Research On Penetration Mechanism And Application Of Free Fall Penetration In Offshore Engineering

With the implementation of China’s maritime power strategy and“the Belt and Road Initiatives”,marine engineering construction has attracted more attention,such as land reclamation from the sea,cross-sea bridges,subsea tunnels,offshore wind power,offshore oil drilling platforms,nature gas hydrate drilling platforms are increasing.At the same time,marine engineering is constantly developing from offshore to far sea,from shallow sea to deep sea,and the scale of the project is increasing.The marine engineering puts forward higher requirements for accurate acquisition of marine geotechnical engineering parameters.However,the methods of borehole sampling and laboratory testing in traditional survey technology have problems,such as stress release,sample disturbance,and high cost,which cannot meet the requirements of marine geotechnical engineering.In the world,Ocean in-situ testing techniques represented by Piezocone Penetration Testing(CPTU)and Full-Flow Penetration Testing(FFPT)are effective methods to obtain geotechnical engineering parameters.These methods all use marine static penetration penetrating equipment,and require large ships equipped with heavy cranes as the carrying and operating platform.Therefore,these methods have the disadvantages of heavy equipment,complex operations,high overall cost,long time-consuming,extremely high cost,and the influence of weather.These problems have greatly restricted the application of conventional ocean static cone penetration technology in ocean engineering.Free Fall Penetration Testing(FFP),as the new portable dynamic penetration testing technology,can be used as an effective supplementary method for conventional offshore static penetration testing.In recent years,it has been widely used in marine sediment stratification,marine slope stability analysis,marine environment investigation,marine disaster prevention,port dredging,anchor foundation design of floating wind power system,pipeline and cable survey,freshwater lake paleontology research and other fields.Under this background,this paper takes the Free Fall Penetration Testing as the research object,and combines domestic and foreign literature,firstly independently designs the free fall penetrometer.Then,based on Newton’s second law of motion,the mechanical model and theoretical system of FFP penetration process are established,the test process is defined,and the test data processing process is established.Based on the laboratory model test,the penetration mechanism of FFP was studied.Finally,based on the Taihu Lake Tunnel Project of Suzhou-Wuxi-Changzhou Southern Expressway,the field test of FFP is carried out with typical lacustrine mucky soft soil as the research object,and the evaluation method of undrained shear strength of soft soil based on FFP is established.The main research contents of this paper are as follows:(1)The s free fall penetrometer is designed and developed independently,including six parts:tip,probe body,counterweight module,sensor module,stabilizer tail and lifting ring.The tip is divided into conical tip,hemispherical tip and cylindrical tip.IMU integrated with high-precision three-axis accelerometer,gyroscope and geomagnetic field sensor is developed.It can obtain the acceleration,angular velocity and other motion data of FFP,and it is equipped with data acquisition software to display and record the motion data of FFP in real time.(2)Based on Newton’s second law of motion,the forces in the process of penetration are analyzed,including the submerged weight,bearing resistance at the tip,frictional resistance along the shaft,drag resistance and buoyancy force.The mechanical model and theoretical system of FFP penetration process are established,the FFP test process is defined,and the processing framework of test data is established.(3)The research of the free fall penetration testing laboratory model test is carried out,the undrained shear strength of the test soil is determined and the standard value of the strength of the test soil is provided by vane shear test(VST).By carrying out CPTU tests on three different tips,the bearing capacity coefficient N_c of different tips and interfacial friction ratioαare studied respectively.FFP tests with different release heights,masses and ends are carried out to study the effects of different factors on penetration depth,and the optimal strain rate parameterβis determined.An evaluation method of undrained shear strength of soil is established based on FFP.(4)Based on the Wuxi Taihu Tunnel Project of the Suzhou-Wuxi-Changzhou Southern Expressway,the typical mucky lacustrine soft soil is selected as the study soil to carry out the FFP field test.Combined with CPTU field test,Ball and T-bar full-flow penetration testing and previous site geological data,the curves of acceleration and velocity during FFP,CPTU and full-flow penetration testing are obtained,and the relevant physical and mechanical parameters of the field soil are obtained.The undrained shear strength curve of the soil is obtained through a variety of testing techniques and the evaluation method of the undrained shear strength of soil is verified by the FFP.