| Pile driving and pulling based on high-frequency vibration is a method that is mainly used to provide temporary support to foundation pits,The drivability of steel sheet piles,the vertical and horizontal capacities of the piles buried in soil,and the characteristics of the interface between the pile and soil during pile-sinking have been extensively studied: however how to the improve the construction efficiency of pile driving and pulling,and decrease the adverse effects on the environment warrant further research.In this research,this problem was investigated by integrating the analysis of the high-frequency vibratory characteristics of soil,piles,and vibratory hammers,and the in situ testing and analysis of pile driving and pulling based on high-frequency vibration.A high-frequency dynamic triaxial apparatus was adopted to investigate the dynamic characteristics of saturated soils subjected to high-frequency vibration load.A hybrid model integrating lumped elements and a continuous bar was established using the high-frequency vibration-based pile driving and pulling technique: the modal frequencies were then used to reveal the dynamic characteristics of the piles and the vibratory hammers when subjected to high-frequency vibration by an integrated parametric study.A full-scale,in situ test of a high-frequency vibration-based pile driving and pulling operation was carried out on a highspeed railway construction site,in Qingdao City,Shandong Province,China.Moreover,the changes in driving and pulling speeds of the piles,their influencing factors,the ground vibration and noise propagation during pile sinking and pulling,as well as variations in surface displacement,horizontal deformation,and pore water pressure of soils as the pile sinking and pulling was completed,were analysed.The main contributions of this study included the following:(1)High-frequency dynamic triaxial tests were conducted on a saturated sandy soil to obtain the dynamic characteristics of the soil under the high-frequency vibration load.An increase in vibration frequency can cause an increase in the peak particle velocity;the dynamic pore water pressure decreased with the number of cycle of vibratory loading),and an increased number of cycles was required for soil liquefaction: however,it could also cause the opposite effect and shorten the time to soil liquefaction.Similarly,an increase in vibration frequency reduced the dynamic axial strain on the soils under liquefaction,increased the axial strain with time,while reduced the increase in axial strain with the number cycles.Vibration frequency adjustment also affected the relationships between the dynamic pore water pressure ratio and the axial strain ratio with number of cycles(the dynamic pore water pressure ratio,axial strain ratio,and the ratio of the number of cycles respectively refer to the ratio of the pore water pressure,the ratio of axial strain,and the ratio of the number,of cycles to those at onset of soil liquefaction.The test data were fitted to acquire the relationship between the number of cycles to liquefaction of saturated sands,and the dynamic elastic modulus of saturated soft clay with vibration frequency.According to the aforementioned analysis,the vibration frequency of high-frequency vibration based pile driving and pulling ranged between 20 Hz to 60 Hz,which was beneficial to increasing pile driving and pulling velocity efficiency as soil liquefaction occurred.(2)The hybrid model,integrating lumped elements and continuous bars,was established to investigate the dynamic characteristics of piles and vibratory hammers under high-frequency vibratory load.By taking the piles as constant cross-section homogeneous elastomers,resistance parameters of the soils at pile sides and pile tops were used to deduce the equivalent stiffness parameter of the pile top according to the longitudinal vibration theory of the piles and the transmissivity of soil,furthermore,this two-degree-of-freedom,lumped parameter model of the piles and vibrating hammers was used to calculate the amplitude-frequency relationship between the exciter and the damping beams of the vibrating hammers.Additionally,using the hybrid model,the change in peak particle velocity of the pile body with the embedded depth of the piles in the soil was then determined based on the vibration characteristics of the head of the pile.On this basis,the influences of the parameters of the pile body,the vibrating hammers,and soil characteristics,on the vibration characteristics of piles and the hammers were studied.The results showed that,increasing the vibration frequency of the hammers within their normal working frequency range could cause an increase in the vibration velocity at various points on the pile body and thus increased the pile driving and pulling velocities.Moreover,as the vibrations of both the vibratory hammer,and the vibration damping of the beams,increased,more attention to the maintenance of related equipment and construction safety was required.(3)The full-scale in situ field testing of high-frequency vibration-based driving and pulling of steel sheet piles and the numerical simulation of the pile sinking process were carried out to demonstrate the changes in pile driving and pulling velocity and diverse environmental issues induced by pile driving and pulling.In addition,seven factors affecting the pile driving and pulling velocities were summarised to analyse the influence of pile parameters,equipment performance indices,and soil characteristics thereon during different stages of pile driving and pulling.In practical construction work,the change in vibration frequency and mass-moment of vibration hammers can be exploited to promote the exciting force delivered by the hammers,consequently increasing construction efficiency.The pile driving more significantly influenced the soil than pile pulling: pile driving led to surface heave,increased the pore water pressure in the soil surrounding the piles,and generated movement towards the opposite side of the pile;while pile pulling resulted in ground settlement,an increase in the pore water pressure in the soil,and movement towards one side of the pile.The maximum disturbance on the surface occurred at 2.5 m from the pile,while the soil deformation and pore water pressure change were maximised at a horizontal distance of about 5 m from the pile.Surface vibration amplitudes,in both pile driving and pulling processes,were similar as the embedded depth of the piles increased: the amplitude decayed exponentially over increasing horizontal distance from the pile head.Furthermore,the noise produced by pile driving was greater than that caused by pile pulling.The noise pollution from both pile driving and pulling processes exhibited different trends over changing embedded pile depths,however the decay characteristics were the same over increasing horizontal distance.It was suggested that excavator pile drivers be used for high-frequency,vibration-based pile driving and pulling during the construction of an urban pipe gallery,trenches,and grooves,due to the smaller disturbance of the soil,reduced surface vibration,and reduced noise pollution. |
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