The site of the Ishigaki New Airport had several karstic caves, which crosses the runway at several locations. The bats in these caves were claimed to be precious species, the Okinawa Prefectural Government decided to construct the reinforced protection arch structures above the caves against their possible collapses. The authors carried out a series of computations using 2D and 3D numerical methods to check the static and dynamic stability assessment of natural underground openings with its protection arch structure. In this article, the authors would present the outcomes of the numerical studies and discuss their implications.
The natural underground openings are generally caused by the dissolution and/or erosion of rocks by sea waves, winds, river flow or percolating rain water and they may present some engineering problems especially in urbanized areas above karstic regions. The stability problems may arise in the form of sinkholes. However, the studies are very few for evaluating the stability of such natural underground openings in literature.
Ryukyu limestone is widely distributed all over Ryukyu Islands. Large engineering projects have been recently increasing in the Ryukyu Islands and natural underground openings present some stability problems to the superstructures on the ground surface. Although the numerical methods are superior over empirical methods and simplified analytical methods, the authors also explored the applicability of quick stability assessment techniques based on either empirical methods or simplified analytical methods for natural caves.
The site of the Ishigaki New Airport had several karstic caves, which crosses the runway at several locations. The bats in these caves were claimed to be precious species, the Okinawa Prefectural Government decided to construct the reinforced protection arch structures above the caves against their possible collapses. The construction of the protection structures was quite unusual in civil engineering and their design was a quite challenging problem. The width of the protection structures was decided on the basis of past case histories, model test under static and dynamic conditions. Analytical and numerical studies were performed to check the stability of the caves beneath and protection arch structures using 2D and 3D static and dynamic analyses. During construction and afterwards, continuous real-time monitoring was undertaken to check the response of the structures. Furthermore, strong motion measurements for the response of the structure during earthquakes were also implemented and it is still continued. The authors explain this integrated study on this very unique structure and present the outcomes. Furthermore, they discuss their implications in rock dynamics field.
