Experimental Analysis of the Slurry Diffusion Behavior Characteristics of Point Source Grouting and Perforated Pipe Grouting in Sandy Soil

Abstract

Grouting technology is widely used in foundation treatment to achieve the uplifting and correction of buildings. In this context, analyzing the slurry diffusion mechanism and the resulting behavioral characteristics is crucial for guiding precise engineering design practices. This study utilized an independently developed grouting model testing system to conduct grouting experiments on sandy soil employing diverse grouting methodologies and infiltration diffusion patterns. The objectives were to elucidate the characteristics of grouting pressure, lifting displacement, and stress variations within the sandy soil. The findings indicate that slurry diffusion in sandy soil typically progresses through three distinct stages, exhibiting a cyclic pattern of “compaction–splitting–compaction”. We observed that the slurry diffusion pattern closely aligns with the trend of uplift displacement changes. Furthermore, a general downward trend was observed in the stress attenuation of sand during the grouting process. Marked disparities exist in the slurry diffusion mechanism and stress characteristics between point source and perforated pipe grouting. These research outcomes contribute significantly to advancing the theoretical understanding and experimental design of grouting techniques in sandy soil.

1. Introduction

Grouting technology has undergone refinement and advancement over a period of more than two hundred years since its inception. This progress is primarily attributed to its ease of operation, simple technology, exceptional outcomes, and numerous other notable advantages [1,2,3]. As a result, it has been widely applied in various engineering domains, such as tunnels, water conservancy projects, construction, and other engineering fields. This technology imparts substantial benefits for foundation lifting and correction, seepage and leakage control, and foundation reinforcement, among others [4,5,6,7]. Grouting methods can be categorized based on different criteria. Among them, filling grouting, infiltration grouting, compacting grouting, and splitting grouting are more prevalent depending on how the slurry permeates within the soil mass [8,9,10]. In recent decades, scholars have extensively studied slurry diffusion theory, including the well-known Maag theory, column diffusion theory, and Baker’s formula [11,12]. However, the concealment nature and complexity of grouting engineering impose significant limitations on the accuracy of relevant theoretical research. Consequently, the program design predominantly relies on past engineering experience, indicating that theoretical research on grouting significantly lags behind engineering practice [13].

The theoretical research methods for grouting mainly fall into the following three categories: theoretical derivation, model testing, and numerical simulation. Among these, the model testing method is extensively utilized in simulation research on grouting engineering due to its significant advantages, such as ease of operation, low cost, and an intuitive simulation effect. This method offers robust guidance for the design of engineering practices [14,15]. The effect of the relationship between the influence of grouting parameters and soil properties on the slurry diffusion patterns can be elucidated through modeling tests [16,17,18]. Moreover, grouting modeling tests are frequently employed to verify theoretical outcomes derived from diffusion theory and numerical simulations of grouting principles [19,20,21]. Experimental analyses usually focus on the final state after grouting. Methods like regression analyses are employed to derive influence function relationship equations [22,23]. However, there exists a notable dearth of studies examining the time-varying processes that characterize the dynamics of slurry diffusion [24,25,26].

In the context of model testing research, the shape of slurry diffusion is closely related to the injection technique used. Specifically, spherical diffusion is observed during end-hole injection, whereas columnar diffusion occurs with perforated pipe injection [27,28]. These two grouting methods have significant impacts on the diffusion behavior and action effects of the slurry within a soil matrix because of their different operational characteristics. Unfortunately, these differences are frequently neglected in the design of many model experiments, leading to a lack of research outcomes in this area. To address the limitations of previous research, in this study, we conducted model tests of grouting with both perforated pipes and point source methods in sandy soil employing a self-developed grouting model experimental system. The diffusion characteristics of Ordinary Portland Cement (OPC) grout and ultrafine cement grout in sandy soil, as well as their interaction effects with the soil, were examined. The outcomes of these experiments not only contribute to enriching the understanding of the grouting diffusion mechanisms in sandy soil but also offer valuable insights and practical guidance for field applications.

2. Test Materials

(1) Sandy soil

The pore size of sandy soil medium has a direct effect on the migration of slurry particles. To maintain consistent performance parameters of the sand and soil in repeated tests, the standard sand (compliant with American standard ASTM C778-21) used for cementitious sand tests was selected for this experiment, and its production control indices are presented in

Table 1. Control indices for standard sand.

Silicon Dioxide	Moisture Content	Mud Content	Heat Loss	Chloride Ion Content	Floatation Content
>98%	≤0.18%	≤0.18%	<0.47%	≤0.0070%	≤0.0020%

. The standard sets clear requirements for the particle size of standard sand, thus providing robust assurance of the consistency of the basic physical properties of the sand from the source.

To accurately assess the particle size distribution characteristics of the standard sand used to test sandy soil and thereby determine the engineering properties of this type of soil, this study strictly adhered to the relevant requirements of the Standard for Geotechnical Testing. A sieving test was conducted to obtain the particle size distribution curve, and the specific results are presented in Figure 1. The unevenness coefficient (C_u) of sandy soil was 9.31, and the curvature coefficient (C_c) was 2.31. These values indicate that the sandy soil medium’s grading was good, as evidenced by the criteria C_u ≥ 5 and C_c = 1~3. This type of soil possesses high compactness and favorable engineering properties. Additionally, the specific gravity of the test sand was 2.64, and the angle of internal friction was 39°. To ensure the reproducibility of the test, we developed a standardized operational procedure for filling sand samples into the model bucket. Based on predetermined standards of 6.0% moisture content and a density of 1.60 g/cm³ (corresponding to a porosity of 0.75), the sand rain method was used for layered filling, with the sand filler being compacted every 10 cm. The mass of the sand sample to be filled each time was accurately calculated according to the scientific relationship among moisture content, soil particle specific gravity, density, and the void ratio. Uniform compaction was carried out according to predetermined compaction parameters to maintain the basic performance consistency of the sand in each test. After compacting each layer, a scraper was used to roughen the surface of the soil sample such that there were no obvious boundaries between layers after compaction. This standardized filling method minimized differences in soil properties caused by improper filling operations to the greatest extent, ensuring a high degree of consistency in the sand properties for each test.

(2) Cement slurry

The significant difference in particle size distributions between ultrafine cement and ordinary cement results in distinct injectability properties of their slurries and grouting effects [29,30]. As indicated by the research of Sha et al. [31], the type of grout and particle size are key factors determining the injectability of fine sand, while the influence of grouting pressure is not significant. In the grouting model test, two types of cement were utilized—P.O 42.5 cement and 1340-mesh ultrafine cement. The aim of this test was to compare the performance of these two cements during grouting. To enable a direct comparison, a standard sand grouting model test was conducted with a water–cement ratio of 1.0. Specifically, 10 kg of water was mixed with an equivalent amount of cement (based on the water–cement ratio) to prepare the slurry. This slurry was then injected into the sandy soil, ensuring that the same volume of slurry was used for both types of cement. The results of this test allowed a comparative analysis of the grouting effectiveness and performance of P.O 42.5 cement and 1340-mesh ultrafine cement in sandy soil conditions.

3. Test Set-Up Systems

The grouting test system comprised two primary modules—the grouting model apparatus and the data acquisition system. The grouting model apparatus primarily included a mixer, a grouting pump, grouting pipelines, a model barrel, and related accessories. Meanwhile, the data acquisition equipment primarily comprised a pressure transmitter, a displacement meter, a soil pressure sensor, a dynamic stress–strain collector, a computer, and associated accessories. Specifically, this grouting testing device provided a three-dimensional model testing apparatus and method for grouting uplift, allowing simulation of grouting model tests under different environmental water occurrence conditions. It circumvented issues such as inconvenience in hole formation within the test soil sample in the test tank, difficulties in grouting sealing, and inconvenience in surface displacement measurement. Consequently, it enhanced the success rate of the experiments and eliminated the influence of preferential grout diffusion channels formed by gases or liquids in the grouting pipe on grout diffusion in the grouting model test.

The electric mixer (Hangzhou Jingfei Instrument Technology Co., Ltd., Hangzhou, China) utilized had a power of 300 W and a rotational speed of up to 3000 rpm; the high-pressure grouting pump (Shanghai Shizhao Industrial Co., Ltd., Shanghai, China) had a power of 1300 W, a working pressure range of 0–70 MPa, and a stable flow rate of approximately 0.54 L/min. In this study, the grouting tests were conducted with a constant grouting flow rate and a gradually increasing grouting pressure. The grouting pipelines and grouting tubes were made of stainless-steel tubes, which are capable of withstanding high pressures. The pipelines were connected using quick-fit clamp connectors, enabling easy assembly, disassembly, and cleaning. Moreover, the model barrel was fabricated from high-transparency plexiglass, which possesses both high strength and the capacity to endure high-pressure grouting.

The grouting pressure sensor (Chengdu Hongsida Technology Co., Ltd., Chengdu, China) was installed at the bottom of the test model barrel near the grouting pipe to effectively monitor the grouting pressure and obtain real-time pressure data for the grout inside the grouting pipe as accurately as possible. Simultaneously, earth pressure sensors were arranged inside the model barrel at specific distances of 15 cm, 30 cm, and 45 cm from the bottom of the barrel in the longitudinal direction and at distances of 5 cm, 10 cm, and 15 cm from the center of the grouting pipe in the transverse direction. A total of nine earth pressure sensors were deployed. The displacement meters were used to monitor real-time changes in the uplift displacement of the overlying sandy soil surface during the grouting process. Specifically, the surface displacement meters were arranged to measure the surface displacement values at different horizontal distances from the center of the grouting hole, namely, 0 mm (displacement meter 1, DS 1), 55 mm (displacement meter 2, DS 2), 110 mm (displacement meter 3, DS 3), and 165 mm (displacement meter 4, DS 4). The data-monitoring instruments employed were connected to the dynamic stress–strain collector via data lines, with the parameter change curve displayed in real time on a computer. A diagram depicting the assembly of the test apparatus system is provided in Figure 2, while a photograph or graphic representation of the device is shown in Figure 3.

4. Analysis of the Test Results

4.1. Analysis of Grouted Solids

The injectability of a slurry into sand directly influences its diffusion mode. Evaluation of injectability in porous media primarily relies on the particle size ratio of the medium particles to the grouting material particles. In the context of this research, where standard sand with uniform parameters was used, the particle size of the cement particles emerged as the paramount factor dictating the slurry’s diffusion mode. As shown in Figure 4a, the P.O 42.5 cement particles exhibited inadequate penetration into the pores of the standard sand, resulting in a predominantly squeeze grouting mode. In contrast, Figure 4b illustrates that the ultrafine cement particles efficiently infiltrated the pore space, manifesting the coexistence of squeezing and osmotic diffusion. Examination of the stone body’s morphology revealed that the diffusion pattern of “flower pipe” grouting primarily occurs in the transverse direction, whereas point source grouting demonstrates a diffusion trajectory primarily along the longitudinal axis, which was highly consistent with the direction of the grouting action.

4.2. Analysis of the Slurry Diffusion Action Mechanism

4.2.1. Analysis of the Diffusion Mechanism of Perforated Pipe Grouting

After a comprehensive analysis of the temporal variation in the grouting pressure curve, it became clear that the grouting process can be distinctly divided into the following three primary phases: the infiltration and diffusion phase, the squeezing and splitting phase, and the squeezing and accelerated lifting phase. As depicted in Figure 5, the P–t curve for P.O 42.5 cement grouting illustrates that during the initial stage of grouting, the slurry, under low pressure (ranging from 0 to 63 kPa), contacted the sand and soil, infiltrating into their pore spaces in all directions. However, as cement particles rapidly accumulated, blocking the diffusion channels, the slurry’s penetration ability was impeded, leading to a short-lived stage that lasts only 58 s. As slurry injection continued, the permeability of the sand and soil gradually decreased, as indicated by a significant drop in pressure from 104 kPa to 68 kPa at 93 s into the process. Subsequently, the slurry initiated splitting within the sand layer. This repeated cycle of splitting–filling–re-splitting characterized the second stage of the grouting process, where the primary action of the slurry was dominated by squeezing and splitting mechanisms. Notably, this stage entailed a total of five distinct, large splits.

As slurry injection continued, the sand underwent continuous compression, necessitating a gradual increase in the energy required to split the sand layer. Consequently, the diffusion space formed after fracturing expanded until the sand’s compression capacity reached its upper limit. The extent of pressure reduction, observed during five cycles of splitting and grouting, exhibited a pattern of initial escalation followed by a decline (with percentage decreases of 34.6%, 36.9%, 46.6%, 32.2%, and 26.1%, respectively), indicating that the compression capacity of the sand increased and then decreased as the grouting process progressed. Upon completion of five cleavage cycles and the filling of novel diffusion channels, the sand reached its maximum compressibility threshold. At this point, the grouting energy mainly contributed to the uplift of the overlying sand strata. As the uplift process advanced, gradually overcoming the resistive forces imposed by the sand layer (including model barrel boundary resistance and overlying sand pressure), the requisite lifting energy diminished, as evidenced by a steadily declining grouting pressure trend. This observation aligned well with the real-time monitoring of the lifting displacement variation curve.

As shown in Figure 6, the P–t curve of ultrafine cement grouting indicates the occurrence of numerous minute splitting effects during the penetration and diffusion phase, with these stages exhibiting prolonged durations. Subsequently, within the squeezing and splitting stage, only two discernible pressure drops were observed, and the peak grouting pressure (P_max) was significantly higher than that recorded during P.O 42.5 cement grouting. The uniqueness of the grouting process was due to the small particle size of ultrafine cement, which facilitated its diffusion and penetration into the pore spaces of standard sand. Moreover, the larger specific surface area of ultrafine cement particles, along with the increased viscosity of the slurry, means that the diffusion process needed to overcome greater resistance. This aligns with the research findings of Zhang et al. [32] regarding the diffusion characteristics of ultrafine cement grout in coral sand.

4.2.2. Analysis of the Diffusion Mechanism of Point Source Grouting

There is a significant difference between the mechanisms of “point-like” grout diffusion and “column-like” grout diffusion. As depicted in Figure 7, which presents the P–t relationship curve for P.O 42.5 cement grouting, the slurry diffusion process was predominantly governed by compression and density effects, with minimal contributions from infiltration and cleavage. Consequently, the sand layer quickly reached its compression limit within a short time, causing the pressure curve to peak at approximately 130 s and subsequently exhibit a steep decline. This was due to the “point-like” grouting pressure direction being vertical and the sand layer having a direct compression effect; at the same time, the P.O 42.5 cement particles’ size was large, complicating their effective entry into the sand’s pore spaces, and extrusion of the sand layer was more significant. In contrast, due to the small particle size of ultrafine cement particles, they easily entered the pore spaces of the sand and soil, and the direction of the grouting pressure was vertical. In the absence of boundary conditions, the grouting pressure for the ultrafine cement slurry was in the vertical direction, allowing full penetration, cleavage, and diffusion. Therefore, compared with perforated pipe grouting, ultrafine cement point source grouting not only entails longer penetration and diffusion times but also involves many significant fracturing events, as shown in Figure 8.

4.3. Mechanism Analysis of Grouting Lifting Action

4.3.1. Analysis of the Mechanism of Lifting Action in Perforated Pipe Grouting

The curve demonstrating the relationship between lifting displacement (L) and time (t) for P.O 42.5 cement perforated pipe grouting, as shown in Figure 9, was generated using displacement meters at points 1–4 to measure the lifting values at different positions centered around the sand surface and relative to the central lifting values of these positions (the same applies to the following descriptions). In general, the evolution of lifting displacement in sandy soil exhibited a discernible trend, transitioning from a gradual increase to an accelerated lifting pattern. During the infiltration and diffusion stage, the lifting displacement curve remained stable. As the grouting process progressed and the grouting pressure increased, the lifting displacement started to change at the second splitting node. The growth rate of the lift displacement after each splitting became significantly higher. After completion of the fifth splitting, the lift displacement increased steadily at a high rate, which can be clearly observed from the change in the slope of the L–t variation curve. Zooming in on the L–t variation curve locally at the splitting node revealed that the lifting displacement curve underwent a transient horizontal change after grouting splitting, with the third large splitting event being the most obvious. The time needed for the horizontal change in the lift displacement curve to occur corresponded to the time required for the slurry to fill the new diffusion channel, which is consistent with the conclusion of the analysis of the changes in the grouting pressure curve mentioned above. At the same time, the sandy soil lifting displacement obviously showed the phenomena of a high center and a low edge, which is consistent with the morphology change in the stone body taken out after excavation, indicating that the direction of slurry diffusion has a direct influence on lifting changes in the upper layer of sandy soil. The sand grouting uplift effect exhibited a distribution characteristic of “higher in the center and lower at the edges”, which was mainly the result of the combined action of multiple factors such as the mechanical properties of the sand, the energy distribution of the grouted body, and the grout diffusion mode [10]. Therefore, the grouting-induced lifting mechanism in sandy soil involved a particular process. First, the compression volume of the sandy soil absorbed the pre-slurry diffusion volume. Then, as slurry injection continued, the compression of the sandy soil gradually reached its limit, and at the same time, the grouting resistance tended to stabilize. As the grouting process continued to progress, the volume of the injected slurry was redirected to accommodate the ongoing displacement within the overlying sand layers.

Upon analyzing the L–t curve of ultrafine cement grouting shown in Figure 10, it became evident that the evolution of its lifting displacement significantly diverged from that observed for P.O 42.5 cement grouting. Mainly, ultrafine cement grouting exhibited no significant lifting displacement changes in the infiltration–diffusion and compacting–splitting stages, indicating that the slurry was mainly infiltrating and diffusing inside the sandy soil, the compression effect on the sandy soil was small, and the compression process was slow. In the compacting–lifting stage, the lifting displacement showed a high-speed development trend, and the slope of the displacement curve was significantly steeper than that of P.O 42.5 cement grouting. However, the overall displacement value was relatively small. Furthermore, the perforated pipe grouting method necessitates a sequential process involving lateral diffusion compression followed by vertical lifting, thereby delaying the time node at which significant lifting changes occur in the overlying sandy soil.

4.3.2. Analysis of the Lifting Mechanism of Point Source Grouting

The lifting mechanism exhibited by point source grouting demonstrated notable differences compared to that of perforated pipe grouting. As shown in Figure 11, the point source grouting with P.O 42.5 cement responded more quickly to the lifting effect of the overlying sand and soil. This is because the action of the slurry was mainly in the vertical direction, as reflected in the significant upward trend of the lifting displacement curve at the early stage and a greater growth rate of the slope of the displacement curve. However, due to the grouting process in which the slurry mainly played a squeezing role, with almost no fracturing performance, the lifting displacement curve was relatively smooth unlike the horizontal development phenomenon observed for perforated pipe grouting. As depicted in Figure 12, ultrafine cement point source grouting also responded faster than perforated pipe grouting, and the lifting effect of the slurry on the overlying sand was more significant. Furthermore, the horizontal development of the displacement curve at the splitting point matched very well with the aforementioned lifting characteristics observed during perforated pipe grouting, as mentioned earlier.

4.4. Stress State Analysis of the Grouting Process

By embedding soil pressure sensors at various spatial locations within the soil mass, the characteristics of stress variation in sandy soil during the grouting process were obtained. The soil stresses at specific points on the P~t curve during grouting—namely, the initial sharp drop in grouting pressure (first significant splitting), the peak grouting pressure (maximum grouting pressure), and the point at which significant surface uplift displacement occurred (10% of the maximum uplift displacement)—were selected for comparison and analysis of the soil stress variation characteristics under different grouting methods using P.O 42.5 cement and ultrafine cement. Simultaneously, to facilitate a comparative assessment of energy dissipation during the grouting process, the percentage of soil stress relative to the grouting pressure was used as an analytical indicator.

4.4.1. Stress State Analysis of the Perforated Pipe Grouting Process

The test results showed that the soil pressure experienced insignificant stress changes at a height of 45 cm. This was due to the lifting effect of the sand layer; accordingly, the stress analysis only needed to be conducted at heights of 15 cm and 30 cm. Based on the real-time stress data acquired from the soil pressure sensors positioned laterally at 5 cm, 10 cm, and 15 cm from the grouting center, the soil stress at the points of the initial sharp drop in grouting pressure, peak grouting pressure, and significant development of uplift displacement can be obtained, as illustrated in Figure 13. Notably, the P.O 42.5 cement was unable to effectively fill the pore spaces in the sand, which allowed the slurry to rapidly and directly compress these spaces in the lateral direction. In contrast, ultrafine cement had a relatively slower compression effect on the sand. As a result, the P.O 42.5 cement grouting process caused lower energy losses than ultrafine cement throughout the entire grouting process. Moreover, since the sand compression effect was relatively slow, the grouting energy loss with P.O 42.5 cement was smaller than that with ultrafine cement throughout the entire grouting process. It is also worth noting that at a height of 30 cm, both grouting processes experienced significant energy attenuation, with soil pressure constituting merely 0.5–20% of the initial grouting pressure.

As depicted in Figure 13a, at the start of grouting, the stress attenuation degree of the two kinds of cement grouting exhibited a declining trend. When the peak grouting pressure was reached, due to the full diffusion of ultrafine cement in the lateral direction and the stress transfer being limited by the boundary conditions, the stress attenuation of the bottom sand layer manifested a unique characteristic. Specifically, the stress attenuation persisted in a decreasing manner at a height of 30 cm, whereas the ultrafine cement grouting sustained its stress attenuation trend across varying heights, as shown in Figure 13b,c.

4.4.2. Stress State Analysis of the Point Source Grouting Process

Given the minimally significant cleavage effect associated with the point source grouting diffusion process of P.O 42.5 cement, to facilitate a comparative analysis, this study focused solely on the peak pressure point (P–t) and notable displacement points of the sand layer, which were situated at varying locations, to analyze soil pressure variation patterns. As illustrated in Figure 14, owing primarily to the vertical force exerted by the point source grouting on the sandy soil, a greater distance from the center of grouting at a height of 15 cm corresponded to a greater degree of stress attenuation, while at a height of 30 cm, an increasing trend was observed. This notable divergence from the soil pressure patterns observed in perforated pipe grouting can be attributed to the primary vertical direction of slurry diffusion, which induced significant horizontal compression in the surrounding sand layer, resulting in reduced vertical soil pressure. When the sand layer was compressed towards the barrel wall area, the lateral constraint imposed by the barrel redirected the slurry force upwards. Consequently, the relative attenuation of the corresponding force diminished with increasing distance from the grouting center. As depicted in Figure 14a, due to the predominantly vertical nature of the slurry action and the ease with which ultrafine cement particles infiltrated the pore spaces of sandy soil, the stress transfer distance was reduced, resulting in relatively minimal energy attenuation. In contrast, P.O 42.5 cement failed to efficiently penetrate the pore spaces of the sand, ultimately diminishing the role of stress transfer, and its direction of action was mainly vertical, resulting in substantial energy attenuation with the compression of sand pore spaces. When sandy soil compression reached the upper limit of significant lifting, the P.O 42.5 cement grouting pressure was able to have a greater degree of direct action on the soil particles such that its energy attenuation was small. Conversely, in the case of ultrafine cement grouting, the opposite scenario emerged, as illustrated in Figure 14b.

5. Conclusions

(1)

During the grouting process, variations in the diffusion patterns and characteristics of the slurry resulted in distinct behavioral traits. Nevertheless, the underlying diffusion mechanism can be broadly encapsulated as follows: sand compression absorbs the pre-slurry diffusion volume, and with the subsequent injection of slurry, the sand compression gradually reaches the limit, and the resistance to grouting tends to be stabilized. As the grouting process advances further, the injected slurry volume subsequently compensates for the continuous displacement incurred by the overlying sand layers. The horizontally developed “step” feature on the uplift displacement curve is congruent with the slurry’s splitting behavior.

(2)

The grouting energy of P.O 42.5 cement consistently exhibited a gradual attenuation trend within the bottom sand layer. Conversely, ultrafine cement, owing to its finer particle size, was able to effectively enter the pore spaces of sand and soil. However, when utilizing the perforated pipe grouting method, the lateral boundary conditions imposed limitations on the penetration of the ultrafine cement, resulting in a “multi-peak” characteristic in its grouting energy attenuation. In contrast, when employing the point source grouting method, the grouting energy attenuation trends for these two cement types reversed as the grouting process progressed. Notably, the lateral boundary conditions significantly constrained energy attenuation during perforated pipe grouting. Furthermore, the vertical direction of grouting energy showed a substantial reduction, yet the stress characteristics of the two types of grouting methods diverged markedly due to their distinct directions of action.