Pile-soil Interface Research Articles

Numerical evaluation of slope effect on soil–pile interaction in seismic analysis

This study aims to evaluate the slope effect on the behavior of soil–pile interface in the lateral direction, which is a factor governing the seismic response of pile-supported structures. A series of numerical simulations of a single pile in cohesionless soil subjected to lateral loading were performed by considering various slope configurations, soil densities, and loading directions. The lateral and confining forces acting at the soil–pile interface increased at a comparable rate under increasingly lateral loading. The ratio of the ultimate lateral force to the ultimate confining force was defined as a lateral interface-force ratio, which was introduced to quantify the slope effect on the strength parameter of the lateral soil–pile interface. The lateral interface–force ratio increased together with embedment depth and maintained a constant value below a certain depth corresponding to the change in failure mechanism from the wedge mode to the flow mode. In addition, an interface model of the soil–pile separation due to the slope failure was developed for the numerical analysis of piles in the cohesionless soil. Finally, the proposed lateral soil–pile interface was implemented via a three-dimensional numerical simulation of a dynamic centrifuge test for the validation work. The good agreement between the numerical simulation and experimental data indicates the reasonable applicability of the proposed interface properties.

Modified Interaction Method for Response of Group Piles Considering Pile–Soil Slip

The existing model for calculating the settlements of group piles is based on the principle of superposition, which fails to calculate the interaction between piles more comprehensively and to take into consideration the influence of slip between pile and soil. In this paper, the interaction between group piles is analyzed from a novel perspective. It is assumed that the interaction between piles is a dynamic equilibrium process, i.e., additional shear forces and additional displacements are continuously transferred between piles until a state of equilibrium is reached. On this basis, we propose a new model for calculating the settlements of group piles considering pile–soil slip. First, a calculation method for pile–side resistance is developed considering the influence of slip. Based on experience with the pile–soil interface, pile–side soils can be categorized as near–pile soil and far–pile soil, and different load–transfer models are applied to describe their mechanical states. By equating pile–side soils into a nonlinear spring and connecting them in series to determine the overall equivalent stiffness considering the effect of pile–soil slip, the pile–side resistance under different loading conditions can be accurately determined. Secondly, equilibrium analysis of the pile unit is carried out when the equilibrium condition is reached, and the stiffness matrix for load transfer is derived. Therefore, in this paper, the interaction between piles is concentrated in this matrix, which makes the proposed model for pile settlement calculation clearer and more concise. Compared with measured data, the proposed method can capture the main features of the load–settlement behavior of group piles.

Analysis of Load-Settlement Curve Based on Load Transfer at Pile-Soil Interface

A field pile loading test was carried out on the Peshawar–Karachi Motorway (PKM) project in Pakistan to show the settling mechanism of bored pile foundation in pulverized soil and the force characteristics of frictional resistance at the pile-soil interface. The changes in pile lateral frictional resistance and pile settlement during the loading-unloading process of test piles were measured and analyzed, as well as the load-settlement distribution characteristics of test piles in different soil layers, the distribution of test pile internal forces, and the changes in pile-soil relative displacement. It was established that there was considerable deterioration of pile lateral frictional resistance and residual deformation of pile tip displacement throughout the test pile load-settlement process, and the association between the pile-soil interface frictional resistance and pile-soil relative displacement was addressed. The results reveal that the frictional resistance at the pile-soil interface is directly connected to the nature of the soil layer, with a positive connection between the natural density, specific gravity, compression deformation, and the plastic index under immediate load, and a negative correlation between the natural moisture content, compression coefficient, and settlement variations after unloading. The load-settlement of the pile rose in a non-linear proportion during the loading-unloading operation, with a maximum settlement value at the pile top of 8.14 mm and a residual deformation at the pile bottom of 1.94 mm. The frictional resistance of the pile perimeter was distributed non-linearly throughout the pile depth, and the frictional resistance of the pile-soil interface was severely deteriorated at an embedded depth of 15 m, with the degradation degree of the silty soil layer being significantly smaller than that of the silty clay soil. The relative pile-soil displacement was positively linked with the lateral frictional resistance of the pile under the same load, and the correlation coefficient in silty soil was much greater than that in sandy soil.

Vertical vibration of a floating pile considering the incomplete bonding effect of the pile-soil interface

Based on the dynamic Winkler model and fictitious soil pile model to consider the relative sliding at the pile-soil interface and the propagation effect of soil beneath the pile toe, respectively, a more rigorous analytical model for the vertical vibration problems of a floating pile is established. The general solution for the displacement of soil is derived by combining the Laplace transform and variable separation methods. Then, the vertical dynamic solution of the floating pile is obtained by introducing the pile-soil interface condition and transfer function recursion method. Furthermore, extensive parametric analyses are performed to investigate the effects of the stiffness coefficient for the dynamic Winkler model, the length of the fictitious soil pile, and the inhomogeneity of the soils and pile on the vertical vibration for the floating pile. This indicates that the proposed analytical model provides a wider range and more rigorous application for the vertical vibration problems of floating piles embedded in layered and limited thickness soils. Moreover, the reduced form of the obtained solution can also be applied to investigate the corresponding vibration problems of end-bearing piles.

Numerical model for dynamic installation of large diameter monopiles

This paper presents the numerical code VibPile for simulation of the nonlinear dynamic response of large monopiles under harmonic loading and installation by vibration or impact driving. VibPile is based on a nonlinear FE model of the pile-soil interaction along the shaft and the tip by elasto-plastic springs representing the near field and elasto-dynamic elements (spring and dashpot) for the far field. While for the shaft friction and tip resistance the classical engineering solutions, such as those by standards or guidelines (e.g. API) and literature, are used in the numerical simulations, new computational models are developed for the dynamic response of the soil inside the pile and the far-field elasto-dynamic springs.What primarily differentiates the present model from the existing ones is the treatment of the soil inside the monopile together with the stiffness at the pile tip. Most existing models are based on the solutions for piles with solid sections or pipe piles in which the soil inside the pile follows the pile vibrations like a solid section. For large diameter monopiles commonly used for offshore wind turbines, both the soil response inside the pile and the stiffness of the pile tip are different. The developed model is verified against the vibro-pile test data collected at Altenwalde, Germany. The results include axial pile strains, accelerations, and shear stresses along the pile shaft and at the pile tip. Both the strains and accelerations as well as the rate of pile driving measured at Altenwalde are reproduced by VibPile with reasonable accuracy. Sensitivity analyses are presented to highlight the effects of the shear wave velocity and shear strength at the pile-soil interface on the pile driving rate. Moreover, the effect of the natural period of the inner soil on the pile driving is investigated.

Rigorous solution for kinematic response of end-bearing pile under vertically incident P-waves

Considering the pile–soil interaction, this study investigates a displacement model for the steady-state kinematic response of a single pile to vertically incident P-waves based on a continuum model. A rigorous solution for the vertical kinematic response of an end-bearing pile is presented herein. The scattered waves were uncoupled from the total wavefield using a reasonable displacement assumption. According to the governing equations and boundary conditions, scattered waves can be easily expressed in an infinite series form, representing the superposition of each vibration order. The generalized expression of the pile–soil dynamic resistance is obtained using Hamilton’s principle. The pile–soil resistance is divided into the Winkler-spring part (produced by elastic shear force) and the dynamic-membrane-spring part (produced by normal elastic stress and inertial force) along the pile–soil interface. Similar to scattered waves, the displacement of the pile and free-field can be expressed in an infinite series form. According to the assumption of the displacement field, all undetermined constants are determined, and the final series solution is provided. The presented has a more concise and definite physical meaning and has excellent application prospects in pile seismic design. Based on the given rigorous analysed solution, a parametric study was conducted to investigate the effects of the pile and soil parameters on the dynamic response of the pile–soil system. Comparing the responses of piles with different stiffnesses, the characteristic stiffness represented a rigid pile (ℜ≤2) and a flexible pile (ℜ≥3).

Analysis of Bored Pile Subjected to Machine Vibration: an Insight into the Influence of the Soil-Pile Interface Coefficient

Analysis of Bored Pile Subjected to Machine Vibration: an Insight into the Influence of the Soil-Pile Interface Coefficient

Analysis on Response of a Single Pile Subjected to Tension Load Considering Excavation Effects

With the development of urbanization, numerous excavations are carried out to facilitate the development of underground space. As a support for tunnel structures, uplift piles are often installed prior to tunnel excavation. The excavation inevitably causes disturbance to the soil below the excavation surface, changing the soil’s mechanical behavior and stress state significantly. However, there is still a lack of a method to evaluate the change in pile capacity due to excavation. This paper proposes a semi-analytical approach for estimating the change in load-settlement behavior of an uplift pile considering the effects of excavation. A hyperbolic model was adopted to simulate the nonlinear interaction of the pile–soil interface. The nonlinear shear-induced soil displacement outside the pile–soil interface is introduced to obtain a more realistic load-displacement behavior of the uplift pile. An effective iterative program was implemented based on the proposed semi-analytical approach. The comparisons between the results from the proposed methods, well-documented field tests, centrifuge tests, and other analytical methods showed that the proposed approach is suitable for analyzing an uplift pile considering excavation effects. A parametric study was conducted to investigate the effects of main soil properties on the pile capacity loss caused by excavation. The results showed that the soil friction angle and the ratio of the excavation depth to the pile effective length have a great influence on the loss in pile uplift capacity caused by excavation. However, the loss of pile uplift capacity caused by excavation is not affected by the soil’s shear modulus or Poisson’s ratio.

Study on the nonlinear behavior of soil–pile interaction in liquefiable soil using 3D numerical method

An interface contact model was proposed to study the nonlinear behavior of soil–pile interactions during soil liquefaction in earthquakes. The soil–pile interface was modeled using joint elements that exhibit linear deformation in the elastic stage and slippage in the plastic stage. The joint elements can potentially model the separation between the soil and the pile. Based on the effective stress theory, an advanced fully coupled 3-D soil–water dynamic finite element–finite difference analysis was performed with different ground motions. In the analysis, a cyclic mobility model properly described the structure, consolidation, and anisotropy characteristics of the soil; the pile was modeled using a hybrid element with a beam and a column to consider the volume effect of the pile. Parametric studies of the interface were conducted to quantify the factors influencing the pile response. The potential soil–pile separation was also discussed. The results indicate that the soil–pile interface has a greater influence on the pile response in strong ground motion; consideration of pile volume is necessary for both small and strong ground motions. The potential soil–pile separation is indispensable in clarifying the soil–pile interaction mechanism and quantitatively estimating the pile response in earthquakes.

Cyclic t-z model for the long-term thermomechanical analysis of energy piles

Many elastoplastic and nonlinear models have simulated the long-term behavior of energy piles under cyclic thermomechanical loads. Although they considered the strain–stress hysteretic relationship at the pile–soil interface, the interface strength was assumed independent of the number of thermomechanical cycles. This simplification may cause errors in predicting the long-term responses of energy piles. Therefore, we proposed a cyclic t-z model for the long-term thermomechanical analysis of an energy pile. Here, the interface strength of the model changes with the cycle number of thermomechanical loads. In addition, the accumulation of irreversible deformation at the pile–soil interface can be modeled. Moreover, the cyclic performance of interface shear tests and the long-term behavior of an in situ energy pile were simulated. Comparisons between the measured and simulated results indicated several vital aspects of the energy pile–soil interaction, including strength degradation, strain ratcheting phenomenon of the interface, and long-term pile responses under cyclic thermomechanical loading. Furthermore, numerical simulations were used to evaluate the effects of model parameters on the long-term behavior of energy piles under thermomechanical loads.

Load-Settlement Behaviour Analysis Based on the Characteristics of the Vertical Loads under a Pile Group

The nonlinear load-settlement behaviours of vertically loaded pile groups containing various numbers of piles (up to a few hundred piles in a group) are analysed using the method proposed by Lee and Xiao. This back-analysis method assumes the “local shear displacement” of a thin layer of disturbed soil along the pile–soil interface and the soil outside the interface is assumed to be linear elastic. Parametric studies are conducted to examine the load-displacement behaviours of the rigidly capped pile groups. Factors such as the number of piles in a group, pile slenderness (L/D), and pile spacing (S/D) are examined to study the effects on the performance of pile groups up to the failure state. Some phenomenological features of large pile groups under nonlinear conditions, which are difficult to obtain through an elastic analysis, are revealed in the present analysis.

Modeling Seismic-Soil-Pile Interaction (SSPI) Problems for Large Pile Groups

This paper presents a critical review of the current challenges in analyzing large, realistic pile groups in soft clay soil under earthquake shaking. The review focuses on three main essentials of numerical analyses, namely soil behavior and constitutive model, pile and pile-soil interface modeling and computational framework.

Failure mechanism of fine-grained soil-structure interface for energy piles

The response of the soil-structure interface can significantly affect the performance of any geotechnical structure. Thermal cycles are a new factor that influence the response to all structures that have an energy function in addition to the structural one, such as energy piles foundations. In this study, a theoretical interpretation of the failure mechanism at the pile-soil interface subjected to cyclic thermal loads and a dedicated constitutive model is presented. The phenomenon is characterised by large localised strains concentrated in a thin layer around the pile surrounded by soil behaving under oedometric conditions. The theoretical framework refers to direct shear tests at a constant normal stiffness, as well as oedometric tests on clayey soil. Observations indicate a negligible effect of the temperature cycles on the soil-structure interface response for typical temperature ranges of energy piles (ΔT= −10, +20 °C). Therefore, an isothermal mathematical formulation in the framework of elastoplasticity is proposed for the analysis of both conventional and energy piles. The effectiveness of the interface model in reproducing the observed behaviour is confirmed via experimental tests. The proposed constitutive model is applicable in engineering practice via the finite-element or the load-transfer method. Additionally, its calibration requires exclusively conventional geotechnical testing.

Thermal resistance analysis of an energy pile and adjacent soil using radial temperature gradients

This paper examines the thermal resistances of energy piles and surrounding soils. A field-scale bored energy pile was installed through stiff sandy clay and dense sand and instrumented to measure temperatures on the external walls of the heat exchanger pipes, at the pile-soil interface, and in the soil. The radial temperature gradients between the pipes and the pile-soil interface, and the pile-soil interface and the soil, were used to evaluate the pile and soil thermal resistances, respectively. The thermal resistances were on the same order of magnitude with values of 0.053 mK/W, 0.072 mK/W, and 0.066 mK/W for the pile, stiff sandy clay, and dense sand due to similarities in their thermal properties. The analysis suggests that pile and soil thermal resistances are influenced by the pile dimensions, number of pipes, concrete cover, soil type and duration of heating. Hence, meticulous interpretation of thermal resistances considering these parameters should be conducted to understand heat transfer processes in energy piles accurately. Estimates of the pile thermal resistance from the equivalent diameter and thermal response test methods were found to be inconsistent with each other, highlighting the significance of considering steady state in-situ radial temperature gradients in designing energy pile systems.

Study on Shear Characteristics of Interface between Frozen Soil and Pile during Thawing Process in Permafrost Area

In permafrost areas, the degradation of permafrost greatly affects the stability of concrete pile composite foundations. Hence, direct shear tests were carried out to analyze the effect of the rising frozen temperature, moisture content, and normal stress on the mechanical properties of the frozen soil-pile interface during the thawing process of permafrost. A constitutive model was established to describe the shear stress-displacement variation law of interface, considering the hydrothermal coupling effect. The results show that the frozen strength of the interface was provided by the ice crystal structure formed at the interface, and its area increases with increasing water content. The whole shear process can be divided into three stages: the prepeak stage with growing shear stress, the postpeak stage with deep dropping shear stress, and the shear stress reconstruction stage. The peak frozen strength was positively correlated with water content and normal stress, however, it was negatively correlated with the rising frozen temperature. The residual frozen strength has a linear relationship with normal stress and water content, however, it shows different regularity with rising frozen temperature at different water content. Moreover, the Gompertz model prediction results are in good agreement with the experimental results. This model can describe well the stress-displacement variation law of interface with different rising frozen temperature and water content.

Study on Behavior and Bearing Capacity Computation Method of Shallow Rock-Socketed Short Piles Based on the Self-Balanced Loading Test

The self-balanced loading test is a state-of-art pile testing method, but its suitability to pile bearing capacity determination in transformer substation engineering in mountainous and hilly areas is not yet clear. In this study, a two-dimensional axisymmetric numerical model is established by the PLAXIS software to simulate the behavior and bearing mechanism of shallow rock-socketed short piles based on the self-balanced loading test. The model is first validated by simulating the field tests of two adjacent piles under self-balanced loading. Then the influence factors of the load-displacement curves of piles are analyzed. Thereafter, the mechanical mechanism of the self-balanced loading tests is simulated and compared with the conventional static loading tests. It is observed that the rock modulus, rock-socketed depth of piles, and burial depth of the Osterberg Cell affect the load-displacement significantly, but the cohesion of the rocks affects little. Moreover, compared with the conventional static loading tests, the shear stress of the pile-soil interface distributes less uniformly under self-balanced loading conditions. On this basis, a bearing capacity computation method of shallow rock-socketed short piles based on the self-balanced loading test is proposed.

Mortar contact discretisation methods incorporating interface models based on Hypoplasticity and Sanisand: Application to vibratory pile driving

A segment-based mortar contact discretisation technique for the analysis of hydro-mechanically coupled dynamic processes with quadratically interpolated finite elements and incorporating advanced interface models is presented. The proposed mortar technique divides the surface pair into finite segments with arbitrary order of integration, allowing, opposite to simpler contact discretisation techniques, for an exact integration of contact stress. The performance of the segment-based in comparison to a (simpler) element-based mortar technique is evaluated by means of simulations of vibratory pile driving model tests in water-saturated sand. For the mechanical modelling of the soil–pile interface either a simple Coulomb friction model, the hypoplastic model with intergranular strain extension, or the elastoplastic Sanisand model is utilised. It is concluded that the segment-based mortar method is superior to the element-based technique in terms of numerical stability if the number of integration points per segment is larger than the number of integration points per element edge for the element-based technique. However, the techniques are found to perform similarly when a comparable number of integration points is used. While the implementation of the segment-based method is found to be more complex in general, the implementation of parallelisation is simpler compared to the element-based mortar method.

Thermally induced ratcheting of a thermo-active reinforced concrete pile in sand under sustained lateral load

Thermally induced ratcheting of a thermo-active pile is the accumulation of net and irreversible pile head displacement upon heating–cooling cycles. Although this kind of phenomenon has been observed in vertically loaded piles in sand, it is unknown whether this exists in laterally loaded cases, and also what underlying mechanisms occur under the thermomechanical flexural soil–pile interaction. This study presents a series of centrifuge tests and finite-element simulations of the thermomechanical behaviour of a laterally loaded thermo-active pile in sand. A new model reinforced concrete (RC) was used in the centrifuge tests to mimic the thermal and mechanical properties of a prototype RC pile realistically. Ratcheting was evident in laterally loaded piles and its extent was more significant when the working horizontal load was higher. The ratcheting phenomenon was attributed to the accumulation of soil plastic strain due to the cyclic mechanical loading induced by pile thermal horizontal expansion and contraction, soil dilation upon soil–pile interface shearing and creep. The additional bending moment induced by the thermal action did not induce yielding within the pile. A subsequent numerical sensitivity study suggested that ignoring the softening behaviour of the sand would lead to underestimation of the magnitude of the accumulative thermally induced pile head lateral displacement.

Field investigation into the performance of pipe pile in soft clay under static and cyclic axial loads

We performed a field investigation into the effective stress, the shaft friction at the pile–soil interface, and the pile-end base resistance of a 29.5 m long prestressed concrete pipe pile in soft clay under static and cyclic loadings (50 000 cycles). Our results indicate that under low-level cyclic loading (CLR ≤ 0.2, where CLR is the ratio of cyclic load amplitude to the ultimate capacity), the pore water pressure at the pile–soil interface initially accumulates with the number of cycles, followed by gradual dissipation after reaching its maximum value. The effective stress also initially decreases and then increases. The shaft friction increases after cyclic loading. Owing to axial load redistribution, the base resistance decreases with the number of cycles. When the pile is subjected to high-level cyclic loading (CLR ≥ 0.5), the pore water pressure at the pile–soil interface keeps accumulating while the effective stress continues to decrease, leading to the degradation of shaft friction. The base resistance increases as a result of axial load redistribution. In particular, we analyzed the effects of the static load level and the cyclic load level on the changes shaft friction.

Theoretical analysis of the barrier effect of embedded isolation piles on tunneling-induced vertical ground displacements

This paper proposes an elastic solution for evaluating tunneling-induced vertical ground displacements due to the restraint of embedded isolation piles. Based on the modified Loganathan-Poulos (L&P) formula and the general form of the Melan/Mindlin solution, the restrained tunneling-induced vertical ground displacements can be calculated according to the displacement compatibility condition at the pile-soil interface. The solution can consider the relative vertical slip between the pile shaft and the surrounding soil and the embedding of the pile tip into the supported soil. The method is verified through comparisons with existing analytical solutions, boundary element program and field-measured data. Parametric analysis indicates that the parameters associated with the isolation pile (the distance of the tunnel axis from the pile axis, the pile length and the relative pile axis stiffness) and the soil (Poisson's ratio, the friction angle and the ground volume loss) have important influences on the ground surface settlement troughs and the barrier efficiency. The results also show that the pile skin-soil shear stiffness, which characterizes the relative slip between the pile shaft and the surrounding soil, determines the barrier efficiency. When the pile-soil interface is completely bonded, the contact state of the pile tip with the supported soil also has a nonnegligible impact on the barrier efficiency.