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What is Road Subgrade?

The foundation of the pavement structure is known as subgrade. Preparation of subgrade consists of all operations before the pavement structure could be placed over it and compacted. The subgrade may be situated on embankment or excavation or at the existing ground surface. In all the above cases, Site Clearance -Clearing Grubbing Operation should be done before starting the pavement structure construction. After that, the grading operation is started as per the design and drawing of the highway plan and profile.

Scope of the Methodology :

  • Excavation is necessary for the construction of roadway complete as per Technical Specifications.
  • Loosening and compaction of the original ground/subgrade up to the required depths as directed by the Engineer.
  • Construction of subgrade layer with the help of approved borrow area material, including all leads and lifts, completed. 
  • Construction of road subgrade with suitable approved material available from the roadway excavation or any other excavation, completed.
  • Construction of subgrade with approved material satisfying soaked CBR equal to or greater than 8% and complete as per MORTH. 

Subgrade Construction Equipment:

The following sets of equipment are necessary for the planned progress of road subgrade construction work.

  1. Excavator
  2. Air Compressor & Pavement Breaker /Jack Hammer
  3. Tippers /Dumpers
  4. Motor Graders
  5. Vibratory Compactor
  6. Tractors Dozer with Disc Harrows/Spreading Blade/Ploughs
  7. Water Bowser with Sprinklers etc.
  8. Crawler Dozers
Motor Grader for Subgrade Construction
Motor Grader for Subgrade Construction

Steps for Preparation of Subgrade:

  • The road subgrade shall be prepared as per the MORTH specifications if it is not mentioned in contract technical specifications.
  • The limits of filling shall be marked by fixing batter pegs at regular intervals on both sides of the layer and working line with a white the help of lime powder.
  • The layer shall be built 300mm wider than the designed and drawing dimensions so that after proper compaction is achieved up to the toe, the surplus materials shall be trimmed to get the properly compacted slopes of subgrade. Where the fill is to be deposited against an existing subgrade, continuous horizontal benches of 300 mm wide shall be cut into the old slope.
  • In cutting section, where cutting is to be done up to subgrade top its top layer, shall be loosened and re-compacted as per technical specification.
  • The topsoil in the borrow area shall be removed by grubbing or stripping, so that earth without vegetation is excavated and loaded. 
  • Soil from approved borrow areas shall be excavated with Excavators and loaded onto tippers or dumpers for transportation to the stretch ready to receive fill Material. 
  • The material shall be dumped between the limiting lines marked with lime powder.
  • The material shall be spread in layers of a uniform thickness not exceeding 250mm of compacted thickness. Grader or a combination of dozer and grader can be used for this activity.
Crawler Dozers for subgrade construction
Crawler Dozers for Road subgrade construction
  • The grader will initially spread the heap of earth dumped over a stretch maintaining an approximate line and level. 
  • At this stage, the material should have an Optimum Moisture Content(OMC), ranging from +1% to –2%. The following mixing or drying process should be adopted if it is not found within the permissible limit.
  1. The material at the site is too Dry: Additional water shall be added to increase the moisture content up to the permitted limit. After sprinkling water with the browser, the material shall be thoroughly mixed with the help of grader to obtain a homogenous mix. After that, the grader shall carry out the final precise grading. 
  2. The material at the site is too Wet: If the material at the site is too wet, it shall be dried by aeration and exposure to the sun until the moisture content is acceptable.
  • When a combination of grader and dozer is used, the dozer shall carry out the initial spreading of borrowed material to the approximate line and level. After that, the grader shall carry out the final precise grading.
  • The in-situ moisture content shall be checked with the help of rapid moisture meter.
  • Compaction of fill shall start immediately after achieving the required moisture content. Compaction shall be done with the help of vibratory compactor. The compaction pattern, which includes the number of passes required, shall be finalized after full-scale trials at the site to achieve the required degree of compaction as per technical specification. The general pattern shall be as follows.
  1. Initial rolling, two static passes with a vibratory roller.
  2. Subsequent rolling, four vibratory passes(One pass includes both forward and reverse movement of the roller).
  • The compaction shall commence from the lower end and move to the upper-end width by width.
  • Further layer shall commence after the underlying layer is tested and approved.
  • The rain cuts shall be repaired before placing further layers of subgrade.
  • The compaction behind the structure shall be done with the use of vibratory roller or plate compactor to achieve 98% of MDD as per MORTH specification.
  • The surface of embankment shall have suitable cross fall to enable efficient surface drainage.

The purpose of this Methodology for road subgrade is to provide details of the Quality Control System/Procedures, and Safety and Environment Management Plans to be adopted.

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Summary
Subgrade - Steps for Preparation of Subgrade
Article Name
Subgrade - Steps for Preparation of Subgrade
Description
The subgrade may be situated on embankment or excavation or at the existing ground surface. In all the above cases, Site Clearance -Clearing Grubbing Operation should be done before starting the pavement structure construction.
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Publisher Name
Construction Civil
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