GRANULAR SUB-BASE
SCOPE
This work shall consist of laying and compacting well-graded material on prepared subgrade in accordance with the requirements of Specifications. The material shall be laid in one or more layers as sub-base or lower sub-base and upper sub-base (termed as sub base hereinafter) as necessary according to lines, grades and cross sections shown on the drawings or as directed by the Engineer.
MATERIALS
The material to be used for the work shall be natural sand, gravel, crushed stone, or combinations thereof depending upon the grading required. The material shall be free from organic or other deleterious constituents and conform to gradings (given below).
While the gradings in Table -1 are in respect of close-graded granular sub-base materials, one each for maximum particle size of 75 mm, 53 mm and 26.5 mm, the corresponding gradings for the coarse-graded materials for each of the three maximum particle sizes are given at Table -2. The grading to be adopted for a project shall be as specified in the Contract.
Physical requirements
The material shall have a 10 per cent fines value of 50 kN or more (for sample in soaked condition) when tested in compliance with BS : 812 (Part 111). The water absorption value of the coarse aggregate shall be determined as per IS :2386 (Part 3); if this value is greater than 2 per cent, the soundness test shall be carried out on the material delivered to site as per IS : 383. For Grading II and III materials, the CBR shall be determined at the density and moisture content
likely to be developed in equilibrium conditions which shall be taken as being the density relating to a uniform air voids content of 5 per cent.
Table -1. Grading for Close-graded Granular Sub-Base Materials
Table -2. Grading for Coarse Graded Granular Sub-Base Materials
Note : The material passing 425 micron (0.425 mm) sieve for all the three gradings when tested according to IS 2720 (Part -5) shall have liquid limit and plasticity index not more than 25 per cent and 6 per cent respectively.
STRENGTH OF SUBBASE
It shall be ensured prior to actual execution that the material to be used in the sub-base satisfies the requirements of CBR and other physical requirements when compacted and finished.
When directed by the Engineer, this shall be verified by performing CBR tests in the laboratory as required on specimens remoulded at field dry density and moisture content and any other tests for the “quality” of materials, as may be necessary.
CONSTRUCTION OPERATIONS
Preparation of sub grade
Immediately prior to the laying of sub-base, the subgrade already finished as applicable shall be prepared by removing all vegetation and other extraneous matter, lightly sprinkled with water if necessary and rolled with two passes of 8 -10 Ton smooth wheeled roller.
Spreading and compacting
The sub-base material of grading specified in the Contract shall be spread on the prepared sub grade with the help of a motor grader of adequate capacity, its blade having hydraulic controls suitable for initial adjustment and for maintaining the required slope and grade during the operation or other means as approved by the Engineer.
When the sub-base material consists of combination of materials mentioned above, mixing shall be done mechanically by the mix-in-place method.
Manual mixing shall be permitted only where the width of laying is not adequate for mechanical operations. The equipment used for mix-in-place construction shall be a rotator or similar approved equipment capable of mixing the material to the desired degree. If so desired by the Engineer, trial runs with the equipment shall be carried out to establish its suitability for the work.
Moisture content of the loose material shall be checked in accordance with IS : 2720 (Part II) and suitably adjusted by sprinkling additional water from a truck mounted or trailer mounted water tank and suitable for applying water uniformly and at controlled quantities to variable widths of surface or other means approved by the Engineer so that, at the time of compaction it is from 1 per cent above to 2 per cent below the optimum moisture content corresponding to IS 2720 (Part VIII). While adding water, due allowance shall be made for evaporation losses. After water, has been added, the material shall be processed by mechanical or other approved means like disc horrows, rotators until the layer is uniformly wet.
Immediately thereafter, rolling shall start. If the thickness of the compacted layer does not exceed 100 mm, a smooth wheeled roller of 8 to 10 Ton weight may be used. For a compacted single layer up to 225 mm the compaction shall be done with the help of a vibratory roller of minimum 8 to 10 Ton static weight with plain drum or pad foot-drum or heavy pneumatic tyred roller of minimum 200 to 300 kN weight having a minimum tyre pressure of 20.7 MN/m or equivalent capacity roller capable of achieving the required compaction. Rolling shall commence at the lower edge and proceed towards the upper edge longitudinally for portions having unidirectional cross-fall and super elevation and shall commence at the edges and progress towards the center for portions having cross-fall on the both sides.
Each pass of the roller shall uniformly overlap not less than one third of the track made in the preceding pass. During rolling, the grade and camber shall be checked and any high spots or depressions which become apparent corrected by removing or adding fresh material. The speed of the roller shall not exceed 5 km per hour.
Rolling shall be continued till the density achieved is at least 98% of the maximum dry density for the material determined as per IS : 2720 (Part 7). The surface of any layer of material on completion of compaction shall be well closed, free from movement under compaction equipment and from compaction planes, ridges, cracks or loose material. All loose, segregated or otherwise defective areas shall be made good to the full thickness of layer and recompacted.
Surface Finish and quality Control of Work
The finished surface shall be checked for lines, levels and regularity. The surface evenness of completed surface in longitudinal and transverse direction shall be within the tolerances specified.
MODE OF MEASUREMENTS:
The finished area will be measured. Deduction for openings etc. will be made as applicable to flooring as per IS 1200.
The surface finish of construction shall conform to the requirements. Granular sub-base shall be measured as finished work in position in cubic meters.
The protection of edges of granular sub-base extended over the full formation as shown in the drawing shall be considered incidental to the work providing granular sub-base and as such no extra payment shall be made for the same.
SCOPE
This work shall consist of laying and compacting well-graded material on prepared subgrade in accordance with the requirements of Specifications. The material shall be laid in one or more layers as sub-base or lower sub-base and upper sub-base (termed as sub base hereinafter) as necessary according to lines, grades and cross sections shown on the drawings or as directed by the Engineer.
MATERIALS
The material to be used for the work shall be natural sand, gravel, crushed stone, or combinations thereof depending upon the grading required. The material shall be free from organic or other deleterious constituents and conform to gradings (given below).
While the gradings in Table -1 are in respect of close-graded granular sub-base materials, one each for maximum particle size of 75 mm, 53 mm and 26.5 mm, the corresponding gradings for the coarse-graded materials for each of the three maximum particle sizes are given at Table -2. The grading to be adopted for a project shall be as specified in the Contract.
Physical requirements
The material shall have a 10 per cent fines value of 50 kN or more (for sample in soaked condition) when tested in compliance with BS : 812 (Part 111). The water absorption value of the coarse aggregate shall be determined as per IS :2386 (Part 3); if this value is greater than 2 per cent, the soundness test shall be carried out on the material delivered to site as per IS : 383. For Grading II and III materials, the CBR shall be determined at the density and moisture content
likely to be developed in equilibrium conditions which shall be taken as being the density relating to a uniform air voids content of 5 per cent.
Table -1. Grading for Close-graded Granular Sub-Base Materials
| IS Sieve Designation | Grading I | Grading II | Grading III |
|---|---|---|---|
75.0 mm | 100 | - | - |
53.0 mm | 80-100 | 100 | - |
26.5mm | 55-90 | 70-100 | 100 |
9.50mm | 35-65 | 50-80 | 65-95 |
4.75 mm | 25-55 | 40-65 | 50-80 |
2.36 mm | 20-40 | 30-50 | 40-65 |
0.425 mm | 10-25 | 15-25 | 20-35 |
0.075 mm | 3-10 | 3-10 | 3-10 |
CBR Value (Min.) | 30 | 25 | 20 |
Table -2. Grading for Coarse Graded Granular Sub-Base Materials
| IS Sieve Designation | Grading I | Grading II | Grading II |
|---|---|---|---|
75.0 mm | 100 | | |
53.0 mm | | 100 | |
26.5 mm | 55-75 | 50-80 | 100 |
4.75 mm | 10-30 | 15-35 | 25-45 |
0.075 mm | <10 | <10 | <10 |
CBR Value (Min.) | 30 | 25 | 20 |
Note : The material passing 425 micron (0.425 mm) sieve for all the three gradings when tested according to IS 2720 (Part -5) shall have liquid limit and plasticity index not more than 25 per cent and 6 per cent respectively.
STRENGTH OF SUBBASE
It shall be ensured prior to actual execution that the material to be used in the sub-base satisfies the requirements of CBR and other physical requirements when compacted and finished.
When directed by the Engineer, this shall be verified by performing CBR tests in the laboratory as required on specimens remoulded at field dry density and moisture content and any other tests for the “quality” of materials, as may be necessary.
CONSTRUCTION OPERATIONS
Preparation of sub grade
Immediately prior to the laying of sub-base, the subgrade already finished as applicable shall be prepared by removing all vegetation and other extraneous matter, lightly sprinkled with water if necessary and rolled with two passes of 8 -10 Ton smooth wheeled roller.
Spreading and compacting
The sub-base material of grading specified in the Contract shall be spread on the prepared sub grade with the help of a motor grader of adequate capacity, its blade having hydraulic controls suitable for initial adjustment and for maintaining the required slope and grade during the operation or other means as approved by the Engineer.
When the sub-base material consists of combination of materials mentioned above, mixing shall be done mechanically by the mix-in-place method.
Manual mixing shall be permitted only where the width of laying is not adequate for mechanical operations. The equipment used for mix-in-place construction shall be a rotator or similar approved equipment capable of mixing the material to the desired degree. If so desired by the Engineer, trial runs with the equipment shall be carried out to establish its suitability for the work.
Moisture content of the loose material shall be checked in accordance with IS : 2720 (Part II) and suitably adjusted by sprinkling additional water from a truck mounted or trailer mounted water tank and suitable for applying water uniformly and at controlled quantities to variable widths of surface or other means approved by the Engineer so that, at the time of compaction it is from 1 per cent above to 2 per cent below the optimum moisture content corresponding to IS 2720 (Part VIII). While adding water, due allowance shall be made for evaporation losses. After water, has been added, the material shall be processed by mechanical or other approved means like disc horrows, rotators until the layer is uniformly wet.
Immediately thereafter, rolling shall start. If the thickness of the compacted layer does not exceed 100 mm, a smooth wheeled roller of 8 to 10 Ton weight may be used. For a compacted single layer up to 225 mm the compaction shall be done with the help of a vibratory roller of minimum 8 to 10 Ton static weight with plain drum or pad foot-drum or heavy pneumatic tyred roller of minimum 200 to 300 kN weight having a minimum tyre pressure of 20.7 MN/m or equivalent capacity roller capable of achieving the required compaction. Rolling shall commence at the lower edge and proceed towards the upper edge longitudinally for portions having unidirectional cross-fall and super elevation and shall commence at the edges and progress towards the center for portions having cross-fall on the both sides.
Each pass of the roller shall uniformly overlap not less than one third of the track made in the preceding pass. During rolling, the grade and camber shall be checked and any high spots or depressions which become apparent corrected by removing or adding fresh material. The speed of the roller shall not exceed 5 km per hour.
Rolling shall be continued till the density achieved is at least 98% of the maximum dry density for the material determined as per IS : 2720 (Part 7). The surface of any layer of material on completion of compaction shall be well closed, free from movement under compaction equipment and from compaction planes, ridges, cracks or loose material. All loose, segregated or otherwise defective areas shall be made good to the full thickness of layer and recompacted.
Surface Finish and quality Control of Work
The finished surface shall be checked for lines, levels and regularity. The surface evenness of completed surface in longitudinal and transverse direction shall be within the tolerances specified.
MODE OF MEASUREMENTS:
The finished area will be measured. Deduction for openings etc. will be made as applicable to flooring as per IS 1200.
The surface finish of construction shall conform to the requirements. Granular sub-base shall be measured as finished work in position in cubic meters.
The protection of edges of granular sub-base extended over the full formation as shown in the drawing shall be considered incidental to the work providing granular sub-base and as such no extra payment shall be made for the same.
