Effects of rock strength and location heights on growth rates of tafoni-like depressions at sandstone blocks used for a masonry bridge pier in the coastal spray zone

摘要

Yayoi Bridge, connecting Aoshima Island with Kyushu main island in Japan, is supported by four piers whose surface is composed of sandstone blocks. The base of the piers are situated at Mean Tide Level (mean tidal range: 1.6 in), and the side walls of the piers have a height of 3 m. Each sandstone block has developed a tafoni-like depression in all blocks due to salt weathering and wave erosion. The greatest depth of each depression in all blocks was measured in 1971, 1989, and 2001 corresponding to 20, 38, and 50 years, respectively, since the construction of the bridge. Rock strength in all sandstone blocks was also measured using an Equotip hardness tester. These data showed that the depth of depressions was smaller in the blocks having larger strength. The depth of depression, D, was described as a function of the salt weathering susceptibility of sandstone blocks W-s, the height above mean high water level h* (m) located the blocks, and time t (years); D = 68.6 (W-s) e(-0.0038 h*) (1-e(-0.031 t)) where W-s = P/S-t (P is the total potential pressure of salt crystallization generated in unit volume of the rock, and S-t is the tensile strength estimated from Equotip hardness).