Study On Optimization Design Of South Anchorage Foundation Of Nanning Liangqing Bridge

As an important bearing member of long-span suspension bridge,gravity anchorage has been widely used and developed.There are few research reports on the anchorage on uneven soft foundation.This paper takes the South Anchorage Foundation of Liangqing bridge in Nanning as the research object,and systematically studies the settlement characteristics and overall stability of gravity anchor foundation on the silicon shale foundation in the research area.Because of the development of karst in the South Bank of Liangqing bridge,the karst of strongly weathered siliceous shale and moderately weathered limestone is relatively developed,the original geological survey data can not accurately judge whether the groundwater of the south bank anchor pit is connected with Yongjiang water,which makes the bridge fail to deal with it in time during the excavation of the anchor,which leads to the change of water level in the foundation pit with the change of water level in the Yongjiang river.Through observation and research,it is confirmed that the karst development in strongly weathered siliceous shale and limestone is developed,and there is a phenomenon of connecting with Yongjiang water.If the bored pile is constructed according to the original design scheme,the water in Yongjiang River may burst into the foundation pit after the drilling pile enters the bearing stratum due to the general head difference,which will eventually lead to the complete flooding of the foundation pit,However,the construction of anchor foundation pit cannot be further carried out.Therefore,this paper mainly studies the optimization design of the foundation of the south bank anchor foundation of Liangqing bridge,and mainly obtains the following research results:(1)The karst of strongly weathered siliceous shale and moderately weathered limestone on the South Bank of Liangqing bridge is relatively developed.During the excavation of anchor,the water level in the foundation pit changes with the water level of Yongjiang river.Through observation and research,it is determined that the karst development in the strongly weathered siliceous shale and limestone is connected with Yongjiang water.Under this geological condition,if the bored pile is constructed according to the original design scheme,there is a risk that the water in Yongjiang River will burst into the foundation pit after the bored pile enters the bearing layer.(2)The South Anchorage excavation is mainly strongly weathered siliceous shale,with developed joint fissures and strong permeability.The original design scheme can not be carried out normally.The design scheme for foundation treatment is optimized as follows:vertical anchor bolt is set at the excavation base surface(about 54m),with the bolt spacing of2 m,drilling to 0-0.5m above the moderately weathered limestone surface,and no moderately weathered limestone is driven to prevent the fissure water and pressure water from pouring into the foundation pit.The anchor rod is adopted φ 28 reinforcement is inserted,and the anchor rod extends into 20 cm thick anchor concrete cushion,then high pressure grouting(grouting to the elevation of current base surface is 54m),and grouting pressure is 2MPa.After grouting,the cylinder with diameter of 0.5m-1.0m is formed around the anchor rod,which improves the compressive strength and shear strength of soil.(3)The results show that the basic allowable value of composite ground foundation bearing capacity is 1134 kpa,The compressive strength of composite foundation at the test point can be taken as 2000 kpa,which meets the design requirements of bearing capacity of anchorage system;Characteristic value of shear strength of composite foundation at test point:internal friction angle is 18.78 °～ twenty point eight one ° The cohesion force is between48.3kpa and 63.0kpa;The characteristic value of shear strength of the contact surface between concrete and rock mass at the test point: the friction coefficient is between 0.598-0.643,and the cohesion is between 22.9kpa and 62.7kpa;The characteristic value of shear strength between concrete and rock mass: the friction coefficient is between 0.458-0.582,and the cohesion is between 16.4kpa and 46.9kpa;Grouting effect test results: grouting filling consolidation effect is good.(4)The dynamic simulation analysis of the South Anchorage construction and operation period of Liangqing bridge by FLAC 3D software shows that:(1)the foundation treatment effect is obvious,and the whole rigidity and settlement deformation characteristics of siliceous shale can be improved obviously by using soil nail and grouting method.(2)The change of anchor foundation displacement before and after foundation treatment can be seen that the foundation treatment can effectively reduce the settlement,horizontal displacement and foundation stress of the anchor foundation,and ensure the safety of the anchorage and bridge.During the operation of the bridge,the observation results of displacement observation point of main tower in 2019 are smaller than that of November 2018,which indicates that there is no obvious displacement of the main tower of the bridge.The difference is mainly caused by sunshine,temperature and measurement error,which verifies the suitability of the optimal design of anchor foundation.(3)The stability of the South Anchorage Foundation of Liangqing bridge is calculated by combining the theoretical formula and numerical simulation.The stability coefficients of the anchorage against sliding and overturning are 2.020 and 3.288respectively;The results show that the stability of the foundation after foundation treatment meets the design requirements.