Highway Alignment

 Highway Alignment

Highway alignment is the position or layout on the ground along the center line of the highway. Horizontal alignment includes straights and curves whereas change in grade and     vertical curves are covered in vertical alignment.

Basic requirement of Highway alignment

Following are basic requirement of an ideal highway alignment:

•        The alignment between two terminals should be as short as possible and as straight     as possible.

•   The alignment must be easy to construct and maintain.

•        The alignment should be safe.

•        The alignment should be economic in long term. Vehicle operation should be easy.

 

 General Guiding Principle for Highway Alignment:-

 The road alignment selection is influenced by following factors:

I. Alignment should be selected in such a way that it connects the starting point and end point with short distance and with fewer curves.

ii. The location through which alignment need to be pass should give easy gradient with easy curves. Sharp curve and steep slopes are avoided.

 iii. Obligatory points are those through which road alignment should or should not pass through. Alignment should pass nearby the village or town. This helps in the development of rural as well as urban area. The alignment should not pass through national parks, school, hospital, offices, where peaceful environment is needed.

 

 

Figure: Obligatory Point

iv. If the alignment is passed crossing the water supply pipeline, then the alignment need to be shifted.

v. The alignment crossing the bridge should be perpendicular to the river stream flow. The alignment line should pass through stable bank. Bridge alignment should not deviate with great distance with adjoining road alignment. Entry and exit alignment of bridge structure should be kept straight as far as possible to avoid accident due to not sufficient sight distance in hilly region.

Figure:-Typical Bridge Alignment

vi. The alignment should be placed such that the operating road condition does not affect the surrounding environment.

vii. The highway should, as far as possible, be located along edges of properties rather than through their middle so as to cause least interference to cultivation and other activities and to avoid the need for frequent crossing of the highway by the local people.

viii. The alignment should be placed near the location where the availability of road construction materials is high.

ix. Hard soil strata and balanced slope for cut and fill is perfect for best alignment

x. Location with drainage problems, high water table level, weak soil strata, susceptible to flood and landslide should be avoided.

xi. If the alignment is passing parallel to the river bank then the toe erosion condition due to river water should be considered for the design. Also, the HFL for past 50 years shall be kept in view.

Special Consideration in Hill Road Design

General considerations

When designing hill roads the route is located along valleys, hill sides and if required over mountain passes. Due to complex topography, the length of the route is automatically increased. Due to harsh geological conditions, special structures also have to be provided. Apart from the highly broken relief which has a fixed role in determining the alignment and location of special structures, climatic and geological conditions are also important. In locating the alignment special consideration should be made in respect to the variations in temperature, rainfall, atmospheric pressure and winds, and geological conditions

Temperature: Air temperature is in the hills is lower than in the valley. The temperature drop being approximately 0.5° per 100 m of rising. On slopes facing south and southwest snow disappears rapidly and rain water evaporates quickly while on slopes facing north and northeast rain water or snow may remain for the longer time. Unequal warming of slopes, sharp temperature variations and erosion by water are the causes of slope facing south and southwest.

Rainfall: Rainfall increases with increase in sea level. 2. The maximum rainfall is in the zone of intensive cloud formation at 1500-2500 m above sea level. Generally, the increase of rainfall for every 100 m of elevation averages 40 to 60 mm. 3. In summer very heavy storms may occur in the hills and about 15 to 25% of the annual may occur in a single rainfall. The effects of these types of rainfall are serious and should be considered well.

Atmospheric pressure and winds:

1. It decreases with increase in elevation.

2. At high altitudes, the wind velocities may reach up to 25- 30 m/s and depth of frost penetration is also 1.5 to 2 m.

3. Intensive weathering of rocks because of sharp temperature variations which cause high winds.

Geological conditions: The inclination of folds may vary from horizontal to vertical stratification of rock. These folds often have faults. Limestone or sandstone folds may be interleaved with layers of clay which when wetted may cause fracturing along their surface. This may result in shear or slip fold.

The degree of stability of hill slopes depends on types of rock, degree of strata inclination or dip, occurrence of clay seams, the hardness of the rocks and presence of ground water.

Route location in hills

Ridge route: Generally, ridge route is not preferred as the gradient necessary is high. It increases the economic cost as well as is time consuming and unsafe. The design speed is low and construction cost is high.

River route: This route is taken along the river and is the safest route. The design speed is high and the construction cost is low. The gradient can be kept low.

Special problem of roads locating in Hilly areas

I .Steep slopes are avoided. Slope should be kept below the ruling gradient.

II. Avoid Steep slope as far as possible

III. Unstable slope, ground with settlement problem, high flood land are avoided as far as possible.

 IV. Locations, along a river valley have the inherent advantage of comparatively gentle gradients, proximity of inhabited villages, and easy supply of water for construction purposes. However, this solution is be-set with disadvantages, such as, the need for large number of cross-drainage structures and protective works against erosion. It would, therefore, be necessary to take the various aspects into account before making the final selection.

 V. Minimize the number of hear pin bend as far as possible.

 

 VI. Ridge crossing should be preferred from the lowest elevation of the ridge. Unnecessary rise and fall in the alignment should be avoided.