Study On Frost Heaving Force And Deformation Mechanism Of Water Saturated Red Sandstone Fractures

With the national economic construction strategy gradually advancing to the west,there are more and more infrastructure construction and resource development in the West.During the construction process in the cold region of the west,freezing and thawing damaged rock masses are often involved.After a long-term freezing and thawing cycle,these rock masses are damaged to varying degrees,which seriously affects the safety and production efficiency in the construction process.Therefore,it is necessary to study the frost heaving damage mechanism of fractured rock.Based on this,this paper takes red sandstone as the research object,carries out its mechanical properties and physical properties tests,monitors the real-time changes of temperature,strain and frost heaving force of fractured red sandstone in the process of freezing and thawing,analyzes the rock on both sides and tip of the fracture before and after freezing and thawing,and theoretically analyzes the changes of frost heaving force and strain of waterless fracture of fractured rock with different widths The main achievements are as follows(1)Through uniaxial compressive strength test and Brazilian splitting test,the basic mechanical parameters and deformation characteristics of red sandstone are mastered;the rock porosity and permeability tests before and after freezing and thawing are carried out,and it is found that the quality of rock skeleton basically does not change before and after freezing and thawing.After freezing and thawing,the average effective porosity of rock increases by 5.78%,and the average permeability increases by28.08% X-ray diffraction was used to analyze the mineral composition of red sandstone.It is found that the content of montmorillonite is as high as 22.09%.(2)Through the frost heave test of red sandstone with water filled fissure,it is found that during the freezing process,there are water samples in the fissures,and the temperature difference between the surface and the crack is large,while in the anhydrous fractures,the temperature of the side and the crack is similar to that in the cracks;the obvious constant temperature platform appears at the crack of 20 mm of the red sandstone with the larger fracture width and the earlier the constant temperature platform appears;when the constant temperature platform appears,the rock will be in the rock The surface strain of rock increases rapidly,and the crack strain increases rapidly after the constant temperature platform lasts for a period of time.It is mainly caused by the joint action of rock skeleton shrinkage and pore water frost heaving.The frost heaving effect of fissure water is lagging behind.This conclusion is fully proved by the change of strain of crack water test and the monitoring of frost heave force in the water fissure;the fracture strain inflection point lags behind the temperature turning point The point,but the crack strain is in sync with the maximum frost heaving force;during the melting process,the frost heaving force and strain both climb up again and appear the secondary high point,mainly because the ice in the deep of the fracture is not melted,and the shallow ice melts into water and then migrates to the deep for re freezing.For the cracks with width of 2-4mm,the peak value of maximum frost heaving force increases with the increase of crack width;the frost heaving process of fractured rock is mainly as follows: firstly,the frozen layer is formed around the rock block,and then the ice plug is formed gradually when the crack water freezes.When the ice plug no longer slips and reaches the full closed state,the crack strain rises rapidly and the frost heaving force suddenly rises to the maximum value,and the crack tip should be stressed The force is concentrated and micro cracks are produced.(3)Through the nuclear magnetic resonance(NMR)test of freeze-thaw red sandstone,it is shown that the red sandstone is dominated by medium and small-sized pores,and micro cracks are formed after freeze-thaw.Based on the nitrogen adsorption test before and after freeze-thaw of rock on both sides of the fracture,it is shown that the rock pores are mainly ink bottle shaped,slit flat shaped,closed cylinder shaped and open cylinder shaped.The freeze-thaw effect does not affect the basic structure of the pores,but the pore size distribution of small and medium size increases.Through the scanning electron microscope test before and after freeze-thaw of rock at fracture tip,it is found that after freeze-thaw of red sandstone,the pores between mineral particles increase and the pore diameter becomes larger,and micro cracks appear on the surface of mineral particles and connect with the boundary pores of particles.Through the above research,the macroscopic and microscopic relationship of frost heaving damage of fractured rock is established.(4)According to the comparison of the applicable conditions of the freeze-thaw damage theory,the volume expansion theory is selected to analyze the frost heaving damage of fractured rock.It is considered that the frost heaving force in the fracture increases with the increase of the width within a certain width range,and the residual content of fracture water after the first stabilization and the thickness of ice plug formed in the fracture after stabilization have an impact on it.For the phenomenon of rapid increase of strain in the waterless fracture,it is mainly because there is no water in the fracture,which will accelerate the freezing of the fracture surface and form a closed condition.At the same time,because there is no water in the fracture,there will be no ice plug in the fracture,so there will be no pressure on the fracture surface.