Pile Interface Research Articles

Time-dependent behaviors of energy piles embedded in multilayered saturated transversely isotropic soils

This paper presents a solution for the time-dependent behaviors of energy piles embedded in transversely isotropic soils, which considers the mechanical and thermal consolidation. By using the transformed differential quadrature method, kernel functions of coupled thermal-hydro-mechanical solution on the soil-energy pile interface are obtained and the boundary integration is conducted. Then, the energy pile is discretized into finite elements. After introducing the displacement coordination and boundary conditions, matrix equations to reflect the interaction between the surrounding soils and energy piles are formulated and solved. Since the consolidation is considered, the solution for energy pile behaviors with time including displacements and thermal stresses are achieved. Computational results are compared with data of existed literatures and field tests to validate the theory in this study. Finally, numerical examples are conducted to discuss the effects of transverse isotropy of soils, consolidation process and the length-diameter ratio of the energy pile.

An enhanced micromechanical rock–pile interface model with application to rock‐socketed pile modeling

AbstractThe increasing use of rock‐socketed piles highlights the importance of developing a suitable design method for their bearing capacity. This study quantifies the shear behavior of the rock–pile interface, which generally dominates the bearing capacity of rock‐socketed piles under service load. A micromechanics‐based rock–pile interface model with idealized nonuniform profile is proposed with two enhancements: (1) the slip line method together with nonlinear Hoek–Brown failure criteria is integrated to identify the critical shear displacement of rock asperity; and (2) the residual stage of shear behavior is properly considered with the rounding progress of sheared rubbles. The enhanced interface model is first validated by the direct shear test results under constant normal stiffness. Then, the interface model is implemented via user‐defined FRIC into the finite element code ABAQUS without the need of explicitly building the rock–pile interface profile. Accordingly, the model is applied to simulate and analyze two field cases involving rock‐socketed piles. Comparison between the predictions and field observed results shows this method can well capture the axial load transfer behavior of pile socket into weak rock.

Numerical Simulation of Double-row Piles in Deep Foundation Pit Excavation

This paper aims to investigate the stress and deformation characteristics of double-row piles in deep foundation pit excavation by selecting representative strata in the Changchun area. The changes in bending moment, displacement, and earth pressure of double-row piles during the excavation are established using the FALC 3D software. The findings indicate that the deformation of the front pile is larger than that of the back pile, and the earth pressure distribution of the fornt pile deviates from the conventional Rankine earth pressure theory. Moreover, there are noticeable variations in earth pressure at the soil layer interface of the rear pile.

Experimental study of concrete shear keys encased by precast RC blocks

Shear keys at the interface of RC pile (hereafter refereed to RCP)) and RC blocks (hereafter referred to RCB) in integrated precast breakwater system (IPBS) have important roles in combining the RCBs into a single integrated body. Hence, a detailed estimation of shear strength between the RCB and RCP with proper shear key design is required. In the present work, the shear (joint) strength between RCB and RCP has been evaluated with different types of shear keys by conducting experiments on two different types of shear keys (full and partial shear keys). For the full shear key specimens, the maximum loads obtained increased linearly with increase in reinforcement ratios of RCB. The failure mode of full shear key specimen was found to be massive destruction failure of RCB without any failure of the shear key. On the other hand, the failure mode of partial shear key specimens showed direct shear failure at minimal failure surface of the shear key. The experimental investigation of concrete shear keys has confirmed that the failure modes vary depending on the amount of installed shear keys, highlighting the need to accurately predict the appropriate shear strength for the shear key encased by precast concrete blocks without post-tensioning. Based on the observations from experiments, a predictive equation on shear strength between RCB and RCB with partial shear keys was developed. The predictive equation considered the contribution of both shear keys and frictional shear behavior between contacted flat surfaces.

Semi-numerical analysis of negative skin friction on permeable pile due to consolidation induced by piling

A semi-numerical approach is proposed to estimate the dissipation of excess pore water pressure and the development of negative skin friction at the normal impervious pile and the permeable pile after piling. An impeded drainage boundary associated with the opening ratio and opening size is introduced to simulate the drainage condition at the soil-permeable pile interface. In the proposed mathematical framework, analytical approaches are adopted to solve the linear equations, and numerical techniques are utilized to address the nonlinear problems so that the adaptivity and efficiency of the mathematical framework can be well balanced. Through comparisons with the field tests, the model tests, and the theoretical answers, the correctness and rationality of the proposed method are validated. Through a parametric study, the key design parameters of the permeable pile are investigated, and the following design tips are obtained: 1. The opening ratio plays the most important role in the drainage efficiency of the permeable pile; 2. Increasing the number of openings is preferred rather than increasing the size of openings, especially when the opening ratio is determined.

Full-scale tests on the grouting effectiveness of offshore bored piles with various bearing strata

The bearing capacity of the post-grouted bored piles in sea-crossing bridges is greatly impacted by the kind of bearing stratum. Engineers are urgently concerned with the reinforcement effectiveness, improvement mechanism, and grout diffusion mechanism of tip-grouting of offshore bored piles in various bearing strata. Two groups of core-drilling inspections and six groups of full-scale static load tests were performed in this research. According to the test results, post-grouting significantly improved the load-settlement response of bored piles with various bearing strata. The improvement mechanism of tip-grouting included the formation of an enlarged pile tip, the pre-mobilization effect, and the improvement of bearing strata and soil–pile interface. The reinforcement effect of post-grouting in a coarse-grained soil bearing stratum was better than that of a fine-grained soil bearing stratum. The diffusion range of upward-flowing cement mortar in the coarse-grained soil bearing stratum and fine-grained soil bearing stratum was 14–15 m and 5–7.5 m respectively, and the diffusion range was significantly affected by the type of bearing stratum. The grout exhibited spherical penetration diffusion in the coarse-grained soil bearing stratum and splitting diffusion in the fine-grained soil bearing stratum, respectively. The investigation findings provided references for amending the existing construction method of post-grouted offshore bored piles in sea-crossing bridges.

Comparative Investigation of Axial Bearing Performance and Mechanism of Continuous Flight Auger Pile in Weathered Granitic Soils

Axial bearing performance and mechanism of continuous flight auger (CFA) pile in weathered granitic soils, i.e., a widespread special soil in South China, were investigated by field test in this study. Load–settlement responses of four CFA piles were examined, and evolutions of shaft/base resistances were captured by ultra-weak fiber Bragg gratings (UWFBG) with a reflectivity ≤−40 dB. Performances of CFA piles were compared with those of a slurry displacement (SD) pile at the same site, thirteen pretensioned spun high-strength concrete (PHC) piles in the literature and empirical data in design code. Test results show that the ultimate bearing capacity of the CFA pile is highest among different pile types, and typically is twice that of the SD pile. Again, CFA pile produces the highest shaft resistances at 140 kPa and 153 kPa in two weathered granitic soils, while the base resistance of 3080 kPa is between those of the SD pile and the PHC pile. By field excavation, the superior mechanism of the CFA pile is suggested to avoid the formation of in-between bentonite layers and prevent preferential baseflow along fissures, both of which can weaken the soil–pile interface. Overall, this study provides fundamental data through UWFBG and explanations based on field observations which underpin the need for developing a design code specified for CFA piles in South China.

Effect of thermal cycles on volumetric and shear behavior of sand–concrete interface

Energy piles are dual-purpose foundation elements that transfer the building loads to the supporting soil and act as heat exchangers for heating and cooling. Energy piles are subjected to heating and cooling cycles over days and seasons, resulting in cyclic expansion and contraction. The cyclic expansion and contraction bring about the modification of vertical stress and the increase of vertical displacements on the piles, in other words, the thermo-mechanical behavior of the pile, the soil, and the soil–pile interface is changed by the cyclic thermal cycles. Investigation to know the effect of temperature cycles on soil–pile interface characteristics has high academic and significant engineering values. This paper intends to investigate the thermo-mechanical response of soil–pile interface under different types of cyclic thermal loadings, to better understand the behavior of energy piles and their surrounding soils after the buildings are constructed and the geothermal activation. Direct shear tests are performed by using a new interface direct shear test machine composed of a temperature controlling device to assess the influence of different cyclic thermal loadings on the shear behavior of Fontainebleau sand–concrete interface. Direct shear tests are carried out after three different thermal loadings, i.e., a constant temperature of 13 °C, and two different kinds of cyclic cooling–heating​ (8–18 °C) loadings with 10 cycles (namely CT-1 and CT-2). The horizontal stress and horizontal displacement are evaluated after the 10 thermal cycles in CT-1 and CT-2 programs, and the shear results are then compared. The results reveal that the cyclic thermal cycles increase the peak shear strength of CT-2, however they do not significantly affect the residual shear strength of both CT-1 and CT-2. The peak/residual interface friction angle of the reference test is 29.5∘/28.4°, after the 10 cyclic thermal cycles, it becomes to 28.9∘/27.9° for CT-1, and 31.7∘/28.5° for CT-2.

An analytical solution for the load-settlement relation of hollow composite piles with ultra-large diameters

The load transfer mechanism is significantly more complex due to its heterogeneous radial distribution properties. To solve this problem, smart sensors and actuators are the good candidates for the improvement of structural reliability and robustness. This study investigates the load transfer characteristics of hollow composite piles with ultra-large diameters, and finds out the load transfer path. Based on the shear deformation transfer method and the assumptions of no relative slide between hollow pile, grouting body and cement mixing pile interface, a mechanical analysis model of hollow piles with ultra-large diameters-grouting body-cement mixing pile, is then developed. The basic equations of load-settlement relation of piles are deduced from the mechanical equilibrium conditions. Based on the hyperbolic model, the equivalent elastic modulus is introduced to improve the transfer function method. Afterwards, analytical solutions for the load-settlement relation of hollow composite piles with ultra-large diameters are presented. Finally, the analytical solution based on the two methods is compared with the finite element solution in order to verify their rationality and feasibility. The results indicate that under vertical load, the load transfer path to pile side (radial) is as follows: hollow pile → grouting body → cement mixing pile → foundation soil. Moreover, the transfer path of load to pile tip (vertical) is directly transferred to foundation soil, which is consistent with the traditional large-diameter solid pile. Compared with the shear deformation transfer method, the transfer function method can better simulate the nonlinear soil, which is more consistent with the finite element results.

Soil–pile interaction in vertical vibration in inhomogeneous soils

AbstractThis paper develops a novel reference analytical solution for axially loaded piles in inhomogeneous soils, extending the pioneering elastodynamic model of Nogami and Novak (1976) to piles embedded in vertically inhomogeneous soils. Following the classical earlier model, the pile is modelled as a rod, using the strength‐of‐materials solution, and the soil layer as an approximate continuum, which rest on rigid rock. The approximation lies in reducing the number of dependent variables by eliminating certain stresses and displacements in the governing elastodynamic equations: the vertical normal and vertical shear stresses in the soil are controlled exclusively by the vertical component of the soil displacement. Soil inhomogeneity is introduced via a power law variation of shear modulus with depth, and perfect bonding is assumed at the soil–pile interface. The proposed generalized formulation treats two types of inhomogeneity by employing pertinent eigen expansions of the dependent variables over the vertical coordinate. The response is expressed in terms of generalized Fourier series and includes: (i) displacements and stresses along the pile and the pile–soil interface; and (ii) displacement and stress in the soil. Contrary to available models for homogeneous soils, the associated Fourier coefficients are coupled, obtained as solutions to a set of simultaneous algebraic equations of equal rank to the number of modes considered.

An integrated model for offshore wind turbine monopile in porous seabed under multi-directional seismic excitations

This study proposes an integrated three-dimensional time-domain finite element model for offshore wind turbine (OWT) monopile in porous seabed under multi-directional seismic excitations. Biot's u-p formulation is used to describe the dynamic interaction between the soil skeleton and pore water. Potential-based fluid elements are used for seawater to accurately model fluid–structure interaction (FSI). Moreover, FSI interface elements are applied to the interface of seawater and pile and to the interface of seawater and mud line. To analyze the response of the OWT-monopile-seabed-seawater system to multi-directional seismic excitations, a viscous-spring artificial boundary is implemented in the finite element model. The integrated model was sufficiently verified in terms of the vibration characteristics of OWTs, the displacement and the excess pore water pressure using implemented viscoelastic artificial boundary, and the FSI interaction under both horizontal and vertical excitation. The results indicated that: (1) The hydrodynamic pressure and the excess pore water pressure varied with different positions along the OWT and the pile; (2) Adding the excitation of the EW component ground motion caused the change of the distribution of transient liquefaction with respect to that under the excitation of only NS component ground motion; (3) The vertical ground motion caused a substantial fluctuation of the vertical displacement of the OWT and larger area of transient liquefaction near the surface of the seabed and near the tip of the pile; (4) resonance occurred when the frequency of input motion was approaching to the first natural frequency of OWT, leading to extremely large acceleration and displacement of OWT.

Laboratory tests on the frictional capacity of core pile–cemented soil interface

Pre-bored grouted planted (PGP) piles are a new type of composite pile in which a core pre-stressed high-strength concrete (PHC) pile is surrounded by a cemented soil (CS) layer. The frictional capacity of the core PHC pile–CS interface has a significant effect on the bearing capacity of a PGP pile. In this work, a series of shear tests was carried out to determine the frictional capacity of pile–CS interfaces for both smooth square piles (SPs) and smooth pipe piles (PPs). The shear test results revealed the following. The peak skin friction (PSF) of the SP–CS and PP–CS interfaces both increased with the CS strength. The PSF of the SP–CS interface was smaller than the PSF of the PP–CS interface of identical pile (equivalent) diameter due to the occurrence of a stress concentration at the corners of the SP. The PSF of the SP–CS and PP–CS interfaces both decreased with the pile (equivalent) diameter. The occurrence of cracks in the CS caused brittle failure of both the SP–CS and PP–CS interfaces.

Influence of soil–pile interface characteristics on the seismic response of single pile foundations: shaking table testing and numerical simulation

Influence of soil–pile interface characteristics on the seismic response of single pile foundations: shaking table testing and numerical simulation

Numerical Simulation of Piles in a Liquefied Slope Using a Modified Soil–Pile Interface Model

The liquefaction of soil surrounding a pile significantly affects the dynamic interaction between the soil and the pile. In particular, liquefaction of the sloping ground can induce permanent deformation and a bending moment on the pile due to the lateral displacement of the liquefied soil in the downslope direction. However, numerical analysis studies on piles installed in a liquefiable slope have been very limited and have not properly simulated the behavior of the pile. Therefore, a modified soil–pile interface model was proposed, which linearly decreases the interface friction angle with the increase in the excess pore pressure ratio. The proposed model was validated by comparing it with the centrifuge test results of Yoo et al. (2023). Simulation results on the slope crest settlement and the pile-bending moment showed good agreement with the centrifuge test results. A parametric study was conducted by applying the validated model to analyze the effect of slope inclinations and the amplitude of input motions on the slope displacement and the pile moment. The simulation results showed that the slope inclinations affected the area of the sliding mass, causing a larger pile-bending moment with a larger inclination. When the amplitude of the input motion was sufficiently large to trigger the failure of the liquefied slope, the slope displacement and the pile-bending moment did not increase any further.

An approach for 2D modelling of laterally loaded piles

Despite a considerable progress in the analysis and design of monopiles, many methods are based on complex mathematical structures with doubtful or hard assumptions to verify. Therefore, there is still a need for simple and yet accurate methods for the analysis of monopiles under drained and undrained lateral cyclic loading conditions. In this work, a simple yet efficient two-dimensional modelling approach for the analysis of monopiles is proposed. To account for out-of-plane frictional forces, counter-forces derived from virtual frictional forces generated at the out-of-plane pile interface are applied along the pile length together with the scaled pile stiffness. The predictive capabilities of the proposed approach were validated by back-calculating two different experimental sets. The first consists of a small-scale field monopile test on a coarse-grained soil subjected to lateral cyclic loading under drained conditions. The second is a centrifuge test involving a fine-grained soil subjected to lateral cyclic loading under practically undrained conditions. Simulation results with the proposed approach suggest an accurate prediction of pile displacements and bending moments under both drained and undrained lateral cyclic conditions. The method is, however, unable to reproduce pore water pressures generated behind the pile in low permeability materials.

Effect of eccentricity on ultimate lateral unit earth pressure of tetrapod piles in clay

This study investigates the ultimate lateral unit earth pressure of rigid tetrapod foundation imposed by eccentric loads, based on a couple of two-dimensional adaptive finite-element limit analyses with lower-bound, upper-bound and 15-node mixed Gauss element formulations. A parameter study is conducted to emphatically explore the influence of eccentricity as well as to assess the role of additional parameters such as soil–pile interface strength and pile spacing on the normalised lateral earth pressure factor Np. Results from the numerical analyses reflect that the presence of loading eccentricity may lead to significant reduction in lateral soil resistance, the maximum diminution is up to 45% for the case that eccentricity equals to 0·5 times spacing. Three representative failure mechanisms are identified associated with various eccentricities. Pile spacing and interface strength additionally affect the variation of failure mechanisms with eccentricity increases. Finally, the eccentricity reduction factor, defined as the ratio of the Np considering the effect of loading eccentricity to the Np omitting this effect, is proposed, and the variation of this factor with eccentricity is presented for different values of pile spacing and soil–pile adhesion factor.

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.

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.

Experimental Investigation of the Mechanical Properties of the Sand-Concrete Pile Interface Considering Roughness and Relative Density.

The aim of this study was to investigate the effect of roughness and relative density on the mechanical properties of sand–concrete pile interface. A series of direct shear tests were carried out on the interface using a large-scale direct shear apparatus with various relative densities of sand (73%, 47%, and 23%) and concrete blocks with four roughness values (I = 0, 10, 20, and 30 mm). Various mechanical properties (such as shear stress, volume change, peak shear strength, secant friction angle, and normalized friction coefficient) from the interface tests were compared with trends obtained from the pure sand direct shear test. For the smooth interface, the shear stress–horizontal displacement curves of the dense sand specimen exhibited a slight softening response, which became more apparent as the roughness increased. The curves of the loose sand specimen demonstrated a hardening response. The volumetric response was influenced by the combination of normal stress, relative density, and roughness. The peak shear strength demonstrated a nonlinear increasing trend as the normal stress increased. With an increase in the normal stress, the secant friction angle and peak friction coefficient decreased as exponential and power functions, respectively. Additionally, a critical roughness value Icr resulted from the tests, which halted the upward trend of the peak friction coefficient and normalized the secant friction angle when I exceeded Icr.

Effect of a filter cake on shear behavior of sand-concrete pile interface

Effect of a filter cake on shear behavior of sand-concrete pile interface