Hill Roads

Hill roads are built in steep, unstable and rain-fed terrain, so they face many more problems than roads in the plain. The main problems are:

Design problems

1. Alignment and geometry: steep gradients (ruling about 6%, limiting 7 to 8%), many sharp bends (hairpin bends) with small radius and extra widening and superelevation, and short sight distances.
2. Gradient compensation on curves and the need to limit the maximum gradient to reduce vehicle operating costs.
3. Cross-section: limited width, partial cut and fill and retaining structures, and hard choice of formation width.
4. Drainage design: heavy runoff, many streams and gullies; many culverts and catch drains are needed.
5. Slope stability and the design of retaining and breast walls, with limited site data.
6. Choice of cut-fill balance and disposal of spoil.

Construction problems

1. Difficult access: transporting materials, equipment and labour to remote steep sites, so costs are high.
2. Rock and boulder excavation, blasting needs care to avoid shattering of the hill.
3. Landslides, rockfalls and slope failure, especially in the monsoon in weak geology (Siwalik and Himalayan fold belt), and earthquakes.
4. Heavy rain and short working season, flooding and erosion.
5. Spoil disposal and the environmental damage to slopes and rivers.
6. Shortage of suitable material, labour and skills; quality control is hard.
7. Maintenance: frequent landslide clearing, drain cleaning and repairs; high recurrent cost.
8. Environmental and social issues: deforestation, impact on water sources and villages.

Good practice follows NRS 2070 and DoR's Environmental and Social Manual and the principles of "green roads": simple, low-cost construction, bio-engineering, and proper drainage.

The cross-section of a hill road depends on the slope of the natural ground, the type of soil or rock, and the cost. The typical types are:

1. Cross-section in cutting (full cutting)

The road is wholly cut into the hill. The cut slope is made stable by flattening (1:4 in rock and 1:1 or 1:1.5 or flatter in soil) and benching, with a catch drain above. It is suited to steep, rocky slopes; it is the safest in fill terms, but spoil must be disposed of and the cost of excavation is high.

        catch drain
          \_/___
            \    \  cut slope
             \    \
    side drain\____\___ road ___

2. Cross-section in filling (full embankment)

The road is wholly on fill, retained by a toe wall or side slope. It is used on gentle ground and in valleys, and may need toe walls and slope protection. On steep ground it is avoided, since fill on steep slopes may slide.

        ___ road ___
       /            \  fill slope 1.5:1
 _____/              \_____
 natural ground, benched

3. Balanced cut and fill (side-hill / half-cut half-fill)

Half of the road width is excavated and the spoil is used to build the other half, with a retaining wall (breast wall on the hill side, retaining wall on the valley side). This is the most economical section on moderate slopes (up to about 25 to 30°) since it minimises haulage and avoids spoil.

          breast wall
        ___/|
 cut   /    |__road__|
 _____/     fill  |retaining wall
 hill slope         \_______

4. Cross-section with retaining wall (full width on a shelf)

Used on steep ground with unstable slope or where land is limited. The wall retains the cut on the hill side and the fill on the valley side.

5. Cross-section on rock (shelf or half-tunnel / gallery)

On very steep rock, the road may be a shelf cut with an overhead protection, or a half tunnel. This is expensive and only for exceptional places.

Common elements

• Side drain on the hill side (a minimum width of about 0.6 to 1.0 m), catch drain above the cut, camber or inward slope (reverse superelevation) of the road towards the hill for drainage, with parapet or guard stones on the valley side.
• Cut slopes, fill slopes and wall heights chosen by soil and geology.

In hill road location, the alignment may follow a river (valley) route along the stream or a ridge route along the crest of the ridge.

Merits and demerits in brief

River route: merits easy gradient, direct access to water, materials and settlements; demerits many cross drainage works, flood and scour risk, landslides on the bank, unstable slopes and costly protection.

Ridge route: merits few cross drainage structures, stable and well-drained ground, low earthwork and maintenance cost, wide view; demerits poor access to water and settlements, long detour to reach villages, steep climbs at saddles, exposure to wind, and scarcity of construction water and material.

In practice a combination is used, following the river at lower levels and climbing to a ridge by gradual hairpin bends or switch-backs.

Hill road construction is more challenging than in the plain because of steep terrain, weak geology, heavy rainfall, and limited access. The following special considerations apply (IRC SP:48 and NRS 2070).

1. Alignment and survey: study the geology and drainage and avoid unstable slopes, landslide zones and slip-prone geology. Choose the alignment on stable (rock) slopes and ridges where possible, with the minimum number of hairpin bends and cross-drainage crossings. Follow the contour with a balanced grade.
2. Gradient: ruling gradient is about 5 to 6% (limiting 7%, exceptional 8%) in the mountains; grade compensation on curves; and gradients are kept as steady as possible, with a rest (flat) every 3 km of rising road.
3. Geometric design: reduced design speeds (20 to 40 km/h) and narrow widths; hairpin bends with minimum radius about 12 to 14 m, with a flat approach and a minimum 60 m distance between bends; extra widening on curves; sight distance with setbacks, and parapets or guard stones for safety.
4. Cross-section: cut-and-fill balanced sections, with retaining and breast walls; the road is given an inward slope towards the hill (about 4 to 5%) to drain.
5. Drainage: more extensive than in plain, with catch drains, frequent cross drains and mitre drains, lined side drains, energy dissipation and prevention of concentrated discharge on slopes.
6. Slope stability and protection: use correct cut slopes for each soil or rock, benches, retaining walls, gabions, bio-engineering (vegetation, "green road" approach), and control of spoil disposal (put in proper tipping sites).
7. Construction methods: controlled blasting, labour-based methods, and phased works; plan to avoid monsoon.
8. Pavement: use light, flexible pavements with good sub-surface drainage; stage construction where necessary.
9. Hazards: allow for landslides, debris flows, seismic effects and flash floods; include protective works, warning signs and maintenance plans.
10. Environmental and social issues: minimise forest clearance and slope disturbance, and follow DoR's environmental guidelines.

These considerations make hill roads more costly and slower to build, and need competent geotechnical judgement and regular maintenance.

A retaining wall is a structure built to hold back earth (or water) on one side where the ground level changes abruptly, to carry the road on the hill side or to protect the valley-side fill. Types used in road construction are:

1. Gravity walls

Resist the earth pressure by their own weight.

• Dry stone masonry: stone placed without mortar. Cheap, flexible and drains freely, for low heights (up to about 3 m).
• Stone masonry in cement/lime mortar: for heights up to about 6 m, with weep holes.
• Plain concrete: where stone is not available.

2. Semi-gravity walls

Plain concrete with a little reinforcement, thinner than a gravity wall.

3. Cantilever RCC walls

An inverted T or L shaped RCC wall; the stem acts as a cantilever, and the weight of the soil on the heel provides stability. Economic up to about 6 to 8 m.

4. Counterfort and buttress walls

RCC walls with vertical ribs (counterforts, behind the stem; buttresses, in front) at regular spacing, which support the stem. For heights above about 8 m.

5. Gabion walls

Rectangular wire-mesh boxes filled with stones, stacked in steps. They are flexible, permeable, easy to build with local labour and tolerate settlement; very popular in Nepal on hill roads.

6. Crib walls

Interlocking precast concrete or timber members filled with soil or gravel; are flexible.

7. Reinforced earth (MSE) walls

Layers of compacted fill with strips, grids or geotextiles tied to facing panels. They are economical for high walls and perform well in earthquakes.

8. Anchored / soil-nailed walls and sheet piles

For steep unstable cut slopes; use ground anchors or nails.

9. Breast walls

Built on the hill side to support a cut face, and prevent rock fall (as opposed to retaining walls on the valley side).

  Gravity wall     Cantilever RCC       Gabion
     /|              |  stem               ___
    / |              |                    |___|__
   /  |   soil    ___|__ heel             |_____|__
  /___|          |toe  |                  steps

Basis of selection

• Height of the wall and the type and strength of the backfill.
• Bearing capacity and depth of the foundation soil, and settlement.
• Availability of local materials (stone, sand, cement) and skilled labour.
• Cost and construction time.
• Drainage conditions (provide weep holes, filter) and ground water.
• Seismic conditions and flexibility.
• Space available and aesthetics.
• Ease of maintenance and repair.

Drainage of a hill road must remove (a) rain falling on the carriageway and hillside above it and (b) groundwater seeping into the slope. Poor drainage is the main cause of landslides, slips and pavement failure in the hills. Structures follow IRC:SP:48 (Hill Road Manual) and Nepal Road Standard (NRS 2070) / DoR hill-road guidelines.

Surface drainage structures

1. Catch water drain - cut along the hill side, about 5-10 m above the top of the cut slope, parallel to the road. It intercepts runoff from the upper catchment. Lined (stone masonry/concrete) in erodible soil; bed slope generally 1-6%.
2. Side (longitudinal) drain - on the hill side of the carriageway; V-shaped or trapezoidal, lined with masonry/concrete; carries water from the road surface and cut face to the nearest cross-drain.
3. Cross drainage works - culverts (pipe, slab, box), causeways/Irish bridges and small bridges, placed at every natural stream and, on long sections, at spacing of about 50-100 m so that side-drain flow does not build up.
4. Chutes / cascades and drop structures - stepped masonry channels that carry water safely down steep valley-side slopes from culvert outlets.
5. Camber/crossfall - 2.5-3% on the bituminous surface (3-4% on gravel), or inward slope on the hill side, to shed water quickly.
6. Berms, kerbs and spouts on the valley side to direct water away from fill slopes.

Sub-surface drainage structures

1. Cut-off (intercepting) drains - trenches filled with graded filter material and a perforated pipe, placed across the seepage flow to lower the water table.
2. French drains - gravel-filled trenches (with filter fabric or graded sand) for seepage from the cut slope.
3. Horizontal (weep) drains - 50-100 mm perforated pipes driven 10-30 m into the slope with 5-10% upward inclination.
4. Sand/gravel blanket under the subgrade and weep holes (75-100 mm at 2-3 m c/c) in breast and retaining walls.

Layout

   Hill slope (uphill)
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
   \ catch water drain  (5-10 m above cut)
    \_____ 
       \  cut slope
        \ breast wall
   ======\=side drain=====  <- road
   ____camber____ 
        shoulder  \  fill slope
                    \ retaining wall
   culvert  ====>  chute -> natural stream

Water collected in the catch and side drains is led to a cross-drain, passed under the road and discharged by chute or masonry channel into a natural watercourse, never onto the fill slope.

A hairpin bend is a sharp turn of about 180 degrees (deflection angle larger than about 120 degrees) that is introduced on a hill road to gain height when the natural ground slope is too steep for the road to climb within the ruling gradient. The road doubles back on itself in a "U" shape on a spur or gentle spot.

Types

• Circular type - one circular arc of radius R with straight approaches.
• Compound/Transition type - a central circular arc with two transition curves; used for larger radii and where the road is wide.
• Symmetrical and unsymmetrical hairpins depending on layout around the apex.

Design and location (IRC:SP:48; NRS 2070 follows similar limits)

• Locate on stable, gentle (preferably < 30 degrees) and well-drained hill slope, on a spur or bench, never on a landslide-prone or unstable slope; avoid cutting a deep slice.
• Minimum radius of the inner curve about 14 m (IRC). Curves are widened (for example 3-4 m at inner edge) so that vehicles can turn.
• Gradient on the curve: not steeper than 1 in 40 (2.5%) on the hairpin; approaches within the ruling gradient (about 5-6%).
• Superelevation up to 10% (max), not more than 1 in 10; the full superelevation is achieved by rotating about the inner edge.
• Minimum 'intermediate' straight of 60 m between consecutive hairpins (and between hairpin and other curves) if possible.
• Provide good drainage, retaining/breast walls, and parapets; sight distance and adequate carriageway width (7.5 m or more including widening).
• Avoid more than a few hairpins in a series; if needed, space them well apart.

Hairpins increase the road length, construction cost and vehicle running cost, so they should be used only when a longer-than-direct alignment (developing length by switchbacks) cannot be avoided.

A river route (valley route) is a highway alignment that follows a river valley instead of crossing the ridges. It is common in Nepal's hills because river banks give a gentle, continuous gradient and fewer cross-drainage works.

Basic considerations in locating it

1. Above flood level - the road formation should be kept above the Highest Flood Level (HFL) with adequate freeboard (about 0.6-1.0 m); avoid sections that are frequently flooded.
2. Bank stability - choose the stable (concave/outer, rocky, or terraced) bank; avoid sections where the river is cutting, shifting or undercutting the bank. Provide bank protection (gabion, spurs, retaining walls).
3. Gentle ground and gradient - follow a gradient close to that of the river, avoiding unnecessary ups and downs. Good valley slopes give easy construction.
4. Geology and slope stability - avoid landslide, rock-fall and debris-flow areas, old slips and weak rock; cut as little as possible.
5. Crossing of tributaries - cross them at the upper, narrow, stable reaches at the best bridge site (rock abutments, a narrow channel) and at right angles to the flow; minimise the number of crossings.
6. Choose the side with less cross drainage, less settlements, and with a bridge-free side wherever possible.
7. River training - the road should not constrict the river; keep a safe distance from river meanders.
8. Material and economy - local aggregates from the riverbed, low earthwork, least cost over the life cycle, minimum land acquisition, and sustained access to settlements.
9. Environment and flood plains - avoid prime agricultural land, protect the river ecology, and follow the NRS 2070 and IRC:SP:48 environmental requirements.

Hill road

A hill road is a road built in hilly (mountainous and steep) terrain where the natural cross slope of the ground is steeper than 25% (that is, mountainous terrain 25-60% and steep terrain above 60%), as per the terrain classification of IRC and the Nepal Road Standard (NRS 2070). Such a road needs cutting and filling on side slopes, retaining structures, many drainage works and hairpin bends.

Factors to be considered in design

1. Terrain and topography - cross slope, ridge and valley pattern, and available spurs and benches decide alignment and earthwork; the terrain class decides design speed (typically 20-40 km/h for mountainous and steep terrain).
2. Geology and soil - rock type, bedding, dip, joints, faults and weak zones control slope stability and cut slope angles. Avoid landslide zones; choose the more stable side of the hill.
3. Hydrology and drainage - rainfall intensity, stream crossings, springs and seepage determine number and size of culverts, catch drains and sub-surface drains. Drainage is the most critical element in hill roads.
4. Climate - temperature, snow, frost, rainfall and wind affect alignment (sunny slopes preferred), gradient and pavement choice.
5. Geometric standards - ruling gradient 5% (limiting 6%, exceptional 7%; hairpin 1 in 40), minimum radius (IRC: 14 m on hairpins), curve widening, superelevation up to 10%, and sight distance for the low design speed. Roadway width is reduced (single lane 3.75 m with passing places, or intermediate width) where traffic is low.
6. Traffic - volume, type, axle loads, future growth; decides single/double lane, carriageway and pavement type.
7. Stability measures - slope protection (bio-engineering), breast and retaining walls, gully control.
8. Construction and materials - access for machinery, availability of local stone and aggregate, labour, balance of cut and fill, spoil disposal.
9. Economy and environment - least cost over life cycle, low environmental damage, social benefit, land acquisition and maintenance cost.

The alignment of a hill road is controlled by the following factors (IRC:SP:48, NRS 2070).

Factors controlling alignment

1. Topography - an alignment on a gentle slope with a minimum of cut and fill is chosen; use spurs and benches; avoid crossing deep gullies.
2. Geology and soil - stable rock or soil, no slip zones, favourable dip of strata.
3. Drainage - minimum number of stream crossings at good bridge sites; avoid water-logged and seepage areas.
4. Hydrology/rainfall and climate (below).
5. Traffic and design speed which fix the maximum gradient and the geometric standards.
6. Gradient and curves - keep the ruling gradient; use hairpins only where height must be gained.
7. Economy and construction - cut-fill balance, access for equipment, availability of materials.
8. Environment and settlements - villages served, forests, farmland and cultural sites.

Effect of temperature, rainfall, pressure and geology

• Temperature: in cold regions sun-facing (southern) slopes are preferred so that snow and ice melt early, and frost action and icing are reduced. Frost heave needs a good sub-base and drainage. On hot, sunny slopes the road is drier and more stable.
• Rainfall: heavy rainfall demands good drainage and stable slopes. Choose slopes that are not receiving large runoff; avoid slopes with heavy rainfall-triggered landslides; roads on the leeward (less rainy) side are less affected. Keep the road away from gullies and streams, and provide catch drains.
• Pressure (wind/atmospheric): at high altitude the air pressure is low, so engine power drops and vehicles climb poorly, so a flatter ruling gradient (reduced with altitude above 3000 m) is used. Strong wind and snow drifting favour sheltered, lee-side alignment.
• Geology: choose hard, sound, well-bedded rock with joints dipping into the hill; avoid fractured rock, faults, loose scree, old landslides and clay beds that dip towards the valley. Stability governs cut slope angle and the number of retaining structures.

How surface water is collected and disposed of

Rain falling on the hill above the road and on the carriageway is collected before it can enter the road structure and then carried across or away from it.

1. Catch water drain (uphill, 5-10 m above the cut) intercepts the water from the upper hillside and leads it to a natural stream or cross-drain.
2. Side drain (V or trapezoidal, lined) along the hill-side edge of the road collects water from the cut slope and the carriageway (camber 2.5-3% towards the hill, or crown with cross-fall).
3. Cross drainage - culverts and small bridges take the water of the side and catch drains across the road at close intervals (about 50-100 m or less, and at all natural nallahs).
4. Outlet - the culvert outlet is connected to a chute/cascade, drop wall, or stilling basin to discharge into the natural gully without scouring the fill slope.
5. Valley side - berms and kerbs on high fills guide water to outlets; the road surface is kept sealed.

 UPHILL ~~~~~
   catch drain =====>--+
        \              |
   cut slope           v
   ----side drain--> culvert ---> chute
   =======road=======      \
        fill slope          \__ stream

Special problems of surface-water drainage in hill roads

• Steep slopes give high velocity and scouring of drains, fill slopes and outlets; lining and drop structures are needed.
• Concentrated flow and heavy rainfall in short time; many natural streams and gullies to cross.
• Water entering cracks and cut faces causes landslides, slips and erosion.
• Large sediment/debris load blocks drains and culverts; frequent cleaning is needed.
• Limited space for drains on narrow benches; difficult disposal because no place to discharge safely.
• Snow melt and springs add flow in cold areas.
• The cross-drainage works are many, costly and difficult to build on steep terrain.

A catch water drain (catch drain) is a lined or unlined open channel excavated on the hillside above the top of a cut slope, roughly parallel to the road, which intercepts the surface runoff from the upper catchment before it reaches the road.

Role and necessity

• Protects the cut slope from erosion and from saturation by running water; the slope remains stable, which reduces landslides.
• Reduces the load on the side drain and cross-drainage structures, because only water from the road and cut face has to be carried by them.
• Prevents water from entering cracks and tension cracks on top of the slope, which could trigger slips.
• Keeps the pavement and formation dry, protecting the subgrade strength and the road life.
• Diverts the water safely to the nearest natural stream or cross-drain through a stable outlet.

Practical details (IRC:SP:48 / NRS 2070)

• Placed about 5-10 m (at least 3-5 m) beyond the crest of the cut.
• Trapezoidal or rectangular, lined with stone masonry or concrete in soft soil; bed gradient about 1-6% so that it neither silts nor scours.
• The hill-side edge is bunded with the excavated material, and the outlets are protected by chutes.

Gully control and land stabilisation

A gully is a deep channel cut by concentrated surface flow on a slope. Gully control means stopping the growth of such channels and filling them, by reducing the flow velocity and trapping sediment. Land stabilisation means making the unstable slope or gully bank stable by engineering structures and vegetation (bio-engineering) so that erosion and landslides do not continue and endanger the road.

Gully control structures / methods (any four)

1. Check dams (gabion, stone masonry or brushwood) - small cross-barriers placed in the gully at intervals so that the toe of the upper dam is at the same level as the crest of the next lower dam. They reduce the bed slope, slow the flow, and trap sediment.
2. Drop structures / chutes - masonry or concrete stepped channels that lead the flow safely down a steep gully; energy is dissipated in stilling pools.
3. Diversion drains and catch drains - intercept and divert the runoff from the gully head, reducing the discharge entering it.
4. Vegetative and bio-engineering measures - planting grass (vetiver, napier), shrubs and trees on gully banks and bed; brush layering, live check dams, fascines and jute/coir netting hold the soil and bind the slope.
5. Gabion walls and re-grading of gully sides - regrade steep banks to a stable slope (1:1.5 to 1:2) and protect the toe.

  upper  ___           flow ->
        |   \__ check dam 1
         \       \___ 
           \  silt   check dam 2
 toe of dam 1 level with crest of dam 2

These works are used with proper drainage so that the road is not damaged by gully extension.

A breast wall is a wall built on the hill side of a hill road, against a cut slope, to support the slope face and prevent small slips, rock fall, and weathering or erosion of the cut face. It is not intended to hold a full thrust of a retained mass of fill, but to stabilise the cut face and the slope material over it (as required by IRC:SP:48).

Features

• Constructed in stone masonry (dry or in cement/lime mortar), plain/reinforced concrete or gabion; the wall rests on firm rock or soil.
• Height typically 3-5 m; if the cut is higher the wall is built in tiers with berms.
• Weep holes (75-100 mm, at about 2-3 m spacing) and a filter/drainage layer behind it release the pore water pressure.
• The back face is kept very close to the cut slope (face batter about 1 in 4 to 1 in 3 front slope); the space behind is filled with the excavated material or packed with stones.
• The top of the wall is connected to the drain system, or the side drain is at its base; the top of the cut above is protected by a catch drain and vegetation.

   hill slope  \
                \ cut face
                 |#| breast wall
                 |#|  weep holes
   ==============|#|===  road

Functions

• Supports the hillside cut, preventing slides.
• Protects against erosion and weathering.
• Reduces the width needed for cut slopes, saving land and excavation. Where the road is on the valley side, a retaining wall is used instead.

Hill road alignment is located by the following methods (IRC:SP:48; NRS 2070 also recommends an engineering survey procedure).

1. Reconnaissance and desk study

Study of topographic maps, aerial photographs and satellite images to find controlling points (passes, river crossings, settlements) and possible routes. A walk-over reconnaissance then examines the geology, slope, drainage, existing trails and soil conditions.

2. Preliminary survey - "contour/grade line" method

A gradient (grade) line is set on the ground using an abney level/clinometer and ranging rods so that the road stays at the chosen gradient (usually 5-6%) with minimum cut and fill. The method gives a trial line along the contour:

• Ruling-gradient line is set from the starting point; the contour spacing equals the rise per length (for example, 5% gradient at 10 m contour interval gives 200 m of road per contour).
• Where the natural slope is gentler than the ruling gradient, the line is free; where it is steeper, the length is developed using hairpin bends.

3. Method of ridge/valley (route classification)

• Ridge line (spur) alignment - follows the ridge; stable and well drained, but long.
• Valley / river route - follows a river; low gradient but flood and bank problems.
• Mid-slope alignment - along the hillside; most common; requires side-hill cutting and retaining walls.

4. Final location survey

The chosen route is surveyed by traverse and levelling, fixed with pegs; the centre line is marked on the ground with pegs at 20 m (or less) intervals, curves are set out, and the final alignment is checked for the geometric standards.

5. Modern methods

Photogrammetry, GPS, Total Station and GIS/DEM-based route selection can be used for the study and optimisation of routes.

Route location in hill roads is the process of choosing a stable, economical alignment between the fixed end points over difficult, steep and unstable terrain.

Steps

1. Identify control points - the end points, mountain passes, saddles, bridge sites on rivers, settlements and market centres, and points to avoid (landslide zones, unstable slopes, forests, religious sites).
2. Study of maps and photographs - topographical maps, aerial photographs, satellite imagery and geological maps give possible routes.
3. Reconnaissance - walk the corridor to assess slope, geology, drainage, water sources and construction material.
4. Selection of route - the choice between a ridge, a mid-hill or a valley (river) route according to the following.

Main principles of hill road location

• Prefer sunny, stable and dry slopes; avoid shaded, wet slopes.
• Gradient near the ruling gradient (5%; limiting 6%); use hairpins only to gain height (spaced well, on stable ground).
• Keep the cross-drainage crossings as few as possible and at the best sites (right angle to the flow).
• Keep cut and fill balanced and the cutting height lower than the stability limit; avoid deep cuts and high fills.
• Avoid landslides, scree, rock falls, springs and unstable geology.
• Take minimum land and environmental damage, and keep the cost low for construction and maintenance.
• Provide for future widening and stage construction (NRS 2070 recommends the engineering survey followed by a detailed geotechnical study).

The formation (roadbed) and the cross slopes (cut slope above and fill slope below) of a hill road are unstable because of steep ground, rain and the removal of toe support. The following measures are used (IRC:SP:48, DoR/NRS 2070 guidelines).

1. Drainage measures (most important)

• Catch water drains, side drains, cross-drains, chutes and cascades to keep surface water away.
• Sub-surface drains (French drains, horizontal drains, cut-off drains) and weep holes to reduce pore pressure.

2. Cut slope stabilisation

• Flatten the cut slope to a stable angle (for example 1:4 to 1:1 depending on rock/soil), cut in benches/berms (about 1.5-2 m wide every 6-9 m height) with a drain on each berm.
• Breast walls (stone masonry or gabion), rock bolts, shotcrete and wire-mesh for loose rock; rock-fall nets and catch ditches.

3. Fill slope and formation stabilisation

• Retaining walls (masonry, gabion, reinforced earth) at the toe of high fills; toe walls and buttresses.
• Fill compacted in thin layers on benched ground; use rock-fill or free-draining material; keep fill slope 1:1.5 to 1:2; provide a berm on high fills.
• Avoid dumping spoil on the downhill slope.

4. Bio-engineering and vegetation

Grass planting (vetiver, napier), shrubs and trees, brush layering, live fascines, jute netting, turfing and bamboo planting bind the soil, reduce erosion and are cheap, environment-friendly and recommended in NRS 2070 and the DoR Green Roads approach.

5. Other measures

• Gully control - check dams and drop structures.
• Slope protection - stone pitching, shotcrete, gabion mattress, geotextile.
• Control of landslides by removal of load at the head, loading at the toe, and grouting.
• Regular maintenance and monitoring after the monsoon.

Hill roads need structures to hold the soil and to protect slopes from erosion and failure (IRC:SP:48; Nepal DoR hill-road guidelines).

A. Retaining structures

1. Retaining walls - on the valley side to support fill and the road; types:
   • Stone masonry gravity wall (most common; dry or in cement mortar, height up to 6-8 m);
   • Gabion wall (wire crates filled with stone; flexible, permeable, cheap and good for settling ground);
   • Concrete/RCC cantilever or counterfort wall for greater heights;
   • Reinforced earth / geosynthetic wall.
2. Breast walls - on the hill side, supporting the cut face (see below).
3. Toe walls and buttresses - stabilise the base of the slope.
4. Crib walls (timber/concrete) - interlocking members filled with soil. All walls must have weep holes and a filter backfill, and be founded on firm ground; designed against sliding, overturning and bearing failure.

B. Slope protection structures

1. Stone pitching/turfing on embankment and cut slopes.
2. Shotcrete/guniting and wire-mesh on rocky cut slopes.
3. Gabion mattresses and river training works (spurs, revetments) along rivers.
4. Rock-fall protection - rock-fall nets, catch ditches, and fences.
5. Bio-engineering - grass, shrubs, tree planting, brush layering, fascines.
6. Check dams, drop structures and chutes for gully control.
7. Drainage works - catch drains, side drains, French drains.

 hill ->  |breast wall|  road  |retaining wall| <- valley
          cut slope             fill slope

Selection depends on the height, soil/rock type, availability of stone, water, and cost; gabion and masonry walls are the commonly used in Nepal.

Road-side structures for stability of the roadway

1. Retaining walls (masonry, gabion, RCC) on the valley side to hold fills.
2. Breast walls on the hill side to support cuts.
3. Drainage structures - catch water drains, side drains, culverts, chutes, French drains and horizontal drains to keep water out of the slope.
4. Toe walls, buttresses and parapets (guard walls) at the valley edge.
5. Rock-fall and slope protection - rock nets, shotcrete, catch ditches, turfing, stone pitching.
6. Gully control works - check dams, drop structures.
7. Bio-engineering works over the exposed slopes.
8. Berms and benches on high cuts.

Stabilising landslides along the road

First investigate: type of slide (rotational, translational, debris flow, rock-fall), cause (water, loading, toe cutting), extent, depth of slip plane, and monitor movement.

1. Drainage control (most effective) - divert surface water with catch drains and lined channels around the slide; remove subsurface water with horizontal drains, French drains, and cut-off trenches; seal cracks.
2. Geometric changes - unload the head (remove soil from the top), load the toe with a berm, flatten the slope, or construct benches.
3. Retaining structures at the toe - gabion/masonry walls, piles, anchors, and buttresses to resist the slide.
4. Soil reinforcement - soil nails, rock bolts, ground anchors, geogrids; grouting of fractured rock.
5. Bio-engineering - grasses, shrubs and trees to bind soil and absorb water, plus jute net, bamboo, and brush layering.
6. Realignment - if the slide is large, shift the road away or bridge over it; or use a bypass.
7. Maintenance - regular inspection, cleaning of drains and early repair after each monsoon.

Hairpin bend

A hairpin bend is a sharp bend of about 180 degrees on a hill road that is used to gain height when the road has to turn back along a spur. It consists of a central circular curve (radius $R$, minimum about 14 m as per IRC:SP:48) with a transition curve of length $L_s$ at each end.

Symmetrical hairpin with transitions: elements

Let the two straight approaches meet at the apex (intersection point) $I$ with an interior angle $\alpha$. The deflection angle is

$$\Delta = 180^\circ - \alpha$$

For a hairpin $\Delta$ is large (about 120 to 180 degrees).

1. Angle turned through by each transition

For a spiral of length $L_s$ ending in radius $R$, the spiral angle is

$$\theta_s = \frac{L_s}{2R} \ \text{(radian)}$$

2. Central circular arc angle

$$\Delta_c = \Delta - 2\theta_s$$

3. Shift of the circular curve

$$s = \frac{L_s^2}{24R}$$

4. Tangent length (from apex $I$ to the start of the transition, $T_s$)

The shifted circle has radius $R$ and the tangent to it from $I$ is $(R+s)\tan\frac{\Delta}{2}$. Adding the spiral offset $L_s/2$:

$$T_s = (R+s)\tan\frac{\Delta}{2} + \frac{L_s}{2}$$

5. Length of the circular arc and total curve length

$$L_c = R\,\Delta_c = R\,(\Delta - 2\theta_s)$$ $$L = L_c + 2L_s = R\Delta - L_s + 2L_s = R\Delta + L_s$$

(with $\Delta$ in radians).

6. External distance (apex distance)

$$E = (R+s)\sec\frac{\Delta}{2} - R$$

7. Chainages

$$\text{Start of curve} = \text{Ch}(I) - T_s,\qquad \text{End of curve} = \text{Start} + L$$

   straight          apex I
 -----------\       /------
             \  __/
   TS ~~~~~~~ SC  CS ~~~~ ST
              ( R )

Design checks (IRC:SP:48)

Superelevation up to 10% (1 in 10) developed over $L_s$; widening provided on the inner side; gradient on the curve not more than 1 in 40; minimum radius of the inner curve 14 m (IRC) and at least 60 m of straight between consecutive hairpins.

Special structures in hill roads

1. Retaining walls - valley side, to hold fill and to widen the road.
2. Breast walls - hill side, to support cut faces and avoid slips.
3. Revetments and toe walls - protect slopes and river banks from scour.
4. Gabion structures - walls, mattresses and check dams in wire crates; flexible and permeable.
5. Check dams, drop structures, chutes/cascades - gully control and safe disposal of culvert flow.
6. Cross-drainage works - culverts, causeways, Irish bridges and small bridges.
7. Retaining/catch ditches, rock-fall nets and shotcrete for rock slopes.
8. Hairpin bends and passing places (lay-bys) on narrow roads.
9. Parapet/guard walls and crash barriers on the valley edge.
10. Bio-engineering works - grass, shrubs and brush layering.
11. Sub-surface drainage - French drains, horizontal drains, weep holes.
12. Tunnels, snow galleries and avalanche shelters in very difficult or snow-bound areas.

Sketch of the drainage layout in hill road

  hill ~~~~~~~~~~~~~~~~~~~~~~
   catch water drain ===>   (to nala)
         \
          \ cut slope
           \ breast wall
    side drain ---> culvert ---+
    ==== road =================  |
          \ retaining wall      v
           \ fill slope      chute
  valley ~~~~~~~~ stream <------+

Flow path: hillside water is intercepted by the catch drain; road and cut-face water goes to the side drain; both are carried through culverts and chutes to a natural stream. Sub-surface drains (French/horizontal) discharge into the side drain or through weep holes.

When a road alignment meets a river, the following crossing alternatives are considered and chosen on traffic, river conditions and cost (IRC and NRS 2070).

1. Ford / drift - the road is taken through the river bed, with a stabilised (paved) section. Cheapest, but can be used only when the river is shallow, the flow is small and the traffic is light; closed during floods.
2. Causeway / Irish bridge (submersible bridge) - a road across the stream with pipes or vents, overtopped in floods. Suitable for seasonal, flashy rivers and low traffic.
3. Culvert / small bridge - for small streams; the cheapest permanent crossing for small flows.
4. Major bridge - slab, girder, truss, arch, cable-stayed or suspension, for all-weather crossing of wide, deep or perennial rivers. High cost.
5. Ferry or ropeway/ suspension footbridge - for very low traffic, temporary or in remote areas (where road cost is not justified).
6. Realignment to a better site - move the crossing to the best site of the river: narrow, straight, stable banks, rock foundation, right angle crossing.

Selection criteria

• Traffic volume and type, and the importance of the road
• Width, depth, discharge and flood characteristics
• Bank stability, foundation conditions
• Cost of construction and maintenance and the delay or interruption from floods
• Skew (best at right angle to the flow), approach roads and bridge length
• Environmental effects

Superelevation (cant) is the raising of the outer edge of the carriageway at a horizontal curve above the inner edge, so that the component of the vehicle's weight balances the centrifugal force.

Superelevation on hill roads (IRC:SP:48; NRS 2070 follows)

• The superelevation rate is

$$e + f = \frac{v^2}{127R}$$

with $v$ in km/h and $R$ in m; $f$ = side friction factor, taken as 0.15.

• Maximum value: 10% (1 in 10) on hill roads for non-snow-bound areas; in snow-bound/ice-prone areas not more than 7% (to avoid sliding of vehicles at low speed on ice).
• Minimum: equal to the camber, 2.5 to 3% (cross-slope of the road surface).
• On a hairpin, the combined gradient (longitudinal grade plus superelevation) must stay within limits, so the longitudinal grade is reduced on the curve.
• Attainment: the superelevation is introduced gradually in the length of the transition (or the length of at least 2/3 on the straight and 1/3 on the curve if no transition), by rotating the pavement about the inner edge (so that the hill-side cut and drain are not affected) at a rate of about 1 in 60 to 1 in 100 (the relative gradient between the edge and the centre line).
• For radii above a limit (when $e$ required is less than camber) the road section remains the normal camber.
• Extra widening is provided on sharp curves along with the superelevation.

For a hairpin, with a small radius (14 m or so), the full 10% superelevation is provided, speed is restricted to 20 km/h, and the curve is on a gentle gradient of 1 in 40 maximum.