Abstract: The weak interlayer soil between the marlstone and limestone in the third member of the Badong formation, which widely exists in Badong County of the Three Gorges Reservoir Area, was taken as research object. The effect of CaCO₃ content (0%, 2%, 4%, 6%) on the shear strength of weak interlayer soil were studied via unconsolidated undrained triaxial test (UU). Moreover, the changes of internal porosity and cementation degree of soil samples before and after the triaxial test were observed using CT scanning. On such basis, the mechanism of shear strength variation of calcium-containing weak interlayer soil was analyzed. Results indicated that the shear strength of interlayer soil increased with the rising of CaCO₃ content in soil samples because CaCO₃ enhanced the internal friction angle of soil by reducing soil porosity and changing soil particle morphology. In the meantime, the cohesion of the soil was strengthened by improving the cementing ability of the soil, thereby increasing the shear strength. In addition, the change of cohesion was more affected by the content of CaCO₃, while the change of internal friction angle was less affected by the CaCO₃ content.

Key words: weak interlayer soil, shear strength, CaCO₃, triaxial test, CT scanning, Badong formation