Practical soil classification methods in China based on piezocone penetration tests

The parameters obtained from the piezocone penetration tests (CPTU) are commonly used for soil profiling and geostratigraphy, along with the parameters used for assessment of geotechnical design. Numerous classification charts of soil behaviors are developed, but most of them are on the basis of ASTM Unified Soil Classification System (USCS), which is different from China' standards of soil classification. The relationship between CPTU measurements and engineering classification standards of soils in China needs to be established to take full advantage of piezocone soundings. Piezocone penetration tests, borings and laboratory tests are carried out at 95 locations of typical geologic formations in Jiangsu Province, China, in which, the distances between CPTU soundings and adjacent boreholes are less than 5 meters. Thus the CPTU data are fully analyzed and validated for providing a reasonable soil classification. Based on a systematic review of literatures, seven classification charts of soil behaviors are chosen to determine the soil types from the CPTU data at all the test sites, and these results are further validated with the soil classification types determined from the field samples and laboratory test results according to China's standards. It is shown that compared with the other six classification charts, the soil behavior classification based on the soil behavior type index I_c(RW) proposed by Robertson (2009) can give the best results, but the chart needs to be modified to adapt the region-specific feature. Thus, a soil classification chart based on the CPTU modified from Robertson (2009) and coordinated with China's standards is developed. In this chart, soils are classified as middle sand, fine sand, silty sand, silt, silty clay, clay, muck and mucky soils based on a normalized cone parameter Q_tnwith a variable stress exponent and normalized friction ratio F_r.