An adaptive prediction method for mechanical properties deterioration of sandstone under freeze–thaw cycles: a case study of Yungang Grottoes

Due to the location of the Yungang Grottoes, freeze–thaw cycles contribute significantly to the degradation of the mechanical properties of the sandstone. The factors influencing the freeze–thaw cycle are classified into two categories: external environmental conditions and the inherent properties of the rock itself. Since the parameters of rock properties are inherent to each rock, the effect of rock properties on freeze–thaw degradation cannot be investigated by the control variates method. An adaptive multi-output gradient boosting decision trees (AMGBDT) algorithm is proposed to fit nonlinear relationships between mechanical properties and physical factors. The hyperparameters in the GBDT algorithm are set as variables, and the Sequential quadratic programming (SQP) algorithm is applied to solve the hyperparameter optimization, which means finding the maximum Score. The case study illustrates that the AMGBDT algorithm can precisely determine the effect of each independent factor on the output. The patterns of mechanical properties are similar when the number of freeze–thaw cycles and porosity are used as variables separately and when both are used simultaneously. The uniaxial compressive strength decay rate is positively correlated with the number of freeze–thaw cycles and porosity. The modulus of elasticity is negatively correlated with the number of freeze–thaw cycles and porosity. The results show that the number of freeze–thaw cycles is the main factor influencing the freeze–thaw cycling action, and the porosity is minor. In addition, the fitting accuracy of the AMGBDT algorithm is generally higher than neural networks (NN) and random forests (RF). Studying the influence of porosity and other rock properties on the freeze–thaw cycle will help to understand the failure mechanism of rock freeze–thaw cycles.

RESEARCH ON EVALUATION OF THE SPALLING DISEASE OF NICHE FOR BUDDHA IN GROTTOES

The niche for Buddha is an important part of the cultural relics in the grottoes, but it has caused serious spalling disease due to human activities and natural disasters. This paper presents a new quantitative evaluation method for the spalling disease of niche for Buddha. First, the random forest-based method was used to identify the niche for Buddha . Then, the spatial clustering of the head of the niche for Buddha was performed by DBSCAN, and the reference surface was determined by the RANSAC algorithm. Based on this, the spallation depth of the grotto surface was extracted and the relevant index elements were established. The experiment of point cloud data in Cave 18 of Yungang Grottoes proves the rationality of this method, which is of great significance to the protection and virtual restoration of cultural relics in grottoes.

Effect of Sulfate Crystallization on Sandstone in the Yungang Grottoes

Subjected to various weathering in nature over long time, sandstone has been deteriorated in the Yungang Grottoes. The repeated dissolution and crystallization of soluble salts are important factors that cause the deterioration of the grotto sandstone. In order to study the action mechanism of sulfate crystallization in the Yungang Grottoes sandstone, deterioration tests were carried out by the “soaking-evaporation” cycle of saturated magnesium sulfate (MgSO4) and saturated sodium sulfate (Na2SO4) solutions in laboratory. The test results show that the participation of sulfate greatly accelerated the weathering speed of sandstone. The ultrasonic velocity of the specimens circulated in both Na2SO4 and MgSO4 solutions show a trend of first increasing and then decreasing, and changes more obviously in Na2SO4 solution. In the evaporation stage of the circulation, the peeling mass of the specimens circulated in Na2SO4 solution is significantly greater than that in MgSO4 solution. From the evaporation tests on saturated Na2SO4 and MgSO4 solutions and observation with optical microscope and scanning electron microscope, it is found that a dense crystal layer was formed on the surface during the evaporation of saturated MgSO4 solution, and it gradually seals the internal solution, slowing down the further evaporation. During the solution evaporation, Na2SO4 crystallizes into powdery crystals in a flocculent and soft state, and there are pores between the crystal grains. The change in the ultrasonic velocity of specimens in the "soaking-evaporation" cycle tests can be explained by the evaporation test results.

Similarity Index Based Approach for Identifying Similar Grotto Statues to Support Virtual Restoration

Grottoes, with caves and statues, are an important part of immovable heritage. Statues in a particular grotto setting are often similar in geometric form and artistic style, and identifying the similarity between these statues can help provide important references for value recognition, condition assessment, repair, and the virtual restoration of statues. Traditionally, such reference information mainly depended on expert empirical judgment, which is highly subjective, lacks quantitative analysis, and cannot provide effective scientific support for the virtual restoration of grotto statues. This paper presents a similarity index based approach for identifying similarities between grotto statues by studying 11 small Buddhist statues carved on the 18th cave in the Yungang Grottoes, located in Datong, China. The similarity index is determined according to the hash values calculated based on the pHash method using the orthophoto images of Buddhist statues to identify similar statues. Similar feature points between the identified statues are then matched using the Scale Invariant Feature Transform (SIFT) operator to support the repair and reconstruction of damaged statues. The experimental results show that the variation of similarity index values confirms the visual inspection of the statues’ appearance in the orthophotos. The additional analysis of three-dimensional (3D) point clouds also confirms that the similarity index based approach is accurate in the initial screening of similar grotto statues.

Study on the Seismic Performance of Small-Diameter Bolts Reinforced in Grottoes

The Yungang Grottoes, a World Heritage Site in Datong, consist of 252 caves that are noted for their collection of 5th- and 6th-century Buddhist grotto sculptures and reliefs. Various diseases have appeared in the grottoes under the general influence of natural and artificial factors. Bolt support is a commonly employed method for grotto reinforcement and has been widely applied in many projects. Small-diameter bolts have also been used in the reinforcement projects at the Yungang Grottoes, but the corresponding effect on the seismic performance of grottoes is still unclear. In this paper, a dynamic analysis via the numerical modelling of an ear grotto of the 19th grotto in Yungang is established, and the rock displacement, acceleration, and bolt axial force responses under a seismic wave are analyzed. The results show that the seismic dynamic responses of grottoes are greatly affected by the cliff structure. The displacement and acceleration responses of the cliff body vary greatly within the abrupt transition of the cliff structure. Based on this variation, the seismic capacity of small-diameter bolts in the vertical direction is greater than that in the horizontal direction. The axial force of a bolt is small at both ends of the bolt, large in the middle of the bolt, small on the top of a cliff, and large at the bottom of the cliff. Although the axial force is small, the upper rock mass of the grotto has a tendency to undergo relative movement compared with the outer rock mass. The results also indicate that based on the structural defects in the vertical direction of the cliff body caused by grotto excavation, the inclined angle of the bolt should be increased as far as possible or vertical support should be adopted to enhance the stability of the rock mass at the top of the grotto.

Study on Strength Damage Evolution Regulation of Sandstone under Cyclic Erosion

Owing to salt erosion, the sandstone of Yungang Grottoes has widespread weathering diseases. The soluble salt develops reciprocating crystallization pressure under the action of dry and wet cycles to diminish the sandstone strength. Finally, several pore-like and powder-like weathering phenomena are formed. To explore the change in sandstone strength during this process, the sandstone of Yungang Grottoes was taken as the research object. Herein, the uniaxial compression test, XRD test, and other methods were employed to study the samples under different salt erosion cycles. As the number of salt damage cycles increased, the phenomenon of sand particles on the sandstone surface gradually amplified. The compressive strength, tensile strength, and elastic modulus of sandstone decreased with the increase in erosion cycles. The curve was divided into two stages, and the macroscopic and microscopic damage equations of sandstone after erosion were established. During the entire damage process, erosion damage served as the basis of load damage and was affected by different cycles, impacting the development process of load damage. Through the establishment of numerical simulation meso-parameter evolution equations, the PFC2D particle flow model was used to conduct uniaxial simulation tests, and the simulation results were close to the macroscopic test results. Using the particle flow simulation test parameters under salt erosion, the development of the distribution of “displacement-force chain-crack” was analyzed under different salt erosion cycles. These results further revealed the meso-fracture damage characteristics of the Yungang Grottoes sandstone under the action of salt damage and provided a theoretical basis and a novel method for the protection of Yungang Grottoes against weathering.

Research on the Space transformation of Yungang Grottoes Art Heritage and the Design of wisdom Museum from the perspective of digital Humanity

The pace transformation and innovative design of Yungang art heritage should keep pace with times on the setting of “digital humanity” times, make the most of new research approaches which is given by “digital humanity” and explore a new way of pace transformation of art heritage actively. The research object of this topic is lineage master in space and form and the transformation of promotion and Creativity of Yungang Grotto art heritage. The goal is taking advantage of big data basics and the information sample collection and integration in the context of the full media era. Transferring Yungang Grotto art materially and creatively by making use of the world's advanced "art + science and technology" means, building a new type of modern sapiential museum and explore the construction mode of it and the upgraded version of modern educational functions.