| With the further development of marine power strategy,more widespread attention has been paid to the safe service of submarine geotechnical facilities.One of the research hotspots is to accurately assess the in-situ undrained shear strength characteristics of marine soil.Existing T-bar and spherical penetrometers and other commonly used seabed soft soil in-situ testing techniques are one of the important means to determine the design parameters of marine geotechnical engineering.However,the evolution of soil failure mechanism,soil strain softening and strain rate effects are caused by these types of probing devices in the process of large deformation penetration,the inversion of soil strength in situ using the resistance coefficient models of the existing plastic solutions will be distorted.In view of this problem,the following research work has been carried out: the sequential limit analysis(SLA)method for geotechnical engineering developed by my supervisor has been extended so that it can:(1)analyze rotational axisymmetric problems and(2)deal with more extreme soil large deformation;on this basis,the large deformation interaction between soil and structure during the penetration process of T-bar and spherical penetrometers is studied and a new dynamic probing device have been designed.The main research work is briefly described as follows:(1)The finite element limit analysis(FELA)solver in the numerical platform Optum G2 is used to replace the relatively niche FELA solver Ox Lim in the original SLA method,making the method more general and universal,and the original plane strain SLA method is compatible with rotating axisymmetric problems for large deformation simulation,making it possible to simulate pile,bucket,spudcan foundation and Ball and CPT probing device penetration problems.The methods for dealing with rigid body and soil contact interface and upper bound solution maneuver field in special conditions are improved,which make up the shortcomings of the original method in dealing with extreme large deformation conditions(such as soil slope accumulation and collapse of heavy pipe horizontal buckling).(2)Using the newly developed SLA method which can consider soil strain softening effect and strain rate effect,the vertical penetration analysis of cone,wedge,spherical penetrometer and spudcan foundation and the vertical installation and horizontal buckling analysis of seabed pipeline are carried out.The simulation results are compared in detail with the analytical solutions,numerical solutions of other large deformation methods and existing model test results.The calculation accuracy and reliability of the proposed method are verified in the cases of rotational axisymmetric penetration problems and extreme large deformation conditions.(3)A vertical large displacement loading model test of circular plate is carried out,and the limitations of SLA method based on rigid-plastic assumption in simulating overconsolidated soft clay problems with strong elastic behavior in the early stage are discussed.(4)The large deformation process of T-bar and Ball penetrating from mud surface to deep layer is simulated.The influence of soil parameters,strain softening coefficient and strain rate coefficient on penetration resistance are quantified.The transformation law of soil failure mechanism and resistance variation during probe penetration from shallow layer to deep layer are revealed.The errors caused by using traditional coefficient calculation model are evaluated and are corrected based on simulation results.A calculation model for accurately inverting soil true strength from in-situ test results in actual engineering is provided.(5)The dynamic characteristics of rigid body are considered in SLA method,and the soil rate effect is used to reflect the effect of penetration rate on soil strength.An example is given to illustrate the effectiveness of the quasi-static method to simulate the dynamic problem.Combining the characteristics of seabed type and self-falling type penetration devices,a new type of deep-sea in-situ probing device that needs to be mounted on a deep submersible is designed.SLA method is used to simulate the penetration process of self-weight probe,the variation range of sensor parameters is evaluated,which provides a new solution for deep-sea in-situ testing. |
