Rock resistance and the development of horizontal grooves on Danxia slopes

Experimental analyses of uniaxial mechanical (compressive) strength, resistance against sulfuric acid, and freezing and thawing properties were performed on 137 sandstone cores collected from the Danxiashan Mountain (Guangdong Province), Langshan Mountain (Hunan Province), Taining (Fujian Province), and Longhushan Mountain (Jiangxi Province). In addition, 42 rock slices were collected for analysis under a polarizing microscope. The results show that the sandstone samples from the Longhu Mountain are the weakest in terms of the uniaxial mechanical strength and in the resistance against sulfuric acid, freezing, and thawing followed by Danxiashan, Langshan, and Taining. As the conglomerates, they are the weakest in Taining in the uniaxial mechanical strength and in the resistance against sulfuric acid, freezing, and thawing followed by Danxiashan, Longhushan and Langshan. As proved by the experiment, in the same area of the Danxia landscape, the uniaxial mechanical strength of the conglomerates over the flat grooves is universally higher than the sandstones in the flat grooves. In addition, the uniaxial mechanical strength without saturated water of both the sandstone and conglomerate are generally higher than those stones with saturated water, implying that in rainy and water-saturated season, Danxia rocks tend to crack and collapse more easily. Also discovered in this research, the developments of flat grooves in the strata of sandstones have certain reasons. The uniaxial mechanical strength, and the resistance against sulfuric acid, freezing, and thawing of the sandstones in the four sites of the Danxia landscape are universally lower than that of the conglomerates over the grooves. In terms of the physical composition and structure of the rocks, sandstone universally has a higher content of calcium fragments, more tensile cracks, and can be more easily penetrated into calcite veins; and the crystal fragments can be easily carbonated and are mostly cemented in the style of pores and contacts. In contrast, the conglomerates contain more high-temperature recrystallized fragments of volcanic tuff lavas, glass lavas containing quartz crystallized fragments, and fragments of metamorphic rocks. In addition, the carbonated cementations can be found between crystallized fragments so that the uniaxial mechanical (compressive) strengths and the resistance against sulfuric acid, freezing, and thawing of conglomerates are universally higher than that of sandstones. As also indicated by the experiments, the sandstones and conglomerates in the four research sites are highly sensitive to the action of acid erosion: various rocks will become immediately and completely loose after having been immersed in a 10% sulfuric acid solution for 72 h, and some rocks will even become immediately and completely loose after having been immersed in a 2% sulfuric acid solution for 1 h. Results show that the environment with acid rains tended to come into being more easily in the late Cretaceous period because of the frequent eruptions of volcanoes; and the environment of the frequent alternations of intensive freezing and thawing in the Quaternary glacial stage and the interglacial stage should have highly promoted the development of many flat grooves in the Danxia landscape.