Abstract: In geotechnical engineering practice, the specific classification of a clay soil is generally determined by measuring its liquidity and plasticity index, and the basic engineering properties of the soil are determined accordingly. However, the liquid-plasticity limit (LPL) method recommended in the Standard for Geotechnical Test Methods (GB/T 50123—2019) has some shortcomings as the soil samples produced by the method differ significantly in dry density, which makes the data points obtained during the test discrete in the double logarithmic coordinate axis. In this paper, a new test method, the vertical penetration stratified sampling liquid-plastic limit joint test method, is developed by improving the liquid-plastic limit joint test method recommended in the national standard. The proposed method is more rigorous in the preparation of soil samples and shows a good linear relationship on the double logarithmic axis. The boundary moisture content of four soil samples, namely, reticulated red soil, brown caliche, silt loam and tidal soil, is tested in house to verify the uniformity of dry density and the richness of moisture content distribution levels of the prepared samples. Comparison between the proposed method and standard-recommended method as well as plate shear test result further verify the rationality of the present method.

Key words: liquid-plastic limit, liquid-plastic limit combined testing, dry density, moisture content, depth of cone penetration