Study On Theory And Application Of In-situ State Characteristics And Liquefaction Evaluation Of Cohesiveless Soils Based On Multifunctional Piezocone Penetration Test(CPTU)

The large-scale development and utilization of urban underground space,which puts forward higher requirements for the safety,the performance of long-term service and the capacity of disaster prevention and mitigation of these infrastructures.The liquefaction evaluation becomes the primary task of the prevention of liquefaction earthquake damage,where spatial distribution and in-situ state of cohesionless soils(sand,silt)are very important contents.Multifunctional piezocone penetration test(CPTU)is a kind of measurement technology for in-situ evaluation of soil engineering properties at natural locations,which is widely used internationally in the state characteristic evaluation of cohesiveness soils and liquefaction discrimination with the advantages of high precision,multiple testing parameters,etc.Compared with the relative density,state parameter can represent the double effects of the density and the stress level of cohesionless soil.Its determination method and related application are widely concerned by scholars at home and abroad.In order to provide theoretical basis and technical support for in-situ state evaluation and application,further researches of the determination of in-situ state parameters and the application of liquefaction foundation design level were carried outBased on this background,with the funding of the National Key R&D Program of China,the National Science Foundation Project and the Postgraduate Research&Practice Innovation Program of Jiangsu Province,aiming at the problem of state parameter evaluation,the evaluation and application of state characteristics of saturated cohesionless soils were systematically studied by Multi-function CPTU in situ testing,laboratory tests,and theoretical analysis.The main works and conclusions of this dissertation are shown as following:The determination method of sand state parameter?is systematically studied through the calibration chamber CPT test on Nanjing river sand and the intelligent algorithm of Group Method of Data Handing(GMDH).The results show that the cone tip resistance,q_t has a significant"critical depth",and its value increases with the increase of D_r,about 0.45-0.60 m.In the semi-logarithmic coordinate,good linear relationships between CPT parameters and the unit weight?and D_r are showed.The range of CPT penetration mechanism will be enlarged with the increase of D_r.The state parameter calculated by the GMDH model with q_t1N,??_vo and I_c,BJas input parameters is the preferred model from the perspective of performance evaluation and convenience.By applying to the soil state characterization and liquefaction evaluation of the calibration chamber CPT test on Nanjing river sand and Tangshan earthquake liquefaction site,the?results given are basically consistent with the existing classical methods,and the liquefaction identification results are also consistent.In view of the typical geological cohesionless soil in Jiangsu Province,an in-situ state parameter evaluation method of saturated cohesionless soil based on resistivity CPTU test is proposed.The results indicate that soil behavior type index,I_c is an effective index to connect state performance and electrical characteristics of saturated cohesionless soils.The values of state parameter calculated by the resistivity CPTU method and the Plewes method,the Been and Jefferies method,and Robertson method have the same change trend along the depth and the values are close to each other.The change trend of state parameter?and relative density D_ralong the depth is the opposite,and the linear correlation is good,thus,the?can be quantified by the D_r.The cyclic resistance ratio,CRR calculated by the?is basically the same as that calculated by the international general NCEER method.The electrical dilation parameter has a good power function correlation with the normalized cone tip resistance and relative density,and the resistivity CPTU can be used as a powerful tool for in-situ state evaluation and ground compaction control.In view of the typical geological cohesionless soil in Jiangsu Province,an in-situ state parameter evaluation method of saturated cohesionless soil based on seismic CPTU test is proposed.The results show that the change curve of the state parameter along the depth evaluated by the V_s method is basically the same as those by the CPTU methods,but there are differences in the numerical value.The in situ?by the V_s method is more close to CPTU method,and has much stronger consistency.Under the field stress condition,V_s can be expressed as a function of the state parameter?,the effective overburden stress??_v0 and the coefficient of lateral earth pressure at rest K₀.The contour map of state parameter profile in the plane of V_s-??_v0for Jiangsu area is drawn,and the state(loose and dense)and liquefaction sensitive zone of cohesionless soils at any depth are given.The V_s has a good power function relationship with normalized tip resistance q_t1N,the contour map of V_s in Robertson's SBTn soil classification chart is constructed.Combining the tip resistance with the state parameter can provide a reasonable,comprehensive and unified new method for predicting G_max of cohesionless soil.Based on laboratory tests,the liquefaction dynamic characteristics of silt sand mixture with different sand content and state parameter index-based liquefaction evaluation method were studied.The results indicate that the liquefaction and cyclic softening of sandy silt occur in the range of excess pore water pressure ratio 0.75-0.85,and the cumulative shear strain is about5%.Then liquefaction is occurred by the large shear strain and excess pore water pressure ratio.The influence of silt content is not obvious,which is significantly diluted by other control factors such as the relative density under a certain void ratio e.Besides,there is a good linear relationship between CRR and e.With the increase of silt content,the microstructure of mixture of silt and sand and the contact characteristics of solid particles have been changed,and the cumulative pore distribution curve of sandy silt has good gradient and multi peak characteristics.The state parameter?and CRR_tx,15 have exponential function relationship,and CRR_tx,15gradually decreases with the increase of?.The modified factor model between CRR_tx,15 and field CRR_7.5 based on?has been established.The CPTU liquefaction evaluation method under simplified stress framework was established based on algorithm of combination Kernel Extreme learning machine(KELM)and particle swarm optimization(PSO).The results show that the CPTU model is more applicable to a wider range of soil,including the previous understanding that"too clay rich to liquefy";for silty soil,B_q has a certain effect on CRR,and is presented in the form of graphs.The proposed CPTU model for liquefaction assessment is consistent with the field observation results.The calculation model of P_L based on F_S given by maximum likelihood estimate can consider the inherent uncertainty of the model.Finally,a unified theoretical system of liquefaction potential evaluation based on CPTU simplified stress model is formed.Based on CPTU testing technology,the feasibility and superiority of CPTU liquefaction discrimination are demonstrated,the evaluation method system of treatment effect of liquefied ground and CPTU multiple methods of liquefaction discrimination are studied.The results show that the pore pressure distribution of soils can be directly measured by the CPTU,which directly evaluates the in-situ state and mechanical properties of the soil.The soil classification chart of CPTU is able to give satisfactory results for liquefied and non-liquefied soil.A chart method of combined normalization of excess pore pressure and state parameter difference has been proposed and can effectively evaluate the in-situ state and potential dilative and contractive characteristics.Densification evaluation indexes of liquefied ground treatment are put forward:relative increase index,resistivity index,small strain stiffness,state parameter and so on.According to the theory of random field,the liquefaction potential map is used to evaluate the effect of liquefaction ground treatment,and a visual,quantitative and accurate evaluation system for anti-liquefaction ground treatment is established.Based on combined test evaluation theory of SPT and shear wave velocity,a multi-component liquefaction CPTU evaluation method for cohesionless soil was established.