Relationship between Iron Sand Mining and Slope Failures in Granitic Areas

Abstract

In the granitic areas of the Sanin district, Japan, an older style of iron manufacturing using iron sand, a process called Tatara, was widely used up until the Taisho era early in the 20th century. This study compares the distribution of the old iron sand quarries with the sites of slope failures associated with heavy rainfall in the Chugoku Mountains on the border between the Tottori and Shimane Prefectures. Field investigations and laboratory testing of rock samples shows that the weathering product of granite is characterized by extensive micro-cracking, which allows for easy mining of iron sand using simple tools. However, this character is also an important factor in slope failure. The change in slope stability with the development of the weathered layer is calculated using a slope model, and it is shown that higher slopes are susceptible to failure as weathering of the slope surface progresses. In contrast, lower slopes are resistant to failure even when weathering is advanced. Therefore, while deep wethering of granite facilitated the mining of iron sand, failure occurred on higher slopes with even very shallow weathering. The old iron-sand mining sites are considered to be related to slope failure by a joint dependence on weathering and the development of topography.