Study On Engineering Properties And Consolidation Settlement Regularity Of Soft-soil In Wenzhou Shoal

Soft-soil is one kind of especial soil mass, that is formed by marine and lacustrine sedimentations in the areas of coast, lakes, swamp, overflowland and valley in modern times. Soft-soil has very poor engineering properties with high natural water content, large void ratio, bad penetrability, great compressibility, low shear strength, long consolidation period, high sensitivity, easy disturbance and visible rheology. Soft-soil exists widely in the world, which is mainly located in coasts, where are bulit cities, factories, airports, wharfs, highways etc. Therefore, the study on engineering properties and foundation treatment of soft-soils is becoming one of the most important aspects in geotechnical engineering.The two key points in soft foundation treatment are deformation and stability. For the deep soft-soil foundations, the settlement calculation and residual settlement controlling of foundation are the core problems in foundation treatment. Because of the complexity of soft-soil, the accuracy of deformation and settlement calculation is still inadequate. The calculation theory develops far behind the engineering practice, and great difference is existing between them, which may cause such problems as construction period delaying, economy losing, even engineering accident happening. For this reason, in order to study the foundation design theory, the new settlement calculation methods should be developed to solve the settlement problem. In addition, the numerical computation method and settlement prediction method based on the measured settlement data play an important role in solving the settlement problem for soft foundations. The study of consolidation settlement characteristics and regularities is closely related to selection of physical and mechanical parameters of soft-soil. The selection of parameters and their correlation analysis are very critical to foundation treatment design, and such research is helpful to similar projects.Wenzhou shoal is the largest and most complete tidal flat resources around estuary area of Ou river. The whole embankment of Lingni seawall in Wenzhou shoal reposes on the deep silt foundation. The silt, that with high water content, large compressibility, low shear strength, low bearing capacity and low penetrability, could cause to large deformation and differential settlement. The duration of silt deformation will be long. Therefore, the silt foundation in Wenzhou shoal must be reinforced effectively. The foundation treatment methods used here includes drainage consolidation method with plastic drainage plate and preloading method with ripraps. According to the construction process, monitoring data and operation of the seawall, there is certain difference between the preliminary design computation and practical situation.Based on above, in this dissertation, taking silt in Wenzhou shoal and soft-soil foundation treatment of Lingni seawall as an example, the following scientific researches were carried out:(1) Analysis on areal geology and engineering geologic conditions in Wenzhou shoalBy means of literature reading, analysis of original data in geoexploration, design, construction, monitoring stages and on-site exploration, the general situation of areal geology, sedimentary environment of silt, engineering geologic conditions, hydrogeologic conditions and soft-soil foundation treatment project in Wenzhou shoal were studied in detail.(2) Reliability study on engineering properties of soft-soil in Wenzhou shoalAccording to the geoexploration materials and data of geotechnical test and in situ test, the reliability of each physical-mechanical parameter of silt was studied. Basing on the theory of Vanmarcke random field, adopting the "recurrence spatial method", autocorrelation distance of each soil parameter was figured out. In this way, the "point properties" could be extended to the "spatial properties" of each soil parameter, so the "spatial variability" could be gotten. The regression analysis among soil parameters was done, and also the experimental formulas were obtained among them. Moreover, dynamic variation regularity of soil parameters with depth was studied.(3) Study on consolidation settlement regularity of soft-soil foundation in Wenzhou shoalBy using the on-site monitoring data of representative sections (survey section of in-situ instruments) of Lingni seawall, the surface settlement, layered settlement,horizontal displacement and dissipation regularity of pore pressure in the process of consolidation were analyzed detailedly. The average stress degree of consolidation was calculated by the transformation form of Barron analytical solution which applied to solve the pore pressure according to the actual measured pore pressure data. The calculation and statistical analysis of vertical and radial coefficient of consolidation were carried out by "time square root method" based on the laboratory experiment data. The inversion of vertical and radial coefficient of consolidation was performed by "companion method of exponential curve" and "improved Mentian method" proposed in this dissertation on the basis of actual measured settlement curves. Finally, the contrast of inversive results and calculated results was promopt.(4) Study on the settlement prediction of soft-soil foundation in Wenzhou shoalFirstly, the comparative study of all kinds of settlement prediction methods for soft-soil foundation was carried out. And then, the actual measured settlement data were treated with mathematical method, and the settlement prediction was performed for the representative section of Lingni seawall respectively by hyperbola method, exponential curve method, Pearl curve model, grey Verhulst model and improved BP artificial neural network arithmetic (improved BP-ANN model). Finally, the comparison of prediction values and measured settlement data was carried on. The applicability and predicted efficiency of each prediction method were obtained. The optimal settlement prediction model and method were put forward for Wenzhou shoal.(5) Study on the settlement calculation of uni-directional compression with axisymmetric consolidation of soft-soil foundation in Wenzhou shoalCentering on the settlement calculation of uni-directional compression with axisymmetric consolidation of soft-soil foundation, combining with the abundant settlement monitoring data of Lingni seawall project, the computation and statistical analysis on the calculation method of degree of consolidation, the calculation method of consolidation settlement, the control standards of the compression layer depth, the value of the empirical correction coefficient for settlement and the inversion of compressibility index were proceeded. Based on the above, the "fitting formula method" and the "iterative layerwise summation method of slope-toe" for the final settlement calculation were proposed. The computing formulas and solution procedures of both methods were given in this dissertation. Case verification was made as well.(6) Study on the analytic calculation of one-dimensional and three-dimensional consolidation settlement of soft-soil foundationStarting from the stress and strain status in foundation soil at any time and in any point, considering the preconsolidation state (the value of OCR) and stress-history, by using the secant modulu and effective stress curve, the conversion relation formula between stress degree of consolidation and strain degree of consolidation was derived. The theoretic calculation systems were established for one-dimensional and three-dimensional consolidation settlement calculation which could consider the preconsolidation state (the value of OCR) and stress-history, such as one-dimensional compression modulu (Es) method, one-dimensional compression index (Cc) method, three-dimensional deformation modulu (E) method and three-dimensional compression index(Cc)method. In the end, taking the Lingni seawall for example, the contrast verification of calculation results by the above methods and measured settlement data was made.In the process of research in this dissertation, based on the abundant and dependable test data and on-site monitoring data, through the systemic study and analysis, the main conclusions and research achievements are drawn as follows:(1) The research results of reliability study on engineering properties of soft-soil in Wenzhou shoal show that:①The main engineering properties of soft-soil in Wenzhou shoal could be summarized for "4 high 2 low", that is high natural water content, high void ratio, high compressibility, high sensitivity and low penetrability, low shear strength.②For the soft-soil in Wenzhou shoal, the autocorrelation distance of each soil parameter computed by the laboratory experiment indexes is significant larger than the one computed by the in-situ test indexes. In order to reduce the influence caused by the perturbation and space of sampling, could adopt cone penetration test (CPT) data to determine the autocorrelation distance of silt, and take the average result as a standard value for the silt. In this dissertation, the average autocorrelation distance of silt in Wenzhou shoal is 0.46m. ③The "spatial coefficient of variation" of each physical-mechanical parameter of silt in Wenzhou shoal is obvious less than the "point coefficient of variation". The variability of parameters is decreased in deep degree, which shows that, the soft-soil horizon is relatively homogeneous, and the actual variability of silt parameters in Wenzhou shoal is very low.(2) The analysis results of on-site monitoring data for survey section of Lingni seawall show that:①The surface settlement P～S～t curve and layered settlement curve are characterized as stages and stepladders form. The maximal accumulated monitoring surface settlement has achieved 447.9cm, and the minimum settlement is 170.2cm, the mean value is about 311.9cm. The compressibility of each foundation layer is in the range of 0.005-0.268. The main deformation presents in the treatment area of plastic drainage plate, the amount of compression accounts for 80.4-87.4% of the total settlement with an average at 84.7%. The amount of compression below the treatment area of plastic drainage plate accounts for 12.6-19.6% of the total settlement with an average at 15.3%. Therein, the compressibility of surface layer of silt with the depth 4-8m is maximum in soft-soil foundation.②The monitoring data of incipient pore pressure by pore-water pressure meter shows that, the natural foundation of Wenzhou shoal belongs to normal consolidated soft clay foundation. Accumulative pore pressure-accumulative load curve of soft foundation and pore pressure coefficient-accumulative load curve in each loading step are manifested in three different developing stages (that is, initial loading stage, middle loading stage, terminal loading stage).③The calculation results of vertical and radial coefficient of consolidation gotten by "time square root method" based on the laboratory experiment data show that, under the same loading conditions, thereis Cv Pearl model>grey Verhulst model>exponential method>hyperbolic method. The prediction precisions of the first three methods are similar to each other. The predicted settlement is slightly less than the measured settlement only when using the improved BP-ANN model among the five methods. In the opposite, when using the other four methods, the predicted settlement is more than the measured settlement at different degree. According to the error analysis and convergence rate of prediction curves, the hyperbolic method, exponential method and improved BP-ANN model are applicable to the short-term and medium-term prediction, yet, the Pearl model and grey Verhulst model are suitable for all the short-term, medium-term and long-trem prediction. The prediction results of Pearl model and grey Verhulst model are highly similar, which is related to the functions of both model buildings on basis of S-form curve. It is proved that the development tendency of settlement with time of soft-soil foundation in Wenzhou shoal varies in S-form.(4) The statistic results of a series of calculations and analyses for 63 monitoring sections of Lingni seawall show that:①According to the inversion of compressibility indexes based on the measured settlement data, the linear regression equations of compression modulu (Es) and compression index (Cc) are established between inversive values and testing values. The regression equations could be used to modify the design compressibility indexes.②For the soft-soil foundation of Lingni seawall in Wenzhou shoal, the average value of the empirical correction coefficient for settlement is 1.62 when adopting the compression modulu (Es) method (e～σ'curve method); and the average value of the empirical correction coefficient for settlement is 1.44 when adopting the compression index (Cc) method (e～lgσ' curve method), which anastomosed with the regional empirical value 1.4. Therefore, the compression index (Cc) method is suggested for the settlement calculation.The research also shows that, there is not a simple proportion relation between the final consolidation settlement and final total settlement, the relationship between them could not be described only by the empirical correction coefficient for settlement. In fact, there is a linear relationship between them with high correlation. By means of regression analysis of the final consolidation settlement and final total settlement, the linear fitting formulas between them has been established for soft-soil foundation in Wenzhou shoal to calculate the final total settlement. The calculated results by "fitting formula method" are slightly larger than the measured standard value, and the difference is small. The results gotten by the fitting formulas are more exact and secure.③Due to the shortages of traditional layerwise summation method, a new method called "iterative layerwise summation method of slope-toe" is proposed to calculate final total settlement of soft soil foundation by promoting the traditional layerwise summation method. This new method can consider the settlement caused by weight gain. In order to reduce errors of settlement calculation, the calculation height in the center of the embankment is repeatedly modified (additional stress is modified as well) by means of iteration the settlement of slope-toe in this method. There is an optimum and ultimate iteration times through which the calculated results by iterative layerwise summation method of slope-toe are the most close to the actual measured settlement data.The statistics analysis of calculated final total settlement values by the new method for Lingni seawall shows that, the new method may be potential to calculate foundation final total settlements with its accurate results, and the optimum iteration times is different for different consolidation settlement calculation methods. When adopting the compression modulu (Es) method to calculate the consolidation settlement in the new method, the optimum iteration times is 4 times; and when adopting the compression index (Cc) method, the optimum iteration times is 3 times.(5) The conclusions of the analytic calculation of one-dimensional and three-dimensional consolidation settlement of soft-soil foundation show that:①According to the stress and strain status in foundation soil at any time and in any point, considering the influence of pre-consolidation pressure on effective stress which caused to the deformation of soil mass, by using the secant modulu-effective stress curve, the conversion relation formula between stress degree of consolidation and strain degree of consolidation is derived in different over-consolidation ratio (OCR) conditions. In the beginning and at the end of consolidation of foundation, stress degree of consolidation is equal to the strain degree of consolidation. At any time in the process of consolidation, the strain degree of consolidation is larger than the stress degree of consolidation. The conversion relation formula between them is related to the preconsolidation state, self-weight stress, additional stress and compressibility indexes of soil mass.②Starting from the stress and strain status in foundation soil at any time and in any point, according to the hooke law and generalized hooke law, considering the preconsolidation state (the value of OCR) and stress-history, the analytic calculation formulas of one-dimensional and three-dimensional consolidation settlement and final settlement calculation represented by stress degree of consolidation and strain degree of consolidation of soft-soil foundation at any time are deduced. The theoretic calculation systems are also established for one-dimensional and three-dimensional consolidation settlement calculation, such as one-dimensional compression modulu (Es) method, one-dimensional compression index (Cc) method, three-dimensional deformation modulu (E) method and three-dimensional compression index (Cc) method.The practical calculation results show that, the average strain degree of consolidation of soft-soil foundation in Wenzhou shoal is larger than the average stress degree of consolidation in the range of 0-15%. The difference between the one-dimensional consolidation settlement calculation values and the actual measured settlement values is not small. The contrastive analysis of settlement curve between the three-dimensional consolidation settlement calculation and the actual measured settlement shows that, the three-dimensional consolidation settlement calculation method is better than the one-dimensional method, and the three-dimensional compression index (Cc) method is superior to the three-dimensional deformation modulu (E) method, its results even more close to the actual measured settlement data.