Bridges are the structures that are constructed with the purpose of connecting two places separated by deep valleys or gorges or rivers and streams. Bridges are usually the cross drainage and hence a part of roads making them shorter and hence economical. For places, where the ground is uneven and undulated and where the number of rivers is large, bridges are the most economic and efficient way. It is a very convenient way.

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Objectives

• To select the possible bridge site and axis for the construction of bridge.
• To carry out surveying for topographical mapping, longitudinal and cross sections at both the upstream and downstream side of the river.

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Brief description of the site

The bridge site was surrounded by bushes. The ground was damp and swampy. The soil was soft. The hill slopes on both sides were very steep.

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Technical Specifications (Norms)

The following norms were followed while performing the bridge site survey in the field:

• Control point fixing as well as determining the length of the bridge axis had to be done by the method of triangulation. While forming triangles, proper care had to be taken such that the triangles were well conditioned, i.e., none of the angles of the triangle were greater than 120° or less than 30°.
• In triangulation, distance of Base Line must be measured in an accuracy of 1:2000.
• The triangulation angle had to be measured on two sets of readings by Theodolite and the difference between the mean angles of two sets of readings had to be within a minute.
• Carry out reciprocal levelling to transfer level from one bank to other bank of the river within a precision of ±25*sqrt(k) mm. Determine the RL of the other triangulation stations by fly levelling from the end point of bridge axis.
• Plot a topographic map indicating contour lines at suitable interval (contour interval = 1m).
• The scale for plotting the topographical map was given to be 1:500

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Equipment & Accessories

The equipment used in the survey during the preparation of topographic map in bridge site are as follows:

• Theodolite
• Levelling Machine
• Ranging Rods
• Measuring Tapes
• Leveling Staffs
• Pegs & Arrows
• Marker Pen
• Compass

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Methodology

The various methods performed during the bridge site survey were site selection, triangulation, leveling (fly levelling and reciprocal levelling), detailing by theodolite, cross section, and L-section. The brief descriptions of these methodologies are given below:

Site Selection

Site selection is the first and foremost step for the construction of bridge. Several governing factors are there for the site selection of the bridge. Geological condition, socio-economic and ecological aspect etc. guides the way of selection of bridge site. Therefore, the site was chosen such that it is laid on the very stable rocks at the bed of river as far as possible and not affect the ecological balance of the flora and fauna of the site area. The location of the bridge was selected in such a way that the heights of the roads joined by the proposed bridge were almost the same. This prevented a lot of cutting and filling to maintain a gentle gradient. The bridge site was chosen in such a way that the bridge axis was perpendicular to the flow direction and was also shorter in span so as to make the construction economical.

Topographic Survey

For the topographic survey of bridge site, triangulation was done. Triangulation is the process of measuring the angles of a chain or a network. The main purpose of the triangulation was to determine the length of the bridge axis. The triangulation also serves the control points for detailing. The bridge axis was set and horizontal control stations were fixed on either side. Distances between stations on the same sides of river i.e. base lines were measured with tape precisely. Then the interconnecting triangles were formed and angles were measured with the theodolite. The bridge axis length or span was calculated by solving the triangles using the sine rule. For vertical control, the level was transferred from the given arbitrary benchmark and RL was transferred to the stations on the next bank by reciprocal leveling while direct level transfer method was used or the same bank.

L-Section & Cross Section

For gaining an idea about bed slope, nature of the riverbed, and the variation in the elevations of the different points along the length of the river, L-section is carried out. Keeping the instrument at the control (traverse) stations on the river banks, the staff readings were taken at different points along the center line of the river up to a 150 meters upstream and 50 m downstream. Then the L-Section of the riverbed was plotted on a graph paper on scale for vertical and horizontal.

Cross-section of a river at a particular point is the profile of the lateral sides from the centerline of the river cut transverse to the L-Section at that point. The cross section can be used to calculate the volume and discharge of water at the particular section if the velocity at the cross section is known.

Levelling

a. Transferring R.L. from B.M. to control points:

The benchmark was on the rock. R.L. was transferred to the triangular station from the B.M. by fly leveling by taking the back sight-reading to the bench mark which should be within the given accuracy. The R.L. was transferred to the opposite bank of the river by reciprocal leveling.

b. Reciprocal Leveling:

This method is applicable when taping is obstructed but not the vision. For transferring the RL across the bridge reciprocal leveling was performed. This method eliminates the error due to focusing, collimation, earth’s curvature and refraction of atmosphere etc.

Taking the average of the two differences we get the difference in elevation between A and B.

Detailing

Theodolite was used for detailing of the entire bridge site. The reading was taken from the different station set up. The detailing was done with respect to the skeleton formed by triangulation. The vertices of triangles serve as a control point. The details were booked, up to 150m upstream and 50m downstream. The data and the calculations have been tabulated in a systematic way.

Comments and Conclusion

Economy and durability determine the way how a bridge is designed. The bridge axis should be designed such that the span length should be minimum and the location is safe. The bridge axis should not be below the highest flood level. The bridge span was found out to be …… meters. The cross-section was taken at the banks of river and at the middle of the river to get the profile of the flowing river.

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.

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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.

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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.

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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

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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. 

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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

Temporary adjustments of Level

The temporary adjustments for a level consist of the following:

a) Setting up the level: The operation of setting up includes fixing the instrument on the stand and levelling the instrument approximately.

b) Leveling up: Accurate leveling is done with the help of foot screws and with reference to the plate levels. The purpose of leveling is to make the vertical axis truly vertical. It is done by adjusting the screws.

c) Removal of parallax: Parallax is a condition when the image formed by the objective is not in the plane of the cross hairs. Parallax is eliminated by focusing the eye-piece for distinct vision of the cross hairs and by focusing the objective to bring the image of the object in the plane of cross hairs.

Permanent adjustments of Level

To check for the permanent adjustments of level two-peg test method should be performed. Two staffs were placed at A and B of known length (about 50 m). First the instrument was setup on the line near A and both staff readings (Top, Middle, and Bottom) were taken. Then, the instrument was setup at the middle C on the line and again both staff readings on A and B was taken. Then computation was done in order to check whether the adjustment was within the required accuracy or not.

Levelling is an art of determining relative altitudes of points on the surface of the earth or beneath the surface of the earth. It is used to find the elevation of given points with respect to a given or assumed datum and to establish points at a given elevation or at different elevations with respect to a given or assumed datum. Levelling deals with measurements in a vertical plane. Finding out elevation is necessary to enable the work and establishing point, that are necessary in the setting out of works.

• Simple Levelling:

The operation of levelling for determining the difference in elevation, if not too great, between two points visible from a single position of the level, is known as simple levelling.

• Differential Levelling:

The method of levelling for determining the difference in elevation of two points either too far or obstructed by an intervening ground, is known as differential levelling. The level is set up at number of points and the difference in elevation of successive points, is determined in this method.

• Check Levelling

After the completion of fly levelling, level lines are run to check the accuracy of the bench marks previously fixed which is called check levelling.

• Profile Levelling:

The operation of levelling carried out to determine the elevations of the points at known distances apart, and also salient features, along a given straight line is called profile levelling. It is also known as longitudinal levelling.

• Cross-section levelling

The operation of levelling which is carried out to provide levels on either side of the main line at right angles, in order to determine the vertical section of the earth surface on the ground is called cross section levelling.

• Reciprocal levelling:

When the level is not possible to be set up between two points due to an intervening obstruction as large water bodies, reciprocal levelling is carried out. The two sets of reciprocal levelling are done to find out the difference in elevation between two points accurately.

 

A contour is defined as an imaginary line passing through the points of equal elevation. Thus contour lines on a plan illustrates the configuration of the ground. The method of representing the relief of the ground by the help of contour is called contouring. The vertical distance between two consecutive contours is called contour interval. Every 5^th contour which is 5 times of the contour interval is the index contour which is generally darkened in the contour and is known as Index Contour. The least horizontal distance between two consecutive contours is called the horizontal equivalent.

Methods of contouring

There are two ways of contouring. They are namely

1. The Direct method

2. The Indirect method

1. The direct method:

In this direct method, the equal elevated points are joined. For this, firstly the points with the same elevations are found out by setting out the instrument at a point and by hit and trial method of searching the points which gives the same required staff reading.

2. The indirect method:

In this method, some suitable guide points are selected and surveyed, the guide points need not necessarily be on the contours. There are some of the indirect methods of locating the ground points:

a. By squares

b. By cross-sections

c. By tacheometric method

Contour Interpolation

The process of drawing contours proportionately between the plotted ground points or in between the plotted contours is called interpolation of the contours. Interpolation of contours between points is done assuming that the slope of ground between two points is uniform. It may be done by anyone of following methods.

• Estimation
• Arithmetic calculation
• Graphical method

Contour Characteristics

• Two contour lines do not intersect each other except in the case of overhanging cliff.
• A contour line must close onto itself not necessarily within the limits of a map.
• Contours of different elevations do not unite to form one contour except in the case of a vertical cliff.
• Two contour lines do not unite to form a single one except in the case of perpendicular cliff.
• Contours drawn closer depict a steep slope and if drawn apart, represent a gentle slope.
• Contours equally spaced depict a uniform slope. When contours are parallel, equidistant and straight, these represent an inclined plane surface.
• A set ring contours with higher values inside depict a hill whereas a set of ring contours with lower values inside depict a pond or a depression without an outlet.
• When contours cross a ridge or V-shaped valley, they form sharp V-shapes across them. Contours represent a ridge line, if the concavity of higher value contour lies towards the next lower value contour and on the other hand these represent a valley if the concavity of the lower value contour, lies toward the higher value contours.
• The same contour must appear on both the sides of a ridge or a valley.
• Contours do not have sharp turnings.