Abstract: The efficiency of the current combined determination method for liquid and plastic limits for the limit moisture content of fine-grained soil is low, and the automation of the test process is difficult to be realized. Hence, an extended application of the test principle of current combined determination method of liquid and plastic limits is discussed. It is concluded that the water content at each grade of soil paste can be obtained by monitoring the water increment in the process of preparation of soil paste, and it is not necessary to use only the drying method to test the water content of each
grade of soil paste. Moreover, the calculation methods for moisture content of soil paste at all levels by using both dry and wet samples are put forward. Thus, on the basis of a large number of observed test operation timing results via the combined determination method of liquid and plastic limits of fine-grained soil in the survey process of Kunming Rail Transit Line 4, the evolution analysis of the current test method system function is made, and the optimized limit moisture content test process is obtained. The optimized process can reduce the man-machine operation time by 75%, and the time required to complete the limit moisture content test of single group of samples can be decreased from 16. 5 h to 6. 5 h. According to the characteristics of the optimized test flow and test method, the automatic realization scheme of liquid and plastic limit closed joint test method is put forward, and the test application conditions are preliminarily discussed.

Key words: soil test, limit moisture content, combined determination of liquid and plastic limits, closed-type combined
determination of liquid and plastic limits