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Roads and highway works

Road/Highway Construction For Embankment and Sub-grade

Roads and highway works plays an important role in prosperity of any country. These are constructed through various of steps. In this article we will discuss about the very first step of highway construction, EARTHWORK EXECUTION.

Typical cross section of a road/highway is very important to understand:

Road and highway works

After knowing the different layers of road, the topics that we are going to discuss here are:

  • Embankment
  • Subgrade

Machineries for Construction

1. Roller/Compactor

2. Grader

3. Tipper/Dumping Truck

4. Water Tanker

5. Excavator

Specifications of Machineries:

Roller:

Vibratory Roller:-

  • Used in soil which requires great pressure for compaction
  • Drums are made to vibrate employing rotating & reciprocating mass
  • Gross weight= 8-10 ton
  • Diameter of drum= 1220mm
roller

Vibrating Plate Compactor

  • Compacting small areas by providing impact load to soil
  • Gross weight= 100 Kg to 2 ton
Plate compactor

Grader:

  • Motor grader is a consumption machine with a long blade use to create a flat surface during grading process.
  • Used to prepare base course to create a wide flat surface.
  • Position of blade is between front or rear axle.
  • Blade width = 2.5m to 7.3m
Grader

Tipper/Dumping Truck:

  • A dumping truck use for transporting loose material (such as soil, sand) of construction.
  • It is equipped with open-box bed, hinged at one end and equipped with hydraulic pistons to raise the front allowing material to be deposited.
Dumper

Water Tanker:

  • A water tanker use to transport water from a source.
  • to site and to sprinkle for the preparation of a pavement surface.
Water tanker

Material Requirement for Embankment & Subgrade

The material used in embankments, shoulders and miscellaneous backfills is as per Morth 5 shall be

i) Soil

ii) Moorum

iii) Gravel

iv) Reclaimed material from pavement

v) Fly ash, pond ash

vi) Mixture of these or any other material

vii) Such material should be free of logs, vegetation and any other ingredient which deteriorate the stability of embankment.

viii) Size of the material should not exceed 75mm.

Physical Requirement of Material for Embankment:

Test Performed Frequency No. of (Test/m³) Relative IS Code Limits
Grain Size Analysis 2/3000 IS:2720 (Part4)
Free Swell Index 1/3000 IS:2720(Part40) <50% (Emb & SG)
Atterberg’s Limit 2/3000 IS:2720(Part5) LL<50% &PI<25%
MDD & OMC 2/3000 IS:2720(Part8) As per Table 300-1
CBR 1/3000 IS:2720(Part16) >10% or as per design
Deleterious Content As required IS:2720(Part27) Na2SO4 <0.2% 
Moisture Content 2/3000 IS:2720(Part2) ±2% Of MDD
FDD(Embankment) 1/3000 IS:2720(Part28) >95% of MDD(Emb)
FDD(Subgrade) 1/2000 IS:2720(Part28) >95% of MDD (SG)
FDD(OGL/NGL) 1/3000 IS:2720(Part28) >95% of MDD (Emb) >97% of MDD (SG)

*Emb-Embankment   SG-Subgrade       MDD-Maximum Dry Density

CBR- California Bearing Ratio                OGL-Original Ground Level

EMBANKMENT

A series of compacted layers of soil or lifts of suitable material place on the top of each other until the level of sub-grade surface reach.

Function:

  • Gives support to the other layers of road pavement system.
  • Raise the grade of roadway above the existing ground surface as per FRL (finished road level)

Construction Procedure for Embankment:

Borrow Area Identification:

  • For earthwork construction borrow area to be chosen within the closet proximity of the working site.
  • Borrow pits to be made of size 25m*25m*1.5.
  • Samples collected will be checked at site lab for various parameters of soil i.e. FSI, LL & PI, MDD & OMC, CBR.

OGL Bed Preparation:

Unwanted material shall be removed and checklist should be prepared

  • Toe line should be marked with respect to vertical profile.
  • Removal of bushes and trees girth less than 300mm will be done by using proper equipment like excavators.
  • Clearing and grubbing shall be followed up to the thickness of up to 150mm.
  • Unwanted material shall be removed and checklist should be prepared.

Material and Spreading:

  • Toe line of the embankment shall be marked.
  • The original ground shall be levelled, scarified, mixed with water and compacted to achieve 95% of lab MDD for embankment.
  • Material is taken from borrow area and shifted, spread via dumpers with uniform heaps and levelled by grader.
  • The embankment is constructed having slope then the same shall be suitably benched and jointed with the adjacent layer.

Water Application:

  • If the moisture content is less than the OMC, the water shall be added by sprinklers via water tanker and should be ±2% to the OMC.
  • Calculate the water required for specified area and 250mm thickness.
  • If normal moisture content is greater than the OMC, material can be allowed to expose to the Sun.

Compaction:

  • After finishing by motor grader, the layer shall be compacted by steel drum or vibratory roller.
  • Each layer shall not be more than 250mm with vibratory roller and 200mm with static roller used to achieve 95% of FDD.
  • The rolling shall commence from lower to upper edge and each pass should overlap 1/3rd of previous pass.
  • Generally, as per trial 6-8 passes are enough to achieved required compaction.
  • If benching is required it should be done. Width of the bench should be two times the layer thickness.

SUBGRADE

  • The sub-grade surface is the top of the embankment and the surface upon which the sub-base is placed.
  • Sub-grade is the foundation of the pavement surface.

Function:

  • Adequate support to road pavement.
  • Stability to road pavement.

Construction Procedure for Sub-grade

Material and Spreading:

  • Before commencement, Toe width of sub-grade soil shall be marked.
  • The approved Borrow area/Cut section material as per Morth 305.2.1 is brought to stretch.
  • The same material shall be approved before sending for filling purpose.
  • The dumped material shall be spread uniformly by means of grader/mechanical means to achieve specified slopes and grades.
  • Clods and hard lumps of earth will be broken to a maximum size of 50 mm for sub-grade.

Watering:

  • If the moisture content is less than OMC, the water shall be added by sprinkling, considering evaporation losses, so that at the time of compaction it remains in the range of ±2% to the OMC.
  • If NMC is greater than OMC, the material can be allowed to expose to sun.
  • The stretch is to be watered with required quantity of water with water tanker uniformly at control rate of flow.

Compaction:

  • After finishing by motor grader/dozer, the layer shall be compact by steel drum or vibratory roller either individually or suitable combination.
  • The compacted thickness of each layer shall be 250mm when 80-100 KN static roller use to achieve 97% of MDD.
  • The rolling shall commence form lower to upper edge and shall overlap 1/3rd of preceding pass.
  • Generally, 6-8 passes are enough to achieve required compaction of 97% of MDD.

Productivity of Machines

  • Embankment and Subgrade Productivity: 25000 m³/month/team/shift
  • Grader Sub base productivity :                  12000 m³/month/team/shift
  • Roller Productivity :                                  100 m³/hr

* The data considers for 5 km lead and 8-10 hours of working.