NEPAL ENGINEERING SERVICES

Thursday, June 13, 2019

Road Costruction Technology














Road construction technology is the branch of highway engineering which deals with all kinds of activities and technology or operations for changing the existing ground to the designed shape, slope. It provides all the necessary facilities for smooth, safe  and efficient traffic operation. It also includes the reconstruction of existing roads. The type of technology depends upon the available resources which are available equipment, plant and human resources. 

Various activities can be divided into 
1. Earthwork
* Site clearance, earthwork in filling for embankment
* Excavation for cutting
* Excavation for borrow pit
* Excavation for structural foundation
* Disposal of surplus earth

2. Drainage works
Side drains,
Culverts,
Sub- surface drainage,
Causeways,
Minor bridges,
Other water management structures

3. Structural works
Earth retaining structures, 
Gully control works, 
Landslide stabilization works, 
River training works
Bridge protection works
Anchor walls

4. Pavement works
Sub-grade preparation,
Sub base preparation,
Wearing course

5. Miscellaneous works
Road furniture,
Traffic sing,
 marking signal,
 Bio engineering


Tools, equipments and plants used in road construction
Since road construction done manually takes lots of time and it is difficult to achieve required degree of quality, construction equipments are extensively used.  For small works labour force may be more effective, but for large projects construction equipments are most.

Tool
Hand shovel,
 chisel,
 spade,
hand hammer,
brushes,
trowel,
wheel barrow

Equipments

1 Earth moving equipments
Dozer,
Scraper,
Loader,
Excavator,
Backhoe,
Dragline,
Clamshell,
Trench digger

2 Compaction
Smooth wheel roller,
Vibrating roller,
 Pneumatic roller,
Sheep foot roller,
Rammers of various capacities

3 Leveling equipment
Motor grader

4. Paving 
Binder storage tank with heating device,
Binder spreader/distributor,
Aggregate spreader,
Cement concrete mixture,
Bituminous paver,
Concrete paver

5 Lifting 
Backhoe,
Crane

6 Transporting 
Dumping trucks,
Trucks flat body,
Mini dumpers

7 Miscellaneous equipments
Rock driller,
Water tanker,
Drilling machines,
Blasting equipments


Plant
Cement concrete plant,
Asphalt concrete plant,
Cold mix plant,
Aggregate crushing plant,
Screening plant,
washing plant,
Sand blowing unit


Earthwork
The process of preparing the sub-grade to maintain level, designed grade by compaction is termed as earthwork. It can be either filling or excavation depending on the nature of original ground and finalized section of the road. It is economical to plan the movement of earth from cuts to the nearest fill and so on.

Planning of Earthwork

Planning of earth movement during earthwork in road construction is important and essential for efficient usage and minimizing the cost. Highway construction involves a large amount of earthwork in cut and fill. Planning is  best done by drawing mass haul diagram.


Mass haul diagram- 
A graphical representation of the cumulative amount of earthwork moved along the centerline and distances over which the earth and materials are to be transported  after correction due to soil condition. It is the graphical representation of the amount of earthwork involved in road construction and the manner in which may be most economically handled. Each ordinate in the diagram is the balance of materials obtained form cut and used in fill. 


  Shrinkage or swell factor: It is well known that one cubic meter of excavation on amount will not occupies exactly 1 m 3 of space in the fill, so adjusting is required. This can be done by using the shrinkage or swell factor. 
- Borrow: It is the location away from the Right of Way (R.O.W.) and it is chosen by the Engineer. The borrow pits soil should be comply with the followed specification (preferably out of R.O.W.). 
Note: there is a problem in urban areas because of borrow cost. 
- Waste: It is the unwanted excavation material which should be disposed out of R.O.W.

Site clearance
It is the first operation after the completion of survey works for fixing the road alignment. Some works included are- clearing grass, weeds, bushes, shrubs and top organic soil at least covering toe width
Removal of trees, stumps, and roots along the alignment up to right of way 
Removal of existing structures along the alignment
Embankment for filling
When it becomes necessary to raise the sub-grade of a road above the existing ground level embankments are constructed. Raising of sub grade may be due to – water table, to prevent pavement due to surface and capillary water
To maintain the design standards wrt vertical alignment.

Various elements of embankment are

Height, fill material, settlement, stability of foundation and slopes

Excavation for cutting

Is the process of cutting and removing the earth including rock form its original position. The cut material is transported and dumped as a fill or spoil bank. Depending on the nature of soil or rock cutting equipments are selected.
Elements are – depth, stability of foundation, slopes, accommodation of side drains

Pavement construction:-


Types of pavement construction
  earthen road
  gravel road
  water bound macadam[WBM]
  soil stabilized roads- mechanical stabilization, 
                                    soil cement stabilization, 
                                    soil lime stabilization,
                                    soil bitumen stabilization
  bituminous/ black topped roads- prime coat, tack coat, surface dressing, otta seal, grouted/penetration macadam premixes- BBM bituminous bound macadam, bituminous carpet, asphalt concrete, mastic asphalt cement concrete roads- cement grouted layers, rolled concrete layers, cement concrete slabs


Before the construction of pavement layers, sub-grade is prepared [refer earthwork page3]. The site should be cleared off and grading is done to bring the vertical profile to meet the design standards. It is essential to compact the top of formation adequately starting from edge to the centre on straight section and from inner edge to outer edge where super elevation exists.


Earthen road it is the cheapest type of road. General camber is 4 to 5%. Steep camber is preferred for quick removal of water. 


Equipment- grader, roller, tipper and water tanker or manually for small projects


Procedure-

Soil survey- beyond the right of way, extracted material from borrow pit should be free from organic matter
Centre line and reference points are fixed with wooden pegs
Prepare the sub-grade as stated earlier

The borrow soil [if necessary mix different types of soils to desired proportion] is dumped on the prepared sub grade and pulverized. The field moisture condition is checked and water added if needed to bring the moisture content to OMC.
Spread the soil over the carriage way and rolled in compacted thickness
Open to the traffic after setting of compaction
For quality control- conduct Atternberg limit and proctor density test
Check for camber
Check filed moisture density and dry density [ minimum 95%]
Gravel road
Superior to earthen road, carry heavier traffic, normal camber 3 to 4%. 
Two types of construction- feather type and trench type
Material- clean, hard, strong, tough, durable varieties of crushed stones or gravel of specified gradation. Rounded and river gravel is not preferred as they have poor interlocking. CBR>60%
Equipment- as above
Procedure- 
Gravel is stacked along the sides of proposed road
Wooden pegs fro centre line and reference points are marked
Preparation of sub-grade- as above
Placing of gravel as per types of construction
Spread with greater thickness at centre and less towards the edges to obtain desired camber
Compaction with smooth wheel roller or vibratory roller. Half width of roller overlapping during compaction
Open to traffic after completion
For quality- as above [ density >98%

Water bound Macadam [WBM]

Is known after the name of John Macadam. The main principle is- crushed or broken stone aggregates are bound together by the action of rolling or traffic compaction. The binding is achieved by using stone dust as filler material in presence of water. Usually thickness is 8 to 30cm with compaction done on thickness of 8 to 15 cm as one layer. Normal camber 2.5 to 3.5%
Material- WBM roads with gravel including crushed gravel, hard broken stones or soft broken stone with  brick ballast, blast furnace slag etc
Coarse material, screening material and stone dust are required. Clean, hard, strong durable and free from excess of flaky, elongated or soft and dirt. LAAV =max 40%
Grading Size range Screening size
1 90 to 40 mm 12.5mm
2 63 to 40 mm 10mm
3 50 to 25mm 10mm
Equipment- aggregate spreader, roller, tipper, water tanker
Procedure-
Preparation of sub-grad as mentioned above
Materials with compaction factor [nearly 20 % extra] are stacked along the road
Arrangement of lateral confinement
Spreading of coarse aggregates with compacted thickness of 8 to 15 cm
Compaction with smooth wheel roller of 6 to 10 tonnes
Application of screening aggregates and wet rolling to fill about 50% voids
Application of filler materials with PI≤9% in tow thin layers
Finishing of the surface with 6cm sand sprinkling water and rolled
Making of shoulders
Open to traffic after few days of completion
For quality- LAAV, CBR others as above
Soil stabilized roads
Due to financial limitations developing countries are compelled to strengthen their road network by stage construction. The construction cost can be dramatically lowered by selecting local materials. If the stability of the soil is not adequate to support the wheel loads soil properties are improved by soil stabilization methods. The main principle is the effective use of local soil and other material suitable stabilizing agents with low cost.

Mechanics of soil stabilization
It is the improvement of stability or bearing capacity of the soil by using controlled compaction, proportioning and addition of suitable admixture. It deals with physical and chemical methods to make stabilized soil serve as a pavement component.
Evaluate the properties of available soil, decide the method of stabilization, and design the stabilized mix, adequate compaction.

Techniques of soil stabilization

Proportioning- locally available soils and aggregate mixed in suitable proportion
Cement agents- adding Portland cement, lime, bituminous materials
Modifying agent- Portland cement, lime for highly clayey soil
Water proofing agents- bituminous materials
Water repelling agents- organic compounds [resinous materials]
Water retaining agents- calcium chloride for non-cohesive soil
Heat treatment- heat treated soil
Chemicals- several chemicals <0.5% by weight of soil
In all the above technique adequate compaction is a must.



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