Californian Bearing Ratio Test - Lab Report Example

Californian Bearing Ratio Test Objectives The test results are used along with some empirical relationships to determine the thickness of the different layers of pavement which the soil can sustain. 2. The results and test value are used as basis of the decision of the usage of soils materials such as backfill, sub grade etc. 3. This test also educates the design engineer of the response of soil of with the increase or decrease of moisture content of soil and then relates this moisture content with the CBR value of the soil. 4. The CBR value indirectly provides the level of strength and ability of resistance of the soil subgrade and its deformation under loading (heavy/light) Apparatus: 1. Electronic Weighing balance with precision of 0.1 g 2. Motorized Compression Machine with a speed of 1mm/ min 3. Calibrated Proving Ring for load measurement within a good range of 100 kN 4. Dial Gauge for measurement of penetration through displacement 5. Compactor of 2.5 Kg rammer 6. Length measuring devices, such as vernier caliper, steel ruler 7. Standard CBR mould and all fittings 8. Drying Oven 9. Sieves for Sieve analysis and sample preparation. Procedure The step in the procedure of this test is the preparation of sample soil that is to be tested. The soil is passed trough the 20 mm test sieve and a sample of about 6 kg is acquired. As shown in the picture below. After this, the Compaction of the soil sample is done considering the BS 1377(ordinary tests) as reference. According to which the soil sample is placed in the mould and hammered with a rammer of 2.5 kg, from a height of 300mm about 62 times. There are three layers prepared with similar manner to acquire a finished standard sample. As shown in the figure below. Then the next stage is the penetration, the mould prepared is placed in the compression machine and a surcharge plate is placed at top of the sample. Then a bedding load is applied which is 50 kN, after which all dial gauges are reset to zero. Then compression machine starts to apply load at a rate of precisely 1mm / min. Keep the load increment of 0.25mm and record the dial gauge readings at each interval until a total of 7.5 mm of penetration is seen.As shown in the picture below. After the complete penetration of 7.5mm unload the sample from the machine and acquire some wet soil sample and after recording its weight place it oven to dry and later calculate its moisture content. Then the calculation are done as following. Calculations and Analysis of Data: Same formula is used for 5mm penetration Penetration Load Standard load CBR mm kN kN % 2.5 3.7 13.2 28.0 5 7.61 20 38.05 Density and moisture content Diameter of the Mould = 152 mm Depth of the Mould = 127mm Volume of the Mould = 2304521 mm3   2.30x10-3 m3 Density of Soil Tested = 1993.49 kg.m-3 Moisture Content Mass of container = 544 g Mass of moisture = 18.6 g Mass of wet soil + container = 1014.1 g Mass of dry soil = 451.5 g Mass of dry soil + container 995.5 g Moisture Content = 4.11 % Plotting of Data Discussions and Conclusions (a) The points during the experiment where there is a possibility of errors are as following i) Make sure the sample preparation is done taking all the consideration of the standards and the no of blows by the rammer must be carefully monitored. ii) Then the surcharge disc must be perfectly leveled on to the face of the soil and mould. iii) The face of the plunger must be perfectly leveled to the face of the soil sample before the commencement of the test, as uneven faces will result into improper application of force. iv) All gauges must be reset to zero before the test and all the reading during the test should be taken carefully without parallax errors. v) The CBR test can only be standardized if the penetration rate of plunger is exactly equal to 1 mm /min. vi) The soil that is to be tested must be cleared from any contaminations that is pebbles, leaves or any such materials. This is initially done by sieving the sample from 20mm sieve. (b) It is seen that with the increase of moisture content the compaction of the soil is also increased but after a certain point which is know an OMC (optimum moisture content) the increase in moisture results into decrease in compaction. So the increase in moisture content will increase the CBR value till the OMC is acquired after which the CBR will decrease. (c) The design value can be decided on the basis of testing and analysis of those tests. The steps would go as follows i) Different soil specimen are acquired of the same type and different moisture content are introduced into them ii) For these samples the bulk unit weights and dry unit weights are recorded. iii) A plotting of the above data is done between percentage moisture and dry density of sample. The maximum/peak point of this graph is the OMC optimum moisture content and against this there is a dry density value as well. iv) In the next steps would be to make different samples with different compactive efforts that is, the no of blows by the rammer is reduced and varied eg 10 blows, 30blows, and 56 blows . Then CBR test would be performed at each of these samples and values at 2.5 mm and 5.0 mm penetration is recorded and CBR value calculated. v) Then a graph is plotted between dry density and CBR value. vi) As we are aware that we have maximum dry density (one from the graph of moisture content and dry density and this is value corresponding to OMC) vii) We will locate the maximum dry density which is normally 95% value of the original maximum dry density, on the graph (CBR vs dry density ) from this we will finally get the design CBR . Concluding the design CBR value will be the point on intersection of 95% max dry density and the CBR curve. (d) The general reason for this is that these countries have very high water tables/levels. This might be due to several reasons which could be very high precipitation or may be areas of depression. They are so high that they rise to the level of the road sub grade and the soil gets fully saturated with water. In order to simulate the field condition in the lab, the samples are soaked samples. So that the response of the soil can be recorded and the pavement could be designed accordingly and remedial measures diagnosed. Conclusions: The CBR value for our sample is about 38 % this is a very good value for sub grade and it could be considered as high strength sub grade. It is suggested that this soil can be used as sub grade in a “Rigid Pavement”. Variations for rigid pavement section (NCHRP 1-37A, 2002) 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. United States Department of Transportation - Federal Highway Administration. (04/07/2011). Geotechnical Aspects of Pavements Reference Manual. Available: http://www.fhwa.dot.gov/engineering/geotech/pubs/05037/01.cfm. Last accessed 03/12/11. 3. BS EN 130364:2003 Road and airfield surface characteristic test methods British Standards Institution. 4. Highways agency( 2009)."design manual for roads and bridges vol.1 -15 Published by the highways agency, transport scotland welsh assembly Government and the northern lreland department for regional development 5. Civil Engineering Portal. (.). Determine The Maximum Dry Density And The Optimum Moisture Content Of Soil. Available: http://www.engineeringcivil.com/determine-the-maximum-dry-density-and-the-optimum-moisture-content-of-soil.html. Last accessed 03/12/11. Read More