Pile-soil Interface Research Articles

Evaluation of the soil–pile interface properties in the lateral direction for seismic analysis in sand

This study aimed to numerically evaluate the strength parameter of the soil–pile interface in the lateral direction. A 3D numerical model of a single pile in sandy soil was built to analyse the interface strength parameter, which was defined as the ratio of the lateral force mobilised on the interface to the confining stress multiplied by the pile perimeter. Results showed that the interface strength parameter increased with the embedment depth ratio and converged to a constant value at a depth of four times the pile diameter. This relationship can be expressed using a bilinear formula that can be adopted to define the strength of springs connecting soil and pile elements in the lateral direction. The applicability of the proposed strength parameter to seismic analysis was validated by performing 3D numerical simulations of centrifuge tests. The simulation targeted single and group piles installed in a level ground of dry silica sand. The simulation results agreed well with the measured data, indicating the reasonable applicability of the proposed strength parameter of the soil–pile interface in the lateral direction.

Mathematical characterization of pile–soil interface boundary for consolidation analysis of soil around permeable pipe pile

A permeable pipe pile is proposed to accelerate the dissipation of excess pore-water pressure through drainage holes around the pile circumference into the pile’s cavity. This investigation generalizes the permeable pile–soil interface as an impeded boundary, based on which a mathematical model for soil consolidation around the permeable pile is derived. Subsequently, the influence of opening pattern, drainage hole-to-pipe pile radius ratio, and opening ratio on the consolidation efficiency is numerically investigated. A total of 240 numerical cases with different drainage hole-to-pipe pile radius ratios, opening ratios, and hydraulic conductivity ratios are calculated to determine the permeable pile–soil interface parameter using back-analysis, and an approximate expression for interface parameter is obtained. Comparing against experimentally measured excess pore-water pressure dissipation, the proposed technique with impeded drainage boundary can provide good evaluations for drainage characteristics at the permeable pile–soil interface and consolidation behavior of the surrounding soil. It is found that the spatial distribution pattern of drainage holes has negligible influence on the consolidation efficiency. Increasing the opening ratio (with constant opening size) or reducing the drainage hole-to-pipe pile radius ratio (with constant opening ratio) can both accelerate the consolidation efficiency.

Use of digital correlation techniques for investigating behaviour of geothermal driven piles

Geothermal energy has established its huge potential as renewable source of energy for general building establishments. A key feature here is the use of energy piles which serve as major energy exchange system which is sustainable and low-cost. Energy piles are basically driven precast piles employed to provide structural support to structures and act as heat exchange units to provide heating / cooling to buildings. The behaviour of energy piles is influenced by temperature, physical and engineering properties of soil. However, uncertainty still exists at pile soil interface irrespective of its heating or cooling conditions. The present study examines the suitability of using digital correlation technique (DIC) namely GeoPIV-RG software to study the soil deformation at pile soil interface in sandy soil. A rectangular test tank made of steel with a size of 60 × 20 × 45 cm is used for experimental studies. Hollow half section aluminum piles closed at the ends with two different pile length to outer pile diameter ratios viz., 10 and 15 of same flexural rigidity are used for the experimental studies. The piles are subjected to sustained loading simulated through Universal testing machine and movements of soil around the pile are captured using a digital camera. The image analysis software named GeoPIV-RG is used to determine the shear strains and soil displacements. Analysis of results indicated that this technique could capture deformation characteristics and surface heave formation in the surrounding infill medium.

Thermal Stability Evaluation Method Based on Pile’s Bearing Capacity in a Permafrost Region

Pile foundations are widely adopted in the Qinghai–Tibet Railway. The pile’s bearing capacity must be influenced the trends of global warming, rising foundation soil temperatures, expanding active layers, ground ice melting, etc. Thus, the pile’s long-term bearing capacity must be understood, and methods for evaluating its stability in permafrost regions must be developed. The bearing capacity of a pile foundation in a permafrost region is caused by the freezing strength of the pile–soil interface and support force at the pile bottom. Based on the formation mechanism of the bearing capacity of pile foundations in permafrost regions, the stability ratio of the freezing area and the freezing strength between the pile and soil are proposed to evaluate the thermal stability of the pile’s bearing capacity in permafrost. Using the method, a bearing capacity study on a single pile in typical moist permafrost and two piles in a permafrost region where warm groundwater is underground is performed. Compared with the field-measured value, the evaluation method proves to be correct and effective. Using the two parameters dependent on the soil–pile interface temperature, the effect of the thermal stability of pile bearing capacity may be described intuitively. Thus, both parameters are conducive to the analysis of pile bearing capacity formation mechanism in permafrost.

Inclined single piles under vertical loadings in cohesionless soil

Physical-scaled model testing under 1 g conditions is carried out in obtaining the vertical response of fixed head floating-inclined single piles embedded in dry sand. Practical pile inclinations of 5° and 10° besides a vertical pile (0°) subjected to static and dynamic vertical pile head loadings are considered. To account for the effects of soil nonlinearity as well as the soil–pile interface nonlinearity on the response of piles, a range of low-to-high magnitude of pile head displacements is considered for the static case while a varying amplitude of harmonic accelerations for a wide range of frequencies is considered for the dynamic case. Experimental results are obtained in the form of pile head stiffnesses and strains generated in the pile under both the static and dynamic loadings. Results suggest that the nonlinear behavior of soil as well as the nonlinearity generated at the interface between the soil and the pile as the result of applied loading considerably affect the response of piles. The soil–pile interface nonlinearity that governs the slippage of pile shows a clear influence on the pile head stiffnesses by providing two distinct values of stiffnesses corresponding to the push and the pull directional movement of piles; the two values are significantly different. Axial and bending strains generated in the piles show expected dependency on the amplitude of applied loading; the pile head-level bending strain increases almost linearly with the increase in the angle of pile inclination.

Application of Miniature FBG-MEMS Pressure Sensor in Penetration Process of Jacked Pile.

In order to study the penetration mechanism of jacked piles in viscous soil foundation, the stress variation law of the pile–soil interface was obtained by installing silicon piezoresistive earth pressure and pore water pressure sensors, and fiber Bragg grating (FBG) sensors in a model pile body, and the penetration characteristics of jacked piles in homogeneous viscous soil were defined. The test results show that: Fiber Bragg grating and silicon piezoresistive sensing technology can better meet the requirements of testing the characteristics of jacked pile in viscous soil. The ratio of pile lateral resistance to pile end resistance varies when pile is jacked in homogeneous viscous soil. In the early stage of pile jacking, the ratio of pile lateral resistance is small, and in the later stage of pile jacking, the ratio of pile lateral resistance increases, but the ratio of pile end resistance is still higher than that of pile lateral resistance. The ratio of the effective stress to the total radial stress is high, and the variation law of the two is consistent with the depth. The total radial stress, pore water pressure, and effective radial stress all exhibit the degradation phenomenon, and the degradation degree decreases gradually with the increase in penetration depth at the same depth. The ratio of excess pore water pressure to overburden weight decreases with the increase in depth, and the maximum value is 87%. The research results can provide a reference for the engineering practice of jacked pile in viscous soil foundation.

Dynamic analysis of laterally loaded single piles in sandy soils considering sliding and debonding on the pile-soil interface

This paper proposes an analytical solution for the dynamic analysis of laterally loaded single piles in sandy soils that considers the effects of sliding and debonding on the pile-soil interface. These effects have received little attention in earlier studies and usually lead to an overestimate of the dynamic impedances of piles and soil. In this paper, the pile-soil interface is decomposed into three components, the bonding, sliding and debonding components, and the spread of the bonding and sliding areas is associated with the inertial friction angle of the soil around the pile. The effects of sliding and debonding of the pile-soil interface are investigated by using the radiant stress theory and the convolution method of Fourier coefficients. The displacement fields and attenuation functions of the soil are then derived by the potential functions. The dynamic impedances of the pile top are calculated by the Adomian decomposition method. The results are presented by contours in both the horizontal and vertical planes, and the present solution is validated through comparisons against a finite element simulation and other analytical solutions. The influences of related parameters, including the load frequency, effective inertial friction angle of the soil, modulus ratio between the pile and soil and pile slenderness ratio, have also been studied in this paper.

Time-dependent behaviour of clay-concrete interfaces with different contact surface roughnesses under shear loading

Mechanical properties of the interface between a pile and soil greatly affect the bearing capacity of a pile. The creep of soil causes a long-term strength of the pile–soil interface to be smaller than its instantaneous shear strength. It is helpful to understand the settlement deformation and strength characteristics of friction piles from the micromechanism by studying the shear creep mechanical properties of the clay–concrete interface. In this work, we carry out shear creep tests on the interfaces between clay and concrete slabs with different roughnesses under different normal stresses. We analyze the influence of the joint roughness coefficient (JRC) on the long-term cohesion and long-term friction angle of the interface. We propose an instantaneous damage factor of the shear modulus and improve the Nishihara creep model with the instantaneous damage effect of the shear modulus. The experimental results show: (1) the whole process test curve of the shear creep shows that the specimen is rapidly destroyed after the transient creep stage and the steady-state creep stage, but the tertiary creep stage is not obvious. (2) The long-term shear strength of the clay-concrete interface is greatly affected by the change in the normal stress under the same JRC conditions, and the long-term shear strength of the interface increases linearly with increasing normal stress. (3) The relationship between the long-term shear strength and normal stress of the interface can be described by the Mohr–Coulomb failure criterion, and the long-term cohesion and long-term friction angle under different JRC conditions can be calculated accordingly. There exists a critical value JRCcr for the long-term cohesion, and the long-term cohesion of the interface first increases and then decreases as the JRC increases. When the long-term cohesion of the interface decreases to a certain extent, it fluctuates around the long-term cohesion value of clay. Moreover, the long-term friction angle of the interface fluctuates slightly with varying JRC, and there is no obvious change rule. (4) Considering the instantaneous damage of the shear modulus, we establish a relationship between the instantaneous shear modulus and the damage factor and introduce the damage factor into the Nishihara creep model. The shear creep mechanical parameters of the clay–concrete specimens under multistage loading are inversed.

Heat Transfer Performance in Energy Piles in Urban Areas: Case Studies for Lambeth College and Shell Centre UK

A ground source heat pump system is a highly efficient renewable heating, cooling, and ventilation system that utilizes the ground as a heat source or sink via ground heat exchangers. Energy pile is an energy geotechnical structure that couples a ground heat exchanger with a geotechnical structure, leading to low capital cost. The design of energy piles can be challengeable due to their complicated geometries and the requirement of mechanical load. This study focuses on the heat transfer across the concrete–soil interface of energy piles in urban areas. Case studies from two projects, the Lambeth College and Shell Centre projects, are presented and discussed. The back analysis of two energy pile cases illustrated that the heat transfer coefficient at the pile–soil interface can differ between the cooling mode and the heating mode. It can be concluded that the difference in the heat transfer coefficient is influenced by a number of factors such as soil properties, concrete (grout) properties, and the installation method.

Analysis of nonlinear load-displacement behaviour of pile groups in clay considering installation effects

This paper presents an analytical approach for predicting the nonlinear load-displacement behaviour of pile groups consisting of displacement piles in normally consolidated and lightly overconsolidated clayey soils. The effects of squeezing and remolding on the soil around the piles caused by pile installation are reflected by the soil strength and shear modulus. The nonlinear load-displacement behaviour at the pile-soil interface is modeled by the load-transfer method with exponential load-transfer functions and the pile-pile interaction effects are reflected by the shear displacement method. A hybrid analytical method, incorporating the developed soil parameters and combining the load transfer method and the shear displacement method, is proposed for the problem considered. Four sets of centrifuge model tests, involving pile groups with different pile diameters, pile lengths and pile spacings, were conducted to verify the proposed analytical framework. The validity of the proposed method, the significance of the installation effects as well as the nonlinear behaviour of the pile groups are demonstrated by comparing the load-displacement curves from the centrifuge model tests with those from the proposed method and other relevant methods. Parametric studies are conducted to investigate the pile diameter, pile length, pile spacing and pile number on the stiffness efficiency of pile groups. The results indicate that increasing the pile diameter and length can effectively improve the stiffness efficiency and the load carrying behaviour of displacement pile groups.

Drained-Undrained Shaft Resistance of Piles in Soft Clays

The determination of the stress distribution around driven piles in soft clays constitutes a complex problem. The stress distribution is affected by a range of factors such as soil permeability, soil strength, sensitivity, remoulding, distance from adjacent piles and number of piles. The mechanisms of pile installation and subsequent consolidation are investigated by considering that pile installation may be represented by the expansion of a long vertical cylindrical cavity. The stress paths followed by typical soil elements at the soil–pile interface are analyzed by means of the theoretical relationships obtained during the undrained expansion of a cylindrical cavity in soft cohesive soils and modified to take into account the severe remoulding caused by pile installation. It is shown that the shaft resistance of driven piles may be calculated by considering either a drained stress path or an effective stress path during undrained loading. It is also shown that soft clay remoulding caused by pile driving is a major factor that must be taken into account for a reasonable estimation of the limiting skin friction.

Equivalent linear model for the lateral dynamic analysis of pile foundations considering pile-soil interface degradation

An equivalent linear BEM-FEM model for the approximate analysis of the time harmonic lateral response of piles considering soil degradation along the soil-pile interface is proposed. The non-degraded soil around the foundation is modelled with boundary elements as a continuum, semi-infinite, isotropic, homogeneous or zoned homogeneous, linear, viscoelastic medium. Piles are modelled with finite elements as beams according to the Bernoulli hypothesis. Differently from what is considered in other BEM-FEM coupled numerical schemes, welded contact conditions are not assumed herein between piles and soil. On the contrary, displacements along FEM piles and within BEM soil are related through distributed springs and dashpots whose properties vary along the pile and that represent the varying stiffness and energy dissipation properties of a pile-soil interface with different levels of degradation at different depths. The developed numerical model is verified by comparison against a more rigorous but much more costly multi-domain boundary element model in which the same degraded soil regions around the pile are modelled as three-dimensional regions.

Influence of soil mass on the vertical dynamic characteristics of pipe piles

A new soil-pile interaction model, called ‘additional soil model’, is developed to investigate the influence of soil mass on the dynamic characteristics of pipe piles. The soil is divided into an inner region in the immediate vicinity of the pile and an outer region at greater distance. The soil in the inner region is assumed to be an additional mass to participate with the vibration of the pile. The viscous damping at the soil-pile interface and the radiation damping in the soil are calculated separately. An analytical solution for the vertical dynamic characteristics of pipe piles is derived based on the developed soil model. The rationality and accuracy of the additional soil model have been validated through comparison of theoretical predictions and measured driving stress waves of two full-scale steel pipe piles. The approach makes a useful improvement of soil-pile interaction modeling for dynamic applications. The main conclusions of this study include: 1) both the soil plug and the pile surrounding soil can lead to a decrease of apparent wave velocity of pipe piles; 2) the pile impedance will be underestimated if calculated without the consideration of soil mass; 3) the resonant amplitude and natural frequency of piles will be significantly overestimated if the soil mass is ignored.

Study on the Effects of Grouting and Roughness on the Shear Behavior of Cohesive Soil-Concrete Interfaces.

Grouted soil–concrete interfaces exist in bored piles with post-grouting in pile tip or sides and they have a substantial influence on pile skin friction. To study the effect of grouting volume on the shearing characteristics of the interface between cohesive soil and concrete piles with different roughness, grouting equipment and a direct shear apparatus were combined to carry out a total of 48 groups of direct shear tests on cohesive soil–concrete interfaces incorporating the grouting process. The test results showed that the shear behavior of the grouted cohesive soil–concrete interface was improved mainly because increasing the grouting volume and roughness increased the interfacial apparent cohesion. In contrast, increasing the grouting volume and roughness had no obvious increasing effects on the interfacial friction angle. Interfacial grouting contributed to the transition in the grouted cohesive soil from shrinkage to dilation: as the grouting volume increased, the shrinkage became weaker and the dilation became more obvious. The shear band exhibited a parabolic distribution rather than a uniform distribution along the shearing direction and that the shear band thickness was greater in the shearing direction, and it will become thicker with increasing grouting volume or roughness. The analysis can help to understand the shear characteristics of soil–pile interface in studying the vertical bearing properties of pile with post-grouting in tip or sides.

A thermo-hydro-mechanical model for energy piles under cyclic thermal loading

This paper introduces a thermo-hydro-mechanical finite element model for energy piles subjected to cyclic thermal loading. We address four particular features pertaining to the physics of energy piles: three-dimensionality, embedded heat exchangers, soil constitutive modeling and pile–soil interface. The model is designed to capture the strong coupling between all important physical and thermomechanical processes occurring in a concrete pile embedding U-tubes heat exchangers and surrounded by a saturated soil mass. It encompasses solid and fluid compressibility, fluid and heat flow, thermoplastic deformation of soil, buoyancy, phase change, volume change, pore expansion, melting point depression, cryogenic suction and permeability reduction due to ice formation. The model is distinct from existing energy pile models in at least two features: (1) it can simulate the detailed convection-conduction heat flow in the heat exchanger and the associated unsymmetrical thermal interactions with concrete and soil mass; and (2) it can simulate cyclic freezing and thawing in the system and the associated changes in physical and mechanical properties of the soil mass that likely lead to thermoplasticity and deterioration of pile shaft resistance. The performance of the model is demonstrated through a numerical experiment addressing all its features.

Compression load tests on bored and barrette piles founded in gravelly sediments

Compressive tests on piles socketed in gravelly sediments were conducted. Six bored piles and four barrette piles, with or without base grouting, were studied using back-analysis. A spline function was used to fit the measured data for pile load along the depth. Using the fitted spline function and assuming that behaviour at the pile–soil interface is hyperbolic, back-analysis was used to determine the model parameters by calibrating the design value of β (the product of the coefficient of horizontal soil stress and the value of the tangent of the effective angle of shearing resistance of the soil–shaft interface) and comparing the results of back-analysis and the measured data. The results of this comparison revealed that the results of back-analysis were sensitive to the measured data. A back-calculated β value that is, in general, larger than that found by current methods was obtained.

Effect of a ventilated open structure on the stability of bored piles in permafrost regions of the Tibetan Plateau

Temperature is a key factor that affects the adfreeze strength between frozen soil and a structure. Under the influence of climate change, permafrost degradation reduces the bearing capacity of bored piles in permafrost regions. Based on the monitoring results of bored piles at a site in Beiluhe and related simulations, the effect of a ventilated open structure (VOS) on the temperature at the pile-soil interface and the bearing capacity of bored piles was discussed. The results showed that climate warming can significantly reduce the bearing capacity of bored piles. The higher thermal conductivity of concrete than frozen soil had a negligible influence on thermal stability of pile. However, the temperature at the pile-soil interface decrease drastically within a year due to the lower surface boundary temperature beneath a VOS. Numerical simulation indicated that the bearing capacity of bored piles with a VOS increases during the first 25 years and then decreased. While the bearing capacity without a VOS decreases over 50 years. A VOS can improve the stability of bored piles, delay bearing capacity degradation of bored piles for at least 50 years, and significantly reduce the design pile length.

A computation model for pile-soil stress ratio of geosynthetic-reinforced pile-supported embankments based on soil consolidation settlement

The pile-soil stress ratio (PSSR) is the key to the design and construction of geosynthetic-reinforced pile-supported (GRPS) embankments. However, the computation models for the PSSR which takes account of the effect of consolidation settlement of soil among piles (SAP) is not complete. This paper splits the piles and the SAP into multiple small elements for iterative calculation, according to the principle of the collaborative actions between pile, soil, embankment and reinforcement. The calculation method for pile-soil interaction was improved by Davis’ 1D nonlinear consolidation theory, and the hyperbolic load transfer model of pile-soil interface proposed by Wong and Teh. Then, a novel computation model for the PSSR of GRPS embankments was established, under the joint impact of soil arching effect, tension membrane effect and improved pile-soil interaction. Finally, this proposed computation model was proved reasonable by the comparison with tests and other methods, and the effects of key parameters were investigated. The results show that the settlement difference between piles and SAP increased with the degree of consolidation of the SAP and the elapse of time; the PSSR below the reinforcement net (RN) is slightly higher than that above the RN; the PSSRs above and below the RN are both positively correlated with the degree of consolidation and initial bulk compressibility of the SAP.

GEOTHERMAL ENERGY PILE-SOIL INTERACTION UNDER MECHANICAL LOADING AND THERMAL CYCLES

Harvesting shallow geothermal energy for heating and cooling building spaces in winter and summer is considered environmentally friendly and renewable. Recently, geothermal energy piles have been used as heat exchanger elements in ground source heat pump systems to exchange heat with the ground underneath buildings for heating and cooling purposes. However, imposing thermal cycles on such piles may result in possible adverse effects on their structural and geotechnical performance. A comprehensive understanding of the behavior of geothermal energy piles is therefore vital for the successful applications of such systems. This paper aims to investigate the interaction between the soil and geothermal energy pile subjected to a combination of mechanical loading and thermal cycles. Coupled thermo-hydro-mechanical (THM) finite element analyses were carried out on a hypothetical geothermal energy pile using climatic and geological conditions in Winnipeg, Manitoba, Canada. Numerical results were presented in this paper in terms of pile head displacements, strains, and stresses developed in the pile, as well as shaft friction and effective radial stresses along the pile-soil interface. The effects of heating and cooling on the ultimate geotechnical pile capacity were also presented. Based on the numerical results, it was found that the thermo-mechanical loads have considerable effects on the geothermal energy pile responses.

A Parametric Study on the Evolution of Cyclic Clay-Pile Interface Friction for Large Numbers of Cycles

The present experimental study investigates the influence of cyclic displacement amplitude and effective consolidation stress on the evolution of mobilized local shaft friction along piles submitted to large number of cycles (up to 105 cycles). Two-way cyclic displacement-controlled tests were performed on an instrumented pile-probe installed and loaded in a calibration chamber. Tests were performed on reconstituted specimens of saturated clay to examine the shaft friction evolution in the soil-pile interface during cyclic loading. Displacement-controlled static tests were also performed before and after the cyclic loading in order to quantify the influence of cyclic parameters on post-cyclic static response. It was found that the amplitude of cyclic displacement and the initial state of stress have an influence on the evolution of local friction during cyclic loading. The degradation rate of local friction increased for larger cyclic displacement amplitudes whereas with increasing the effective consolidation stress, the degradation rate decreased. The application of displacement- controlled cycles resulted in a modification in the behaviour of the interface. A significant peak of static friction followed by strain softening was observed during post cyclic static tests, which was not the case for pre-cyclic static tests. The peak value of friction obtained upon post-cyclic static loadings found to be more important for higher values of applied displacement amplitude during cyclic loading. Finally, a brief synthesis of the results is given.