Study On The Lateral Performance And Excavation-Induced Response Of Pile Foundations In Soft Soils

Pile foundation is an important foundation type in the fields of construction,transportation,ocean and underground engineering.With the development of major infrastructure construction,the pile foundation is characterized by super-long,large-diameter construction and complex service environment.The horizontal bearing capacity of pile foundations is the fundamental guarantee for the construction?structure?to resist earthquakes,wind and wave loads,and underground space excavation unloading.Improper analysis of the lateral pile performance in soft soils often leads to major engineering safety accidents.Therefore,it is significant for geotechnical engineers to reasonably evaluate the horizontal bearing capacity of pile foundations and predict their variation response characteristics and propose corresponding safety control method under complex service environment.Based on the National Key Research Project of China?2016YFC0800201?,the National Natural Science Foundation Project?51878157?,the Construction Science and Technology Research Project of Jiangsu Province?2014ZD66?and the Graduate Student Scientific Research Innovation Program of Jiangsu Province?KYLX16₀244?,this paper focuses on the horizontal bearing theory and technology of pile foundations in soft soils.The lateral performance and excavation-induced response mechanism of piles in soft soils are systematically studied through theoretical analysis,in-situ testing,field observation and numerical simulation.The main research contents and achievements are as follows:?1?This paper comprehensively summarizes the research status of laterally loaded piles at home and abroad,especially in the past 20a,analyzes the research results of lateral pile calculation methods and excavation unloading responses,and points out its existence and problems to be solved.?2?For the complexity of the lateral bearing characteristic of piles in soft soils,the corresponding calculation methods are proposed.By constructing the relationship between piezocone penetration test?CPTU?parameters and p-y curve parameters,the practical analysis method and displacement control standard of laterally loaded piles based on CPTU test are proposed.Aiming at the"large deformation"of piles in soft soils,the large-deformation p-y analysis method based on the stress increment model is proposed.The proposed method can comprehensively reflect the effects of radial soil stress increment,longitudinal soil stress increment and deep pile deflection on the lateral soil resistance,and has significant advantages in the calculation of large lateral-deformation of pile foundations.Aiming at the"large diameter"issue,the horizontal bearing analysis method?f_m?of large-diameter piles considering friction effect is proposed.The f_m-method considers the additional bending moments generated by the friction force on the pile,including the frictional bending moment caused by the deflection of the pile axis and the frictional bending moment caused by the pile size effect,which greatly improves the calculation accuracy of horizontal bearing capacity of large diameter piles.?3?Based on the combination of field observation and in-situ testing,the effect of foundation pit excavation on the horizontal bearing capacity of adjacent pile foundations was studied systematically.The evaluation method of the lateral pile response induced by excavation unloading is proposed.Excavation unloading of the foundation pit can release stress on the soils around the pile,thus affecting the horizontal bearing capacity of the pile foundation.The in-situ testing results show that the cone tip resistance is weakened after the foundation pit excavation.The attenuation of the cone tip resistance is closely related to the soil unloading stress path,and the horizontal bearing capacity of the pile after excavation unloading is lower than that of the free field.Besides,excavation of the foundation pit causes the lateral soil movements,which in turn causes the adjacent piles to be passively loaded.Due to the excavation unloading process,the horizontal deformation law of the passive pile is complex,and the pile-soil interaction changes continuously with the excavation process.To calculate the lateral deformation of the passive pile and its internal force characteristics,an analysis method for passive piles considering the whole process of adjacent excavation unloading is proposed and verified.?4?The experimental study on the effect of foundation pit excavation on the horizontal bearing capacity of the pile foundation at the bottom of the pit was carried out,and the variation law of the in-situ test parameters of the soil before and after excavations and the lateral pile response at the bottom of the pit was determined.The results indicate that excavation unloading causes the stress release of the soil at the bottom of the pit and affects the characteristics of the p-y curve of the pile-soil interaction.The determination of the horizontal bearing capacity of the pile foundation at the bottom of the pit shall take into account the joint effects of excavation-induced stratum change and soil stress release.Regardless of the unloading effect,the horizontal bearing capacity of the pile foundation at the bottom of the pit was overestimated.Therefore,using in-situ test parameters of the actual soil layers after excavation unloading can accurately calculate the horizontal bearing capacity of the pile foundation at the bottom of the pit,which is in good agreement with the field test results.?5?By constructing the numerical model of the pile-soil-excavation system,the weakening mechanism of the horizontal bearing capacity and its influencing factors of the passive piles caused by the adjacent excavation unloading were further studied.The pile-soil interaction characteristics due to the excavation-induced soil movement accompanied by stress release?stress-deformation coupling?were obtained.The influence of excavation mode,soil modulus,drainage state and different loading sequence on p-y curves was clarified.Results indicate that the passive pile-soil interaction is mutated due to the excavation of the adjacent pit.The passive pile p-y curve is different from the active pile p-y curve,showing obvious softening characteristics,and the faster the excavation rate,the faster the p-y curve falls.Accordingly,the internal pile-soil interaction mechanism of the horizontal bearing capacity evolution of the passive piles due to the excavation unloading process is revealed.?6?Based on the cap-pile coupling model,the mechanism of the cap constraint effect on the horizontal bearing capacity of passive piles due to adjacent excavation was further analyzed.A design method and design reference standard for passive pile caps are proposed,which considers the embedded depth of the pile head into the pile cap,the relative modulus of pile cap and pile head and the pile-cap contact stiffness together.The research results indicate that the pile cap constraint can significantly affect the horizontal bearing capacity of the passive pile adjacent to an excavation.Under the single influencing factor,the embedded depth of the pile head into the pile cap should be controlled at?5¹5?cm,and the relative modulus ratio of the cap-pile should be controlled at?0.1⁵?.If considering the comprehensive impact,pile engineers should follow the cap constraint coefficient method and control K_c within the range of?0.1¹?%.?7?The key technologies for the lateral capacity enhancement and safety control of engineering piles in soft soils are established.Through field test and theoretical calculation,the improvement effects of the post-grouting technology in soft clays and the resonance compaction method in liquefiable soils for laterally loaded piles are confirmed,which provides a technical reference for the enhanced construction of pile foundations in soft soils.Further,the safety control method of active piles from the perspective of parameter sensitivity and the active control method of horizontal bearing capacity of passive piles from the perspective of excavation effect are given,respectively.The research results indicate that post-grouting technology can significantly improve the horizontal bearing capacity of the bored pile,which is about 20%higher than that of the ordinary bored pile.The resonance compaction technology can greatly improve the soil properties and increase the lateral pile capacity by about 30%.For active piles,the sensitivity of the horizontal bearing parameters is:pile head fixity condition>size effect>vertical load>pile construction inclination.For passive piles,reasonable control of excavation rate and cap constraint degree and right loading sequence during the excavation process,is the significant way to ensure the horizontal bearing performance of passive piles and the stability of whole constructions.