Construction Methodology of Sub Grade
Sub Grade work shall consist of laying, spreading and compaction subgrade layer as per contract drawings with approved and specified material following cause 305 of MORT&H specification.
- Motor Grader
- Vibratory roller
- Water tanker with sprinkler
- Survey kit
As per clause 305.2 of MORT&H, the material to be used for the construction of the sub Grade shall be satisfying the following requirements.
- The particle size of the coarse material in the mixture of the earth shall not be more than ⅓ of the layer thickness.
- While testing as per IS: 2720 (Part 40), The free swell index of the material shall not be more than 50 per cent.
- While testing as per IS: 2720 (Part 8), The material’s dry unit weight shall not be less than 1.75gm/cc.
- The liquid limit shall not be more than 50 per cent.
- The plasticity index shall not be more than 25 per cent.
- CBR for sub Grade material shall be as drawing/design.
- Where the excavation reveals a combination of acceptable and unacceptable materials, the acceptable materials are excavated separately for use in permanent works without contamination by the unacceptable materials. The acceptable materials shall be stocked separately.
- The embankment top shall be cleaned off if there is any extraneous material. If the surface is dry, it should be made wet by sprinkling water.
- The approved material from the barrow area and the approved sources shall be transported utilizing end dumpers and dumped in the form of heaps.
- Material shall be spread using the motor grader to its loose thickness and specified grade.
- Roots, stamps and any foreign material including oversized clods etc. shall be removed if any while spreading.
- Moisture content shall be checked before commencing the compaction, if it is less than OMC, water shall be added by sprinkler and if it is more than OMC it shall be dried by iteration and exposure to the sun till to reach the OMC.
- Rolling shall be carried out from lower edge to upper edge with 8-10ton static weight vibratory roller till to achieve the required compaction and no tyre impression or drum impression should not be found on the layer surface.
- Field density shall be achieved in the manner and frequency as specified in the specification
- Loose rockets or surface foliation will be removed by mechanical means only. To avoid any loose pockets or foliation the final level will be reached by cutting in excess and never by adding layers less than 100mm. In case of material is missing the layer will be loosened by scarifying, moisturizing before the dumping of new material.
- In the case where the difference between the subgrade level (top of the subgrade on thick pavement rests) and ground-level is less than 0.5m and the ground does not have 97 per cent relative compaction concerning the dry density, the ground shall be lessened up to a level 0.5m below the subgrade level, watered and compacted in layers to achieve dry density not less than 97 per cent relative compaction.
- As per the design, Cross fall shall be maintained.
Quality control for the above work shall be as per table 900-3 of section 900 of MORT&H specification.
- Moisture content shall be checked before compaction preceding the work. The same should be -2 to +1 of OMC.
- The surface shall be checked for levels at as per MORT&H intervals longitudinally and transversely and the same shall be within +20/-25mm of the designed level.
- The compacted surface shall be checked for field density at a frequency of 1 test per 2000sqm.
- The degree of compaction shall not be less than 97%.
- Moisture content will be checked before compaction for preceding the work.
- No traffic will be allowed except for GSB purpose.
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