Pile End Research Articles

An Iterative Computing Method for Load Effect of Bidirectional Reinforced Embankment

Bidirectional reinforced embankment (BRE) technology, pile supporting in vertical and geosynthetic reinforcement in horizontal, has been widely adopted in engineering practice to control total and/or differential settlement. In design, the load distribution of BRE is essential for determining pile size (i.e., diameter and length), pile spacing, pile cap details (if any), and properties of geosynthetic reinforcement, et al. Several methods have been proposed in previous studies to investigate the load bearing mechanism in the BRE platform. However, resistance of foundation soil is typically neglected, which implies that the load on piles would be overestimated if the resistance of foundation soil is neglected or over weakened. The reason for this overestimation is that larger geosynthetic deflection would be expected without the reaction of foundation soil, which would cause a more significant soil arching and thus more load would be transferred to piles. To minimize this overestimation, a method for analyzing load transfer mechanism in the BRE was proposed, considering the subsoil resistance as well as the existence of neutral plane (i.e., zero skin friction). An equal settlement was assumed for soils at the embankment bottom and the foundation top. The iteration process was adopted to solve equations to determine the load shared by piles and soils. In derivation, the soil arch height in the embankment was calculated instead of being assumed in existing methods. A flow chart was developed to solve equations of the proposed method and two case histories from literatures were used to compose the rationality verification. Compared with the field measurement and numerical calculation, this study can be more effective in calculating loads shared by piles and soils in the BRE for the working condition with better pile end bearing stratum.

Nonlinear analysis of single pile settlement based on stress bubble fictitious soil pile model

AbstractTaking both pile end stress diffusion effect and pile‐soil interface softening effect into account, a new nonlinear method for the vertical settlement of pile is proposed in this paper. In detail, a new model, namely stress bubble fictitious soil pile model, is proposed to simulate the support from the pile bottom soil. The nonlinear soil softening model is introduced to establish the relationship between unit pile shaft friction and pile‐soil relative displacement. Then, a nonlinear calculation method for load‐settlement analysis in multilayered soil is developed with the integration of these two models. The validity and accuracy of the proposed method are verified through the comparisons against experimental and existing simplified methods. At last, a detailed parametric study is conducted to assess the influence of the parameters related to the proposed model and pile‐soil system on the load‐displacement behavior of single pile. The promotion of this method could hugely boost the prediction of the single pile settlement for the preliminary design of pile foundation in layered soil.

The macroscopic and mesoscopic study on strengthening mechanisms of the single pile with raft under pile-soil-raft combined interaction

Previous studies found that the ultimate bearing capacity of single pile with raft (SPWR) is greater than the sum of the ultimate bearing capacities for single pile and raft foundation. However, the mechanism of this strengthening mechanism is still unclear. This paper studies the bearing capacity characteristics of three different foundation models (i.e., SPWR, single pile and raft foundation) through laboratory tests and discrete element method (DEM) simulations. Specifically, in laboratory tests, the analysis of displacement contour map is conducted to explain the macroscale strengthening mechanism. Subsequently, in DEM simulations, the deformation, force chain and stress transmission of three foundation models are examined, aiming to reveal the mesoscopic strengthening mechanism. The results show that the wedge-shaped elastic core is observed under the raft of SPWR, which is a key element in the strengthening mechanism. On the one hand, the elastic core can strengthen the compactness of soil near the pile side and pile end, which improves the lateral friction resistance and the end bearing capacity of SPWR. On the other hand, the elastic core can increase the range of disturbance to the SPWR, which makes a larger range of soil to bear the load.

Analytical modeling for the load-transfer behavior of stiffened deep cement mixing (SDCM) pile with rigid cap in layer soils

A new composite structure named stiffened deep cement mixing (SDCM) piles has recently been considered and applied in many geotechnical engineering projects. The SDCM pile was formed by inserting a stiffened core into the deep cement mixing (DCM) pile that has just finished construction, thereby combining the advantages of DCM piles and concrete piles. This study developed a load-transfer analytical approach for SDCM pile with rigid cap in layer soils from the theoretical solution view. Herein, the nonlinear mechanical behaviors of the DCM pile–soil and DCM pile–stiffened core interfaces were characterized by a tri-linear bond-slip model, and the action of soil on the pile end was simulated by an elastic-perfectly plastic model. Three-dimensional finite element simulations were conducted for SDCM piles embedded in homogeneous soil and double-layer soils. The reasonable matches between the predictions and the results from FE simulations and physical model tests confirmed the applicability of the theoretical approach. The full-range load-transfer behavior and the bearing performance of SDCM piles were further discussed. Parametric studies were finally conducted, providing insights into the design practice of SDCM piles.

A simple method for analyzing thermomechanical response of an energy pile in a group

This study develops a simple method for rapid estimation of the thermomechanical response of an energy pile in a group. Based on the equilibrium condition of the force, the governing equations for a single energy pile are established using the finite difference method. An exponential model is used to describe the relationship between the unit skin friction and pile–soil relative displacement as well as the relationship between the unit end resistance and displacement at the pile end. Based on the Pyke model for the loading–unloading behavior of the pile–soil interface, the incremental relationship between the unit skin friction and pile shaft displacement, and the incremental relationship between the unit base resistance and pile end displacement, are presented in the unloading and reverse loading stages. Considering the mechanical and thermal loads and the reinforcing effect of neighboring piles, a simple method is proposed to analyze the thermomechanical response of an individual energy pile in a group. Comparisons between the measured and computed results of six well-documented cases are presented to verify the reliability of the proposed method.

Experimental Investigation of the Impact of Necking Position on Pile Capacity Assisted with Transparent Soil Technology

Necking in different positions of a pile body has a significant influence on the bearing performance of a single pile. The transparent soil model experiment was adopted to investigate the impacts of necking positions on the vertical bearing capacity of a single pile and the soil deformation around the pile, and subsequently, the causes of the variation of bearing capacity were analyzed. The results show that the existence of necking does not change the bearing behavior of single pile as pile friction resistance. The bearing capacity of piles decreased by 8.21% and increased by 7.30% when the necking is located in the shallow and middle pile body, respectively, while it did not change significantly when the defects were in the deep part of the pile. The necking position has a significant effect on the deformation and deformation range of soil. The soils at the top, side, and near the end of a pile were mainly influenced by shallow necking, middle necking, and deep necking, respectively. The soils at the location of necking had apparent settlement phenomenon as the piles subsided. The soils at shallow necking’s locations were relatively loose, which reduced the side friction force of piles and finally resulted in a reduction of bearing capacity of a single pile. With the increase of the load, the soil around piles gradually developed penetration phenomenon (large deformation), while the soil at the end of pile moved up against pile settlement, which made the necking soil in the middle denser. The loss of side friction resistance was less than the resistance from the necking, which gave birth to an increase in the bearing capacity of a single pile. It was finally found that the side friction resistance and the resistance caused by necking were the main factors governing the bearing capacity of necking single piles.

Technology and application of static drilling rooted geothermal energy pile

As a kind of building energy saving technology utilizing the shallow geothermal energy, energy pile is one of the ways to achieve the goal of reducing carbon emissions during building operation. Considering the shortcomings of construction technology for traditional energy pile, we propose the static drilling rooted geothermal energy pile (SDRGEP) and construction technology based on its characteristic. The short-term and long-term tests of SDREP are carried out by a test pile of a building in Ningbo. The variation properties of pile temperature, coefficient of performance (COP), thermally induced axial load and mobilized shaft resistance are analyzed. The test results show that the pile temperature gradually increases with time. COP value gradually decreases over time and then tends to be constant. Particularly, the performance of thermal axial stress and mobilized shaft resistance are determined by the constraint conditions of pile top and pile end. In the short-term test, the thermal axial stress shows the form of large in the middle and small in both ends, and there is a null point (NP). In the long-term test, the thermal axial stress at the top of the pile are larger. The mobilized shaft resistance is related to the time, temperature and constraint conditions, etc. It is varied gradually from one NP to three NPs along the pile. Therefore, the test results are related to the service state and construction technology of pile, which provides the theoretical basis for promoting the application of the SDRGEP.

Laboratory Tests on Open‐Close Pile Jacking and Load Bearing Characteristics in Saturated Clay Soil

The stress characteristics of the pile‐soil interface have an important influence on the effect and long‐term bearing capacity of jacked piles. In order to obtain the stress characteristics of the pile‐soil interface of the jacked pile in clay, the development law of pore water pressure and earth pressure for open‐ and closed‐end jacked piles during the pile jacking process was studied using indoor model tests based on a saturated soft clay foundation. Test results show that, during the pile jacking process, the excess pore water pressure, earth pressure, and effective earth pressure at the pile‐soil interface all increase gradually along penetration depth, and the increasing trend first increases linearly and then increases sharply at the pile end. The excess pore water pressure at the pile‐soil interface of an open‐end pile is smaller than that of a closed‐end pile. The earth pressure and effective earth pressure at the pile‐soil interface of the closed‐end pile show a “lateral pressure degradation,” and the degradation becomes more and more significant with increasing depth. The pile‐soil interface adhesion coefficient increases with increasing penetration depth, and the greater the adhesion coefficient is, the higher the tightness between the pile and soil will be. As a method of determining the interfacial tightness between the pile and soil, the adhesion coefficient is closely related to the excess pore water pressure and earth pressure generated during the pile jacking process and can accurately reflect the mechanical mechanism of pile jacking in saturated viscous soil. Results of this contribution can provide reference for theoretical research on the mechanical pile jacking mechanism in saturated viscous soil.

Failure Mechanism of the Bearing Stratum at the End of a Pile Induced by Shield Tunnel Excavation Beneath a Piled Building

Planned tunnel excavation beneath piled buildings is common in actual projects. The excavation-induced deformation of the bearing stratum at the end of an existing pile end may decrease the ultimate bearing capacity of the pile, affecting its mechanical properties. Since previous studies on this topic have focused on the pile deformation induced by tunneling, the deformation mode of the bearing stratum at a pile end has not been thoroughly investigated. In this study, a new two-dimensional failure mechanism describing the failure characteristics of a bearing stratum at the end of an existing single pile induced by shield tunnel drilling beneath the pile is established. Based on this mechanism and the kinematic approach, an analytical equation for the failure surface of the bearing stratum during tunnel drilling beneath a single pile is obtained via a variational principle, and the shapes of the rupture surface of the bearing stratum at the pile end are plotted for various parameters. To validate the validity of the method in practice, an analytical expression for the failure surface of the bearing stratum is obtained and compared with numerical simulation solutions. The results of this comparison show that the presented method is effective for studying the failure characteristics of a bearing stratum at a pile end and can be used for predicting these failure characteristics in practice.

Pile Foundation Design and Analysis of Teaching Building in A High School

The foundation design should adhere to the principles of adapting measures to local conditions, using local materials, protecting the environment and saving resources. Based on the geotechnical engineering survey data, the design should be carried out by comprehensively considering the factors such as structure type, material situation and construction conditions. The pile foundation design is adopted in this project. includes the determination of pile end bearing layer and pile length, the determination of vertical bearing capacity of single pile, the calculation of pile number and pile layout form, the checking calculation of pile foundation bearing capacity and settlement, the design and checking calculation of pile body structure, the design and checking calculation of pile cap. Until all requirements of the specification are met.

Investigation on in-situ test of penetration characteristics of open and closed PHC pipe piles

To investigate the load transfer mechanism of open and closed Pre-stressed High-strength Concrete (PHC) piles jacked into layered soil, a full-scale in-situ test was conducted. In this test, the axial stress experienced by one open and two closed PHC pipe piles during jacking into layered soil was monitored using Fiber Bragg Grating (FBG) sensors mounted on pile shaft. The test results showed that (1) The jacking force on pile depended on the soil mechanical properties at the end of pile; (2) The variation of axial force on pile reflected that the compaction effect of soil for closed pile was larger than that for open pile; (3) At the beginning of penetration depth, the variation of shaft and end resistances on the open pile were different from those on the closed piles; (4) The variation of average shaft resistance relied on the mechanical properties of soil at the side of pile.

Corrosion initiation life of laterally loaded PHC pipe piles served in marine environment: Theoretical prediction and analysis

In this paper, a theoretical approach is presented for evaluation of the corrosion initiation life of laterally loaded PHC pipe piles in marine environment. The bending moment of the pile is determined by the p-y method that considers the effects of the nonlinear behaviour of the soil surrounding the pile. Then mechanical analysis in terms of a cross-section of the pile is performed to model the extension and expansion of the original incomplete microcracks under the external lateral load and the internal pre-stress. Considering the propagation of the original microcracks, the equivalent coefficient of chloride diffusion of the region with original microcracks is determined based on the principle of equal flux. The diffusion of the chloride ion in the PHC pipe pile is modeled by a compound radial diffusion model, which is well validated by an ABAQUS model. Extensive parametric studies are conducted to investigate the effects of the lateral load, pre-stress, cover thickness, the width of the original incomplete microcracks and the pile end type on the corrosion initiation life of the pile. The results show that the corrosion initiation life decreases with the increase of the lateral load and the pre-stress play a positive role in inhibiting the diffusion of the chloride ion. The proposed approach properly incorporates the effects of the external lateral load and the internal pre-stress on the coefficient of chloride diffusion, which provides an appropriate and reliable method for predicting the service life of laterally loaded PHC pipe piles in marine environments.

Monitoring and Analysis of Pore and Earth Pressure at the Pile Surface Using Piezoresistive Silicon Pressure Transducers

The piezoresistive silicon pressure transducers based on the piezoresistive effect have demonstrated their potential in the accurate monitoring of pressure. However, their usage in the pore and earth pressure monitoring at the pile surface under hydraulic jacking has not yet been explored. In this study, two self-made model piles (one is a closed-ended pile and the other is an open-ended pile) were instrumented with piezoresistive silicon earth transducers and pore pressure transducers and then driven into the soil using a hydraulic jack. A comprehensive investigation was first carried out for the structure of the model piles, the installation procedure of the transducers, and the composition of the test system. The pore and earth pressure measurements of the transducers were used for the evaluation of the distribution of the pore pressure, excess pore pressure, radial earth pressure, and radial effective earth pressure. The model test results indicate that the piezoresistive silicon pressure transducers are suitable for monitoring the pore and earth pressure at the pile surface during jacking. In addition, the pore pressure, excess pore pressure, radial earth pressure, and effective radial earth pressure along the test piles were affected by the penetration depth and the pile end form.

Model Test of Jacked Pile Penetration Process Considering Influence of Pile Diameter

In order to investigate the influence of different diameters on pile end resistance, pile side resistance, pile axial force and pile force transmission law of jacked pile penetration, two pairs of embedded sensitized microfiber grating sensors were installed by slotting the pile body. The pile-jacking process of static-pressing viscous foundation soil with different diameters of closed-tube model piles was successfully monitored. The test results show that the pile pressure, pile end resistance and pile side resistance of the two test piles increase linearly with the increase of pile depth. When the pile jacks, the final pressure of the test pile TP1 is higher than that of the test pile. TP2 is 31% higher, pile end resistance is 18% higher, and total side resistance is 57% higher. The results show that increasing the pile diameter can significantly increase the pile side resistance; under different penetration depths, the pile side resistance is from top to end. Continuously exerted, the axial force of the pile body decreases with the depth of the pile and the slope of the distribution curve of the axial forcegradually decreases. At the maximum penetration depth, the axial force of the pile TP1 is 18% larger than that of the test pile TP2; As the depth increases, the unit side resistance at the same penetration depth gradually decreases, that is, the side resistance has a “degradation effect”; at the end of the pile jacking, the percentage of the pile end resistance to the pile force exceeds 50%, that is, the pile end resistance bears most of the load. This research can be used as a reference for the study of pile driving mechanism in clayey and layered clayey soils.

Foundations for light structures

Due to developments in soil mechanics, the price of foundations for ordinary structures has decreased to 5 to 10 per cent of the overall price of the building, but it still remains relatively high, sometimes exceeding 30 %. In Estonia, this issue has been relevant for the past years. The first solution was to use wedged piles under agricultural buildings (barns, storage units etc). As a result, the concrete content of foundations decreased 30 to 60 %, and the rate of steel use went up by 50 %. Currently the cooperation between wedged piles and grillage is being researched, with the objective of lowering the price of foundations. ln weak soils, 2-8 m injection piles have been used under private houses, holiday homes and storage structures, but one can save in the cost of foundations that way. Light structures and storage units have often been erected on screw piles (the steel pipe of the pile 100x100x6 filled with concrete, the pile end a steel sheet 15-20 mm thick and (depending on load bearing capacity) 25-60 cm in diameter. These piles can receive pressure as tension and if designed with the cooperation of piles and grillage in mind, the cost of foundations will decrease 50-60%. All of these solutions have been in use in Estonian silt and sands, the screw piles also in weak soils, where they stand on the dense intermediary layers alternating in weak soils.

Field test of earth pressure at pile-soil interface by single pile penetration in silty soil and silty clay

Prestressed high-strength concrete (PHC) pipe pile with static press-in method is widely used in recent years. The earth pressure at the pile-soil interface caused by the process of pile jacking has an important influence on the jacked pile construction and even the bearing characteristics. Based on a construction project in Dongying, the change law of earth pressure at pile-soil interface caused by the penetration of PHC pipe pile is analyzed by monitoring the earth pressure at pile-soil interface during pile jacking, and the evolution characteristics of earth pressure at pile-soil interface in viscous soil foundation are revealed. The results show that the earth pressure at the pile-soil interface at different positions (h/B) caused by the penetration of PHC pipe pile gradually increases with time. In the process of pile jacking, the influence of pile jacking on the earth pressure at the pile-soil interface on the pile body and part that exceeds the relative distance of effective pile end (h/B) is weak. In the process of pile jacking, the earth pressure at the pile-soil interface is greatly affected by the relative height of the pile end (h/B), so it is necessary to consider the effect of the pile length to calculate the earth pressure at the pile-soil interface. The phenomenon of "lateral pressure degradation" exists with the increase of h/B in the earth pressure at the pile-soil interface. The research results have certain engineering guiding significance for construction and bearing capacity determination of jacked pile in viscous soil foundation.

Temperature effects on pile–soil interface behaviour in the case of piles in sand

The effects of increased temperature on the stress–strain response of geothermal piles and supporting soil were studied in this work. Geothermal piles with different pile end supports were investigated numerically using the finite-element software Abaqus. A critical-state-based constitutive model (clay and sand model) was adopted for simulating the elasto-plastic thermomechanical behaviour of soil using two user-defined material subroutines (UMAT and UMATHT). In order to understand the additional stresses and strains in piles and soil due to thermal loading of piles, a set of parametric investigations was carried out for different values of the restraint at the pile base, the relative density of the sand, pile length, pile diameter and the coefficient of lateral earth pressure of the soil. An increased coefficient of lateral earth pressure in the soil at the pile–soil interface was observed due to circumferential thermal radial strains over the pile length. Furthermore, the thermal response of the piles and soil was found to depend on the soil's relative density and the magnitude of thermal load experienced by the piles. Cyclic thermal loading resulted in increased axial stresses in the piles.

STRENGTHENING OF THE FOUNDATIONS OF RENOVATED BUILDINGS WITH INJECTION PILES

The paper describes the technique of strengthening shallow foundations of reconstructed buildings using injection piles. First, the constructive solution of the existing foundations, the structural scheme of the building, as well as the loads transferred to the building structures before and after its reconstruction are established. At the same time, an assessment of the soil conditions of the construction site of the reconstructed building is carried out; a bearing soil layer is revealed for deepening the lower ends of injection piles. Based on the data obtained, the loading of the base of the foundations of the reconstructed building is assessed and the need for their reinforcement (or further operation without reinforcement) is established. In the case of strengthening the foundations of the building, the method of transferring the additional load to the injection piles is selected. Then their bearing capacity and design loads allowed on the piles are substantiated. The construction of foundations is carried out, taking into account their reinforcement with injection piles, which are hereinafter called combined. Verificationcalculations of the base of the combined foundations are performed for the firstand second groups of limit states. In accordance with the regulatory documents, strength calculations of the main structural elements of foundations are carried out, which are necessary to ensure their full operation, taking into account the reinforcement. At the finalstage, working documentation is developed to strengthen the foundations of the reconstructed building. The stages of the design of strengthening the foundations of reconstructed buildings using injection piles presented in the work allow to properly and consistently organize the work of specialists.

The Pile End Mechanism of Jacked Pile in Layered Soils

The research on penetration mechanism of jacked pile has been the focus of pile engineering. However, the majority of literature was limited in scope to homogeneous foundation and the layered effect was commonly ignored. With this consideration, the aim of this paper is to investigate the layered effects on pile end resistance for jacked pile penetration. For this purpose, an indoor model test is conducted to simulate the pile penetration process. In this test, the variation law of the pile end resistance with the penetration depth in different layered soil during the jacking process is revealed and the influence zone of various soil layers on pile end resistance is discussed in detail. Based on the model test, the discrete element numerical simulation of pile driving process is performed in layered soils and results are compared with experimental data. Results from this study show that the zone of influence in different soil layers can be influenced by soil properties, such as strength and stiffness. When the pile end is outside the influence zone of soil layers, the pile end resistance infinitely approaches that of homogeneous foundation at the same depth. Besides, there are approximate linear relations between the ratio of zone of influence in soil layer to pile diameter ( $$Z_{{\text{s}}} /D$$ , $$Z_{{\text{w}}} /D$$ ) and the minimum value of pile end resistance ratio ( $$\eta_{\min }$$ ). Numerical simulation results are well consistent with the model test results. The research results are helpful to estimate the variation law of pile end resistance in layered foundation and have guide significance for the design and construction in jacked pile engineering.

Thermomechanical behaviour of an energy pile–raft foundation under intermittent cooling operation

This paper presents two in situ experimental investigations of the influence of intermittent and continuous cooling operation on thermal and mechanical responses of a foundation. The experiment was conducted with 16 h of cooling operation and 8 h of rest (C-16 mode) for 46 days and with 24 h of cooling operation (C-24 mode) for 32 days. A lower number of daily operating hours promoted natural ground thermal recovery and led to higher heat exchange rates with lower thermal loads on the foundation. The constant heat exchange rate in C-16 was approximately 115 W/m, which was 53.3% higher than the constant heat exchange rate at the same time in C-24. The pile temperatures at all depths in C-16 were decreased by 29.0–36.8% compared to those in C-24. Thermally induced stresses of the pile and raft in C-16 were 25.8–35.7% and 85.9% lower than the thermally induced stresses of the pile and raft in C-24. The mechanical responses of the energy pile in C-16 were reversible. During the cyclic cooling and recovery of C-16, stable changes in axial strain and stress with pile temperature were observed over further cycles, without significant variations in pile end displacement and the vertical stress at the pile toe.