California Bearing Ratio at Optimum Moisture Content - Lab Report Example

Determination of California Bearing ratio at Optimum Moisture Content Level Group Sunday, 11 September Determination of California Bearing ratio at Optimum Moisture Content Abstract The experiment was aimed at the determination of the California bearing ratio of a soil sample. After carrying out the experiment, the California bearing value of the soil sample was determined to be 2.123 %, the soil moisture content of 34.49024 %, bulk density of 1941.1978 kg/m2 and a dry density of 1443.3745 kg/m2. Introduction Construction of pavements is done by constructing a number of layers over each other. During the design process. It is essential to ensure that none of the layers gets stressed under application of load. The design of the pavement should be such that sections of the pavement are not subjected to excessive deformation that would otherwise result in settlement of the section or localized deformation. One of the empirical methods used in the design of pavements is the California bearing ratio test abbreviated as CBR test. CBR value is determined by carrying out a CBR test on the subgrade soil. The CBR value depicts an index that shows the soil bearing capacity and the soil strength. Using the CBR value, sub – base and base material of a pavement to be constructed can be easily designed (Knapton). Using the CBR value, evaluations of the strengths of the soils can be established by using CBR value as an indicator. The CBR value typically represents a percentage of the soil with a standard crushed soil from California. Ultimately, the CBR value is mainly used in the quantification of the response of the subgrade and pavement to loading (Indian Institute of Technology Kanpur). There are various factors that affect the CBR test. These include the soil density, the soil texture and the soil moisture. Thus, the testing procedure to be used for a specific CBR test is completely dependent on the material type being tested. A drawback to the CBR test is that it gives the total thickness of material to be applied above the subgrade. The thickness is constant for a particular pavement irrespective of the materials that are used in the construction of the layers (Yoder and Witczak). In the California bearing Ration test, the objectives are the determination of the CBR value of the soil that is being considered to be used as a foundation of a pavements. This is mainly done to facilitate the evaluation of the subgrade soil strength (Purushothama). It should be noted that CBR does not provide information about the soil properties such as cohesion shearing resistance etc. Experimental procedure Apparatus required to carry out the CBR test are; i. Compaction mould ii. Loading frame iii. Compacting hammer iv. Oven v. Two dial gauges Procedure 4.5 Kg of the sample to be tested was collected from the field. A soil sample was then taken from this sample a compacted in a CBR mould in 3 layers using 2.5 Kg hammer with 260 blows. After the compaction, the mould was then placed in platen of the testing machine after which the seating force was applied to the specimens. The specimen is then penetrated by the plunger at a rate of 1 mm/min while taking the readings of the applied forces ta an interval of 0.25 mm. These were done up to a maximum penetration of 7.5 mm. Immediately the penetration test is completed on the specimens, a soil sample that is about 350 g is taken form the sample and the sample is used in the determination of moisture contents which is used in the calculation of the dry density of the soil. Results and Calculations. The results for the test were as shown in the table 1 below Penetration of plunger (mm) Force Gauge Reading div Force on Plunger KN 0.00 0 0 0.25 4 0.10192 0.50 5 0.1274 0.75 6.5 0.16562 1.00 8 0.20384 1.25 8.5 0.21658 1.50 9 0.22932 1.75 9.5 0.24206 2.00 10 0.2548 2.25 10.5 0.26754 2.50 11 0.28028 2.75 11.5 0.29302 3.00 12 0.30576 3.25 12.1 0.308308 3.50 12.5 0.3185 3.75 12.8 0.326144 4.00 13 0.33124 4.25 13.1 0.333788 4.50 13.4 0.341432 4.75 13.7 0.349076 5.00 14 0.35672 5.25 14.1 0.359268 5.50 14.2 0.361816 5.75 14.4 0.366912 6.00 14.7 0.374556 6.25 14.9 0.379652 6.50 15 0.3822 6.75 15.1 0.384748 7.00 15.2 0.387296 7.25 15.3 0.389844 7.50 15.4 0.392392 Calculations and analysis The force applied was calculated using the formula below. The units of the force were in kN. The calculation of the force for all the entries was done in excel and the values recorded in table 1 above. A sample calculation of the force for a single entry is like as shown below. Force gauge reading of 4 has a plunger penetration of 0.5mm. Since the mean gauge calibration is 25.48 N/Div., the force is Using the results, a graph of force on the plunger vs. the plunger penetration was drawn as below. There was no correction required on the results since the graph curve is as expected. From the graph, there is the determination of the California bearing Ration. This is done by getting the forces at a plunger penetration of 2.5 mm and 5.0 mm then reporting the higher value. In this case, the penetration ate 2.5 mm has a force on the plunger of 0.28028 kN., while at 5.0 mm plunger penetration, the force on the plunger is 0.35672 kN. Thus the CBR values are calculates as For a penetration of 2.5 mm For a penetration of 5 mm Since the higher value is reported, then the CBR value for the sample is 2.123 % Results for the determination of the Moisture contents Container No. 1 Mass of wet soil + container (m2) g 389.2 Mass of dry soil + container (m3) g 341.5 Mass container (m1) g 203.2 Mass of moisture (m2-m3) g 47.7 Mass of dry soil (m3-m1) g 138.3 Moisture content % 34.49024 Average moisture content % 34.49024 The determination of the dry density of the soil sample uses the volume of the mould, and the bulk density of the soil. The volume of the mould is calculated as below; Formula for the mould volume is Since the diameter (D) of the mould was 0.152 m and the height (h) of the mould was 0.127 m, then the volume of the mould is Bulk density of the sample is calculated using the formula Mass of the sample in the mould was 4.5 kg and the volume of the mould is 0.0152 m2 thus the bulk density is Bulk density and moisture content is used to give the dry density of the soil sample. Thus the dry density of the soil is calculated as The drawing of the dry density against moisture graph was not done since in drawing such a graph, different moisture content has to be used. But in this case, only a single moisture content was done which was used to determine the dry density at that particular point. Discussion The CBR value of the sample being tested was 2.123 %. The soil has a moisture content of 34.49024 %, bulk density of 1941.1978 kg/m2 and a dry density of 1443.3745 kg/m2. A CBR of 2.123 % is considered to be small. This gives an indication that soil sample is from a tilled farm land. Using the value of the given CBR, it is now possible to determine the required thickness of a pavement if the design load is known. The curve on the graph of the force on the plunger vs. the plunger penetration is not smooth. This is an indication that there were some errors in carrying the experiments. This errors may include both systematic errors and random errors. The errors may due to parallax errors in reading the measurements, unwanted deflection due to the surface vibrations among other errors. Conclusion In the laboratory test, the California Bearing test value has been determined. This was the Amin objective of the experiments and it was done successfully. From the flow of the experiment, it can be concluded that California bearing ratio is the most cost effective, time saving method that can be used in the design of pavements as compare red to other methods. References Indian Institute of Technology Kanpur. CALIFORNIA BEARING RATIO TEST . 2004. Electronic. 8 April 2014. Knapton, John. Ground Bearing Concrete Slabs: Specification, Design, Construction and Behaviour. London: Thomas Telford, 2003. Electronic. Purushothama, Raj. Soil Mechanics & Foundation Engineering. new Delhi: Pearson Education India, 2008. Electronic. Yoder, E. J. and M. W. Witczak. Principles of Pavement Design. Ontario: John Wiley & Sons, 1975. Electronic. Appendix I Working safely 1. Before starting the test Getting acknowledged with the procedure of the test Wearing safety gears Students getting oriented on actions to take in case of emergency Orienting students to carry out the test on the best practices Checking the equipment to ensure they are working well before operation. 2. Safety during the process of taking the experiment When turning on and off of the machinery, hands must remain dry at all times The mould should be used with a lot of care since it is prone to causing injuries to users Carrying of the hammer should be carried with both hands and held firmly to prevent it from falling since this can injure feet. 3. After completing the test and preparing to leave the laboratory Clean the equipment and return the equipment to their respective storing places. Cleaning of the working benches Turning off of all running electrical equipment 4. Responsibilities of the students Students to ensure personal safety Reporting all incidents of accidents and injuries to the lab technicians Keeping the laboratory clean. Ensuring safety of other working partners Appendix II. Risk assessment Title CBR Susceptible people Mitigation measures Activity Likely hazard Work. Vis. Pub. Sev. Prob. CBR Electricity shock    3 1 Ensure hands are very dry Back hurt   2 2 Carrying the hammer with both hands ensuring it’s held firmly Read More