Pile Shaft Research Articles

Transient Horizontal Response of a Pipe Pile in Saturated Soil with a Flexible Support at the Pile Head

This study examines the horizontal transient response of pipe piles in saturated soil, assuming a two-stage equivalent linear relationship between the bending moment and the rotation at the pile head. The potential function is introduced, and the three-dimensional wave equation for saturated soils is decoupled using operator decomposition and the method of separation of variables. By applying the appropriate initial and boundary conditions, the horizontal lateral forces on the pile from both the surrounding soil and the soil within the pile shaft are calculated. The pipe pile is modeled as a Timoshenko beam. Continuity conditions at the pile–-soil interface are applied, and a time-domain solution for horizontal transient vibrations is derived. This solution accounts for various pile top constraint conditions and is obtained through the inverse Laplace transform. Validation against existing results demonstrates the accuracy of the proposed model. Finally, a parametric study investigates the effects of factors such as impact load, permeability, pile diameter, and pile head constraints on pile displacement.

Effect of Different Static Load Test Methods on the Performance of Combined Post-Grouted Piles: A Case Study in the Dongting Lake Area

To investigate the effect of combined end-and-shaft post-grouting on the vertical load-bearing performance of bridge-bored piles in the Dongting Lake area of Hunan, two post-grouted piles were subjected to bi-directional O-cell and top-down load tests before and after combined end-and-shaft grouting, based on the Wushi to Yiyang Expressway project. A comparative analysis was conducted on the bearing capacity, deformation characteristics, and load transfer behavior of the piles before and after grouting. This study also examined the conversion coefficient γ values of different soil layers obtained from the bi-directional O-cell test for bearing capacity calculations. Additionally, the characteristic values of the end bearing capacity, obtained from the bi-directional O-cell and top-down load tests, were compared with the values calculated using the relevant formulas in the current standards, which validated the accuracy of existing regulations and traditional loading methods. The results indicate that the stress distribution along the pile shaft differed between the two test methods. In the bi-directional O-cell test, the side resistance developed from the end to the head, while in the top-down load test, it developed from the head to the end. After combined post-grouting, the ultimate bearing capacity of the piles significantly increased, with side resistance increasing by up to 81.03% and end resistance by up to 105.66%. The conversion coefficients for the side resistance in silty sand and gravel before and after grouting are 0.86 and 0.80 and 0.81 and 0.69, respectively. The characteristic values of the end bearing capacity, as measured by the bi-directional O-cell and top-down load tests, were substantially higher than those calculated using the current highway bridge and culvert standards, showing increases of 133.63% and 86.15%, respectively. These findings suggest that the current standard formulas are overly conservative. Additionally, the measured values from the top-down load test may underestimate the actual bearing capacity of piles in engineering projects. Therefore, it is recommended that future pile foundation designs incorporate both bi-directional O-cell testing and combined post-grouting techniques to optimize design solutions.

An analytical solution for settlement of pile-supported reinforced low embankment considering lateral friction along pile shaft

An analytical solution for settlement of pile-supported reinforced low embankment considering lateral friction along pile shaft

Numerical modelling of pile jacking in highly sensitive clays

This paper presents large deformation finite element (FE) modelling of penetration of solid cylindrical piles into highly sensitive soft clay. The simulations are performed using a Coupled Eulerian–Lagrangian (CEL) FE modeling technique. The pile is penetrated at a constant rate, and the analyses are performed for undrained conditions to simulate the response during penetration. The soil model considers the effects of strain softening and strain rate on the undrained shear strength. The FE-calculated results are compared with available analytical and numerical solutions for idealized soil profiles. Simulations are also performed for two instrumented piles previously installed into highly sensitive clay at Saint-Alban in Québec, Canada. The installation-induced changes in stresses, degradation of undrained shear strength, and tip resistance obtained from FE analyses are consistent with the field test results. Large plastic shear strains develop near the pile, which can significantly remould the soil near the pile shaft. A parametric study shows that a quicker post-peak degradation of undrained shear strength of highly sensitive clay creates a smaller zone of high plastic shear strain near the pile, while the plastic zone is wider for low- to non-sensitive clays.

Research on Load Transfer Mechanism of Pre-Stressed High-Strength Concrete Nodular Pile Embedded in Deep Soft Soil

The pre-stressed high-strength concrete (PHC) nodular pile is a type of PHC pile with a variable cross-section of the pile shaft, and it has normally been applied in ground treatment projects in recent years. The PHC nodular pile shaft consists of nodules, which introduce differences for the load transfer mechanism of the PHC nodular pile compared to the conventional PHC pipe pile. In this paper, the load transfer mechanism and influencing factors of the bearing capacity of the PHC nodular pile were investigated based on a group of field tests and numerical simulations. The following conclusions were obtained based on the analysis of the field test and simulation results: the nodules along the pile could effectively increase the ultimate capacity of the PHC nodular pile, and the field test results showed that the ultimate capacity of 450 (500) mm PHC nodular piles was about 1.23–1.38 times of the 450 mm PHC pipe pile after being cured for 40 days, which can be used for the design of PHC nodular pile. The simulation results showed that the bearing capacity of the PHC nodular pile would decrease with the increase in nodular spacing and nodular length along the pile shaft, while increasing with the increase in nodular diameter, and the diameter of the nodule can be increased moderately to improve the ultimate capacity of the PHC nodular pile.

Research and application of a new method for axial force test of prestressed high-strength concrete pipe piles

In order to figure out actual shaft resistance, axial forces of pile shaft have to be calculated based on strain test, which is a challenge for heat and steam cured prestressed high-strength concrete pipe piles. Disadvantages of three common methods of strain test and the empirical linear equation between axial force and strain are analyzed. A new strain test method, namely the slot-forming wood method, is proposed and applied in laboratory and field test. Taking pipe pile PHC500AB-125 as an example, laboratory tests were performed to establish the nonlinear function between axial force and strain. Field tests were performed and strains were tested by the slot-forming wood method. Results calculated by the nonlinear function between axial force and strain were compared with that by traditional linear equation to reveal the difference. The tests indicated that the new method of strain test was applicable and useful since the survival rate of strain gauges was high and the accuracy has been improved. Overcoming the disadvantages of the traditional linear equation, the proposed function between axial force and strain is more accurate. The study provides theoretical basis for technology innovation in pile test and for revision of related technical standards.

Impact-Driven Penetration of Multi-Strength Fiber Concrete Pyramid-Prismatic Piles

The article focuses on studying the impact-driven penetration of multi-strength fibroconcrete pyramid-prismatic piles. The research object includes multi-strength pyramid-prismatic piles with varying types of reinforcement and different levels of concrete compressive strength. The aim of the study is to experimentally investigate the enhancement of pile impact resistance through the differentiated selection of concrete strength based on the dynamic stresses in the pile shaft caused by impact forces. As a result of the experimental studies on the piles, it was found that the difference in energy costs for driving them does not exceed 3.7–4.1%, proving the insignificant influence of the type of reinforcement and fiber concrete strength on the energy expenditure during driving. At the same time, it was established that the type of reinforcement and the type of fiber significantly affect the strength and impact-resistant properties of the pile shaft, ensuring defect-free driving. For example, the defectiveness (e.g., chips, cracks, potholes, spalling) of the head of the steel fiber concrete (SFC) pile reaches 57.5%, while for the polypropylene fiber concrete (PFC) pile it does not exceed 5.2%, demonstrating the advantages of using polypropylene fiber under impact conditions.

Analysis of the screw pile group subjected to combined V–H loading for energy application

ABSTRACT The present study investigates the behaviour of screw piles in very stiff clay under the combined vertical and horizontal loads using a three-dimensional finite element analysis. The soil is modelled using the Mohr–Coulomb elastic, perfectly plastic material, whereas the pile shaft and helices are modelled as a linear elastic material. This study examines the effect of the number of helices, helix diameter, inter-helix spacing, and pile spacing on the performance of screw piles under both vertical compression and lateral loads. The study shows that the effect of the number of helices is negligible under the combined load. The increase in the helix diameter (D h) improves the pile performance. The results also reveal that the optimum interhelix spacing is 3D h, and the optimum pile spacing is 2D h. Thus, the present study demonstrates that optimizing the geometry of the screw pile enhances its performance, making it suitable for offshore wind turbines.

Theoretical investigation of the resistance effect of embedded isolation piles on tunneling-induced lateral ground movements

This paper presents an isolation pile–soil lateral interaction model that considers not only the relative linear sliding at the pile-soil interface but also the pile group–soil interaction. The model allows for the calculation of the pile–soil interaction force using the elastic continuum method combined with the displacement compatibility relationship at the pile–soil interface, thus enabling the total lateral ground displacement to be obtained. The proposed method is validated by comparisons with the elastic foundation method (EFM) and the boundary element method (BEM). The advantages of the proposed method in calculating the lateral ground deformation resisted by isolation piles are demonstrated. The mechanical mechanism of the isolation pile’s lateral resistance effect on ground deformation can be summarized as follows: the combination of positive and negative resistance effects drives the lateral ground displacements along the depth direction from the original inhomogeneity caused by the tunnel excavation to a relatively homogeneous state. The soil parameters including Poisson’s ratio, internal friction angle and ground volume loss; the isolation pile parameters including the distance of the tunnel axis from the pile axis, the pile length, the buried depth of the pile top and the relative pile bending stiffness; and isolation pile–soil interface parameters including relative pile shaft–soil stiffness and relative pile tip–soil stiffness all exert a significant influence on the lateral resistance effect of the isolation pile. In light of that the resistance effects at different depths below the surface are not uniform, it is of the utmost importance to assess the resistance performance of the isolation pile in accordance with the location and lateral deformation requirements of the protected structure in actual engineering.

Editorial Note, Issue 2, Vol 18 – 2024

This series of papers feature a very practical array of applied research studies and case histories with findings to advance the deep foundation industry. Volume 18 Issue 2 includes interesting numerical 3D studies on the performance of onshore wind turbine foundations written by an author team affiliated with the Southwest Jiaotong University in Chengdu, China. This issue also highlights papers focused on a practical study of driven steel piles in sandy soils; critical insight into better characterizing the residual strength in p-y curves for rock-socketed pile shafts, particularly the Strong Rock (SR) p-y curves commonly used in practical applications; design aids for selecting steel HP sections for driven piles, based on the AISC Specification 360-22 for Structural Steel Buildings; and an investigation of the behavior of walls reinforced by screw and grouted nails in both sandy and clayey soils through 3D numerical modeling with Abaqus.

Characterizing residual strength of p-y curves for rock-socketed pile shafts

Rock-socketed pile shafts are an effective choice of foundation for heavy structures (e.g., bridges, wind turbines, etc.). One of the primary challenges in the design of rock-socketed shafts for lateral loads is the characterization of residual strength in p-y curves in rock. For example, in the Strong Rock p-y curves that are commonly used in practical applications, the assumed brittle response of fractured rock results in nearly zero residual strength. However, there is no experimental evidence for this drastic drop in the residual strength of fractured rock. The drastic drop in the lateral reaction of p-y curves may result in unreasonably long rock-socket lengths causing constructability issues and unnecessarily increase the construction costs. This paper proposes modifications to the residual strength of p-y curves that can be implemented in existing rock p-y curves. Data from a lateral load test was used to develop a three-dimensional numerical model in FLAC3D using the Hoek-Brown constitutive model for rock. The 3D numerical model was then used to characterize the residual strength of rock p-y curves. The proposed residual strength was implemented in Strong Rock p-y curves in LPILE to model a lateral load test and to illustrate the application of the proposed modifications in a practice-oriented numerical tool.

An Analysis of the Flexural Stiffness and Horizontal Bearing Capacity of CFRP Composite Pipe Piles in Marine Environments

Carbon-fiber-reinforced polymer (CFRP) is a composite material consisting of a resin matrix reinforced with carbon fibers. This study focuses on CFRP composite pipe piles as the subject of investigation, exploring the impact of substituting steel bars with CFRP bars on the bending performance of pipe piles through rigorous three-point bending tests. The attenuation of flexural stiffness in CFRP pipe piles under a chloride salt environment was anticipated. The lateral bearing capacity of CFRP pipe piles was calculated by introducing a stiffness degradation coefficient for the piles and utilizing the finite difference method. The findings of the analysis suggest that as the CFRP reinforcement replacement rate increases, the initial bending stiffness of the composite pipe pile experiences a corresponding decrease. After serving for 28.15 years, the steel reinforcement within the pipe pile commences rusting, resulting in a nonlinear decline in the bending stiffness of the composite pipe pile. As the service time of pipe piles increases, a higher replacement rate of CFRP reinforcement results in a slower attenuation of pile stiffness. Consequently, both the horizontal displacement at the top of the pile and the bending moment along the body of the composite pipe pile gradually increase over time. During the same service period, the higher the rate of CFRP reinforcement, the less noticeable the attenuation in the horizontal bearing capacity of the pile shaft.

Stability analysis of deep foundation pit with a double-row cast-in-place piles and diagonal steel lattice braces under sloped excavation conditions

Existing deep foundation pit support structures are commonly composed of external earth-retaining structures, internal horizontal bracings, and vertical columns. A closed bracing system, often formed by a horizontal support through a bracket board, frequently impedes vertical excavation and soil removal operations in the foundation pit, and the processes of assembly and dismantling are complex and time-consuming. This study presents a combined support system and construction method consisting of cast-in-place piles and diagonal steel lattice braces. For sloped excavation, diagonal braces were constructed by slotting through the reserved soil, allowing the use of a single layer of support within the excavation depth. This approach significantly optimizes the construction process, reducing both project duration and overall cost. The field monitoring results indicated that the support method effectively controlled the lateral displacement of the pile bodies. Field monitoring results demonstrated that the proposed support system effectively controlled the lateral displacement of the pile bodies. The adoption of a support-first, excavation-second approach significantly controlled the settlement of the ground surface around the foundation pit, thereby preventing excessive increments in the axial force of the supports due to the large longitudinal depth excavation. The calculation results of the three-dimensional finite element model for foundation pit excavation and support indicate that the proposed support method results in a decreasing ratio of the maximum lateral deformation depth of the pile body, denoted as δh-m, to the excavation depth He as the excavation depth increases. This implied that the displacement of the pile body was strictly controlled. When the depth of the foundation pit excavation exceeded 10 m, the maximum lateral deformation occurred below 10 m along the pile shaft. The diagonal steel lattice braces transferred the load to the top of the cast-in-place piles at the bottom of the pit, where the stress concentration occurred. During construction, special attention must be paid to the strength of the connection between the pile top and the connecting beams.

Large deformation analysis of intermittent pile penetration into dense sand incorporating a state-dependent Mohr–Coulomb model

The challenges of predicting the stresses acting around piles driven in sand limits meaningful analysis of their single and group loading responses, aging processes and scale/in-situ stress level dependency. Benchmark local stresses measurements made in the sand mass and pile shaft in Calibration Chamber (CC) models show that sharply different regimes act when the pile is either penetrating or temporarily stationary. This paper presents an Arbitrary Lagrangian–Eulerian (ALE) finite element simulation of installation into dense sand, highlighting the stress changes developed between intermittent penetration stages and exploring the effects of initial stress level. The installation by cyclic jacking of closed-ended model piles studied in a highly instrumented CC experiments is simulated with a state-dependent Mohr–Coulomb model calibrated to high-quality element tests on the sand employed. The predicted pile head loads and local stresses are compared with the CC experiments, showing generally good agreement over the lower parts of the pile shaft while also matching key field-scale trends from CPT-based practical design approaches. More advanced constitutive modeling appears necessary to eliminate discrepancies noted over the higher shaft levels, which may be linked to currently neglected aspects of the sands’ highly nonlinear behavior, including grain crushing and hysteresis under cyclic loading.

Small-scale physical modelling of vertically loaded, cyclically thermally-activated helical piles

To investigate the use of thermally-activated helical piles in shallow geothermal energy systems, a 1-g modelling study was conducted. Helical piles with either a single- or double- helix were installed in a medium dense, dry sand, and subjected to mechanical, thermal only and thermo-mechanical loading. The results indicate that during the thermal tests (1 – 3 cycles), a small upwards residual displacement was observed and pile head movements ranged between about 90% and 100% of the free expansion of the pile shaft above the shallowest helix, suggesting that the helices fixed the shaft and little restraint was offered by the surrounding soil. In the thermo-mechanical tests (30 thermal cycles), the pile head developed irrecoverable settlement as a function of the number of helices (more helices, less settlement) and initial load (higher load, greater settlement). No significant alteration in pile axial stiffness or resistance was found for piles with zero mechanical load that underwent only a few thermal cycles; however, an increase in stiffness and resistance, beyond that due to inherent variability in the test setup, was observed for piles with an initial load and following a large number of thermal cycles. The testing of thermally-activated helical piles in sand has confirmed that the response is similar to conventional piles and that thermal ratcheting effects can be managed by the application of suitable margins of safety in design and/or the use of multi-helix piles.

Scours effects on the lateral bearing capacity of monopile-friction wheel composite foundations in sand-overlaying-clay deposit

The monopile-friction wheel composite foundation is recognized as an innovative offshore wind turbines (OWTs) foundation type, distinguished by its superior lateral bearing capacity and resistance to local scouring. In order to investigate the influence of local scour in sand-overlaying-clay deposit on the lateral bearing capacity of this new type of composite foundations, a series of numerical simulations are conducted to explore the effects of potential parameters including different scour parameters, overlying sandy layer thickness and loading point height on the lateral bearing performance. The results indicate that the greatest reduction in lateral bearing capacity of the composite foundation due to scour depth can reach approximately 35%, while the scour extent and angle have relatively small effects. Moreover, the lateral bearing capacity of the composite foundation increases with the thickness of overlying sandy layer. Local scour and the sand layer thickness also affect the lateral deformation and the soil resistance along the pile shaft. Furthermore, as the loading point height increases, the lateral resistance of the composite foundation decreases rapidly. These findings offer valuable insights for the design of monopile-friction wheel composite foundations in sand-overlying-clay deposits.

The Influence of Pile Shaft Distributed Grouting on the Horizontal Load Response of Rectangular Piles

The Influence of Pile Shaft Distributed Grouting on the Horizontal Load Response of Rectangular Piles

New Design Criteria for Long, Large-Diameter Bored Piles in Near-Shore Interbedded Geomaterials: Insights from Static and Dynamic Test Analysis

This paper presents an analysis of long, large-diameter bored piles’ behavior under static and dynamic load tests for a megaproject located in El Alamein, on the northern shoreline of Egypt. Site investigations depict an abundance of limestone fragments and weak argillaceous limestone interlaid with gravelly, silty sands and silty, gravelly clay layers. These layers are classified as intermediate geomaterials, IGMs, and soil layers. The project consists of high-rise buildings founded on long bored piles of 1200 mm and 800 mm in diameter. Forty-four (44) static and dynamic compression load tests were performed in this study. During the pile testing, it was recognized that the pile load–settlement behavior is very conservative. Settlement did not exceed 1.6% of the pile diameter at twice the design load. This indicates that the available design manual does not provide reasonable parameters for IGM layers. The study was performed to investigate the efficiency of different approaches for determining the design load of bored piles in IGMs. These approaches are statistical, predictions from static pile load tests, numerical, and dynamic wave analysis via a case pile wave analysis program, CAPWAP, a method that calculates friction stresses along the pile shaft. The predicted ultimate capacities range from 5.5 to 10.0 times the pile design capacity. Settlement analysis indicates that the large-diameter pile behaves as a friction pile. The dynamic pile load test results were calibrated relative to the static pile load test. The dynamic load test could be used to validate the pile capacity. Settlement from the dynamic load test has been shown to be about 25% higher than that from the static load test. This can be attributed to the possible development of high pore water pressure in cohesive IGMs. The case study analysis and the parametric study indicate that AASHTO LRFD is conservative in estimating skin friction, tip, and load test resistance factors in IGMs. A new load–settlement response equation for 600 mm to 2000 mm diameter piles and new recommendations for resistance factors φqp, φqs, and φload were proposed to be 0.65, 0.70, and 0.80, respectively.

Estimation of Pile Shaft Friction in Expansive Soil upon Water Infiltration

Estimation of Pile Shaft Friction in Expansive Soil upon Water Infiltration

Mechanical Properties of Adjacent Pile Bases in Collapsible Loess under Metro Depot

Metro transit construction has begun to develop rapidly in northwest China because of the acceleration of urbanization. Accordingly, metro depots are also regarded as an essential auxiliary facility for stopping, operation, and maintenance of trains. Meanwhile, many commercial buildings are constructed over metro depots to improve the utilization rate of land due to the increasingly scarce urban land resources, known as transit-oriented development (TOD). These buildings have a large covered area and transfer concentrated loads to the bases. Therefore, pile bases under metro depots have the bearing characteristics of undertaking large concentrated loads, while lesser loads are placed on the soil between the adjacent pile bases. Additionally, the main ground in northwest China is collapsible loess, so the collapsibility should also be considered. Based on the above background, this research performed static loading tests with and without immersion in a reduced scale of adjacent pile bases under a metro depot in Xi’an. The remolding process of natural loess could destroy its structure and the anisotropy of natural loess could also affect the test results. Therefore, four kinds of artificial collapsible loess with different mass ratios of barite powder, kaolin, river sand, cement, industrial salt, and calcium oxide were made by the free-drop method. This method could make the artificial loess simulate the structure of natural loess reasonably. Then, the artificial loess with the most similar properties to intact loess was selected by comparison. Finally, static loading tests with this artificial loess were implemented. The results showed that the ultimate bearing capacity was 4.5 kN. At the same time, the axial force decreased along depth, since the pile shaft friction was positive, and the load sharing ratio of pile tip force increased to 0.58 when the load exceeded 4.5 kN in the situation without immersion; the settlement of pile bases increased significantly after immersion, while the negative shaft friction occurred at the depth of −8 cm~−35 cm, and the load sharing ratio of pile tip force reached 0.92.