Pile-soil Interaction Model And Multi-point Dynamic Testing Method Along The Pile Under High-strain Conditions

As a fast mobilization of dynamic testing method,the pile high-strain test method provides a new approach for testing the compressive load capacity of piles.This method not only overcomes the shortcomings of conventional static load test method including being time-consuming,relatively expensive and difficult to implement,but also can be efficient in more various sites(e.g.narrow sites as well as offshore environment);more piles can be inspected by the high-strain test,which can greatly improve the reliability of the overall evaluation of engineering piles.However,since the high-strain dynamic problem involves complex subjects including the soil stiffness nonlinearity,elasto-plasticity,hysteresis during loaing and unloading process and different types of dynamic soil damping as well as the slippage at pile-soil interface,there is a lack of in-depth and systematic research on the coupled vibration of the pile-soil system and its simulation.Instead,some over-simplified models are often used to simulate the pile-soil interaction in the dynamic analysis,which not only differs greatly from the actual mechanism of dynamic soil resistance,but also causes a strong empirism and uncertainty for the selection of model parameters.Besides,the calculation models for different types of piles have not been distinguished,and there is less investigation on the soil response around the pile,all of which greatly affects the reliability of the high-strain dynamic test analysis,and therefore,making the application of high-strain test greatly restricted.In view of this,an innovative method for high-strain dynamic analysis of pile is proposed based on the discrete segment fictitious-soil pile(FSP)model.Based on the FSP model,the simulation methods of coupled vibration of different types of pile-soil systems undergoing high-strain dynamic impact loading have been investigated.Besides,the characteristics of pile-soil dynamic response,the wave attenuation law in the soil of pile shaft and pile base,and the slippage pattern of soil plug inside pipe piles under dynamic load conditions are studied.Moreover,an innovative high-strain multi-point dynamic analysis method is proposed for predicting the compressive bearing capacity curve of the pile,and its accuracy and engineering application potential are explored.The main work and innovations of this manuscript are summarized as follows:(1)A novel coupled vibration model of rigid disc-FSP(RD-FSP model)undergoing high-strain impact loading conditions is proposed.The model introduces the FSP method into the high-strain dynamic analysis by considering the soil stiffness nonlinearity,elastoplasticity and hysteresis during the vibration process.The discrete-segment difference iteration algorithm method is adopted to solve the dynamic model combined with MATLAB programming.The feasibility of FSP method for calculating dynamic response of rigid disc under high-strain conditions is verified,and the soil response at different depths in the soil are investigated based on the proposed model,which can give deep insight into the wave propagation and attenuation in the FSP under high-strain dynamic conditions.(2)Based on the RD-FSP model,an innovative solid pile-FSP coupled model(SP-FSP model)under high-strain conditions is proposed.The proposed model takes into account the characteristics of pile shaft resistance,elastic-shortening of pile and the slippage at the pile-soil interface,etc.The solution of the model is given based on the method of dynamic equilibrium equations of the pile-soil system in the form of matrixes and combined with MATLAB programming.Based on the proposed model,the dynamic behaviours of solid piles,the movement of pile base soil under the high-strain conditions and the sensitivity of the relevant parameters are investigated.Besides,the study deepens the understanding of the depth of the affected soil zone of pile base under high-strain conditions as well as the influence of relative model parameters.Recommendations for the selection of parameters for the SP-FSP model and for high strain testing on practical projects are also made in the end.(3)Based on the SP-FSP model,an innovative standard-diameter open ended pipe pile-FSP coupled model(SOEP-FSP model)with soil plug under high-strain conditions is proposed.The proposed model takes into account the difference between the internal and external soil resistance models of the pipe pile,and simulates the one-dimensional vertical propagation of stress waves in the pipe pile,soil plug and pile base soil,which is more consistent to the actual mechanics of soil resistance around the pipe pile than the conventional soil models.Based on discrete-segment difference iterative algorithm and MATLAB programming,the dynamic responses of the pipe pile,soil plug and pile base soil under high-strain conditions are investigated.The slippage pattern of soil plug inside standarddiameter pipe piles under dynamic load conditions and its occurrence conditions are discussed,which gives more insight into the difference in the mechanism of soil plug resistance on the inside wall of pipe piles between static and dynamic tests.(4)Based on the SOEP-FSP model,an innovative large-diameter open ended pipe pile-FSP coupled model(LOEP-FSP model)with soil plug under high-strain conditions is proposed.The proposed model adds consideration to the differences in vertical soil vibration at different radial positions for the internal and external soil for the large-diameter pipe piles,and divides the slip and non-slip zones according to the deformation characteristics of pile shaft soil.Based on discretesegment difference iterative algorithm and MATLAB programming,the dynamic responses of the large-diameter pipe piles and shaft soil and base soil under high-strain conditions are investigated.Besides,the motion characteristics of the soil in the slip zone of pile shaft and the vertical propagation characteristics of the stress wave in the base soil are explored have also been investigated,as well as the radial radiation propagation and attenuation characteristics of the soil motion in the non-slip zone at the shaft soil of large-diameter pipe piles.(5)A novel direct dynamic analysis method based on the axial multi-point measurements of piles,i.e.the high-strain multi-point dynamic test(MPDT)method,is proposed for predicting the compressive bearing capacity curve of piles.The basic assumptions,calculation principles and testing and analysis procedures of MPDT method are given firstly.The theoretical discrete-segment model of single pile based on the FSP method is established and small-scale model experiments are conducted to verify the applicability of the MPDT method in predicting the bearing capacity curve of pile.Besides,the differences between dynamic and static bearing capacity curves as well as related influencing factors are investigated,which provide a theoretical basis for the application of this new dynamic method in practical engineering.