Granular-Sub-base is often used as subbase layer of the pavement . The main role of this layer is to spread evenly the load over the subgrade .The subbase shall consist of a uniform mixture of granular material placed on a subgrade, uniformly moistened, shaped, and compacted, all in accordance with the contract documents.The work shall consist of laying and compacting of well-graded materials on a prepared and approved sub-grade. Materials shall be laid in one or more layers as per line and level, grade and cross section shown in the drawing and as per clause 401 of MORT&H specification.In the following sequence we had to carry out the work:
2. Setting Out.
3. Selection of Material.
5.Methods of Operation.
I) Ministry of Road Transport& Highways Specifications
II) Technical Specifications
III) Relevant contract drawings.
IV) IS 2386 Codes.
2.0 Setting Out
After the layer of Sub-grade has been approved, then line and level are carried out to fix the granular sub-base layer (herein after called GSB layer). Pegs are fixed at regular intervals on the safe side of the road edge where blade of grade will not disturb the peg while blending of the GSB materials. On each pegs top level of GSB layer is marked. The chainage boards and working bench mark shall be set outside the limits of construction areas.
3.0 Selection of Material
The material used in GSB shall be mixture of natural sand, moorum, gravel, crushed stone in specified grading given in Table 400‑1.Crushed slag, crushed concrete, brick ballast and kankar can also be used on the prior approval of the Engineer or competent authority. It shall be free from organic or other deleterious materials. The material for GSB shall be obtained from the approved source and shall be in accordance with clause 401 of MORT&H specification.
3.2 Physical requirement
The material shall have Impact Value 40% Maximum when tested in compliance with IS: 2386(Part 4) or IS 5640. The water absorption value of coarse aggregate shall be determined as per IS: 2386 (Part 3); if the value is greater than 2%, the aggregate shall be tested for wet Impact value (IS 5640).
The Contractor shall before the commencement of the construction of the sub-base course, submit to the Engineer, the results for approval of the laboratory testing on the physical properties defined above. The construction of the sub-base course shall be taken up only upon the Engineer’s approval of the material.
3.3 Strength of GSB
Prior to actual execution, it shall be ensured the material to be used in sub-base satisfies the requirement of CBR and other physical requirements when compacted and finished as per table 400-2.The minimum CBR 30% is required.
The following equipments shall be carried out for the Sub grade Construction:
- Motor Grader
- Vibro Roller (80 to 100 KN) and pneumatic tire roller (200 to 300 KN weight with minimum tire pressure of 0.7 Mpa)
- Water Tanker
- Tractor trolleys
- Survey Kit / instrument
5.0 Method of Operation
5.1 Preparation of GSB
Prior to laying of GSB material, the sub-grade already finished to clause: 301 or 305 of MORT&H as applicable shall be prepared. Approved sub-grade layer is rolled with two passes by smooth wheel roller with slight sprinkle of water.
5.2 Spreading and compacting
The GSB Material is spread with the help of motor grader on the approved layer of Sub-grade in single layer of 200 mm with Grading either IV or V. During spreading and mixing by grader in site, water is sprinkled over the material by water browser mounted on water tanker. Sufficient water is added, taken into account of evaporation losses so that Moisture content of the materials should lie in between 1% to 2% of below optimum moisture content. Immediately rolling starts with vibratory roller of 80 to 100 KN static weight with plain drum or with pneumatic tire rollers of 200 to 300 KN weight having tire pressure of minimum 0.7 Mpa. Rolling is done from lower edge towards upper edge longitudinally for the portion having unidirectional cross fall and super elevation.
Each pass of the roller shall uniformly overlap not less than one third of the track made in the previous pass. During rolling, grade and cross fall shall be checked and any high spot or depressions, which become apparent, corrected by adding or removing fresh materials. The speed of roller shall not exceed 5km/h. Rolling shall be continued till the density of the layer is achieved at 98% of the Maximum Dry Density.
5.3 Arrangement of traffic
During the period of construction, arrangement of traffic shall be maintained as per clause 112.
6.0 Quality Control
Control Tests for Granular Sub-Base: Post Construction
- Tolerance in Surface Level : +10 mm, -20 mm
- permissible undulation (with 3 m straight edge) : 8 mm
- no. of undulations permitted in any 300 m length, exceeding 16 mm:20
Control on the quality of materials and works shall be exercised by the Engineer in accordance with section 900 of MORT&H.
7.0 Work Safety : Safety will be provided as per location specific safety plan. Following safety point should be kept in mind when using Motor Grader
1.Drive at a slow speed in congested areas.
2.Give the right-of-way to loaded vehicles. Watch for overhead dangers.
3.Know your work area: check weight limitations, types of surfaces, and clearances.
4.Report defective equipment immediately Select a safe parking area.
5.Remove ignition key when leaving grader.
6.Ground the blade when leaving grader unattended.
7.Use colored flags at each end of mould board when blading
8.0 Environmental Safety
1.Nearby Streams, water courses, lakes, reservoir shall be protected from contamination by the soil erosion from areas exposed during excavation by constructing temporary berms, dykes, sediment Basins, slopes, drains & by the use of temporary mulches, seeding or other control devices.
2.Vehicles / tracks shall be kept moist to prevent flying of dust particles with the vehicle movement.