The Research On Red Sandstone Modified Soil Characteristics And Construction Quality Control Technology

Red sandstone has the characteristics of easy weathering and decomposition when encountering water.Periodic wetting-drying cycles caused by heavy summer rainfall and arid climate in Hunan area will cause the strength of red sandstone roadbed soil to deteriorate,which adversely affect the stability of the highway structure.In order to ensure the safety and stability of the red sandstone roadbed,the filler needs to be improved.When the improved red sandstone soil is used for roadbed filling,research on the PFWD-based rapid detection and evaluation standard to ensure the construction quality is the key.In view of this,the red sandstone along the Huai-zhi Expressway was selected as the research object,and the unsaturated wetting-drying cycle environment in which the red sandstone subgrade soil was subjected to alternating heavy rainfall and strong drought was simulated.The test and study of macromechanics,mineral composition and micro-mechanical changes of red sandstone plain soil and cement-modified soil under wetting-drying cycles were carried out,and the parameters of basic physical properties and macro-mechanical strength of red sandstone soil were obtained with the changes of dry-wet cycle,compaction,moisture content,and cement content.And through the analysis of mineral composition,the physical mechanism of mechanical strength degradation was revealed.Subsequently,the research on the quality control indicators of red sandstone roadbed construction based on dynamic strain control under the effect of wetting-drying cycles was carried out,and the ABAQUS was used to simulate the wheel dynamic load and Portable Falling Weight Deflectometer(PFWD)detection process,and the quality control standards and rapid detection indicators for the construction quality of the improved soil were obtained.The specific research content and research results are summarized as follows.(1)The red sandstone plain soil is subjected to the effects of dry and wet cycling,and the deterioration of characteristics is apparent.After undergoing 7 wettng-drying cycles,the compressive strength,bearing capacity and shear strength of red sandstone plain soil can not meet requirements.Analysis of the mineral composition shows that the feldspar,calcite and montmorillonite components contained in red sandstone are prone to chemical reactions during the wet and dry cycle,and generate soluble mineral salt components.This results in cracks and pores in the geotechnical structure,resulting in red sandstone disintegration and reduced strength.(2)The strength change of cement-modified soil under dry and wet cycling conditions was studied.It shows that the unconfined compressive strength and rebound modulus of the improved soil are reduced first and then rebounded after the first wetting-drying cycle.It reveals that the addition of cement could not only improve the initial strength of red sandstone,but also alleviate the deterioration caused by the wet and dry cycle.(3)7 wet and dry cycles were simulated,and direct shear tests were performed on the samples in each humidification path,and the effects of moisture content,compaction,normal stress,and numbers of wetting-drying cycles on the shear characteristics of the improved soil were analyzed.It revealed that the shear strength,cohesion c value,internal friction angle φ value,and shear strength loss of the improved soil decrease with increasing water content.And it showed that the lower the water content,the more obvious the brittle failure characteristics of the sample.With the increase of the number of wetting-drying cycles,the failure type of high-water-content specimens gradually transitioned from brittle failure to plastic failure.Under the same conditions,the shear strength,cohesion c value,and internal friction angle φ value of the 96%compaction specimen are higher than the 90%compaction specimen.The cohesive force c increases during the humidification process and the number of dry-wet cycles,while the internal friction angle φ rises slowly from 10%to 15%as a whole,and drop sharply during subsequent humidification.(4)SEM and ImagePro-plus software were used for data acquisition and analysis,in order to qualitatively and quantitatively analyze the degradation mechanism of the wet-dry cycle effect at the micro level.With the increase of wetting-drying cycles,the large plate-like particles gradually disintegrate into loose fragments.The particles are rearranged and combined,and the porosity gradually increases,it showed the number of large and medium pores gradually increased,while the number of small and micro pores decreased.Fractal dimension of morphological distribution appears to decrease first,then increase in the remaining wetting-drying cycles.The average circularity of the particles gradually increases,revealed that the shape tended to be rounded.As a result,the intercalation between soil particles educed and the internal friction angle reduced.(5)The design of subgrade surface strength based on the dynamic strain control technology is studied.Using ABAQUS finite element software,the dynamic load model of the highway wheel is established,and the critical value of the rebound modulus of the modified layer,Ed,is obtained under the different embankment modulus and the filling height roadbed.On this basis,combined with laboratory tests of improved soil rebound modulus,the optimal value of cement content was determined to be 5%;(6)A finite element operation model of PFWD was created based on the wheel dynamic load frequency,and the PFWD detection standard E’vd corresponding to the critical value of the rebound modulus of the modified layer was calculated.It proved that Ed and E’vd had good linear regression correlation.Finally,a multiple nonlinear regression formula for E0 and Hd for predicting E’vd is established,which provides a reference for engineering practice.