| The large-scale construction of infrastructure in coastal areas has gradually brought ultra soft soil into the research scope of geotechnical engineering in China.The particularity and complexity of the soil's engineering characteristics bring great difficulties to the treatment of ultra soft soil foundation,which seriously affects the safety,stability and economic benefits of the infrastructure construction in the ultra soft soil areas.Therefore,in order to further understand the soil,a reasonable definiting principle was presented first through the statistical analysis of collected domestic datum of the soil,and then taking as an example the remolded marine ultra soft soil in Lianyungang,its characteristics of permeability and consolidation were studied in point through many lab tests,as well as the strength,unloading and dynamic characteristics briefly.The main contents and conclusions are as follows:?1?On the basis of engineering practice,"Similar Pascal Effect"was put forward as the principle to select the definition indexes,which were divided into primary indexes and secondary indexes after statistical analysis of the datum of the soil,and the specific range of each index was obtained according to the determination method of standard value of geotechnical parameters.The recommended values of the primary indexes are dry density less than 1.2 g/cm3,liquid index greater than 1.2,degree of saturation greater than 92.0%,permeability coefficient less than 1×10-7cm/s,compression modulus less than 2.5 MPa,internal friction angle less than 3.5°and cone tip resistance less than 0.37 MPa.The recommended values of the secondary indexes are natural water content greater than 50%,natural density less than 1.8 g/cm3,plasticity index greater than 10,the void ratio greater than 1.3,the compression coefficient less than 1.0 MPa-1,the cohesion less than 10.0 k Pa,and the side friction less than 15 k Pa.Finally,the defining principle of ultra soil soil was proposed that all the main indexes meet the requirements above,as well as one of the secondary indexes.?2?Given that it is difficult to measure the shear strength of ultra soft soil by conventional direct shear test,an improved direct shear test method with low normal stress was proposed to study its strength.The results showed that the shear stress increases first and then stabilizes with greater shear displacement.The stress range of2.5 k Pa?7.5 k Pa was reasonable to calculate the strength of the soil because the samples were extruded to damage with the vertical stress too large,then the internal friction angle and cohesion of three groups of samples of different water content are calculated respectively.Meanwhile,a direct shear test method with low vertical stress was summarized,two key points of which are using a measuring ring with smaller elastic coefficient to test and flexibly selecting an appropriate vertical pressure range?the recommended value is 2.5 k Pa?7.5 k Pa?according to the Mohr Coulomb Strength Theory.?3?The characteristic of permeability and consolidation was studied through the permeability tests of variable head and one-dimensional consolidation tests.The results of the form showed that the permeability coefficient of the remolded soil with water content ranging from 48.3%to 83.4%has a good exponential relationship with the initial void ratio.Through the latter were the laws of consolidation deformation studied with time,consolidation pressure,moisture content or dry density.The calculation results of compression parameters showed that the compression coefficient av1-2 is about 1.15 MPa-1?1.51 MPa-1,the compression modulus Es1-2about 1.41 MPa?1.75 MPa,and the compression index about 0.37?0.52,all of which have good linear relationships with water content or dry density.Besides,the calculation showed that the compression coefficient av0.25-0.5 is about 2.65?4.77 times of av1-2,which indicated that the soil has strong compressibility under low pressure,and the compressibility is stronger with the worse properties.After comparative analysis of the consolidation coefficient of the remolded soil calculated by four methods as the formula method,direct solution method,Casagrande's logt method and Taylor's?t method,it was found that Taylor's?t method is more suitable and the value is about 10-5 cm2/s?10-4cm2/s.?4?The unloading characteristic was studied by one-dimensional unloading tests and triaxial lateral unloading tests.Through the former,the law of springback deformation changing with time and vertical pressure was obtained.Then the resilient modulus was redefined in order to study the deformation characteristic in the whole unloading process and the calculations was that the maximum of resilient modulus is about 40.78 MPa?53.32 MPa and the minimum about 0.13 MPa?0.29 MPa.Besides,the results of triaxial unloading test showed that the principal stress difference increases with the axial strain in the early stage,and the cohesion and internal friction angle of the samples with moisture content of 55.3%and 65.0%are 25 k Pa,15.33 k Pa and 0°,1.08°respectively.?5?By dynamic triaxial test were the dynamic stress-strain relationship,dynamic elastic modulus and dynamic strength of the remolded soil with different water content and confining pressure studied.The results showed that the dynamic stress increases with the dynamic strain showing hardening characteristic,and the dynamic elastic modulus decreases,and the dynamic strength decreases rapidly with the the vibration times of the load.Then the dynamic cohesion and dynamic internal friction angle and their variation with moisture content or dry density were obtained. |
PDF Full Text Request