Research On Bearing Characteristics Of Rock-socketed Piles Of High-speed Railway Bridge In Soft Rock

Soft rock formations are widely distributed in China.With the rapid development of high-speed railway construction,soft rock-socketed piles for bridge foundations across rivers and mountainous areas have been widely used.The pile-rock system,especially the mechanical properties of the pile-soft rock interface,are quite different from the pile-soil and pile-hard rock interfaces due to the specific physical and mechanical characteristics of soft rock.The current design criterion for pile foundations do not clearly indicate the design standards for soft rock-socketed piles,but refer to the design standards for hard rock-socketed piles.There are large differences in the design standards for soft-rocksocketed piles in different types of criterions.In addition,the pile foundation of the highspeed railway also needs to bear the cyclic load caused by the upper train operation,this will also cause the bearing capacity of pile foundation to be weaken and greater settlement under the cyclic load,which will affect the stability of the structure.Therefore,an objective understanding of the bearing characteristics of soft rock-socketed piles and the cumulative settlement mechanism under cyclic loading has important value for improving the theoretical system and engineering design specifications of pile foundation.It can also ensure the safety of high-speed railway and other traffic operations.Aiming at the engineering geological characteristics of soft rock-socketed piles in high-speed railway projects,the thesis is set in the soft rock-socketed pile foundation of the Taihe Ganjiang River Bridge.The main research is focused on the bearing mechanism and cumulative settlement laws of soft rock-socketed piles under cyclic loads.The main research contents and conclusions are as follows:(1)Experimental study on shear deformation characteristics of pile-soft rock cementation interface.The sample preparation molds with different roughness interfaces have been designed and manufactured to prepare concrete-soft rock cemented specimens.And then the shear tests on pile-soft rock bonded specimens with different roughness undulation angles ? were carried out.The influence of factors,such as undulation angle?,the uniaxial compressive strength of the soft rock and the initial normal stress of the interface on the shear characteristics of the pile-soft rock interface has been analyzed based on the shear tests.The test results show that:(a)The shear stress-shear displacement curve of the pilesoft rock bonded interface is mainly divided into three stages.The shear stress increases approximately linearly with the increase of the shear displacement in the first stage.The growth amplitude and growth rate are both small in this period.In the next stage,the growth rate is significantly increased.After reaching the peak shear stress,the shear stress gradually tends to the residual level in the residual friction stage.(b)The peak shear stress and residual friction of the pile-soft rock bonded interface both increase with the increase of the uniaxial compressive strength of the soft rock and the roughness undulation angle?,but the growth rate of them shows an opposite law.The maximum bond strength of the interface increases significantly with the increase of the uniaxial compressive strength of the soft rock,but the growth rate shows a weakening trend at the same time.The maximum bond strength is hardly affected by the roughness undulation angle ? of the interface roughness.The peak shear stress and residual friction of the interface both increase with the increase of the initial normal stress of the interface.(c)The shear failure of the pile-soft rock bonded interface mainly occurs in the convex part of the soft rock,and the concrete part of the pile body hardly suffers from shear failure.When the uniaxial compressive strength of the soft rock is constant,the greater the undulation angle of the interface roughness,the more the roughness will be sheared.When the interface roughness is constant,as the uniaxial compressive strength of the soft rock increases,the shear ratio of the rough body gradually decreases,and the interface failure part gradually changes from the root to the rough top.The smaller the roughness undulation angle,the lower the damage.The shear ratio of the soft rock increases correspondingly with the increase of the initial normal stress.(2)A theoretical study on load transfer of soft rock-socketed piles based on shear characteristics of pile-soft rock interface.From the microscopic mechanism of shear mechanical properties of pile-soft rock interface,the shear mechanics process of the pilesoft rock interface is divided into three stages of bonded elastic deformation,dilation and residual friction based on the shear tests results of pile-soft rock interface.A load transfer model of pile side resistance combining with the interface cementation and post-peak softening characteristics is proposed.And an analytical solution to reveal the load-transfer mechanism of soft rock-socketed piles is derived.The analysis results show that:(a)The side resistance of soft rock-socketed piles increases with the increase of uniaxial compressive strength of soft rock in the dilation stage,and the interface bond strength will increase the side resistance correspondingly.The more obvious the effect of bond strength on side resistance caused by increasing uniaxial compressive strength of soft rock,the more fully the side resistance of the rocksocketed pile will be exerted.(b)The ultimate side resistance and residual friction of the pile-soft rock increase with the decrease of the pile-soft rock elastic modulus ratio.The interface bond strength will increase the ultimate side resistance but weaken the residual friction due to the reduced ultimate shear displacement and the normal stress of the pilesoft rock interface,which are caused by interface bond strength.(c)With the increase of the roughness undulation angle of the pile-soft rock interface,the ultimate side resistance of the interface shows a trend of first increasing and then decreasing,and the residual side resistance gradually decreases.There is an optimal interface roughness that maximizes the ultimate side resistance,but it is also affected by the uniaxial compressive strength of the soft rock.(3)Numerical simulation study on the mechanical characteristics of soft rocksocketed piles.Based on the secondary development platform of the ABAQUS,the contact subroutine UINTER considering the characteristics of the bond strength and softening of the pile-soft rock interface is written,the numerical model of the soft rocksocketed pile is established to analyze the mechanical characteristics of rock-socketed piles under vertical loads.The results show that:(a)the lower the uniaxial compressive strength of the soft rock,the faster the rate of pile top settlement decreases with increasing pile top load at a high level.(b)The higher the pile-soft rock elastic modulus ratio,the faster the pile top settlement decreases with increasing pile top load,and the corresponding the distribution ratio of base resistance sharing is higher.(c)At a lower load level,the settlement decreases with increasing roughness of pile-soft rock interface,and at a higher load level,the settlement tends to increase and then decrease with increasing undulation angle ? of pile-soft rock interface,but distribution ratio of end resistance gradually decreases.(d)When the embedded depth is certain,the load-settlement curve changes from slowdeclining to steep-declining with the increasing embedment ratio,and the ultimate bearing capacity of the soft rock-socketed pile is also decreased.when the pile diameter is certain,the ultimate bearing capacity of pile increases with the increase of embedded depth,but there is a critical embedded depth.When the critical value is exceeded,the ultimate bearing capacity of pile changes less with the increase of embedded depth,and the change of settlement of pile top is almost zero.It is concluded that the impact of changing the pile diameter on the ultimate bearing capacity of the soft rock-socketed pile is more obvious than the embedded depth.(4)Research on the theoretical calculation method of the ultimate end resistance of soft rock-socketed piles.According to the different failure modes of the soft rocksocketed pile bottom,the theoretical calculation methods of the ultimate end resistance of the soft rock-socketed pile under bottom shear failure mode and the spherical cavity expansion failure mode are respectively studied,and the sensitivity of each parameter is analyzed.Considering the effect of embedment ratio,the reliability of the theoretical calculation method is verified by numerical simulation results,and the theoretical calculation method of ultimate end resistance of soft rock-socketed piles and iterative calculation procedure of pile top settlement are proposed.The research shows that:(a)The calculated value of the ultimate end resistance based on the shear failure mode of the rock-socketed pile is highly influenced by the rock Geological Strength Index(GSI).The ultimate end resistance increases with the increase of GSI and the growth rate increases continuously.(b)The calculated values of ultimate end resistance based on the spherical cavity expansion theory increase with the increase of cohesion and internal friction angle and elastic modulus of soft rock,respectively.The growth rate gradually decreases.The friction angle of the soft rock has the least influence on the ultimate end resistance,followed by the cohesion,and the elastic modulus has the greatest influence on the ultimate end resistance.(c)When the embedment ratio n?10,it is recommended to calculate the ultimate end resistance based on the spherical cavity expansion theory.When the embedment ratio n?15,the ultimate end resistance calculated by the shear failure mode is closer to the numerical simulation results.When the embedment ratio is 10<n<15,the ultimate end resistance is interpolated by taking the larger value obtained by the two theoretical methods for n=10 and n=15,respectively.(5)Research on cumulative settlement mechanism of soft rock-socketed piles under cyclic loading.Based on the cyclic shear weakening mechanism of the rock joint surface,a cyclic shear load transfer model considering the wear characteristics of the interface roughness is proposed,and the influence of various factors on the cyclic shear load transfer characteristics of the pile-soft rock interface is analyzed.Subsequently,a calculation program for the cumulative settlement of the soft rock-socketed piles under cyclic loading was compiled on the bases of the cyclic shear load transfer model,and the influence of various factors on the cyclic loading effect of the soft rock-socketed piles was further studied.Research shows that:(a)The undulation angle ? of the pile-soft rock interface decays exponentially with the increase of the number of cyclic shears,the larger the ?,the faster the ? decay rate.But the trend of the ultimate side resistance changes from exponential decay to an increasing trend and then decreasing.The residual side resistance shows an opposite trend with the ultimate side resistance,both of which eventually tend to the ultimate value.The attenuation rate of the ultimate side resistance and undulation angle? decrease with the increase of the uniaxial compressive strength of the soft rock.The larger the elastic modulus of the soft rock,the smaller the ? attenuation limit value,but the larger the limit value of the limit side resistance.The attenuation rate of the undulation angle ? is faster with the larger the initial normal stress at the interface,and growth rate of the ultimate shear displacement ratio is faster with the increase of the number of cycles.The limit value of the ultimate side resistance shows a pattern of decreasing and then increasing.(b)The load distribution law of the pile is obviously affected by the peak of the cyclic load.When the peak of cyclic load is small,the side resistance gradually decreases downwards along the pile,and the side resistance near the bottom increases with the cyclic numbers.But the axial load distribution of the pile shows a little change,which is similar to the distribution ratio of end resistance and the pile top cumulative settlement.When the peak of cyclic load is large,the side resistance gradually increases downwards along the pile,and the more the number of cycles,the faster the attenuation rate of the side resistance.The axial load near the bottom of the pile also gradually increases.And the growth rate of distribution ratio of end resistance and the pile top cumulative settlement are faster with the increased number of cycles.(c)The embedment ratio has the less effect on the pile top cumulative settlement and the growth rate of distribution ratio of end resistance under the cyclic loading.The smaller the uniaxial compressive strength of the soft rock,the more obvious the pile top cumulative settlement is affected by the number of cyclic loadings,and the faster growth rate of cumulative settlement and distribution ratio of end resistance with the increased number of cycles.The effect of pile-rock elastic modulus ratio on the cumulative settlement becomes more obvious as the number of cycles increases,and the larger the pile-rock elastic modulus ratio,the greater the growth rate of the cumulative settlement and distribution ratio of end resistance.