Unit 2: Hill Road Alignment Note (Hill Road)

2.1. Introduction

uThe position of the center line of the road in the ground is called the alignment of the hill road

uAim of Hill Roads: Alignment should be - short, easy, economical & safe, line of communication between the obligatory points considering the physical features of the region and traffic needs as well as least disturbance to the eco-system_.

uMinimize Rise and Fall: by following a direct line as far as possible between the obligatory points

2.2. Factors affecting hill road alignment

 The following points are to be considered while aligning the hill roads:

i) Temperature

uThe temperature of air varies inversely with altitude. Air temperature in the Hill is Lower than in the Valley.

u Amount of solar heat received with orientation.

u Temperature drops about 0.5 0C per 100 m of rise.

uSimilarly, the amount of heat received by hill slopes varies enormously with their orientation in relation to exposure to the sun.

u Slopes facing South or South West:

a.Snow disappears rapidly and rainwater evaporates quickly

b.Unequal warming

c.Sharp temperature variations and erosion by water

d.Greatest destruction

ii. Rainfall

u Amount of rainfall in hilly regions is inversely proportional to the altitude      As the elevation increases rainfall also increases.

uIn hilly regions, wind often flows along the valley and gorges, as consequence of which the rainfall in the valley is substantially higher than on highlands and water-sheds.

u Also, maximum rainfall occurs in a few months only. So these all uneven situations create problems in the construction and maintenancof the road.

u  Maximum rainfall
Zone of intensive cloud formation (1500 - 2500 m above the mean sea level) 40- 60 mm increase in rainfall for every 1000 m of elevation

uIn summer there is very heavy rainfall having a serious effect on the construction and maintenance of roads.

iii) Atmospheric Pressure and Winds

u  Atmospheric pressure is inversely proportional to altitude it decreases with increase in elevation 

uAt high altitudes (3000 -4000 m):
❑ wind velocity is frequently coming at 25-30 m/s
❑ freezing temperatures
❑ Depth of frost penetration 1.5-2.0 m
u The change in the character of wind is due to appreciable differences in atmospheric pressure in valleys and on mountain passes.
u Therefore, there is intensive rock weathering and the wind contributes to the damage of low-cost road pavements.

iv) Geological Conditions

u Horizontal or almost vertical stratification of rock

uThe tendency of sedimentary rocks is to slip under the influence of force parallel to the layer.

uThe degree of stability of hill slopes depends upon the type of rock, the degree of strata inclination or dip, the hardness of the rock, and the presence of groundwater.

uThe instability of the road may be due to groundwater, landslides, and unstable folds.

uTherefore we should follow the stable slope with no groundwater and the alignment should pass through the area where the dip of strata is as small as possible

V) Route location

uHill road is characterized by longer routes with many sharp curves.

uThere are mainly two route location possibilities in case of hill roads.

uThe approach to the location of hill road alignment varies for the sections along the valley bottom and along the mountain pass.

uThe first is called ‘river route’ and second is called ‘ridge route’

a) River Route

u The location of a route along the river valley is known as the river route.

uRiver route is frequently used in hill road due to comparatively gentle gradient.

uIt is advantageous that availability of water and other construction materials in the vicinity.

uHowever, a river route may involve numerous horizontal curves, the construction of large bridges over tributaries, and on stretches along steeply sloping hillsides.

uIt may also be necessary to construct special structures on the hillside for the safe of the road against landslides.

. Characteristics

1.Location: along a river valley
2.Most frequent case of hill road alignment
3.Comparatively gentle gradient

Merits

a.Serves the rural settlement
b.Low vehicle operating cost, availability of water and other construction materials

Demerits

c.Numerous horizontal curves

d.Construction of special retaining and protection of walls on the hillsides

e.Extensive earthwork

f.Construction of a large number of cross drainage structures

g.Steep-sloping hillside may be insufficiently stable

h.Massive river training and protection works on the riverside

b) Ridge Route

➢A ridge route is characterized by a very steep gradient, numerous sharp curves including hairpin bends and the expensive rock works.

➢The road usually follows the top section of the hill system and crosses successively mountain pass.

➢Geologically stable and comparatively mild slope sections are
selected for the artificial development of the route

➢Extensive rock works

➢Construction of special structures

➢Necessity of long length away from the air route

➢The geologically stable and comparatively mild slope should be selected.

2.3. Survey Methods

Sequences of Survey Method in Hill Road 
a)Map Study
b)Reconnaissance
c)Preliminary Survey
d)Determination of Final Center Line
e)Final Location Survey Aerial Surveys, Photogrammetry, and Remote Sensing Can be taken to facilitate surveying for the selection of alternative alignment

Alignment survey

• Reconnaissance

➢Tentative alignment after map study-topographical, geological and meteorological

• Trace Cut:

➢A trace cut consists of 1 to 1.2 m wide track constructed along the selected alignment to facilitate access to the area for inspection and further surveys. In general, gradients along the trace cut should be easier by 10 to 20% than the standard limiting gradients.

• Detailed survey

➢Fixing of benchmarks

➢Cross section and longitudinal sections – 15 m on straight, 30 m at sharp curves

➢Design of horizontal curves and hairpin bends

➢Soil investigation, geology, and hydrology study

Preliminary Survey:

u A preliminary should cover a strip of 15 m on either side of the center line on straight reaches, and 30 m on either side on curved reaches. A traverse survey should be run along the trace with a theodolite.

uCross-sections should be taken at intervals of at least 20 m. Contouring should be done at sharp curves and difficult locations, the contour intervals being 2 m or less.

Final Centre Line:

uAlso known as paper location, this should be marked on the plan.

uHorizontal and vertical curves should be fully designed.

Final Location Survey:

uThe final center line should be laid out on the ground in the field. This involves staking out the center line using a theodolite and detailed leveling. The stakes should be at 20 m intervals. Benchmarks should be established at intervals of 250 m and at all drainage crossings.

uLongitudinal sections and cross-sections shall be taken at all staked stations. Cross-sections should extend up to the right of way and levels should be taken at 2 to 5 m intervals.
In the last stage of the alignment survey, hydrological and soil investigations for the route should be carried out; this helps in deciding on the drainage and protection works