Study On The Basic Characteristics And Formation Mechanism Of The Sinian Sandstone Saprolite

The southwest region is warm and humid,and with complex geological conditions,especially Yunnan Province,which is one of the three major sandstone producing areas in China,the problem of sandstone saprolization is more prominent in this area.Sandstone saprolite is a special structure between rock and soil,which retains the mineral structure and structural characteristics of sandstone,but its strength is basically lost.Existing studies have little understanding of this kind of sandstone structure,which is rock-like but not rock,soil-like but not soil,and its research results can not explain the formation and development mechanism of saprolite.More and more engineering problems related to sandstone saprolite are gradually exposed as the construction activities are gradually promoted to mountainous areas.Therefore,identifying the characteristics of sandstone saprolite and its formation and development mechanism can deepen people’s knowledge and understanding of such problems.Based on the Sinian sandstone and its saprolite in Dacun Tunnel,Tonghai County,Yuxi City,Yunnan Province and Tuanjie Township,Kunming City,this paper studies the characteristics of Sinian sandstone and its saprolite,the formation mechanism,geological background and development conditions of sandstone and saprolite in detail by means of field investigation,indoor test and multi-scale observation,data and theoretical analysis.The main conclusions of the paper are as follows:1.The Sinian strata are covered by the upper thousands of meters of rock cover,providing rich provenance for the development of sandstone saprolite and other saprolite.The large-scale tectonic movement and some deep and large faults developed in the study area and its periphery have played an important role in the denudation,tension,destruction and secondary pore development of the Sinian and upper rock cap in the area,and are the main reasons for the rock fragmentation in the area,which provides a rich place for the development of saprolite.2.The upper limit particle size of sandstone saprolite is about 0.5-1 mm,and its color is generally white,yellowish white or earthy yellow.The strength of the structure is low,the structure is loose,and the intergranular pores can be identified.The radius of pore throat and the degree of pore throat development are much larger than those of its parent rock,and the pore structure is significantly different from its mother rock,among which the through-type large pore is formed by long-term saprolization of its native rock.Secondly,when the saprolite is leached by the later infiltration water,some dissolution pores may also develop.3.Under scanning electron microscope,some authigenic dolomites which are easy to undergo redox reaction in sandstone saprolite still maintain relatively high content,with sharp edges and corners,and no obvious signs of dissolution were observed.The results obtained by using the chemical weathering indicators CIA,CIW,PIA,V,MWPI and STI show that the sandstone has not undergone chemical weathering or only slightly chemical weathering.4.The overall transformation from sandstone to saprolite is gradually realized through the self-driven expansion of the "saprolite crust front".The saprolite crust is formed by the fatigue or failure of the structural connection caused by the periodic,repetitive and long-term changes of the matrix suction induced by the diurnal and seasonal changes of the air humidity in the unsaturated zone,and is the product of the physical weathering of sandstone.5.The Sinian sandstone saprolite is developed in the unsaturated zone where the crustal dynamic environment is stable for a long time and the air humidity fluctuates significantly.The diagenetic characteristics dominated by cementation and the porous structural characteristics of sandstone itself are the basis for the saprolization of sandstone.The soil layer rich in clay minerals above the rock structure and in the fracture wall,or the interbedding of rock masses of different lithology,can create a variable humidity environment for the rock structure that can maintain a long time in both high and low humidity states,and is the key to the structural fatigue effect caused by periodic changes in matrix suction.The saprolization of sandstone occurs in multiscale rock structures,the higher the degree of fragmentation of the rock mass,the larger the specific surface area,and the faster and higher the transformation rate from parent rock to saprolite.