In This Section, I am going to discuss on the topic of Gravel road construction.
Various terminologies related with surface road construction have the following definitions:
“Sub-grade” means the zone immediately below the pavement.
“Pavement” means any part or all of the construction comprising base course and surface course
“Base Course” means the material which is placed on the compacted sub-grade to form the lower part of the pavement.
“Surface Course” means the material placed over the base course to form the wearing surface.
“Shoulders” means the fill placed either side of the wearing surface above the base course.
Tolerances for gravel road construction
- The surface level of the layer making up the road are constructed to the levels and lines shown on the drawings or ordered by the Engineer within the following limits:
|Above required Level||Below required Level|
|Surface Course||15 mm||15 mm|
|Base Course||10 mm||10 mm|
|Sub-grade||20 mm||20 mm|
|Side slopes from positions shown on the drawing||0.15 m to one side or other|
|Centre-line of roads||0.25 m to one side or other|
- Where any tolerances are exceeded, the Contractor have to determine the full extent of the area which is out of tolerance and have make good the surface of the pavement course or formation as follows:
If the formation level is too high, it should be retrimmed and recompacted as specified. If the formation level is too low the deficiency are corrected by the addition of suitable material of the same classification or other approved by the Employer’s Representative laid and compacted to Specification.
The layer is scarified to a depth of 75 mm, reshaped with material added or removed as necessary and recompacted all to the Specification.
Grade and Alignment Control of Gravel Road
Grade and alignment control stakes is supplied, set and maintained by the Contractor subject to checking by the Engineer in order that the work is conform to the lines, grades and cross-sections shown on the drawings or ordered by the engineer. The stakes have set in rows on and parallel with the centreline of the roadway and spaced so that string lines may be stretched between them.
Curve radii, super elevation profiles and vertical and horizontal transitions is as shown on the drawings or as directed by the engineer.
Works to be Kept Free of Water
The Contractor make prior arrangements for the rapid removal of any water which may fall on or be discharged into the site of roadworks under active construction or in a partially completed state.
The Contractor should provide where necessary temporary drains, pumps or other means of maintaining the roadworks free from water. Such provision include forming the embankments in such a manner that their surfaces have at all times a sufficient cross fall to enable them to shed water. The Contractor have to ensure that there is no ponding of water adjacent to the road and side drains.
Sub-grade of Gravel Road
Materials for sub-grade is excavated from road side drains or from approved borrow areas or be excess material from excavation in cut to sub-grade level. All unsound material are removed from the sub-grade, which is then be compacted in accordance with the requirement for road embankment in specification.
The sub-grade once it has been finally compacted, shaped and approved, shall be protected and kept well drained. Storage or stockpiling of materials or contractor’s equipment on the formation shall not be permitted.
Traffic permitted to pass over the compacted sub-grade only with the approval of the Employer’s Representative. Use of prepared sub-grade by Contractor’s equipment or public traffic which can damage or cause the sub-grade to deteriorate is not permitted and the Contractor should, at his own expense, repair any soft spots or any damage caused by his operations or by public traffic.
Base Course of Gravel Road
Base course should natural river sand and gravel mixture of consistent grading and strength with the following properties:
The base course material should have following textural gradation:
Nominal Maximum Size 3″
BS Sieve % Passing by weight
No. 7 20–50
No. 36 10–30
No. 200 0 –15
The base course only be constructed provided that the underlying layer conforms to the requirements specified for the layer concerned. Immediately before placing the material, the underlying layer are checked for any damage or deficiencies which shall be made good.
The Contractor should arrange his sequence of transporting, spreading and compacting the pavement so that segregation is avoided and the required compacting is achieved over the full width. Before commencing work the Contractor have to submit his proposals for the working and compaction of road materials to the Engineer for approval. Contractor’s equipment for compaction may consist of sheep foot roller, rubber tyre or vibrating rollers or other suitable plant which will compact the respective materials to their specified densities and as approved by the Engineer.
Surface Course of Gravel Road
The material for the surface course should consist of a well graded crushed stone or natural river gravel, free from vegetable matter, lumps of clay or other deleterious substances. The material conform to the particle size distribution given hereunder:
Sieve Size (mm) % Passing
All clay lumps is excluded but dust or powdered silt shall be added in controlled amounts to achieve the required grading and plasticity modulus.
The surface course should only be constructed provided that the underlying base course layer conforms to the requirements specified for the layer concerned. Immediately before placing the material, the underlying layer is checked for any damage or deficiencies, which have to made good.
The Contractor should arrange his sequence of transporting, spreading and compacting the pavement, so that segregation is avoided and the required compaction is achieved over the full width. Before commencing work, the Contractor have to submit his proposals for the working and compacting of road materials to the Engineer for approval.
Contractor’s equipment for compaction may consist of sheep foot roller, rubber tyre or vibrating rollers or other suitable plant which can compact the respective materials to their specified densities and as approved by the Engineer.
Inspection, Sampling and Testing
While the construction work is in progress, regular inspection, sampling and testing of the Sub-grade base course and surface course should be carried out by the Contractor under supervision of the Engineer.
Five samples, for each 500 m length of road or as determined by the Engineer, have to taken for determining field density. The field density shall not be less than ninety five (95) percent of the optimum dry density determined by Standard Proctor Compaction Test in the Laboratory for the material being compacted. The cost of such tests shall be born by the Contractor.
The Compacted base course and surface course should have a surface smoothness such that waves and irregularities do not exceed 10 mm in 3 m as measured against straight edge, for surface course and 15 mm in 3 m for base course.
The compacted base course and surface thickness is not less than that shown on the drawings. Any areas of the compacted base course or surface course having a compacted thickness or smoothness less than those specified is corrected by scarifying the surface, adding approved material, reshaping, re-compacting and finishing as specified and approved by the Employer’s Representative. Skin patching an area without scarifying the surface to permit proper bonding of the added material are not permitted.
Measurement and Payment of Gravel Road
Measurement for payment for base course and surface course are made of the materials in place to the prescribed lines, grades and dimensions shown on the drawings or as directed by the Engineer to the required degree of compaction.
The volume is measured by taking the area of the compacted course at regular interval of 30 m or as directed by the Engineer and following average end area method. The thickness is measured by making holes into the compacted layer at three points at each cross-section; two on the sides and one at the centre. The average thickness so measured is taken as the thickness of the compacted layer. Alternatively, the thickness can also be determined by taking the differences in the level before and after the construction of the layer in consideration.
Payment for base course and surface course should made at the unit rate per cubic meter of the respective item in the Bill of Quantities. The unit rate should include all cost for preparation of formation, selection of materials, supply control of moisture content and compaction in layers as specified for pavement and shoulder materials; testing and forming to tolerances.