Open AccessArticle

Model Tests of Concrete-Filled Fiber Reinforced Polymer Tube Composite Pile Under Cyclic Lateral Loading

Chao Yang

^1,*,

Guoliang Dai

²,

Weiming Gong

²,

Yuxuan Wang

³,

Mingxing Zhu

and

Shaolei Huo

⁴

School of Architectural Engineering, Yangzhou Polytechnic Institute, Yangzhou 225127, China

School of Civil Engineering, Southeast University, Nanjing 210096, China

School of Civil Engineering and Architecture, Jiangsu University of Science and Technology, Zhenjiang 212003, China

⁴

China Power Engineering Consulting Group Co., Ltd., Beijing 100029, China

Author to whom correspondence should be addressed.

Buildings 2025, 15(4), 563; https://doi.org/10.3390/buildings15040563

Submission received: 16 January 2025 / Revised: 9 February 2025 / Accepted: 11 February 2025 / Published: 12 February 2025

(This article belongs to the Special Issue Research on Rock Mechanics and Rock Engineering, Geotechnical Engineering and Mining Sciences in Construction—2nd Edition)

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Figure 1
Degradation of traditional piles [<a href="#B17-buildings-15-00563" class="html-bibr">17</a>]. "> Figure 2
Two kinds of FRP model piles (left: 1.6 m, right: 1.2 m). "> Figure 3
Particle size distribution curves. "> Figure 4
Layout of strain gauge. "> Figure 5
Loading device. "> Figure 6
Moment distribution of pile. "> Figure 7
Relationship between the maximum bending moment of the pile shaft and the number of cycles under diverse loads (with a cycle period of 13 s). "> Figure 8
Lateral load carrying curves at the pile top in the cyclic loading test of the single pile. "> Figure 9
Bending moments of pile shafts at different loading frequencies (100 cycles of loading). "> Figure 10
Displacement of pile shafts under varying loading frequencies (100 cycles of loading). "> Figure 11
Distribution of pile shaft bending moments under diverse loading modalities. "> Figure 12
Distribution of bending moments along the pile shaft for piles with different embedment depths (after 100 cycles of loading). "> Figure 13
Distribution of lateral displacement along the pile shaft for piles with varying embedment depths (after 100 cycles of loading). ">

Versions Notes

Abstract

Concrete-filled FRP (Fiber Reinforced Polymer) tube composite piles offer superior corrosion resistance, making them a promising alternative to traditional piles in marine environments. However, their performance under cyclic lateral loads, such as those induced by waves and currents, requires further investigation. This study conducted model tests on 11 FRP composite piles embedded in sand to evaluate their behavior under cyclic lateral loading. Key parameters, including loading frequency, cycle count, loading mode, and embedment depth, were systematically analyzed. The results revealed that cyclic loading induces cumulative plastic deformation in the surrounding soil, leading to a progressive reduction in the lateral stiffness of the pile–soil system and redistribution of lateral loads among piles. Higher loading frequencies enhanced soil densification and temporarily improved bearing capacity, while increased cycle counts caused soil degradation and reduced ultimate capacity—evidenced by an 8.4% decrease (from 1.19 kN to 1.09 kN) after 700 cycles under a 13 s period, with degradation rates spanning 8.4–11.2% across frequencies. Deeper embedment depths significantly decreased the maximum bending moment (by ~50%) and lateral displacement, highlighting their critical role in optimizing performance. These findings directly inform the design of marine structures by optimizing embedment depth and load frequency to mitigate cyclic degradation, ensuring the long-term serviceability of FRP composite piles in corrosive, high-cycle marine environments.

Keywords:

FRP composite pile; cyclic lateral loading; bearing capacity; model test; soil–pile interaction

1. Introduction

The pile foundation is one of the most common foundation types for structures such as offshore drilling platforms, port terminals, transmission towers, and wind turbine generators [1,2,3]. Analyzing its stress deformation under lateral static loads and its cyclic cumulative deformation under wind and wave cyclic loads is a crucial aspect of its design [4,5,6]. Therefore, numerous scholars at home and abroad have conducted related research and proposed several common calculation methods, including the m-method and the p-y curve method [7,8]. Poulos [9] introduced an analytical method that considers the attenuation of soil stiffness and strength around the pile with increasing cycles and utilizes the p-y curve method to analyze the cyclic lateral cumulative deformation of pile foundations. Based on existing research findings, he also suggested calculation curves for the weakening factors of soil stiffness and strength around piles under cyclic conditions. Li et al. [10] proposed a simplified method for estimating the interface mechanical behavior of monopiles under initial lateral loads. Li et al. [11,12,13,14] presented an extensive usage of the energy-based variational method in laterally loaded deep foundations (caissons or monopiles) with very large diameters. Wang et al. [15] conducted research on the stress-deformation behavior of pile foundations under cyclic lateral loading using centrifuge model tests. Zhu et al. [16] conducted field lateral monotonic and cyclic loading tests on two large-diameter pile foundations, revealing the pile–soil interaction patterns and the development trends of lateral displacement and bending moments of pile foundations under monotonic and cyclic lateral loads. These research findings indicate that load characteristics, including cyclic loading amplitude, number of cycles, and frequency, significantly influence the load-bearing behavior of piles, along with pile construction, layout, and soil properties.

The aforementioned pile foundations not only need to withstand cyclic lateral loads generated by waves and currents but are also exposed to the corrosive effects of seawater and other marine environments. Traditional pile materials, such as steel, concrete, and wood, have limited service lives in such corrosive environments. For instance, steel piles are prone to rusting, concrete piles are susceptible to chloride ion erosion, and wooden piles are vulnerable to marine borer attacks, as shown in Figure 1.

Steel piles exhibit the highest corrosion rate (0.3–0.5 mm/year) [18], leading to significant structural degradation. Concrete piles, while initially resistant, suffer from chloride-induced reinforcement corrosion (0.1–0.4 mm/year) and eventual spalling [19]. Timber piles are most vulnerable to bioerosion, with up to 30% cross-sectional loss after 10 years of exposure [20]. In contrast, FRP composite piles show no measurable corrosion (<0.01 mm/year), confirming their superior durability [21].

To address the aforementioned issues, some scholars have begun to explore the feasibility of applying new composite pile materials in such harsh environments. Nowadays, composite piles have already been utilized in many military and civilian engineering projects abroad [18,19,20,21,22,23]. Among them, the most commonly used composite pile is the FRP (Fiber Reinforced Polymer) composite pile, which is constructed with an FRP shell filled with concrete inside. Ashford et al. [24] conducted a comparative analysis of the mechanical properties of FRP composite piles and traditional reinforced concrete piles under seismic loads. Pando [25] proposed a simplified model for calculating the long-term axial and bending capacities of FRP composite piles in marine environments. Murugan et al. [26] performed lateral load tests on FRP composite piles and analyzed the enhancement effect of FRP on the pile’s bearing capacity. A state-of-the-art review by Abdulla et al. [27] highlighted the materials, methods, and applications of PVC-FRP-confined concrete, showing significant improvements in compressive strength and ductility. Similarly, Jiang et al. [28] investigated the strength enhancement of coarse aggregate-free concretes due to FRP confinement, revealing that FRP wrapping can increase the load-bearing capacity by up to 50%. These studies underscore the potential of FRP confinement as a versatile and efficient technique for structural reinforcement. These studies collectively demonstrate that FRP composite piles address traditional materials’ limitations by combining corrosion resistance (durability) with enhanced axial/lateral capacities (performance), offering a viable solution for cyclic lateral loading in corrosive environments.

In this paper, model tests were conducted on two types of FRP composite piles with different lengths. By observing the bearing behavior of the FRP model piles under cyclic lateral loads, the coupled effects of cyclic loading frequency, cycle count, and burial depth were systematically analyzed—a critical gap in existing studies that predominantly focus on static or single-factor conditions. The results reveal how frequency-dependent soil drainage and burial depth synergistically govern cyclic degradation rates (8.4–11.2%), providing a novel framework for optimizing FRP pile designs in marine environments where multi-parameter interactions dominate long-term performance.

2. Experiment Overview

2.1. Design and Manufacture of Specimens

For this experiment, a total of 11 model piles were designed, including 9 piles with a length of 1.6 m and 2 piles with a length of 1.2 m. The outer diameter and wall thickness of the model piles were 70 mm and 7 mm, respectively. The model piles were constructed using GFRP (Glass Fiber Reinforced Polymer) tubes and concrete pouring. The FRP tubes were custom-manufactured by an industrial partner based on the specified geometric parameters (diameter, wall thickness, and length). The manufacturer’s process involved filament winding with epoxy resin, and the concrete used had a strength grade of C30. The concrete mix was designed in accordance with the Chinese standard JGJ 55-2011 (Specification for Mix Proportion Design of Ordinary Concrete) [29]. The composition included: Cement: Portland cement (Type P·O 42.5, complying with GB 175-2007) [30], with a water-to-cement (w/c) ratio of 0.41. Fine aggregate: Natural medium sand with a fineness modulus of 2.3–3.0 and a silt content of ≤3%. Coarse aggregate: Crushed limestone with a nominal maximum particle size of 20 mm and a continuous gradation of 5–20 mm. The concrete was subjected to natural curing (ambient temperature: 10 ± 3 °C, relative humidity ≥ 90%) for 28 days. During the concrete pouring process, an appropriate amount of expanding agent was added, and timely vibration was applied to ensure compactness. The model piles were cured for 28 days before testing. Prior to the experiment, the bending stiffness EI of the FRP composite piles was calibrated. The two kinds of FRP model piles are shown in Figure 2.

2.2. Test Soil Sample Preparation

The model foundation was tested using sandy soil. Soil tests revealed that the sand used for the test was medium sand with a particle size range of 0.1 to 2 mm, a specific gravity of 2.68, and a relative density of 0.55. During the test, the sand was first filled to the pile base position where the model pile was to be buried. The model pile was then vertically placed at the designated location, and its position was adjusted using a level and a plumb line. Once the model pile was completely vertical, it was fixed in place with a support. Sand backfilling was then carried out, with each layer being filled to a depth of 30 cm, leveled, and compacted to ensure a consistent density throughout the model soil. Finally, the support fixing the base pile was removed, completing the pile embedding process.

According to the standard soil test methods, the specific gravity of the air-dried sand samples was determined using the pycnometer method. The minimum dry density was measured using the inverted graduated cylinder method. The maximum dry density was determined by both the vibrating hammer method and the compaction test, and the higher value obtained from the compaction test was taken as the maximum dry density of the sand samples (Table 1).

Figure 3 presents the particle size distribution curve of sandy soil. Through calculations, it is found that the coefficient of uniformity (C_u) of the sandy soil in the test is 3.436, and the coefficient of curvature (C_c) is 0.904.

2.3. Layout of Measurement Points

The strain gauge layouts for the 1.6 m and 1.2 m piles are shown in Figure 4. To prevent the strain gauges attached to the outer surface of the model piles from being damaged due to friction with the sandy soil, the model piles were installed during the preparation of the foundation.

The strain gauges were adhered using ethyl-cyanoacrylate adhesive (commercially designated as 502 glue, compliant with QB/T 2567-2002 Standard for Ethyl-2-Cyanoacrylate Instant Adhesives) [31], followed by wire soldering. A protective coating of polydimethylsiloxane-based silicone rubber sealant (commercially designated as 703 glue, meeting the technical requirements of HG/T 3313-2000 for Room-Temperature Vulcanized Methyl Silicone Rubber) was subsequently applied [32].

During the experiment, since the strain gauges were attached to the outer surface of the model piles, if the piles were driven into place, the strain gauges on the pile surface might be damaged due to friction with the sandy soil. Therefore, for this experiment, the FRP model piles were embedded in place. First, the sandy soil in the steel trough was excavated to the pile base position where the piles were to be buried. The model piles were then vertically placed at the designated locations, and their positions were adjusted using a level and a plumb line. Once the model piles were completely vertical, they were fixed in place with supports. Subsequently, the excavated sandy soil was backfilled, with each layer being filled to a depth of 30 cm, leveled, and manually tamped twice. Compaction was then performed point by point, and the soil was filled and compacted layer by layer, with consistent tamping force and time to ensure a uniform density of the model soil. Finally, the supports fixing the base piles were removed, completing the embedding of the model piles.

2.4. Loading Device and Method

The test model tank for this experiment was welded from 15 mm thick steel plates, with dimensions of 2.5 m × 2.5 m × 2 m (length × width × depth). Stiffeners were installed on all four sides of the model tank to prevent local deformation. Due to the relatively small lateral ultimate load of the model piles (approximately 1.2 kN), the conventional hydraulic servo loading system, which has a minimum loading capacity of 50 kN, was not suitable for this test. Therefore, a self-made cyclic lateral loading device was used for lateral loading. This device consists of a motor, steel beams, pulleys, steel wire ropes, etc., as shown in Figure 5.

The working principle of this device is as follows: The lateral load applied to the top of the model pile is the difference between F₁ and F₃ (as shown in Figure 4), where F₃ is provided by the mass block on the left loading tray, and F₁ is provided by the lever system on the right. When the variable-speed motor rotates, it drives the rotation of the rotating rod, which in turn drives the mass block on the right loading tray to perform circular motion in the lateral plane. At this time, the moment of the mass block on the lever connected to the variable-speed motor changes as it performs the circular motion. According to the principle of moment balance, the force F₁ outputted by the lever system at this time varies sinusoidally. Consequently, the resultant force applied to the pile top, which is the difference between F₁ and F₃, also varies sinusoidally. By adjusting the rotation cycle of the variable-speed motor, any loading frequency can be achieved.

Sinusoidal loading formulas are as follows:

F_{j} = F_{1} - F_{3}

F_{1} = \frac{F_{l} d_{l} + F_{r} d_{r} + F_{2} d_{r}}{a} + \frac{F_{2} b}{a} \cos θ

Let \frac{F_{l} d_{l} + F_{r} d_{r} + F_{2} d_{r}}{a} - F_{3} = 0

F_{j} = \frac{F_{2} b}{a} \cos θ

d_{r} = a + c

Cyclic loading is applied in stages: Each stage of loading is 1/6 to 1/8 of the estimated load, with the first stage being 1 to 2 times the stage loading. The experiments are grouped according to different cyclic periods and numbers of cycles. The specific grouping scheme for the experiments is shown in Table 2.

The termination of this test is controlled by displacement. The model pile did not suffer damage when the ultimate load was reached. The termination criteria for the test are as follows: the displacement of the foundation pile suddenly increases drastically with a relatively fast rate of change, and the difference in displacement change over three consecutive loading cycles exceeds 0.4 mm; the displacement at the soil surface level of the pile reaches approximately 10 mm. When these conditions are met, loading is terminated, and the corresponding load at this point is defined as the ultimate load specified in this test.

3. Experimental Results and Analysis

3.1. The Impact of Cyclic Loading Cycles on Pile–Soil Interaction

Figure 6 and Figure 7, respectively, provide the distribution of the bending moment along the pile depth of the 1.6 m long model pile under different load levels and the relationship curve of the maximum bending moment of the pile body varying with the number of cyclic loads. As depicted in Figure 6, at the initial loading stage (0.29 kN), the soil surrounding the pile is primarily in the elastic phase. The maximum bending moment of the pile shaft lies at a position four times the pile diameter d below the mud surface. As the number of lateral load cycles increases, the magnitude and distribution of the bending moment of the pile shaft undergo insignificant changes.

When the lateral loading increases to 0.59 kN, as the number of load cycles increases, the soil around the pile undergoes plastic deformation, and the number of cycles exerts a considerable influence on pile–soil interaction. The overall bending moment of the pile shaft undergoes a relatively notable increase with the growth of the cycle number. The increment of the maximum bending moment of the pile shaft after 700 cycles, compared to that after 50 cycles, is approximately 20%, and the increment is mainly concentrated in the first 200 cycles, approaching 16%.

When the cyclic lateral load approaches the ultimate state, the maximum bending moment of the pile shaft decreases as the number of cycles increases. Figure 7 illustrates more intuitively the relationship between the maximum bending moment of the pile shaft and the loading cycles under different loads. The maximum bending moment of the pile shaft after 700 load cycles is decreased by approximately 15% compared with that after 50 load cycles. It is necessary to clarify that under a cyclic period of 13 s, the ultimate lateral bearing capacity decreases from 1.19 kN (at 50 cycles) to 1.09 kN (at 700 cycles), corresponding to a reduction of approximately 8.4% (see Table 3). The observed variability in degradation rates (8.4–11.2%) across different cyclic periods suggests that loading frequency interacts with soil drainage conditions. Higher-frequency loading (2 s period) may accelerate fatigue damage accumulation due to limited soil creep recovery, whereas longer periods (13 s) allow partial stress relaxation, moderating the degradation rate. The observed bending moment reduction at high load levels (e.g., a 15% decrease in Figure 6) may stem from soil degradation mechanisms: cyclic loading induces pore pressure buildup in cohesive soils, reducing effective stress and soil confinement, thereby lowering passive resistance and bending moment peaks. Notably, shorter cyclic periods (2 s) exacerbate this effect due to limited drainage time (Table 3), while longer periods (13 s) allow partial pore pressure dissipation, moderating degradation rates.

Figure 8 depicts the lateral load–displacement curve at the pile top of FRP composite piles under the ultimate loads of different cycle periods. As depicted in the figure, the lateral load–displacement curves at the top of the FRP composite piles all present rather distinct nonlinear characteristics. As the number of cyclic loading cycles increases, the lateral displacement at the pile top corresponding to the same single-pile load continuously enlarges, and the increase in the amplitude of the lateral displacement decreases with the increase in the cyclic load period. Chen et al. [33] defined the ratio of the load amplitude to the displacement amplitude under the current load (such as the slope of the straight line in the figure) as the lateral secant stiffness of the pile foundation. According to this definition, it can be found that the lateral secant stiffness of a single pile continuously decreases with the increase in the number of cycles. This may be because, as the number of load cycles increases, the plastic deformation of the soil around the model pile accumulates continuously, thereby causing a continuous decrease in the lateral stiffness of the pile foundation.

3.2. The Impact of Cyclic Loading Frequency on Pile–Soil Interaction

Figure 9 and Figure 10, respectively, provide the distribution of the bending moment along the model pile with a length of 1.6 m under different load grades and the relationship curve of the lateral displacement of the pile shaft varying with the cyclic loading frequency. It can be seen from Figure 9 that the overall bending moment of the pile shaft and the maximum bending moment increase as the loading frequency increases (the cycle period decreases from 14 s to 5 s). Under relatively low load grades, the maximum bending moment of the pile shaft increases from 171 kN·m to 304 kN·m (a 78% increase) with rising loading frequency, while its position shifts downward by approximately 60 mm. When the load grade is close to the limit state, the increase extent is merely around 30%, and the position of the maximum bending moment remains essentially unchanged. When the load is relatively low, the cumulative displacement in the shallow layers of the soil increases as the frequency of cyclic loading increases. As a result, the resistance of the deeper soil layers begins to play a more significant role earlier in the loading process. This shift in resistance distribution along the pile shaft leads to a corresponding downward movement of the point where the maximum bending moment occurs.

It can be observed from Figure 10 that the displacement of the pile shaft decreases as the loading frequency increases. The higher the load level, the greater the extent of the reduction. This is due to the fact that soil possesses distinct anisotropic properties (structural anisotropy and stress history anisotropy). Coupled with the influence of pore water in the soil, hysteresis becomes one of the principal characteristics of the dynamic stress–strain relation of soil. When the loading frequency rises, the hysteresis phenomenon of the stress–strain relation of the soil surrounding the pile becomes more pronounced, and consequently, the displacement of the pile shaft will decrease. At the same time, it is also demonstrated that for pile foundations subjected to bidirectional cyclic lateral loads, the ultimate lateral bearing capacity increases as the loading frequency rises.

3.3. The Impact of Different Loading Methods on Pile–Soil Interaction

Table 3 lists the ultimate lateral bearing capacity of FRP composite piles with a length of 1.6 m under static loading and partial cyclic loading. The ultimate bearing capacity of FRP composite piles under cyclic loading is typically greater than that under static loading. The smaller the number of cycles and the higher the loading frequency, the more significant the increase in ultimate bearing capacity. As the number of cycles increases, the enhancement effect of the bearing capacity reduces, as listed in Table 3.

Figure 11 provides the curve of the bending moment distribution along the pile shaft of the 1.6 m long model pile of the FRP composite pile under the static load at the load level immediately preceding the ultimate load.

The possible reasons for the analysis might be as follows: Under cyclic loading, when the number of cycles is quite limited, the vibrations induced by high-frequency loading can densify the soil around the pile [34,35], thereby increasing the bearing capacity of the pile foundation. Meanwhile, due to the rapid loading, the stress–strain relationship of the soil exhibits a pronounced hysteresis phenomenon. However, as the number of cycles increases, cyclic loading leads to the continuous accumulation of plastic deformation in the soil around the pile. Soil degradation occurs under the action of cyclic loads. The lateral stiffness of the pile–soil system will constantly decline with the growing number of cycles, thereby causing a decrease in bearing capacity and an increase in displacement. This analysis of the test results is in accordance with the exposition in Section 2 concerning the dynamic strength of soil and its variation rules under cyclic loading.

3.4. The Impact of Different Embedment Depths on Pile–Soil Interaction

Figure 12 and Figure 13, respectively, present the bending moments of the FRP composite piles with two distinct pile lengths under the cyclic loading of the same level, as well as the relationship curves of the lateral displacement of the pile shaft and the distance from the mud surface (with embedment depths of 1.2 m and 0.8 m, respectively). As depicted in the figures, when the loads are equivalent, the distribution of bending moment along the shaft of the pile with a greater embedment depth is notably smaller compared to that of the pile with a shallower embedment depth, resulting in a reduction of approximately 50% in the maximum bending moment of the pile shaft. The location of the displacement zero point also differs, with the former situated in the middle of the pile shaft and the latter close to the pile bottom, indicating that the influence scope of the cyclic lateral load on the lateral displacement of the pile shaft is limited. This divergence originates from the spatial propagation of soil weakening. For shallow piles (0.8 m), the sharp increase in bending moment decay rate reflects weakened shallow soil constraints propagating downward, driving rotation center migration and progressive instability. Meanwhile, deeper piles (1.2 m) maintain stable moment decay, where intact soil–pile friction suppresses rotation, confirming the “elastic deep soil confinement” mechanism. At this loading level, the lateral displacements of the pile shafts at the mud surface for the two piles are 7.1 mm and 9.8 mm, respectively. At this juncture, the pile with a shallower embedment depth exhibits excessively large lateral displacement, and its bearing capacity is approaching the ultimate load, whereas the deep pile can still continue to operate. This implies that, under the same circumstances, the deeper the embedment depth, the greater the ultimate bearing capacity of the pile.

4. Conclusions

(1) The number of cyclic loads has a significant impact on the ultimate lateral bearing capacity and bending moment distribution of the model pile. As the number of cycles increases, even at low load levels, the ultimate lateral bearing capacity of the model pile gradually decreases, with the magnitude of reduction increasing with higher loading frequencies. Additionally, the bending moment increases most significantly within the first 200 cycles, with subsequent changes becoming more gradual.

(2) The impact of the loading frequency of cyclic loads on the interaction between piles and soil is manifested in the following ways: The higher the loading frequency, the more pronounced the lag phenomenon in the dynamic stress–strain relationship of the soil. The lateral displacement of the foundation pile will decrease, and a higher cyclic frequency will densify the soil around the pile and enhance the lateral bearing capacity of the foundation pile.

(3) The influence scope of cyclic lateral loads on the lateral displacement of pile bodies is restricted. Under the identical cyclic load, the shallower the burial depth is, the larger the maximum bending moment of the pile body will be.

(4) Consequently, in the design of FRP pile foundations subjected to cyclic lateral loading, the lateral bearing capacity can be influenced in two opposite directions: it may increase compared to static loading conditions due to frequency effects, or it may decrease due to cumulative cyclic degradation. The specific influence of these two factors on the bearing capacity should be comprehensively evaluated based on the loading frequency, number of cycles, and other relevant design parameters.

Author Contributions

Conceptualization, C.Y.; methodology, C.Y., G.D. and W.G.; validation, S.H.; investigation, Y.W.; resources, G.D. and M.Z.; writing—original draft preparation, C.Y., G.D. and W.G.; writing—review and editing, Y.W., M.Z. and S.H.; supervision, W.G. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (NO. 52178317). The authors are grateful for their support.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

Author Shaolei Huo was employed by the company China Power Engineering Consulting Group Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Degradation of traditional piles [17].

Figure 2. Two kinds of FRP model piles (left: 1.6 m, right: 1.2 m).

Figure 3. Particle size distribution curves.

Figure 4. Layout of strain gauge.

Figure 5. Loading device.

Figure 6. Moment distribution of pile.

Figure 7. Relationship between the maximum bending moment of the pile shaft and the number of cycles under diverse loads (with a cycle period of 13 s).

Figure 8. Lateral load carrying curves at the pile top in the cyclic loading test of the single pile.

Figure 9. Bending moments of pile shafts at different loading frequencies (100 cycles of loading).

Figure 10. Displacement of pile shafts under varying loading frequencies (100 cycles of loading).

Figure 11. Distribution of pile shaft bending moments under diverse loading modalities.

Figure 12. Distribution of bending moments along the pile shaft for piles with different embedment depths (after 100 cycles of loading).

Figure 13. Distribution of lateral displacement along the pile shaft for piles with varying embedment depths (after 100 cycles of loading).

Table 1. Physical characteristics of soil.

Specific Gravity of Soil Particles	Dry Density ρ_d /(g·cm⁻³)		Cohesion c /(kPa)	Angle of Internal Friction φ /(°)
Specific Gravity of Soil Particles	ρ_dmin	ρ_dmax	Cohesion c /(kPa)	Angle of Internal Friction φ /(°)
2.68	1.39	1.69	8	32

Table 2. Grouping of model tests.

Test Pile Number	Pile Length L/m	Cycle Count	Cyclic Loading Period /s	Test Specification
0	1.6	Static Load	-	After applying a load and maintaining it for 4 min, a strain measurement is taken. Subsequently, the load is unloaded, and after a waiting period of 2 min, the residual strain is measured.
1	1.6	50	2	After every 10 cycles, a strain measurement is taken.
2			6
3			13
4	1.6	200	2	For the first 100 cycles of cyclic loading, a strain measurement is taken every 10 cycles. After this, the frequency of strain measurements is reduced to once every 20 cycles.
5			6
6			13
7	1.6	700	2
8			6
9			13
10	1.2	100	6	Perform the same cyclic loading for 50 cycles
11	1.2	700	6	Ditto

Table 3. Ultimate bearing capacity of FRP piles under different cyclic loading cycles.

Loading Mode	Static Load/kN	Number of Cyclic Loads		Loading Period /s
Loading Mode	Static Load/kN	50	700	Loading Period /s
Ultimate Bearing Capacity /kN	1.04	1.49	1.34	2 s
		1.34	1.19	6 s
		1.19	1.09	13 s

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Yang, C.; Dai, G.; Gong, W.; Wang, Y.; Zhu, M.; Huo, S. Model Tests of Concrete-Filled Fiber Reinforced Polymer Tube Composite Pile Under Cyclic Lateral Loading. Buildings 2025, 15, 563. https://doi.org/10.3390/buildings15040563

AMA Style

Yang C, Dai G, Gong W, Wang Y, Zhu M, Huo S. Model Tests of Concrete-Filled Fiber Reinforced Polymer Tube Composite Pile Under Cyclic Lateral Loading. Buildings. 2025; 15(4):563. https://doi.org/10.3390/buildings15040563

Chicago/Turabian Style

Yang, Chao, Guoliang Dai, Weiming Gong, Yuxuan Wang, Mingxing Zhu, and Shaolei Huo. 2025. "Model Tests of Concrete-Filled Fiber Reinforced Polymer Tube Composite Pile Under Cyclic Lateral Loading" Buildings 15, no. 4: 563. https://doi.org/10.3390/buildings15040563

APA Style

Yang, C., Dai, G., Gong, W., Wang, Y., Zhu, M., & Huo, S. (2025). Model Tests of Concrete-Filled Fiber Reinforced Polymer Tube Composite Pile Under Cyclic Lateral Loading. Buildings, 15(4), 563. https://doi.org/10.3390/buildings15040563

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