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Hill Roads

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Introduction• A hill road is one which passes through a terrain with a cross

slope of 25 percent or more.• A terrain can be classified into four groups based on the cross

slope of the country i.e slope approximately perpendicular tothe centre line of the highway alignmento Level or Plain terrain - Cross slope 0 to 10 percento Rolling terrain - Cross slope Greater than 10 upto 25 percent

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o Mountainous terrain and - Cross slope Greater than 25 upto 60 percento Steep terrain - Cross slope Greater than 60 percent

Classification of Hill roads (As per IRC:52-1981)

National HighwaysState HighwaysMajor District RoadsOther District RoadsVillage Roads

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Fig. Classification of Terrain

Classification of Hill roads (Border Roads organization)Class 9 roads (6m wide for 3 tonnes vehicles)Class 5 roads (4.9m wide for 1 tonne vehicles)Class 3 roads (2.45 to 3.65m wide for jeeps)

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• Ruling gradient is the maximum gradient within which the designerattempts to design the vertical profile of a road. Gradients up to theruling gradient are adopted as a normal course in design of verticalalignment and accordingly the quantities of cut and fill are balanced.Hence ruling gradient is also known as design gradient

• The selection of ruling gradient for purpose of design is a complex jobas several factors such as type of terrain, the length of grade, the speed,pulling power of vehicles and presence of horizontal curves areconsidered.

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• IRC Recommendation for ruling gradient values for design in differentterrains• For plain and rolling terrain – 1 in 30 (3.3 percent)• For mountainous terrain – 1 in 20 (5 percent)• For steep terrain – 1 in 16.7 (6 percent)

Floating gradient- This may be defined as a longitudinal slope at whichno tractive effort is involved on the part of the vehicle to maintain designspeed

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5Ref: IRC:SP:48-1988

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6Fig. Elements of roadway (in hills and plains)

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7Fig. Road Land

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Sight DistanceTwo types of sight distance are considered in design of hill roads, these are

a) Stopping sight distance

b) Intermediate sight distance

Design values for stopping and intermediate sight distance for various speed

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Note: The overtaking sight distance is not considered in designof hill roads as it is not normally feasible/practicable on hillroads.

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Width of Carriageway Shoulder and Roadway

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Note:•The roadway widths are exclusive of parapets (0.6m width) and side drain (0.6mwidth)

•In hard rock stretches or unstable locations where excessive cutting may lead toslope failure, width may be reduced by 0.8m on two lane and 0.4m on other roads

•On horizontal curve extra width should be provided on curve

•On roads subject to heavy snow fall, where snow clearance is done over longperiods, roadway width may be increased by 1.5m

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Camber/Cross Fall

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Fig: Camber/Cross slope

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Fig: Typical details of vertical /horizontal clearance

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Horizontal Alignment

Fig. Bad alignment

General

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Note: Breaks in horizontal alignment at cross-drainage structures and sharpcurves at the end of long tangent/or straight section should be avoided.

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Fig. Reverse Curve

Note: Reverse curves if needed should be ensured that there is sufficientlength between the two curves for introduction of requisite transition curve

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Fig. Broken-Back Curve

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Note:Curves in the same direction separated by short tangents is known asbroken-back-curve.

It should be possibly avoided in the interest of aesthetics and safety and

be replaced by a single curve.If this is not feasible, a tangent length corresponding to 10 seconds travel

time must at least be ensured between two curves.

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Note:Compound curves may be used in difficult topography but only when it

is impossible to fit in a single circular curve.To ensure safe and smooth transition from one curve to the other, the

radius of the flatter curve should not be disproportional to the radius of thesharper curve. A ratio of 1.5:1 should be considered the limiting value.

Fig. Compound Curve

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Set-Back Distance

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Fig. Visibility at Horizontal Curve

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Vision Berms

Where there is a cut slope on the inside of the horizontalcurve, average height of sight line can be used as anapproximation for deciding the extent of clearance.

For stopping sight distance , this may be taken as 0.7m.

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line demarcating the set-back distance envelope, either bycutting back the slope or benching suitably. Such aprovision is generally known as better benching or visionberms.

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Fig. Vision Berms

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Blind curves

• Blind curves are those on which sight distance available is less than thesafe stopping sight distance i.e. absolute minimum from safety point of view.

• While clearance of obstructions within the minimum set-back distance

is expected to ensure the minimum sight distance required as perstandards, in hill roads it may not always be possible to ensure this dueto terrain conditions.

• In such cases certain curves will have sight distance less than minimum

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as per standards and hence blind.• In a blind curve there is always the danger of a vehicle not being able to

come to stop before reaching danger point or a vehicle coming from theopposite direction which is likely to collide with it, due to lack of adequate sight distance.

• It has to be ensured that blind curves are accepted only where it is unavoidable and that also rarely in any stretch of road since trafficabilityand safety of a hill road considerably reduced by blind curves.

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Hair-Pin Bends

• This is three-centered compound curve which is designedas a circular curve with transition curves at each end.

• A hair pin bend is located on a hill side having minimum

slope and maximum stability.• Inner and outer edges of roadway of hair-pin-bend should

be concentric with respect to centre line of the pavement.

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• s recommen e y a w ere a num er o a a r-pin-bend have to be introduced a minimum interveninglength of 60m should be provided between the successivebends to enable driver to negotiate the alignment

smoothly.• At hair-pin-bends, preferably, the full roadway width

should be surfaced.

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Compound

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Fig. Hair-Pin Bends

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Hair-pin-bend design criteria (IRC:SP-48-1998

It may be designed either as a circular curve with transition at each end,or as compound circular curve. The following design criteria should befollowed

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Passing Places

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Fig. Passing Places for ODR and VR

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• Passing places are required on single lane hill roads tofacilitate crossing of vehicles approaching from theopposite direction and to tow aside a disabled vehicle so

that it does not obstruct traffic.• They should be provided at the rate of 2-3 per km.• Normal size of passing place is 3.75m wide, 30m long on

Passing Places

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ns e e ge an m ong on e ar er s e.• The exact location of passing places should be judiciously

determined taking into consideration the available extrawidth and visibility.