| Yi-Wan Railway is a trunk railway of China, which runs through Wulin mountain range hinterland, the complex geological conditions. 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) Residual strain of socketed piles is the result of coupling of contraction, own weight, creep and temperature, in the earlier age of concrete, various factors are involved in the role. First, own weight and dilatancy of warming produce compressive strain, the subsequent contraction of the concrete and the temperature gradually decline part of the pile under tensile strain. With growth of concrete age, pile side resistance has been strengthened, the compressive strain produced by own weight decreases, the contraction increased tensile strain and the whole pile all the tension gradually and become more stable. In the process of residual strain forming, because uneven cooling has led to the uneven temperature distribution, and construction has led to the pore wall rough, which affect the residual strain development and distribution. Therefore, we should pay attention to the hydrological and geological conditions of the pile around, a reasonable choice of the construction method and good construction quality control.2) 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.3) 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.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 optimization analysis on the effect on the pile axial load distribution related to thickness and elastic modulus of the subgrade closed to platform, it was found that the total pile axial load is proportional to the thickness, and inversely proportional to elastic modulus, Pile total side resistance and the end of the pile axial force as well. However, the effect on pile axial force distribution related to thickness more significant than that related to weakening of subgrade. In general, to take full advantage of the bearing capacity of the subgrade, it should be kept strictly on subgrade for the erosion thickness, meanwhile maximize their elastic modulus as far as possible.6) Through finite element analysis on composite pile foundation in inhomogeneous subgrade, law on axial load distribution and displacement was found as following: the axial force distribution of composite pile foundation significantly effected by the subgrade uneven impact, therefore, the monitoring results of this paper can be explain by the situation; Because it do not know that the specifics of the geological and hydrogeological conditions and degree of construction disturbance, to achieve accurate simulation more needs to be done to increase the assumptions and the complexity of finite element and difficulty of calculating; Although there is effect marks on internal forces and displacement to the other direction including those under pile bottom by subgrade inhomogeneity, but overall, the effects are not great, for the effect on pier top deviation are also within the scope of control; displacement distribution of the subgrade closed to platform bottom and for the pile top is not related to that of the stress and the axial force respectively, but related to effects of side resistance of pile (including adjacent pile); The subgrade settlement under pile bottom is related to its stress, stress value increases with increasing of corresponding settlement value.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 forst time, monitoring and thoery anlysis was conducted on residual strain of socketed piles, and its mechanism was explained, influence to bearing capability was simply analyse.2) 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.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) Influence arose by soften and inhomogeneous soften of bedrock to composite pilegroup's bearing behavior was analyse, and difference between theory analysis and practice results was qualitative explained according to data compare.
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