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 structura lly important arch stone from the bridge is studied comprehensively by phys ical, chemical, and mechanical methods. As a result of this study, a physic al weathering degree index (PWD), a chemical weathering degree index (CWD) and related experimental methods are proposed for a subsequent study of roc k weathering. The PWD and CWD indexes are represented respectively by the e ffective porosity of and the combined water content in the weathered rocks. Considering that chemical and physical weathering simultaneously affect th e mechanical properties of rocks, a total weathering degree index (TWD) is proposed to enable the comprehensive study of rock weathering. At the surfa ce of the arch stone, PWD, CWD, and TWD were 7.35, 2.43, and 6.12%, respect ively. Physical weathering was characterized by an increase of effective po rosity 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 leach ing or clay alteration caused by weathering was observed. The physical weat hering is the major weathering mode in the 150 years of weathering history of the bridge. Both dynamic and static mechanical properties appeared to de crease linearly with the total weathering degree. At the surface, the dynam ic elastic modulus had decreased ca. 36%; wall strength of the joints had d ecreased by ca. 17%; and normal stiffness had decreased by ca. 68%. (C) 199 9 Elsevier Science B.V. All rights reserved.