Some Researches On Characteristics Of Marine Deposit And Analysis Of Ground Settlement

Ground settlement calculating is always an important subject in geotechnical engineering research, and, in actual engineering practice, settlement calculating results are quite inaccurate because of unreasonable methods and inexactness of soil parameter chosen. Especially for coastal soft ground, because of its special properties such as consolidating with drainage, creep deformation, nonlinear characteristics of settlement, et al, settlement calculating methods that commonly used can hardly describe the actual development rule of ground settlement. From angles of both theory and reality, it will be important and significant to take some researches on deformation properties of marine deposit clay and calculating methods of nonlinear ground settlement.In this dissertation, some questions as nonlinear settlement calculation, drainage and consolidation properties of marine deposit clay, and its creep deformation characteristics, and so on, will be studied with emphases through methods of summarizing research documents, collecting and analyzing actual project data, theoretical deducing, numerical analyzing, laboratory test, field monitoring, et al. And the rationality and feasibility of research results will be tested and verified in actual project examples. The main contents are included as follows:The geotechnical test data of typical projects on west coast Shenzhen region are collected, and nearly a thousand samples of marine deposit clay are analyzed with its physical and mechanical parameters using statistical method. As a result, the former research works draw conclusions such as distribution of physical and mechanical parameters of marine deposit clay with depth, characteristic of correlations between parameters, and the rule how indexes of consolidation or secondary consolidation vary with load. There are specific regional disparities in engineering properties of soft soil, and the statistical results in this paper have practical significance for construction in Shenzhen coastal soft soil region.Through considering carefully about the computation theory and processes of the stratified summation method which is commonly used, theoretical limitations and existing problems in applying the method to calculate ground settlement are pointed out. As for the problem of nonlinear compression characteristics for soft soil, starting from the linear relationship between compression modulus and pressure, which is advised by Wei Ru-Long and Shi Jian-Yong, the hyperbolic model for e-p curve of laboratory consolidation test is brought forward, and its reasonableness is testified by actual ground settlement analysis of marine clay subgrade in large area loading. As for the hyperbolic model, the calculation parameters can be determined through different ways, and the cockamamie process of plotting and interplotation in traditional methods, which use e-p curve to calculate settlement, can be avoided, and the settlement calculation becomes easier.Based on simplification of Duncan-Chang constitutive model, parameters of the simplified model can be obtained from laboratory compression test data in general projects, so the nonlinear soft ground settlements can be calculated with incremental approach. Compared to general method, the simplified method is more suitable for nonlinear characteristics of soft ground settlement, and no special test is needed to determine calculation parameters, and it provides a more practical way. The subgrade of a large oil tank on deep soft soil ground in Zhuhai region is analyzed by using the simplified method mentioned above, and the calculating result is compared with those obtained from stratified summation method and Huang Wen-Xi method. As a research result, calculation with the common stratified summation method for construction subgrade settlements in soft area will bring distinct error to actual situation. There is difficulty in taking calculation parameters when applying Huang Wen-Xi method in actual projects. Calculating results by using the simplified method advised in this paper will fit better to actual situation.With application of the tangent modulus method which is proposed by professor YANG Guang-Hua, the nonlinear settlement of five in-situ plate loading tests on Texas A&M University Riverside Campus in USA is analyzed, and how to determine calculation parameters of tangent modulus method through some in-situ tests is researched. Through theoretical deduction and actual example analysis, the method obtaining initial tangent modulus from pressuremeter test data is put forward when soil properties of subgrade vary unevenly with depth. According to numerical simulating results of cone penetration test (CPT), correlation of power function between strength parameter and specific penetration resistance for sandy soil subgrade is obtained, and this provides a way to gain strength parameters of different ground layers with CPT data. Based on calculating parameters of tangent modulus method obtained from the in-situ test mentioned above, nonlinear ground settlements are analyzed with actual examples. As an analysis result, the ground settlement calculation method, which uses some simple in-situ test to determine calculation parameters of the tangent modulus method, is feasible and more suitable for nonlinear ground settlement calculation than normal methods.The variation rules of permeability during consolidating process is studied about marine clay, and a new empirical formula is proposed to calculate seepage coefficient for soft soil in consolidating process. Based on the above work, the varying regulation of consolidation coefficient for soft soil is deducted, and an empirical formula for calculating the consolidation coefficient is obtained. A practical differential program is compiled to calculate consolidation degree of soft ground with considering the variation of consolidation coefficient. As actual example analysis shows, consolidation analysis with considering variable consolidation coefficient will be more reasonable than classical consolidation theary.Based on the Buisuman formula, an improved calculation model of secondary consolidation for soft ground settlement is provided with considering the limitation of secondary consolidation settlement, and the rationality of the calculation model is testified by long duration field monitoring data of soft ground settlement in actual project.Fitting and forecasting approaches of settlement curve are studied within ground treatment of preloading drainage consolidation method, and an improved method is advised to forecast ground settlement with segmented fitting curves. For the improved method considers the different characteristics of settlement curve between primary and secondary consolidation phases, it is more suitable for long term forecasting of soft ground settlement. According to the analysis of observational data of settlement plate with actual examples in typical project, the improved method is more correspond to actual situation than normal forecasting method.