The caverns are located in the central part of the Jinqu Basin, where the strata are observed to be monolithic. They are within the Upper Cretaceous Quxian formation, and the lithologies are brick-red thick layered argillaceous siltstone sandstones. This type of rock exhibits low hardness and loose structure and is easily weathered. The caverns have characteristics of artificial construction, large span, and ultra-shallow burial. Due to tourism development, the ancient caverns have undergone huge environmental changes to transform them from being filled completely with water to no water. The ancient underground caverns are affected due to changes in the humidity of the ground surface and groundwater, and they are obviously affected by weathering, human activities, and periodic temperature changes. After conducting an observation, it was established that vault shedding exists in caverns No. 12, 18, and 24 and that the vault of cavern No. 24 has collapsed. Additionally, a few cracks have been observed on the vault of cavern No. 2, which have extended and expanded. Due to joints cutting the vault of the cavern this can collapse at any time. The vault of cavern No. 3 also contains cracks, and there is a dangerous rock mass with three sides hanging in the air, which continues to deteriorate.
Therefore, to ensure long-term stability of the cavern, investigating the mechanism of vault cracks is of considerable importance. If the causes of vault cracks can be analyzed from different perspectives, it will be beneficial to effectively control the direction, length, and width of crack development and will play a vital role in the long-term protection of the cavern.
Crack Vault Office 2008 Key
Several scholars have investigated the expansion of vault cracks in the ancient underground cavern of Longyou; however, their conclusions still need to be improved. The excavation method of the Longyou ancient cavern is similar to bord and pillar-type mining in coal mines. The main causes of coal mine roof damage are the large span, high horizontal stress, and small pillars (Gregory et al., 2001). Similar factors exist in the ancient underground caverns of Longyou and lead to cracks in the vault of the caverns to a certain extent. Based on three-dimensional (3D) numerical calculation, Li Lihui used Fast Lagrangian Analysis of Continua in 3D (FLAC3D) to analyze the stress characteristics of the vaults in the Longyou ancient cavern (Li et al., 2005a). Zhu Jiewang analyzed the cracking mode and characteristics of the vault in cavern No. 1 from the perspective of engineering mechanics (Zhu et al., 2009). These studies have laid the foundation for subsequent studies; however, each study has been simultaneously conducted from a single perspective. The causes of development of the cracks needs to be studied from many aspects, rather than through a single factor or method. The causes of cracks in different areas of the cave are also different. So far, the analysis of mechanics, approximate structural form and numerical calculation has been carried out from a single aspect and the related research factors are also onesided. Therefore, these methods cannot comprehensively analyze the causes of vault cracks. Based on the actual location of the cavern, the factors affecting the stability of the cavern, the structural stress and displacement changes of the cavern should be combined to comprehensively analyze the crack development mechanism. The coupling analysis of force field, temperature field, crack, weathering and other factors will be carried out to ensure the long-term stability of the caves through further research. This study will continue to analyze the causes of vault crack development from the perspective of rock mechanics and ancient cavern environment. The following two methods have been proposed to investigate the aforementioned crack expansion mechanism. First, the factors that cause crack expansion will be analyzed. Second, the principle of crack generation on the vault will be analyzed based on the aspects of rock mechanics and fracture mechanics and the reliability will be verified by combining with the monitoring data. At the same time, the application of Building Information Modeling (BIM) technology to the reinforcement measures of the caverns has considerable theoretical significance and engineering application value to ensure further protection of the ancient underground caverns of Longyou.
The vault between the 3-1 and 3-2 rock pillars in cavern No. 3, tensile cracks and arc cracks exhibited a trend that ranged from 0 to 90 (Wang et al., 2010). By comparatively analyzing the surveys and inspection records of cavern No. 3 (Fig. 3) in 2001, 2008, and 2017, it can be observed that the vault crack of cavern No. 3 are obviously continuously developing, especially due to an increase in the number of vault cracks around the upper part of the rock pillar. Additionally, the following points should be noted: (1) there is a partial separation on the northeast side of the vault of cavern No. 3 and (2) five new cracks were observed on the vault of the rock pillar 3-1 near the entrance in 2017. Similarly, new cracks also appear in other caverns, e.g., cavern No. 1. Fig. 4 shows a comparison of the expanding cracks in cavern No. 1 in 2001, 2008, 2009, and 2017.
To limit the further expansion of cracks, the scenic area management department used steel beams and columns to reinforce and support the vault near the rock pillar 1-1 of cavern No. 1 in 2007 (Fig. 5); the vault of rock pillar 3-1 in cavern No. 3 was reinforced with a combined steel column that comprised three reinforced steel columns (Fig. 6), and the changes in each column were observed. Additionally, a dial gauge coulometer was installed at key parts of the crack expansion points, e.g., at the top of the cavern and the side of the wall, to monitor the settlement and cracking conditions. Simultaneously, a thermometer and hygrometer were provided in each cavern to inspect and record the temperature and humidity, respectively, in the cavern. Thus, all of the aforementioned measures have been adopted to protect and monitor the deformation of the caverns in cavern Nos. 1 and 3.
The rock pillar 3-1 in cavern No. 3, which is the pillar near the entrance of the cavern, has an free surface at the top south portion of the pillar (the rock along the northern edge of the cavern). Additionally, there is a part of the vault that is suspended on the north side of the cavern. Furthermore, as shown in Figs. 8 and 9, there is a layer of rocks in the upper part of the cantilever at the cavern vault that has been formed as a result of weathering (Figs. 7 and 8). Further, the upper weathering rock, the artificial sheltered ceiling and the wall increase the pressure on the cavern vault (the whole load acts on the vault and the pillar 3-1, which causes the sinking of the vault). However, an investigation based on structural mechanics shows that the loads will be considerably increased in the curvature of the cantilevered part of the vault. Therefore, the rocks, the vault of the cavern, and the load on the wall are key factors for the development of cracks in the vault of the cavern.
Furthermore, the air flow inside the caverns changes with the number of tourists, and human activities have a corresponding impact on the environment of the caverns, including the behaviors of the tourists, e.g., engraving on and touching the surrounding rocks. These factors together have a corresponding influence on the crack generation on the cavern vault, which must be considered while analyzing the treatment of the vault cracks.
Before the growth of tourism, the caverns were filled with water thus excluding air. The caverns were stable for thousands of years. However, after the growth of tourism, the water in the cavern has been drained and the vault and the wall of the caverns have become free surfaces that are exposed to air. The original stress balance of the whole cavern is broken, and stress redistribution has occurred. The strain direction is directed into the cavern. Cracks occur on the vault and walls of the cavern until a new stress balance is reached. The unloading stress cut the rock mass and produce various fractures, which is conducive to greater strength of weathering. At the same time, the cracks remain a main channel for water flows, which promotes the formation of water seepage damage.
Real-time monitoring plays a crucial role in the overall stability of the ancient underground cavern of Longyou. The Longyou Grottoes Research Institute has employed a variety of instruments to monitor the cracking and sinking of the vault of ancient caverns from the end of 2008 to 2017.
The following conclusions can be obtained on analyzing Fig. 13. First, the three cracks are in the process of expansion. Among them, the cracking tendency of G3-R-C01 and G3-R-C02 was more obvious, whereas that of G3-R-C03 was relatively minor. Second, the slopes of the curves of G3-R-C01 and G3-R-C02 were larger before 2015, which indicates fast cracking speed, whereas the degrees of cracking at G3-R-C02 and G3-R-C03 gradually decreased and tended to remain stable after 2015. Third, the curve of G3-R-C03 decreased before 2015; however, the crack width widened after 2015. Simultaneously, G3-RC01 exhibited the reverse phenomenon, i.e., it expanded before 2015 but decreased after 2015, which demonstrates that the top plate between G3-R-C01 and G3-R-C03 was constantly changing. Fourth, the cracking degree of G3-R-C01 decreased after 2016, but it does not tend to remain stable, which is consistent with the conclusion in Section 3 that the top plate at the free surface is unstable. Further, the crack of the vault near the free surface continued to expand due to multiple factors. Therefore, it is still necessary to reinforce and support the vault of the cavern entrance.
(1) In this study, the factors affecting the expansion of cracks and the characteristics of cracks under the influence of multiple factors must be analyzed. Further, the influence of these factors on the cracks of the top plate must be comprehensively analyzed. Through follow-up monitoring and inspection, the proportion of various factors affecting crack generation on the vault can be studied. (2) By employing the theory of rock mechanics and fracture mechanics to analyze the mechanism of cracks on the vault of cavern No. 3, the regular pattern of crack expansion could be better studied by combining theoretical analysis and onsite monitoring. At the same time, it provides significant insights related to the study about crack expansion on the vaults of other caverns. In addition, it is recommended to adopt the reinforcement scheme mentioned herein. (3) The objectives of the proposed introduction of BIM technology to investigate the ancient underground project of Longyou are as follows: 1) it introduces new ideas and 3D data to perform research and ensure protection of the underground project, 2) it can reduce the reinforcement cost and provide guidance for subsequent construction, and 3) it will plays an important role in developing the application of the BIM technology to geotechnical projects more generally. 2ff7e9595c
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