5. Hill Roads

Topics As Per Syllabus5.1 Introduction5.2 Special Consideration in Hill Roads5.2.1 Alignment of Hill Road Design: General Consideration, Route

Location in Hills, Gradient, Design and Types of Hair Pin Bends, Different Types of Hill Road Cross-sections

5.3 Special Structures in Hill Roads5.3.1 Types of Retaining Structures, River Training Structures, Landslide

Stabilization Structures and Gully Control Structures

1. Introduction

• A hill road is usually defined on the basis of terrain. • As per NRS the cross slope may be classified as:

• Road which passes through with a cross slope of 25% or more i.e. mountainous or steep is termed as “Hill Road”. • Hill road-one which passes through terrain with cross slope of 25% or more. • There are sections along hill road with cross slope less than 25%, esp. when the

road follows river route. • It is the overall terrain that must be considered.

Why Hill Road in Nepal?• Nepal area = 140,000 sq. km approx. • 66% covered with thinly/sparsely populated hills, mountains. • 90% population reside in hilly areas and depends on agro products. • Waterways, railways, airways - difficult • Hilly Regions – Extremes of Climatic Conditions, Difficult and Hazardous Terrain,

Topography and Vast High Altitude Areas• Challenges of Hilly Areas: Floods, Landslides, Snowfall, etc. – Fairly Weathered Roads

Possible • But Hilly Areas Rich: in Natural Resources, Flora and Fauna – Important to launch

development Projects (e.g. Hydropower, etc.), tourism, etc.• For Transport, Economy, Social Reasons – Justify the Construction of Hill Roads in the Hilly

Regions. • Where Tunnels, Viaduct – Possible but Not Economical

5.2 Special Consideration in Hill Roads

5.2.1 Design and Construction Problems of Hill Roads

• Characterized by a highly broken relief with widely differing elevations and steep slopes, deep gorges and several watercourses — unnecessarily increase in road length • Complex geology - different rock bed- geology differs from place to place –

assessment for road foundation is not easy • Environmental impact - removal of vegetation etc. - stable slope may

change to unstable slope after road construction• Great Variation in hydrological condition [ground water condition] vary

from place to place – may lead to damages after road construction • New earth fill for road embankment may overload the relatively weak

underlying soil layer on hill slope – may trigger new and recurrent slides

• Requires installation of various types of special various road structures - cost may rise upto 50 - 60% of total construction • Steep slopes – high speed of surface runoff - provision of erosion protection

works • Construction along relatively small approach stretches– different

construction technology may be needed • Need of special safety precaution during construction • Variations of climatic conditions - precipitation [rain and snow] , velocity of

wind etc. - need considerations • Frequent blasting• Design of hairpin bends to get heights

लमजुङ र मनाङ सीमामा पन मार्दी�भीरको कच्ची बाटोमा रै्दीनिनक उपभोग्य वस्तु पुर्‍याएर फक! रै्दी यातु्रवाहक जिजपहरू । यनितखेर मनाङमा ५० वटा जिजपले उपभोग्य वस्तु, स्थानीयवासी र पय,टक ओसारिररहेका छन्

http://bit.ly/1P8Nrd2

5.2.2 Special Consideration in Hill Roads

1. Selection of Road Alignment2. Special Geometric Standards (Gradient and Hari Pin Bends)3. Cross-section Design4. Design of Drainage Structures (Special Structures may Require)5. Stability of Slopes

1. Selection of Road Alignment

• Alignment through hilly areas is slightly different from aligning through a flat terrain. • For the purpose of efficient and safe operation of vehicles through a hilly

terrain special care should be taken while aligning the highway. • Alignment should be - short, easy, economical & safe

• Alignment survey • Reconnaissance- • Tentative alignment after map study-topographical, geological and

meteorological • Trace cut-

• route selected in step 1 is translated on the ground to provide an access for subsequent detailed survey. A 1-1.2 m wide track is constructed with easier gradient

• Detailed survey • Fixing of bench marks • Cross section and longitudinal sections – 15 m on straight, 30 m at sharp

curves • Design of horizontal curves and hair pin bends • Soil investigation, geology, hydrology study

2. Special Geometric Standards

• The geometric standards followed in hilly areas are different from those in at terrain. • The alignment chosen should enable the ruling gradient to be

attained in minimum of the length, minimizing steep gradient, hairpin bends and needless rise and fall.

3. Cross-section Design

4. Hill Side Drainage

• Adequate drainage facility should be provided across the road. • Attempts should be made to align the roads in such a way where the number

of cross drainage structures required are minimum. • This will reduce the construction cost.

5. Stability of Slopes

• For hilly areas, the road should be aligned through the side of the hill that is stable. • The common problem with hilly areas is that of landslides. • Excessive cutting and filling for road constructions give way to steepening of

slopes which in turn will affect the stability.

Factors Affecting Selection of Alignment in Mountainous and Hilly Areas

• Following points to be considered while aligning the hill roads: i) Temperature• The temperature of air varies inversely with altitude. • The temperature drop being about 0.50C per 100m of rise. • Similarly, the amount of heat received by hill slopes varies enormously with their orientation in

relation to the exposure to sun. • Temperature - lower temperature in hill – drop by 0.5 0C per 100m rise - slopes facing south and north

ii) Rainfall• The amount of rainfall in hilly region is inversely proportional to the altitude. • In hilly regions, wind often flows along the valley and gorges, as consequence of which the rainfall in

the valley is substantially higher than on high-lands and water-sheds. • Also, maximum rainfall occurs in few months only. So these all uneven situation create the problem in

construction and maintenance of the road.• Rainfall- heavy rainfall affects construction and maintenance

iii) Atmospheric pressure and Winds• Atmospheric pressure is inversely proportional to altitude. • At high altitudes, the velocity of wind is frequently coming at 25 – 30 m/s. • The change in character of wind is due to appreciable difference of atmospheric pressure in valleys

and on mountain passes.• Wind-high wind velocity contribute to damage –weathering of rocks, blowing away binding material

in valley iv) Geological condition • The tendency of sedimentary rocks is to slip under the influence of force parallel to the layer. • The degree of stability of hill slopes depends upon the type of rock, the degree of strata inclination

or dip, the occurrence of clay seem, the hardness of rock and presence of ground water. • The instability of road may be due to ground water, landslides and unstable folds.v) Route location • The approach to the location of hill road alignment varies for the sections along the valley bottom

and along the mountain pass. • The first is called ‘river route’ and second is called ‘ridge route’.

Route Location

• Hill road follow twist and turns- curves

• Location of alignment varies for sections along valley and mountain pass

• Alignment through valley - river route; mountain - ridge route

• A hill road may be only river route, ridge route or combination of both

River Route• Most frequent Gentle slope

• Serves rural settlement situated next to the water course• Low vehicle operation cost

• Availability of construction material and water• Involves numerous horizontal curves,

construction of large bridges• Special retaining protection on hill sides

• River training works may be extensive- toe cutting, etc.

Ridge Route

• Steep gradient, sharp curves including hair pin bends • Expensive rock works, successive mountain

pass • Route climbs up continuously from the

valley till mountain pass and descends down• Construction of special structures, tunnels,

snow fences etc.• Are most stable

a) River Route • The location of a route along the river valley is known as river route. River

route is frequently used in hill road due to comparatively gentle gradient. - It is advantageous that availability of water and other construction material

in vicinity. - However, a river route may involve numerous horizontal curves,

construction of large bridges over tributaries and on stretches along steeply sloping hill sides.

- It may also be necessary to construct special structures on hill side for the safe of road against landslides.

b) Ridge Route

• A ridge route is characterized by very steep gradient, numerous sharp curves including hair pin 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.