Study On Weathering Characteristics And Freeze-thaw Damage Mechanism Of Metamorphic Sandstone In Helanmouth's Rock Paintings Carrier

A total of 5,500 monolithic rock paintings of five major types of animals,plants,human bodies,human faces and symbols are distributed on the slopes of the 1000m range on the inner and outer sides of the Helanmouth in Helan Mountain,Ningxia.It is rich in form,concentrated in distribution,and has a profound cultural connotation.However,due to the open-air preservation of rock paintings,the weathering process of metamorphic sandstones of rock paintings accelerates the flaking and shedding of the sandstones,and the shallowing of the nicks seriously threatens the long-term preservation of the Helanmouth's rock paintings.Especially in recent years,due to the drastic changes in the environment,the accelerated weathering of rocks leads to the danger of rock paintings on the verge of disappearing Therefore,it has become an urgent problem to study the weathering characteristics,weathering mechanism and scientific protection measures of rock-bearing carrier rocks.Based on the previous research theories,firstly,site surveys on the engineering geology,occurrence environment characteristics and rock painting diseases of the Helanmouth's rock painting area were conducted in this thesis.Secondly,through the on-site acoustic wave test and the on-site rebound strength test,the weathering characteristics of the Rock painting carrier rock on both sides of north and south of the Helanmouth were studied,and the factors affecting the weathering were summarized.Finally,the chemical freeze-thaw cycle test was carried out in combination with the actual environmental characteristics of the Helanmouth's rock paintings area and its influencing factors of rock weathering.The exper imental study firstly immersed the solution in distilled water and Na ₂SO₄,Na HCO₃ and Ca Cl₂solutions with a concentration of 0.5 mol/L.The rock samples after 0,15,30,45 and60 freeze-thaw cycles were tested for mass,elastic wave velocity and uniaxial compressive strength.The system explored the damage characteristics of the rock sandstone metamorphic sandstone under the combined action of chemical and freeze-thaw.The qualitative and quantitative joint analysis of the cumulative damage mechanism of the samples was established by scanning electron microscopy(SEM),X-ray diffraction analysis(XRD)and porosity-based changes.The conclusions are as follows:(1)According to the site survey results,the climatic and environmental conditions of Helanmouth are poor,mainly due to the longer frost period,lower temperature and larger daily temperature difference.The chemical composition of runoff in Helanmouth is complex and the aqueous solution is corrosive.(2)Rock paintings carrier rock weathering causes the rock surface to rise and fall off,and the damage and nicking bec ome the root cause of rock painting damage.The weathering characteristics are mainly manifested as follows:(1)The weathering degree of the rock mass of the south bank r ock paintings were generally greater than that of the north bank.(2)The weathering degree of the rock at the ditch mouth of Helanmouth is the largest,and the degree of weathering decreases with the increase of the depth of the ditch.(3)The weathering of the rock at the foot of the slope is serious.With the increase of the elevation,the degree of weathering of the carrier rock is decreasing.(4)The weathering characteristics of the rock are consistent with the degree of disease in the site survey.(3)The main factors affecting rock weathering of rock painting s carrier are:(1)three physical weathering effects of freezing,temperature difference and salt crystallization caused by climatic and environm ental conditions.(2)three chemical weatherings of dissolution,hydrolysis and carbonation caused by the water chemical environment.Physical weathering is mainly caused by frost heaving and crystallization,which causes the surface of the ro ck to rise and fall off,and the chemical weathering is mainly caused by chemical dissolution,which causes the rock of the rock painting to become shallow.(4)After the freeze-thaw cycle test of rock-bearing carrier metamorphic sandstone under different chemical solutions,the macroscopic properties,such as mass change,longitudinal wave velocity and compressive strength of the sample show different degrees of cumulative damage,and the damage is caused by freezing.Different types of expansion,dissolution,and salt crystallization result in differences in the type,number,and connectivity of the sample pores.