Study On Sinking Mechanism Of Anchorage Caisson For Suspension Bridge

With the development of large-span suspension bridges in China,more and more large-scale anchorage caisson foundations are used.However,the design and construction of large-scale caissons are still immature: most current codes for design of large-scale caissons are based on research results of large-diameter piles or small-scale caissons;due to the unclear sinking mechanism during construction,problems appear frequently,such as insufficient foundation bearing capacity,tension crack,sinking difficulties,sudden sinking,large displacement under the main cable tension.Therefore,further study on sinking mechanism of large-scale caissons to solve problems during its design and construction period and ensure its safety during the construction process and the bridge operation process,is of great significance to the rapid development of bridge engineering in China.Supported by the Project of Science and Technology Research and Development Program of China Railway Corporation “Research on Key Technologies for Construction Control,Operation and Maintenance of Wufengshan Railcum-road Suspension Bridge of Lianzhen Railway”(Grant NO.2017G006-A)and mainly based on the north anchorage caisson of Wufengshan Yangtze River Bridge,this paper is trying to solve the problems that may be encountered during the design and construction of large-scale caissons mentioned above through theoretical analysis,field test,site monitoring,numerical simulation and other methods.The main work and conclusions of this paper are as follows:(1)From the aspects of bridge type,span,foundation type,etc.,all the cross-river bridges on the Yangtze River were statistically analyzed,and the application of large-scale anchorage caisson foundations in the Yangtze River bridges was studied.The results show that cablestayed bridges and suspension bridges are the main bridge types;the span of Yangtze River bridges is mostly concentrated at 400?600m;a total of 10 large-scale anchorage caisson foundations are used in Yangtze River bridges,accounting for 15.1% of all anchorage foundations.(2)Based on the south anchorage caisson of Wenzhou Oujiang Estuary Bridge and the north anchorage caisson of the Wufengshan Yangtze River Bridge respectively,field tests were carried out for the common ground treatment form of large-scale caissons—composite foundation with thick cushion and group sand piles.The factors affecting the bearing capacity and the reinforcement effect of this composite foundation,the mutual influence of sand pile construction were analyzed.The results show that the composite foundation with thick cushion and group sand piles can strengthen the soft soil very well;the bearing capacity of the composite foundation can be improved by reducing the moisture content of the soil,using good cushion materials,and increasing the thickness of the cushion;sand pile construction has a great impact on the surrounding sand piles,which can be reduced by increasing the distance between sand piles and using the barrier effect;(3)Based on the Timoshenko deep beam theory,an internal force calculation model of largescale caissons was established,an internal force calculation formula was derived,and the influencing factors of tensile stress at the bottom of large-scale caissons were analyzed.The results show that the calculation formula of the large-scale caisson deflection and internal force derived from Timoshenko deep beam theory are consistent with the finite element simulation results,and the calculation result is reliable;after sinking to a certain depth,increase of height and earth pressure around the caisson can reduce the deflection and internal force of large-scale caissons;increasing the initial height and adopting a reasonable sinking method are more feasible and economical measures to control the deflection and tensile stress of large-scale caissons;(4)Based on the limit analysis theory,the calculation formula of ultimate bearing capacity of two-layer soil foundation beneath the blades and inner partition wall was derived.The distribution pattern of the sidewall friction of large caissons was deeply analyzed and the calculation method of the critical depth was proposed.The results show that the calculation method proposed by this paper can effectively calculate the ultimate bearing capacity of twolayer soil foundation beneath caissons;according to the relationship between the sinking depth and the critical depth,the calculation of the sidewall friction of the large-scale caisson can be divided into two stages: in the first stage(the sinking depth is less than the critical depth),the sidewall friction increases linearly with the increase of the sinking depth;in the second stage(the sinking depth is greater than the critical depth),the distribution pattern of sidewall friction is related to soil properties and the sinking depth;(5)The monitoring results during the first sinking of the north anchorage caisson of Wufengshan Yangtze River Bridge were analyzed,and the relevant parameters of soil were inverted based on the monitoring results.The results show that when the sinking of a caisson enters a stable state,the sinking rate is inversely proportional to the soil properties;the internal force of the caisson is greatly affected by the sinking method;the sidewall friction strength inversed from the measured data of this caisson is about 0.8 times of the recommended friction strength;the settlement of the Yangtze River embankment is related to the distance from the monitoring point to the caisson,for this caisson,there is still settlement at a distance of three times of the sinking depth.(6)Based on Rankine?s earth pressure theory and two-parameter method,a calculation method of earth pressure considering the influence of displacement is proposed,and a response calculation method of large-scale caissons under combined load is established based on Winkler model.The results show that: the displacement required to generate the limit passive earth pressure is large,which makes it difficult for the actual lateral earth pressure to reach the limit state in actual engineering;the earth pressure calculation method in this paper can fit the measured earth pressure value well;the response calculation method of large-scale caissons given in this paper can well calculate the rotation angle and rotation position of the caisson under combined load;the friction between large-scale caisson and soil accounts for a small proportion of the resistance.