| Fine-grained marine sediments containing bio-gas are widely distributed in five continents all over the world.The gas is largely presented in the form of discrete bubbles in the soil.Unlike the conventional unsaturated soils,the fine-grained gassy soil contains discontinuous phase of gas and continuous water phase,with the degree of saturation exceeding 85%.The constitutive behaviour of this soil cannot be readily predicted by the classical unsaturated soil mechanics.Nor can its behavior being properly described by saturated soil mechanics considering a compressible fluid,as the size of bubbles(hundreds ?m)is much larger than the soil particle(4.2?m).The previous experimental investigation has revealed that the presence of gas phase could either increase or decrease the strength of soil skeleton,depending on the gas content and pore gas pressure(related to pore water pressure).To date,the following three issues of fine-grained gassy soil still remain poorly understood or unsolved:(a)the morphology of the gas bubbles in an in-situ stressed fine-grained soil and its evolution in response to loading;(b)the effect of gas bubbles on the yield surface shape and stress-dilatancy of the soil;(c)a unified modelling for the constitutive behavior of fine-grained gassy soil considering both detrimental and beneficial effect imposed by the gas bubbles.In light of the afore-mentioned issues,the work performed in this study includes:development of in-situ mini-loading equipment in ?CT,high-resolution ?CT scanning tests on incrementally loaded fine-grained gassy soils,triaxial tests,development of a constitutive model for the soil and the associated analytical solution for cylindrical cavity expansion in fine-grained gassy seabed.These have led to the following key conclusions:1)Under the one-dimensional consolidation condition,the bubbles in the gassy soil are nearly ellipsoidal shaped,with the sphericity ranging approximately between 0.35 and 0.75.For each given loading,the distribution of sphericity and major-axis diameter cannot be fitted by the conventional probabilistic model(Normal,Lognormal,Weibull and Gamma distribution).While Gaussian mixture model that consists of 2 to 4 components of Gaussian distributions can serve the purpose.2)With the increase of the vertical load,the bubbles are mainly compressed in the direction of the vertical load,while the horizontal deformation is limit.This is because the majority of localized soil failure(and thus cavity collapse occurred near the roof of each bubble cavity,as revealed from elliptical cavity contraction analyses considering stress anisotropy.The parameter analysis illustrated that the level of stress anisotropy of soil is the key factor controlling the shape of bubbles.The CT images and the anisotropic cavity contraction analysis did not show any development of tension cracks and tensile stress around the mature gas bubbles(with no tendency to grow)in the consolidated sediments.This differs from the case of very soft consolidating sediments with growing gas bubbles,which may fracture through the seabed due to the tensile stress developed around the bubbles.3)A series of GDS triaxial stress path tests were performed to investigate the shape of yield surface,the direction of plastic strain increment and the stress-dilatancy of fine-grained gassy soil.It is revealed that compared to the stress-dilatancy relation of saturated soils,the gassy soils are either substantially more contractive or less contractive,depending on the initial pore water pressures(uw0)and gas volume fractions(?0).Meanwhile,the presence of bubbles is found to significantly affect the shape of the yield locus of gassy soils("bullet-shaped","elliptical","teardrop-shaped"),depending on uw0 and ?0.In light of these experimental observations,new stress-dilatancy relation and yield surface function were proposed for the fine-grained gassy soil.4)By incorporating the yield function and dilatancy function of gassy soils into the elastoplastic framework in conjunction with the concept of critical state,a new elastoplastic critical state constitutive model for fine-grained gassy soil was presented.Compared with the MCC model,five additional model parameters are introduced in the proposed model,with two new parameters controlling effect of gas on yield function,two new parameters governing the stress-dilatancy,and one new parameter relating to the compression behavior of gas bubbles.The new model captures both the damaging and beneficial effects of gas bubbles on the stress-strain behavior of gassy soils in a unified manner.And at ?0=0,the proposed model is recovered to a conventional critical state model for saturated fine-grained soil(similar to MCC).The model has been verified against the compression and shear behaviour of three types of fine-grained gassy soils with different plasticity.5)By incorporating the new model into a three-dimensional cylindrical cavity expansion framework,a semi-analytical solution was derived to predict the behavior of offshore foundation or cylindrical penetrometer penetrating into fine-grained gassy seabed.Parametric study was performed to quantify the influence of uw0 and ?0 on the changes in stress,pore pressure and strain around the penetrating cylindrical structure.6)The sampling disturbance due to undrained unloading of fine-grained gassy soil was also investigated via a series of triaxial tests.It was found that the undrained unloading has led to an increase in the undrained shear strength of gassy silt,opposing to the reported observations made from gassy clay.This is because the undrained unloading is accompanied by a reduction in total mean stress p and pore water pressure uw,while the pore gas pressure ug remains relatively unchanged.Two competing mechanisms are resulted,a)when the value of ug-uw exceeds the air-entry values(AEV)of soil(mainly for silt),the behavior is dominated by gas encroachment into the surrounding saturated matrix that would increase the shear strength via desaturation;b)when the value of ug-uw stays below the AEV(mainly for clay),the behavior is governed by gas cavity expansion that bubbles would cause failure of the surrounding saturated matrix and thus a reduced shear strength.These mechanisms have been re-produced numerically via coupled hydro-mechanical analyses in Code-bright. |
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