NEPAL ENGINEERING SERVICES

Sunday, May 26, 2019

PROFESSIONAL ROAD ALIGNMENT SURVEY AND GEOMETRIC DESIGN

ROAD ALIGNMENTS AND GEOMETRIC DESIGN

            An identifiable route, way or path connecting two or more places s called road. Roads are  smoothed, paved, or allow easy travel, though they need not be, and historically many roads were simply recognizable routes without any formal construction or maintenance.
            Before the construction of the road, preliminary survey is done. Road alignment is the preliminary stage of road construction. Selection of Intersection Points (IP) is the foundation of construction of the road. After that cross section, longitudinal section and formation level are required.

EQUIPMENTS & ACCESSORIES:

The equipment used in the survey-during the preparation of topography map are as follows:
1. Theodolite
2. Leveling Staffs
3. Ranging rods
4. Measuring Tapes 30m & 5m
5. Leveling instruments
6. Compass
7. Abney level
8. Pegs
9. Marker
HORIZONTAL ALIGNMENT:
Horizontal alignment is done for fixing the road direction in horizonal plane. For this, the bearing of initial line connecting two initial stations are measured using compass. Theodolite at each IP and then deflection angles are calculated.
                                Deflection angle = (360° or 180°) - observed angle
                   If +ve, the survey line deflects right (clockwise) with the prolongation of preceding line and deflects left if -ve (anti-clockwise). The radius is assume according to the deflection angle. Then the tangent length, EC, BC, apex distance along with their chainage are found by using following formulae,
                Tangent length (TL) = R x Tan (Δ/2)
                 Length of curve (LC) = 3.142 x R x Δ/180
                 Apex distance= R x 1/(Cos (A/2)-1)
                 Chainage of BC= Chainage of IP -TL
                 Chainage of MC= Chainage of BC+LC/2
                 Chainage of EC =Chainage of MC + LC/2
The BC and EC points are located along the line by measuring the tangent length from the apex and the põints were marked distinctly. The radius was chosen such that the tangent does not overlap. The apex was fixed at the length of apex distance from IP along the line bisecting the interior angle.


VERTICAL ALIGNMENT:

Vertical profile of the Road alignment is known by the vertical alignment. In the L-section of the Road alignment, vertical alignment is plotted with maximum gradient of 12 %. According to Nepal Road Standard, Gradient of the Road cannot be taken more than 12 %. In the vertical alignment, we set the vertical curve with proper design. Vertical curve may be either summit curve or valley curve. While setting the vertical alignment, it should keep in min whether cutting and filling were balanced or not.

LONGITUDINAL SECTION:

For the longitudinal section of the road the staff reading is taken at the interval of every 15m along the center-line of the road. Besides, these staff  readings at beginning of the curve, ending of the curve and apex were also taken. The RL of each point are calculated.
The profile is plotted on the graph at the horizontal scale of 1:1000 and vertical scale of 1:100; chainage of each point along the horizontal direction and RL in the vertical direction.


CROSS SECTION:

Cross section  run at right angles to the longitudinal profile on either side up to 15m distances wherever possible and the change in the slope is  directly measured using the staff intercept made by the horizontal tape i.e. the stepping method.
Horizontal scale-1:100
Vertical scale 1:100


 STRUCTURES:

The main structures provided for road constructions are retaining structures, cross drain, side-drain, bio-engineering structures etc. retaining  structures are provided where the slope is critical. Gabion structure, dry masonry structures are the example. The camber of the road is made perfectly by putting 4% of stage for gravel road so as to avoid any collection of water on it.

The maximum gradient of the road is about 9% and the minimum gradient of road is about 0% so as to facilitate the flow of drainage to specified direction. However the maximum of 12% is taken wherever not possible. Longitudinal drain is provided on the sides of the road. Retaining walls are
provided on required places.
Construction of hill roads involves many special structures. These may include wide range of structures which are used to retain soil mass, to increase stability of road embankment slopes as well as natural hill slopes, to accommodate road bed in steep slope, to penetrate deep through mountain
pass and so on. Integration of bio-engineering measures with engineering structures is yet another sector requiring special attention. The following types of structures are used normally on the hill road:
1. Retaining structures
2. Drainage structures
3. Slope protection structures

RETAINING STRUCTURES:

A retaining structure is usually a wall constructed for the purpose of  supporting or retaining a vertical or nearly vertical earth bank, which in turn may support, vertical loads along with the self-weight of it. It provides adequate stability to the road way and to the slope. Retaining walls are
constructed on the valley side on the roadway and also on the cut hillside to prevent slide towards the roadway. Types of retaining wall are:
1. Gravity walls
2. Semi gravity walls
3. Cantilever walls
4. Counter fort walls
5. Buttressed walls
6. Crib walls
7. Breast walls
8. Reinforced soil walls

 CONCLUSION:

Survey of the road alignment is done to make most economical, comfortable, and durable. Extra case is taken to avoid any soil erosion and any other ecological damage. Vertical and horizontal curves are set according to Road Design Standards for comfort and other factors.
While setting the road alignment, it should be kept in mind that the minimum IP points should be taken as far as possible and deflection angles should be minimal as far as possible. The task may  be challengeable and tough due to the route high altitude.

The format for road curve calculation is here

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