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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. [ABSTRACT FROM AUTHOR]

The hollowing deterioration of stone relics required effective non-destructive testing (NDT) methods for their timely restoration and maintenance. To this end, a new NDT method based on terahertz (THz) technology by using support vector machine (SVM)-based machine learning models was developed to assess and diagnose the hollowing deterioration of the Yungang Grottoes. According to experiment design, a series of hollowing deterioration samples with various thicknesses of hollowing deterioration were prepared and then measured by using THz time-domain spectroscopy (THz-TDS). Based on the THz-TDS results of 30 randomly selected samples, a SVM-based hollowing deterioration prediction model (SVM-HDPM) was established by analyzing the relationship between the hollowing samples and the THz spectral information. The reliability and accuracy of the model was further proved by verified and compared with using the THz spectral data of the remaining 10 samples. The experimental results with the linear kernel function greatly demonstrated that the SVM-HDPM can have superior prediction accuracy, implying that the model is feasible for the prediction the hollowing deterioration of the stone relics. Moreover, one data preprocess was introduced into SVM-HDPM to meet the needs of field-based test. The predicted results of five different hollowing deterioration with different flaked stone thickness revealed good performance with very low mean square error (MSE) value. Therefore, it is believed that the proposed method can be regarded as an effective NDT technique with practical applications in analyzing cultural relics and have promising future prospects in inspection stone relics-like ancient heritage for hidden flaws. [ABSTRACT FROM AUTHOR]

The long-term strength of rock load-bearing elements is an important mechanical index to determine its bearing capacity and preservation when protecting cultural relics made of stone. In this paper, uniaxial compressive creep experiments of medium-coarse sandstone from the columns of the Yungang Grottoes were performed on both dried and water-saturated samples. The effects of rock-water interaction on the long-term strength of rocks and on the bearing capacity to evaluate safety were determined. The experimental results show that the instantaneous strength and long-term strength of the medium-coarse sandstone in a water-saturated state were lower than the dried samples. The creep properties of the saturated rock were more obvious and the time needed to achieve a steady state creep was longer. The safety factors of the long-term bearing capacity of water-saturated rocks were not high enough, which indicates the possibility of unstable damage to the columns. It is recommended to strengthen the column in a timely manner to prevent damage and potential safety issues, particularly waterproofing the columns for addition protection. Finally, these experimental results provide a reference for the long-term strength evaluation of rock columns, and provide a theoretical basis for waterproofing the World Cultural Heritage, Yungang Grottoes. [ABSTRACT FROM AUTHOR]

The article focuses on the study conducted by the authors on influence of water on the weathering processes of sandstone by using Sandstone from the Yungang Grottoes in China. Topics discussed include deterioration of sandstone which begins from weaker points of the samples, changes in mineral composition of the samples such as quartz grains, feldspar and lithic fragments, and sulfates of calcium and magnesium salt.

Salt crystallization in pore spaces will usually cause major damage to natural sandstone. Salt distribution and movement are hard to detect in the field within a small scale. Understanding salt generation and transportation in semi-arid regions can help to protect natural cultural relics and control the salt weathering damage. Salt transportation behavior and deposition processes in Yungang Grotto were studied through a detailed hydrochemistry analysis within the context of the atmosphere-rock-water environmental system. Changes of salt types in different media were analyzed according to the sequence of water and salt transportation, integrating the results of atmospheric dry and wet deposition, soil in the aeration area and sandstone, and salt on the rock surface. The study shows the generation of salt is related to vertical transport in the soil and rock weathering zone. Salt transportation originates from soil cover, passes through the sandstone weathering zone, and finally accumulates in the grottoes. Due to coal burning pollution, acid deposits leading to sulfate bind to the soil. Salt generation in soil is the result of neutralization among sulfate, carbonate, and carbonate minerals, leading to cations of soluble salt that give priority to bonding with Ca and Mg, while the anions are mainly HCO and SO. Salt in sandstone weathered crust is not only impacted by soil, but also carbonate dissolution from CO-rich water, its inverse process, and feldspar dissolution. Rock surfaces in the inner grottoes suffer intense evaporation, resulting in salt accumulation, and carbonate and sulfate mineral deposition or dissolution, producing the salt type of MgSO or NaSO, while HCO and Ca are heavily consumed. The result can provide a scientific basis for engineering measures that reduce the hazard of salt accumulation. [ABSTRACT FROM AUTHOR]

Deterioration of grottoes induced by the negative impacts of the variable temperature and moisture conditions has been an important issue in the conservation of heritage sites in China. In this case study, the spatial distributions and varying patterns of moisture and temperature in the caves of Yungang Grottoes, China, were investigated. The relative air humidity was approximately 100 % in the deep zone of the unsaturated surrounding rocks of the grottoes where the temperature remained almost steady at around 9.4 °C. However, the indoor air temperature, relative humidity, and rock surface temperature in the caves varied significantly because of the active exchange of the air with the outside atmosphere. The condensation water appeared on cave walls in the summer when the dew point in the air was higher than the rock surface temperature. Preliminary assessments with a special collection device indicated that the thickness of the transient condensation water could reach 0.03-0.10 mm on the back wall of a cave. The occurrence of this condensation water is expected to worsen salt deterioration of the sculptures in the grottoes. As revealed by long-term observations, a wooden building in front of a cave can weaken the indoor air temperature fluctuation and reduce the condensation water, and consequently, prevent the negative effects of the microclimate for the sculptures. [ABSTRACT FROM AUTHOR]

To understand the effects of an acidic environment on the internal structure of sandstone from the Yungang Grottoes, Datong, China, the physicochemical properties of fresh and weathered sandstone samples and their compressional wave (P-wave) velocities in response to different concentrations of H2SO4 and HNO3 solution were investigated. X-ray diffraction (XRD) and a scanning electron microscope equipped with energy dispersive X-ray spectrometer (SEM-EDX) were used to determine grain morphology and chemical composition. The results show that the sandstone, which mainly consists of silicon dioxide and calcium carbonate, became more complicated in composition after weathering. A nonmetallic ultrasonic detector was used to measure the P-wave velocities of sandstones in a natural state and soaked with ultrapure water and with H2SO4 and HNO3 at concentrations of 0.2 mol L−1, 0.4 mol L−1, and 0.8 mol L−1, respectively. For the acid-treated groups, with increasing acidity, P-wave velocity decreased significantly, compressive strength decreased and effective porosity increased; these behaviours are different from those of the water-treat group, implying that the acid solutions damaged the microstructure of the sandstone. The results suggest that the deterioration risk of H2SO4 and HNO3, or sulfates and nitrates converted from ambient SO2, NOx and PM2.5, on the stone relics in the Yungang Grottoes should be a cause for concern. [ABSTRACT FROM AUTHOR]