Seismic Performance Evaluation of Pipelines Buried in Sandy Soils Reinforced with FRP Micropiles: A Numerical Study

Abstract

Unstable sandy soil poses significant challenges for buried pipelines, particularly due to the increased risk of displacement and stress-induced fractures resulting from soil settlement and earthquake-induced ground deformation. These concerns are especially critical in seismically active regions where underground infrastructure is at higher risk. Fiber-reinforced polymer (FRP) composites present a promising and sustainable alternative for deep foundations, offering durability and reduced maintenance costs compared to conventional materials. This study introduces a novel approach to enhancing the seismic performance of pipelines buried in sandy soils by numerically investigating a three-dimensional (3D) multipipe grouting micro anti-slide pile system, utilizing a polyurethane polymer slurry as the grouting material. Key parameters such as pile spacing, diameter, and length, along with the effects of soil wetting and various earthquake intensities, were examined under the influence of surface loads exerted by a fully loaded truck. The results demonstrate that using polymer micropiles significantly reduces soil and pipeline settlement by 15% to 50%, with larger pile diameters and lengths further decreasing settlement and strain on pipelines. While seismic excitation increases settlement, polymer grouting effectively mitigates this impact, leading to substantial reductions in settlement.

1. Introduction

Underground pipelines transport a variety of fluids, including potable and wastewater, oil, and both liquid and gaseous hydrocarbons, whether in their crude or refined state. These pipes have various diameters, ranging from sizes smaller than 300 mm to sizes as large as 1200 mm in nominal size. The seismic response of oil pipelines is a critical concern in regions prone to seismic activity. The potential for ground motion and soil liquefaction poses significant challenges to the structural integrity of these vital conduits for hydrocarbon transport.

Earthquakes are natural disasters that can significantly damage critical infrastructure, including water supplies, sewage systems, and oil and gas pipelines. As urban areas continue to expand, the risk of earthquake-related damage increases. Therefore, it is crucial to initiate recovery efforts to restore essential services. When these essential utilities break during an earthquake, the earthquake’s effects can even worsen. For example, oil pipelines can cause damage, loss of money, and environmental damage. If the pipeline breaks and oil spills, it could catch fire with electricity sparks [1]. Many researchers have studied how underground pipes behave during earthquakes [2,3,4,5]. Understanding how buried pipes react to ground movement is essential for designing and assessing risks in various difficult situations, including when earthquakes cause the ground to shift, as researchers have noted [6,7].

Micropiles are commonly constructed from various traditional materials such as wood, steel, or concrete. They have been extensively utilized for many years [8]. The challenges associated with these materials involve issues such as wood decay, steel corrosion, and concrete deterioration. These traditional materials have restricted operational lifespans and result in significant maintenance expenses when exposed to harsh conditions. Additionally, preserving wooden piles with chemical treatments is becoming more difficult because of their hazardous properties. The highest recorded corrosion rate for steel is 0.03 mm/year for piles, which were removed after 25 years of use [9]. Concrete degradation occurs in environments with elevated sulfate and chloride concentrations and low pH levels in both soil and groundwater [10]. In the United States, marine waterfront communities spend nearly 1.0 billion dollars annually to replace or repair deteriorated structures [11].

Recently, polymer materials have been used for geotechnical problems [12,13], and unlike concrete materials, a polymer mixture with specific proportions demonstrates notable fluidity, expansibility, rapid formation, high strength, and prolonged durability after being formed [9]. Polymer micropiles (as shown in Figure 1) are created by the pressurized injection of a polymer slurry through grouting. During construction, a grouting machine injects a high-polymer slurry into predetermined areas, taking advantage of its favorable fluidity and quick formation characteristics [14]. This process results in the development of a solid structure resembling an anchor around the pile. Shaia (2013) [15] examined fiber-reinforced polymer (FRP) composites as bearing piles instead of traditional materials such as wood, concrete, or steel in highways and related facility substructures. The use of FRPs in engineering has the potential to replace traditional pile materials, especially because traditional materials deteriorate quickly under challenging conditions. Recently, interest in using FRP composite materials for deep foundations has increased [16]. This is due to their advantageous inherent characteristics, including their lightweight nature, high specific strength, durability, and quick application. However, the technique involving FRP composites introduces a promising and emerging approach for various structural and geotechnical purposes, wherein the FRP tube functions as a structural framework within piling foundation systems. While the adoption of FRP composite materials in geotechnical engineering started a few years ago, their use remains somewhat restricted compared to traditional materials [15]. This limitation is attributed primarily to the relatively high initial costs associated with FRP composites, the lack of an extensive database, and insufficient knowledge, particularly for applications in challenging marine environments. Limited research and databases concerning using FRP composites as piling materials have exhibited certain positive indications, motivating researchers to advance their investigations within the geotechnical domain. Compared with their concrete counterparts, FRP piles can be considered sustainable in terms of CO₂ emissions. Furthermore, the durability of FRP piles is greater than that of concrete or steel piles, especially under harsh environmental conditions.

The engineering behavior of FRP piles has traditionally been studied with a focus on lateral loading performance, particularly as fender piles [15,17,18], while their application in deep foundations remains underexplored, and existing research on using FRP composites for piling foundations is limited [17]. This study addresses this gap by introducing a novel 3D multipipe grouting micro anti-slide pile system to enhance the seismic response of oil pipelines. The innovation lies not only in the structural configuration but also in using polyurethane polymer slurry as the grouting material, a combination previously not investigated. This research comprehensively examines parameters such as pile spacing, diameter, and length, alongside the effects of soil wetting and varying earthquake intensities, using 50 real earthquake records. Additionally, it uniquely considers the impact of surface loads from a fully loaded truck on the pipeline, offering a practical and realistic assessment. By exploring these novel aspects, this study significantly advances the application of FRP composites in seismic mitigation for oil pipelines.

2. Methodology

2.1. Current Numerical Models

In this research, computational models were developed and validated, guided by the investigations conducted by Radwan (2022) [19].

Model Description

As previously mentioned, the use of polyurethane polymers has gained popularity in engineering applications because of their exceptional mechanical properties. Building upon these qualities, a new concept, a multipipe grouting micropile, has been introduced. This pile is formed by using a polymer slurry as the grouting material. In the finite element analysis software, the polymer pile is modeled as a circular solid, as depicted in Figure 2. By utilizing the distinctive diffusion properties of polymer slurries highlighted by Wang et al. (2014) [20] and Hao et al. (2018) [21], the finite element strength reduction method is employed to assess the stability of a pipeline with polymer micropiles. The model includes a pile with a diameter of D = 30 cm, and a high-polymer soil ring pile with a thickness of 2.5 cm surrounding the pile is established for analysis.

The suggested numerical model, as confirmed by the validation process in this study (refer to Section 3), effectively replicates the cyclic and nonlinear response of the soil by closely representing the actual stress-strain response during cyclic loading. This method presents advantages over equivalent linear approaches not only by capturing nonlinearity effects but also by considering the average strain-dependent soil modulus and damping coefficients. The simulation accounts for both dry and saturated ground conditions via the modified Mohr–Coulomb (MMC) and modified UBCSAND models, respectively. The modified Newton–Raphson technique has been applied to solve the finite element equations. The MMC model has been found to provide a reasonable representation of the response of dry cohesionless soil to dynamic loads, as highlighted by al-Jeznawi (2024) [22]. Furthermore, the modified UBCSAND model is employed to model the increase in pore water pressure, consequently capturing the potential liquefaction of saturated cohesionless soil, as discussed by al-Jeznawi et al. [22,23,24,25]. A damping ratio of 5% is considered for incorporating Rayleigh damping.

The soil, pavement surface, and piles were each modeled via four nodes with tetrahedral elements. Figure 2 illustrates the implementation of a fine mesh to ensure precise modeling. The accuracy of the mesh size was assessed, and the results are elaborated upon in Section 3.

A series of nonlinear static-dynamic time-history coupled investigations were conducted across multiple design phases to achieve an accurate simulation. The interactions at the soil-pile interface were addressed by applying a strength reduction technique and an interface reduction coefficient. Importantly, the analyses considered the undrained effective stress response of the soil. In line with the recommendations of al-Jeznawi et al. (2021) [12] and Sharifi et al. (2020) [26], finite element analysis was carried out in several stages. These stages represent the transition from elastic to plastic behavior of the ground. The static phases involved multiple load increments (ranging from 25 to 40). Additionally, a time step of 0.01 s was applied to ensure numerical convergence and an accurate representation of pore water pressure generation within the simulation.

2.2. Validation of the Finite Element Model

The study previously published by Radwan (2022) [19] was used to validate the 3D finite element model used in this study. This author focused on using concrete micropiles to improve the safety of pipelines constructed on loose sandy soil. This study considered the application of micropiles to improve the performance of the soil around a pipeline by investigating the ideal micropile arrangement for maximizing the foundation-bearing capacity. The research employed PLAXIS 3D Ver.2020 numerical analysis software to explore this enhancement. Numerous factors, including the spacing, diameter, and length of the piles, as well as the influence of soil moisture, were considered.

The configuration of the numerical model is described in Figure 3. The following criteria were employed to establish the overall dimensions of the model boundaries: the height was set at six times the pavement width, the width was matched to six times the pavement width, and the length was correspondingly set at six times the pavement width. These measurements were selected to minimize any boundary-related effects. The external border of the mesh was fixed to prevent any displacement. The soil was subjected to load simulating the weight of a standard fully loaded large concrete truck with a two-wheel load (refer to Figure 4). In this case, a total force of 66,000 pounds (equivalent to 293 kN) was exerted on the pavement, with 28,000 pounds (approximately 124.5 kN) applied to each of its rear axles. To simplify the scenario, we determined the distribution of truck wheel loads over a 30-cm-radius circle. The loads were supported by the pavement surface, which consisted of a five-centimeter-thick layer of asphalt concrete. The soil, in turn, was characterized via the well-known Mohr–Coulomb model, which requires input parameters, including the dilatancy angle (ψ), friction angle (φ), cohesion (c), and elastic modulus (E) for soil elasticity, as well as Poisson’s ratio (ν). In a previous study, the suitability of the Mohr–Coulomb model for simulating issues related to the interaction of soil with micropiles was confirmed [27].

The asphalt concrete pavement was simulated as a soil element via a linear-elastic model. The micropiles were modeled as incorporated beam elements. The PVC pipeline was modeled via a plate element and the polycurve tool. The pipeline was situated one meter below the ground surface. A variation in the pile spacing, ranging from 25 to 100 cm, was considered in the layout of the confining piles, as shown in Figure 3. Pile lengths ranging from 2 to 4 m and pile diameters of 10, 15, and 20 cm were used to investigate the influence of the pile diameter and length in this study.

Geotechnical and Structural Characteristics of the Models

The current study uses polymer micropiles with varying spacings (25, 50, 75, and 100 cm), different diameters (10, 15, and 20 cm), varying pile lengths (2, 3, and 4 m), and 50 different earthquake records. Figure 5 provides a graphical representation outlining the detailed aspects of the study plan and the parameters considered throughout the research. These micropiles are installed in cohesionless soil with a relative density (Dr) of 25%, with soil properties extracted from the study of Radwan (2022) [19]. These properties have been extracted via the methodology outlined in the study of Beaty and Byrne (2011) [28]. The calibration process is based on the equivalent SPT blow count for clean sand, known as (N₁)₆₀, as determined following the ASTM D 1586-99 standard [29]. This (N₁)₆₀ value forms the basis for the primary calibration equations proposed by Beaty and Byrne (2011) [28]. Additionally, the simulation includes components representing the interaction between the soil and the piles. The key input parameters governing the ground materials are listed in

Table 1. Ground and structural characteristics.

Parameter	Ground Material	Micropile			PVC Pipeline	Asphalt Concrete Pavement
		Concrete	Polymer	Polymeric Soil
ν	0.3	0.1	0.35	0.28	0.45	0.35
γ (kN/m3)	18	25	1.46	2.5	14.6	25
Ø (°)	30	-	-	30	-	-
E (kPa)	10,000	3 × 107	5 × 106	2 × 105	93 × 104	54 × 105
ψ (°)	0	-	-	-	-	-
C (kPa)	1	-	-	300	-	-
Dimension (cm)	(22.8 × 22.8 × 22.8) ×106	10 (Dia.)	20 (Dia.)	25 (Dia.)	20 (Dia.)	350 (width)
Thickness (cm)	-	-	-	2.5	0.5	5

.

The micropile models were tested via a standard fully loaded truck, which was carrying a heavy concrete truck equipped with dual wheels. The truck used for the analysis is shown in Figure 4. A typical fully loaded concrete truck exerts a total force of 66,000 pounds (293.5 kN) on the pavement, with 28,000 pounds (124.5 kN) distributed on each rear axle. The truck’s wheel loads were uniformly spread across a circular area with a radius of 30 cm to simplify the scenario, as shown in Figure 2 (top view).

In this research, the behavior of the soil-pile model is evaluated under the influence of varying acceleration levels via 50 earthquake records (collected from the United States Geological Survey (USGS) website), which cover a range of intensities. The detailed characteristics of the ground motions are provided in

Table 2. Earthquake records used in this study (Ref. The United States Geological Survey, USGS website).

No.	Station	Duration (s)	PGA (g)	Mw	No.	Station	Duration (s)	PGA (g)	Mw
1	1940, El Centro, 270 Deg	53.72	0.360	6.9	26	H24_T1-II-1 (2003, Tokachi-Coast, EW)	120.00	0.630	8.3
2	1940, El Centro, 180 Deg	53.46	0.211	6.9	27	H24_T1-II-2 (2011, Tohoku-Coast, EW)	240.00	0.690	9.1
3	1940, El Centro, Vertical	53.78	0.250	6.9	28	H24_T1-II-3 (2011, Tohoku-Coast, NS)	240.00	0.520	9.1
4	1952, Taft Lincoln School, 69 Deg	54.38	0.156	7.7	29	H24_T1-III-1 (2003, Tokachi-Coast, EW)	120.00	0.516	8.3
5	1952, Taft Lincoln School, 339 Deg	54.40	0.180	7.7	30	H24_T1-III-3 (2011, Tohoku-Coast, NS)	240.00	0.70	9.1
6	1952, Taft Lincoln School, Vertical	54.26	0.105	7.7	31	Bonds Corner El Centro (1979)	37.68	0.770	6.5
7	1952, Hollywood Storage P.E., 270 Deg	78.62	0.060	7.7	32	Parkfield Cholame, Shandon (1966)	26.06	0.237	5.5
8	1952, Hollywood Storage P.E., 0 Deg	78.62	0.042	7.7	33	T1-I-3 (1993, Hokkaido-S/W_Coast, LG)	40.00	0.330	7.7
9	1952, Hollywood Storage P.E., Vertical	78.58	0.020	7.7	34	T1-II-1 (1968, Hyuganada-Coast, LG)	40.00	0.370	7.5
10	1971, San Fernando, 69 Deg	61.84	0.315	6.6	35	T1-II-2 (1968, Hyuganada-Coast, TR)	40.00	0.390	7.5
11	1971, San Fernando, 159 Deg	61.88	0.271	6.6	36	T1-II-3 (1994, Hokkaido-East Coast, TR)	65.00	0.370	8.1
12	1979, James RD. El Centro, 310 Deg	37.82	0.600	6.5	37	T1-III-1 (1983, Nihonkai-Central, TR)	60.00	0.442	7.8
13	1979, James RD. El Centro, Up	39.36	0.480	6.5	38	T1-III-2 (1983, Nihonkai-Central, LG)	60.00	0.430	7.8
14	1979, James RD. El Centro, 220 Deg	37.68	0.780	6.5	39	T1-III-3 (1994, Hokkaido-East Coast, LG)	60.00	0.447	8.1
15	1979, James RD. El Centro, Up	37.84	0.330	6.5	40	T2-I-1 (1995, Hyogoken_South, NS)	30.00	0.830	7.2
16	T1-I-1 (1978, Miyagi-Coast, LG)	30.00	0.330	7.7	41	T2-I-2 (1995, Hyogoken_South, EW)	30.00	0.780	7.2
17	T1-I-2 (1978, Miyagi-Coast, TR)	30.00	0.326	7.7	42	T2-I-3 (1995, Hyogoken_South, NS)	30.00	0.800	7.2
18	1989, Loma Prieta, Oakland Outer Wharf, 0 Deg	39.98	0.220	6.9	43	T2-II-1 (1995, Hyogoken_South, NS)	40.00	0.700	7.2
19	1971, San Fernando Pocoima Dam, 196 Deg	41.58	1.070	6.6	44	T2-II-3 (1995, Hyogoken_South, N30W)	40.00	0.750	7.2
20	1971, San Fernando Pocoima Dam, 286 Deg	41.74	1.170	6.6	45	T2-III-1 (1995, Hyogoken_South, N12W)	50.00	0.600	7.2
21	1971, San Fernando 8244 Orion Blvd., 90 Deg	59.34	0.250	6.6	46	1985, Mexico City, Station 1, 180 Deg	180.10	0.170	8.1
22	T2-III-3 (1995, Hyogoken_South, EW)	50.00	0.630	7.2	47	1985, Mexico City, Station 1, 270 Deg	180.10	0.100	8.1
23	H24_T1-I-1 (2003, Tokachi-Coast, EW)	120.00	0.550	8.3	48	1994, Northridge, Sylmar County Hosp., 90 Deg	59.98	0.600	6.7
24	H24_T1-I-2 (2011, Tohoku-Coast, EW)	240.00	0.810	9.1	49	San Fernando 8244 Orion Blvd (1971)	59.50	0.134	6.6
25	H24_T1-I-3 (2011, Tohoku-Coast, NS)	240.00	0.710	9.1	50	Ōsaka-Kōbe (Hanshin) metropolitan area, (1995)	48.00	0.820	6.9

.

Figure 5 presents a visual illustration that outlines the specific details of the study plan and the parameters taken into account throughout the study.

3. Results and Discussion

3.1. Model Validation

To validate the suggested modeling approach, we compared the outputs of the FE models with the findings of Radwan (2022) [19] for confirmation. This investigation focused on the application of confining micropiles. The findings, illustrated in Figure 6, indicate that employing micropiles as a confining configuration enhances the performance of buried pipelines. This enhancement is attributed primarily to the confining effect, which is particularly noticeable with a pile spacing of 25 cm. Additionally, micropiles tolerate a portion of the load through shaft friction and end bearing, transferring it to deeper soil. This mechanism reduces stresses on the pipeline, resulting in diminished settlement. The data presented in Figure 6 demonstrate a significant improvement in the pipeline response through the utilization of polymer pilings. The settlement under truck loading is significantly reduced, with decreases of approximately 42% and 30% observed when the pile spacings are 25 cm and 100 cm, respectively. Despite using concrete-reinforced micropilings to strengthen the ground, the pipeline settlement still decreased significantly with polymer micropiles, exhibiting reductions of approximately 29% and 19% for pile spacings of 25 cm and 100 cm, respectively.

The influence of saturated soil on pipeline settlement was investigated. In situations where water leakage occurs, the modulus of elasticity of loose sand significantly decreases, resulting in additional settlement of the pipeline. The findings presented in Figure 7 illustrate a considerable increase in pipeline settlement when the soil is fully saturated. The settlement experiences a substantial reduction, with decreases of approximately 54% and 36% when the pile spacing is 25 cm and 100 cm, respectively. Even though concrete-reinforced micropilings are employed to support the ground, there is still a notable decrease in pipeline settlement with the use of polymer micropiles, demonstrating reductions of approximately 41% and 25% for pile spacings of 25 cm and 100 cm, respectively.

To assess the influence of diameter on the behavior of a buried pipeline, we conducted numerical tests with pile diameters set at 10 cm, 15 cm, and 20 cm while maintaining a constant pile spacing of 1 m. As depicted in Figure 8, an increase in the concrete micropile diameter results in a marginal improvement in the behavior of the buried pipeline, with a slight reduction in settlement observed as the pile diameter increases. However, with the utilization of polymer pilings, the settlement of the pipeline decreases as the pile diameter increases. For example, settlement is reduced by approximately 7% when the diameter doubles, as illustrated in Figure 8. Figure 9 shows one case in which the settlement profile of the pipeline

To investigate the influence of the pile length on the behavior of the buried pipeline, we analyzed and compared piles with lengths of 2 m, 3 m, and 4 m with previous work by Radwan (2022) [19]. The results depicted in Figure 10 indicate a significant reduction in the settlement of the pipeline as the concrete micropile length increases. Similar behavior was observed when polymer piles were used, with a significant decrease in the overall settlement of the pipeline compared with that of the soil reinforced with concrete micropiles, as shown in Figure 10.

3.2. Seismic Response

The analysis involved subjecting an oil pipeline to a suite of 50 ground motion records via MIDAS GTS NX software (version 2022R1), which employs nonlinear time history techniques. The soil underneath the pipeline was reinforced with polymer micropiles, and for comparison purposes, concrete micropiles were also considered. Factors such as soil conditions, micropile diameter, spacing between piles, pile length, and ground motion intensity were considered during the analysis. Subsequent subsections present and discuss the results obtained from the time history analysis.

3.2.1. Pile Material Type and Pile Spacing

Figure 11 shows the results for the FRP piles, and Figure 12 shows the results for the concrete piles. In these analyses, the diameter of the pile was chosen to be constant at 10 cm. The results demonstrate that as the intensity of ground motion increases, the vertical displacement of the pipeline also increases, indicating that stronger seismic activity results in greater settlement and deformation. This trend holds the same across all the scenarios. Moreover, transitioning from dry to saturated soil conditions significantly increases the vertical displacement of the pipeline, as does an increase in the pile spacing. Furthermore, the impact of heightened ground motion intensity was quantified by assessing the regression relationship between earthquake acceleration and deformation. The R-square values of the linear fitting lines varied between 0.65 and 0.87, which shows a good relationship. The observed behavior can be attributed to a combination of factors. Stronger seismic activity leads to higher ground motion intensities, resulting in increased shaking and deformation of the ground, which in turn causes greater displacement and settlement of the pipeline. Transitioning from dry to saturated soil conditions alters the properties of the soil, increasing its susceptibility to deformation under seismic loading. Additionally, increasing the spacing between piles reduces lateral support, allowing more movement of the soil and contributing to greater deformation of the pipeline.

In general, using polymer micropiles instead of concrete micropiles decreases the settlement of oil pipelines under seismic loads by more than 20%. This is due to several factors: polymer micropiles, owing to their greater elasticity, ductility, and lower density than concrete micropiles, are more effective at absorbing and dissipating seismic energy, minimizing stress on the surrounding soil, and reducing settlement. Additionally, they offer better resistance to dynamic loading and can dissipate seismic energy more effectively than concrete micropiles. Furthermore, their lightweight nature reduces inertial forces on the pipeline during seismic events, further minimizing settlement.

3.2.2. Pile Dimension

As shown in Figure 13, decreasing the pile diameter causes an increase in settlement. Specifically, the settlement increased by approximately 62% when the diameter decreased from 20 cm to 10 cm when the polymer piles were used (Figure 13a). However, the increase in settlement was reduced when concrete piles were used, where the settlement increased by approximately 37%, with a similar decrease in the pile diameter (Figure 13b). Importantly, with respect to the concrete piles, the settlement was already greater than that observed with the polymer piles under all 50 recorded earthquake datasets. Furthermore, as shown in Figure 14, decreasing the length of the piles leads to a significant increase in pipeline settlement due to reduced lateral support and bearing capacity. In particular, the settlement increased by approximately 63% when the pile length decreased from 4 m to 2 m when polymer piles were used (Figure 14a). However, the settlement approximately doubled when concrete piles with a similar decrease in pile length were used (Figure 14b). Importantly, with respect to the concrete piles, the settlement was significantly greater than that observed with the polymer piles under all 50 recorded earthquake datasets. Smaller piles offer less restraint against soil movement, allowing for greater deformation around the pipeline. Additionally, their diminished capacity to resist loads results in increased settlement. Furthermore, shorter or smaller piles may not provide adequate foundation support, further exacerbating settlement. Figure 13 and Figure 14 illustrate a consistent trend. As the intensity of ground motion increases, the vertical displacement of the pipeline also increases, indicating that stronger seismic activity leads to greater settlement and deformation. This pattern holds across all the scenarios. Moreover, transitioning from dry to saturated soil conditions or decreasing the pile diameter or length results in a notable increase in pipeline displacement. Additionally, the impact of heightened ground motion intensity was evaluated by examining the R² values of the fitting lines, which fluctuated between 0.57 and 0.84, as depicted in Figure 13 and Figure 14.

3.2.3. Specific Case Study

Two earthquakes, El Centro and Kobe, were selected for a close investigation into the effect of shaking on the behavior of both supported and unsupported pipelines. Figure 15 shows a comparison of the behaviors of pipelines under unreinforced soil, reinforced soil with concrete micropiles, and soil supported by polymer micropiles. The results indicate that pipelines supported by polymer piles experience less settlement under seismic excitation, regardless of the soil conditions and pile spacing. For example, under dry soil conditions, settlement decreases by approximately 43% and 37% when polymer piles are used, compared with unreinforced soil and concrete piles. Figure 16 shows the effects of different pile diameters on pipeline behavior during the El Centro and Kobe earthquakes. Similar behavior is observed with increasing pile diameter for both the concrete and the polymer piles.

3.2.4. Time History Analysis

Figure 17 shows examples of pipeline settlement for various scenarios under the influence of the El Centro earthquake. The acceleration data are also depicted within the same figure to demonstrate that the maximum settlement of the pipeline corresponds to the peak ground acceleration (PGA). Additionally, this presentation depicts the pipeline’s behavior over time during a seismic event. The relationship between pipeline settlement and the PGA is influenced by higher ground accelerations imposing greater dynamic loads on the pipeline, resulting in increased deformation. As the ground acceleration increases, the forces acting on the pipeline intensify, leading to more pronounced settlement. Additionally, depicting a pipeline’s behavior over time offers insight into its response to seismic shaking, revealing any transient effects or changes in settlement patterns during an earthquake.

4. Conclusions

Addressing the challenges of unstable sandy soil on buried pipelines is crucial for mitigating the risks of displacement and stress-induced fractures. This study demonstrates that the utilization of micropiles made from polyurethane polymer and fiber-reinforced polymer (FRP) composites offers a promising solution for enhancing soil stability beneath pipelines, providing a more sustainable reinforcement approach. The novel 3D multipipe grouting micro anti-slide pile system introduced in this study, which employs a polyurethane polymer slurry as the grouting material, has been shown to improve the seismic response of oil pipelines significantly. The numerical analysis reveals that using polymer micropiles reduces soil and pipeline settlement by approximately 20–30% in dry soil conditions and further demonstrates that larger pile diameters and lengths contribute to a reduction in settlement and deformation. Additionally, polymer grouting effectively mitigates settlement caused by seismic excitation, with a notable 30% decrease in settlement observed under conditions such as the El Centro earthquake.

Despite these promising findings, this study has limitations that must be acknowledged. The reliance on finite element analysis means that the results, while encouraging, require experimental validation to confirm their applicability in real-world scenarios. Future research should focus on conducting experiments to validate these numerical results and explore the long-term performance of polymer micropiles under varying environmental conditions. Additionally, the complex interplay between seismic activity, soil characteristics, and structural configurations underscores the need for more comprehensive studies to optimize the design and utilization of polymer micropiles in pipeline foundations, ensuring their resilience against seismic hazards.