Experimental Study On Vibration Characteristics Of Pre-Stressed Pile Anchor System Under Train Vibration Load

Taken the pile-anchor reinforcement system of deep and large foundation pit laocated adjacent to the existing Beijing Guangzhou railway as project background,the strata vibration characteristics and the change rule of pile anchor system were investigated by using theoretical analysis,field test,laboratory model test and numerical simulation in this paper.The influence factors of anchor cable pre-stress loss were analyzed,and the pres-tress variation characteristics were studied.The research achievements are beneficial for enginnering practice.The main contents are as follows:(1)The influences of train vibration on the ground surface and in stratum were analyzed according to field vibration test.With the increase of formation thickness,the train vibration wave energy is absorbed by the stratum,and the vibration velocity in stratum presents a decreasing trend.The regression analysis waas carried out on the vibration law of strata and retaining piles with different types of trains,and the vibration velocity expressions of strata and retaining piles according to different axle loads of locomotive were obtained.(2)By analying the field monitoring data of pre-stressed pile anchor support system,the prestress loss law of anchor cable was obtained,and the influencing factors were assessed.Then,the short and long term influence factors of cable pre-stress loss were determined.The influence of train vibration load on the pile anchor support system in the actual engineering of Beijing-Guangzhou railway was revealed.The additional anchor cable pre-stress loss rates of three layers caused by the local train vibration of Beijing-Guangzhou railway are 1.59%,0.96%,0.09%respectively;the additional ground surface subsidence caused by train vibration is 2mm;and the additional earth pressure of the retaining structure is 9.3k Pa.(3)In order to investigate the influences of foundation pit excavation and vibration load on the prestress loss of anchor cable pre-stress,indoor test material,method and device for simulating formation and pile anchor support were developed according to orthogonal combination test analysis.The method of calibrating the stress and strain of the anchor cable was determined.And the fitting expression of the stress and strain is ?=-3×10^-11?³+1.0×10^-7?²+0.0226?-0.0358Through the static load model test(model one and model two),the surface settlement,the pile body horizontal displacement characteristics and the law of anchor cable pre-stress loss were analyzed during the foundation pit excavation.The analyses results prove that the pile anchor system can effectively control surface settlement and horizontal displacement of pile body,which can ensure the stability of foundation pit.According to the vibration characteristics of the train load,model test with continuous vibration for 162 days(similarity)after foundation excavation was carried out.The law of prestress loss in anchor cables,the subsidence and horizontal displacement of the pile under different vibration frequencies,and the effect of anchor depth on prestress loss were analyzed when vibration were1.852,3.086,4.629HZ.Test results reveal that the prestress loss of anchor cables and the subsidence and horizontal displacement of the pile greatly increased with the increase of vibration frequency.The average loss rate of prestressed anchor cable is 2%when driving for 28 days(prototype)continuously at the speed of 120km/h.The average loss rate increases to 8%and 23%when driving for 53 days(prototype)continuously at the speed of 200km/h and 300km/h,respectively.The prestress loss of anchor cables in the anchorage section decreased along the anchorage depth of the anchor cables;the prestress is almost zero at the bottom of the anchorage section.(4)The stress field and plastic zone of the soil and the variation of the anchor cable pre-stress in the static load and the train dynamic load were simulated by using FLAC-3D software.The simulation results reveal that the embedded depth of anchor cable is greatly influenced by the creep of soil,and the stress of anchor cable shows a gradual attenuation trend.With the increase of time,the soil becomes dense,which results in the stress loss of anchor cable slowing down,and even an increase of stresss.The stress of foundation pit is complex,and the stress concentration is obvious.With the increase of the excavation depth,the influence range of stress becomes larger.The dynamic response of pile anchor system was analyzed with four-parameter dynamic model.The numerical results reveal that the prestress of the first and second anchor cable decreases gradually and increases of the third anchor cable with time.Because of the complexities of the anchoring mechanism and the influence of train dynamic load on the anchor cable,only vibration frequency is considered in model test,it is necessary to further study the influence of other vibration factors(such as amplitude and interval time of moving load)on the pres-tress loss of anchor cable in order to obtain more extensive application values.