Determination of the California Bearing Ratio CBR of an Engineering Soil sample - Lab Report Example

Determination of the California Bearing Ratio CBR of an Engineering Soil sample Abstract: The CBR test a test whose basic working principle is the penetration of plunger in to the soil sample and assess the pressure required to do so giving a relationship between penetration and force. Through the results of the test strength and bearing capacity can be indicated and eventually the type of pavement in which the soil can be used can be safely recommended. Concluding with the fact that the empirical relations have been developed between the test results and the design of pavements. Introduction This test is done mainly to evaluate mechanical strength and load bearing capacity of the soil samples. This value serves as a recommendation to the usage of the engineering soil in subgrades and basecourses in pavements. The CBR value is a Rating or percentage which is calculated by comparing the measured pressure with the standard pressure. Then compared with the recommendation given by different engineering practicing bodies and authorities. The test is carried according to BS 1377: Part 4 : 1990. Experimental Procedure As procedure is already discussed in detail in the Laboratory Manual and the BSI Standard. Where as the brief outline is that the first step is to prepare the sample to be tested, then the soil sample is compacted using the standard method. Then the sample is placed in the compression machine and after applying the seating load, the test is initiated and reading is taken for every 0.25 mm interval. Lastly the value against 2.5 mm and 5.0 mm penetration is recorded, and finally the CBR value is calculated. Calculation and Results: Test results of the Sample (TOP) Penetration of Plunger mm Force Gauge Reading div Force on Plunger kN Penetration of Plunger mm Force Gauge Reading div Force on Plunger kN 0.00 0 0.00 4.00 35 0.90 0.25 2 0.05 4.25 35 0.90 0.50 3 0.08 4.50 38 0.97 0.75 4 0.10 4.75 39 1.00 1.00 7 0.18 5.00 40 1.02 1.25 8 0.20 5.25 41 1.04 1.50 10 0.25 5.50 42 1.07 1.75 11 0.28 5.75 43 1.10 2.00 13 0.33 6.00 43 1.10 2.25 15 0.38 6.25 44 1.12 2.50 17 0.43 6.50 44 1.12 2.75 18 0.46 6.75 45 1.15 3.00 20 0.51 7.00 46 1.17 3.25 22 0.56 7.25 46 1.17 3.50 24 0.61 7.50 47 1.20 3.75 28 0.71 Test results of the Sample (Bottom) Penetration of Plunger mm Force Gauge Reading div Force on Plunger kN Penetration of Plunger mm Force Gauge Reading div Force on Plunger kN 0.00 0 0.00 4.00 710 18.09 0.25 142 3.61 4.25 734 18.70 0.50 232 5.91 4.50 755 19.24 0.75 278 7.08 4.75 773 19.70 1.00 327 8.33 5.00 791 20.15 1.25 378 9.63 5.25 809 20.61 1.50 417 10.63 5.50 829 21.12 1.75 458 11.67 5.75 850 21.68 2.00 494 12.59 6.00 863 21.99 2.25 527 13.43 6.25 886 22.58 2.50 557 14.19 6.50 908 23.14 2.75 588 14.98 6.75 924 23.54 3.00 613 15.62 7.00 942 24.00 3.25 642 16.36 7.25 960 24.46 3.50 665 16.94 7.50 973 24.76 3.75 689 17.56 Calculation of CBR for top Penetration Load Standard load CBR   mm kN kN %   2.5 0.43 13.2 3.26   5 1.02 20 5.1 Accepted Calculation of CBR for Bottom Penetration Load Standard load CBR   mm kN kN %   2.5 14.19 13.2 107.50 NA 5 20.15 20 100.75 NA The CBR is 5.1% where as the values of the bottom are to high this might be due to any experimental reasons. Hence these readings are discarded. Moisture Content     A1 A2 Mass of dry soil + container = g 114.58 113.67 Mass of wet soil + container = g 131.64 126.09 Mass of container = g 23.9 38.82 Mass of moisture = g 17.06 14.42 Mass of dry soil = g 90.68 74.88 Moisture Content = % 18.81 19.27 Average Moisture Content = % 19.04 Discussion It was expected that the curves of the CBR test results will come out to be as shown below, in figure (b) But the results our test show that the curve for the bottom is above the curve of the standard this is not correct. Hence there is some source of error. These errors can be due to any reason; this could be due to the faulty apparatus, and there might be a permanent error in the dial gauges. There might be some initial reading that could have been considered and not eliminated later. There is a large error seen in the readings of the bottom of the CBR test, the result shows that the CBR value is more than 100% that is not possible. So these reading are not considered and discarded. Therefore the CBR value of 5% is found as appropriate value. Then there are certain considerations to be taken while the experiment to have accurate readings. Firstly the face of the plunger must be leveled onto the sample face and no kern should be there. Then the surcharge plate must also be placed which will restrict from the heaving of the soil. After this the sample must be made through standard compaction methods and no of blows must be 62 and the speed of penetration must be precisely 1mm/min. The reading of the moisture content along with all other reading is must this is because the increase or decrease in moisture content will affect the compaction of soil sample and hence the readings of penetration of the plunger. Conclusions The experiment was valid for the readings of the top layer and due to some reason the readings for the bottom were faulty and hence the CBR value is 5%. The Optimum moisture content should also be determined so that it can be compared with the actual moisture content. References 1. BS 1377-4:1990, British Standards Institution. (August 1990). Methods of test for soils for civil engineering purposes. Compaction-related tests . BSI. BS 1377-4:1990 2. BS EN 130364:2003 Road and airfield surface characteristic test methods British Standards Institution. 3. C.R.I. Clayton, M.C. Matthews and N.E. Simons . (1196). Chapter 8 Laboratory testing. In: . "Site Investigation". 2nd ed. Department of Civil Engineering, University of Surrey: .http://www.geotechnique.info/SI/SI%20Book%20Chapter%208.pdf Read More