Introduction

Post-grouting technology is a highly practical and widely applicable engineering technique that has been extensively used in underground construction, bridge foundations, underground rail transit, transmission tower foundations, and other fields1,2. It is primarily employed to enhance the mechanical properties of rock and soil-especially their strength and stiffness, thereby improving stability and load-bearing capacity3,4. When post-grouting technology is applied to cast-in-place pile construction, the completed pile becomes a post-grouted pile. As a new type of pile foundation, the post-grouted pile is characterized by injecting cementitious grout at the pile side and pile tip to form a strengthened zone, which increases side resistance and tip bearing resistance5,6,7,8. Post-grouting offers several significant technical advantages in pile-foundation engineering: (i) grouting does not interfere with pile construction or the construction schedule; (ii) under the condition of ensuring load-bearing performance, the technology can significantly shorten the originally designed pile length, reduce carbon emissions, and lower overall cost9,13 Post-grouted pile have been widely implemented in engineering practice and have effectively enhanced the bearing capacity of single piles, enabling them to sustain larger superstructure loads14,15,16,17,18. Therefore, studying post-grouting technology to address the reduction in pile bearing capacity caused by construction processes holds substantial practical significance19,20. Xu et al. clarified the deformation law of the membrane under uniform grouting pressure via casting tests, derived an estimation model for the bulb length related to grout pressure. The loading tests indicated that the membrane-confined grouted pile has a 1.5 fold higher bearing capacity and a 50% reduction in settlement compared with the conventional pile21. Liu et al. established a load transfer model considering the time-varying viscosity of grout for piles with grouting, and proposed an iterative algorithm in layered soils, the reliability of the proposed method was verified by three well-documented engineering cases22.

On the other hand, the types of grouting materials used for ground improvement are becoming increasingly diverse and can be broadly classified into inorganic and organic categories. Among them, cement-based inorganic grouts are the most widely used in practical engineering, as they offer multiple advantages such as abundant raw material sources, non-toxicity, good solidification performance, and ease of preparation23,24. The grouting material used for post-grouting of pile foundations is typically cement slurry. To further reduce costs, respond to national policies promoting green and low-carbon development, and enhance the effectiveness of pile post-grouting, alternative grouting materials are being explored for use in pile foundations.

Geopolymers are inorganic cementitious materials synthesized from silicon-aluminum-rich materials activated by alkaline activators, which exhibit the characteristics of fast curing rate, high early strength, excellent corrosion resistance and high temperature resistance. In 1992, Palomo et al.25,26 successfully prepared geopolymer cementitious materials by blending metakaolin and silica sand, with a curing time of only 24 h and an uniaxial compressive strength of up to 84 MPa at 28 days. Owing to the advantages of short curing time and high compressive strength, numerous scholars have further investigated the approaches to improve the mechanical properties of geopolymers. Susan et al.27 found in their research that the compressive strength of geopolymers could be enhanced by mixing slag with metakaolin. Kuenzel et al.28 revealed that the cracking resistance of geopolymers was improved with the addition of sand. In addition, many scholars have verified through laboratory tests that the incorporation of a certain amount of geopolymers into concrete can rapidly improve the early compressive strength29,30. Correspondingly, the total porosity and macropore volume of the blocks are significantly reduced31,32,33, while the content of cementitious hydration products increases substantially34,35. Fernández-Jiménez et al.36 conducted a study on fly ash-based geopolymers, characterized the reaction process of precursors by scanning electron microscopy (SEM), proposed the concept of fly ash activation using hydroxides, and established the corresponding microstructure model. Wu et al. found that nanomaterials (NA) could significantly enhance the strength of the cement-fly ash system (NCBS)37. Li et al. prepared coal-based solid waste grouting material (CSWG) and four types of fiber-modified variants (F-CSWG), optimized the composite cement ratio and fiber parameters, and investigated the evolution of their properties. The results showed that the composite cement ratio significantly affected the workability of CSWG; the incorporation of fibers caused the mechanical strength and impermeability of F-CSWG to first increase and then decrease, and the F-CSWG achieved optimal performance when the glass fiber content was 0.4%, with compressive strength and impermeability improved by 25.1% and 57.1%, respectively38.

Currently, research on grouting materials mainly focuses on applications such as tunnels, mines, slopes, and ground improvement, whereas studies related to post-grouting of pile foundations remain scarce. Previous model tests have mostly focused on the influence of grouting parameters on the bearing capacity of pile, but have lacked experimental investigations under complex conditions such as groundwater flow and long-term loading39,40. In addition, the mechanism of action of materials used in pile foundation grouting reinforcement has not yet received attention. In view of this, to address this research gap, a series of model tests on post-grouted piles were carried out under the complex scenarios involving groundwater flow and long-term loading in this study. The bearing performance, axial force mobilization, as well as the mobilization characteristics of side resistance and tip resistance of post-grouted piles with different grouting materials were systematically investigated. Furthermore, the reinforcement mechanisms of different grouting materials were revealed by means of model pile excavation tests and SEM tests.

Experimental materials and methods

Materials

The main materials used in this study include cement, fly ash, and sodium silicate. The test materials are shown in Fig. 1. First, considering that cement can reduce the influence of mineral admixtures on the research results, P·O 42.5 cement without any admixtures was selected. This cement has a specific surface area of 342 m2/kg, an apparent density of 3.16 g/cm3, and a standard consistency water requirement of 26.4%.

Fig. 1
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Photograph of raw materials.

Second, since fly ash is a low-activity mineral admixture, its incorporation into concrete can improve concrete performance, reduce shrinkage, and lower hydration heat, making it widely used in large concrete structures. In this study, Grade II fly ash was used, with a water requirement ratio of 98.8% and an activity index of 84.6. In addition, a pre-prepared liquid sodium silicate solution was adopted for the experiments. The geopolymer was prepared by mixing water glass (20% by volume) and fly ash for five minutes.

The simulated soil layer material used in the experiment was artificially screened sand. The particle-size distribution of the sand used in this study was characterized by a specified particle diameter d₆₀ = 1 mm, median particle size d₃₀ = 0.74 mm, effective particle size d₁₀ = 0.22 mm. These parameters indicate that the soil sample has a continuous gradation, a relatively smooth gradation curve, and uniform particle sizes, which are favorable for grout penetration into the soil.

Preliminary tests were conducted to obtain a cement-sodium silicate grout with good injectability. The optimal mix ratio was determined as cement:sodium silicate = 1:0.6, with a sodium silicate Baumé degree of 41.0°, and the water-to-binder ratio of the grout was set to 0.7. To ensure the accuracy of the experiment, a layered compaction method was used to control the density of the foundation sand. The sand was compacted every 10 cm until the designed height was reached. An iron plate was then placed on the sand surface, and a vertical pressure of 6.6 kPa was applied using a jack. During the loading process, the drainage holes at the bottom of the model box remained open. After 24 h, the load was removed, and soil samples were taken for geotechnical testing. Table 1 presents the basic mechanical parameters of the foundation soil.

Table 1 Basic mechanical parameters of the test soil.
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Experimental scheme design

Five types of model test piles were designed in this study, including a ungrouted pile, a cement-grouted pile, a cement-sodium silicate grouted pile, a fly ash–cement grouted pile, and a geopolymer grouted pile. All test piles were subjected to vertical static load tests and long-term loading tests. Among them, the cement grouted pile and cement-sodium silicate grouted pile were tested under conditions with groundwater flow. Table 2 presents the test scheme for the model piles.

Table 2 The test scheme for the model piles.
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Design of the model box and model piles

A cylindrical steel model box was used in this experiment, with drainage holes installed at the bottom and water inlet holes at the top, as shown in Fig. 2. The distance between the model pile and the model box boundary was set to 455 mm (22.75 D) to ensure that the boundary effect was negligible.

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Model container capable of simulating groundwater flow.

As shown in Fig. 3, in this experiment, small-scale steel pipe piles were selected as the model piles. Before attaching the strain gauges, the pile surface was cleaned with alcohol. High-strength adhesive was then used to bond the strain gauges at the designated positions, ensuring full adhesion. Finally, a layer of 704 waterproof silicone rubber was added, and waterproof tape was wrapped around the gauges to ensure their proper functioning in an underground flowing-water environment.

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Grouted model pile.

In addition, a steel plate (2 mm thick) made of the same material as the steel pipe was welded to seal the pile tip. Two grouting tubes (8 mm in diameter) were installed inside the pile to conduct the post-grouting test along the pile side. Connection holes (8 mm in diameter) were drilled between the bottom of the grouting tubes and the inner wall of the pile, and these holes were temporarily sealed using thumbtacks. The detailed arrangement of the side post-grouting tubes is shown in Fig. 3b.

Grouting system

The post-grouting model tests in this study were carried out using a self-designed grouting system, which mainly consists of a grout pump, a grout mixing tank, a pressure-relief valve, and grouting valves, as shown in Fig. 4. Before injecting the grouting material, clean water was first pumped through the system to flush the grouting tubes and ensure unobstructed flow throughout the entire grouting pipeline.

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Schematic diagram of the grouting system.

The grouting materials were prepared in advance according to the selected mix proportions and water-binder ratios, and the prepared slurry was placed into the grout mixing tank for injection. Figure 4 illustrates the tee joint connected to the grouting pipe, which ensures that the grout can be simultaneously injected into the two grouting holes on the pile side. Finally, the valve of the grout pump was opened, allowing the slurry to be pumped from the grout mixing tank through the grouting tubes and into the soil around the pile. During the grouting process, the injection pressure and flow rate were controlled using the pressure-relief valve and the grouting valve.

Grouting test and vertical loading test

The grouting test was conducted 24 h after the model piles were installed in the soil.

  1. (1)

    Clean water grouting was fist performed to flush and unblock the grouting tubes, ensuring smooth flow throughout the entire grouting system.

  2. (2)

    A preliminary grouting test was then carried out to determine the required grouting pressure for successful slurry discharge and to identify a suitable grouting volume for the post-grouted model piles.

  3. (3)

    The grout was prepared according to the experimental design, and formal grouting was conducted. The grouting time for a single grouting pipe is approximately 2 min.

Preliminary test results indicated that when the grouting pressure was 0.6 MPa, the slurry could be successfully injected. A grouting volume greater than 720 mL provided good differentiation among test results. Therefore, the final grouting pressure and grouting volume were set to 0.6 MPa and 720 mL, respectively. The grouting volume of each grouting pipe is 360 mL.

The load increment for the vertical loading tests was determined based on the preliminary tests. The load increment was set as one-tenth of the ultimate bearing capacity obtained from the preliminary tests, with the first stage load specified as twice the magnitude of this increment. Each loading stage was maintained for a minimum of 60 min, and the subsequent load stage could only be applied when the settlement rate of the pile head tended to stabilize under the current load. At a given load level, if the pile top settlement exceeds twice the settlement recorded at the preceding load level and fails to reach a relatively stable state even after 24 h, the preceding load level shall be taken as the vertical ultimate compressive bearing capacity of the pile foundation. After each loading level, the pile head settlement was recorded at 5 min, 15 min, 30 min, 45 min, and 60 min. The vertical static load test for the ungrouted pile was conducted 24 h after pile installation, whereas the grouted piles were tested 20 days after the completion of the grouting process. Figures 5 present the schematic illustration and loading setup for the vertical static load test, respectively.

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Photograph of the vertical load test setup.

Scanning electron microscopy test

Scanning electron microscopy (SEM) utilizes an electron beam to scan the surface of a sample, generating signals such as secondary electrons. The surface morphology of the sample is then obtained by detecting these signals. SEM has the advantages of high resolution, which allows observation of nanoscale details, large depth of field, and excellent imaging quality, enabling clear visualization of three-dimensional morphology. In this study, SEM was employed to observe the micromorphology of slurry stone treated with different grouting materials, and the action mechanisms of these grouting materials were revealed based on their microstructural characteristics. The test equipment is illustrated in Fig. 6.

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The SEM test equipment.

Analysis of test results

Load-settlement curves at the pile head

Figure 7 presents the load-settlement (Q–S) curves of the five groups of test piles conducted in this study. The Q–S curve of the ungrouted pile exhibits a pronounced inflection point. This is because the pile lacks sufficient side resistance without grouting, once the load reaches a certain level, significant deformation occurs, thereby leading to a rapid decline in its bearing capacity. The Q–S curves of the fly ash–cement grouted pile and the geopolymer grouted pile also show clear inflection points. However, unlike the ungrouted pile, the settlement of these two grouted piles does not increase abruptly, as grouting enhances pile stability, controls vertical settlement at the pile head, and improves the overall bearing capacity.

Fig. 7
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The Q–S curve of grouted pile.

In contrast, the Q–S curves of the cement grouted pile and the cement-sodium silicate grouted pile are smoother compared with the ungrouted pile, mainly because the formation of cementitious and cement-sodium silicate slurry stone alters the load-bearing behavior of the single pile41.

When the ungrouted pile reaches the ultimate load, its Q–S curve exhibits a steep drop characteristic with a rapid increase in pile top settlement, indicating the occurrence of punching shear failure. Under flowing-water conditions, the bearing capacity of the pile foundation can be significantly improved through grouting treatment. Injecting cement-sodium silicate grout into permeable sand layers effectively improves the soil properties. In permeable sandy ground, the Q–S curves of the cement-sodium silicate grouted pile and the cement-grouted pile show similar trends, however, the cement-sodium silicate grouted pile exhibits smaller settlement under the same load. This demonstrates the clear advantage of cement-sodium silicate dual-liquid grouting in enhancing pile bearing behavior. Under flowing-water conditions, cement-sodium silicate grouting is more suitable than cement grouting for improving pile bearing performance.

For the fly ash–cement grouted pile and the geopolymer grouted pile, the grout forms a reinforcement body around the pile, improving the mechanical properties of the surrounding soil to some extent, however, the failure mode of the pile is not altered. When loaded to the ultimate load, punching-type failure still occurs, although the bearing capacity is significantly higher than that of the ungrouted pile. In contrast, for the cement-grouted pile and the cement-sodium silicate grouted pile, effective slurry stone are also formed under flowing-water conditions, greatly enhancing the bearing capacity of single piles. Even when loaded to the final loading stage, no sharp drop occurs in their Q–S curves. However, since the settlement did not reach a stable state within 1 h, the bearing capacity was taken as the load level immediately below the final stage. Therefore, the bearing capacities of the ungrouted pile, geopolymer grouted pile, fly ash–cement grouted pile, cement grouted pile, and cement-sodium silicate grouted pile are 1764 N, 3504 N, 4088 N, 5840 N, and 8760 N, respectively.

From Fig. 7, it can also be observed that all types of grouting materials can enhance the bearing capacity of pile foundations. Compared with the ungrouted pile, the ultimate bearing capacities of the geopolymer grouted pile and the fly ash–cement grouted pile increased by 93.6% and 131.7%, respectively. The ultimate capacity of the fly ash–cement grouted pile is slightly higher than that of the geopolymer-grouted pile, mainly because the bonding ability of geopolymer to sand is weaker than that of the fly ash–cement, resulting in a relatively lower improvement in the strengthening of the pile-side soil. Cement-sodium silicate grout is an effective grouting material that can improve the permeability and mechanical properties of sand around the pile. Grouting with cement-sodium silicate can significantly enhance the bearing capacity of a single pile under flowing-water conditions. It is noteworthy that the bearing capacity of the cement-grouted pile under flowing-water conditions is higher than those of the ungrouted pile, the geopolymer grouted pile, and the fly-ash cement grouted pile under static-water conditions, indicating that cement exhibits superior reinforcement performance in sandy soils.

Compared with the ungrouted pile, the bearing capacities of the cement grouted pile and the cement-sodium silicate grouted pile increased by 231.1% and 396.6%, respectively. The test results indicate that under flowing-water conditions, the cement-sodium silicate dual-liquid grouting method exhibits a clear advantage over single-liquid cement grouting in improving the bearing behavior of pile. The advantage of cement-sodium silicate dual-liquid grouting lies in its short gelation and solidification time, which allows it to rapidly bond with the soil through physical and chemical interactions to form reinforced soil. This method can effectively reduce grout loss under flowing-water conditions, thereby enhancing the physical and mechanical properties of the soil around the pile and reducing its permeability.

Axial force transfer characteristics of the pile

The distribution of axial force under different load levels is illustrated in Fig. 8. The steepness of the axial force curves varies among the different grouted piles, which is due to the differing effects of the grouting materials on the mechanical properties of the soil surrounding the piles. Compared with the ungrouted pile, the axial force curves of the grouted piles have steeper slopes, indicating a faster reduction of axial force along the pile length. This reflects the more rapid mobilization of side resistance and highlights the influence of side grouting on the surrounding soil as well as its impact on the overall bearing behavior of the pile foundation.

Fig. 8
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Distribution of pile axial force at various loading stages.

Development characteristics of side resistance

The average side resistance at each pile section can be indirectly obtained from the reduction in axial force between sections. Figure 9 shows the distribution of side resistance along the pile under different load levels. The side resistance develops progressively from the upper soil layers to the lower layers. The resistance in the upper layers is mobilized first, followed by the lower layers. As the vertical load increases, the side resistance in the shallow upper layers gradually stabilizes, while the resistance in the middle and lower soil layers continues to increase.

Fig. 9
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Distribution of side resistance along the pile.

For the ungrouted pile, side resistance gradually increases with depth. In contrast, grouted piles exhibit significantly higher side resistance, exceeding that of the ungrouted pile. Under the ultimate load, the average side resistances of the ungrouted pile, geopolymer grouted pile, and fly ash–cement grouted pile are 64.46 kPa, 135.88 kPa, and 150.79 kPa, respectively. The average side resistances of the cement grouted pile and the cement-sodium silicate grouted pile are 234.27 kPa and 350.75 kPa, respectively. Under flowing-water conditions, the side resistance of the cement grouted pile is lower than that of the cement-sodium silicate grouted pile, primarily due to grout loss and pressure dissipation during grouting. The grout improves the side resistance of the surrounding soil, enhancing its strength and stiffness. Notably, after side grouting, the grout solidifies and bonds with sand particles to form a stable reinforced-grout structure, creating a new pile-grout–soil interaction system, which significantly increases the side resistance in the sandy soil.

In addition, the mobilization of side resistance exhibits different characteristics under various conditions. Under flowing-water conditions, the reinforcement effect in the side-grouted region is more pronounced. The reinforced zone is approximately within ± 5 cm of the grouting hole, with a peak in side resistance at the hole. Below the grouting hole, the side resistance decreases significantly due to partial loss of soil outside the reinforced segment. The performance of different types of grouted piles varies under different conditions. For the geopolymer grouted pile, the side resistance increases most significantly in the 0.05–0.1 m range below the soil surface, making it suitable for projects requiring enhanced bearing capacity in the shallow soil layers. Under conditions without groundwater flow, the geopolymer grouted pile exhibits the greatest increase in side resistance, indicating that this type of grouting achieves optimal reinforcement in a dry environment. In contrast, the side resistance of the fly ash–cement grouted pile increases with pile embedment depth, making it suitable for projects requiring improved bearing capacity in deeper soil layers.

Development characteristics of pile tip resistance

After the loading tests, the data of tip resistance and tip settlement were processed to plot the tip resistance-tip settlement relationship curves of ungrouted piles and various grouted piles, facilitating further analysis of the bearing performance of pile foundations. Figure 10 presents the tip resistance-tip settlement curves of each single pile.

Fig. 10
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Curve of pile tip resistance versus tip displacement.

As shown in Fig. 10, the pile tip resistance versus pile tip settlement curves of the piles exhibit different trends, but overall, grouting enhances the development of pile tip resistance. In the initial loading stage, small pile tip settlement generate relatively large pile tip resistances, indicating that pile tip resistance is highly sensitive to vertical loads at the early stage. With the increase in applied load, the pile tip settlement increases and the pile tip resistance is further mobilized, thus enhancing the bearing capacity of the pile foundation. Under the ultimate load, the pile tip resistance of the ungrouted pile is lower than that of the grouted piles.

In addition, post-grouting reinforces the soil around the pile side, which further facilitates the mobilization of pile tip resistance and improves the overall bearing capacity of the pile. Beyond the ultimate load, the relationship between pile tip resistance and pile tip settlement exhibits a strain-hardening behavior, indicating that the pile foundation has reached its ultimate bearing capacity and is incapable of sustaining additional loads.

As shown in Table 3, the application of different grouting materials can effectively improve the bearing capacity of pile. Under the ultimate load, the pile tip resistance contributes relatively little to the bearing capacity, and the single pile mainly relies on side resistance, thus behaving as a friction pile. The total side resistance of geopolymer-grouted piles and fly ash–cement grouted piles increases significantly, with a relatively high increase in tip resistance as well. The total side resistance of geopolymer-grouted and fly ash–cement grouted piles increases by 110.80–133.93%, while the tip resistance increases by 23.58–118.29%. The values in parentheses are the tip bearing ratio (%).

Table 3 Measured values and improvement ratios of ultimate bearing capacity, total side resistance, and base resistance for individual piles.
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Under flowing-water conditions, the total side resistance of cement grouted piles and cement-sodium silicate grouted piles increases significantly, with a relatively high increase in tip resistance. The total side resistance of these piles increases by 263.44–439.58%, and the tip resistance increases by 31.30–131.71%. The experimental results indicate that side grouting can substantially enhance the bearing capacity of pile foundations, demonstrating a significant effect on improving pile performance. The tip resistance of fly ash–cement grouted piles also shows a notable increase, further contributing to the pile’s bearing capacity. The degree of improvement in bearing capacity varies among different grouting materials, highlighting the need to select the most suitable grouting material and parameters based on specific project conditions. Compared with ungrouted piles, the tip resistance of grouted piles is significantly higher, indicating that grouting effectively enhances pile bearing capacity. This is because side grouting not only strengthens the surrounding soil but also partially reinforces the mobilization of pile tip resistance.

Analysis of slurry stone formation

The objective of this section is to bserve the penetration and diffusion of the grouting slurry in the model foundation. As shown in Fig. 11, excavation results show that under flowing-water conditions, the cement slurry spreads laterally along the pile side and forms a stable cement–soil reinforced structure with the surrounding sand, creating a relatively wide lateral reinforcement zone. However, a small amount of grout remains in an unhydrated state.

Fig. 11
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Schematic diagram of grout distribution under groundwater flow conditions.

The cement slurry stone is shown in Fig. 12a, and the penetration reinforcement beneath the slurry stone is shown in Fig. 12b, where unhydrated slurry can be clearly observed. The unhydrated slurry is influenced by water flow and moves toward the lower part of the pile side. For cement-sodium silicate grouted piles, the injected slurry primarily forms a dense slurry stone, which is firmly attached to the pile side. Its surface bonds with the surrounding sand to create an integrated reinforced structure, allowing the pile-grout–soil interaction to reach a new equilibrium state. The distribution of cement-sodium silicate slurry in the model foundation is shown in Fig. 12c. The upward return height of both cement slurry and cement-sodium silicate slurry is approximately 8–10 cm, and the diameter of the slurry stone is about 4 cm. Compared with the cement slurry stone, the cement-sodium silicate slurry stone has a larger volume, due to its shorter gelation time, higher consolidation rate, and reduced influence from water flow. Notably, the sodium silicate not only accelerates the hydration of cement but also reacts with cement hydration products Ca(OH)₂ to form a viscous calcium silicate hydrate gel with appreciable strength. This contributes to increased slurry viscosity and density, further controlling slurry loss and enhancing the stability of the pile-grout–soil reinforced body. From the perspective of grout formation, using cement-sodium silicate slurry is more effective in achieving the intended post-grouting performance under flowing-water conditions.

Fig. 12
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Grout distribution under groundwater flow conditions.

As shown in Fig. 13, under non-flowing water conditions, the volume of the extracted slurry stones after excavation is relatively small. This is because geopolymer and fly ash–cement slurries have good permeability, and the reinforcement primarily occurs through penetration. The slurry mixes extensively with the surrounding soil, resulting in a smaller retained intact slurry stone.

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Slurry distribution under non-flowing water conditions.

Analysis of SEM test results

Unreinforced sand samples and grout-reinforced sand samples were observed under SEM at magnifications of 100 × , 500 × , and 2000 × , respectively. The micromorphology images of the unreinforced sand sample and the slurry stone treated with different grouting materials are shown in Figs. 14, 15, 16, 17 and 18, respectively.

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Image of sand particles.

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Image of cement-stabilized slurry stone.

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Image of cement-sodium silicate stabilized slurry stone.

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Image of fly ash–cement stabilized slurry stone.

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Image of geopolymer stabilized slurry stone.

As observed from the sand particle images, the unreinforced sand particles exhibit distinct angularity in their structural morphology at low magnifications. At high magnifications, the surface of the unreinforced sand appears rough and uneven yet relatively dense. After the injection of different grouting materials into the sand, a large amount of cementitious material adheres to the surface of sand particles, resulting in mutual bonding of the particles and a significant improvement in overall integrity. Hydration reactions of the various grouting materials generate abundant flocculent C–S–H gels, flaky CH crystals, and a large number of acicular and prismatic ettringite (AFt) crystals42. These hydration products fill the voids between sand particles and enhance the overall integrity of the particles through bonding effects, thereby improving the comprehensive mechanical properties of the sand consolidated body.

Notably, the flocculent C–S–H gels and flaky CH crystals in the cement-sodium silicate slurry stone interweave with each other, forming a denser microstructure than that of the cement slurry stone under high-magnification SEM observation43. This is the primary reason why cement-sodium silicate grouting material is more effective in improving the physical properties of sand around piles under flowing water conditions. Abundant hydration products are observed on the surfaces of both geopolymer slurry stone and fly ash–cement c slurry stone. It is worth noting that a large number of unreacted spherical fly ash particles remain on the surface of fly ash–cement slurry stone, indicating that the hydration reaction is not fully completed. Fly ash only exists in the slurry stone through a filling effect, which results in its strength being lower than that of cement slurry stone and geopolymer slurry stone.

Conclusions

This study conducted model tests of post-grouted piles under vertical loading in sandy foundations. Four types of grouted piles using different grouting materials were designed, and both groundwater flow conditions and long-term loading scenarios were simulated. The study comparatively analyzed the load-settlement behavior, axial force transfer characteristics, side resistance mobilization, and pile tip resistance of single piles under vertical loads. In addition, the load-settlement behavior of the piles under long-term loading was further investigated. Finally, the formation and distribution of grout-reinforced bodies for each grouting material were comparatively analyzed. The main conclusions are summarized as follows.

  1. (1)

    All grouting materials can enhance the bearing capacity of the piles. Among them, the cement-sodium silicate double-liquid grout offers advantages such as short gelation and consolidation time, rapid physical and chemical bonding with the surrounding soil to form a reinforced body, and effectively improves the physical and mechanical properties of the soil around the pile under flowing water conditions.

  2. (2)

    The development of pile side resistance causes the axial force along the pile to gradually decrease from head to bottom, resulting in relatively small axial force transmitted to the pile tip. Compared with ungrouted piles, the pile side resistance of grouted piles is significantly increased. Under flowing water conditions, the pile-side resistance of cement-grouted piles is lower than that of cement–sodium silicate grouted piles, as the cement–sodium silicate grout has a short gelation time and provides a pronounced reinforcement advantage.

  3. (3)

    Grouting can effectively control pile head settlement. Under long-term loading, the average pile head displacement increment of grouted piles is only 45.43% of that of ungrouted piles. Cement–sodium silicate grouted piles can control settlement even under higher pile head loads, showing better reinforcement performance than cement-grouted piles. The cement-sodium silicate double-liquid grout demonstrates remarkable reinforcement effectiveness under flowing water conditions and maintains a stable strengthening effect.

  4. (4)

    Cement grout spreads laterally along the pile, providing a large reinforced area and effectively improving the overall stability of the pile foundation. In contrast, fly ash–cement grout and geopolymer grout produce smaller reinforced volumes, primarily relying on penetration strengthening. Cement-grouted piles and cement-sodium silicate grouted piles form larger slurry stone, creating a stable, integral bearing structure.

  5. (5)

    All grouting materials generate hydration products after being injected into sand, which enhance the integrity and mechanical properties of the slurry stone by bonding and filling the gaps between sand particles. Among these materials, the cement-sodium silicate slurry stone features more densely interwoven products, the geopolymer slurry stone undergoes sufficient hydration, and the fly ash–cement slurry stone exhibits lower strength than the other two types due to the presence of unreacted particles and incomplete hydration.