flexible & rigid pavement design
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Croney (1977) Design is the process of developing the most economical combination of pavement layers in relation to both thickness and type of wheel load to suit to the soil foundation and the cumulative traffic to be carried during the design life.
Design of Highway Pavements Types of pavements 1) Flexible pavements. 2) Rigid pavements. 3) Semi rigid pavements. Design factors for pavements. 1) Design wheel load. 2) Subgrade soil. 3) Climatic factor. 4) Pavement component material. 5) Environmental factors.
Flexible pavement design methods 1) Group Index (G.I) method. 2) CBR method. 3) California R-value or Stabilometer method. 4) Mc-Leod method. 5) Tri axial test method. 6) Burmister method.
1.INTRODUCTION THE DESIGN OF FLEXIBLE PAVEMENT INVOLVES THE INTERPLAY OF SEVERAL VARIABLES,SUCH AS,THE WHEEL LOADS,TRAFFIC CLIMATE,TERRAIN AND SUBGRADE CONDITIONS. THE IRC FIRST BROUGHT GUIDELINES IN 1970,THESE WERE BASED ON CALIFORNIA BEARING RATIO METHOD. TO HANDLE LARGE SPECTRUM OF AXLE LOAD,THESE GUIDELINES WERE REVISED IN 1984.
THE EQUIVALENT AXLE LOAD CONCEPT WAS USED IN THIS APPROACH,PAVEMENT THICKNESS WAS RELATED TO THE CUMULATIVE NUMBER OF STANDARD AXLES TO BE CARRIED OUT FOR DIFFERENT SUBGRADE STRENGTHS. DESIGN CURVES WERE DEVELOPED TO CATER UPTO 30 MILLION STANDARD AXLES. WITH THE RAPID GROWTH OF TRAFFIC,THE PAVEMENTS ARE REQUIRED TO BE DESIGNED FOR HEAVY VOLUME OF TRAFFICOF THE ORDER OF 150 MILLION STANDARD AXLES.
ON THE BASIS OF MATHEMATICAL MODELLING OF THE PAVEMENT STRUCTURE USING MULTIPLE LAYER ELASTIC THEORY AND FEEDBACK ON THE PERFORMANCE OF THE EXISTING DESIGNS,GUIDELINES FOR THE DESIGN OF FLEXIBLE PAVEMENT WAS FINALLY REVISED IN 2001.
THESE GUIDELINES ARE APPLICABLE TO EXPRESSWAYS,NATIONAL HIGHWAYS,STATE HIGHWAYS,MAJOR DISTRICT ROADS AND ROADS CARRYING MOTOTISED VEHICLES. FLEXIBLE PAVEMENTS ARE THOSE WHICH HAVE BITUMINOUS SURFACING AND GRANULAR BASE AND SUBBASE COURSES CONFIRMING TO MORTH SPECIFICATIONS SECTION 400 .(SUB BASES,BASES,NON BITUMINOUS AND SHOULDERS.) SECTION 500 BASES AND SURFACE COURSES BITUMINOUS.
THESE GUIDELINES APPLY TO NEW PAVEMENTS. FOR DESIGN OF STRENGTHENING MEASURES OR OVERLAY FOR EXISTING PAVEMENTS IRC 81 TENTATIVE GUIDELINES FOR STRENGTHENING OF FLEXIBLE ROAD PAVEMENTS USING BENKELMEN BEAM DEFLECTION TCCHINIQUE FOR TIME TO TIME REVISION OF THIS CODE IT IS SUGGESTED THAT ALL THE ORGANISATIONS INTENDING TO USE THE GUIDELINES,SHOULD KEEP A DETAIL RECORD OF THE YEAR OF CONCTRUCTION ,SUBGRADE CBR,SOIL CHARACTERISTICS,TRAFFIC,PAVEMENT PERFORMANCE,OVERLAY HISTORY,CLIMATIC CONDITIONS,ETC.
3.RECOMMENDED METHOD OF DESIGN. PAVEMENT DESIGN GIVEN IN IRC 37 1984 WERE APPLICABLE TO DESIGN TRAFFIC UPTO 30 MSA. NEW SET OF DESIGN ARE TO CATER TRAFFIC UPTO 150 MSA. DESIGN APPROACH AND CRITERIA. FLEXIBLE PAVEMENT HAS BEEN MODELLED AS A THREE LAYER STRUCTURE STRESSES AND STRAINS AT CRITICAL LOCATIONS (ANNEXURE 1) HAVE BEEN COMPUTED USING THE LINEAR ELASTIC MODEL FPAVE DEVELOPED UNDER THE MORT&H RESEARCH SCHEME R-56 ANALYTICAL DESIGN OF FLEXIBLE PAVEMENT.
ANNEXURE-1CRITICAL LOCATION IN PAVEMENT.
TO GIVE PROPER CONSIDERATION TO THE ASPECT OF PERFORMANCE,THE FOLLOWING THREE TYPES OF PAVEMENT DISTRESS RESULTING FROM REPEATED APPLICATIONS OF TRAFFIC LAODS ARE CONSIDERED : 1) VERTICAL COMPRESSIVE STRAIN AT THE TOP OF THE SUBGRADE.IF THE STRAIN IS EXCESSIVE,THE SUBGRADE WILL DEFORM RESULTING IN PERMANENT DEFORMATION AT THE PAVEMENT SURFACE DURING THE DESIGN LIFE.
2) HORIZONTAL TENSILE STRAIN AT THE BOTTOM OF THE BITUMINOUS LAYER .LARGE TENSILE STRAIN S CAUSE FRACTURE OF THE BITUMINOUS LAYER DURING THE DESIGN LIFE. 3) PAVEMENT DEFORMATION WITHIN THE BITUMINOUS LAYER.
THE PERMANENT DEFORMATION WITHIN THE BITUMINOUS LAYER CAN BE CONTROLLED BY MEETING THE MIX DESIGN REQUIREMENTS AS PER THE MORT&H SPECIFICATIONS,THICKNESS OF THE GRANULAR AND BITUMINOUS LAYER ARE SELECTED USING THE ANALYTICAL DESIGN APPROACH SO THAT STRAINS AT CRITICAL POINTS ARE WITHIN THE ALLOWABLE LIMITS.
FOR CALCULATING THE TENSILE STRAINS AT THE BOTTOM OF BITUMINOUS LAYER AND THE RELATIONSHIP FOR ASSESSINGTHE ELASTIC MODULI OF SUBGRADE,GRANULAR SUB-BASE AND BASE LAYERS ARE GIVEN IN ANNEXURE-1.
THE PAVEMENT DESIGNS ARE GIVEN FOR SUBGRADE CBR VALUES RANGING FROM 2% TO 10% AND DESIGN TRAFFIC RANGING FROM 1 MSA TO 150 MSA for an annual pavement temperature of 35c.
For design purposes 1) design traffic in terms of cumulative number of standard axles(8160kg);and. 2) CBR value of subgrade. Is required. For estimating design traffic following information is needed: 1) initial traffic after construction in terms of CVPD. 2) traffic growth rate during the design life. 3) design life in number of years.
4) vehicle damage factor (VDF). 5) distribution of commercial traffic over the carriageway. For the purpose of structural design ,only the number of commercial vehicles of gross weight of 3 tonnes or more and their axle loading is considered. To obtain a realistic estimate of design traffic,due consideration should be given to to the existing traffic or that anticipated based on possible changes in the road network and land use of the area served,the probable growth of traffic and design life.
Estimate of the initial daily average traffic flow for any road should normally based on atleast 7 days 24 hour classified traffic counts. Traffic estimate can be made on the basis of potential land use and the traffic on the existing routes in the area. Traffic growth rate may be estimated by 1) by studying the past trends of traffic growth. 2) IRC 108 GUIDELINES FOR TRAFFIC PREDICTION ON RURAL HIGHWAYS.
If adequate data is not available it is recommended that an average annual growth rate of 7.5 % may be adopted. Design life. Design life is defined in terms of the cumulative number of standard axles that can be carried before strengthening of the pavement is necessary. It is recommended that pavements for national highways and state highways should be designed for a life of 15 years.Expressways and urban roads may be designed for a longer life of 20 years. For other categories of roads, a design life of 10 to 15 years may be adopted.
It is a multiplier to convert the no. of commercial vehicles of different axle loads and axle configurations to the number of standard axle load repetitions. It is defined as equivalent number of standard axles per commercial vehicle. The VDF varies with the vehicle axle configuration,axle loading,terrain,type of road and from region to region.the VDF is arrived at from axle load surveys on typical road sections so as to cover various influencing factors,such as traffic mix,mode of transportation,commodities carried,time of the year,terrain,road conditions and degree of enforcement.
Vehicle damage factor (VDF)
The axle load equivalency factors recommended in the AASHTO guide are given in Annexure-2.
These are used to convert different axle load repetitions into equivalent axle load repetitions. It is recommended that designer should take the realistic values of VDF after conducting the axle load survey. Where sufficient information on axle loads is not available and the project size does not warrant conducting an axle load survey, the indicative value of VDF are as :
Distribution of commercial traffic over the carriageway A realistic assessment of distribution of commercial traffic by direction and by lane is necessary as it directly affects the total equivalent standard axle load applications used in design.In the absence of adequate and conclusive data following distribution may be assumed . 1) Single lane roads traffic tends to be more channelised on single lane roads than two lane roads and to allow for this concentration of wheel load repetitions,the design should be based on total number of commercial vehicles in both directions.
Two lane single carriageway roads: 75% of the total number of commercial vehicles in both directions. Four lane single carriageway roads: 40% of commercial vehicles in both directions. Dual carriageway roads: 75% of the commercial vehicles in each direction,for dual three lane carriageway & dual four lane carriageway distribution factor will be 60% and 45% respectively.
Where significant difference between the two streams can occur,condition in the more heavily trafficked lane should be considered for design.
Computation of design traffic. The design traffic is considered in terms of the cumulative number of standard axles (in the lane carrying maximum traffic) to be carried during the design life.
Subgrade Subgrade in cut or fill should be well compacted. Expressways, NH, SH, MDR 97% of dry density achieved by IS 2720 part 8. Other roads - 97% dry density by IS 2720 part 7. IRC:36 Recommended practice for the construction of earth embankment for road works. Material used as subgrade in expressways,national highways,state highways should have the dry density not less than 1.75gm/cc.
CBR gives the strength of subgrade soil in cut and fill at most critical moisture condition likely to occur in situ. CBR can be determined as per IS 2720 part 16 in lab.static/dynamic compaction can be adopted. In-situ CBR is not recommended for design purposes.
Soaking for 4 days may be unrealistically severe moisture condition,in certain cases were the climate is arid throughout the year ie the annual rainfall is less than 500mm is less and the water table is too deep,CBR value may be prepared at the natural moisture content of soil at subgrade depth immediately after recession of the monsoon. Use of expansive soil is not allowed for subgrade construction particularly for heavily trafficked roads.
When expansive soils are unavoidable compaction req