Analysis On The Vertical Bearing Mechanism Of Permanent Fully-cased Rock-socketed Filling Pile For Deep Soft Ground

The coastal cities in China are dominated by marine sedimentary plain.These areas are often encountered with deep flowing soft soil geological conditions,which is not conducive to their engineering construction.Taking a pile foundation project in Hengqin District of Zhuhai as an example,the average depth of moderately weathered granite in this project is about28 m,and the average thickness of soft soil layer is about 24 m.The geological condition is very special.A lot of pile foundation projects in Zhuhai have improved the common rock socketed cast-in-place pile,and the new pile type of permanent fully-cased rock-socketed filling pile has been obtained.The practice shows that the permanent fully-cased rock-socketed filling pile can not only meet the requirement of bearing capacity,but also ensure the construction quality.It is well suited to the geological condition of such a deep soft soil and solves the problem of pile formation in deep soft soil.Permanent fully-cased rock-socketed filling pile refers to a type of rock-socketed steel tube-concrete composite structure in deep soft soil which adopts full steel casing tube protection when drilling or cleaning hole and pouring concrete,and its steel casing tube is not pulled out to stay in the soil for bearing force after the pile is formed.In this paper,the construction technology of rock socketed pile in permanent full retaining tube is summarized,and then the thickness selection,bearing capacity and settlement calculation method and influencing factors of bearing capacity are studied.The main contents and achievements are as follows:(1)Combining with a pile foundation engineering example in the Hengqin District of Zhuhai City,summarized the construction technology,construction points,quality controlling and engineering application of permanent fully-cased rock-socketed filling pile,then analyzed its engineering advantages.(2)Based on the static equilibrium theory,analysed the stability of steel casing tube under the action of internal ring pressure and the strength of steel casing tube under external ring pressure during construction,then derived the formula for calculating the thickness of the steel casing tube of permanent fully-cased rock-socketed filling pile.It provides a theoretical basis for the selection of the thickness of its steel casing tube.(3)Analyzed the influence factors of load transfer of the permanent fully-cased rock-socketed filling pile,then selected the reasonable load transfer model to derive the load-settlement calculation formula based on the load transfer function method;considering Buckling instability and strength of pile body,deduced the formula for calculating the ultimate bearing capacity of the permanent fully-cased rock-socketed filling pile in deep soft soil;Finally,proved the rationality of these formulas by the static load test data for real measurements.(4)Based on the pile foundation engineering project in Hengqin District of Zhuhai city,using MIDAS GTS NX finite element software,established the three-dimensional solid element model to analysis of permanent fully-cased rock-socketed filling pile bearing characteristics.Then analyzed the influence of several main factors on its bearing characteristics,including pile diameter,pile length,thickness of steel casing tube,soil conditions,and rock-socked depth.