The Research On Strength Characteristics Of Improved Red Sandstone Soil Under Drying-wetting Cycles

There are a large number of red sandstone distributed in the south of China.This kind of rock has low strength and poor water stability.Combined with the climate characteristics of humid,hot and rainy in the south of China,how to effectively use red sandstone soil as subgrade filler is an urgent problem to be solved.In this paper,through unconfined compressive strength test and triaxial test,the change rule of red sandstone soil strength under the action of dry and wet cycles is analyzed,and the improvement mechanism is clarified combining with the analysis of soil microstructure;finally,the field test of cement modified red sandstone subgrade filling is carried out to demonstrate the improvement performance.The research results are as follows:(1)Through mineral composition analysis,disintegration,and CBR tests,it was found that red sandstone is easily disintegrated because it contains hydrophilic mineral components such as montmorillonite.The average mass loss after 6 cycles of water soaking disintegration is 10.21%.The softening coefficient of compressive strength of red sandstone is 54%,which belongs to soft rock.The CBR value of red sandstone soil is 7.1%,which does not meet the requirements of highway roadbed filling.Therefore,it is necessary to improve the red sandstone soil.(2)The cement and lime are used as modifiers of red sandstone soil respectively,and unconfined compressive strength test is carried out on the improved red sandstone soil.The test results showed that the red sandstone cement modified soil has better strength and water stability under dry and wet cycling conditions,so cement was selected.As an improver for red sandstone soil.(3)Triaxial compression tests were performed on red sandstone plain soil and cement-modified soil.It was found that the stress-strain curve of red sandstone soil was strain-weak.The addition of cement did not change the form of the stress-strain curve of red sandstone soil.At all stages,the force characteristics of cement on red sandstone soil have been improved to a certain extent;cement has transformed the failure mode of red sandstone soil from plastic failure to brittle failure.With the increase of the number of dry-wet cycles,the elastic modulus and peak strength of the cement-modified test specimens decreased first and then increased;after multiple dry-wet cycles,the internal friction angle and cohesion of the improved soil were 0 times compared to the cycle.There is a certain attenuation at time,but the overall attenuation is not large;the attenuation rate of the friction angle in the improved soil with 4%cement dosage is the smallest.The multivariate nonlinear relations of elastic modulus,peak strength and shear strength index with respect to the number of wet and dry cycles,cement dosage and confining pressure are established,and the fitting effect is ideal.(4)Using scanning electron microscopy(SEM)and Image-pro Plus software to analyze the microstructure of the cement-improved soil,it was found that cement as an improver can significantly improve the phenomenon of loose soil structure and enlarged pores caused by wet and dry cycles.;The hydration products of cement have a large change in the distribution of particle pore morphology under the effect of dry and wet cycles.As the amount of cement increases,the stripe pores continue to increase,and the fractal dimension is relatively increased.(5)By comparing the strength test results of red sandstone improved soil subgrade before and after rainstorm,it can be seen that the red sandstone improved soil with cement dosage of 4%can meet the requirements of subgrade strength.