California bearing ratio (CBR) and strength behaviour of compacted Dhaka clay as road subgrade

Abstract

In this study Califol11ia Bearing Ratio (CBR) and other strength characteristics are investigated for compacted Dhaka clay. The clay samples were collected from Pallabi Phase II, land project area of Eastel11Housing Ltd. on the east side of National Botanical Garden. After air drying, the soil samples were broken down for use in different laboratory tests. The samples were compacted at modified compaction energy at different percentage of water content. Unconfined Compression Strength test and Califol11ia Bearing Ratio (CBR) test for both soaked and unsoaked conditions were conducted. Swelling was measured during soaking and Vane Shear test was performed for soaked samples using a torvane. The results indicate that unconfined compressive strength, dry density and CBR value increase with the increase of water content on the dry side upto Optimum Moisture Content (OMC), but with the addition of more water beyond the OMC these values decrease. During preparation of unconfined compression strength test samples and also from the failure pattel11, it is observed that sample prepared with water content less than OMC becomes more stiff and rate of strength gain is high. Deformation at failure in unconfined compression test is lower for samples with lower percentage of moulding water. With the increase of water content, deformation at failure increases. At twenty percent or more water content there is no peak value and the stress-strain curves run parallel to the abscissa. Unsoaked CBR value is quite high as compared to soaked CBR value. At 9.5, 12.0, 15.3, 18.5, 19.9, 22.4 and 27.5 percentages of water content, soaked CBR values are 5.65, 16.67,21.37,17.42,13.05,6.31 and 3.60 percentages, whereas unsoaked CBR values are 37.41,45.57,53.29,32.13, 17.66,8.10 and 5.50 percentages respectively. During soaking swelling occurs. With the increase of moulding water content, percentage of swell decreases and at twenty percentage or more moulding water content swelling becomes zero due to better saturation, but percentage of swell increases with the decrease of moulding water content. At 9.5% moulding water content percentage of swell is 2.8 and it is observed to be the maximum value. Based on the above experimental tests results correlation are developed among Unconfined Compressive Strength, Water Content, Califol11ia Bearing Ratio, Dry Density, Vane Shear Strength and Swelling index. Some relationships are found linear and some are found nonlinear.