Experimental Study On Combined Test Method To Determine The Liquid-plastic Limit Of Fine Grained Soil

A series combined test method tests were performed to determine the liquid-plastic limits of different samples of fine grained soil and silty sand. The result showed that the loglog relationship between the depth of penetration of fall cone and the water content of some samples were clearly not linear, and the liquid-plastic limit could not be obtained according to the linear relation. Effective test data could not be measured within the three arranges of water contents proposed by the present standards. The dependability was necessary to discuss about the liquid-plastic limit test results obtained from three arranges. It was possible that the contradictory and wrong classification results were attained using the liquid-plastic limit obtained from the test data within straight line. The reliability of the method to measure the plastic limit and liquid limit of fine grained soil, which were the necessary parameters for the classification and the physical condition determination, was correlative with the engineering application. There are great academic significance and referenced practical value to study deeply into the applicability of the combined test method for fine grained soil.Making references to abundant domestic and overseas literature, the principle and the applicability of the combined test method were discussed from the it's theory according the above phenomena and deduced that, the results of the test with this method only reflect the shear strength of the soil. The combined test method was suitable for the soil with enough clay particles content in which the shear strength was mainly controlled by the cohesion.At first, some kinds of test were made consisted of the combined test method tests and the mechanical composition analysis with different kind of fine grained soil from different source. The results showed that the lowest clay particles content of the soil which was suitable with the combined test method was 13%. The test was suitable for the samples over 13% clay particles content since the shear strength was mainly controlled by the cohesion and the plasticity was available from the plasticity indexes.To validate the above conclusion, a series of mixture tests were performed and different samples were made by mixing each of three kinds of nature silt and the nature clay with different mixture ratio. The liquid-plastic limits of all the samples, in which the clay particles content were between the nature silt and the nature clay, were attained using the combined test method. The results showed that the lowest clay particles content of the silt, when it was suitable with the combined test method, is 12% to 15% with little differences. If the point of 3～4mm depth of penetration of fall cone exist, and the loglog relationship between the depth of penetration of fall cone and the water content of all the points were clearly linear, it could be concluded that the clay particles content of the sample is over the lowest value when the combined test was suitable.At last some opinions about the methods of fine grained soil classification and the test step were given. The combined test method could be first test. If the result showed that the point of 3～4mm depth of penetration of fall cone exist, and the loglog relationship between the depth of penetration of fall cone and the water content of all the points were clearly linear, it could be concluded that the test method was suitable. The plasticity was available from the plasticity indexes. The mechanical composition analysis could not be required and the classification could be made using the liquid-plastic limit with the plasticity chart. Otherwise the combined test was not suitable and the classification should be made using the results of the mechanical composition analysis.