Theoretical And Experimental Study On Bearing Characteristics Of Root Anchorage For Suspension Bridge

The design and construction of anchorage is a key issue to design and construct the suspension bridge in soft soil and thick overburden areas.It has also attracted more and more attention of engineering technicians.In order to solve the problems of large volume,high cost and difficult construction of traditional gravity anchorages,Anhui Expressway Corporation proposed a new type of anchorage,namely root anchorage which was first applied in Chizhou Qiupu River Suspension Bridge in the plat area.The root anchorage consists of several large diameter hollow root piles and a lightly cushion cap.The main cable load from superstructure is supported by the vertical and horizontal bearing capacity of large diameter hollow root piles.As a new type of anchorage,the research on its bearing characteristics is still insufficient.The theoretical analysis of the vertical and horizontal bearing characteristics of the root piles are worthy of further study.The theoretical calculation method of the whole anchorage structure also needs to be studied accordingly.On the other hand,the safety and long-term displacement characteristics of the root anchorage under the action of working load are the concerns of both designer and constructor.For this reason,with the support from the Key project of Anhui Provincial Department of Transportation entitled "Research on engineering application of root anchorage"(NO.KJ2014-0024),this thesis is focused on the methodology,characteristics of root anchorage and its real case application.The corresponding researches are carried out on theoretical calculation,numerical simulation,model tests in the laboratory,field experiments and long-term monitoring on site.The main contributions and conclusions of this dissertation are outlined as follows:1)The nonlinear bearing characteristics of root piles under vertical loads are studied.The root-soil nonlinear load transfer model has been established by using hyperbolic function.The compaction effect caused by the jacking construction of roots in the root pile is analyzed by using cylindrical expansion theory,and then an enhancement coefficient is introduced to consider the compaction effect of root.Based on the pile-soil load transfer model,the equilibrium differential equation of root pile under vertical load is derived.The nonlinear solution of load settlements is given by using iterative method.Numerical examples and static load test of Chizhou Yangtze River Highway Bridge are used to verify the proposed method.The results show the reliability of the method.Based on this method,The key root parameters in the root pile such as the number,size and embedded depth are investigated so that the nonlinear bearing characteristics of the root pile under vertical loads can be calculated in the multilayer soils.2)As the anchorage foundation of suspension bridges bears not only vertical loads but horizontal loads,the nonlinear bearing characteristics of root pile under combined load are studied.The interaction of roots in root pile under combined load is decomposed into two parts: horizontal resistance and resistance moment.The nonlinear root-soil interaction is considered by hyperbolic function.Based on the mechanism of the root,the equilibrium differential equation of the root pile under the combined load considering the influence of the vertical component is derived,and then the nonlinear solution of the root pile under the combined load is obtained by using the iterative method.The proposed method is verified with two horizontal static load tests of root piles on site for Wangdong Yangtze River Highway Bridge.The results show that the proposed method is effective and agrees well with field test results.Based on this method,the key root parameters in the root piles such as root number,root size,root arrangement angle,root embedded depth and vertical component are studied accordingly.3)The model tests of root pile are designed and carried out under combined loads in the laboratory.The tests are divided into three different test conditions,namely horizontal load,30 ° inclined load and 60 ° inclined load.For the comparison,the corresponding tests of anchorage foundation with traditional ordinary piles are also carried out.By using the dial indicators to measure the horizontal displacement of the pile head and the strain gauge to measure the stresses of the pile,the bending moment curve of the pile is obtained by integrating.Thereafter,the horizontal bearing characteristics of the root pile under the combined load and the influence of the vertical component load on the horizontal bearing characteristics of the root pile can be calculated so that the horizontal bearing characteristics of the root pile under the combined loads can be revealed.4)The mechanical balance formula of root anchorage foundation is deduced and established.The nonlinear displacement calculation method of root anchorage is proposed accordingly.The proposed method is then implemented in the North Root Anchorage of Qiupu River Bridge under the anchorage forming condition and the design cable load condition.Compared with the field measurements,it is demonstrated that proposed method can achieve a good calculation accuracy.This method is further used to analyze the bearing characteristics of root anchorage under combined load.The results show that the horizontal displacement-load curve of the root anchorage increases nonlinearly with the increase of the calculated cable load.The rotation trend of the root anchorage is very small and basically in a balanced posture after the construction of the anchorage or under the cable load.With the increase in the load,the possible failure mode of root anchorage is overturning failure.At this time,the pile behind the anchorage gradually changes from compression to possible uplift.5)The triaxial creep tests were carried out to obtain the creep characteristic parameters of soil samples.The parameters of the triaxial creep test results are determined by using the three-dimensional Burgers model.Based on the soil test results,the three-dimensional viscoelastic-plastic numerical analysis of the North Anchorage of Qiupu River Bridge was performed.On the other hand,the long-term displacement monitoring of the North Anchorage of Qiupu River Bridge was carried out in the field for two years.Both numerical calculations and field measurements have shown that the root anchorage of the Qiupu River Bridge showed good stability under both the anchorage forming condition and the cable load condition.The back tilt angle of the root anchorage is only-0.0052° under the anchorage forming condition,while the forward tilt angle of the main cable load condition is only 0.0063°.The horizontal displacement of the root anchorage is small during the operation period.The horizontal displacement increment is about 11 mm while the overall value is about 21 mm.All those are less than the allowable horizontal displacement recommended in the specification.The numerical analysis and on-site long-term monitoring test results showed a good consistency.The results showed that the root anchorage exhibit stable creep characteristics under working loads and the displacement changes can be stabilized when the bridge is completed and operated.It is demonstrated that such a new root anchorage of suspension bridges has a good stability so that the design scheme of the root anchorage is feasible and safe.Through reasonable design and construction,such a new type of root anchorage can meet the requirements of design and service for the anchorage foundation of suspension bridges.