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The position or the layout of the centre line of the highway on the ground is called the alignment.
It includes, Horizontal alignment: - straight path, horizontal deviation and curves.
Vertical alignment: - changes in gradient and vertical curves
Requisites of ideal Alignment
An ideal alignment between two station should offer maximum utility by serving maximum population and products and also should possess following requirements:
i) Short: it is desirable to have a short alignment between two stations. A straight path between the two terminals would provide this.
ii) Easy: it is should be easy to construct and maintain the road with minimum problems and also the alignment should be easy for vehicle to operate with easy gradient and curves.
iii) Safe: it should be safe enough for construction and maintenance from the view point of stability of natural hill slopes, embankment and cut slopes. It should be safe for the traffic operation with safe geometric features.
iv) Economical: The road alignment could be considered economical only if the total cost including initial cost, maintenance cost and VOC is lowest.
Factors Controlling Alignment
The various factors which control the highway alignment are:
i) Obligatory points
iii) Geometric design
v) Other considerations
In hill roads additional care has to be given for:
iii) Geometric standards of hill roads
iv) Resisting length.
A) Obligatory Points: there are control points governing the alignment of the highways. These control points may be divided broadly into two categories.
i) Points through which the alignment is to pass: This may cause the alignment to often deviate from the shortest or easiest path. The various examples of this category may be bridge site, intermediate town, a mountain pass or quarry.
Case 1: When it is necessary to cross hill range, mountains the various alternatives are to cut a tunnel or to go around the hill. This suitability depends on many factor like site condition, topography, cost consideration etc. The figure shows how the alignment AB deviated to ACB.
Case 2: due to the position of the bridge the alignment deviated from CD to CFD. Because the bridge are constructed at narrow gap and where the intensity of water force is low.
Case 3: when connecting a road network between two station. It always beneficial to provide a link road or to connect the proposed alignment through near by station
ii) Points through which the alignment should not pass: also make it necessary to deviate from the proposed shortest alignment. The obligatory points which includes religious places( temples church, mosque and grave), very costly structure, unsuitable land ( lakes, ponds, marshy soil)
B) Traffic: the alignment should suit traffic requirements. Origin and destination study should be carried out in the area and the desire lines be drawn showing the traffic flow. The new alignment should keep in view of desired lines, flow patterns and future trends.
C) Geometric design: geometric design factors such as gradient, radius of curve and sight distance, overtaking sight distance, ruling gradient on hilly region also would govern the final alignment of the highway.
D) Economics: the alignment finalised based on the above factors should be economical compared to other alignment. The safety, saving and returns should be more compared to investment.
It is based on the initial cost of construction and maintenance cost of the road, if it a shortest path the cost of construction will be reduced.(Decision is based on Quantity of Cutting and Filling of Earth.)
E) Other consideration: factors like drainage consideration, hydrological factors, water table, seepage flow, high flood level, political considerations and monotony also affect in deciding the alignment.
The stages of Engineering surveys for Highway locations:
i) Map study
iii) Preliminary surveys
iv) Final location and detailed surveys
A) Map study: By careful study of topographical map, it is possible to have an idea of several possible alternate routes so that further details of these may be studied later at the site. The features like river, hills valleys, and counter intervals can be observed. By knowing these feature it can fairly assign the alignment avoiding valleys, lakes and possible location of bridge (avoiding sharp turns etc.). It is also possible to suggest permissible gradient considering counter intervals.
B) Reconnaissance: it is to examine the general character of the area for deciding the most feasible routes for detailed studies. A field survey party may inspect a fairly broad stretch of land along the proposed alternative routes of the map in the field. Only few simple instruments like abney level tangent clinometers, barometer or GPS are used by the reconnaissance party to collect additional details rapidly.
Some of the following details are collected;
i) Valleys, ponds, lakes, marshy land, hills, permanent structures and other obstruction along the route which are not available in the map.
ii) Approximate values of gradient, length of gradient and radius of curves of alternate alignments.
iii) Number and type of cross drainage structures, maximum flood level and natural ground water level along the probable routes.
iv) Sources of construction materials, water and location of stone quarries v) When the alignment passes through hill, additional details like type of rocks, dip of strata, seepage flow.
C) Preliminary Survey: this survey can be done either by Conventional approach or aerial survey if the area is more. The main objectives of preliminary survey are:
i) To survey the various alternate alignments proposed after the reconnaissance and to collect all the necessary physical information and details of topography, drainage and soil.
ii) To compare the different proposals in view of the requirements of a good alignment.
iii) To estimate quantity of earth work materials and other construction aspects and to work out the cost of alternate proposals.
iv) To finalise the best alignment from all consideration.
D) Final Location and Detailed survey:
i) The alignment finalized at the design office after the preliminary survey is to be located on the field by establishing the centre line.
ii) The centre line of the road finalized is to be translated on the ground during the location survey
iii) The centre line stakes are driven at suitable intervals say 50 m in plain and rolling terrains and at 20 m in hilly terrain.
iv) Temporary bench marks are fixed at intervals of about 250 m and at all drainage and under pass structures
v) The levels are taken along longitudinal section and cross section at very 50 – 100 m intervals. The cross section should taken at curves and where there is a gradient change.
vi) The data collected during the detailed survey should be elaborate and complete for preparation of detailed plans, design and estimate of the project.
Drawings and Reports
The following drawings are usually prepared in a highway projects
i) Key map : Proposed and existing roads, important places
ii) Index Map : topography, size being 32 x 20 cm
iii) Preliminary survey plans : alternate alignment, other information
iv) Detailed plan : alignment, boundaries, counter intervals, A2 size sheet
v) longitudinal section: Datum line, existing ground, vertical profile, scale 1 H: 10 V
vi) Detailed cross section: level at every 100 m interval
vii) Land acquisition plans; details of buildings, well, other details
viii) Drawing of cross drainage and other retaining structures: scale 1:1, structural details
ix) Drawings of road intersections: Intersection details and traffic flows
x) Land plans showing quarries etc.
Project Reports .
The project report forms an important part of the project document. It should contains information such as
i) General details of the project and its importance
ii) Feature of the road including selection of the route, alignment, traffic etc.
iii) Road design and specifications
iv) Drainage facilities and cross drainage structures
v) Materials, labour and equipments
vii) Construction programming
viii) Other miscellaneous items like diversion of traffic, road side amenities, rest houses etc.
Steps in New Highway Project
i) Map Study:
ii) Reconnaissance survey:
iii) Preliminary survey
iv) Location of final alignment
v) Detailed survey
vi) Material survey
ix) Pavement construction: preparation of sub grade, Construction of sub base, surface courses.
x) Construction control: quality check during construction and test on pavement like unevenness, camber, super elevation, extra widening.
Necessity of Re - Alignment:
i) Improvement of horizontal alignment design elements, such as radius, superelevation, transition curve, clearance on inner side of the curve of shifting the curve to provide adequate sight distance, elimination of reverse curve and undesirable zigzag.
ii) Improvement of vertical alignment design elements like steep gradients, changes in summit curves to increase sight distance, correction of undesirable undulations like humps.
iii) Raising the level of a portion of a road which is subjected to flooding, sub mergence or water-logging during monsoons..
iv) Re-construction of weak and narrow bridges and culverts and changes in water way at locations slightly away from the existing site.
v) Construction of over bridges or under bridges at suitable locations across a railway line in place of level crossing or across another road to provide grade separated intersection. vi) Re-alignment required due to a portion of road being submerged under water at the reservoir area on account of construction of a new dam.
vii) Construction of a bypass to avoid the road running through a town or city
viii) Defence requirement.
Steps in Re - Alignment
i) Reconnaissance of the stretch of road to be re-aligned, study of the deficiency and possible changes in alignment.
ii) Survey of existing road recording the topographic features and all other existing features including drainage conditions along a strip of land on either side of the road.
iii) Observation of spot level along the centre line of the road and cross section levels at suitable intervals.
iv) Soil survey along the stretches of land through which the re-aligned road may possibly pass.
v) Comparison of economics and feasibility of alternate proposal of realignment.
vi) Finalization of the design features and realigned road stretch
vii) Preparation of drawings.
viii) Marking out centre line
ix) Earthwork and preparation of sub grade
x) Checking of geometric design elements
xi) Design and construction of new pavemen