Research On Sinking Resistance And Its Distribution Characteristics Of Deep And Large Open Caisson

Open caisson has been widely used in the cross-river and cross-sea bridge engineering with its superior bearing capacity.However,so far,the distribution law of sinking resistance and sinking design calculation theory of deep and large open caisson is not perfect,so it is necessary to study the sinking resistance and its distribution characteristics in depth and systematically.Based on the project of Science and Technology Research and Development Program of China Railway Corporation,“Research on the New Structure and Key Techniques for Construction of Long Span Combined Road with Railway Cable-Stayed Bridge-Research on the Interactions of Large-Sized Caisson and Soil in Deep Water”(NO.2013G001-A-2),this dissertation takes the construction of the main pier sinking foundation of Hutong Changjiang River Bridge as engineering background and research object.Through field monitoring,centrifugal model test,theoretical analysis and numerical simulation,the research on sinking resistance and distribution characteristics of deep and large caisson is systematically carried out,the distribution law of sinking resistance is revealed,the computational theory and method of sinking resistance of caisson is proposed.The main contents and conclusions are as follows:1.The piecewise function is used to describe the distribution of frictional resistance of the straight sidewall and stepped sidewall.The practical calculation model of the frictional resistance of the sidewall of the caisson is proposed,and the rationality of the calculation model were verified by the centrifugal model test results.This practical calculation model can be used for sinking calculations in saturated sand environment.2.Based on the theory of cylindrical cavity expansion considering dilation effects,a theoretical solution for the ultimate soil resistance beneath the cutting curb is proposed,considering the characteristics of unilateral failure of cutting curb foundation and centrifugal model test and field monitoring are used to verify the calculation results.The comparison verification proves the rationality and effectiveness of the theoretical calculation method.3.Based on the soil arching effect theory,the magnitude and distribution of earth pressure on sidewall when the depth of sinking is large was analyzed by using theory of non-limit earth pressure and horizontal differential element method.The theoretical formula was deduced and the formula calculation results are compared with the field measured data and centrifuge model test data.The comparison verification proves the rationality and effectiveness of the theoretical calculation method.The formula is applicable to the working conditions in which the sinking depth of the caisson is large and the caisson is in a relatively slow sinking state.4.Study on the distribution law of the soil resistance of the cutting curb(1)The earth pressure of the cutting curb is related to the soil properties,the depth of the cutting curb and the depth of the caisson.The earth pressure of the cutting curb fluctuates greatly when sinking,and stable when heightening;(2)The soil resistance of the cutting curb tread is positively correlated with the depth of the cutting curb and the depth of the caisson;(3)The earth pressure on the inclined surface of the cutting curb increases with the increase of the depth of the cutting curb.The lower the position is the greater the increase.The earth pressure on the inclined surface of the cutting curb increases with the increase of the depth of the caisson.The lower the position is the greater the increase.The earth pressure on the inclined surface of the cutting curb is in the form of cubic polynomial distribution approximately along the inclined surface;(4)When the shape of the cutting curb is constant,the soil resistance ratio of the unit projection area of the inclined surface and the tread of the cutting curb is a certain value,and the value is 0.8 in the centrifugal model test.5.Study on the distribution law of frictional resistance of the sidewall(1)The earth pressure of the sidewall is related to the depth of the caisson,the attitude of the caisson and the stress relaxation effect.The earth pressure of the sidewall fluctuates greatly when sinking,and stable when heightening;(2)The earth pressure of the straight sidewall shows a significant nonlinearity under the action of stress relaxation effect,and there is a maximum value at about 2/3 times of the depth of the caisson.Above the position of the maximum value,the earth pressure of the sidewall increases with the increase of the depth of the caisson.Below the position of the maximum value,it decreases with the increase of the depth of the caisson,and finally decreases to about2/3 of the maximum value.Generally,it presents an approximate parabolic distribution that the upper and lower sides are relatively small,and the middle sides are relatively large.The height of the stress relaxation zone above the cutting curb is greater than 5 meters.The closer to the cutting curb,the more obvious the effect of the stress relaxation effect;(3)The step on sidewall will cause the friction resistance of the sidewall to decrease by about 50%above the step and nearly below the step and the maximum value of the friction of the sidewall will move down.With the increase of time,except for the friction resistance of the sidewall nearly above the step is almost unchanged,the frictional resistance of the sidewall gradually recovers,and the frictional resistance of the sidewall nearly below the step is even larger than that of the straight sidewall.When calculating the sinking of the stepped sidewall,the different sidewall friction reduction factorslshould be selected according to the sinking time t_z.The relationship between the sidewall friction reduction factor and the sinking time is:l=2.85×10^-4t_z+0.5.The sidewall friction reduction factor is small at the initial stage of sinking,and the value can be 0.5.If the sinking time of the caisson is longer,the value of the sidewall friction reduction factor in the late sinking period increases accordingly;(4)Under the condition that the caisson sinks to different depths,the earth pressure gradually decreases with the increase of the active translational displacement of the caisson,and the initial decrease was faster.When the displacement is greater than 0.8Sc,the change of earth pressure tends to be stable,and the active earth pressure is close to the limit state.As the translational displacement of the caisson increases,the earth pressure no longer changes significantly.The earth pressure gradually increases with the increase of the passive translational displacement of the caisson,and the initial decrease was faster.When the displacement is greater than 0.8Sc,the change of earth pressure tends to be stable,and the passive earth pressure is close to the limit state.As the translational displacement of the caisson increases,the earth pressure no longer changes significantly.