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

This paper deals with the problematic weathering of a 150 year old historic stone bridge made of welded tuff. The weathered condition of the structurally important arch stone from the bridge is studied comprehensively by physical, chemical, and mechanical methods. As a result of this study, a physical weathering degree index (PWD), a chemical weathering degree index (CWD) and related experimental methods are proposed for a subsequent study of rock weathering. The PWD and CWD indexes are represented respectively by the effective porosity of and the combined water content in the weathered rocks. Considering that chemical and physical weathering simultaneously affect the mechanical properties of rocks, a total weathering degree index (TWD) is proposed to enable the comprehensive study of rock weathering. At the surface of the arch stone, PWD, CWD, and TWD were 7.35, 2.43, and 6.12%, respectively. Physical weathering was characterized by an increase of effective porosity within a zone of ca 5 cm depth. Chemical weathering had advanced to ca 10 cm depth by processes of hydration and oxidation. No chemical leaching or clay alteration caused by weathering was observed. The physical weathering is the major weathering mode in the 150 years of weathering history of the bridge. Both dynamic and static mechanical properties appeared to decrease linearly with the total weathering degree. At the surface, the dynamic elastic modulus had decreased ca 36%; wall strength of the joints had decreased by ca 17%; and normal stiffness had decreased by ca 68%.