# Natural Creation of Large Rock Cavern: Can We Construct Them? Jenolan Caves as a Case Study
## 1 Introduction  
Natural caves are found throughout the world, and some of them have been explored and investigated from different perspectives [1, 2]. A large number of case studies focus on physical surveying [3, 4, 5], geological and engineering geological formations mapping [6, 7, 8, 9], cave environment investigation [10, 11, 12], and stability assessment of natural caves and old relic man-made caves [13, 14, 15]. The Jenolan Caves are some of the well-documented and most spectacular limestone caves in Australia, as well as one of the oldest caves formations in the world to date [8]. According to Showcaves [16], the Grand Arch of the Jenolan Caves system is the largest natural open cave in Australia, standing at 24 m high, 55 m wide, and 127 m long. A cave with a similar size to that of the Grand Arch could possibly host small city streets, buildings, and facilities. Interestingly, some local governments, such as those of Hong Kong [17], Singapore [18], and Finland [19], aim to build city infrastructures below the ground to release more land on the surface for a better living environment and better development in the future. Generally, geological formation profiles, weathering grade of rock, geological structures (i.e., joints, folds, faults, and dyke contact), and in situ stress field are considered prime factors to control the stability of both natural and man-made underground rock openings [20]. Theoretically, empirical rock and structural mechanics principles [20, 21, 22, 23, 24] could be used to interpret these physical phenomena of stability in regard to large-span rock-opening structures. A majority of large-size man-made rock-cavern studies [25; 26; 27; 28; 29] were based on project-specific ground conditions to develop. A general parametric relationship among rock mass parameters and opening shape with the specific ground conditions was shown in these captioned studies. However, a unified solution to interpret a maximum size of stable opening created in a range of rock mass conditions with different opening shapes is limited. There are number of studies focusing on the different shapes of rock openings, such as rectangular [30; 31], circular [30; 32; 33], elliptical [32; 34; 35], or half-rounded horseshoe shapes [30; 31; 36]; however, all of these studies consider the opening created in continuum media or considered with continuous infinite joints. Importantly, there is a limited number of relevant investigations and studies specifically focusing on what is the difference of ground responses between different roof arch shapes for rock opening created in a range of rock mass conditions. To obtain a clear understanding of how these factors interact with each other to maintain a stable rock opening created for different rock mass structures, rock strengths, and in situ stress fields, a stability simulation of the Grand Arch, as a learning object, is conducted based on available and published geological information. Seven models are examined in the first part of this study. The investigation exercise is then extended to examine the creation of the same size of man-made rock openings, with two different roof arch shapes that are unsupported and using a rock-reinforcement technique [37; 38; 39] as the stabilization measure. The results of this study provide a clear explanation to describe the impact of various geotechnical parameters for maintaining a stable underground opening with different shapes under different ground and rock mass conditions. It should be highlighted that the purpose of this case study is not to prove the structural stability of the Grand Arch. This paper concludes with a number of questions about the creation of a large-sized stable opening using a rock-reinforcement technique with different geometries of the arch roof design formed in a range of rock mass conditions. For example, how wide can a rock-reinforced cavern be opened in a range of rock mass conditions?