The caves and cantilever-like cliffs, which are called natural rock structures in this article, are caused by the dissolution and/or erosion of rock masses by sea waves, winds, river flow or percolating rainwater. They may present some engineering problems especially in urbanized areas along shorelines and riversides. The stability problems may arise in the form of huge sinkholes and cliff failures. The authors have been recently involved with the stability assessment of natural rock engineering structures in Ryukyu limestone formation as well as the intact and in-situ properties of Ryukyu limestone. The present article presents some procedures for the assessment of stability of the natural rock structures in Ryukyu limestone formation. The stability estimations from various methods are described and compared with each other and their implications in geo-engineering field are discussed in this article..
The caves and cantilever-like cliffs, which are called as natural rock structures, are caused by the dissolution and/or erosion of limestone by sea waves, winds, river flow or percolating rain water and they may present some engineering problems especially in urbanized areas along shorelines and riversides. The stability problems may arise in the form of bending failure, shearing failure and toppling failure depending upon the natural discontinuities of rock mass as well as the geometry of natural rock structures. However, there are very few studies for evaluating the stability of rock cliffs and natural rock caves. Ryukyu limestone is widely distributed all over Ryukyu Islands and they form very steep cliffs along the shorelines of Ryukyu Islands. The toe of these cliffs is often eroded by sea waves and they result in overhanging rock cliffs. When the erosion depth reaches to a given depth, they topple as seen in Figure 1. One can see such examples along the shorelines of Ryukyu Islands. Similarly, the percolating ground water along faults and rock fractures can create natural caves of various scales. There are many spectacular examples in the Islands of Ryukyu. Some of the carstic caves are Gyokusendo, Ishigaki, Nakabari and Kin. Figure 1 shows an example of toppled and overhanging cliffs together with two caves at the site of Gushikawajo remains in Itoman city of Okinawa island.
The Ryukyu limestone is porous and its porosity varies between 4% to 30%. It is generally classified as sandy limestone and coral limestone and their physical and mechanical properties are 1516 given in Tables 1 and 2 [1,2,3]. Although coral Ryukyu limestone is quite porous it may be classified as medium strength rock and it is less prone to water content variations.
The authors carried out visual observations and explorations on Ryukyu limestone cliffs. The cliffs and caves at Gushikawajo site are constitued by coral limestone and bedding thickness is generally greater than 1–2m. There are widely spaced subvertical joints in Ryukyu limestone layers.