Engineering geological characteristics, failure modes and protective measures of Longyou rock caverns of 2000 years old

The Longyou rock caverns are a group of large ancient underground caverns. They were caved manually in pelitic siltstone in the Quxian Formation of Upper Cretaceous. They have the following five characteristics: more than 2000 years old, man-caved, large spanned, near ground surface and medium to hard surrounding rock. They were unearthed by local farmers who were pumping water out of five small pools on a rough-flat ground in 1992. This discovery called the attention of many specialists from China, Japan, Poland, Singapore and USA. One of the most interesting and challenging questions is why the five caverns have been able to keep their integrity for more than 2000 years. To address this question, this paper presents the engineering geological conditions of the caverns and their adjacent areas. They include the following five conditions: (a) geological setting, (b) hydrogeological conditions, (c) seepage in integrity caverns, (d) fractures in integrity caverns and (e) failed caverns. Based on these investigation results, the paper further examines the potential failures of the integrity caverns and their protective measures.