Abstract: Water-rock interaction has a significant influence on the strength characteristics of soft rock. In this paper, the uniaxial compression test and point load test were conducted on typical Cenozoic red sandstone that was taken from Chizhou City, Anhui Province, China. The empirical conversion coefficient between point load strength (PLS) and uniaxial compression strength (UCS) and the softening law of water content on PLS were studied. Meanwhile, the softening mechanism of this sandstone was also discussed based on the analyses of mineral composition and microscopic structure as well as the results of water absorption tests. The resultes show that: (1) in the natural state, the empirical coefficient for converting the point load strength to uniaxial compressive strength of the Cenozoic red sandstone should be appropriately reduced, and it is recommended to take 9.0; (2) The PLS and penetration ratio decreased with the increase of water content, showing a trend of negative logarithmic decay; (3) The characteristic value of saturation of water absorption softening is put forward. This value divides the water absorption softening area into sensitive and non-sensitive areas, and the strength in the sensitive area changes dramatically with the change of water content. (4) The swelling-softening effect of clay minerals is the micromechanism appearing strength softening. Therefore, the characteristic saturation value should be universally suitable for soft rocks containing lots of clay minerals. (5) The Cenozoic red sandstone has a high clay mineral content and more internal microfractures. Because of clay minerals' water-absorbing expansion, crack expanding, and developing, the overall cementation ability of the rock is weakened and the structural system is destroyed, resulting in the reduction of the overall rock strength. This study provides a reference and basis for analyzing strength softening mechanism and strength softening parameters selection in the red sandstone field.

Key words: Cenozoic red sandstone, point load strength, empirical conversion coefficient, softening law, softening mechanism