Research On Penetration Mechanism Of T-bar Penetrometer And Its Application On Offshore Engineering

With the development of marine resources in China and the national“Maritime Silk Road”development strategy,China's marine engineering construction continues to advance,forming a trend of development from offshore,shallow seas to distant seas and deep seas.The scale and difficulty of marine engineering has put forward higher requirements for the sensitivity,safety,and durability of the marine engineering,especially for the evaluation of dynamic loading and cyclic loading effects.The development of more reliable offshore engineering survey technology is imminent.Piezocone Penetration Test?CPTU?technology is one of the most important in-situ test method used in marine engineering.It has the characteristics of rapidness,economy,and reliability.However,a large number of foreign test results indicate that under the high confining pressure stress environment under the seabed,the traditional CPTU test technology is difficult to control the influence of high pore water pressure environment on the measurement accuracy,and the uncertainty of unequal area effect correction and overlying soil stress correction often results in the test as the water depth increases.On the basis of this,foreign scholars have proposed several new full-flow penetrometers based on the full-flow theory to solve the above problems.The full-flow penetration test technology mainly includes T-bar and ball,which can effectively increase the test accuracy,reduce the correction effect of hydrostatic pressure and overlying soil weight,and reduce the effect of soil stiffness and stress anisotropy during testing.And this test technology can measure the remodeled undrained shear strength of soft soil through cyclic penetration test,and further evaluate the sensitivity of soft soil.The research and application of T-bar penetration test in marine engineering in China has the potential to improve the reliability and ensure the safety of marine engineering.CPTU test system and T-bar full-flow penetrating test equipment independently developed by Southeast University were used to rely on the construction of marine soft soil sites along the coast of Fujian Province,through numerical simulation studies,laboratory chamber tests and on-site penetration tests.Through these studies,the methods of T-bar full-flow penetration testing in practical engineering applications were summarized,and the evaluation of engineering properties such as undrained shear strength and sensitivity of soft soil was established.The main contents and conclusions of this paper are as follows:?1?Based on the ABAQUS element analysis software,the penetration process of T-bar full-flow penetration in soft soil is simulated,and the strength characteristics and penetration apparatus of the soil in the penetration process were analyzed.Based on the analysis of the relationship between the resistances and the value of the penetration resistance N_T-bar of the T-bar penetrator was initially obtained.Furthermore,through the stress and strain changes of the soil surrounding the penetrometer in the penetration process,the destruction of the soil and the influencing range of the penetration process were studied.?2?Laboratory chamber tests of T-bar penetrometer were conducted,including single penetration tests,cycle penetration tests,and twitch penetration tests.By comparing CPTU,T-bar,ball penetrometer and vane shear test data,the reliability of undrained shear strength s_u of soft soil was evaluated by T-bar penetrometer.Analyze the results of single penetration test and cyclic penetration test,and put forward the evaluation method of undrained shear strength and sensitivity of soft soil by T-bar penetration instrument.The twitch penetration test was analyzed to study the influence of penetration rate on the penetration resistance of the T-bar penetrometer.?3?Based on the site engineering construction of coastal muddy soft soil along the coast of Fujian Province,T-bar penetration tests and CPTU tests were carried out.By analyzing the test data and the results of numerical simulation and laboratory chamber test,a method for evaluating the mechanical properties of soft soil,such as undrained shear strength and sensitivity,was established.From the results of the cycle penetration tests and summarize,the relationship between the measured penetration resistance q_m,the net penetration resistance q_net,and the circulation penetration resistance value q_cyc and the cycle number n was analyzed.Finally,the empirical relationship of soil strength attenuation in T-bar full-flow penetrating test was established for coastal soft soil sites in Fujian Province.