. Every meter of elevation gained is maintained. 5) The slopes may be within the limits of the ruling gradient unless it is absolutely unavoidable. 6) Heavy rock cuts and locations likely to be affected by landslides are avoided. The cost of road construction may also be reduced by detours to avoid deep cuttings and high embankments. 7) Wherever possible, hairpin bends and zigzags are avoided as these are both difficult to construct and maintain. 8) Halting places are provided at regular intervals in long continuously rising roads. 9) It is preferable to align roads on forested slopes than on bare mountain sides; as such alignments are stable and make the roads less costly to maintain. (Krishnasharma.H;1992,&.Negi. SS 1994)
Gradient and its importance
Gradient refers to the longitudinal slope of the center of the roadway. It is usually expressed in terms of the horizontal distance in which the road rises or falls by one unit. It is the degree of elevation or depression of the road surface with reference to the horizontal plane. The terrain is classified as steep terrain, Mountainous terrain, Rolling terrain and level terrain. Steep terrain is a terrain with cross slope greater than 60 per cent. Mountainous terrain is a terrain with cross slope from 25 to 60 percent. Rolling terrain is a terrain with a cross slope varying from 10 to 25%. Level terrain is a terrain with cross slope less than 10 percent. A truck that is capable of handling a load of 3 ton on the level stretch of an earth road can haul on the same gear a load of only 2 tons up a gradient of 1 in 20 and only 1 ton up a gradient of 1 in 10. The effect of gradient is even more on the hard surface of a metalled road. Only half the load can be hauled up on a 1 in 20 gradient as compared with a load on the level surface. The disadvantages of the steep gradient are a) wastage of power b) increased erosion. c) considerable wear and tear of the vehicle. d) more fuel consumption. The different kinds of highway gradients are 1) ruling gradient, 2) limiting gradient, 3) exceptional gradient, 4) minimum gradient (Krishnasharma.H;1992,&.Negi. SS 1994).
Ruling Gradient
It is the desirable upper limit of a gradient. The designer should aim to provide a gradient within the ruling gradient. Mode of transport in the locality will govern the ruling gradients to be adopted. Indian Road Congress (I.R.C.) recommended the limits for plains and hills are 1 in 30(3.3%) and 1 in 20(5%) respectively.
Limiting Gradient
It is the limit of steepest (or maximum) gradient. It is allowed in difficult terrain to reduce earth work. Limiting gradients when used should be separated by flat and easy gradients. I.R.C. suggested the limits for plains and hills are 1 in 20(5%) and 1 in 15(6.66%).
Exceptional Gradients
These are gradients steeper than the limiting gradients which are provided in exceptional cases such as approaches to causeways, near hairpin bends, very difficult terrains to avoid deep cutting. They should be used only in very difficult situation where they cannot be avoided. I.R.C. recommended the exceptional gradients for plains and hills are 1 in 15(6.7%) and 1 in 12(8.3%) respectively. Exceptional gradients should not be adopted for distances greater than 100 m in a stretch of 1.6 km. They should be separated by easy and flat gradients. I.R.C. has recommended gradients according to different terrains.
Minimum Gradient
It is the gradient given to drain away the water longitudinally where there is no sufficient camber. If the side drains are lined it may be taken as 0.5% and if not lined it is taken as 1%.
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