The CBR methods has one serious dis-advantage. It may be seen that the total thickness of construction remains the same, that is 55 cm, though the pavement component layers in cases (a) and (b) are of different materials with different CBR values. The thickness of construction over compacted soil of CBR value 7% is the same in both the cases equal to 40 cm, though in one case poorly graded gravel of CBR 20% is used where as in the second case, it has been replaced by well graded gravel of CBR value 95%.
The CBR method of pavement design gives the total thickness requirement of the pavement above a sub-grade and this value of thickness would remain the same irrespective of the quality of materials used in the component layers.
Equivalency Factors:
Bound type of bases have superior load spreading properties and therefore a certain reduction in the overall pavement thickenss could be permitted. Well constructed bituminous macadam base will be considered of twice the strength of water bound macadam road. For other bound type of base courses like lean concrete, lime pozzolona concrete and soil cement, the equivalency factor may be taken as 1.5. Corresponding reduction in thickness may be made.
9.4.3. C.B.R. Method of Pavement Design Based on Cumulative Standard Axle Load.
(IRC : 37 - 1984) : In the C.B.R. method of design discussed so far, the traffic is considered in units of heavy vehicles (laden weight greater than 3 tonnes) per day in both directions irrespective of whether the design is for a two lane or a dual carriage way. However, it has been recognised by the field engineers and research workers alike that other factors like repetition of axle loads, distribution of traffic over the pavement width (depending on the
number of lanes) type of terrain and’environmental factors affect considerably the thickness of pavement. Accordingly the I.R.C. has revised the method of design of flexible pavements.
In the revised method, two entirely different apporaches have been given for arriving at the total thickness of pavement based on traffic intensity. The two approaches are discussed in the following paragraphs.
a) Design procedure when the Traffic Intensity is less than 1500 Commercial Vehicles / day - C.B.R. Method.
This method is similar to method already discussed in para 9.4.2. The C.B.R. design curves for use in this method are developed for a legal axle load of 10.2 Tonnes as against 8.165 Tonnes used in design curves have been provided for arriving at the total thickness of the pavement for each of the categories of traffic. The traffic is arrived at by using equation 9.5. By choosing proper curve based on the designed traffic volume, knowing the CBR values of the sub-grade (soaked condition), sub-base and base course materials, the thickness of component layers of the pavement are obtained by repeated use of the design curves. The minimum thickness of sub-base and base courses should be 150mm. For the materials to be used for base course a minimum CBR of 80 has been recommended.
b) Design procedure when the Traffic Intensity is more than 1500 Vehicles / day - The Cumulative Axle Load Method
In this method the ‘Equivalent Axle Load Concept’ is used. For Design purpose, the cumulative number of standard axles (8160 kg) to be carried by the road during its life is considered as the design traffic. The cumulative number of standard axles to be catered for in the design is arrived at from the initial volume of traffic, by the following formula :
NS= 365×A
[ (
1+)
x 1]
F 9.69.7 CBR Method of Pavement Design by Cumulative Standard Axle Load.
where NS = The cumulative number of standard axles to be catered for in design.
r = Annual Growth Rate of Commercial Vehicles x = Design life in years
F = Vehicle Damage Factor (recommended by the IRC) based on initial traffic volume and terrain)
A = Initial volume of traffic in the year of completion of
construction, in terms of commerical vehicles / day duly modified to account for lane distribution (Distribution factors have also been recommended by IRC)
For the value of the designed cumulative number of standard axles (NS) total thickness of pavement may be arrived at for the CBR value of the sub-grade referring to Fig.
9.7. Thickness of component layers - surface course, Granular base course and sub-base
course, and their requirements, based on the value of NShas been recommended by IRC (IRC : 37 - 1984).
9.5. Self Assessment Questions :
1. Explain ‘Flexible and Rigid pavements and bring out the points of difference.
2. What is ‘ESWL’? Explain the concept in determination of the equivalent load.
3. What is the basis on which thickness of pavement is determined based on Group Index method ? What are the limitations of this method?
4. Explain the merits and demerits of the CBR method of flexible pavement design.
5. The CBR value of sub-grade soil is 5%. Calculate the thickness of pavement required given that the wheel load is 4082 kg and tyre pressure is 7kg/cm2using the U.S. Corps of
Engineers Formula. (Ans. 35.4 cm).
12.6. SUMMARY :
Based on their structural behaviours, pavements may be divided into two classes -1) Flexible pavements and 2) Rigid pavements. The term flexible pavement is associated with those pavements which reflect the deformation of the sub-grade and subsequent layers on to the surface. The design of flexible pavements is based on the load distribution characteristics of the component layers. The rigid pavements, because of its rigidity and high modulus of elasticity, tend to distribute the load over a relatively large area of soil. thus a major portion of the structural capacity is supplied by the slab itself.
Various factors to be considered for the design of pavements are (i) Design wheel load (ii) Strength characteristics of pavement materials and (iii) climatic variations, have been discussed.
Design of flexible pavements based on (i) the Group Index Method and (ii) the C.B.R.
method are only explained in this unit.
The group index of a soil is believed to rate the sub-grade soil and hence indicates the pavement thickness needed to protect the sub-grade. This method of design also takes into account the traffic intensity in arriving at the thickness of pavement.
The IRC (IRC: 37-1970) recommends a design method based on the CBR values of the pavement material with a given CBR value requires a certain thickness of pavement layer as a cover. A higher load needs a thicker pavement layer to protect the sub-grade. Design charts
are prepared by the IRC relating the thickness of construction required and the CBR value of the pavement material based on the traffic intensity. For the seven groups A to G of traffic, seven curves are available in the design chart. The traffic intensity to be taken into account for design purpose in terms of the number of vehicles per day exceeding 3 tonees laden weight at the end of design period which is normally taken as 10 years. Design of Flexible pavements based on the cumulative number of standard axle (IRC : 37 - 1984) is also explained.
12.7. REFERENCES :
1. IRC : 37 - 1970 ; Guide Lines for Design of Flexible Pavements.
2. IRC : 37 - 1984 ; Guide Lines for Design of Flexible Pavements.
3. Khanna, Dr.S.K. and Justo, C.E.J. (1984) - Highway Engineering, Nemchand and Bros., Roorkee
4. Ramana Sastry, Dr. M.V.B.R. and Gopal M.S.P., - Computer Oriented Design For Flexible Pavements - Indian Highways - June, 1987.
Yoder, E.J. and Witczak (1975) - Principles of pavement Design - John Wiley and Sons, INC., New - York..
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