What are hill roads?
The term hill road can be explained with reference to the cross slope, i.e., the slope approximately perpendicular to the centerline of the highway alignment. Thus a road is termed as a hill road if it passes through a terrain with a cross slope of 25% or more and it is characterized by widely differing elevations, deep gorges, a number of watercourses, and steep slopes. The hill roads are also sometimes referred to as ghat roads.
In this chapter, the salient aspects of hill roads such as their alignment, design, construction, drainage and maintenance will be discussed.
IMPORTANCE OF HILL ROADS
There are possibly two modes of transport for mountainous or hilly areas, namely, roads and railways. The choice between the two should be based on the relative economics and the following factors are certainly in favour of hilly roads:
(1) Development in stages: A road of small width involving less expenditure can open out the area of immediate economic development and the improvements in the roadway system can be carried out as and when the traffic develops.
(2) Initial cost: There is no doubt that the initial cost of construction of railways is much more than that of roads in hilly areas.
(3) Length: The roads can be constructed with comparatively steeper grades which will result in the reduction of the length of road as compared to the length of the railway track required with milder slopes for rail traction for the same height.
The importance of hill roads can be imagined by understanding the following purposes which they serve:
(i) Economic development: The hilly areas are backward as far as modern civilization, culture and education are concerned and hence, they require tremendous economic development. The main activity of the people in these areas is agriculture. The lands in hills are ideally suited for a variety of crops like the apples, apricots, cherry, etc. among fruits and potatoes, ginger, etc. among the vegetables. If these paying crops are grown in place of maize and other local food-grains and economically transported outside, the economic life of the population can be considerably improved.
(ii) Forest wealth: The hilly areas contain huge forest wealth
in the form of structural and other timbers, minerals, stones, etc. and all these items form the basic valuables for developing the country as a whole and the hilly areas in particular, provided there is an efficient transportation system for carrying these valuables to the plains from where they can be processed and sent to the consumers.
(iii) Industrial development: There are certain areas of hills which are ideally suited for growing tea and jute and for bringing up silk-worms. The presence of roads can help in setting up of the industries of these products in the hilly areas.
(iv) Strategic considerations: In case of an emergency such as war, a well layout system of roads in hilly areas helps considerably for moving the army from one place to the other.
(v) Tourism: Some of the hilly areas present immense natural scenic beauty which attracts thousands of local and foreign tourists. The construction of hill roads is probably the main contributing factor for the development of tourism all along the Himalayas from Gulmarg to Darjeeling and other important hill stations of our country.
BASIC PRINCIPLES OF PLANNING OF HILL ROADS
In a broad sense, the main aim of planning a hill road is to establish the shortest, most economical and safe route between the obligatory points, and to achieve this purpose successfully, the following basic principles are to be observed in the planning of hill roads:
(1) Construction work
(2) Existing routes
(3) Intensity of traffic
(4) Master plan
(5) Natural climatic conditions
(6) Use of contours.
(1) Construction work: The construction of hill roads requires considerable period and greater funds as compared to the roads in plains because it involves items such as parapets to demarcate
the roadway boundary, rock cuttings in difficult regions, provision of erosion control measures, greater number of drainage crossings, etc. It is therefore advisable to plan the construction work in stages over a number of years in such a way that each stage of construction improves upon the previous construction stage so as to bring it upto the requirements of the developing traffic.
(2) Existing routes: The existing pedestrian and mule tracks or good animal beaten tracks present the most convenient routes for further improvements and extension and hence, it is one of the essential principles in hill road planning that maximum use should be made of such existing routes. They may however be suitably modified as the traffic requirements increase.
(3) Intensity of traffic: For the purpose of planning, the hill roads may be categorized as jeepable roads and motorable roads from the view point of intensity of traffic. These roads may then be converted into National Highways, State Highways, etc. depending upon the relative importance in the whole set up of planning. The jeepable roads are narrow in width, have comparatively sharper bends and steeper grades and they can be traversed by jeep cars only. The motorable roads can be used by the commercial vehicles in the hilly area. It may be a good policy to aim at jeepable roads first and to provide motorable roads at a later stage after studying the possibility of providing necessary standards of geometric design and construction.
(4) Master plan: It is advisable to draw a master plan for the development of the whole hilly area and work out the priorities instead of starting the work haphazardly. It may avoid the tremendous economic loss in the form of more construction and operation costs due to greater lengths covered by the haphazard planning.
(5) Natural climatic conditions: It is necessary to explore the natural climatic conditions of the hilly area before the planning of road alignment. It is observed that the sunny side of the hills above a height of about 4500 m and the shady side of the hills above a height of about 3600 m are covered with snow. Now the snow melts quickly on the sunny sides as compared to the shady sides of the hills. It is therefore advisable, as far as possible, to align the road on the sunny side of the hills. In a similar way, the convenient slopes are available along the river valleys and it is therefore economical to carry the alignment along the river valleys, as far as possible. The hilly areas subjected to heavy winds having velocity exceeding 100 km p.h. should also be located and avoided, as far as possible.
(6) Use of contours: When a virgin hilly area is to be explored, the use of contour maps should also be made. It should always be kept in mind that whatever height has been gained should never be lost. For instances, let us say that a ridge has to be crossed to go to a valley beyond it. It is then essential to touch the most convenient lowest points on the ridge to have the minimum length of road.
Method of surveying For Hill Road
The conventional surveying methods of reconnaissance survey, preliminary survey and final location survey may also be adopted for hilly areas. But they prove to be time consuming, depending on the type of country and the nature of project. Some other drawbacks of these methods of surveying are as follows:
(1) Delay in work: The surveying work may have to be withheld during rainy or unfavourable climatic conditions.
(2) Details of area: The details to be collected relates to a number of aspects and it is generally found that there is always a possibility of a few details being missed.
(3) Information: The information from maps may only relate to the time or period when the maps were prepared and it is quite likely that considerable changes might have taken place between the time the maps were prepared and the time of carrying out the project. The information as such may not also be comprehensive and lack of comprehensive information may involve a few difficulties at a later stage.
(4) Man-power: These surveys require lot of man-power to work in the field and co-ordination of work in the difficult sections of the whole project is difficult to achieve.
(5) Remote areas: For areas which are inaccessible and remote from habitations, it becomes necessary to establish special camps with facilities of tents, rations for party, guards to protect against wild animals, medical kit, transport arrangements, etc. The quick and most modern method of surveying for hilly areas is the use of aerial photographs. These photographs are obtained by special automatic cameras fitted in the aeroplanes flying at a constant height and they represent the exact images of the objects on the ground.
In fact, there is no chance of any detail being missed when stereo-pairs are viewed through a stereoscope and a model of the ground to the scale of the photographs is prepared for scrutiny. Thus, for reconnaissance survey, the models for possible routes are prepared and they are scrutinized in the office only instead of going to the field. The possible route or routes are decided and for preliminary survey, the best route is decided by low altitude photography with large-scale photographs.
For final location survey, the details may be collected from the field and necessary longitudinal sections and cross-sections may be prepared accordingly. The bench marks are fixed and the centre line is marked by stakes and reference pillars. Necessary hydrological and soil investigations are carried out for the final route before the construction work is commenced.
ALIGNMENT OF HILL ROADS
The success and utility of a hill or ghat road depend on its proper alignment. It is, therefore, necessary to exercise great care in fixing the alignment of hill roads. A good alignment has the following features:
(i) It achieves the minimum costs of construction and maintenance.
(ii) It allows comfortable travel and the expenditures on motive power, as well as wear and tear of vehicles, are also greatly reduced.
(iii) It contains sharp curves having small radius.
(iv) It gives a stable and safe road.
(v) It grants the easiest, shortest, and most economical line of communication between the obligatory points or important centers to be connected by the hill road.
(vi) It has the gradient as easy as possible.
In general, it can be stated that the best and most convenient alignment will be the one having the minimum of cutting and filling; and a minimum of walling and bridging. In many cases, the alignment of the hill road contains two types of sharp curves known as hairpin bends and corner bends. Fig. 1 shows the hairpin bend and fig. 2 shows the corner bend.
If the side of hill contains ridges and valleys, it will have to be provided with salient and re-entrant curves. A salient curve is a convex curve with its convexity on the outer edge of the road at the ridge of hillside. A re-entrant curve is a concave curve at the valley of the hillside. Fig. 3 shows the salient and re-entrant curves. Due to these ridges and valleys, the visibility on a hill road is less and the traffic has to be very careful while negotiating the salient and re-entrant curves in succession. Otherwise, there are chances of fatal accidents to occur at these points. To improve the visibility at a salient curve, some portions of the hill may even be cut down.
GEOMETRIC STANDARDS OF HILL ROADS
The roads in the hilly areas require special attention in fixing up the standards for geometric design because of various factors, such as types of vehicles using the road, total daily tonnage, difficulty in construction, type of surface to be provided, topography of the area, etc.
If the desired geometric standards of a hill road are not permissible due to finance or any other reason, the economy can be achieved in aspects like the width of surfacing which can be improved at a later stage. But it is necessary to comply strictly with the aspects like gradients, curvature, sight distances, etc. which are prohibitive in cost for improvement subsequently.
It is also recommended that at places on a hill road where minimum geometric standards are not provided, proper signs at prominent points should be exhibited to inform the road users well in advance of the reduction in design speed.
The geometric standards of hill roads are to be considered with respect to the following aspects:
(1) Widths of carriageway, shoulder, roadway and land
(3) Stopping sight distance
(4) Overtaking sight distance
(7) Radius of horizontal curve
(8) Widening at curves
(9) Transition curves
(10) Hairpin bends
(11) Cut slopes
(12) Setback distance
(13) Passing places
(14) Vertical clearance
(15) Lateral clearance.
(1) Widths of carriageway, shoulder, roadway and land for hill road:
Table 1 shows the widths of the carriageway, shoulder, roadway, and land for different categories of highways. The following points should be noted:
(i) The roadway widths are exclusive of side drains and parapets. These should be considered separately, when necessary.
(ii) In hard rocky stretches, the shoulders may be reduced by 400 mm on either side on two-lane roads and by 200 mm in other cases.
(iii) The minimum setback for building line beyond the right of way should be 5 m in normal cases and 3 m in exceptional circumstances.
(iv) For roads subjected to heavy snowfall, the roadway width may be increased by 1.5 m
|Highway Classification||Carriage width in m||Shoulder Width in m||Roadway width in m|
|NH and SH or DR|
|Single lane||3.75||2 x 1.25||6.25|
|Double Lane||7.00||2X 0.90||8.80|
The steeper camber or cross-slope is adopted for hill roads and table 10-2 shows its recommended values. However, if the road has a longitudinal gradient greater than 1 in 20, a flatter camber may be provided.
TABLE 2. CAMBER FOR HILL ROADS
|No.||Type of surface||Camber, per cent|
|1||Subgrades, earth roads and shoulders||3.0 to 4.0|
|2||Gravel and W.B.M. surface||2.5 to 3.0|
|3||Thin bituminous surfaces||2.5|
|4||High type bituminous surface and cement concrete surface||2.0|
(3) Stopping sight distance for hill roads:
The stopping sight distance (SSD) is calculated from the following expression:
Length of SSD = v2( 254 f) + 0.278 Vt
V = Design speed in km p.h.
t = Total reaction time to be taken as 3 seconds,
f = Coefficient of friction to be assumed as 0.4.
Table 3. shows the values of SSD for various design speeds on hill roads as recommended by the I.R.C.
TABLE 3. VALUES OF SSD FOR HILL ROADS
|No.||SSD in m||Design speed in km p.h.|
(4) Overtaking sight distance for Hill roads:
The overtaking sight distance (OSD) is calculated from the following expression:
Length of OSD = [d₁ + d₂ + d3]
= [ 0.278 V₂t+ (0.278 V2 to + 2 s) + 0.278 Vto ]
V = Speed of overtaking vehicle in km p.h.
V2 = (V – 16)
= Speed of overtaken vehicle in km p.h.
t = Reaction time of driver = 2 seconds
s = (0.2 V₂ + 6) Spacing of vehicles
to = (14.4 s/ A)^0.5 = Overtaking time in seconds
A = Acceleration in km p.h./sec. to be taken as 4.72, 4.45 and 4.0 for speeds of 30 km p.h.. 40 km p.h. and 50 km p.h. respectively.
(5) Gradients for Hill roads:
Depending upon the type of terrain and height above mean sea level, suitable values of ruling gradient, limiting gradient and exceptional gradient are to be adopted and grade compensation may be provided on chapter of horizontal curves.
(6) Super-elevation for Hill road:
The super-elevation on hill roads is to be provided by adopting the following formula:
e = V2 / 225 R
e= Rate of super-elevation
V= Design speed in km p.h.
R = Radius of curve in m.
It is recommended that the super-elevation should not exceed 7 per cent in sections of hill roads which get snow bound and 10 per cent in other places.
(7) Radius of horizontal curve for hill road:
The minimum radius of horizontal curve is calculated from the following equation:
R = 0.008 V² / (e + f)
R = Minimum radius of horizontal curve in m,
V = Design speed in km p.h.
e = Roadway super-elevation to be taken as zero for snow bound area and 0.10 for snow free area
f = Lateral friction to be assumed as 0.15.
(8) Widening at curves in hill road:
The extra width of carriageway required at the curve is calculated from the following expression:
We = (18n / R) + (0.1 V / ÖR)
We = Extra width in m,
n = Number of lanes,
V = Design speed in km p.h.
R = Radius of curve in m.
(9) Transition curves for hill roads:
The length of transition curve is calculated from the following expression:
Lt = (0.0215 V3) / CR
Lt = Length of transition curve in m
V = Design speed in km p.h.
R = Radius of circular curve in m
C= 80 /(V + 75) subject to a maximum of 0.76 for speeds less than 30 km p.h. and 0.46 for speeds above 100 km p.h.
(10) Hairpin bends:
A hairpin bend is a sharp curve and it is located on a hill side having the minimum slope and maximum stability. It must also be safe from the viewpoint of landslides and groundwater. For reducing the construction problems and expensive protection works, the hairpin bends should be provided with long arms and farther spacing. A hairpin model is shown in fig1.
A hairpin bend is designed as a circular curve with transition curves at each end. Following are the design standards of a hairpin bend:
- Minimum design speed 20 km p.h.
- Minimum radius of the inner curve 14 m.
- Minimum length of transition curve 15 m.
- Minimum gradient = 1 in 200.
- Maximum gradient 1 in 40.
- Super-elevation in circular portion of the curve = 1 in 10.
- Minimum width of carriageway at apex of the curve = 11.5 m for NH and SH having two lanes of traffic
= 9 m for NH and SH having one lane of traffic
= 7.5 m for MDR and ODR = 6.5 m for VR.
- Minimum straight length between two successive hairpin bends 60 m.
- The approach gradient should not be steeper than 5 per cent for 40 m.
- The island portion of the hairpin bend should be cleared of trees, etc. for good visibility.
(11) Cut slopes:
The detailed investigations are necessary in deep cut and at places where the problem of instability is likely to occur. For hard rock, the side slope is provided nearly vertical or half-tunnelling, if the height of cut exceeds 7.5 m.
(12) Setback distance for hill road:
It is not practicable to provide clear visibility corresponding to overtaking sight distance all along the hill road. Hence the alignment is made in such a way that at least the safe stopping distance is provided. For this purpose, the setback is given and the LRC has specified the minimum setback distances from center-line of the inner side of horizontal curves for various speeds and radii of curves corresponding to the stopping sight distance.
The back distance is calculated from the following equation:
m = R – (R-n) cos θ (Ref. fig. 4)
θ = S / (2 (R-n) ) radians
m = the minimum set-back distance from the centre-line of the road to sight obstruction in metres at the middle of the curve.
R= radius of centre-line of the road in metres
n = distance between the centre-line of the road and the inside lane in metres
S = sight distance in metres.
For applying the above relationship, sight distance is measured along the middle of inner lane. However on single-lane roads, sight distance is measured along centre-line of the carriageway and ‘n” is taken as zero.
R = Radius of curve
S= sight distance
m = minimum set back distance.
n = distance between centre-line of carriageway and centre-line of inside lane.
(13) Passing places:
In order to pass a vehicle against a convoy and to toe aside the disabled vehicle, the passing places are to be provided on the hill roads. There should be two passing places in every kilometer length of road and the dimensions of the passing place should be as follows:
Length on the inside edge … 30 m
Length on the outside edge … 15 m
Width …………….. 3.75 m
(14) Vertical clearance:
The minimum vertical clearance of 5 m should be ensured for overhanging cliff or any structure coming above the road. The vertical clearance should be measured from the highest point of the carriageway, i.e., the crown of the super elevated edge of the carriageway or as the case may be.
(15) Lateral clearance:
The full roadway width at the approaches should be carried through the underpass. It means that the minimum lateral clearance (ie., the distance between the extreme edge of the carriageway and the face of nearest support whether a solid abutment, pier or column) should be equal to the normal shoulder width. In low category hill roads having narrow shoulders, it is desirable to increase the roadway width at the underpasses.
PROTECTIVE WORKS FOR HILL ROADS
In order to give stability and a sense of safety to the hill roads, the following three types of protective works are provided:
(1) Retaining walls
(2) Breast walls
(3) Parapet walls.
- Retaining walls:
The formation of a hill road is generally prepared by the excavation of the hill and the material which is excavated is dumped or stacked along the cut portion. The retaining wall is constructed on the valley side of the roadway to prevent the sliding of backfilling as shown in fig.5. Thus the main function of a retaining wall for hill roads is to retain the back filling and it is provided at the following places:
- at all re-entrant curves;
- at places where the hill section is partly in cutting and partly in embankment; and
- at places where the road crosses drainage.
Where stones are economically and easily available, it is customary to construct the retaining walls in dry stone masonry as it permits easy drainage of seeping water. The design of retaining walls is based on rules-of-thumb and the performances of similar existing retaining walls. The minimum width of 600 mm is kept at the top. The rear side is kept vertical. The front side is provided with a batter of 1 in 4. If the height of the retaining wall exceeds 6 m or so, the bands of coursed rubble masonry in cement mortar at vertical and horizontal intervals of about 3 m are constructed to grant additional stability to the wall.
To facilitate the drainage of the water behind the retaining wall, suitable weep holes at vertical height of 1 m and horizontal spacing of 1.2 m are provided with slope outwards.
- Breast walls:
The cut portion of hill is to be prevented from sliding and the wall which is constructed for this purpose is known as breast wall. See fig. 5. The breast walls are provided with a front batter of 1 in 2 and a back batter of 1 in 3. The back batter may be provided either in one straight batter or in the form of projections. If the height of the wall is less than 2 m, the entire section is made in random rubble stone masonry. If the height of wall exceeds 2 m, the top portion of 2 m height alone is made in random rubble masonry and the remaining portion is constructed in cement mortar of proportion (1:6).
The weep holes, as in case of retaining walls, are provided with slope outwards and sometimes, the vertical gutters connecting the weep holes to the side drain are provided.
- Parapet walls:
The parapet walls are usually provided all along the valley side of the road except where the hill slope is very gentle. They are constructed immediately above the retaining wall, as shown in fig.5 and they prevent the wheels of the vehicles from coming on the retaining wall. It is to be noted that the construction of a parapet wall merely gives a sense of security to the driver and the passengers and it is very rare unless constructed in stone masonry with cement mortar that they act as protecting structures in the event of an accident.
The parapet walls are usually of wall type with uniform thickness of 600 mm and height of 600 mm above the berm level. They can also be constructed of R.C.C. posts of 150 mm x 150 mm section with 1 m height above ground level and 450 mm below ground level and spaced at 1 m centre to centre. In case of hard rocky stratum, the parapet walls may be replaced by the railing of cast-iron.
DRAINAGE IN HILL ROADS
The rain falls very heavily on the hills and as the slopes of hills are quite steep, the water reaches the roadside very quickly and creates drainage problems. The water thus collected should be disposed-off in a proper way through the well-planned and designed drainage system.
(1) Sub-surface drainage:
The seepage flow of water on hills creates problems during and after monsoons. The level of seepage water may be at, above, or below the road level depending upon several factors such as depth of hard stratum and its inclination, the quantity of underground flow of water, etc. The seepage flow also causes the weakening of the roadbed and the pavement and it also causes problems of slope stability. It is, therefore, necessary to control the seepage flow by adopting the suitable method of the sub-surface drainage system.
(2) Surface drainage:
For carrying the surface water, the side drains are provided only on the hill side of the road, as shown in fig. 5. There is limitation in the formation width of road and hence, these drains are constructed of such a shape that the vehicles could utilize the space of side drains in case of an emergency for crossing or parking. The side drains are usually of the following three types:
- angle side drains as shown in fig. 6;
- kerb and channel side drain as shown in fig. 7; and
- saucer side drain as shown in fig. 8.
In order to prevent the side drains from overloading and thereby causing the road surface flooding, the following two measures are taken:
- provision of catch water drain or intercepting ditch above the side drain; and
- suitable cross-drainage work to divert the water through the road on downside of the hill.
Fig. 9 shows the layout plan of catch water drain, sloping drain and cross-drainage work. The water from the hill slope is intercepted and diverted through the catch water drains which are running parallel to the roadway. The catch water drains are usually provided with a gradient of 1 in 50 to 1 in 33 to avoid high water velocity and possible wash out. The water from the catch water drains is led to the cross-drainage works through the sloping drains.
The cross-drainage works are in the form of culverts, scuppers or causeways. They are constructed under the road and usually at right angle to it. For collecting the stones and debris and for preventing scour, the catch pits may be provided at the head of small cross drains. The floor level of catch pit may be kept about 300 mm below the sill of the culvert.
MAINTENANCE OF HILL ROADS
The hill roads because of their peculiar location require careful attention in their maintenance. For the purpose of convenience, the maintenance problems of the hill roads can be grouped into the following four categories:
(1) Control of avalanches
(2) Drainage structures
(3) Prevention of land slides
(4) Snow clearance.
Each of the above category will now be briefly described.
- Control of avalanches:
An avalanche indicates a large mass of loosened snow, earth, rocks, etc. which suddenly and swiftly slides down a hill. Where there are chances for an avalanche to occur, suitable remedial measures may be adopted so that minimum damage occurs to the road structures. One of such preventive measure which is commonly adopted is the construction of galleries above the road which permit the avalanche to slide over the gallery roof without inducing impact loads.
- Drainage structures:
The drainage structures such as catch water drains, catch pits, side drains and culverts are to be periodically inspected and cleaned off all the debris and blockages which prevent the smooth flowing of water in such structures during rains.
As a precautionary measure, the upper slopes are planted with trees to reduce considerably scouring action of unstable ground due to rains.
- Prevention of land slides:
The term land slide is used to indicate the downward and outward movement of slope-forming materials composed of natural rock soils, artificial fills or combinations thereof. The landslides move along the surface of separation by falling, sliding and flowing.
When the shear stresses exceed the shear strength of the soil, the movement in the form of land slide occurs. Hence, anything which contributes towards a decrease in shear strength of the soll or an increase in the shear stress can cause a land slide.
The decrease in shear strength of the soil takes place mainly due to the following causes:
- decrease in inter-granular pressure;
- formation of faults in bedding planes of strata:
- hair-cracking due to alternate swelling and shrinkage of the soil structure;
- increase in water content and consequent swelling and increase in pore water pressure;
- seepage pressure of percolating ground water; etc.
The increase in the shear stress takes place mainly due to the following causes:
(i) external loads due to traffic:
(ii) increase in water or moisture content;
(iii increase in weight due to accumulation of snow:
(iv) removal of part of mass of excavation or removal of retaining wall or increase in slope angle: (v) shocks and vibrations due to earthquakes or blasting;
(v) undermining due to excavation or erosion; etc.
For prevention and correction of land slides, the commonly adopted techniques are as follows:
- construction of buttress at toe and providing suitable retaining structures;
- effective drainage measures to intercept and divert water:
- (iii) relocation or changing the position of the highway:
- (iv) slope treatment to minimise the erosion and to improve the stability conditions; etc.
- Snow clearance:
The depth of accumulated compacted snow on the road surface in winter poses a serious problem for its early removal to restore traffic. In the case of heavily snow bound areas, it becomes difficult for the snow clearing party to locate the position of the road and other structures under the snow cover. For this purpose, the snow markers which are in the form of wooden posts with their height marked in meters are fixed before the winter starts along the road next to the parapet walls to mark the outer edge of the road.
The snow clearance is done with the help of machines and extreme care is taken to see that the top surface of the road is not damaged by the movement of such machines. The commonly used machines are motor graders, snow blasts, or wheel dozers. If the thickness of snow is more, the blasting by explosives may also be adopted. On the other hand, if the thickness of snow is less, the snow clearance can be carried out by manual labor only.
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