RET 565: CONSTRUCTION TECHNOLOGY & INFRASTRUCTURE “ WHAT ARE THE FACTORS CONTROLLING ALIGNMENT OF ROAD?” GROUPS MEMBERS : HAFIZATUN NADIAH BT MOHD NOOR JUNAIDAH BT JAILANI ZUHAILA BT ZAKARIA FARHANA BT SAFAR KHAIRUL ANUAR BT MA’AROF
Road Alignment Problem
Geometric design of road
The elements of design are influenced by a wide variety of design controls, engineering criteria and project specific objectives. Such factors include the following:
- functional classification of the roadway
-projected traffic volume and composition
-required design speed
-topography of the surrounding land
-human sensory capacities of roadway users
-vehicle size and performance characteristics
-traffic safety considerations
Design controls and criteria
Most desirable and economical roadway is one that follows the existing natural alignment of the countryside.
the alignment should be consistent and uniform:
- to help reduce problems related to driver expectancy.
- where both vertical and horizontal curvature are configured to satisfy limiting design criteria.
Sudden changes in alignment should be connected with:
- long sweeping curves
- short sharp curves
both, should not be interspersed with long curves of small curvatures.
The optimal alignments achieved when the grade and curvature are balanced.
Terrain has considerable influence on the final choice of alignment. The topography of the surrounding area can be classify as:
1.level country – the alignment is limited by considerations other than grade such as land use ,bridging, existing cross roads etc.
2.rolling country – grade and curvature must be carefully considered. Depth of cut and height of fill, nos of bridges depending on whether the routes follows the ridges, valleys or a cross drainage alignment.
3.mountainous country- horizontal alignment /curvature is controlled by maximum grade criteria.
Referred as geometric alignment
Is developed to accommodate a given design speed and typically the alignment dimensions and distances are tabulated that facilitates construction staking by the surveying crew.
The design and construction of a roadway is developed and dimensioned to a calculated centerline in terms of:
Tangent /straight sections of roadway are connected with circular curves to create a flowing and smooth alignment.
Most high speed roadway have long tangent distances connected by very gradual long sweeping curves.
A circular curve connecting two straight /tangent s Forces acting on a vehicle when travelling round a circular curve
Consists of a series of straight profile lines connected by vertical parabolic curves which known as profile grade line.
Usually ,the designer studies the effect of change on the centerline profile of the roadway to analyze grade and grade controls.
Ideal grades have long distances between points of intersection, with long curves between grade tangents to provide smooth riding qualities and good visibility.
The grade should follow the general terrain and rise or fall in the direction of the existing drainage.
It is necessary to maintain higher grades in rock cuts and in flat ,low-lying or swampy areas with respect to the existing ground line.
Future possible construction and the presence of grade separations or bridge structures can also act as control criteria for the design of a vertical alignment.
Parabolic vertical curve Types of crest and sag vertical curves
Safe roadway is designed to :
- give drivers a sufficient distance of clear vision ahead
- avoid hitting unexpected obstacles
- can pass slower vehicles without danger
Sight distance is the length of roadway visible ahead to the driver of a vehicle.
Stopping sight distance is provide when the distance is not long enough to permit passing of a slower vehicle. It is based on the sum of two distances:
The distances traveled from the time the object is sighted to the instant the brakes are applied
The distance required for stopping the vehicle after the brakes are applied.
Passing sight distance is sight distance which is long enough to enable a vehicle to overtake and pass another vehicle on a two lane roadways without interference from opposing traffic.
Stopping sight distance
Factors that control the alignment of road
FACTORS THAT CONTROL THE ALIGNMENT
The various factors that control the alignment are as follows:
These are the control points governing the road alignment. These points are classified into two categories. Points through which it should pass and points through which it should not pass. Some of the examples are:
bridge site: The bridge can be located only where the river has straight and permanent path and also where the abutment and pier can be strongly founded. The road approach to the bridge should not be curved and skew crossing should be avoided as possible. Thus to locate a bridge the highway alignment may be changed.
mountain: While the alignment passes through a mountain, the various alternatives are to either construct a tunnel or to go round the hills. The suitability of the alternative depends on factors like topography, site conditions and construction and operation cost.
intermediate town: The alignment may be slightly deviated to connect an intermediate town or village nearby.
The alignment should suit the traffic requirements. Based on the origin-destination data of the area, the desire lines should be drawn. The new alignment should be drawn keeping in view the desire lines, traffic flow pattern etc.
Geometric design factors such as gradient, radius of curve, sight distance etc. also govern the alignment of the highway. To keep the radius of curve minimum, it may be required to change the alignment. The alignments should be finalized such that the obstructions to visibility do not restrict the minimum requirements of sight distance. The design standards vary with the class of road and the terrain and accordingly the highway should be aligned.
FACTORS THAT CONTROL THE ALIGNMENT
The alignment finalised should be economical. All the three costs i.e. construction, maintenance, and operating cost should be minimum. The construction cost can be decreased much if it is possible to maintain a balance between cutting and filling. Also try to avoid very high embankments and very deep cuttings as the construction cost will be very higher in these cases.
Other considerations :
various other factors that govern the alignment are drainage considerations, political factors and monotony.
Political: If a foreign territory comes across a straight alignment, we will have to deviate the alignment around the foreign land.
Monotony: For a at terrain it is possible to provide a straight alignment, but it will be monotonous for driving. Hence a slight bend may be provided after a few kilometers of straight road to keep the driver alert by breaking the monotony.