Road Alignment Survey
A road is an identifiable route, way or path between two or more places. Roads are typically smoothed, paved or otherwise prepared to allow easy travel; though they need not be, and historically many roads were simply recognizable routes without any formal construction or maintenance. The road need to pass through positive obligatory points. Positive obligatory points include cities, schools, markets and negative obligatory points include temples, national parks and wild life conservation areas. Road must not pass through such negative obligatory points.
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.
Geology Hydrology & Soil
The land was undulated with no large boulders or rocks of any kind along the proposed site. There are several places where culvert or causeway can exist. The soil is uniform throughout the whole length of the road. Although the road alignment has certain up and downs. Finally, the starting and ending point of the road has significant level differences. Gravel and sandy soil were found along the road course.
Technical Specifications (Norms)
Reece alignment selection was carried out of the road corridor considering permissible gradient, obligatory points and geometry of tentative horizontal and vertical curves. The road setting horizontal curve, cross sectional detail in 5m interval and longitudinal profile were prepared. Geometric curves, road formation width, right of way, crossings and other details were shown in the map.
While performing the road alignment survey, the following norms were strictly followed:
- Carry out reconnaissance survey and alignment selection of a road corridor about 500m or more.
- If the external deflection angle at the I.P. of the road is less than 3°, curves need not be fitted.
- Simple horizontal curves had to be laid out where the road changed its direction, determining and pegging three points on the curve – the beginning of the curve, the middle point of the curve and the end of the curve along the centerline of the road.
- The radius of the curve had to be chosen such that it was convenient and safe. The radius of the curve should not be less than 12m. The radius must be within the multiple of 5 or 10.
- The gradient of the road had to be maintained below 12 %.
- Subsequent reverse curves in road alignment should be avoided.
- The deflection angle should not be greater than 90 degree.
- Two successive curves must not be overlapped.
- Carry out detailing survey for longitudinal section along the center line at 15m interval, at abrupt change point and at all the curve point BC, MC and EC.
- Cross sections had to be taken at 5 m intervals and at the beginning, middle and end of the curve, along the centerline of the road – observations being taken for at least 7.5 m on either side of the centerline.
- The amount of cutting and filling required for the road construction had to be determined from the L-Section and the cross sections. However, the volume of cutting had to be roughly equal to the volume of filling.
Equipment & Accessories:
The following are the instruments used during the road alignment survey in the field:
- Theodolite
- Tripod Stand
- Leveling Staffs
- Ranging Rods
- Measuring Tape
- Pegs and Arrows
- Marker
- Pen
Methodology
The alignment of road includes several ways and procedures that need to be carried out. Following are the listed methodology.
Reconnaissance
The reconnaissance survey was performed along the given route. Tentative estimation was done for the intersection points, where the direction had to be changed. While returning back the route, the IPs was fixed. For this the inter-visibility of the stations was checked and gradient between the two IPs was adjusted such that it does not exceed 12%, using the theodolite. Meanwhile the pegs with IP no. were driven at these points.
Horizontal Alignment
Horizontal alignment is done for fixing the road direction in horizontal plane. For this, the bearing of initial line connecting two initial stations was measured using compass. The interior angles were observed using theodolite at each IP and then deflection angles were calculated.
If the deflection angle is positive the deflection is towards right and if the deflection angle is negative the deflection is towards the left. The radius was assumed according to the deflection angle. Then the tangent length, Beginning of the Curve (BC), End of the Curve (EC), apex distance along with their chainage were found by using the following formulae,
The BC and EC points were located along the line by measuring the tangent length from the apex
and the points 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. To locate MC, for right hand deflection rotate theodolite by 270 + Delta/2 and for left hand deflection rotate by 90 – Delta/2 .Then, by ranging measure the apex distance and locate the MC.
Vertical Alignment
Vertical profile of the Road alignment is known by the vertical alignment. In the L-section of the Road alignment, vertical alignment was 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 mind whether cutting and filling were balanced or not. But in our road alignment survey, we didn’t design the vertical curve.
L-section & Cross Section
Nature of the ground, the variation in the elevations of the different points along the length of road need to be known for the construction of the road. For this L-Section of the road is required. In order to obtain the data for L-Section, staff readings were taken at points at 15m intervals along the centerline of the road. Thus after performing the necessary calculations, the X, Y and Z coordinates were transferred to all those points. Then, finally the L-Section of the road was plotted. The staff readings at BC, EC and apex were also taken.
Cross sections at different points are drawn perpendicular to the longitudinal section of the road on either side of its centerline in order to present the lateral outline of the ground. Cross sections are also equally useful in determining the amount of cut and fill required for the road construction. Cross sections were taken at 5m intervals along the centerline of the road and at points where there was a sharp change in the elevation. While doing so, the horizontal angles, vertical angles and staff readings were taken. Then, cross sections at different sections were plotted.
Curve Setting
A regular curved path followed by highway or railway alignment is curve. It is introduced wherever it is necessary to change the direction of motion due to the nature of terrain. A curve may be circular, parabola or spiral and is always tangential to two straight directions.
There may be different types of curves:
Simple curve, Compound curve, Reverse curve, Transition curve.
Simple Circular Curve
A simple circular curve is the curve, which consists of a single arc of a circle. It is tangential to both the straight lines.
Setting out of Simple Circular Curves
1. Linear method
In this method, only a chain or tape is used. Linear methods are used when a high degree of accuracy is not required and the curve is short.
Linear Methods of Setting out of curves are,
- Offsets from Long Chord
- Offsets form Tangents
- Successive bisection of Chords
- Offsets from Chords produce
2. Angular method
In this method, an instrument like theodolite is used with or without chain or tape.
Angular Methods of setting out of curves are,
- Rankine’s Method of Tangential Deflection Angles
- Two Theodolite Method