Study On Bearing Capacity Of Post Grouting For Bridge Pile Foundation In Karst Area

Mud retaining wall bored piles are the most commonly foundation forms in the construction of high-speed railway bridges.However,it also has the following shortcomings in the wide range of applications:The damage to the perturbed soil during the hole formation,the mud skin of the pile and the residual silt layer at the end of the pile are easily formed,the erosion and disturbance damage to the hole wall and the surrounding soil,and the cracks between the pile and the soil are easily formed by shrinking of the concrete when the concrete is solidified.The post grouting technology can effectively compensate for the inherent defects of the traditional bored piles,improve the bearing capacity of the foundation piles and reduce the settlement of the foundation piles.At present,the application of post grouting to improve the bearing capacity of pile foundation has been widely used in high rise buildings and highway bridges,and there are few examples in the construction of high speed railway.The 189~#and 190~#piers of the Beijing Shenyang high speed rail Shunyi bridge are located in the deep covered karst development area.The overlying karst caves are mainly distributed between the top 59.7~78.3m of the pile,the maximum pore diameter is 6.3m,and the geological conditions are complex.The traditional design of bored pile is adopted in the original design foundation whose length is 85m.It is necessary to cross the karst stratum area,and there are risks such as uncontrollable construction difficulty,sharp increase in construction cost and serious delay in construction period.Pile foundation grouting technology is adopted to improve bearing capacity,shorten pile length and avoid karst formation.In order to verify the reliability of post grouting technology,pile jacking static load test is carried out near the bridge site,and the numerical results are used to fit the test results.The results and conclusions are as follows:(1)The pile length of 189#pier is reduced from 85m to 45m,and the length of 190#pier design is reduced from 85m to 35m by the appling post grouting technology.It successfully avoided getting into the karst stratum area,and it also avoided the risk of leakage of pulp in construction,rapid increase of construction cost and lagging construction period.(2)The post grouting technology of pile foundation has effectively increased the ultimate bearing capacity of pile,reduced the settlement of pile top and increased the ratio of pile side friction resistance.It is found that the process has a better effect on the 10m below the pile top.(3)Some conclusions have been drawn that the increase coefficient of lateral friction and the ultimate bearing capacity of pile tip in different soil layers.(4)The selection of grouting process and the standard of grouting control for this project are obtained.The successful application of pile foundation post grouting technology in Shunyi super bridge of Beijing Shenyang high speed railway significantly reduced the construction difficulty,reduced the cost of the project,shortened the construction period,and ensured the successful completion of the Beijing Shenyang high speed railway control project.The conclusion has some guiding significance for the research and application of post grouting technology in pile foundation engineering of high-speed railway bridges.