Model Piles Research Articles

Influence of Soil Excavation on Bearing Behavior of Pile Group Foundation Composed of Underpinning Piles and Existing Piles

Abstract The behavior of underpinning piles during the soil excavation is critical for constructing new basements under the existing structures in urban areas; however, none of the existing studies has systematically evaluated their performance and interaction with existing piles under loading. This study aims to experimentally evaluate the impact of soil excavation on the bearing capacity of pile group foundation composed of underpinning piles and existing piles under loading. A model pile foundation with four model piles, one pile cap, and retaining structures was first constructed in our laboratory. Different soil excavation depths at 10 cm, 20 cm, and 30 cm (soil type: clayey silt) were used to explore their influence on the pile side friction, the axial force of the pile shaft, and pile settlement. The results were compared with the bearing behavior of the pile group foundation composed of four model piles and five underpinning piles at different pile lengths (500, 600, and 700 mm) and diameters (5, 10, and 15 mm). The results showed that soil excavation activity could immediately activate the different interaction responses between piles and soils and cause foundation settlements. The underpinning piles with sufficient length (more than 67 % of the existing model pile length) and diameter (more than 25 % of the existing model pile diameter) as well as proper pile’s embedded depth significantly reduced the foundation settlement, particularly at high soil exaction depth. The pile-soil interaction mechanism caused by soil excavation is revealed. The work could bring insights to the underpinning pile design for basement excavation beneath existing buildings.

The decrease of the bearing capacity of single pile foundation during earthquake on liquefiable soil (Study Case: Birobuli Area, South Palu)

Birobuli is located South Palu, Indonesia. In Indonesia, earthquakes are frequent events. Consequently, it is crucial to consider the safety of building structures against earthquakes when calculating the factor of safety of the building. It is important to not only analyse the impact of earthquakes on the superstructure but also take into account their effect on the substructure. One potential substructure failure caused by earthquakes is soil liquefaction, which refers to a decrease in soil bearing capacity resulting from seismic activity. To assess this phenomenon, a quantitative approach was employed in this study, involving soil investigations and sampling from the field. The findings from the soil investigation were used to calculate the susceptibility of soil liquefaction in different soil layers and the axial bearing capacity of single pile foundations both before and during earthquake and liquefaction. The study site predominantly consisted of silty sand with ground water table (GWT) level at -4m with low CPT value, making it susceptible to liquefaction from depths ranging between -4 meters and -12 meters. Two single pile models were simulated in the study, with respective reductions in their axial pile bearing capacity of 71,6% and 38,9%

A new p-y model for soil-pile interaction analyses in cohesionless soils under monotonic loading

The most widely used analysis method for the laterally loaded pile problem is the Winkler spring approach. Although researchers have proposed nonlinear formulations for the p-y curves, the contribution of the soil nonlinearity has not been thoroughly studied. The main drawback of the current approach is the use of a single stiffness in the p-y formulation. This study investigates the laterally loaded pile problem by employing the pressure-dependent hardening soil model with small-strain stiffness (HS-Small Model), where the degree of soil nonlinearity is better integrated. The parametric analyses are performed on the verified model for various pile and soil properties. A new p-y model is proposed for pile behaviour under monotonic loading based on the numerical analysis results. The model includes the initial stiffness, ultimate soil resistance, and degree of nonlinearity parameters. The validity of the proposed model is demonstrated by simulating a centrifuge and two field tests from the literature. The proposed model accurately accounts for soil nonlinearity and significantly improves the estimation of lateral displacements.

Research on energy storage charging piles based on improved genetic algorithm

Aiming at the charging demand of electric vehicles, an improved genetic algorithm is proposed to optimize the energy storage charging piles optimization scheme. Firstly, the characteristics of electric load are analyzed, the model of energy storage charging piles is established, the charging volume, power and charging/discharging timing constraints in the charging process are considered, and the optimization is carried out by the improved genetic algorithm, and the profit of charging piles and user charging electricity charges are calculated, so as to obtain the possible optimal parameters, as well as the impact of the energy storage system on the stability of the power grid. Secondly, the analysis of the results shows that the energy storage charging piles can not only improve the profit to reduce the user’s electricity cost, but also reduce the impact of electric vehicle charging on the power grid load.

A practical lateral spreading pressure model for piles in inclined liquefied ground

Liquefaction-induced lateral spreading is a main cause of pile failure during earthquakes because it places considerable soil pressure on piles. The lateral spreading pressure originates not only from liquefied soil but also from nonliquefied soil when it overlays a liquefied soil layer. In this study, we develop a practical lateral spreading pressure model for pseudostatic analysis to analyze pile response in inclined liquefied ground. The model comprises two parts. First, using a laminar flow model, the lateral spreading pressure of liquefied soil is modeled as the equivalent fluid flowing pressure. The coefficient of viscosity of the liquefied soil is constructed on the basis of shaking table tests in the literature. The normalized lateral spreading pressure depends on the liquefied soil thickness-to-pile diameter ratio. Second, for nonliquefied soil, individual p–y curves for uphill and downhill soils are used to simulate their distinctive soil reaction mechanisms. The uphill soil is subjected to lateral spreading soil displacement, whereas the downhill soil is subjected to pile displacement. This approach can elucidate the net soil reactions of the downhill and uphill soils that vary depending on pile stiffness. The proposed model is validated through simulations involving 1-g shaking table tests, centrifuge tests, and case histories reported in the literature.

A Study on the Softening Shear Model of the Energy Pile–Soil Contact Surface

In this paper, a finite element numerical model of thermal-hydro-mechanical of energy piles under multi-layer geological conditions was established, and field tests of ultra-long energy pile (1000-mm-diameter, 44-m-long) were carried out to reveal the temperature distribution and mechanical properties of energy pile under typical working conditions. Based on the analytical results, a softening shear model of the energy–soil interface under the condition of large shear displacement was proposed with the load transfer method, and the reliability of the model was verified. The model can simulate the shear–displacement relationship of the pile–soil interface under different geological conditions.

Experimental and Numerical Simulation Study of Water Infiltration Impact on Soil-Pile Interaction in Expansive Soil

A laboratory model of a single pile embedded in Nanyang expansive soil and subjected to water infiltration is applied in this study to examine the interaction between the expansive soil and pile foundation upon water infiltration. The soil matric suction decreases as a result of the rising soil-water content. The amount of soil ground heave reaches its peak of 10.7 mm after 200 hours of water infiltration. As matric suction decreases, pile shaft friction also declines, which causes more of the load at the pile head to be carried by the pile base resulting in more pile settlements. A new numerical simulation method is provided to simulate this issue by coupling the subsurface flow, soil deformation, and hygroscopic swelling to investigate the expansive soil-pile response upon water infiltration. From the numerical simulation model, hygroscopic strain arises as a result of elevated moisture levels resulting from the entry of water, and due to ground heave and the mobilization of lateral soil swelling, the shear stress at the interface between the soil and the pile gradually increases over time. It reaches its maximum value of 4420 Pa at upper depths around 200 hours after the infiltration. The comparison between the lab model testing data and the numerical model results demonstrates a good level of concurrence.

A bounding-surface-based cyclic “p–y+M–θ” model for unified description of laterally loaded piles with different failure modes in clay

The increasing turbine sizes have necessitated monopiles in soft clay to have larger diameter and rigidity, from early design of flexible piles to recent semi-rigid piles, with future anticipating rigid piles. Existing few cyclic soil–pile interaction models are developed for flexible pile associated with full-flow failure (above the rotation point, RP), with little attention paid to semi-rigid and rigid piles involving rotational-shear failure (below RP). This study aims to unify the description of piles with varying rigidity by proposing a cyclic two-spring model, where lateral resistances above and below RP are described with cyclic p–y and M– θ springs, respectively. It naturally recovers to a cyclic p–y model for flexible piles. The cyclic p–y and M– θ formulations are developed within the bounding-surface plasticity framework, based on numerical results of cyclic soil–pile interaction concerning full-flow and rotational-shear mechanisms, respectively. These numerical analyses are performed using a cyclic plasticity clay model developed and implemented numerically in this study. The cyclic “ p–y+M– θ” model quantitatively reproduces experimental results of cyclic shakedown and ratcheting for flexible, semi-rigid, and rigid piles. Ignorance of the M–θ spring could underestimate cyclic resistance of rigid piles by 25%, suggesting the model’s merit in reducing conservatism for monopiles in feature designs.

Multi-objective optimization of a pipe energy pile with heat exchanger pipes subjected to chaotic advection

Shallow geothermal energy extraction through an energy pile system relies on the constant temperature available below the earth's surface at shallow depths. The energy pile system transfers the heat stored in the earth to a place of utility for space heating and cooling through fluid circulation, thus extracting geothermal energy. In this study, a 3D energy pile model comprising a helical heat exchanger pipe (HHEP) was developed and validated with the existing experimental data using a FEM-based software package. Further, it was hypothesized that replacing HHEP with a chaotic configured helical heat exchanger pipe (CHEP) would enhance thermal performance. The numerical analyses supported the hypothesis, where there was about a 15 % increase in heat release rate for energy piles containing CHEP. The above results form a basis for exploring the simulation results through laboratory scale model tests. Thus, an experimental setup was fabricated to conduct geothermal energy tests on a scaled pipe energy pile (PEP) model. The investigation was carried out by considering three design-input parameters: flow rate, pitch, and inlet temperature at three different levels with water and ethylene glycol as heat exchanger fluid (HEF) in a 4:1 ratio. The heat release rate of the PEP (Qp), heat flux per unit length of HEP (qpl), soil excess temperature (θs), total thermal resistance (TTR), and energy consumed (EC) were analyzed. Furthermore, multi-objective optimization using the MO-Jaya algorithm and multiple attribute decision-making (MADM) R-method were used to predict an optimal set of design-input parameters. Finally, a confirmation test was conducted to test the competence of the predicted parameters. The percentage variation between the predicted design input parameters and the experimental values was below 10 %.

Seismic Failure Mechanisms of Concrete Pile Groups in Layered Soft Soil Profiles

So far, little attention has been paid to the investigation on the seismic failure mechanisms of flexible concrete pile groups embedded in the layered soft soil profiles considering the material non-linearities of soil and concrete piles. The purpose of this study is to investigate seismic failure mechanism models of flexible concrete piles with varied groups in silt layered loose sand profiles under horizontal strong ground motions. Three-dimensional finite element models of the pile–soil interaction systems, which include nonlinearities of soil and concrete piles as well as coupling interactions between the piles and soil, were created for Models I, II, and III of the soil domains, encompassing 1x1, 2x2, and 3x3 flexible pile groups with diameters of 0.80 m and 1.0 m. Model I consists of a homogenous sand layer and a bedrock, Models II and III are composed of a five-layered domain with homogeneous sand and silt soil layers of different thicknesses. The linear elastic perfectly plastic constitutive model with a Mohr–Coulomb failure criterion is considered to represent the behavior of the soil layers, and the Concrete Damage Plasticity (CDP) model is used for the nonlinear behavior of the concrete piles. The interactions between the soil and the pile surfaces are modeled by defining tangential and normal contact behaviors. The models were analyzed for the scaled acceleration records of the 1999 Düzce and Kocaeli earthquakes, considering peak ground accelerations of 0.25 g, 0.50 g, and 0.75 g. The numerical results indicated that failure mechanisms of flexible concrete groups occur near the silt layers, and the silt layers have led to a significant increase in the spread area of the damaged zone and the number of damaged elements.

Pile Driving and the Setup Effect and Underlying Mechanism for Different Pile Types in Calcareous Sand Foundations

The mechanical response and deformation characteristics in calcareous sand foundations during pile driving and setup were studied using model tests combined with the technical methods of tactile pressure sensors and close-range photogrammetry. Different types of piles were considered, including a pipe pile, square pile and semi-closed steel pipe pile. The test results show that during pile driving, the pile tip resistance of different piles increases with an increase in the pile insertion depth, and an obvious fluctuation is also obtained due to the particle breakage of the calcareous sand and energy dissipation. Different degrees of particle breakage generated by different type piles make the internal stress variations different, as with the pile tip resistance. The pile tip resistance of model pile A, which simulates a pipe pile, is the highest, followed by model pile B, the simulated square pile. Model pile C, which simulates a semi-closed steel pipe pile, has the smallest pile tip resistance because its particle breakage is the most obvious and the pile tip energy cannot be continuously accumulated. The induced deformation such as sag or uplift on the surface and the associated influence range for the calcareous sand foundation are the smallest for model pile C, followed by model pile B and then model pile A. Model pile A has the most obvious pile driving effect. During the pile setup process after piling, the increase in the total internal stress of model pile B is the largest, and the improvement of the potential bearing capacity is the most obvious, followed by model pile A and model pile C. During the pile setup, the induced uplift deformation in pile driving is recovered and the potential bearing capacity increases due the redistribution and uniformity of the vertical and radial stress distributions in the calcareous sand foundation. Considering the potential bearing capacity of different model piles, the influence range of pile driving, foundation deformation and the pile setup effect, it is suggested to use a pointed square pile corresponding to model pile B in pile engineering in calcareous sand foundations.

Feasibility assessment of implementing energy pile-based snowmelt system on a practical bridge deck in diverse climate conditions across China

The accumulation of snow on roads poses substantial challenges to traffic flow, incurring profound human safety concerns and considerable financial losses. To address this issue, a cost-effective and environmentally friendly solution is presented in the form of an energy pile-based snowmelt system. This research provides a comprehensive framework, which integrates the principle of bridge deck energy balance and numerical models of energy pile and practical bridge deck, aiming to evaluate the feasibility of the system in diverse climatic zones across China. First, the long-term climate and geography data pertaining to the snowmelt heat flux demand for the investigated climate regions are gathered and correlated. The Pearson Correlation Coefficients (PCCs) of each parameter (99 % confidence interval) elucidate the strongest correlation between ambient temperature and heat flux demand. Employing the proposed framework, the subsequent feasibility assessment demonstrates that the system's snow-melting effect varies with regional climate and snowmelt targets, and when real-time snow-melting is nonessential (snow-free area ratio, Ar, with values of 0 and 0.5), the system is feasible in most cities except Mohe, where the ambient temperature (Ta) reaches −28.74 °C and the snowfall rate water equivalent (s) is 1.56 mm/h. However, the system fails to meet the real-time snow-melting goal (Ar = 1) when it is located in extreme climate (Ta < −17 °C or s > 1 mm/h). The findings also reveal that snowmelt systems exhibit consistent behavior within the same climate zone. The study provides valuable insights into the practical application of energy pile-based snowmelt systems in different climate conditions, and highlights the critical role of climate considerations in the successful implementation of such innovative technologies.

Study of air flow and heat transfer in soybean piles based on CT

The airflow within the grain pile is directly influenced by the 3D structure of the pores. The airflow distribution will further induce temperature changes within the grain pile, ultimately affecting the safety of grain storage. A quantitative analysis of the porosity, pore size and fractal dimension of the soybean grain pile is performed using computed tomography (CT). Then, the geometric modeling of the soybean grain pile is established by iterative reconstruction technology. Finally, the distribution of airflow and temperature fields in soybean grain piles is analyzed by finite element numerical simulation. It is found that the pore equivalent diameter, throat diameter and throat length are mainly within the 2–5 mm, 0–2 mm, and 3–6 mm, respectively. The results of the numerical simulation of the pressure drop of soybean grain piles are well agreed with the Nemec formula and the measurement values. Pressure diminishes along the airflow direction, with a substantially greater drop observed in vertical ventilation compared to horizontal ventilation. The airflow traverses the channel with a larger radius, while the velocity in a single pore gradually diminishes from the center to the wall. The intricate features of the pore structure significantly influence the distribution of airflow within the grain pile. This phenomenon results in an imbalance of heat transfer within the grain pile, thereby manifesting as discernible heterogeneity in both velocity and temperature distributions. The relevant research methods and conclusions can provide theoretical support and guidance for the multifield coupling theory of grain storage and grain storage safety research.

Dynamic response of a large-diameter end-bearing pile in permafrost

Vertically dynamic model of a large-diameter pile in frozen soil is established, in which the frozen soil is described to a saturated frozen porous media, and the large diameter end-bearing pile is simplified to a one-dimensional rod considering the influence of the transverse inertia effect. Analytical solutions of the longitudinal coupling vibration between the end-bearing pile and the frozen soil are obtained using Helmholtz decomposition and variable separation methods in the frequency domain. By comparing the dynamic responses of the longitudinal vibration of the large diameter end-bearing pile with the traditionally one-dimensional pile, as well as the impedance factor of the frozen soil layer induced by the pile vibration, these demonstrate the influence of the transverse inertia effect on the high frequency vibration of large diameter pile is significant, and the influence on the pile with a smaller slenderness ratio is larger. The temperature and the Poisson’s ratio also have significant effects on the vertical vibration of large diameter piles in frozen soil, which cannot be ignored in the analysis.

Numerical modeling of open-ended piles

Deep foundations are generally used when significant loads are applied, and the site conditions do not allow for the implementation of soil reinforcement processes. This research focuses on studying the behavior of piles in a frictional environment, used as foundations for offshore structures in dense sands, involving anchor depths and overloads significantly higher than those encountered in onshore applications. The bearing capacity of open-ended driven piles plays a crucial role in this study. Therefore, a series of tests were conducted in the literature on model piles in a calibration chamber, which is considered a tool for physical modeling, allowing for significant penetration of instrumented model piles under confinement conditions similar to those experienced by real piles. Emphasis is placed on predicting the ultimate load capacity of open-ended piles using numerical methods. Calculations were performed using the finite element code PLAXIS to numerically reproduce the same shear stresses as those measured on the model during various phases of the experiment. The approach aims to validate the documented experiments in the literature and compare the ultimate load capacity of an open-ended pile to that of a closed-ended pile. Our study is limited to the case of a single pile, subject to static and axial force. The results show that it is possible to achieve a good agreement between the experiment and the numerical modeling with a relatively simple model, provided that the soil parameters are chosen correctly and the interface stiffness is adequately simulated.

An innovative experimental device for characterizing the responses of monopiles subjected to complex lateral loading

Offshore wind turbines are usually founded on monopiles. During the operation period, the structure is subjected to complex lateral loading from wind, wave and current. The soils surrounding those monopiles may deform with increasing the number of loading cycles, leading to tilting of the whole structure; hence, it is vital to carry out physical model tests to examine the long-term performance of monopiles. This study proposes an innovative experimental setup for centrifuge modelling of the response of monopiles under complex lateral loading. Hydraulic actuators are adopted to apply lateral loads on model pile, and electrohydraulic servo-valves and associated controllers are used to achieve a closed loop position or load control. A carefully designed spherical hinge and load bars are used to connect the model pile and actuator shafts. This enables that the pile can rotate freely and can move vertically freely. A centrifuge test on a winged monopole subjected to perpendicular lateral loading was carried out at 100g. The experimental results shed light on pile responses in the cyclic loading and constant loading directions.

Numerical modelling of under-reamed scaled-down piles by water jet driving

Water jet pile driving technique has been shown to be viable for driving precast piles in highly resistant soil layers. However, the use of this technique drastically reduces the pile load capacity. On the other hand, the use of under-reamed precast piles improves the vertical load capacity. The objective of the present paper is to show the efficiency of the use of under-ream, through the numerical modelling of load tests carried out in laboratory scaled-down models. For numerical modelling, finite element method (FEM) was used. Through numerical analysis, it was possible to identify the distribution of stresses and strains at the toe, shaft, and under-reams. With that it was possible to identify and check the contribution of these in the total vertical compressive load capacity. It was possible to verify that the under-reams contribute to the vertical load capacity varying from 47% to 57% depending on the configuration, number, and distribution of the underreams along the pile. Thus, increasing the final vertical load capacity, if compared to piles without under-ream (uniform shaft). Numerical modelling proved to be a fundamental tool, which made it possible to show the mechanisms involved in the action of under-ream in increasing the vertical load capacity of piles.

Experimental investigation of model piles subject to lateral load in cohesionless soil

In this research work, the way piles behave in sandy soil and are exposed to lateral forces was examined experimentally by using several lab examinations. These tests were performed on a model of steel piles, including a closed-ended circular pile, a closed-ended square pile, and an H pile. The outcomes of the experimental tests were presented as load-displacement diagrams to examine how pile length and shape affected lateral load capability. Moreover, along the length of the pile, the bending moments were obtained and presented as relationships between the bending moment and the pile length. Therefore, it can be said that piles' capacity to support lateral loads rises with increasing pile length for each of the study's pile shapes. Additionally, it finds that the H-pile has a greater lateral load capacity and maximum bending moment than other piles.

Experimental Evaluations of Model Piles Subject to Inclined Loading in Sand

The structures that subjected to varying loads are supported by pile foundations. These loads consist of inclined and vertical loads. The main objective of this work is to experimentally investigate the effects of an inclined load on the lateral pile reaction and axial pile displacement for piles found in cohesionless soil. For this purpose, the slenderness ratios of model steel piles are varied by values of 11, 14, and 17. Moreover, the pile shapes involved in this study are H-pile, closed end circular and square. The experimental tests were conducted in sandy soil. However, the load inclination angle included in assessing the pile behavior were 0, 30, 45, and 60 from the vertical. In order to study the above parameters on the behavior of model pile, thirty-six experimental tests were carried out. Several findings indicated that the lateral pile deflection was highly influenced by increasing the load inclination. Further, the load inclination effect on the pile's ultimate bearing capacity is high which leads to a reduction in the pile's capacity. Additionally, the vertical displacement of pile is less than horizontal displacement when the pile is subjected to an inclined load of 45° and the model fails by horizontal displacement.

Investigation of laboratory and field tests of piles installed by displacement technology

When compared to conventional pile systems, the Drilled Displacement System (DDS) and Continuous Flight Auger (CFA) piles offer advantages in terms of improved load-carrying capacity, quicker installation, and less spoil generation. In this article, experiments on miniature piles built utilizing the Drilled Displacement System (DDS) and Continuous Flight Auger (CFA) technologies in a soil-filled test tank are described. For the study, model piles with dimensions of 20 mm in diameter and 300 mm in length were used, scaled at 1/20. The model piles are subjected to static loading, and throughout each phase of the loading process, the relevant displacements are measured. For both DDS and CFA piles, the analysis's findings on load-settlement curves and estimates of ultimate bearing capacity were compared. According to the study, the DDS piles outperformed the CFA piles in terms of load-carrying capacity. Additionally, full-scale field tests with static loads were performed on DDS-drilled piles with dimensions of 400 mm in diameter and 6 m in length. The field test's load-settlement response exhibits good agreement with the model testing. Overall, the study's findings offer insightful information about the behavior and performance of DDS piles that may be used to improve the design and installation of these piles in various soil types.