Study On Microstructure And Water Absorption Charactristics Of Mudstones In Xining Area

With the rapid development of high-speed railway in China,it is often seen a series of traffic safety problems caused by expansive mudstone.In the construction projects of Baoji-Lanzhou high-speed railway and Lanzhou-Wulumuqi high-speed railway,there have been some engineering problems such as cracking of inverted arch of tunnel and arch on subgrade caused by mudstone.Therefore,the study of the water absorption characteristics of mudstone has certain significance to the disintegrating effect of rock water absorption deformation.In this paper,the distribution characteristics and geological origin of mudstone in Xining area are analyzed firstly.Taking Neogene mudstone and Cretaceous mudstone in Xining area as research objects,the mineral composition and microstructure of mudstone have been systematically studied.On this basis,the mudstones in Xining area are classified,and the water absorption mechanism of mudstones in Xining area is revealed by studying the microstructure of different types of mudstones under different water absorption conditions.Combined with the water absorption test,the relationship between different microstructure and water absorption characteristics has been analyzed,and the relationship between the microscopic characteristics and the disintegration effect of water absorption deformation has been revealed.The main results achieved are as follows:(1)The mineral composition and microstructure of mudstone in Xining area were analyzed by means of X-ray diffraction(XRD),scanning electron microscope(SEM)and other methods.And the experimental results showed that the clay mineral composition of mudstone in Xining area is mainly illite.Mudstone is mainly divided into two types according to the microstructure unit: one is N-type mudstone dominated by flat-sheet superimposed units,and the other is SN-type mudstone with more clastic particle units.Among them,the diagenetic environment of N-type mudstone is stable,which is less affected by geological structure,and its clay mineral content is relatively high and structure is relatively compact.Using Image-Pro Plus software to analyze the pore characteristics of mudstone in Xining area,it is found that the pore area of N-type mudstone is small and the distribution range of pore diameter is small.(2)According to the analysis of the microstructure of mudstone under different water absorption conditions,the pore development characteristics of N-type and SN-type mudstone are different due to the diverse microstructure.The pore area of N-type mudstone shows an increasing trend as a whole,and its pore area and amount are at a high level.The pore area of SN-type mudstone shows a downward trend as a whole,and its pore area and amount are at a low level.However,the roundness variation trend of N-type and SN-type mudstone is the same,both tending to be close to 2.This is because the continuous expansion of clay minerals and pore deformation caused by extrusion pores with the development of water absorption.(3)According to the water absorption experiment,the water absorption process of mudstone can be divided into two stages: nonlinear deceleration water absorption and constant velocity water absorption.The variation of water absorption rate of mudstone with time can be divided into three stages: the sudden increase of water absorption rate,the descending stage and the stable stage.Mudstone deformation process can be divided into rapid growth stage,slow growth stage and stable stage.The variation of mudstone deformation rate with time can be roughly divided into three stages: sudden increase of deformation rate,the descending stage and the stable stage.(4)Through the correlation analysis,it can be seen that there is a certain correlation between the clay mineral content,pore area and the water absorption characteristics of mudstone.Among them,the clay mineral content has a strong correlation with mudstone water absorption and water absorption rate,but a weak correlation with deformation and deformation rate.The correlation between pore area and water absorption,water absorption rate,deformation and deformation rate of mudstone is strong.It can be seen that the water absorption characteristics of mudstone in Xining area are greatly influenced by its pore structure.Based on the analysis of the relationship between mudstone microstructure and disintegration characteristics,it is concluded that mudstone with low clay mineral content and large pore area has stronger disintegration.