# Study on the Earth Pressure during Sinking Stage of Super Large Caisson Foundation
## 1 Introduction  
In recent years, with the development of economy and transportation construction, the number of bridge planning and construction in China is increasing, and the bridge span is constantly refreshing the world record. Caisson foundation has the advantages of high bearing capacity, good integrity, big stiffness, and small area occupancy [1]. Therefore, it is widely used in large-scale bridge construction. With the requirement improvement of bearing capacity and settlement of large-sized bridge engineering, the caisson foundation is developing towards to large size and deep sinking elevation. Ensuring the safe sinking has been the key issue to sinking construction of caisson foundation. The distribution characteristics of end resistance is different from that of small and medium-sized caisson foundation, which inevitably caused different controlling method of sinking construction for different size caisson foundation [2]. However, at present, the sinking construction of caisson foundation by soil excavation mostly depends on engineering experience, the distribution of earth pressure is not clear during soil excavation, which caused poor controllability of sinking process [3; 4; 5]. The soil excavation only relying on engineering experience has seriously affected the safety of the sinking construction and the reliability of caisson foundation structure, and the sinking process by soil excavation needs to be further studied. The distribution of the sinking resistance can reflect the whole stress state of caisson foundation. The sinking resistance is an important basis to make soil excavation scheme, and the earth pressure at the foot blade is the key factor to study the sinking resistance of caisson foundation. Based on the slip line field theory, [PERSON] et al. [6] established an approximate calculation model of the ultimate soil resistance of caisson foundation during sinkingand derived the calculation formulas of the ultimate bearing capacity for plane strain problem and axisymmetric problem. [PERSON], et al. [7] used similar methods to determine the distribution characteristics of earth pressure of the annular caisson foundation, which was verified by engineering examples. [PERSON], et al. [8] analyzed the sinking process of caisson foundation of Hutong Yangtze River bridge by conventional model test and obtained the earth pressure distribution of foot blade during the sinking process. [PERSON], et al. [9], studied the earth pressure under the foot blade by the centrifugal model test, and found that the ratio of earth pressure almost keeps constant between horizontal position and the oblique position of the foot blade. Besides, the field measurement of earth pressure was used to draw the time-history curve in order to analyze the distribution of sinking resistance and the sinking behavior of caisson foundation by many scholars [2; 10; 11; 12]. [PERSON], et al. [13] and [PERSON], et al. [14], focused on the mechanism of sudden sinking of caisson foundation and found that the earth pressure at the foot blade will significantly decrease before sudden sinking of caisson foundation. At present, most existing studies focused on end resistance of small and medium-sized caisson foundation, while there are few studies on that of large or super large caisson foundation. [PERSON], et.al. [15] pointed out that with the continuous increase of the scale of caisson foundation, the ratio of end resistance and side friction resistance has gradually become the controlling factor to safe sinking of caisson foundation. For small or medium-sized caisson foundation, the side friction resistance played a leading role in the sinking process, which is not possibly suitable for large or super-large caisson foundation. In this paper, the variation characteristics of end resistance by soil excavation was studied based on soil excavation process of super large caisson foundation of the main tower of Changtai Yangtze River Bridge.