Abuchiragama is karstic cave in Ryukyu limestone. There is a increasing tendency that ground water is seeping in from top of cave after rainfalls. For the safety of entrants, it is necessary to evaluate the stability of the cave. This study is concerned with the risk managment of this cave utilizing RMQR as a new evaluation method of the quality of rock mass, and the stability evaluation of the cave was carried out through an empirical method and analytic methods. It is understood that it is necessary to examine in details the areas accessible to entrants, in particular, the entrance and exit areas. These areas are now being investigated in details and we have been considering to review the counter-measures master plan for rehabilitation.

1 Introduction

Itokazu Abuchiragama karstic underground cave is in Nanjyo-city, in south part of Okinawa Island (Figure 1). This cave was used as underground shelter during the battle of Okinawa. Now this cave is used for peace education for trips of the schools, and the number of entrants is about 110 thousand people.

From around 2012, there is an increasing tendency that groundwater infiltrates into the cave from its top of cave after rainfalls. For safety of entrants, it was necessary to evaluate safety and stability of this cave.

We have conducted various investigations. These involves discontinuity surveying, monitoring the crack displacements, acoustic emission measurements and accelerations at ground surface and inside the cave during seismic events. RMQR proposed by Aydan et al. (2014) as a new rock mass classification system is used to assess the quality of rock mass surrounding the cave and the stability assessment of the cave was carried out using empirical method and analytic methods.

2 Geological Overview

Ryukyu limestone is relatively young formation formed during Pleistocene. This formation is widely distributed in middle south area of Okinawa island.

Abuchiragama is located within the west plateau of Ryukyu limestone formation. There are faults around this underground shelter, There is a high possibility that they played a role on the formation of the cave (Figure 2).

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