Impact of Water Saturation on Physico-Mechanical Properties of Fragile Chandpur Phyllite at Varying Foliation Angles

The effect of water saturation in the Himalayan region is a critical factor influencing the physico-mechanical properties of rocks. Typically, the anisotropic and fragile rock formations are vulnerable to it, as it may directly/indirectly cause directional variation of rock properties. One such notable formation is phyllite, within which numerous geotechnical engineering projects are ongoing or completed. This study aimed to assess the impact of water saturation on the physico-mechanical properties of highly foliated Chandpur phyllite at varying foliation angles. The findings revealed a decreasing trend in P-wave velocity with increasing foliation angles, where higher values were observed in saturated specimens. Mechanical properties exhibited a “U-shoulder” type anisotropic curve with the foliation angle, reaching minimum values at β = 30° and maximum values at β = 90°. Although the overall trend of the mechanical properties remained consistent with the foliation angle, the intensity and magnitude were significantly influenced by saturation. At the microscopic level, the failure surface of the specimen having β = 30° displayed a smooth surface with negligible broken fragments indicating reduced cohesion and lower strength with failure along foliation, regardless of saturation. In contrast, specimens at β = 90° exhibited a rough/jagged surface with numerous broken fragments suggesting increased cohesion and higher strength with failure across foliation. The mineralogical analysis further highlighted the role of well-developed micaceous minerals, in the form of foliation planes, with the rock matrix present between them, influencing the directional behavior of phyllite.