Research On Failure Mechanism And Bearing Capacity Of Subsoil Of Deeply Buried Large Dimensional Open Caission

Open caisson is a kind of technique that permit the structured caisson to be progressively sunk either under its own weight or with the aid of caisson jacks in a controlled manner from the surface to a predetermined depth. With the development of infrastructure construction in China, there is no doubt that the open caisson is the first choice of the main tower foundation, horizontal anchors and the maritime constructions of bridges over rivers and bays due to its good integer behavior, high stiffness and so on.The main tower foundation of Hu Tong Yangzi River Bridge lies in the over 200-meter sedimental soil with fluvial facies, lacustrine facies and littoral facies which formed in Quaternary Holocene. These soils have the characteristics of short formation, small preconsolidation pressure, low bearing capability and great settlement deformation. Combined with the design and construction of the main tower foundation of Hu Tong Yangzi River Bridge, this thesis investigates the failure mechanism and bearing capacity of the open caisson which is deep buried and large dimensional by implementing field experiments and monitoring,large scale loading tests, theoretical analysis and numerical simulation. Some useful conclusions are demonstrated as follows.1. Open caisson can be defined as instrumental retaining structure during the sinking and construction stage. In this stage, bearing capacity of open caisson performs through the resistance on footing blade and the side friction.(1) For the bearing capacity of footing blade, assuming failure mechanisms of footing-soil interface,the upper bound solution based on limit analysis is derived, and the rationality of the failure mechanism is verified by theory analysis in this thesis. For the foundation on the weightless soil, the upper bound solution corresponds well with the verified accurate value during the specific stage. However, for the foundation on the weight soil, the current research of the upper bound solution is still on the stage of semi-theory and semi-experience. The upper bound limit solution could provide references for further researches.(2) The upper bound limit solution is verified and analyzed by numeral simulation further in this thesis as well as the influence of the relative width of foot blade and the friction angle of interaction on the bearing capacity of footing blade. The cutting blade bearing capacity firstly increased, and then decreased with the growing of width of footing blade. When the friction angle of interaction was 0.5, the cutting blade bearing capacity varied smaller than when the friction angle of interaction was 0. While the friction angle of interaction maintained a constant value, the bearing capacity showed similar tendency with the width of foot blade. The bearing capacity changed dramatically in different soil. When the friction angle of interaction was 0.5, the capacity was growing rapidly. And it was even obvious when the reaction angles of interaction were different.(3) Combined with the analysis of bearing capacity of open caisson during the sinking and construction stage, the traditional study of sudden sinking has been extended to shallow depth and deep depth cases, and the inducing factors are different. When the buried depth was shallow, soil vertical resistance weakened sharply on one side of the footing blade, which was the main inducing factors of sudden sinking, due to the lack of effective lateral support, which usually cause significant changes of open caisson of attitude. When the buried depth was deep, the suddenly decreased local stress concentration of side friction was the main inducing factors of sudden sinking. Due to the lack of effective lateral support and the effect of effective lateral support of the surrounding soil, open caisson posture will not change significantly after the sudden sinking;(4) The interaction between open caisson and the foundation soil, is difficult to be accurately explained in theory at present due to the difference of different soil engineering characteristics and distribution of uncertainty, and the inevitable posture change in the process of the open caisson sinking. The quantitative reference of field real-time monitoring was directed through the qualitative analysis theory, and the two complement each other, giving relatively reliable construction proposal and real-time warning in the process of open caisson sinking, which provides effective guarantee for safe and orderly sinking.2. Open caisson was used as large buried rigid foundation during built using stage, the bearing capacity is the foundation soil bearing capacity when the settlement does not exceed expectations.(1) The static load model test and numerical simulation calculation showed that destruction of the sandy soil foundation has obvious progressive failure characteristics. There was obvious distribution in contact stress between rigid foundation base and soil, followed as progressive failure of foundation soil.Early in loading, the contact stress of the basement and soil stress distribution present uniform growth.Along with the load increase, based edge began to yield, yield region of the contact stress of the basement continues to grow, no longer contact stress peak value, the horizontal stress in the soil of significant growth.Along with the load continued to increase, yield area expanded from the underlying edge to foundation center, the peak contact stress also move to the foundation center, the peak value of horizontal stress in the soil to the deep direction.(2) In this paper, the different stages of the large-scale base loads and the failure modes of the traditional foundation were unified from the perspective of settlement control, the foundation bearing capacity under different failure modes research was converted into different stages under the same pattern.In the consolidation stage, there is no plastic zone in the foundation soil, overall in the elastic compression state, at this point, if the settlement of foundation have been uncontrolled, this can be regarded as punching shear damage. In the local shear shearing stage, yield of foundation soil area has not been unicom, at this time, if the settlement of foundation have been uncontrolled, this can be regarded as local shear failure.(3) The gradual damage formula of foundation soil is deduced on the basis of the principle of passive earth pressure According to the formula, the width of yield area can determined, and then the basal contact stress distribution can be determined. When q0? cNc + qNq , the foundation soil as a whole is in elastic state, the contact stress is evenly distributed. When cNc + qNq?q0?qu, foundation soil is in local shear state, the contact stress of the yield area basement is the combination of uniform distribution and combination of trapezoidal distribution.Yield area soil under different load gradually reach limit state from edge to center, then present trapezoid distribution, while elastic region are evenly distributed.(4) Through the tri-axial test of sandy soil under high confining pressure, the compression of soil was found to be divided into two stages, i.e. elastic compression stage and yield compression stage. The sandy soil compression modulus under different load and confining pressure of simple correction formula was proposed according to test data, which provide a reference for bearing capacity calculation method that was based on the subsidence control for the validation of this article.(5) The large open caisson foundation bearing capacity design method that was based on the subsidence control is put forward. When the contact stress distribution of basement was combined, the bearing capacity of the yield area in the calculation was not considered. A certain amount of iterative calculation was used in the sandy soil compression modulus after using the load increment and load. Then the foundation bearing capacity design value with basic long-term settlement as control index was obtained.