Field Test and Numerical Simulation Study of Bearing Characteristics of Combined Post-Grouted Piles for Railway Bridges

Abstract

Utilizing the results of static load tests using the self-balancing method on two large-diameter bored piles from the Huaiyang Left Line Special Bridge Project of the Lianyungang–Zhenjiang Railway, this study aims to investigate the effect of combined tip-and-side post-grouting on the bearing characteristics of post-grouted piles in railway bridges. The difference in bearing performance between individual piles before and after grouting was evaluated using a comparative analysis. The results show that the bearing capacity of the pile foundations is greatly increased by combined tip-and-side post-grouting. In particular, following grouting, a single pile’s maximum bearing capacity rises from 32.99% to 38.42%. The combined post-grouting produces a compressed grout that enhances the mechanical characteristics of the pile–soil contact, resulting in a significant increase in side resistance all the way along the pile. The combined post-grouting also optimizes the performance of the tip resistance, resulting in a more rapid response as the pile tip displacement increases. Additionally, the combined post-grouting modifies the pile shaft’s load transfer mechanism by increasing the tip resistance’s contribution to the pile foundation’s ultimate bearing capacity and moving the bearing’s center of gravity closer to the pile end.

1. Introduction

Super-long and large-diameter bored piles are frequently utilized in large-scale bridges due to their exceptional single-pile capacity. Nonetheless, conventional bored piles frequently confront challenges, including the formation of mud cakes along their perimeters, the accumulation of drilling residues at their termini, and the disruption of the underlying bearing stratum, all of which negatively influence their load-bearing capabilities. The quality of pile formation is crucial in determining the bearing capacity of bored piles, leading to a relatively variable vertical bearing capacity. A substantial body of research has consistently shown that the application of post-grouting methodologies to pile foundations can proficiently mitigate these aforementioned issues [1,2,3,4,5]. Specifically, pile end post-grouting creates an enlarged pile head at the pile bottom, which pre-compresses and reinforces the surrounding soil, significantly enhancing the single-pile bearing performance. Conversely, pile side post-grouting strengthens the soil adjacent to the pile through upward and downward seepage of the pressurized grout. The formation of a grout–soil blend at both the pile’s terminal and lateral regions effectively removes the residual slag at the base and the mud cake adhering to the pile sides. The resultant benefits of employing post-grouting techniques notably enhance the load-transfer efficiency of the pile foundation, leading to a considerable augmentation in its bearing capacity while concurrently mitigating settlement. Consequently, post-grouting techniques have found widespread application in high-rise buildings, bridges, harbors, railways, and water conservancy projects [6,7,8,9,10,11].

To investigate the bearing characteristics of post-grouting bored piles, numerous scholars have conducted extensive research. Ma et al. [10] investigated the effects of post-grouting at the pile bases by performing static load tests on six drilled piles, and they combined finite element analysis software with an exploration of how the cement grout’s diffusion radius at the pile tip, along with its penetration depth, influences the pile’s load-bearing capacity. Thiyyakkandi et al. [12] detailed the construction process and results of field static load tests conducted on an innovative jet-grouted precast pile in cohesionless soil conditions, and their findings revealed that, when utilizing this grouting technique, the bearing capacity of these pile foundations surpassed that of similar-sized bored piles by a factor of over 2.5 times. However, there is a paucity of reports on the application of combined tip-and-side post-grouting in railway bridge projects.

Traditional static load testing typically encompasses the accumulation load method and anchor pile method. However, in railroad bridge projects, site constraints often render both methods impractical. Furthermore, large-diameter post-grouted bored piles exhibit substantial bearing capacities, rendering traditional static load testing inadequate in providing sufficient upper loads to fulfill testing requirements. Consequently, numerous scholars have adopted the self-balancing method for static load testing in pile foundation projects with significant loading demands. Nguyen and Fellenius [13] observed superior outcomes from a self-balancing static load test conducted on two large-diameter piles that underwent post-grouting at their terminals for the Binh Le Bridge in Vietnam. Similarly, Zhou et al. [14] conducted an on-site self-balancing static load test on large-diameter post-grouted piles at the Guanyin Temple Yangtze River Bridge, showing that the post-grouting technique enhances the ultimate bearing capacity and reduces pile top settlement. In summary, the self-balancing static load test is a proven method for assessing the bearing capacity of large-diameter post-grouted piles with substantial loading requirements in field conditions. This method is especially advantageous for assessing the load-bearing capacity of bored piles that have undergone post-grouting at both their tips and sides in practical railway and bridge construction projects.

This paper focuses on the Huaiyang Left Line Bridge project of the Lianyungang–Zhenjiang Railway. Given the load requirements and site constraints, this study employed a self-balancing method for static load testing on two large-diameter bored piles installed within this project. The objective is to analyze the difference in the bearing performance of the two piles before and after the application of combined post-grouting. By comparing these results and integrating findings from numerical simulations, this study investigated the impact of combined post-grouting on the pile foundation bearing capacity and load transfer mechanisms. The results obtained from these tests offer significant insights that inform the design of pile foundations in comparable railway bridge endeavors, thereby fostering the utilization of the combined tip-and-side post-grouting methodology within the realm of foundation engineering.

2. Project Overview

2.1. Overview of Site Stratigraphy

The Lianyungang–Zhengjiang Railway’s Huaiyang Left Line Bridge is situated in Fairy Town, Jiangdu District, Yangzhou City. A comprehensive analysis of the drilling data and regional geological information revealed that the geological stratigraphy at the bridge site was relatively complex. Based on genesis and age, the strata primarily consisted of Quaternary Holocene (Q₄^al) and Quaternary Upper Pleistocene (Q₃^al) alluvial formations, which encompassed clay and sandy soils. At the bridge site in question, the geological layers were predominantly composed of silty clay and clayey materials. To meet the loading requirements of the superstructure, the project employed large-diameter piles that have undergone post-grouting as the foundational design approach. Table 1 presents the parameters of each soil layer on the test site.

2.2. Design of Test Piles

In accordance with the design specifications, two engineering piles, designated as TP1 and TP2, were selected as test piles at Pier 85# of the site. The pertinent parameters of these two test piles are detailed in Table 2. Both piles were constructed using the rotary excavation technique, and the elastic modulus of the concrete for the test piles was 31.5 GPa. Figure 1 depicts the soil layer distribution within the pile lengths at the test location, along with the arrangement of the testing apparatus. During pile construction, measures were taken to enhance the mud casing on the pile sides and eliminate debris from the pile tips. Furthermore, to bolster the load-bearing capacity of individual piles while minimizing settlement, both test piles underwent reinforcement through the application of combined tip-and-side post-grouting methodologies. The test piles were plugged 12 h after concrete placement, and the combined post-grouting process was initiated subsequent to the completion of the pile bearing capacity test before grouting. To guarantee the quality and uniformity of the grouting, three grouting pipes were arranged for pile end grouting, and two pipes were placed for the pile side grouting. Additionally, grouting reinforcement was performed at the load box following the completion of the testing. The pressurized grout was composed of P.O.42.5 ordinary silicate cement mixed with a water–cement ratio of 0.55. The specific grouting parameters are detailed in Table 2.

3. Field Test Method

3.1. Self-Balancing Static Load Test Inspection Principle

The self-balancing method represents a novel approach for conducting static load tests on foundation piles. Different from traditional pile loading and anchor pile methods, this technique involves embedding a load box (O-cell) and reinforcing cage, which are welded together, inside the pile. The high-pressure oil pipe of the load box is then routed to the surface before pouring concrete into the pile. The load box is subsequently filled with oil and loaded using a high-pressure oil pump located on the ground. This force is transmitted to the pile, with the side resistance of the upper pile segment balancing the side and tip resistance of the lower segment to maintain the applied load. Given the considerable length of the test piles, the rated load capacity of the jack poses limitations on the use of a single load box, making its arrangement challenging. To address this issue, the double load box self-balancing test method was employed in this study. Furthermore, this method offers the following advantages: (1) It enables effective assessments of pile foundation bearing capacity before and after grouting. (2) The loading sequence of the double load box can be flexibly adjusted, allowing for the measurement of bearing capacity across different pile sections. In accordance with the design specifications, the upper load box (Upper O-cell) of test piles TP1 and TP2 was positioned 28.5 m from the pile end, while the lower load box (Lower O-cell) was located 3 m from the pile end. The equipment arrangement for the double load box self-balancing test method is depicted in Figure 2.

3.2. Loading and Unloading Method

Each test pile incorporated two annular load boxes, which were pressurized using a high-pressure oil pump and a precision pressure gauge of grade 0.4. The maximum pressure value indicated by the gauge was 60 MPa. An I32 I-beam served as the reference beam and was driven into the soil to a depth of 2 m. One end of the reference beam was hinged to the reference pile, while the other end was welded to it, with the beam spanning a length of 9 m. During the testing process, six electronic displacement meters were utilized per pile to measure pile displacement. Specifically, two meters were dedicated to measuring the upward displacement of the load box’s top plate, another two to measure the downward displacement of the load box’s bottom plate, and the remaining two meters measured the vertical displacement at the pile base. The test loading was conducted using the slow sustained load method, where each loading increment was set to 1/15 of the estimated ultimate load capacity, with the initial loading increment being double the subsequent levels. Similarly, unloading was performed in increments matching the loading levels, with each unloading step being three times the corresponding loading increment.

4. Analysis of Test Results

4.1. Pile Top Load–Top Displacement Curve

The curve depicting the pile top load (Q) against its corresponding top displacement (s) provides a clear and intuitive illustration of the pile foundation’s bearing capacity and deformation behavior [15]. It serves as a crucial reference for engineering inspectors to assess the safety and stability of the test piles. The results obtained from the self-balancing test were converted into equivalent top load–top displacement curves, which correspond to those derived from traditional static load tests. These conversions yield the equivalent top load (Q) versus top displacement (s) curves for test piles TP1 and TP2, as illustrated in Figure 3.

As depicted in Figure 3, the curves of both test piles exhibit notable steep drop points prior to grouting, whereas the curves after grouting display a gradual change, indicating a significant enhancement in the single-pile bearing characteristics. Under low loading conditions, the variation in pile top displacement prior to and subsequent to grouting is comparatively negligible, as the primary support for the pile top load originates from the lateral resistance of the pile, and the strengthening influence of the grout has yet to manifest itself. However, as the load gradually increases, a significant disparity emerges between the Q-s curves before and after grouting. During this phase, the pile top load is distributed along the pile shaft down to the pile tip, with the pile’s lateral resistance gradually nearing its ultimate capacity, while the contribution of the pile tip resistance starts to increase progressively. Prior to grouting, soil damage occurs upon reaching the ultimate bearing capacity, resulting in a steep decline in the load–displacement curve. Conversely, following the combined grouting procedure, the grout injected into both the pile sides and pile tip significantly improves the mud layer on the pile sides and the slag accumulation at the pile tip, respectively. This reinforcement leads to an improvement in the performance of both the side and tip resistance as the load at the pile top increases. Throughout this process, the grout applied to the pile sides notably boosts the bearing capacity of the individual pile under moderate loads while also reducing the pile top displacement. As the load further increases, the reinforcing effect of the grout at the pile tip becomes increasingly prominent, significantly augmenting the bearing capacity of the individual pile under heavier loads. Notably, upon reaching the ultimate bearing capacity post-grouting, there is no abrupt, steep decline observed.

At a particular loading stage, the Q-s curve manifests a distinct point of inflection, signaling that the pile under test has attained its vertical ultimate bearing capacity. The load value recorded at the stage preceding this inflection point can be considered the pile’s ultimate bearing capacity [16,17]. According to this criterion, the ultimate bearing capacities of test piles TP1 and TP2 before grouting are 27,644 kN and 26,560 kN, respectively. However, the curves of the test piles after grouting do not display any obvious inflection points; thus, the ultimate bearing capacity is taken as the last applied load, which is 36,765 kN for both TP1 and TP2. The ultimate bearing capacities of the test piles in different phases along with their corresponding pile top displacements are presented in Table 3. The ultimate bearing capacities of test piles TP1 and TP2 increase by 32.99% and 38.42%, respectively, after undergoing combined grouting. This underscores the profound influence of combined post-grouting on the load-carrying capacity of individual piles. It is worth noting that the settlement of the pile foundation does not increase in tandem with the augmentation of its load-bearing capacity; indeed, the pile top displacement under the ultimate load condition of test pile TP2, post-combined grouting, undergoes a slight decrement. The test results demonstrate that combined post-grouting improves the bearing performance of bored piles. The cement grout injected at the pile side effectively reduces the impact of the mud skin formed during construction, thereby enhancing the exertion of side resistance. Meanwhile, the grout infused at the pile tip consolidates the slag present at the borehole’s base, resulting in a substantial enhancement in the pile tip resistance’s expression. Additionally, the reinforcing influence of combined post-grouting on the soil adjacent to the pile serves to further elevate the individual pile’s bearing capacity and diminish settlement.

4.2. Distribution of Pile Side Resistance

The combined post-grouting technique represents a significant advancement over traditional pile end grouting as it integrates both pile end and pile side post-grouting technologies. This combination results in substantial improvements in both pile tip-and-side resistance. The bearing capacity of piles is primarily derived from these two components. During the combined post-grouting process, the pile side grout strengthens the surrounding soil and alleviates issues related to the mud skin formed during pile installation. Figure 4 illustrates the variation in pile side resistance (q_si) along depth (H) for each soil layer of test piles at different stages under ultimate load. The figures reveal that, although the distribution patterns of pile side resistance before and after grouting remain largely unchanged, the pile side resistance at the same depth is significantly enhanced due to the injected grout. It is important to note that when a single pile is loaded, the pile side resistance gradually increases from the top to the bottom until it reaches its ultimate value. Under smaller loads, the deep pile side resistance may not be fully exerted. However, as shown in Figure 4, under ultimate load, the pile side resistance at all depths has reached its ultimate value. Table 4 provides statistics on the ultimate pile side resistance at each depth. The data indicate that the pile side resistance within the entire pile length of test piles TP1 and TP2 is increased after grouting compared to before. Due to the similar nature of the soil layer, there is no significant difference in the shallow pile side resistance, and the increase in side resistance is consistent. Notably, the increase in pile side resistance is particularly pronounced at approximately 1 m from the pile end. Specifically, the side resistance of test piles TP1 and TP2 at this depth increased by 78.72% and 80.21%, respectively. This enhancement is attributed to the pre-pressurization effect of the cement grout injected into the pile end, which extends to a certain range of the surrounding soil, thereby increasing the stress in the pile side soil within this range. The results demonstrate that combined post-grouting not only reinforces the soil on the pile side and improves the mud skin through pile side grouting but also further densifies the soil within a specific range of the pile end through the upward return of the grout at the pile end. This comprehensive approach significantly enhances the bearing performance of the pile foundation.

4.3. Tip Resistance–Tip Displacement Curve

Pile tip resistance is a crucial factor in determining the bearing capacity of pile, particularly under heavy loads. Its role becomes even more prominent as it directly influences the pile’s overall load-bearing capacity. Notably, the displacement of the pile tip, which is necessary for pile tip resistance to develop, is significantly impacted by grout compression. Figure 5 presents a comparison of the pile tip resistance (q_b) versus pile tip displacement (s_b) curves for test piles, both before and after grouting. Initially, when the pile top load is minimal, whether before or after grouting, the pile tip resistance for both TP1 and TP2 is relatively low. At this phase, the load is predominantly borne by the lateral resistance of the pile, leading to minimal displacement at the pile’s termination point. As the load applied to the pile’s apex progressively intensifies, the resistance offered by the pile tip starts to become evident, which is accompanied by a corresponding augmentation in the displacement at the pile’s termination. However, once the top load exceeds a certain threshold, the growth rate of the tip resistance for the test pile before grouting slows down and stabilizes, while the pile tip displacement continues to rise. This indicates that the pile top load has surpassed the ultimate bearing capacity of the piles, leading to soil damage. Conversely, after combined grouting, the pile tip resistance of TP1 and TP2 does not exhibit any noticeable slowdown in the growth rate. The grout injected at the pile end consolidates the pile end slag, reinforces the soil, and forms a consolidated grout body at the pile end. This results in a significant improvement in tip resistance. When providing the same pile tip resistance, the pile tip displacement of the test piles after combined grouting is reduced, leading to decreased settlement of the pile foundation under the same pile top loading condition. Furthermore, Figure 5 reveals that after combined grouting, the pile tip resistance responds more swiftly, with a faster growth rate. When the test pile reaches its ultimate bearing capacity, the proportion of pile tip resistance increases. This is attributed to the pre-pressurization effect of the grout injected into the soil at the pile end after grouting, which allows for the tip resistance to develop earlier. This not only enhances the bearing capacity of the single pile but also significantly reduces the settlement of the pile foundation. In summary, combined grouting effectively improves pile tip resistance, reduces pile tip displacement, and enhances the overall bearing capacity and stability of the pile foundation.

4.4. Pile Load Transfer Mechanism

The ratio of pile side and tip resistance to the load capacity of a single pile serves as a quantitative indicator of the pile’s load transfer mechanism [18]. Table 5 presents the proportions of pile tip-and-side resistance relative to the top load of the pile when the test piles, TP1 and TP2, reach their ultimate bearing capacity state. Upon examination of the table, it is evident that following combined post-grouting, both the pile side resistance and tip resistance of the test piles increased to a certain degree in their ultimate bearing capacity state. Notably, the increase in tip resistance is significantly greater than that of pile side resistance, amounting to approximately twice the pile tip resistance before grouting. Consequently, the ratio of pile tip resistance to the ultimate load for TP1 and TP2 increased from 12.49% and 12.99% to 18.65% and 18.48%, respectively. The improvement observed can be attributed to the mitigation of problems such as mud cake formation along the pile sides and slag accumulation at the pile ends through the application of combined post-grouting. Furthermore, the reinforcement of the soil surrounding the pile contributed to this enhancement. These combined factors resulted in a modification of the load transfer mechanism within the pile. Prior to grouting, the pile top loads of TP1 and TP2 were primarily borne by pile side resistance. However, following combined post-grouting, although pile side resistance was effectively improved, its proportion in the ultimate bearing capacity decreased notably. Conversely, pile tip resistance exhibited a more substantial improvement, with its proportion in the ultimate bearing capacity also increasing significantly. This shift indicates that the center of gravity of the pile load bearing moved towards the pile end after combined post-grouting, resulting in an earlier and increased contribution of pile tip resistance to the pile top load. In conclusion, combined post-grouting not only enhances the bearing capacity of a single pile but also alters the load transfer mechanism of the pile. This change enables pile tip resistance to participate in bearing the pile top load earlier and respond more rapidly. Consequently, pile tip resistance is greatly improved, and its proportion of the ultimate bearing capacity is significantly increased. These findings further improve the performance of single-pile bearing capacity in cohesive soils.

5. Finite Element Analysis of Post-Grouted Piles

5.1. Model Building

Due to the limited computational capabilities of computers in the past, numerical simulations in traditional post-grouted pile engineering primarily relied on simplified 2D models [19,20]. However, this approach resulted in a lack of refinement in the models and neglected the three-dimensional geometric shapes of actual objects. With advancements in computer technology, constructing more detailed and accurate 3D models has become the preferred method to conduct numerical simulation tests on post-grouted piles [21]. Compared to 2D models, 3D models can more comprehensively represent the geometric shapes of actual individual piles and lay the foundation for future, even more refined numerical simulation tests of post-grouted piles. Research has indicated that numerical simulation analysis using PLAXIS 3D V22 to evaluate the bearing capacity of pile foundations can yield effective and regular conclusions [21,22]. Therefore, a three-dimensional full-scale finite element model was developed in PLAXIS 3D V22 software using the field-tested pile TP1 as the foundational blueprint. In this model, the soil and additional solids are simulated utilizing the Moore–Cullen model, while the pile is defined as a linear elastic material. Since the bored piles were symmetric cylinders, and the setup of the combined post-grouting device was relatively uniform in the horizontal direction, the monopile models before and after grouting were considered symmetric bodies. The test pile possesses a diameter of 1.5 m and a length of 56.5 m. The dimensions for pile end grout consolidation are established with reference to the coring test results [23], with the diameter and height set to 1.9 times the pile radius. Additionally, the height of the grout’s upward return is designated as 12 m, in accordance with the JTG 3363-2019 “Code for Design of Highway Bridge and Culvert Foundations and Fundamentals” [24]. The grout on the pile side is represented as an interface unit, with a virtual thickness factor of 0.2. To simulate the force exerted by the self-balancing load box in the pile at a distance of 28.5 m from the pile end, a pair of vertically opposed forces is implemented in the model. The calculation domain for the soil extends 20 times the pile diameter (D) horizontally and 1.5 times the pile length (L) vertically from the pile’s tip. PLAXIS 3D automatically meshes the pile by configuring the appropriate parameters. The granularity of mesh delineation has a direct impact on both calculation accuracy and speed; thus, 15-node cells are employed for mesh delineation. After comprehensive consideration, the piles and soil within a specific range are refined and encrypted. Figure 6 illustrates the mesh division of the calculation model. The blue area in the figure is the encrypted area of the mesh division, which improves calculation efficiency while ensuring calculation accuracy. Table 6 shows the specific parameters of each material in the model.

5.2. Analysis of Calculation Results

To validate the rationality of the model, a comparative analysis was conducted between the load–displacement curves of the test piles, as calculated through numerical simulation prior to grouting, and the corresponding curves measured during field testing. The comparison is depicted in Figure 7. Observing Figure 7, it is evident that the load–displacement curves derived from the numerical simulation align well with those recorded in the field, despite the presence of minor discrepancies. Overall, the results from both the numerical simulation and field testing exhibit a consistent trend, indicating that the established calculation model and parameter selection are reasonably accurate. Furthermore, this analysis underscores the feasibility of utilizing PLAXIS 3D software to analyze the force characteristics of pile foundations prior to grout injection.

Numerical simulation serves as an efficient tool for investigating issues related to the post-grouting of pile foundations. As illustrated in Figure 8, load–displacement curves for the self-balancing tests of pile foundations, both before and after grouting, have been computed. Upon examining Figure 8, it is apparent that, after grouting, under identical loading conditions, the displacements of both the upper and lower pile sections are significantly reduced compared to before grouting. Prior to grouting, the upper pile section bears the entire load primarily through pile side resistance. However, post-grouting, this resistance is substantially enhanced due to soil reinforcement surrounding the pile and improved pile–soil interface mechanical properties. Both before and after grouting, the pile side resistance exhibits a notable steep decline as the load increases, eventually reaching a limit value where the upper pile section can no longer sustain additional load, leading to pile–soil displacement and significant relative movement. Differently, the lower pile section supports the load from the load box through a combination of pile side resistance and pile tip resistance. Comparing the load–displacement curves of the lower pile before and after grouting reveals a significant reduction in displacement under the same load following combined grouting, indicating a substantial improvement in the single-pile bearing capacity. Furthermore, Figure 8 demonstrates that the curves for the lower pile section, both pre- and post-grouting, do not exhibit a steep decline. Upon reaching the maximum resistance of the upper pile segment’s lateral surface, the lower pile segment’s bearing capacity remains unmaximized, indicating that the pile tip resistance has yet to be fully utilized and is still in a phase of gradual growth. This observation indicates that pile tip resistance and pile lateral resistance develop asynchronously, and the capacity of a single pile to support heavier loads is predominantly influenced by the extent of pile tip resistance mobilization.

6. Conclusions

This paper presented a comparative analysis of the bearing performance of two large-diameter bored piles in the Huaiyang Left-Line Bridge Project of the Lianyungang–Zhenjiang Railway, focusing on the results obtained from static load tests conducted using the self-balancing method. The analysis compared the pile foundation’s performance before and after the application of combined grouting. It also examined the changes in the pile load transfer mechanism before and after grouting. Additionally, the paper integrated numerical simulation results to investigate the bearing characteristics of piles subjected to combined post-grouting and compared these findings with those from field tests. The key conclusions drawn are as follows:

• (1). The application of the combined post-grouting technique significantly increased the ultimate bearing capacity of the two test piles of the railway bridge project by 32.99% and 38.42%, respectively. Moreover, the pile top displacement is significantly decreased under the same applied load. It is worth noting that the load–displacement curve at the pile top exhibited a gradual and smoothly varying trend, rather than a sharp decline, following the combined grouting treatment. This observation underscores the substantial impact of the post-grouting intervention.

• (2). The injection of grout into the pile sides during the combined grouting process significantly enhances the mechanical characteristics of the pile–soil interface, leading to an increase in pile lateral resistance along the entire length of the pile. The grout injected at the pile end creates a pre-compression effect on the soil within a specific proximity of the pile tip. Consequently, the most pronounced increase in pile side resistance is observed within approximately 1 m from the pile end, with an enhancement ranging from 78.72% to 80.21%.

• (3). Under identical pile tip displacements, test piles subjected to combined post-grouting demonstrate an augmented pile tip resistance. The pile tip resistance responds more promptly to increasing pile tip displacements after grouting. Furthermore, the application of combined tip-and-side post-grouting alters the load transfer mechanism of the pile. Specifically, there is a substantial increase in both pile lateral resistance and pile tip resistance in the vicinity of the pile end. Consequently, the proportion of pile tip resistance to the ultimate bearing capacity of the pile foundation increases, from a range of 12.49% to 12.99% before grouting to a range of 18.14% to 18.65% after grouting. This shift signifies a relocation of the center of gravity of the pile foundation’s bearing capacity towards the pile end.

• (4). The numerical simulation results indicate that the implementation of combined post-grouting enhances the mechanical properties of the pile–soil interface through the introduction of grout into the pile sides, thereby increasing the pile side resistance. Moreover, the consolidation of grout injected into the pile end significantly improves the pile tip resistance performance. It is worth highlighting that the development of pile lateral resistance and pile tip resistance occurs asynchronously; the pile lateral resistance reaches its ultimate capacity before the pile tip resistance is fully mobilized. This observation aligns with the findings obtained from the field testing.

Author Contributions

Methodology, R.Z.; Software, Z.G.; Resources, W.G. and Z.W.; Writing—original draft, R.Z., Z.G. and Z.W.; Writing—review & editing, R.Z., Z.G., W.G. and Z.W.; Project administration, Z.W. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article.

Acknowledgments

Lei Wang of Nanjing Dongtu Construction Technology Co., Ltd. was also acknowledged for his contribution to this work.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. Distribution profile of soil layers of test site and shaft instrumentation.

Figure 2. Schematic diagram of self-balancing test with double-level O-cells.

Figure 3. Equivalent conversion Q-s curves of test piles: (a) TP1; (b) TP2.

Figure 4. Distribution curves of side resistance along the depth of test piles under ultimate load: (a) TP1; (b) TP2.

Figure 5. qb-sb curves of test piles: (a) TP1; (b) TP2.

Figure 6. Computational modeling and mesh segmentation.

Figure 7. Curves of calculated and measured results.

Figure 8. Self-balance test of numerical simulation load–displacement curve.

Table 1

Parameters of each soil layer on the test site.

Layer No.	Soil Name	Basic Permissible Value of Load Capacity (kPa)	Standard Value of Side Resistance (kPa)	Grading of Geotechnical Construction Work
1-1	Miscellaneous fill			I
2-1	Silty clay	120	40	II
2-2	Silty clay	160	55	II
2-3	Silty clay	150	50	II
2-4	Silty clay	160	50	II
2-5	Silty clay	200	65	II
3-1	Clay	180	60	II
3-2	Clay	120	35	II
3-3	Clay	180	60	II
3-4	Clay	180	55	II

Table 2

Parameters of test piles and grouting parameters.

Pile No.	Diameter (m)	Length (m)	Bearing Layer	Grouting Method	Grouting Pressure (MPa)		Water–Cement Ratio	Quantity of Cement (103 kg)
					Pile End	PILE SIDE
TP1	1.5	56.5	Clay	Combined	2.7	3.8	0.55	6.69
TP2	1.5	56.5	Clay	Combined	2.5	3.3	0.55	6.58

Table 3

Ultimate bearing capacity and corresponding pile top settlement of test piles.

Pile No.	Before Grouting		After Grouting
	Ultimate Bearing Capacity (kN)	Displacement of Pile Top (mm)	Ultimate Bearing Capacity (kN)	Displacement of Pile Top (mm)
TP1	27,644	30.33	36,765	33.53
TP2	26,560	33.23	36,765	30.56

Table 4

Pile side resistance statistics.

Soil Name	Elevation of Soil Layer	Average Side Resistance of TP1 (kPa)		Average Side Resistance of TP2 (kPa)
		Before Grouting	After Grouting	Before Grouting	After Grouting
2-1 Silty clay	8.572–7.572 m	77	92	72	92
2-1 Silty clay	7.572–3.84 m	79	94	74	94
2-2 Silty clay	3.84–2.14 m	69	81	68	85
2-3 Silty clay	2.14–2.36 m	61	74	59	78
2-4 Silty clay	−2.36–9.16 m	94	107	85	104
2-5 Silty clay	−9.16–19.26 m	96	109	87	108
3-1 Clay	−19.26–19.428 m	98	115	94	120
3-1 Clay	−19.428–28.16 m	96	112	92	117
3-2 Clay	−28.16–38.46 m	95	109	85	106
3-3 Clay	−38.46–43.26 m	90	103	82	100
3-3 Clay	−43.26–44.928 m	88	98	78	99
3-4 Clay	−44.928–46.928 m	94	168	96	173
3-4 Clay	−46.928–47.928 m	92	163	94	168

Table 5

The ratio of pile resistance to pile top load under ultimate bearing capacity.

Pile No.	Stage	Side Resistance (kN)	Side Resistance as a Proportion of the Ultimate Load	End Resistance (kN)	End Resistance as a Proportion of the Ultimate Load	Ultimate Bearing Capacity (kN)
TP1	Before grouting	24,191	87.51%	3453	12.49%	27,644
	After grouting	29,907	81.35%	6858	18.65%	36,765
TP2	Before grouting	23,110	87.01%	3450	12.99%	26,560
	After grouting	29,972	81.52%	6793	18.48%	36,765

Table 6

Specific parameters of material.

Material Name	Stage	E (MPa)	ν	c (kPa)	φ (°)
Pile		3.15 × 104	0.2
soil		30	0.3	4	30
Grout consolidation	Before grouting	30	0.3	3.2	24
	After grouting	450	0.3	40	35

Note: E, modulus of elasticity of the material; ν, Poisson’s ratio of the material; c, the cohesion of each soil; φ, the internal friction angle of each soil.

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