Road construction activities; otta seal procedure

Activities in road construction

The main activities in constructing a road are:

1. Survey and setting out - final location survey, fixing the centre line, bench marks, and clearing limits (right of way).
2. Site clearance - cutting trees, removing stumps, boulders and topsoil; demolition of structures.
3. Earthwork - excavation in cut, hauling and filling to form the formation level; compaction of embankment in layers (150-200 mm) to 95-97 percent of Proctor density; benching and slope trimming in hills (planned using the mass haul diagram).
4. Drainage and protection works - side drains, catch-water drains, cross-drainage (culverts, causeways), retaining/breast walls, bioengineering for slopes.
5. Subgrade preparation - scarifying, watering, compacting the top 500 mm of soil to specification and checking CBR.
6. Sub-base and base courses - granular sub-base (GSB), WBM/WMM or stabilised base; each layer spread, watered and rolled.
7. Surfacing - prime coat, tack coat, bituminous binder course (DBM/BM) and wearing course (BC, premix carpet, surface dressing, Otta seal), or a cement concrete pavement slab.
8. Shoulders and kerbs - construction of earth/gravel shoulders, kerbs and edge drains.
9. Road furniture and finishing - signs, markings, guard rails, kilometre posts, plantation.
10. Quality control and measurement - field tests, levels, thickness, density and payment measurement, following the Nepal DoR "Standard Specifications for Road and Bridge Works" and IRC/MoRTH specifications.

Construction procedure of Otta seal

Otta seal is a low-cost bituminous surfacing made of a single or double layer of graded natural gravel or crushed stone (maximum size 16-19 mm) rolled into a relatively thick film of soft binder (cutback such as MC-3000, or 150/200 penetration bitumen) sprayed on the base. It originated in Norway (Norwegian Public Roads Administration, Note 31) and is used on low-volume roads in Nepal under DoR guidelines.

Materials: graded aggregate (continuous grading, maximum size 16 or 19 mm; fines below about 10 percent; flakiness below 30 percent) from a crusher or river source; soft binder (150/200 penetration bitumen cut back with kerosene, or emulsion). A prime coat on the base is generally not required, which is one of the advantages of Otta seal.

Construction

1. Prepare the base (WBM, crushed gravel or stabilised base) to a smooth, tight surface; patch defects and sweep it clean (prime only if the specification asks for it).
2. Spray the binder with a distributor at the specified rate (about 1.4 to 2.0 litres per m2 for the first layer, depending on the grading and the base) at its spraying temperature (about 120-150 C for hot binders).
3. Immediately spread the aggregate (about 13-16 litres per m2, slightly damp) by chip spreader or tipper to cover the binder fully.
4. Roll with a pneumatic tyre roller (at least 12 t) with many passes (about 15 or more); the kneading forces the binder up through the aggregate.
5. Broom loose stones back onto the surface during the first days. For a double Otta seal, apply the second layer (binder and a finer graded aggregate) in the same way after the first layer has bedded in under traffic.
6. Allow traffic at low speed. The seal improves with traffic over the first weeks as the binder works up into the aggregate.