| This study takes world’s largest open caisson as the research object,aiming at the problems of the unclear sinking force characteristics encountered,the inaccurate design methods.Especially there is no solution to the world-class problem of sand gushing and sudden sinking,and related research is blank.In this study,with the combination of test(field test,laboratory sinking simulation test and centrifuge sinking simulation test,etc.)and theoretical research,the mechanical characteristics of sinking and the mechanism of sand gushing and sudden sinking are systematically studied.This study reveals the force characteristics and influencing factors of an open caisson.A method for calculating the sinking resistance of caisson considering the coupling effect of multiple factors such as burial depth,construction status,space effect and tilt is proposed.And based on the law of soil migration,combined with calculation method of sinking resistance,a judgment model and early warning mechanism for sand gushing and sudden sinking are established.The main contents and conclusions are as follows:1,The spatial distribution rules and influencing factors of end resistance and side resistance during the sinking of a large deep-water open caisson in sand are revealed.A calculation method of sinking resistance considering the coupling influence of multiple factors such as burial depth,construction status,space effect and tilt is proposed.(See results in published papers [2],[3],[4]and [5])(1)Reveals the distribution law of end resistance under the influence of multiple factors.The end resistance of the caisson increases with the increase of the burial depth,the degree of sand compaction when sinking and the height of the mud surface inside the blade foot.The end resistance reaches the ultimate bearing capacity of the soil when sinking.Tilt will increase the end resistance on the side where the sinking stops.The spatial distribution of end resistance of open caisson is the largest at the corner point and the smallest at the midpoint of the long side.The average stress of the short side is greater than that of the long side.The distribution of resistance on the slope of the blade foot can be described by a cubic function,and the calculated value is 70% of the simplified method of triangular distribution.(2)The vertical pressure of the caisson is between the static earth pressure and the passive earth pressure and the distribution of the side pressure is as follows: The side pressure at the caisson step is close to the theoretical value of static earth pressure;about 0.4h below the step(h is the height of the step)is the maximum point of lateral pressure,which is 50% to 60% of the theoretical value of passive earth pressure,and this ratio relationship decreases with increasing burial depth;there is a stress relaxation zone near the cutting edge,and the lateral pressure is 60%of the maximum value;the pressure above the step increases linearly and there is a stress concentration point,which is twice the theoretical value of static earth pressure.(3)The lateral distribution of side resistance is related to the lateral position.The degree of dispersion increases with the increase of the burial depth;at the same depth,the lateral resistance increases linearly with the increase of the horizontal distance from the midpoint.The distance between the average lateral resistance point and the midpoint of the side is about 0.25L(L is the side length).(4)The setting of steps can significantly reduce the lateral resistance of open caisson.After the burial depth of the caisson exceeds 20 m in the vertical state,the average lateral resistance of the area below the steps is 135 k Pa to 320 k Pa,and the average lateral resistance of the area above the steps is 20 k Pa to 80 k Pa;the overall average lateral resistance of the caisson is 60 to 115 k Pa,which is 3 to 4 times to the standard value.(5)The change of construction state of open caisson has little influence on the average lateral resistance,and has obvious influence on the distribution form.At the same burial depth,the lateral resistance when the caisson is heighting is 0.9 times that of sinking.A stress dissipation model is proposed for the heighting of the caisson.When the burial depth of the caisson is 20 m to 40 m,the stress dissipation causes a 30% reduction in lateral resistance at the stress concentration point below the step.(6)In order to overcome the problem that the existing earth pressure theory of retaining wall is not applicable to the inclination calculation of a large deep-water open caisson,a lateral pressure calculation model based on the horizontal squeezing amount and lateral pressure change law is proposed.Compared with vertical state,when the side wall of the caisson squeezes the soil,the lateral pressure increases linearly,and when it is far away from the soil,the lateral pressure decreases linearly.When the amount of inclined horizontal squeezing is 0.006H(H is the burial depth),the lateral pressure is about twice that of the vertical state,which is close to the theoretical value of passive earth pressure.When 0.0017 H away from the soil,the side pressure is 25% of the vertical state,which is close to the theoretical value of static earth pressure.The research reveals that the ratio of the horizontal squeezing volume to the buried depth when the large deep-water open caisson is inclined is much smaller than that of the classic earth pressure theory of retaining wall.2,For the first time,this study reveals the internal connection between “sand gushing and sudden sinking” and the migration of soil outside the caisson,and points out that permeability is positively related to migration and is the main factor affecting soil migration;blade foot resistance and lateral pressure are negatively correlated with migration and are both affected by influence of penetration.Before and after the “sand gushing and sudden sinking”,the tread resistance,slope resistance,hydraulic gradient and lateral pressure near the blade foot all changed significantly,and these changes are earlier than the occurrence of “sand gushing and sudden sinking”.(See results in published papers [1] and [5])3,In order to overcome the world-class problems of early warning of “sand gushing and sudden sinking”,for the first time,an early warning mechanism of “sand gushing and sudden sinking” in a large deep-water open caisson is proposed,and relevant warning thresholds are given.The mechanism includes the calculation model of migration coefficient,the calculation method of sinking resistance considering the coupling effect of multiple factors,the comprehensive analysis and evaluation of on-site monitoring values(sinking posture,sinking resistance and suction nozzle flow velocity,etc.).When the migration coefficient B is 0.5 ≤B＜1,small-scale sand gushing and sudden sinking may occur;when B≥1,large-scale sand gushing and sudden sinking may occur.Combined with the calculation method of sinking resistance and on-site monitoring results,it is comprehensively judged whether the “sand gushing and sudden sinking” of the will occurred.This achievement has made an important contribution to ensure the sinking of the caisson and prevent the hazard of “sand gushing and sudden”.(See results in published papers [1] and patent [1])4,The laws of soil migration outside the caisson,the affected area,and the characteristics of the subsidence space formed after the migration are revealed.The migration trajectory of the soil outside the caisson is mainly parabolic,which is basically consistent with the influence boundary of migration.In the engineering application,a polyline distribution is used to describe the impact boundary,which is 0.65 H on the river bed surface and 0.55 H at 0.3H below the river bed surface.The subsidence caused by sinking on the river bed surface is 0.15 H.This result provides a basis for evaluating the change of the river bed surface by the construction of a caisson.(See results in published papers [1])5,Considering that the calculation of the bearing capacity of the soil at the blade foot with a bevel has not been solved,this study based on the finite element ALE sinking simulation,reveals that the bearing capacity of the soil at the blade foot is linearly related to the internal friction angle and cohesion.For the same internal friction angle,when the slope of the blade foot is completely inserted into the soil,the bearing capacity of the soil at the blade foot increases nearly linearly with the increase of the height of the mud surface.The effect of internal friction angle on the bearing capacity is greater than the cohesive force.When the friction angle in the soil exceeds 35 °,the effect of cohesion on the bearing capacity of the soil is not obvious.(See results in published papers [7])... |
PDF Full Text Request