Study On Soft Ground And Site Deformation Pattern Of Cangnan Residual Soil Backfill Beach

The coastal area of Wenzhou Cangnan has a large amount of beach soft ground and a wide range of residual soil.The beach soft ground has high water content and strong compressibility,and often needs backfilling treatment.However,the engineering characteristics of both are poor.To ensure the project quality and construction safety,it is particularly important to monitor the deformation of beach soft ground and improve the engineering characteristics of residual soil.In this thesis,the engineering properties of the soft foundation of the beach after backfilling and the basic characteristics of the residual soil were systematically studied through on-site monitoring of the foundation after backfilling and indoor modified residual soil tests(compaction test and infiltration test),and the main results were obtained as follows:(1)Through the monitoring of the site after backfilling,it was found that the maximum vertical settlement deformation and horizontal displacement deformation occurred in the soft layer of silt(2?4 m below ground),and the foundation deformation law and pore water pressure growth rate were different and complicated in different backfilling stages.(2)Five modified materials,fly ash,cement,lime,cement+fly ash,and lime+fly ash,were used to modify the residual soil,and compaction tests were conducted on the modified residual soil under different dosing conditions.The results showed that the maximum dry density increased and the optimum moisture content decreased with the increase of fly ash admixture.The optimum moisture content increased and the maximum dry density decreased with the increase of cement,lime and cement+fly ash dosing.The maximum dry density decreases and then increases,and the optimum moisture content increases and then decreases as the amount of lime+fly ash increases.Comparing with the residual soil before modification:the addition of fly ash can increase the overall density of the residual soil and achieve compaction stability with low water content,while the addition of cement or lime to the residual soil on the contrary requires an increase in water consumption to achieve the target compaction.(3)Under the condition that the aqueous solution is the percolator,the 2%cement+10%fly ash modified soil sample has the best impermeability;under the condition that the salt solution is the percolator,the optimal admixture of each modified soil in the aqueous solution percolation(with the lowest permeability coefficient)is selected for the percolation experiment.The results showed that the impermeability of each modified residual soil in salt solution percolation was better than that of the unmodified residual soil,among which,2%cement+10%fly ash modified soil sample had the best impermeability,while the good impermeability of 6%cement modified soil sample was mainly reflected in the low concentration(50 mg/L and 100 mg/L).(4)The microscopic analysis by XRD and SEM showed that a series of hydration and carbonation products,such as C-S-H gels,Ca(OH)2 and other crystals and silicate colloids,occurred in the modified soils of the five modified materials,and these chemical products are the direct factors to change the granite residual soil particle structure,size and pore space.The piezometric(MIP)analysis of the soil samples with the optimal doping levels showed that the critical pore size and the most available pore size of the five modified granite residuals were smaller than those of the original granite residuals,and the largest change was 2%+10%C-F-GRS,which proved the best impermeability from the microscopic pore structure.Combined with the above study,it is concluded that the deformation of beach soft ground is uneven and complex after backfilling and loading,and the deformation pattern will change in different locations,stages and directions.The best performance of 10%fly ash modified residual soil in terms of compaction characteristics and 2%cement+10%fly ash in terms of impermeability,therefore,the best modification scheme for Cangnan residual soil is 10%fly ash and 2%cement+10%fly ash,and this conclusion is of great significance to the improvement of residual soil backfill quality.