Study On The Dynamic Penetration Resistance And Mechanism Of Free-Running Of Large Diameter And Super Long Ocean Platform Piles

With the ocean development towards to the deep sea areas,large diameter(O.D.greater than 2m)and super long(buried depth greater than 100m)piles have been commonly applied for constructing ocean production platforms.Because of the huge weight of the pile itself and the driving hammer,pile free running often takes place during pile penetration,which may break off the positioning wire rope,damage the pile and the hammer,and even cause the hammer to drop into the sea.Up to now,the existing literatures in this area are rare to find.Based on the study of the mechanism of pile free running,this paper presents the pile running conditions and running distance judgment methods according to the limit equilibrium theory,energy conservation law,the cavity expansion theory and so on.The accuracy of theoretical calculation for prediction the pile free running distance depends on the rationality of the parameters that are adopted in pile penetration analysis.Therefore,the data from laboratory tests and in situ observation are analyzed,in which accumulated pore pressure and fatigue effects on the soil properties and pile-soil interface characteristics are considered,which can help to select reasonable parameters for pile free running analysis.The bearing capacity of a pile with free running taking place during penetration is the most concerned problem.Analysis and researches of the time effect on pile capacity are carried out in this paper by studying the generation and dissipation of pore pressure in the foundation during and after the pile driving.The main research contents and features of this paper can be summarized in the following.1.Mechanism of pile-running is clarified in this paper by using dynamic ultimate balance theory and by inverse analysis of dynamic measured data with CAPWAP pile-driving-analysis-software.Based on the research of mechanism of pile-running,the judge standard of pile-running during pile driving and the computation method of pile-running area are put forward in this paper according to theories including ultimate balance theory,law of conservation of energy and cavity expansion theory.2.Computation of pile driving resistance during pile penetration plays an important role in judging pile running area.The computation method for soil resistance in the API standard which is adopted generally now is safe for the pile capacity in design stage,but it is dangerous for assessing the pile-running area and gains great errors.The method put forward in this paper take the side load effect and the dynamic effect into consideration,and provide reasonable computation methods of static pile tip resistance and static pile lateral friction for non-cohesive soil and cohesive soil respectively.And this method also provides the reduction factor of dynamic lateral friction for both non-cohesive soil and cohesive soil by introducing the concept of sensitivity.Thus,the computed pile running area is more reasonable.3.In-situ test method of cone penetration is a more accurate method to get soil parameters.According to the characteristics of the large diameter and super long pile,a reasonable way is put forward to analyze the pile tip resistance and pile lateral friction by using measured data of cone penetration test,and it is also mentioned to compute the influence range of pile tip resistance and distribution of reduction factor of pile lateral friction along the pile.Combined with the cavity expansion theory,the calculation results of different parameters of cone head resistance are compared with each other in cohesive soil and non-cohesive soil,and the proper cone head resistance parameters can be provided to calculate the tip resistance in cohesive soil and non-cohesive soil according to the comparison.Finite element method simulation is adopted,and the method is also built to calculate the resistance of pile driving by using in-situ measured data.4.To the super long pile,lateral friction is the main part of the pile capacity.The proper usage values in engineering projects are put forward by developing a large scale of direct shear equipment to study the friction characteristics of pile and soil layer and by summarizing the changes of coarseness of different particle diameters of sand and soft clay under different values of density.Considering that the sinking of pile is similar to the process of continuous shearing,residual strength tests under large deformation of typical sand,salty clay and clay are carried out respectively by using ring shear apparatus.The effects of different rates of shear,cyclic load and content of cohesive particles are also studied.Moreover,friction characteristics of pile and soil are sufficiently summarized.5.There are few researches of effect of excess pore pressure generated during pile driving to the pile lateral friction at home or abroad.But,the substantial reduction of lateral friction of the large diameter and super long pile is related closely to the generation of excess pore pressure during pile driving.The regulation of the generation and dissipation of pore pressure during pile driving is established with the data of in-situ monitoring test of pore pressure and the modified cam-clay model,on the basis of which the time effect on pile capacity is studied.6.Taking the Liwan 3-1 engineering project of jacket platform and the Lishui project for examples,pile driving of the piles of platform is studied with the suggested theory and method,and the results of the freely buried depth and the pile-running area are also got,which fit well with the measured data.