Abstract: Although piezocone penetration test (CPTU) is widely used in engineering geological exploration, it has undeniable deficiencies in detection accuracy of shallow gas-bearing strata. Based on CPTU penetration principle, the shear deformation characteristics of soil at cone shoulder during penetration are analyzed. Furthermore, using the consolidated undrained triaxial shear test of dense gassy sand with different initial saturations, the response law and difference of excess pore pressure in soil at cone shoulder are discussed when CPTU probe penetrates into saturated and gas-bearing strata. The results show that, the presence of gas can significantly affect the shear deformation characteristics of sand, and with the increase of gas content, the shear volumetric strain increases, and transitions from shear dilatancy to shrinkage; however, the sensitivity of pore pressure response in soil decreases, leading to a non-negative characteristic phenomenon, which can be regarded as an important symbol of gas-bearing strata identification. According to the characteristics of shallow gas reservoir distributed in Jiangsu and Zhejiang coastal plain in China, a fine identification method of gas-bearing strata based on in situ multi-function piezocone penetration test is developed. In this method, the “dual control” index composed of silt or fine sand soil property identification and non-negative characteristics of excess pore pressure is used for preliminary judgment, and then combined with multi-functional auxiliary parameters for further inspection and correction. The validity of identification method is preliminarily verified in a practical engineering, and the identification accuracy of multi-layer gas-bearing strata is thereby improved.

Key words: piezocone penetration test (CPTU), shallow gas, geological disaster, gassy soil, identification method