Hill Roads design and construction PPTX.pdf

This notes contains notes on Hill roads.

1. Unit-2
Hill Roads

2. Definition

© The road which is constructed at mountainous or hilly region is known as hill road. With
reference to cross slope, Hill road can be defined as the one which passes through a terrain
with a cross slope of 25% or more. Sometimes the road also can be passes through river
route with cross slope less than 25% in such condition also the road is included in hill road.
Hill road is also known as GHAT Road.

Steep terrain 4

Mountainous
60%

— Rolling terrain

Plain terrain

3. Importance of hill roads in Nepal

© The population living in hill area depends upon the agriculture and other products of that
area which are not sufficient for them whereas in plain terrain the agricultural and
industrial products are always surplus. This unbalanced productivity of land makes the
people of hilly area underprivileged. For the overall economic development of the nation
this unbalanced distribution of national product should be curtailed down. Surplus
products in one area should be served in deficit area. Construction of hill roads is
important not only from economic consideration but it is associated with the social
justification for providing facilities to underprivileged mass of the country.

+ The hilly regions generally have extremes of climatic conditions, difficult and hazardous
terrain, topography and vast high altitude areas. The region is sparsely populated and the
basic infrastructural facilities that are available in plain areas are mostly absent. The roads
in these areas are affected by floods, landslides, snowfall etc. compelling certain roads to be
closed in part of the year especially in winter months. But these areas are rich in natural
resources, flora and fauna and are important to launch development projects, tourism etc.

4. Design and construction problems in hill roads

If we review the characteristics of the hill road, there is unnecessary airily increase in the length of roads in hilly
area due to presence of steep slopes, gorges, and number of water courses along with large elevation differences.

In hilly areas, during rainy seasons problem of landslide and flood occur. So, many protection structures are needed
to be built which increases the cost of the road construction.

It is very difficult to maintain the required sight distance in hill roads due to sharp curves & hill side slopes which
impact the safety in vehicle operation.

The variation in the climatic condition such as pressure variation, the change in temperature due to altitude
difference, precipitation increases at greater height etc. which make difficulties in design & construction of hill road.

© Stability of hill slope before construction may not be as stable due to increased human activities.

© The geological condition varies from place to place.

+ Heavy and high-speed runoff makes difficulty in construction and increases maintenance cost as well as drainage

structures cost.

© Itis quite difficult to design hairpin bends to attain height.

© Filling may overload the weak underlying soil layer on hill slope which may trigger new slides. So, Special safety

precautions are to be adopted in hill road.

In hilly terrain many rocks encounter in alignment so, frequent rock cutting, drilling, blasting and clearing activities
are to be carried out which increased the cost and degree of risk level.

Hill roads are costlier than plain roads.

5. Special consideration of hill road geometric design

Due to widely differ in elevation, presence of numbers of water tasks, geographical condition of
hilly area, it is necessary to take great care in Road design.

Some special consideration of Hill Road geometric design are explained below.

1. Selection of road alignment: Short, safe, easy, economic and comfort are the ideal
requirement of the any road alignment. So, the designer must choose a short, easy,
economical, safe and comfort route as far as possible. Besides that, while selecting the road
alignment factors such as altitude, geological condition, temperature, rainfall, atmospheric
pressure etc. have to be considered

2. Stability of Slope: Landslide in hill area is common. While constructing hill road, cutting
and filling in natural slope is carried out. So, for safe and to gain stability, we should
maintain the slope in cutting and filling. Cutting and filling of the Earth to construct Hill
Roads on hillside causes steepening of existing slopes and affect its stability. So, while
aligning Hill Roads special care should be taken to align the road along the side of hill
which is stable.

6. Special consideration of hill road geometric design

3. Drainage: Snow fall and rainfall are common in hilly region. So, to manage the runoff water
we should construct numerous hill side drains and cross drains for adequate drainage facility.
Construction of cross-drainage structure is quite costlier. So, careful consideration has to be
done while aligning hill road. We must attempt to minimize the cross-drainage work as far as
possible while aligning the road.

4. Geometric Design: The total length of road is increased due to the widely differ in elevation
and geological conditions. Serious care should be taken while designing gradients, speed, sight
distance, horizontal and vertical curves etc. Geometric standards given by the government are
followed to design hill roads. These standards are followed with reference to gradient, speed,
curves and they consequently influence the radius of curve, sight distance and other related
features. The Hill Road route should enable ruling gradient to be attained in most of the length
of alignment, minimizing needless rise and fall, steep gradients and hairpin bends.

7. Special consideration of hill road geometric design

5. Resisting Length: Resisting Length concept is mainly used to assess the suitability of
alternative alignments and should be kept as low as possible. It is calculated from the total
work to be done to move the loads along the route taking the horizontal length, the actual
difference in levels between the two stations and the sum of ineffective rise and fall in excess
of floating gradient. So, for resisting length, the ineffective rise and executive fall should be
kept minimum.

6. Others: Besides above, traffic signs, road side safety, length of the vehicles also plays a vital
role in hill roads. Route of road (river or ridge) has to be considered properly. Important flora
and fauna have to be saved. And maintenance cost must be minimized as far as possible.

8. Factors affecting hill road alignment

+ Temperature:

1

2

3.

As altitude increases, temperature decreases.

Amount of solar heat received varies with orientation.

In slopes facing south and south west snow disappears rapidly and evaporates quickly.
Daily temperature fluctuation is caused by movements of warm and cold air steams.
In slope facing south and south west there are:

a. Unequal warming .

b. Sharp temperature variation and erosion by water.

c. Greatest destruction.

9. o

Factors affecting hill road alignment

Rainfall
Rainfall is directly proportional to the altitude

i.e. Rainfall « Altitude

So, Rainfall increases with the increment of elevation.
There is 40-60 mm increases in rainfall for every 100m of elevation.

At 1500m to 2500m above the sea level intensive cloud is formed. This zone is termed as zone
of intensive cloud formation.

In summer or in rainy season there is very heavy rainfall having serious effect on the
construction and maintenance of roads.

Landslides and rock slip are also the serious phenomenon that occurs in hill roads due to rain
fall.

10. Factors affecting hill road alignment

© Atmospheric pressure

> Atmospheric pressure its inversely proportional to the altitude
E 3 1
Km
i.e. Atmospheric pressure TNE
So, Atmospheric pressure decreases with the increase in elevation.

> At high altitudes (3000 4000m), the Wind velocity ranges from 25 to 30 m/s.
At high altitudes there is freezing temperature and the depth of frost penetration is 1.5 to
2.0 m.

> At high altitude there is intensive rock weathering, low-cost and improper road pavements
get damaged easily.

v

11. Factors affecting hill road alignment

© Geological Condition

1

2

3

Geological condition is one of the major factors that affect the alignment of hill road.
Natural slope stability is most important in hill roads operation and maintenance.
Horizontal or almost vertical stratification of rock.

Stability of the hill side slopes depends upon the type of rock, hardness of rock, degree of
strata inclination and ground water presents in that region.

Cutting through hard rocks are very expensive. The alignment of roads may be suitably
deviated to avoid such areas.

There may be variation of inclination of folds from horizontal to vertical stratification of
rock. Faults are often present in these folds. Limestone or sandstone folds may be
interleaved with layers of clay. When they come in contact with rain or when they get
wetted they may fracture along their surface and may result in shear or slip fold.

So, the alignment should pass through the stable slope with no ground water and the area
where there is minimum dip of strata as far as possible.

12. Typical cross sections of hill roads

© The cross section of a road in a hilly terrain is determined by
a. The original ground slope of the site,
b. The slope of the road formation,
e. Width of roadway,
d. Side drain size, shape etc.
Original Hill Stope

Cutting

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Various types of hill road cross-sections

Cut & fill

Bench type

Box cutting

Embankment with retaining wall
Semi bridge

Semi-tunnel

Platforms

Fully in embankment

14. Cut & fill

© In the situation, when hill side slope or road bed slope has a gradient 2% or more then, cut
and fills road bed is cheaper and environmentally friendly. The fill and cut mass are
generally balanced. The fill mass is obtained from cut mass within free haul distance.
Benches are made on the surface of the hill side with a height of 0.5 m and length varying
from 1.5 to 3.0 m depending upon the slope to ensure adequate stability. Sometimes the
longitudinal cracks appear in the pavement due to the settlement of fill portion of the road
bed.

15. Bench type

© A cross section of the bench type although entails some increase in earthwork but ensures
the complete stability of the road bed, if hill side is itself stable.

16. Box cutting

When the location of road bed is unstable or unsuitable along the hillside due to one or
other reasons, the road bed is designed as trench type of cross section. In some case it is
provided to meet the geometric design standards for a given category road. It increases
earthwork to a large extent.

17. Embankments with retaining walls

© On steep slopes of about 30-35°, the earthwork involved in constructing the embankment
increases substantially. The retaining wall is sometimes provided to reduce earthwork's cost
and to increase stability. When embankment soil on steep grounds itself need support, the
retaining wall is provided. They can be introduced on a less steep ground slope to increase
the stability of road bed.

18. If the road is located on a hill slope the retaining wall needs to be at a substantial height. In
such cases, in order to reduce the quantities of work, road bed with a semi-bridge type of
structure may be constructed. Some part of the roadway is accommodated on the semi
bridge and some on bench cut.

19. Semi-tunnel

© When inscribing is to be cut into steep hills in stable rock faces, the rock may be permitted
to overhang the road to reduce rock works. Such a cross section is called a semi-tunnel. For
high altitude hill roads roadway in half tunneling is advantageous as snows do not block
them.

20. Platforms

On the very steep or precipitous slopes, where shifting of the route into the hillside will lead
to enormous rock works, which ultimately increases the cost of road construction and
where semi-tunnel cannot be constructed due to geological condition, platforms are usually
cantilevered out of the rock on which road way is partially located.

7

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21. Fully in Embankment

© When cross slope of the hill is very small, road may be on full embankment. Quantity of
filling may be low or high. In case of high or heavy filling, retaining wall should be
provided on the both side of formation.

22. Special structures in hill roads

+ Construction of hill roads involves many special structures. These may include wide range
of structures which are used to retain soil mass to increase stability of road embankment
slopes as well as natural hill slopes to accommodate road bed in steep slope, to penetrate
deep through mountain pass and so on. Special structures are also required to dissipate
energy of surface water in the hill road drainage system to provide snow avalanche control
and protection system, river training and erosion control to prevent scour under cutting and
the cutting by the river.

+ The following types of special structures are most frequently used in the hill roads in Nepal:
1. Retaining structures
2. Drainage structures

3. Slope protection structures

23. Retaining structures

o A retaining structure is usually a wall constructed for
supporting vertical or nearly vertical earth bank. Retaining
walls are constructed on the valley side on the cut hill side to
prevent the slide towards the roadway. Situations where
construction of retaining walls is required:

1. Places where the valley side surface gets saturated in the
monsoons and is likely to result in slip taking a part of the
road with it.

2. Places where undercutting by a stream or other water course
causes damage to the valley side and the road.

3. In valley point where water flows over the road

4. To achieve roadway width, where cutting into the hill is not
economical or has to be restricted due to other reasons

24. a)
b)

D)

d)

Revetment walls

Revetment or breast walls are provided on the inner side of the road to give support to the
loose and unreliable soil of the cut up hill side. The embankment slopes are normally
protected with rough stone pitching about 30cm thick in order to avoid erosion due to flow
of water. Such walls perform the following functions:

Keep the road edge defined and also protect the drain to some extent.

The hill slope to the extent of breast wall height will remain protected from slips and any
slide above this height will flow over the top of the breast wall.

It would not allow continuity of the flowing mass of soil and would thus facilitate the
clearance of slides.

Assistance in drainage from hill slope through weep holes in breast wall on to the side
drain in front of the wall.

25. Toe wall

© If the slopping length is too long it is preferable to construct a toe wall as to support the
embankment. Where the cutting slope is steep and contains loose soils, slips are likely to
occur. In such locations, revetments walls of dry stone masonry are constructed to retain
the soil on the cutting side to prevent occurrence of any such slip.

+ When culverts/scuppers are constructed and waterfalls above the retaining walls on the
valley side to a considerable height, in the form of free fall, there is considerable erosion at
the toe of the retaining wall. In order to check this, one or a series of toe walls are
constructed in order to break the water force so that the retaining wall does not get eroded.
In some cases, similar structures called check walls are required in the stream bed on the
upstream side of the road to reduce the flow of debris which blocks the road.

26. Slope protection works

In hill roads, landslides are very common due to steep slopes. The basic cause of landslide is the
development of shear stresses more than the shear strength of the soil. Fresh unturfed
embankment and cut slopes are the least stable part of the road bed since the soil on the surface
of the slopes is subjected to the direct action of sun, rain, and wind.

Causes of landslides

a)
b)
o
d)

e)

Increase in moisture content of the soil in hill slopes which increases the pore water pressure.
Alternate swelling and contracting of the soil mass.

Seepage pressure of percolating groundwater.

Steeper slopes.

Human activities like blasting and using heavy vehicles at unstable zones.

Preventive measures

a)
b)
D}

0)

The highway may be realigned at areas more prone to landslides.
Construction of retaining walls must be done at places where required
Adopting easy slopes during design and construction of the road.

Treatment of slopes to increase stability conditions

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