| As China's transport development in the building of more bridges, many reasonable, economic, aesthetic bridge structure has been constantly reserached, introducted and developed. Because of its unique advantages in particular, high-pier and long-span continuous rigid frame bridge is widely used by designer.Pier, beam and base of high-pier and long-span continuous rigid frame bridge are consolidated as one bearing forces together, the pier's type and heigh influenc bearing behavior of structure. Bearing behavior of structure of high-pier and long-span continuous rigid frame bridge is:pier and beam consolidated as one; the lower and super structure bear force together, and which reduce negative moment in top pier; pier's stiffness is lower and larger displacement is allowed; because biridge structure is multi ultra-setting structure, Shrinkage, creep, temperature changes, prestressed of concrete caused uneven settlement of platform, which caused additional internal forces that significantly influenced the structure's bearing behavior.As uneven settlement of foundation of high-pier and long-span continuous rigid frame bridge is strictly control, so based on the prototype observation for pile group foundation of Dukouhe Bridge of Yiwan railway, this paper conduct test and theory annlysis on it, and which offer reference to construction practice. Dukouhe River Grand Bridge located in the middle of Yi-Wan Railway, and its subgrade is made up of soft rock which engineering behavior is unsteady and vulnerability to external disturbances. Because height of No.5 pier is 128 m, and strict with stability of pier and deviation of pier top, so were selected for the prototype observation target. The aim of this paper is to study interactive mechanism of system made up of piles, soil (or rock), bearing platform and pier during the course of construction. The interaction including superstructure and substructure of bridge is a very complicate issue, and directly related to construction process; meanwhile, construction phase of piling is key and its influence run through whole process of construction and operation of bridge. Once there is flaw attributed to piles construction, the remedial cost is very high, study on interaction of bridge has important value of theory and practice. The main contents of the paper are load transfer mechanism of single pile, and loading behavior of composite pile foundation. The research methods of the paper are long years of prototype observation on pile foundation of No 5 pier, supplemented by theoretical analysis and numerical simulation. Throught methods of foregoing, some conclusions were get such as mechanism of residual strain of socketed piles and its influence on load transfer of pile before loading, rules of load trandfer during construction of superstructure, characters of load distribution of composite pile foundation in soft rock, and influence of roughness of shaft wall and inhomogeneity of subgrade to bearing capability of single pile and bearing behaviors of pile group respectively, following is details conclusions:1) Through prototype observation it was found that pile top load distribution of the composite pile is changed with the progress of construction, that is to say, as the weight of the upper structure and the bending stiffness of the platform change. Bending stiffness of the platform actually includes the contribution of the solid part of the pier bottom, which significantly affected the bearing traits of pile group in the early. Softening by soaking and weathering, the subgrade closed to platform bottom can not bearing load, the early upper load mainly borne by pile, with the growth of time, the subgrade closed to platform bottom compressed and gradually share part of the upper load. And the contact situation between the pile caps and platform is changing with time; this also affected the load space distribution of pile foundation.Due to construction process of the bridge has not yet been completed during this monitoring, plus possibility as a result of the uneven compress of subgrade (part of the subgrade closed to river was soften by river water, an the other was fresh rock excavated from hill), it is currently monitoring result that the middle piles (nominated as pile 5-8 and pile 5-15) bearing more load than the corner pile (nominated as pile 5-19) and the corner pile's behavior of bearing load was very complicated, for it first tension further compression. As only the top center of the other pile laid strain gauge, but it was found by monitoring that axial strain distribution of the top of the pile is very uneven, additionally considering the theoretic contact situation between old concrete of piles and new concrete of platform was more distinguishing than the practical one, so pile only planted a strain gauge on its top can not reflect the pile strain distribution of the whole picture.2) Through long-term monitoring of load transfer characteristics of the large diameter pile of pile group foundation, it was found that the load distribution of pile top was complex and varied with spatial location and time changes, and pile top axial load distribution is significant differences with the theoretical results under ideal state, therefore, pile group foundation design should be regulating according to the actual situation.According to analysis of load transfer characteristics of pile socketed in soft rock, it can be considered to be end-bearing friction piles, namely most upper load bearing by side resistance. As the roughness of side wall of hand-dug pile is variation, the distribution of side resistance characteristics of pile is complex, and the side resistance curve has more than one peak. It was found through monitoring that large diameter rock-socketed pile has greater axial stiffness, smaller pile settlement, higher loading capacity, so it is a suitable base type for high, big and heavy structure.3) Method for load transfer function concerning on side resistance nonlinear softening can better fit the test curve, and reflects the gradual destruction of the pile bearing behavior. And because of the flexibility of load transfer function, based on this method, the procedures can simulate for load-settlement curve of pile in multi-storey soil, but accurately simulate needs of the engineering data.4) Interface is different when medium around pile is different; the roughness of interface is remarkably influence bearing capability of rock socketed pile. At present, roughness can be figured by convex concave factor. According to simulation by FEM, the paper not only approved that settlement of pile top are in inverse proportion to convex concave factor, but also proportional to the shape factor of asperity, and influence of height of the asperity is notable greater than that of half wavelength. Pile top settlement is actually increases with the increasing load; therefore, reduction pile displacement is to increase capacity under the same load. In engineering practice, there are two methods to enhance pile bearing capability. The first method is increasing height of asperity, when design length of pile can be lengthening, but this method is not economical; the second method is making "short-wide asperity", this method is economical.5) According to the finite element analysis on T-shaped continuous rigid frame of No 5 pier, it was kown that selfweight of superstructure is the man influence factor on machenical response of pile group foundation; during the construction phase of the bridge piers, the tensile stress caused by wind load on the cantilever and the pier can not be ignored, especially no pre-stressed concrete on bridge pier, it easy to crack; the load distribution of pile groups is that the middle piles bear load more than the pile at Around, which is broadly consistent with the actual monitoring results. According to the finite element stability analysis on T-shaped continuous rigid frame of No 5 pier, we can see under the most unfavorable combination of load in the construction phase, the main mode instability of the bridge is collapse as a whole along the vertical direction; partial softening of the foundation can lower the overall instability eigenvalue of the bridge.At present, the height of No 5 pier is the highest comparing to same type railway bridge in China, according to prototype observations and anlysis to it, some innovation was summarized as follows:1) For the first time, prototype observations on rock socketed composite pile group was completed, the rules of single pile's load trandfer characters and bearing behaviors of pile group was get and it will provide reference to practice engineering.2) In considering the non-linear soil softening soil around pile side, using load-transfer function get better result when analyze pile inthe multi-layer soil.3) Influence of roughness of interface of rock socketed pile was approved by FEM method, and a new concept named shape factor of asperity was bring forward.4) According to the finite element analysis on T-shaped continuous rigid frame of No 5 pier, Strength and stability of it was anlyzed, and which is broadly consistent with the actual monitoring results. Partial softening of the foundation can lower the overall instability eigenvalue of the bridge.
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