SETTING OUT OF THE HORIZONTAL CURVES. The process of route survey comprises of three stages;
The reconnaissance survey is the practical visit to the construction site in order to acquire the physical knowledge of the nature of the area as a whole. Station points are also selected and indivisibility ensured. The survey method is strictly determined by the topography of area. The preliminary survey is the initial survey carried our in order to determine the actual topography and details of the area concerned. At the end of this exercise, the physical undulation and existing features on the ground are provided on the working plan. This information helps the designer to plan and determine the course of the route, taking into consideration, the purpose, safety and economy. The final survey consists of the actual setting out exercise on the ground to locate the course of the route including the course of designed curves.
Example 1: The centre lines of two straights are projected forward to meet at I and the deflection angle is measured to be 300. If the straights are to be connected by a simple circular curve of radius 200m, tabulate all the setting that data taking 20m chords on a through chainage basis. The chainage of I is determined to be 2259.5m.
Solution: Data: Chainage at I = 2259.59m
Radius curve = 200m
Deflection angle = 30^0
Standard chord length = 20m
Tangent length, T1,I = R tan Ø/2 = 200 tan 30/2 = 200 tan 150 = 53.59m Since the chainage of I is known, we can now determine the drainage at T1 Chainage at T1 = chainage I – tangent length = 2259.59 – 53.59
Length of arc= R Ø (radius) = 200 x 300(radians) = 200 x 5.2359878 x 10-1 = 104.72m For a standard chord length of 20.00m, the first sub chord is 14.00m. The second, third, fourth and fifth chords (standard) = 20m each, total = 80.00m The final sub chord = 10.72m Check: 14.00 + 80.00 + 10.72 = 104.72m (ok) Deflection angle, δmm = chord length x 180 x 60 2pR = 1718.9 x chord length/R Hence for the first sub chord, Deflection angle δ mm= 1718.9 x 14.00/200 = 120.30min = 20 00’ 19’’ For standard chord, δ mm = 1718.9 x 20.00/200 = 171.90min = 20 51’ 53’’ For final sub chord δ mm = 1718.9 x 10.72/200 = 92.00min= 10 32’ 08’’ Sum of deflection angles = 140 59’ 59’’ The setting out table is presented as follows;
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