Abstract: The principle of liquid and plastic limits water content joint measurement method is that the depth measured by a cone penetration of a cohesive soil is linear with its water content in the double logarithmic coordinate system. In order to ensure the accuracy of these results or to judge abnormalities, it is needed to measure three different depths of one uniform cohesive soil samples in different water contents to calculate the liquid limit water content and the plastic limit water content. The author’s years’ experience in the liquid and the plastic limits analysis has shown that most types of soil have excellent linearity. Based on the mathematical theorem that two points determine the line, any two points can obtain the line. Further study of the two-point method, the ideal depth range was found. It is shown that the two-point method can ensure the quality better and the lower out-of-tolerance rate by the repeated test to three-point method. In this paper, ideal depth range of the two-point method is obtained by the theoretical deduction and is verified by experiments.

Key words: Limit Water Content, Liquid and Plastic Limits Water Content Joint Measurement, Liquid and Plastic Limits, Two-Point Method