Driven and jacked piles are widely used in modern building practice, nevertheless, the mechanisms of pile installation and pile setup in sand still remain unclear or even unexplored at present. The main objective of this research is to carry out a comprehensive study in order to enhance understanding on pile installation and pile setup and to determine the involved micromechanics.

To explore the mechanisms of a driven pile setup in dry sand, model driven pile tests were carried out in a custom-made pressurized chamber. A bender element system and tactile pressure sensors were used in parallel to monitor the stiffness and stress changes in the soil surrounding the pile during the tests. The experimental results demonstrate that pile setup is not caused by the increase of at-rest...[ Read more ]

Driven and jacked piles are widely used in modern building practice, nevertheless, the mechanisms of pile installation and pile setup in sand still remain unclear or even unexplored at present. The main objective of this research is to carry out a comprehensive study in order to enhance understanding on pile installation and pile setup and to determine the involved micromechanics.

To explore the mechanisms of a driven pile setup in dry sand, model driven pile tests were carried out in a custom-made pressurized chamber. A bender element system and tactile pressure sensors were used in parallel to monitor the stiffness and stress changes in the soil surrounding the pile during the tests. The experimental results demonstrate that pile setup is not caused by the increase of at-rest radial stress σ_ps', rather, it is mainly attributed to the increase in radial stress during pile loading Δσ_pf', as a result of soil aging (or creep). Pile installation pushes the surrounding soil to the side, thereby imposing additional loading on the soil inside the influence zone. This loading action initiates an associated aging (or creep) process during the setup period and the aging effects ultimately give rise to an increase in Δσ_pf' and the pile shaft resistance. The measurements also reveal that the increase in aging-induced soil stiffness is due to the contact normal forces among soil particles gradually becoming more homogenized during the setup period. This suggested setup mechanism can explain the absence of pile setup in bored piles (non-displacement piles) — the loading action induced by the pile installation is too insignificant to trigger the required aging process. In addition, in a similar logic, it can elucidate why the setup rate is higher in large-displacement piles than in small-displacement piles.

To examine the soil responses in terms of the strain and stress distributions, the stress paths, the movement of particles and contact force mobilization during monotonic jacking, a three-dimensional simulation of a centrifuge model pile test was performed using the discrete element method (DEM). The simulation results show great agreement with the measurements from the centrifuge model pile tests. The distribution of the incremental deviatoric strain dε_q indicates the formation of a shear band in the shoulder of the ‘nose cone’ that is beneath the pile base. Based on this effect, the soil mass can be divided into three zones, i.e., the ‘nose cone’ (zone I), the shear band (zone II) and the surrounding soil beyond the shear band (zone III). The distributions of the radial stress σ_r', hoop stress σ_θ', vertical stress σ_v' and shear stress τ_rv' have different modes and high-gradient distributions of mean stress p' and deviatoric stress q are found in zone II. As for the stress paths, the surrounding soil experiences four phases in terms of changes in p' and q during monotonic jacking, and the change in the soil stress state decreases with increasing distance from the pile centerline. From a microscopic point of view, the particles in the shear band rotate strongly in similar directions, fully mobilizing the friction at the associated contacts. This provides a micromechanical explanation for the soil failure during monotonic jacking.

To explore the mechanisms of friction fatigue for jacked piles in sand, model jacked pile tests were carried out in a custom-made pressurized chamber. Tactile pressure sensors were used to monitor the changes in stresses and interparticle contact forces in the soil surrounding the pile during cyclic jacking. The measurements suggest that the influence of the cyclic jacking process on the radial and hoop stresses of the surrounding soils at a certain depth can be categorized into two stages, i.e., Stage-1 where the pile penetration depth z < the transition depth z* and Stage-2 where z > z*. The transition depth z* is associated with the maximum stationary radial stress σ_rs' or hoop stress σ_θs' obtained during the whole pile penetration process. In Stage-1, the values of the stationary radial stress σ_rs', ultimate radial stress σ_ru', and the difference lΔσ_r'l = lσ_ru' - σ_rs'l continue to increase with increasing penetration depth z. In Stage-2, the values of σ_rs', σ_ru', and lΔσ_r'l decrease with increasing z and the associated soil structure gradually becomes unstable. Similar responses are also observed in the measured hoop stress. It is also found that z* increases linearly with increasing distance from the pile centreline. Based on these experimental findings, a simple model is proposed to explain friction fatigue. In response to the continuous jacking cycles and increasing pile penetration depths, the base gradually reaches the transition depth of the soil elements surrounding the pile one by one, followed by the elements gradually entering Stage-2. Hence, the soil structure in the nearer zone becomes more unstable, and the associated ultimate radial stresses gradually decrease, which in turn leads to decreases in the ultimate radial and shear stresses on the pile shaft, causing friction fatigue.

To examine the mechanisms of the time-dependent behavior, i.e., variations of the pile residual resistance under a constant head load, the resistance vs pile-head-settlement response during static compression load tests and pile serviceability performance with time, of jacked piles in sand, a jacked model pile test and associated DEM simulations were conducted. The experimental results demonstrate that for the pile under zero head load, the base residual resistance increases with time. In addition, during the load test, the base resistance Q_b increases with time at a relatively small settlement but decreases with time at a relatively large settlement. As to the shaft resistance Q_s, it decreases and increases with time at a relatively small settlement and relatively large settlement, respectively. Moreover, the corresponding resistance vs the pile-head-settlement responses reveal a worse serviceability performance with elapsed time for the piles mainly supported by the base resistance and a better serviceability performance with elapsed time for the piles mainly supported by the shaft resistance. The DEM simulation results demonstrate different modes of particle movement and changes in the vertical stress in the soil during aging for various magnitudes of the head load. This behavior in the soil is related to the variations of the resistances applied on the pile. Ultimately, the consistency between the experimental and DEM simulations demonstrates that the micromechanical initiation mechanism of all the aforementioned time-dependent behavior appears to be microfracturing at the contacts leading to friction reduction and packing re-equilibrium.