Research On Penetration Mechanism And Application Of Ball Penetrometer In Offshore Engineering

In offshore engineering,the coastal soft soil is widely distributed around the world.In the regional economic development strategy of our country"The Belt and Road"driven by the construction of our country is gradually turning to the direction of 18 thousand km long coastline,coastal and deep-sea resources for domestic use is less than the level of the developed world,imminent construction of various types of marine engineering.In order to ensure the safety of the marine building,fully grasp the seabed soil environment,how to obtain reasonable physical and mechanical parameters of soft soil foundation such as shear strength,sensitivity,consolidation properties become the important problem of marine geotechnical engineering practice.Piezocone Penetration Test?CPTU?technology at home and abroad in the marine geological exploration is one of the most important in-situ measurement methods.However,at the bottom of the sea high pressure stress environment,the precision of the pore pressure components cannot be guaranteed,ordinary cone cannot describe the overlying soil effect on the measured cone resistance value.Therefore,the increase in soft clay in the seabed obtained data accuracy will be decreased with the depth of sea water.On this basis,foreign scholars have put forward T-bar and ball penetrometer testing technology,which can effectively increase the test precision in the marine soft soil moisture content greatly,reduce the hydrostatic pressure and overburden weight correction value.Compared to the CPTU full flow penetrometer is less affected by soil stiffness,and stress anisotropy.Through the cycle of penetration it can determine the strength of reshaped soft clay rapidly and accurately.The ball penetrometer also have the advantage of pore water pressure measurement,the anisotropic symmetric geometry of it is more suitable for marine geotechnical in deep and uneven investigation,so the promotion and application of the ball type probe undoubtedly has a positive meaning for improving the test precision,reducing the geological exploration cost of marine soft clay foundation and improving the marine soil engineering construction safety.This paper is under the situation of vacant research of domestic ball penetrometer technology,combined with domestic and foreign literature.This research first uses large deformation finite element analysis software to simulate the process of ball type probe penetration and stress and strain relationship affected by the surrounding soil.Then according to the relevant international standard ball probe,based on CPTU test equipment and data acquisition system developed by Southeast University,the research develops the ball type probe experimental equipment in accordance with the experimental model of slot size,and studies on indoor model of ball groove type probe.The process and application of ball penetrometer in engineering practice has been concluded and promoted based on the actual project of ball type probe field test.Combined with numerical analysis and experimental data,this research establishes data price method of soft clay mechanics and consolidation characteristics on the basis of experimental studies.The main contents and conclusions of this paper are as follow:?1?Based on finite element analysis software ABAQUS,considering the effect of strain in real soil,the penetration process was simulated respectively considering the effect of the entire spherical probe and the spherical probe rod.The force condition of probe and soil behavior in the process of penetration are simulated by the ideal elastic-plastic model and D-P strength characteristics,and the soil stress and displacement around the probe during penetration,as well as plastic strain distribution are compared.Related to changes in deformation characteristics of probe penetration in surrounding soil and pore water pressure,the research combined with the modified Cambridge model and Standard module summarizes the deformation and pore induction of ball probe penetration in the process of penetration.?2?Based on the ball type probe in the specific experimental model groove penetration process,the experimental equipment and process are improved.The research separately tests circular penetration experiment indoor single penetration test,and strain rate were spherical full flow penetrometer.Combined with collected vane shear data binding groove model,the evaluation of soil shear strength and sensitivity of the ball type probe are studied,and the influence of strain rate on the measured data are analyzed.?3?Analysis the experimental data of circular penetration and penetration rate.The soil degradation cycle intensity in the data are measured and expressed as actual penetration resistance q_in and extraction resistance q_ext step by step along the cycle times of N decline.Combined with the indoor model slot,the typical curve and strength attenuation process of the ball type probe circular penetration test are analyzed.The research studies the obvious differences about penetration and extraction resistance in both undisturbed soft soil and remolded soft soil.The relevant literature is summarized,and influence factors of finishing spherical probe offset value are settled.The transmission rate of CPTU probe penetration experiment determines drained shear condition under different shear strain rate of experimental soil.Respectively,based on the ball type probe,this study uses multiple sets of single penetration and penetration rate of twitch experiment to quantify the effects of probe penetration rate on actual shear strength.?4?Carry out the ball penetrometer test project based on the project in FuJian province.Combined with the CPTU test and vane shear test results,the ball type circular probe penetration experiment data are analyzed,in particular:a)Describe typical curves of ball penetration probe test in Fuzhou combined with cycle data,and on the basis of it,the measured penetration resistance q_m are corrected for pore water pressure u and soil load?_v0;b)According to the relationship between penetration resistance q_m and the number of cycles of N from undisturbed soil is established to completely reshape the strength attenuation curve;c)The relationship between the value of soil sensitivity s_t and the cycling test measured penetration resistance q_in and extraction resistance q_ext is derived.?5?By comparing laboratory or field vane shear test data,the fitting relationship between spherical probe penetration resistance q_net and undrained shear strength of s_u is established,from which the probe experimental data of bearing capacity factor N value can be obtained.Based on the observation on N value extension depth distribution of ball,T-bar and CPTU,the accuracy and reliability of soft clay behaviors evaluation by bearing capacity coefficient N value are verified with the help of the normal distribution and residual analysis method.According to the field and model experiments,regression relationship between measured penetration resistance q_m,modified penetration resistance q_net and penetration resistance value q_cyc,N cycles is established respectively.