CPTU Classification Diagrams for Identification of Sensitive Clays

Abstract

When dealing with slope stability considerations in deposits where sensitive and quick clays might be encountered it is vital to map the extent of these clays. For the geotechnical engineer, the cone penetration test with pore pressure measurement (CPTU) is a powerful tool in this respect. With its combined measurement of tip resistance, pore pressure and sleeve friction, the CPTU holds a great potential for identification of quick and sensitive clays. Such interpretations can be done based on measured data directly or by combining parameters in dimensionless numbers. Amongst the more popular dimensionless numbers are the pore pressure ratio (B q ), the cone resistance number (N m ) and the friction ratio (R f ). Diagrams exist which allow classification of soils based on the combination of such numbers. Robertson (Can Geotech J 27:151–158, 1990) is one widely used example. However, In Norway, it is found that existing diagrams to a large extent fail to identify sensitive and quick clays. Based on a database of 10 Norwegian sites a new set of classification diagrams are presented with focus on identifying quick and sensitive clays. The diagrams are based on a pore pressure ratio where the tip pore pressure is used (u 1 ) rather than the u 2 -position as this is found to better capture the actual collapsible response of sensitive clays. The cone resistance number is modified to also include an effect of overconsolidation (OCR) instead of only accounting for vertical effective overburden. Also, the friction ratio is normalized with pore pressure (u 1 ) rather than the cone resistance. Electrical resistivity values from R-CPTU-soundings are also included in the considerations. The outcome is a set of revised classification diagrams that provides more accurate identification of Norwegian sensitive and quick clays compared to existing classification diagrams.