DEVELOPMENT OF CORRELATION BETWEEN CBR AND MINI SPT VALUES OF SUBGRADE SOIL

Abstract

The prime objective of a pavement design is to determine the thickness of different layers so that the pavement structure can bear the load without any distress. Different parameters are used in the various design methods. One of the most popular methods of flexible pavement design is CBR method, which was originally developed by the California Division of Highway. The design method based on it, is responsible for more than half of all the pavements now under construction throughout the world. CBR test, though simple, requires strict adherence to the test procedure to obtain the reliable and reproduceable results. IS:2720(PartXXXI)-1969 suggest CBR test on 25 cm depth from ground surface. However, the effect of wheel load goes upto a deeper level. Any change in the soil structure below 25CM will adversely affect the pavement performance. The Standard Penetration Test, on the other hand, may be carried out to explore the soil properties upto any depth. The mini SPT developed by Mohan, Handa and Jain (1983) in Geotechnical Engineering Laboratory of Civil Engineering Department of University of Roorkee, Roorkee, is quite handy equipment and can be carried to field easily. In present study field Mt of Subgrade Soil has been correlated with mini SPT value under indentical conditions of test. Tests were carried out under field conditions, so results are those expected under such condition rather than under ideal "Laboratory type" conditions. CBR and Mini SPT tests were conducted for in-situ and saturated conditions of soil. CBR tests were done at 25cm depth as suggested by IS code and Mini SPTs were conducted viz 25 and 50cm adopting procedure as described in by Mohan (1983). The analysis of test results have shown that a non-linear relationship exists between CBR and Mini SPT value of a Subgrade Soil. The relationship was statistically more sound when CBR and mini SPT as determined at same level were correlated in comparison of that between CBR at 25cm and Mini SFr value at 50cm depth. This is attributed to the change in the soil properties and density at deeper level. Further, the correlation between the two parameters under saturated conditions of soil was found to be very poor. This may be due to the difference in penetration level in two tests, more swelling of the top layer of soil than the bottom layer and more susceptibility of CBR to surface moisture. The correlations have also been developed between dry density, CBR and N-value. It is recommended that further study be carried out on different types of soils to have more generalized relationship between CBR and mini SPT value