Effect of geomorphologocal setting on the erosion rates of sandstone in the coastal spray zone

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Yayoi Bridge, connecting Aoshima Island with Kyushu main island in Japan, is supported by four piers, which are masoned with sandstone blocks and are frustum of pyramid in shape, 3 m in height and 70ﾟ in wall angle. Four side walls of the piers face approximately east, south, west, and north, respectively. Each sandstone block has a dish-like depression due to weathering and erosion. The depth of the depression was measured for all blocks of the four piers in 1992 and 1994 corresponding to 41 and 43 years after the completion of the bridge respectively.The results of our research are summarized as follows.1. The average erosion depth is the largest on the south-facing wall, becomes gradually smaller on the west- and east-facing walls in this order. This difference in erosion depth acording to wall aspect mainly due to insolation. The relationship between erosion depth and insolation is expressed by linear function. The regression lines of four piers are parallel on graph paper. The uppermost line is a line of 4th pier and the lowest is of 1st pier.2. A new technique for nondestructive in situ rock-surface moisture monitoring was developed and established. This method and instruments are very useful to clarify the actual conditions and the mechanism of salt weathering.3. The measurement of rock-surface moisture change indicates that the distribution of large fluctuation in rock-surface moisture is coincident with the distribution of deep depression.4. Thermal infrared images of the piers were taken by themo-tracer. From the images, we can obtain useful informations on salt weathering.