CHAPTER4 ROAD ALIGNMENT

4.1 INTRODUCTION

Roads are specially prepared to provide ways between different places for the use of the vehicles, people and the animals. Roads are used in countries like Nepal, where there are less chances of airways and almost negligible chances of the transportation system. Roadalignmentboth the tasks– to runa road between twopoints, i.e. tofix or chose appropriate road alignment between two points which are far apart and to carry out the survey for the safe, economical and appropriate construction along the route. This specific job is essential for an engineer combating with the mountainous topographyof Nepal.

This part of the Surveycamp deals with the roadalignment survey done at Kakani.

4.2 OBJECTIVES

Road alignment survey was done toaccomplish the following objectives:

4.3 BRIEF DESCRIPTION OF THE AREA

The site is surrounded by steep hills, which is covered with densely planted shrubs. There are several rise and falls along the route needinglots ofcuttingandfilling.

4.4 NORMS (TECHNICAL SPECIFICATIONS)

Reconnaissance was carried out for alignment of the road corridor considering permissible gradient, obligatory points, bridge site and the geometry of tentative horizontal and vertical curves.

The topographic map of the scale of 1:1000 of the road corridor was prepared. Geometriccurves, roadformationwidth, rightof way, crossingsandotherdetails were shown in the map.

While performingthe road alignment survey, followingnorms were strictlyfollowed:

Note: If the external deflection angle at the I.P. of the road is less than 3°, curves need not to be fitted.

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6.L-section of the road had to be prepared on a scale of 1:1000 horizontally and 1:100 vertically.

7.The cross-section of the road had to be prepared in a scale of 1:100(both horizontal and vertical).

8.The amount of the cutting and filling required for the road construction had to be determined from the L-section and the cross-sections. However, the volume of cuttinghad to be roughly equal to the volume of filling.

Theequipmentusedinthesurveyingduringthe preparationofthetopographicmapare as follows:

For the selectionof the alignment for hill roads following points are considered.

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7.Hair-pin bends should be avoided as far as possible. But when they become unavoidable, they should locate in the gentle and slopes. Gentle slopes reduce the costof retainingwalls.

Considering all above points alignments should be selected for the hill road. In every change in horizontal direction in horizontal control and every change in gradient in verticalcontrol,intersectionpoint(IP)shouldbefixedalongthealignmentbypegging.

4.6.1 Reconnaissance

The reconnaissance survey was carried along the route from the starting point and the planning of works was made for the intersection points (IP) where the direction of the route has to be changed. After having complete reconnaissance, the (IP) stations were fixed. For this, the inter-visibility of the stations was kept in mind and the gradient or simply the slope between two IPs was maintained such that it didn’t exceed 12 % accordingtoNRS.Atthesametime,pegswiththeIPnumberweredrivenatthesuitable position on the ground with references.

4.6.2 Measurement of Lengths and Deflection Angles

The distances between the IPs were measured with the help of tape. Both forward as well as backward measurement of the distances was carried out. The accuracy for the linear measurement of the distance betweenIPs is 1:2000.

Onesetofhorizontalanglewasmeasuredforthedeflectionangle.Thefaceleftreading wasobservedandthedeflectionangleswerecalculated.Asthetraverseformedasopen traverse, no angular correction could be made. So as far as possible, both the linear measurements as well as the angular measurements were observed carefully and precisely.

4.6.3 Horizontal Alignment

Thecentrelineoftheroadforfixingthedirectionoftheproposedroadinthehorizontal planeisknownashorizontalalignment.Forfixinghorizontalalignment,thebearingof the initial line connecting two initial IPs was measured using compass. The interior angle was measured with the help of Theodolite at each IP and the deflection angles were computed.

Deflectionangle (Δ) = (360° or180°) – Observed angle.

4.6.4 Curve Setting

Curves are generally used on highways and railways where it is necessary to change the direction of the motion of the vehicle. A curve may be circular, parabolic or spiral and isalways tangential to the two straight directions commonly known astangents.

Curves whichare generally usedon highways areas follows:

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4.6.4.1 Simple Circular Curve

A simple circular curve is the one which consists of a single arc of a circle. It is tangential to both of the straight lines namely tangents. During the road survey, it is always kept in mind thatthe radius of the simple circular curve should notbe less than 11m. As far as possible, flat circular curves are preferred to that of the sharp one. Flat curvesarecomfortabletothepassengersandthereislesspossibilityofaccident.Before settingoutthecurve,itselementsareessentialtobecomputed.Someessentialelements of simple circular curve are asfollows:

Length of tangent:R tan (Δ/2)

Where,R = radius of simple circular curve = deflectionangle

Length of long chord: 2R sin(Δ/2)

Apex distance: R (sec(Δ/2) – 1)

Mid ordinate: R (1 – cos(Δ/2))

Length of curve: πRΔ 180°

Chainage of T1:Chainage of IP – Rtan (Δ/2)

Chainage of T2:Chainage of T1 + πRΔ

180°

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Figure 8Simple Circular Curve

SettingOut of Simple Circular Curves

A simple circular curve can be set in the field by various linear and angular methods which are listed as follows:

By ordinates from the longchord

By perpendicular offsetfrom tangents

By radial offsetfrom tangents

By offset from the chords produced

By successive bisection of the curves

Rankine’s method of tangentialangles

Two Theodolite method

Tachometric method

4.6.4.2 Transition Curve

A transition curve is a curve of varying radius introduced between a straight and a circular curve, or between two branches of a compound curve or reverse curve. The functions of a transitioncurve are asfollows:

To accomplish gradually the transition from the tangent to the circular curve, so that thecurve is increased gradually from zero to a specified value.

To provide a medium for the gradual introduction or change of the required super-elevation.

A transition curve introduced between the tangent and the circular curve should fulfill the followingconditions:

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(5)The rate of increase ofcurvature alongthe transition should be the same asthatof increase of cantor super-elevation.

(6)Itslengthshouldbesuchthatfullcantorsuper-elevationisattainedatthejunction with the circular curve.

Super-elevation

When a pavement or a track is sloped upwards the outside of the curve, it is termed as banked or super elevated. Thus, ‘super-elevation or cant’ is the amount by which the outerend of the road or outer rail is raised above the inner one.

The length of transition curve should be such that the required super-elevation or cant is provided at a suitable rate. Thereare three methods for determining its length:

Elements of Transition Curves

Length of tangent: (R+S) tanΔ/2 + L/2

Where,R = Radius of simplecircularcurve joining transitioncurve S= Shift

L = Length of the transition curve

Shift (S): L2/24R

Spiral Angle (Δs): 180L/2ΠR

Central Circular Angle (Δc): (Δ-2Δs)

Length of the circularcurve: ΠR(Δ -2Δs)/180°

Length of the combinedcurve: ΠR (Δ-2Δs)/180° + 2L

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4.6.4.3 Vertical Curve

A vertical curve is used to join two intersecting grade lines of railroads, highways or other routes to smooth out the changes in the vertical motion. An abrupt change in the rate of the grade could otherwise subject a vehicle passing over it to an impact that would be either injurious or dangerous. The vertical curve, thus, contributes to the safety, comfort andappearance.

A grade which is expressed as percentage or 1 vertical is to n horizontal, is said to be upgrade orpositivegradewhentheelevationalongtheroadalignmentincreases,while it is termed as downward grade or negative grade when the elevation decreases along the directionof the motion.

4.6.5 Levelling

Themethodof differentiallevelingwasappliedintransferringthelevelfromthe given B.M. to all the I.P.s as well as other components of the curve. Along with the transfer oftheleveltothechainageattheintervalof20mandthecomponentsofthecurvelevel was also transferredto thecross-section upto the distance of 10m on either side of the chainage and thecomponentsof thecurve.

4.6.5.1 Profile Levelling (Longitudinal Sectioning)

Profile leveling is the process of determining the elevations of the points at the short measured intervals along a fix line or alignment such as the center line of the railway, highway, canal or sewer. The fixed line may be a single straight line or it may be composed of a succession of straight lines or of a series of straight lines connected by curves. It is also known as longitudinal sectioning. By means of such sections the engineer is able to study the relationship between the existing ground surface and the levels of the proposed construction in the direction of its length. The profile is usually plotted on specially prepared profile paper, on which the vertical scale is much larger thanthe horizontal, of costs aremade.

Profile leveling, like differential leveling, requires the establishment of turning points on which both back and fore sights is taken. In addition, any number of intermediate sights may be taken on the points along the line from each set up of the instrument. It isgenerallybesttosetuptheinstrumenttoonesideoftheprofilelinetoavoidtooshort sights on the points near the instrument. For each set up, intermediate sights should be taken after the fore sight on the next turning point has been taken. The position of the intermediate points on the profile is simultaneously located by chaining along the profile andnoting their distances from the point ofcommencement.

For the longitudinal section of the road, the staff reading was taken at the interval of every 20m along the center line of the road. Beside this, staff readings at beginning of the curve, ending of the curve and the apex of the curve were also taken. The R.L. of each point was calculated. The profile was plotted on the graph paper atthe horizontal scale of 1:1000 and the vertical scale of 1:100; chainage of each point along the horizontal directionandR.L. inthe verticaldirection.

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4.6.5.2 Cross Sectioning

Cross-sections are run at right angles to the longitudinal profile and on the either side of it for the purpose of lateral outline of the ground surface. They provide the data for estimating quantities of earth work and for other purposes. The cross-sections are numbered consecutively from the commencement of the center line and are set out at right angles tothe main line of sectionwiththechain and tape.Cross- sections maybe taken ateachchain. The length of cross-sectiondepends upon the nature ofthe work.

The longitudinal and cross-sections may be worked together or separately as per the requirement.

Cross-section was plotted on the graph paper both the horizontal as well as vertical scale of 1:100.

4.6.6 Tachometry

Tachometry is the branch of angular surveying in which the horizontal and vertical distancesofthepointsaredeterminedorobtainedbyopticalmeans.Themethodisvery rapid,convenient.The primaryobjectoftachometryisthepreparationofthecontoured maps or plans requiring both horizontal as well as vertical control. In tachometry, tachometer is used. A tachometer is an ordinary transit Theodolite fitted with a stadia diaphragm. The stadia diaphragm essentially consists of one stadia hair above and below atequaldistance of the horizontalcrosshair.

Thetachometricprocessisappliedtodeterminetheelevationofthepointsatthecross- section. As for cross-section, horizontal control is not needed to define as it is in right angle to the road alignment; only vertical control is to defined or determined which is enabled with the aid of tachometric surveying of cross section points. Fly leveling is carried out todefine the elevation of the IPs.

4.6.7 Structures

The main structures provided for the road construction are retaining structures, cross drains, side drains, bio-engineering structures, etc. Retaining structures are provided where slope is critical. Gabion structure, dry masonry structures are the example. The cross drainage is provided at the interval of 150 to 200m of road mostly at the valley and wherever necessary. Causeways, culverts, and bridges are the example of cross drainage. The side drain is the channel by which the pavement can be protected from the surface water. It is usuallyconstructed along the roadjust below the cut slope. The collectedwaterisdrainedoffbythemeansofcrossdrainage.Thetrapezoidalsidedrain was proposed for this project, cross section of which is 30cm base width, 50cm top width and 30cmdepth.

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4.7 COMMENTS AND CONCLUSION

In spite of the different kinds of obstacles in the field, our group was successful in completing the fieldwork as well as the office work at time. In the field we had spent quite some time in discussing the route of the road and in designing the curves, which led to good results. All the group members were quite conscious and tried our best to get error free data and calculations. The road had to be designed on a sloping ground, so our group members felt the restrictions during the cutting and filling and for the construction of different retaining structures. Moreover, after performing the road alignment survey, we were able to build confidence in designing roads at difficult terrain taking into consideration the factors like economy, convenience and its use.