Experimental Study On Engineering Properties Of Qiantang Alluvial Silts

With Hangzhou entering a new development stage known as Qiangtang Era, Qiangtang alluvial silts have been frequently encountered in the metropolitan construction of Hangzhou. Silt is a kind of complex and transitional soil between sand and clay and its engineering property is different from both of them. For this reason, the thesis studied engineering properties of Qiantangjiang alluvial silts through investigations, laboratory testing and with the self-developed apparatus for testing seepage stability. The main work is as follows:Firstly, the thesis investigated and analyzed the geological characteristics of Qiantang alluvial silts. Based on the test results of a large number of samples from dozens of projects along the Qiantang River in Hangzhou, typical geological sections of silts in Xiasha area, New Qianjiang area and Binjiang area were summarized respectively in terms of geological origins. Components of particles and basic properties of silts were analyzed with statistical methods. The research revealed that Qiangtang alluvial layers were composed of sandy silt and were characterized by "high silty and low clayey". Seepage failure easily occurred in these soil layers under hydrodynamic pressure.Secondly, the thesis studied characteristics of the soil-water system of Qiantang alluvial silts. Through X-ray diffraction and Electron Scanning Microscope tests, the mineralogy and microstructure of silts were analysed. The falling conditions of cones within three ranges of sinking depths (3?4 mm,7?9 mm and 15?17 mm) were measured and analysed in the fall cone tests, which was designed to determine the liquid limit and plastic limit of the samples with different ratios of silt, sand and clay. The relationship between water content and penetration was also studied in the tests. The conventional liquid and plastic limits cannot represent the plasticity characteristics of silts. Based on the hydrometer analysis results of nearly one hundred samples, suggestions on the classification and nomination of silts are proposed.Thirdly, the thesis studied the dilatancy of Qiantang alluvial silts. Drained and undrained triaxial tests were conducted to study the dilatancy of saturated silts. General triaxial drained tests of five groups of silts with different saturation degrees were carried out afterwards to study the dilatancy of the unsaturated silts. The failure of Qiantang alluvial silts usually goes through large deformations. The stress-strain curve of drained tests can be divided into four stages. It is reasonable that the point where the strain can continue without further change of volume is chosen as a failure criterion to define the drained strength of silt.Fourthly, the thesis studied seepage characteristics of Qiantang alluvial silts. Using improved constant head test, the permeability law of silts was studied. Statistical analysis was made of the results of variable water head tests of over 30 projects in Xiasha area, New Qianjiang area and Binjiang area. The seepage characteristics of silts layer were studied and the indexes of permeability of Qiantang alluvial silts were proposed by taking into account both the results of the statistical analysis and the outcomes of the field pumping water tests.Fifthly, the apparatus for testing seepage stability were developed. According to the limit equilibrium analysis of silt samples, the new apparatus were developed for effective measurement of the critical hydraulic gradient of silt undergoing seepage failure in laboratory tests. The changing water level device and the transparent vessel insure the accuracy and dependability of impermeability intensity measuring. Parallel multi-containers were used so that more samples could be tested simultaneously.Lastly, the thesis studied the seepage stability of Qiantang alluvial silts. The apparatus were used to conduct experiments on five typical silt samples, undisturbed soil samples with horizontal bedding and anti-seepage layers with geotextile. Failure mechnisms of silt under seepage were analyzed and corresponding preventive measures were proposed. The research provides more knowledge on the prevention of silt seepage failure, and can be used as an important reference for general engineering.