基于CPTU的中国实用土分类方法研究

国际上已有比较成熟的基于原位孔压静力触探测试（CPTU）的土分类方法,这些方法多以美国统一土质分类标准（USCS）为依据,其土类与中国土的工程分类标准不对应。因此,建立基于CPTU原位测试参数、符合中国标准的土工程分类方法具有重要工程意义。通过选择江苏典型地质成因的试验场地,进行了95孔CPTU测试和相应的钻孔取样与室内土工试验,其CPTU孔和相应钻孔之间距离小于5 m,对采用室内土工试验依据中国国家标准《岩土工程勘察规范》得到的土分类和CPTU测试参数进行了系统对比分析。选择国外7种常用的CPTU土分类图进行应用比较,发现Robertson的土类指数法分类图能提供最好的分类结果,据此建立了中国基于CPTU土类指数法的实用土分类方法,已有测试资料应用表明该分类图的准确率达到95%以上,可用作为中国工程场地采用原位CPTU测试进行土工程分类的方法。

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.