Comprehensive study of the weathered condition of welded tuff from a historic stone bridge in Kagoshima, Japan

摘要

This paper deals with the problematic weathering ofa 150 year old historic stone bridge made of welded tuff. Theweathered condition of the structurally important arch stone fromthe bridge is studied comprehensively by physical, chemical, andmechanical methods. As a result of this study, a physicalweathering degree index (PWD), a chemical weathering degreeindex (CWD) and related experimental methods are proposed for asubsequent study of rock weathering. The PWD and CWD indexesare represented respectively by the effective porosity of and thecombined water content in the weathered rocks. Considering thatchemical and physical weathering simultaneously affect themechanical properties of rocks, a total weathering degree index(TWD) is proposed to enable the comprehensive study of rockweathering. At the surface of the arch stone, PWD, CWD, andTWD were 7.35, 2.43, and 6.12%, respectively. Physicalweathering was characterized by an increase of effective porositywithin a zone of ca. 5 cm depth. Chemical weathering hadadvanced to ca. 10 cm depth by processes of hydration andoxidation. No chemical leaching or clay alteration caused byweathering was observed. The physical weathering is the majorweathering mode in the 150 years of weathering history of thebridge. Both dynamic and static mechanical properties appeared todecrease linearly with the total weathering degree. At the surface,the dynamic elastic modulus had decreased ca. 36%; wall strengthof the joints had decreased by ca. 17%; and normal stiffness haddecreased by ca. 68%.