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In 1928, California Division of Highways developed this method of design. CBR tests were conducted by the California state highway department on existing layers including sub-grade, Sub-base and base course. Based on the extensive data collected on pavements which behaved satisfactorily and those which failed, an empirical design chart was developed correlating the CBR value and the pavement thickness. The basis of the CBR design chart is that a material with a given CBR requires a certain thickness of pavement layer as a cover. A higher load needs a thicker pavement layer to protect the sub-grade.

The Indian Road congress has developed a design chart (fig. 9.6) for use in our country. Different curves have been given based on different volumes of traffic.

Fig. 9.6

Studies carried out by the U.S. Corps of Engineers have shown that there exists a relationship between the pavement thickness, wheel load, tyre pressure and CBR value of the sub-grade within a range of 10 to 12%. It is possible therefore, to extend the CBR design curves for various loading condition using the expression.

t = P CBR1.75 - 1

p

1/2

= 175. P ^{1 2/} CBR - A

(9.4)

Where t = Pavement thickness; cm

P = Wheel load, kg

CBR = California Bearing Ratio, in %.

p = Tyre pressure, kg/cm². A = Area of contact, cm².

Pavement Thickness Determination: For this, soaked CBR value of the sub-grade soil and C.B.R. values of other materials to be used in each layer is a pre-requisite. Depending upon the anticipated traffic intensity appropriate design curve is chosen from fig. 9.6. Sub-grade layer is the bottom most layer in the flexible pavement. Hence according to the CBR value (soaked) of the sub-grade and the curve chosen based on intensity of traffic read the required pavement thickness from the design chart 9.6. This will give the required cover over the sub-grade to protect it. Sub-base layer is laid over the sub-sub-grade. Now according to the CBR value of sub-base material and using the same design curve, the thickness of pavement required above the base course is obtained again from the fig. 9.6. The thickness of sub-base is obtained by deducting the thickness of cover required over sub-sub-base from the thickness of cover required over the sub-grade. Similarly thickness of the other subsequent layers may be determined. The method is illustrated in the worked out example 9.2.

IRC Recommendations: Some of the important recommendations of the IRC for the CBR method of design (IRC: 37-1970) are given below:

1. CBR test should be performed on remoulded soil and not on insitu- soil. The specimens should be prepared by static compaction where-ever possible and otherwise by dynamic compaction.

2. For design of new roads sub-grade sample should be compacted at O.M.C. to I.S. light compaction density, when suitable equipment is available in the field for compaction to this density. Otherwise the soil sample may be compacted to the dry density desired to be achieved in the field. In the case of old existing roads, samples should be compacted to the field density of the sub-grade soil at the field moisture content (F.M.C.)

3. Soil sample may kept soaked in water for 96 hours in the case of new-constrictions before CBR test is made on them. But in cases of areas where rain fall is less than 50 cm and water table is also quite deep, it is not essential to soak the test sample.

4. At least three samples should be tested before assigning CBR value of a particular soil. If the variation in the CBR values in the three tests is beyond the specified limits as least six test

specimen should be tested and average value should be assigned as CBR value. Limits of variation are as follows.

Upto CBR value of 10% 3

10 to 30% CBR values 5

30 to 60% CBR values 10

5. Top 50 cm thickness of sub-grade should be compacted to such an extent that 95 to 100% I.S.

light compaction (Proctor) density is achieved.

6. Thin layers of bituminous wearing course upto a thickness of 2.5 cm should not be considered towards the total thickness of the pavement since they do not increase the structural strength of the pavement.

7. Traffic intensity per day on the road pavement has been divided into seven categories ranging from A to G as shown in fig.9.6. Traffic is considered in terms of vehicles having laiden weight exceeding 3 tonnes.

8. To estimate the value of traffic at the end of life span of the pavement, IRC recommends the following expression.

A = P(1+r)ⁿ⁺¹⁰ (9.5)

where A = number of heavy vehicles per day for design (laiden weight > 3 tonnes).

P = number of vehicles per day at count r = annual rate of increase of heavy vehicles

(taken as 7.5% in the absence of any data).

n = number of years between the last count and the year of completion of construction.

Worked Example :

9.2. Soil sub-grade sample was obtained from the site of a road project and the CBR value was determined as 4%. It is further desired to use the following materials for different pavement layers.

i) Compacted sub-grade 7% CBR

ii) Poorly graded gravel 20% CBR

iii) Well grade gravel 95% CBR

iv) Minimum thickness of bituminous concrete surfacing

with CBR of above 80% 5 cm.

The traffic survey revealed the ADR of commercial vehicles as 1200. The annual growth of traffic is found to be 8%. The pavement construction is to be completed in three years after the last traffic census.

a) Design the pavement section using the CBR method as recommended by the IRC.

b) Suggest alternate design without using poorly graded gravel.

c) Discuss the limitations of the CBR method of pavement design in the light of the above results.

Solution :

Design traffic A = P(1+r)ⁿ⁺¹⁰

= 1200 (1+0.08)⁽³⁺¹⁰⁾ = 3260 vehicles/day.

Curve ‘F’ in the chart 9.6 includes this traffic range. Hence is the design curve.

The thickness of material above different layers of different CBR values are obtained from fig.9.6 as follows.

Sub-grade soil 4% CBR 55 cm

Compacted sub-grade 7% CBR 40 cm

Poorly grades gravel 20% CBR 21 cm

Well graded gravel 95% CBR 8 cm

Bituminous concrete surface 80% CBR 8 cm.

The designed pavement section for cases (a) and (b) are shown in fig.

CASE -A

CASE -B