Construction of Hill Roads
The forming of hill roads and construction steps are given below :-
Formation Works for construction of hill roads
First it is necessary to construct the formation or the finished profile of hill road. So, as to carry out the next stages of construction such as protective works, drainage works and pavement. However special problems are face while constructing hill roads in steep. Also, very steep terrains including through hard rocks and at high altitudes.
The construction of formation works consist of the following steps:
- Trace-cut
- Jungle clearance
- Earth work excavation through soil and soft rocks
- Rock cutting, drilling, blasting and clearing
Rock Cutting
If the rock strata slope downward into the hill-side. The hard rock is permit to over-hang the road forming a half tunnel. Blasting is doing either from face or from one or both sides. The bore holes for ‘end attack’ and ‘face attack’ for blasting the hard rock. If the strata are inclining towards the hill slope. The cutting is continuing until the inner slope is at a safe angle to prevent slipping. In such a case, blasting and cutting is commencing from top. If the rock is soft or loosely stratifying. The cutting is commence at point on the hill-side far above the formation level for obtaining sufficient batter for the inner slope.
Blowing out the cliff face by firing large mines then dressing the slope, is another alternative. In actual construction, there is great difficulty in removing rock exactly to the require levels and grade. Unless special care is taken, blasting may be slightly short of requirement. It can be over-blasting the drain on hill-side is also formed. Blasted stones are using for retaining walls, back-filling high retaining walls and aprons etc.
Precipice Work for Construction of hill roads
Where the time available does not allow for blasting and tunnel work, ‘cliff galleries’ and ‘ cradles ‘ are resorting for the negotiation of cliffs and precipices’ . These are suitable only for light vehicles or foot traffic. It consider only for short term use and not as a permanent roadway for regular traffic. Precipice Work is important that the strata should dip inwards from the face in order to ensure safe attachment for the jumpers. It hold fasts to lessen the risk of rock falls. If the top of the precipice is inaccessible. It is necessary either to start from below or to work forward from the farthest point of the road. A log or light gangway is projecting forward beyond the end to provide a platform from which the holes are drilled. The supports are fixed in advance.
When boring is not practicable, a light roadway may be carried on timber cradles suspend by means of wire cables from jumpers driven at the top of the cliff. The bearer of the cradle should tilt upwards; otherwise when the cradle draws away from the face from the cliff; the surface will slope outwards and become dangerous.
Retaining Walls
Retaining walls are most important structures in hill road construction. To provide adequate stability to the roadway and to the slope. It constructing on the valley side of the roadway. It also on the cut hill side to prevent land slide towards the roadway.
Dry stone masonry prefers to masonry in mortar as the former permits easy drainage of seeping water. The design of retaining walls is generally base on the thumb rules. The performance of existing structures in similar regions. As a general guideline it is suggest that a section of 0.5 h with a minimum width of 0.45 m to 0.60 m at top and a front batter with the rear side vertical. A typical retaining wall of height less than 6.0 m.
Width of retaining wall
The width of retaining wall foundation is taken as (0.4 h +0.3) m for heights up to 6.0 m and as (0.5 h +0.6) m, for heights exceeding 6.0 m, with a top width of 0.75 m. Also bands of course rubble masonry in cement mortar at vertical and horizontal intervals of about 3.0 m each are constructing for high retaining walls. These bands provide add stability in the case of high retaining walls. Here the rear face is keep vertical and fill with boulders and stones to improve drainage. To resist earth pressure for half the height.
In hilly regions, the presence of hard gravelly soil or even soft rock may sometimes be misleading as these may occur in thin section over-lying sloping rocks. So whenever the presence of hard rock at shallow depth is indicating, the foundations should be taken up to the rock bed. In addition to this, dowel bars are also providing. The foundation bed is provided with a downward slope of 1 in 6 or 1 in 4 towards the hill side.
In the case of soil and soft rock, ‘reinforced earth’ retaining walls are also being adopting after appropriate investigations of the stretches.
Revetment Walls for Construction of hill roads
The embankment slopes are normally protect with rough stone pitching about 0.3 m thick in order to avoid erosion due to flow of water. If the sloping length is too long, it is preferable to construct a toe wall to support the embankment, depending upon the slope available. Where the cutting slope is steep. Then, it contains soil which is loose or can get scouring, slips are likely to occur. In such locations, revetment walls of dry stone masonry are constructing to retain the soil on the cutting side to prevent occurrence of any such slips.
