Study On Surface Weathering Mechanism Of Sandstone At Qingyang North Grottoes

Longdong region was an important node of the northern section of the silk road.In the process of spreading Buddhism eastward,it gradually developed into the Longdong grottoes group with Qingyang North Grottoes as the core.However,after more than 1500 years of weathering,the long-term preservation of Qingyang North Grottoes is facing severe challenges.At present,the research on the mechanism of weathering in academic circles mainly starts from the single factor weathering mechanism,and then expands the multi-factor coupling effect.The sandstone of Qingyang North Grottoes is selected as the research object.On the basis of field investigation,laboratory weathering simulation test,such as freezing and thawing cycles,drying and wetting cycles and Salt crystallization cycles,were set in the room respectively.Combined with the weathering characteristics of rock samples in different dimensions,such as macro,local and micro,as well as the surface mineral composition and element migration rules,the whole process of weathering mechanism was tracked and evaluated.The main conclusions of this paper are as follows:(1)Qingyang North Grottoes is located in the Longdong region,belonging to the temperate continental semi-arid climate.The surface is crisscrossed with ravines.It belongs to the typical loess hilly landform.Gentle occurrence of the strata,and the Cretaceous Luohandong Formation(K₁lh)strata are mainly exposed.Due to fluvial facies deposition,fine-grained feldspar lithic sandstone and medium-fine-grained limestone lithic sandstone are produced in rhythm.(2)The existing diseases of Qingyang North Grottoes mainly include crack,salt efflorescence,biological diseases,granular disintegration,honeycomb weathering.It is characterized by various types,wide distribution and serious harm.Through investigation,it is found that the distribution of diseases is closely related to the distribution of alkaline surface water and groundwater in the grotto area.Seasonal and diurnal temperature alternation also plays a role in weathering development.(3)Under the action of freezing and thawing cycles,both fine feldspar lithic sandstone and calcareous medium-fine lithic sandstone weathered in the form of granular disintegration and massive exfoliation.Under the freezing action,water gradually accumulates to the side with higher temperature.This mechanism causes fractures to develop.At this time,the loss rate of mass is basically lower than 5%,and when the crack accelerates,the loss rate of the mass is exponential,which is closer to20-30%.The induction,expansion and cutting of cracks make it show phased characteristics in the weathering proce.The induction of cracks in the early stage is the warning of a large number of granular disintegration in the late stage.Compared with the induction of new cracks,the expansion and cutting of cracks is the key to the rapid weathering.(4)In the process of drying and wetting cycles,granular disintegration is continuous and coherent throughout the whole proce.The evaporation rate of surface sandstone moisture is faster,which leads to the granular disintegration mainly concentrated at 0-0.5cm of the surface layer,while the influence on the interior of sandstone is weak.The weathering rate of drying and wetting cycles is obviously lower than that of freezing and thawing cycles.In the single cycle,the quality loss rate of the GH group and GH group respectively were 0.22%,0.28%.The weathering rate of fine-grained feldspar lithic sandstone is only slightly higher than that of medium-fine grained limestone sandstone under drying and wetting condition.(5)In the process of salt crystallization cycles,the weathering mechanism is slightly different with different salt concentrations.When the salt concentration is higher than 47.5g/L,the fracture of rock samples along the direction of rock strata rapidly expands into its deterioration mode,The loss rate of single-cycle quality is between 5-17%.When the salt concentration is higher than 34.675g/L,the fracture of rock samples along the rock strata rapidly expands into its deterioration mode.When the salt solution concentration is lower than 34.675g/L,the granular disintegration from the outside and the inside becomes dominant.The quality loss rate is maintained at 1-2%.In the later stage of deterioration,when the rock structure is weak and loose,sandstone sample is cracked and then destroyed.(6)In the above three tests,the chemical alteration index cia continued to increase.The dissolution of calcium and magnesium cements,alteration of feldspar and loss of interstitials are the fundamental causes of the loss of intergranular connections.The wetting-drying cycles is dominated by water erosion.The pore volume percentage larger than 40?m increased rapidly from 51.93%in the tenth period to 81.24%in the 10-20?m pore size.The loss of cemented components in freezing and thawing cycles is mainly affected by the tensile and compressive stresses caused by frost heave.CIA increased from 42 at the initial time to 51 at the failure time.The decomposition rate of feldspar and cements by frost heave stress was much higher than that by scour.In the DL formation,the percentage of pore volume larger than 40?m was 98.51%at the 10th period.For salt crystallization cycles,the grain is squeezed because of the crystallization and expansion during the process,which leads to the structural destruction of sandstone and the loss of cementing materials.The stress generated by salt crystallization is much higher than the frost heave stress,and the process not only leads to the connection of fractures,but also further induces new fractures,and the percentage of pore volume less than 10?m is increased,which leads to the destruction of sandstone structure and the loss of cementing material.The above processes mainly occur in the early phase of the cycles,but the loss of residual feldspar and cement due to persistent fatigue damage in the late phase is the key to the damage of particle structure.