Test Research On Large-diameter Cast-in-situ Concrete Thin-wall Cylindrical Pile For Strengthening Soft Soil Below Embankment

Large-diameter cast-in-situ concrete thin-wall cylindrical pile is a newly developed type of pile for strengthening subgrade on the basis of pipe sinking filling pile. It is applied in strengthening the soft soil subgrade for high embankment of Hangzhou-Nanjing Expressway, which is for the first time this type of pile used in expressway construction in China. Based on site tests, this paper makes an extensive research on large-diameter cast-in-situ concrete thin-wall cylindrical pile for strengthening soft soil below embankment.According to site tests and observation, data analysis and theoretical calculation, the paper studies the stressing condition, deformation feature and settlement law of thin-wall cylindrical pile, ground soil and cylindrical pile compound subgrade under action of embankment load; explores the theoretical calculation model of bearing capacity; determines the design method and parameters of cylindrical pile; analyzes the pile and soil stress ratio and strengthening effect by using coverplate on pile top and paving of geogrid; compares the strengthening effect and economic benefit of thin-wall cylindrical pile, dry jet mixing pile and plastic drain plate under the same geological and loading conditions. Finally, main achievements and conclusions for strengthening soft soil subgrade on cast-in-situ concrete thin-wall cylindrical pile are obtained.Main achievements and conclusions: The cylindrical pile compound subgrade has a small deformation and post-construction settlement avoiding the preloading and secondary excavation of road sections as well as additional pavement at bridge head. This kind of pile can increase the strength and rigidity of soft soil ground dramatically. The subgrade is stable and can be filled quickly resulting in shorter construction period. The economic and social benefits are obvious. By using cylindrical pile compound subgrade, the high embankment section has a good antiskid quality. The calculation model and formula of limit bearing capacity of cylindrical pile has been put forward. The theoretical calculation method for strengthening soft soil by cylindrical pile has been worked out. This paper provides a completely new method for solving the quality problems such as uneven settlement, and instability of subgrade and bump at bridge head.