Quality Assurance Handbook for Rural Roads Volume-I - pmgsy

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Quality Assurance Handbook for Rural Roads
2. Ensure that the soil for subgrade meets the specified requirements in terms of physical properties
and the specified CBR value for pavement design.
3. Compact each layer of the material in the subgrade at OMC (±) 2% to at least 100% of Maximum
Dry Density as per IS:2720 (Part 7).
4. If the difference between the subgrade level (top of the sub-grade on which the pavement rests)
and ground level is less than 300 mm and the ground does not have the needed 100% relative
compaction with respect to IS:2720 (Part-7), loosen the ground upto a level 300 mm below the
subgrade level, correct moisture content to OMC (±) 2% and compact in layers to 100% of the
maximum dry density as per IS:2720 (Part 7).
5. If the subgrade soil does not possess the requisite engineering properties like highly plastic black
cotton soil and other weak soils yielding very low soaked CBR values, the same should be improved
in strength (CBR) and workability by treatment with additives like lime/cement etc. as described
in sub-sections 403 and 404 or by mechanical stabilization.
6. In conditions where salt concentration is in excess of 0.2%, a capillary cut-off of coarse sand should
be provided below the subgrade as shown in the drawings to check the upward movement of
moisture from below.
7. For a road in cutting, prepare the subgrade in accordance with subsection 302 to receive a subbase
course.
8. Ensure that the subgrade is compacted and finished to the design strength consistent with other
physical requirements.
9. Maintain the surface of subgrade, at all times during construction, at such a cross fall as will shed
water and prevent ponding.
B. Quality Control Requirements
1. Materials
(i) The material used for subgrades shall be soil, moorum, gravel, a mixture of these or any
other approved material. Material considered unsuitable for embankment construction as
per subsection 301 shall not be used for sub-grade.
(ii) The material for subgrade shall be non-expansive in nature.
(iii) Where an expansive clay with acceptable “free swelling index” value is used as a fill material
in embankment, the sub-grade and top 500 mm portion of the embankment just below the
sub-grade shall be non-expansive in nature.
(iv) Any fill material which yields a maximum dry laboratory unit weight of less than 16.5 kN/
m 3 determined as per IS:2720 (Part 7) shall be considered unsuitable for use in subgrade.
(v) The size of coarse material in the soil shall ordinarily not exceed 50 mm when placed in the
subgrade.
2. Surface Level
The permissible tolerances in surface levels of subgrade shall be (+) 20 mm and (-) 25 mm.