Shear Characteristics Of Interface Between Saturated Sand And Suction Foundation During Penetration

As a new type of Marine engineering foundation,the penetration process of suction foundation is the key step in the installation of suction foundation,and penetration to a preset position is the premise for foundation to normally provide bearing capacity.The penetration process of suction foundation is as follows:firstly,the suction foundation sinks under the action of dead weight,the bottom is cut into the seabed to form a confined space,and then the water in the confined space is extracted to make the suction foundation penetrate into the preset position.Many factors such as penetration rate,soil compaction,soil particle size and gradation affect the lateral friction resistance of suction foundation during penetration.In this paper,interfacial shear apparatus is used to investigate the effects of shear rate,soil compactness,particle size and gradation on the interface shear characteristics of saturated sand-suction foundation by conducting interfacial shear tests.The variation law of interface shear strength index and the variation law of sidewall friction when suction foundation is immersed are obtained.In addition,the shear strength index of soil under the influence of the above factors is obtained through direct shear test,and the relationship between the friction angle of soil and the interface friction angle is obtained.The research results of this paper can provide reference for the design and construction of suction foundation.The main research results were as follows:(1)In the actual project,the penetration rate of suction foundation in sandy soil is 0.17～0.12 cm/min,so five different shear rates are preset in this paper,which are 0.05,0.1,0.5,1.0 and 1.2 cm/min respectively.By means of interface shear test and direct shear test,the variation of interfacial shear strength,soil shear strength and its index of saturated sand-suction foundation at different shear rates was investigated.The results show that the interface shear strength and interface friction angle increase with the decrease of shear rate.When the vertical pressure is 10～200 kPa,the shear rate changes from 1.2 to 1.0 cm/min,the shear strength of saturated fine sand increases by 9.8%～17.7%.And the interface friction angle increases with the decrease of shear rate,and the range of increase under different vertical pressures is 5.1%～10.6%.Under different vertical pressures,the ratio of interface friction angle to internal friction angle ranges from 0.28 to 0.47.Moreover,the interface friction coefficient of saturated fine sand at different shear rates is 0.20～0.38.Under the same condition,the interface shear strength,interfacial friction angle and interfacial friction coefficient of saturated coarse sand are higher than that of saturated fine sand.In addition,the movement of particles is the main reason for the change of shear strength and volume shrinkage of the interface.The movement forms of particles in the early stage of shearing are tumbling and sliding,while in the later stage of shearing,the particles are basically arranged in a stable manner,and the movement forms of particles are mainly sliding.Finally,the friction resistance of inner wall and outer wall increases exponentially with the increase of penetration depth,but the friction resistance of inner wall is less than that of outer wall.At the penetration depth of 5 m,at each shear rate,the friction resistance of the outer wall of suction foundation in fine sand is 1.58～1.72 times that of the inner wall,and that of the outer wall of suction foundation in coarse sand is 1.94～2.21 times that of the inner wall.(2)The degree of soil compaction has an important influence on the interfacial shear strength and the shear strength of soil.In this paper,loose(Dr=0.3),medium density(Dr=0.5)and dense(Dr=0.7,0.9 and 1)soils were tested.The results show that strain softening occurs in saturated sands with different densities during interfacial shear.But direct shear tests found that strain softening occurred in dense sand(Dr=0.7,0.9 and 1),while strain softening occurred in medium dense sand Dr=0.5)before strain hardening.Loose sand(Dr=0.3)only undergoes strain hardening.What is more,the soil strength of sandy soil with different compactness is similar in the late shearing period.And the shear strength of interface and soil increases with the increase of compactness.It is found that the interface friction angle,internal friction angle and interface friction coefficient all increase with the increase of relative density.In addition,when the vertical pressure is 10～200 kPa,the friction resistance of the suction foundation outer wall in the sandy soil with Dr=1 is 1.26～1.38 times that of the corresponding friction resistance in the sandy soil with Dr=0.3.(3)Through interface shear test and direct shear test,the influences of different sand particle sizes and grades on interface shear strength and soil shear strength were investigated.The results show that the interfacial shear strength and soil shear strength increase with the increase of particle size.When the median particle diameter D5o is increased from 0.175 mm to 0.376 mm,the interfacial shear strength and the soil shear strength are significantly increased,and when the vertical pressure is 100 kPa,the increase ranges of both are 110.9%and 44.9%,respectively.Furthermore,the interface friction angle,internal friction angle and interface friction coefficient all increase with the increase of particle size.When D5o increases from 0.175 mm to 0.376 mm,the increase of interface friction angle and internal friction angle is the largest,which are 70.4%and 22.4%respectively.Moreover,the range of interface friction coefficient is 0.17～0.6.On the other hand,the interface shear strength and shear strength of well-graded sand were greater than those of poorly graded sand.Moreover,the interface friction angle and internal friction angle of well-graded sand are 1.14 and 1.1 times of those of poorly graded sand.In addition,at the same depth,the friction resistance of inner and outer walls increases with the increase of particle size.At the penetration depth of 2.5 m,the particle size increased from 0.075～0.25 mm(fine sand)to 0.25～0.5 mm(medium sand),and the friction resistance of inner and outer walls increased by 89%and 117.6%respectively.Likewise,the friction resistance of the inner and outer walls of the well graded sand is greater than that of the poorly graded sand.