Experimental Study on Scour Resistance Performance Enhancement of Chongqing Red Clay

Abstract

To effectively utilize Chongqing’s solid waste red clay for scour protection of local cross-river bridge foundations, this study modified Chongqing red clay using curing agent and cement, focusing on the effects of curing agent dosage, cement content, and water-to-solid ratio on the flowability, anti-dispersion performance, and scour resistance of solidified soil. Microstructural characteristics were observed via SEM, with formula fitting performed for two key parameters. Results indicate that an increased curing agent dosage significantly reduces flowability and suspended solids content of solidified soil while negligibly affecting critical shear stress; elevated cement content markedly enhances critical shear stress, slightly improves short-term flowability with reverse effects over time, and minimally impacts anti-dispersion performance; reduced water-to-solid ratio mitigates free water-induced cohesion weakening, lowering suspended solids content and flowability while increasing critical shear stress. Microstructural analysis reveals that generated C–S–H gels and ettringite (AFt) effectively fill pores, enhance matrix integrity, and improve scour resistance. A suspended solids content–flowability relationship model (R² = 0.977) established through quadratic polynomial regression demonstrates excellent predictive performance. The optimal mix proportion (0.3% curing agent, 10% cement, 0.5 water-to-solid ratio) meets specifications and construction requirements, serving as the optimal solidified soil formulation for scour protection.

1. Introduction

The collapse of a bridge inevitably leads to economic losses and may also result in casualties [1]. Among the numerous factors threatening bridge safety, the most prominent issue is flow-induced scour, which reduces foundation embedment depth and gradually weakens foundation bearing capacity, ultimately leading to instability of bridge piers [2,3,4,5,6,7]. In over 1000 bridge collapse cases in the United States, approximately 60% were associated with pier foundation scour [8]. In the last century, 138 bridge failures in Britain were attributed to scour damage [9]. As a typical mountainous city with abundant rivers, Chongqing faces frequent bridge foundation scour failures along the Yangtze River and Jialing River due to rapid flow velocities and high sediment concentrations, highlighting significant limitations in traditional scour protection technologies.

The current scour protection technologies for bridge piers are primarily categorized into active protection and passive protection [10]. Active protection methods aim to resist scouring by reducing flow velocity, with typical approaches including anti-scour rings [11,12], scour prevention trenches [13], and sacrificial piles [14]. However, these methods generally suffer from defects such as coupling failure at contact interfaces, poor adaptability to multidirectional flow fields, and high maintenance costs [15,16,17,18]. Passive protection methods focus on enhancing the anti-scour capacity of riverbeds, mainly through techniques like enlarged foundations [19], concrete protection, partially grouted riprap, and riprap placement [20]. Nevertheless, they encounter challenges, including difficulties in positioning during deep-water construction, shrinkage strain in mass concrete, accumulation of hydration heat, and sudden stiffness changes at contact interfaces [21,22]. Fundamentally, traditional protection methods predominantly rely on macroscopic engineering experience while neglecting the microscopic mechanisms governing soil–water–material multiphase interface interactions.

The application of pre-mixed flowable solidified soil for scour protection of bridge foundations represents an emerging technology in recent years, utilizing soil, cementitious materials, water, and curing agents as raw materials to overcome the limitations of traditional protective measures through microscopic synergistic mechanisms. Its core advantages include (1) elastic modulus of solidified soil matching the riverbed matrix to avoid interface modulus discontinuity; (2) elimination of hydration heat-induced cracking risks associated with mass concrete; (3) local material utilization, environmental friendliness, and low cost [23]; (4) suppression of particle loss by enhancing the soil’s inherent shear strength. Regarding the use of solidified soil for scour protection of bridge abutment foundations, a number of scholars have conducted studies. Wang et al. [24] investigated the scour resistance of four single-sized soil samples (d = 0.075 mm, 0.25 mm, 0.5 mm, and 2.0 mm) and four mixed soil samples under varying hydrodynamic conditions. Their study revealed that particle composition significantly influences scour resistance, with well-graded soils exhibiting far superior performance compared to poorly graded soils, while compaction degree had less impact than particle size. Ouyang et al. [25] conducted physical flume tests on cement-solidified soil-protected pile foundations, concluding the feasibility of this protection method. For small-diameter piles, the optimal protective range diameter was seven times the pile diameter, whereas for large-diameter piles, it was reduced to five times. Wang et al. [26] evaluated the long-term performance of solidified soft soil through immersion, wet–dry cycle, and freeze–thaw cycle tests. Results showed that soft soil modified with curing agent and recycled fine aggregate remained intact after 28-day water immersion, exhibited minimal mass and strength loss under wet–dry cycles, and demonstrated excellent freeze–thaw resistance. Jianing et al. [27] utilized alkali-activated steel slag as a curing agent to improve the mechanical properties of saline soil. They found that unconfined compressive strength increased significantly at dosages exceeding 15%, accompanied by denser structures and continuous gel-bonded particle networks, confirming its effectiveness for saline soil modification. Meng et al. [28] demonstrated that seawater significantly weakens the mechanical performance of cement-solidified soil while curing agent addition mitigates this effect. Each 0.5% increase in polyacrylamide (PAM) dosage improved 7-day and 14-day compressive strength by 27.03% and 34.61%, respectively. Wu et al. [29] revealed that the flowability of solidified soil increases with water content but decreases with curing agent dosage, while the slurry retention rate declines with higher flow velocity and flowability. Zhou et al. [30] highlighted that strength enhancement in cement-solidified soil primarily stems from interparticle cementation and expansive product filling, where cementation forms the foundation of strength development and filling facilitates its progression. Zhang et al. [31] modified five types of soft clay with varying liquid limits using two curing agents, demonstrating that improvement effects depend critically on water content and curing agent dosage.

Although previous studies have confirmed the excellent performance of solidified soil, the contradiction between its flowability and anti-dispersion under complex hydrodynamic conditions remains unresolved. Specifically, high-flowability solidified soil exhibits severe dispersion upon water entry, while low-flowability solidified soil fails to meet pumping requirements, a technical bottleneck that severely restricts engineering applications. This study focuses on the scour protection project for pile foundations of Piers 15 and 16 (with a maximum pile foundation exposure of 5.3 m) of the Chongqing Jiangjin Yangtze River Highway Bridge, using local red clay as the research material. Through flowability tests, anti-dispersion tests, scour resistance tests, and microstructural analysis, we reveal the performance evolution of solidified soil under multi-factor coupling effects, clarify the critical thresholds for underwater anti-dispersion and construction flowability, and determine the optimal mix proportion that balances scour resistance and construction feasibility, thereby providing theoretical support and design basis for bridge pier foundation protection engineering.

2. Test Plan

2.1. Material

The soil material used in this test was red clay collected from the construction site in Jiangjin District, Chongqing, which was dried, crushed, and then utilized. The particle size distribution curve of the red clay is shown in Figure 1, and its basic physical–mechanical parameters are listed in

Table 1. Physical and mechanical parameters of red clay.

Type of Soil	Natural Moisture Content/%	Liquid Limit/%	Plastic Limit/%	Plasticity Index/%	Density/g·cm−3
Red clay	29.5	34	19.5	14.5	1.73

. Ordinary Portland cement (Grade 42.5), with its primary chemical composition detailed in

Table 2. Performance indicators of cement.

Ignition Loss/%	MgO/%	SO3/%	Cl−/%	Specific Surface Area/m2·kg−1	Initial Setting Time/min	Final Setting Time/min
4.50	2.01	2.29	0.01	395	169	252

, and a laboratory-developed curing agent exhibiting thickening and adsorption effects, were employed as binding materials.

2.2. Test Proportioning

This experimental study investigates the effects of variations in curing agent dosage, cement content, and water-to-solid ratio on the flowability, dispersion resistance, scour resistance, and microstructural properties of solidified soil, with the formulated test scheme detailed in

Table 3. Specimen proportioning design.

Specimen Number	Dosage of Curing Agent	Cement Admixture	Water–Solid Ratio
0-10-0.50	0	10%	0.5
0.1-10-0.50	0.1%	10%	0.5
0.2-10-0.50	0.2%	10%	0.5
0.3-10-0.50	0.3%	10%	0.5
0.4-10-0.50	0.4%	10%	0.5
0.3-6-0.50	0.3%	6%	0.5
0.3-8-0.50	0.3%	8%	0.5
0.3-12-0.50	0.3%	12%	0.5
0.3-14-0.50	0.3%	14%	0.5
0.3-10-0.60	0.3%	10%	0.6
0.3-10-0.55	0.3%	10%	0.55
0.3-10-0.45	0.3%	10%	0.45
0.3-10-0.40	0.3%	10%	0.4

.

The test process is shown in Figure 2. The sample preparation process is as follows: (1) Dry mix the red clay, cement, and curing agent according to the proportion. (2) Add a fixed amount of water for wet mixing, stirring for 5 min to make the solidified soil fully mixed to obtain the solidified soil required for this test.

In this experiment, the water dosage was defined as the mass ratio of water to the combined mass of red clay and cement, the cement dosage as the mass ratio of cement to red clay, and the curing agent dosage as the mass ratio of curing agent to red clay. The experimental parameters included a water-to-solid ratio of 0.4~0.6, a cement dosage of 6~14%, and a curing agent dosage of 0~0.4%.

2.3. Test Methods

2.3.1. Flow Tests

In accordance with JHS A313 “Test Methods for Air-Entrained Mortar and Air-Entrained Grout”, established by the Japan Highway Public Corporation, the test setup employs an acrylic cylinder (80 mm height, 80 mm inner diameter), an acrylic plate, and a steel ruler to evaluate the flowability of solidified soil. The solidified soil meets pumping requirements when its flowability exceeds 160 mm. During testing, the acrylic plate is first wetted, and the cylinder is positioned at its center. The solidified soil is then poured into the cylinder, followed by gentle tapping on the outer wall to compact the sample. After filling, the surface is leveled with a spatula. The cylinder is vertically lifted, and upon stabilization of the solidified soil spreading, the maximum diameter and its perpendicular diameter are measured, with the average value recorded as the flowability of the solidified soil.

2.3.2. Anti-Dispersion Test

According to DL/T 5117-2021 “underwater non-dispersible concrete test procedures”, the solidified soil anti-dispersive properties of the test, the test process is as follows: take 500 g of freshly stirred solidified soil divided into 10 equal portions, sequentially placed into a 2000 mL beaker containing 800 mL water, and left to stand for 3 min. A glass pipette was used to aspirate 600 mL of the supernatant within 1 min, of which 200 mL was used for pH testing (direct reading using a pH pen), and the remaining 400 mL was used to do a suspension content test. Take 79.8 mm diameter filter paper into the 105–110 °C oven baked 1 h (the mass is recorded as m₁), in into the 8 cm inner diameter of the Brinell’s funnel, 200 mL of water samples for filtration, the filter residue and the filter paper after the second drying (105–110 °C, 2 h) for weighing (recorded as m₂). The content of solidified soil suspension is calculated according to the Formula (1), where 0.2 is the specimen volume correction factor.

S = 1000 × (m₂ − m₁)/0.2

(1)

2.3.3. Scour Resistance Test

In this test, the Erosion Function Apparatus (EFA) was used as the test equipment for the scour performance of solidified soil, as shown in Figure 3. The apparatus mainly consists of the following modules: (1) closed-loop water circulation system, including water storage tanks and variable frequency speed centrifugal pumps; (2) visualization observation system, using a rectangular pipe (50.8 mm × 101.6 mm) embedded with a high-strength Plexiglas observation window, with a working flow rate range of 0.3~6.0 m/s; (3) specimen clamping system, including Shelby sampling tube (Φ76.2 mm) and precision, electronically controlled piston propulsion mechanism (displacement resolution ± 0.1 mm); (4) data acquisition system, including electromagnetic flow meter.

The experimental procedure is shown as follows: (1) A 5 cm rubber sleeve was installed at the mouth of the Shelby tube, followed by an additional rubber ring placed approximately 1 cm from the tube end on the outer side of the sleeve. (2) The prepared solidified soil mixture was loaded into the Shelby tube and secured in the piston chamber. (3) The specimen surface was leveled with a spatula, and the tube top was embedded into the lower opening of the rectangular channel by elevating the crank handle, adjusting the height to align the tube top with the channel base; (4) Water flow velocities were incrementally increased from low to high, with each scouring test conducted under 4~6 distinct velocities. Scouring patterns were monitored while maintaining alignment between the specimen surface and channel base, with flow velocity increments adjusted according to the scouring rate (maximum 6 m/s). (5) Scouring rate curves were plotted to determine the critical shear stress of the solidified soil based on the curve characteristics [17].

2.3.4. SEM Tests

Some of the solidified soil specimens, which had been tested for unconfined compressive strength, were soaked in 99% anhydrous ethanol for 24 h, then put into an oven at 105 °C for 8 h, and then taken out. Some small pieces with relatively flat natural sections and a thickness of about 2 mm were selected as the test samples, and in order to improve their electrical conductivity, it is usually necessary to plate a layer of gold film on the surface of the samples. Then, the SEM test was carried out.

2.4. Performance Indicators

According to the Construction Drawing Design Instruction of Jiangjin Highway Yangtze River Bridge Pile Foundation Scour Prevention Project and DL/T 5117-2021 Test Procedure for Underwater Non-dispersed Concrete, the underwater anti-dispersed solidified soil should meet the following requirements: (1) flowability: 1 h flow ≥ 160 mm; (2) anti-dispersed performance: suspended solids content < 150 mg/L, pH < 12; (3) scour-resistant performance: 5 h scour-resistant flow rate ≥ 2 m/s, 7 d anti-washout flow rate ≥ 4 m/s.

3. Results and Discussion

3.1. Mobility Analysis

The effect of curing agent dosage on flowability is shown in Figure 4. Taking the 0.3-10-0.50 group as an example, with experimental photographs shown in Figure 5, the curing agent dosage exhibits a significant negative correlation with flowability, although the rate of influence varies across different dosage ranges. When the curing agent dosage was increased from 0% to 0.1%, the flowability showed a sharp decrease, which was 26.2%, 32.7%, and 15% at 0 h, 1 h, and 2 h, respectively. The high sensitivity at this stage may be attributed to the fact that the curing agent enhanced the cohesion between red clay particles and cement particles and reduced the free water content in the solidified soil, resulting in a sudden increase in the viscosity of the slurry, which significantly weakened the flow properties. When the dosage of the curing agent was further increased to 0.4%, the rate of decrease in flowability was obviously slowed down, and the decreases of 0 h, 1 h, and 2 h were 21.9%, 14.3%, and 19.2%, respectively. This phenomenon may be caused by a combination of two reasons: firstly, the anti-dispersion efficiency of the curing agent decreases with the increase in the curing agent dosage. Secondly, a dense network structure formed inside the solidified soil slurry will be less sensitive to the subsequent dosage. It is worth noting that in the interval of 0.1~0.4%, the decrease in flowability at 0 h (21.9%) is significantly larger than that at 1 h (14.3%), and it is speculated that this phenomenon may occur because, with the prolongation of the reaction time of the solidified soil, the gel structure formed at the initial stage tends to be stabilized gradually, and the inhibition of flowability tends to level off. The test results show that the flow rate of 1 h when the dosage of curing agent is 0.3% is 163 mm to meet the construction requirements, while the flow rate of 1 h when the dosage is 0.4% is 150 mm not to meet the construction requirements, so the dosage of curing agent should be less than or equal 0.3%.

The effect of cement dosage on flowability is shown in Figure 6. The pattern of cement dosage on the flowability of solidified soil shows a significant time-varying characteristic in the initial stage (0~30 min), and the higher the cement dosage, the higher the flowability. At the initial moment, the flowability of solidified soil with 14% cement doping was 250 mm, and the flowability of solidified soil with 6% doping was 230 mm. The possible reasons for this phenomenon are that water is added according to the water–solid ratio, and the increase in cement doping also needs to synchronously increase the mixing water quantity of solidified soil, which results in the decrease in viscosity of the solidified soil slurry, the enhancement in the shear dilution effect, and thus, the improvement in the flowability. And at this time, the cement has not yet entered the period of accelerated hydration, and the inhibitory effect of cementation on flowability has not yet appeared. With the time increase (60~120 min), the higher the cement doping, the smaller the fluidity. The 1 h fluidity of solidified soil with 14% cement doping decreased to 155 mm (38% decrease), while the 1 h fluidity of solidified soil with 6% cement doping decreased to 175 mm (23.9% decrease). The possible reasons for this phenomenon is that this time period of cement hydration reaction into the accelerated period, C₃A and gypsum reaction generated calcium alumina (AFt), C₃S hydration generated C–S–H gel, resulting in the internal network structure of the solidified soil slurry is more dense, the macroscopic phenomenon is manifested in the sharp decline in the degree of mobility. During cement dosing of the high test group, the gelling component is more enriched with the hydration product generation rate. In the test group with high cement dosage, the cementitious components are more enriched, the rate of hydration product generation is faster, and the pore-filling effect between red clay particles is more significant, so the rate of flowability decrease is higher [32]. The test results show that when the cement doping is 10%, the 1 h flow of solidified soil is 163 mm, which meets the construction requirements (≥160 mm); when the cement doping is increased to 12% and 14%, the 1 h flow decreases to 158 mm and 155 mm, respectively, which cannot meet the flow requirements of construction.

The effect of water–solid ratio on flowability is shown in Figure 7, which shows a significant positive correlation between water–solid ratio and flowability, and the degree of influence of water–solid ratio on flowability is higher than the degree of influence of curing agent on flowability. When the water–solid ratio decreased from 0.6 to 0.4, the flow of solidified soil decreased by 49.7%, 51.7%, and 41.2% at 0 h, 1 h, and 2 h, respectively, while the flow of the curing agent increased from 0% to 0.4%, and the flow of the corresponding time period decreased by only 42.3%, 42.3%, and 31.4%. Analyzing the possible reasons for this situation involves the direct regulation of the rheological properties of the solidified soil slurry by the water–solid ratio. Under the condition of a high water–solid ratio, the free water film adsorbed on the surface of soil particles is thickened; lubrication is enhanced, and the fluidity of the solidified soil is significantly improved. The decrease in water–solid ratio leads to an increase in the volume fraction of solid particles in the solidified soil slurry and an increase in the probability of friction and collision between particles, which results in an increase in the viscosity of the slurry, resulting in a decrease in fluidity. Test results show that when the water–solid ratio ≥ 0.5, the 1 h fluidity can reach 163 mm to meet the requirements of the project pumping (≥160 mm). While the water–solid ratio is 0.4 and 0.45, the 1 h fluidity is only 115 mm and 140 mm, which cannot meet the construction pumping requirements. Therefore, the water–solid ratio of 0.5 is the lower limit value to meet the construction mobility, which needs to be strictly controlled in the proportioning design.

3.2. Anti-Dispersion Analysis

The effects of curing agent dosage, cement content, and water-to-solid ratio on the anti-dispersion performance of solidified soil are shown in Figure 8, Figure 9 and Figure 10, respectively, with turbidity levels of different mix proportions illustrated in Figure 11. As shown in Figure 8, curing agent dosage exhibits a significant positive correlation with anti-dispersion performance. When the curing agent dosage increases from 0% to 0.4%, the suspended solids content decreases sharply from 750 mg/L to 50 mg/L (a 93.3% reduction), and the pH value drops from 12.2 to 8.7 (a 22.3% reduction). This phenomenon may be attributed to the thickening and adsorption effects of the curing agent, which inhibit the migration and suspension of red clay particles, promote particle agglomeration and sedimentation, and consequently reduce suspended solids content. Additionally, the dense gel membrane formed by the curing agent effectively encapsulates red clay particles, blocks water infiltration pathways, and slows the rapid dissolution of cement hydration products, thereby lowering pH values. Furthermore, as shown in Figure 11a, visual comparisons of different dosages demonstrate that increasing the curing agent dosage gradually clarifies the solidified soil–water interface and significantly improves water transparency, further confirming the enhanced anti-dispersion performance. Experimental results indicate that the curing agent dosage must reach a critical threshold to meet specification requirements. At a 0.2% dosage, the suspended solids content is 220 mg/L (46.6% exceeding the specification limit) with a pH of 9.3, approaching but still at risk of dispersion. Increasing the dosage to 0.3% drastically reduces suspended solids to 97 mg/L (35.3% below the limit) and stabilizes pH at 9.1, fully complying with specifications. However, at 0.4% dosage, the 1-h flowability of solidified soil is 150 mm, failing to meet construction requirements (≥160 mm) and increasing project costs. Therefore, a curing agent dosage ≥ 0.3% satisfies anti-dispersion specifications, while 0.3% optimally balances both performance and construction feasibility.

As shown in Figure 9, when the cement content increases from 6% to 14%, the suspended solids content rises from 94 mg/L to 100 mg/L (a 6.4% increase), and the pH value slightly increases from 9.1 to 9.2 (a 1.1% increase), both remaining within specification limits. Figure 11b demonstrates that solidified soil samples with varying cement contents exhibit similarly clear soil–water interfaces and comparable water turbidity levels after immersion, showing no significant visual differences. This observation contradicts conventional understanding, where cement as a cementitious material is expected to enhance bonding strength and improve anti-dispersion performance with increased dosage. Potential explanations for this discrepancy include: (1) Cement hydration requires sufficient time to develop effective strength, whereas anti-dispersion testing occurs within the initial 15 min before the initial setting, preventing the formation of spatial networks through hydration products like ettringite (AFt) and C–S–H gels to immobilize red clay particles. (2) Increased cement content proportionally raises mixing water demand under the fixed water-to-solid ratio, reducing slurry viscosity and marginally elevating suspended solids content. Experimental results indicate that cement content variations (6~14%) exert minimal influence on the short-term anti-dispersion performance of solidified soil, constrained by the time-lagged nature of cement hydration and water-to-solid ratio effects.

As shown in Figure 10, when the water-to-solid ratio decreases from 0.6 to 0.4, the suspended solids content sharply declines from 350 mg/L to 30 mg/L (a 91.4% reduction), and the pH value decreases from 11 to 8.6 (a 21.8% reduction). Figure 11c demonstrates that specimens with lower water-to-solid ratios exhibit dense and smooth solidified soil surfaces and high water clarity with distinct interfaces, while those with higher ratios show blurred interfaces and turbid water, indicating greater particle dispersion. This phenomenon may occur because high water-to-solid ratios result in excess free water exceeding the adsorption capacities of red clay particles, cement particles, and hydration consumption. The surplus free water occupies interparticle voids, forming a “lubrication layer” that weakens van der Waals forces and electrostatic attraction, promoting particle dispersion. Conversely, low water-to-solid ratios enable full adsorption of free water by red clay particles, cement particles, and hydration products, reducing interparticle spacing and significantly enhancing cohesion. The denser accumulation of hydration products like C–S–H gels under low ratios forms a continuous network structure that effectively encapsulates red clay particles, improving the anti-dispersion performance of solidified soil. Experimental results indicate that at a water-to-solid ratio of 0.55, the suspended solids content reaches 150 mg/L (approaching but risking exceeding the specification limit of <150 mg/L) with a pH of 10.3. Reducing the ratio to 0.5 achieves 97 mg/L suspended solids and pH 9.1, fully meeting specifications while maintaining pumpable 1-h flowability. A further reduction to 0.45 yields 140 mm 1-h flowability, failing construction requirements. Thus, a water-to-solid ratio of 0.5 optimally balances flowability and anti-dispersion performance, preventing both cohesion loss due to excessive free water and insufficient flowability from inadequate free water.

3.3. Relationship Model Between Fluidity and Suspended Matter Content

Flowability and anti-dispersion performance, as the core parameters for evaluating the construction properties of solidified soil, exhibit a significant inverse interaction mechanism in their intrinsic relationship. As evidenced by previous analytical results, increased flowability correlates with degraded anti-dispersion performance, while enhanced anti-dispersion performance corresponds to reduced flowability. This nonlinear relationship can be precisely described mathematically through a quadratic polynomial:

y = 0.013x² − 3.8482x + 295.65,

(2)

The fitting coefficient R² = 0.977, where x represents initial flowability (unit: mm), and y denotes suspended solids content (unit: mg/L). The formula applies to conditions where x ≥ 150 and x ≤ 400, with the solidified soil required to be red clay. As shown in Figure 12, the fitted curve reveals that anti-dispersion performance initially declines gradually with increasing flowability, followed by accelerated deterioration at higher flowability levels. This phenomenon may stem from the dramatic reduction in interparticle spacing, cohesion, and van der Waals forces as flowability increases, causing drastic alterations to the spatial network structure of cementation products and transitioning the solidified soil from a three-dimensional continuous skeleton to discrete clustered aggregates.

3.4. Scour Resistance Analysis

Critical shear stress refers to the hydrodynamic shear stress corresponding to the critical state at which soil transitions from static to motion under water flow. The experimental results of critical shear stress for solidified soil are shown in Figure 13, Figure 14 and Figure 15, with photographs of the scouring initiation process provided in Figure 16. The results demonstrate that critical shear stress exhibits significant influences from cement content, water-to-solid ratio, and curing duration but minimal impact from curing agent dosage. As shown in Figure 13, the critical shear stress increases with prolonged curing time but exhibits no clear correlation with curing agent dosage. Previous experimental results revealed that while increased curing agent dosage enhances cohesion between red clay particles, it does not proportionally improve scour resistance. This phenomenon may be attributed to the dual effects of the curing agent: although it strengthens interparticle cohesion, it simultaneously forms a membranous coating around cement particles, hindering their contact and hydration reactions with water. While enhancing cohesion, the curing agent also inhibits cement strength development, leading to non-monotonic trends in scour resistance with increased dosage. Increasing curing agent dosage fails to improve scour resistance, raises material costs, and reduces flowability. Therefore, the optimal curing agent dosage should only meet anti-dispersion specifications (≥0.3%) without additional increments for scour protection.

The influence of cement content on critical shear stress is shown in Figure 14. As curing time increases, the critical shear stress of solidified soil gradually rises. At 1-h curing, solidified soils with different cement contents exhibit minimal differences in critical shear stress, but these differences become increasingly pronounced with prolonged curing, demonstrating higher critical shear stress at greater cement contents. This indicates that cement’s solidification effect on red clay develops progressively during curing and intensifies over time. This phenomenon may arise because cement hydration proceeds gradually: minimal early-stage hydration contributes little to scour resistance, while prolonged curing enables cement to participate in hydration reactions, producing calcium silicate hydrate (C–S–H), ettringite (AFt), and other products that form a three-dimensional network structure. This network fills pores between red clay particles, enhances interparticle cementation strength, and improves the overall performance of red clay. Additionally, although red clay inherently possesses strong cohesion, its strength drastically diminishes upon water absorption. Enhancing its scour resistance relies on boosting interparticle cohesion and reducing water adsorption. Cement hydration consumes free water between particles, thereby lowering water adsorption, while hydration products cement particles to strengthen cohesion. Increased curing duration and cement content amplify free water consumption and interparticle cohesion. Consequently, higher cement contents accelerate the growth of scour resistance with curing time, yielding superior scour resistance at equivalent curing durations.

The influence of water-to-solid ratio on critical shear stress is shown in Figure 15. As curing time increases, the critical shear stress of solidified soil gradually rises, while under the same curing duration, higher water-to-solid ratios result in lower critical shear stress, indicating that the scour resistance of solidified soil is highly sensitive to water content. This phenomenon may occur because high water-to-solid ratio conditions increase free water between red clay particles, elevating porosity and weakening interparticle cementation, thereby reducing shear resistance. Conversely, low water-to-solid ratios promote denser cement particle distribution, enabling hydration-generated cementitious materials to better fill pores and encapsulate red clay particles, thereby enhancing shear resistance.

4. SEM Test Results

Figure 17 presents SEM images of red clay specimens and solidified soil specimens from the 0.3-10-0.50 group. The SEM images of red clay specimens in Figure 17a,b reveal a microstructure dominated by blocky clay particles with rough surfaces, interconnected primarily through point-to-face and face-to-face contacts. The loose particle arrangement and abundant pores result in weak cohesion between red clay particles and poor scour resistance. SEM images of solidified soil specimens cured for 3 days Figure 17c,d show irregularly distributed C–S–H gels and sparse needle-like ettringite (AFt), which partially fill pores and cracks between red clay particles, interconnect their internal structures, and enhance overall integrity, thereby improving strength and scour resistance. Dark shadow regions observed in these images indicate incomplete hydration reactions at this stage, with residual micro-pores persisting between particles. SEM images of solidified soil specimens cured for 28 days Figure 17e,f demonstrate evolved microstructures dominated by acicular structures and granular aggregates, accompanied by significantly reduced dark shadow regions. This progression confirms enhanced hydration reactions producing substantial C–S–H gels and AFt that encapsulate red clay particles, dramatically strengthening microstructural integrity and consequently achieving remarkable improvements in mechanical strength and scour resistance.

5. Conclusions

This study conducted flowability tests, anti-dispersion tests, scour resistance tests, and microstructural analyses on Chongqing red clay, investigating the effects of curing agent dosage, cement content, and water-to-solid ratio on these properties. The main conclusions are as follows:

(1) Increasing curing agent dosage significantly reduces the flowability and suspended solids content of solidified soil while showing negligible impact on critical shear stress. When the curing agent dosage increases from 0% to 0.4%, the suspended solids content decreases by 93.3% (from 750 mg/L to 50 mg/L), the initial flowability decreases by 42.3% (from 390 mm to 225 mm), the 1-h flowability decreases by 42.3% (from 260 mm to 150 mm), and the 2-h flowability decreases by 31.4% (from 153 mm to 105 mm);

(2) Increasing cement content markedly enhances the critical shear stress of solidified soil, slightly improves short-term flowability with reverse effects over time, and minimally affects anti-dispersion performance. When cement content increases from 6% to 14%, the critical shear stress at 2 h, 4 h, and 6 h increases by 100%, 114.8%, and 81.9%, respectively;

(3) Reduced water-to-solid ratio mitigates the weakening effect of free water on interparticle cohesion in solidified soil, decreasing both suspended solids content and flowability while increasing critical shear stress. When the water-to-solid ratio decreased from 0.6 to 0.4, suspended solids content reduced by 91.4%, with 0-h, 1-h, and 2-h flowability decreasing by 49.7%, 51.7%, and 41.2%, respectively, while 2-h, 4-h, and 6-h critical shear stress increased by 307.6%, 163.2%, and 119.5%;

(4) The optimal mix proportion (0.3% curing agent, 10% cement, 0.5 water-to-solid ratio) meets both specification and construction requirements, serving as the optimal formulation for solidified soil’s scour resistance. SEM analysis demonstrates that red clay particles exhibit loose arrangement with abundant pores and weak cohesion, resulting in poor scour resistance. The generated C–S–H gels and ettringite (AFt) in solidified soil fill interparticle voids, enhancing the integrity of red clay and interparticle cohesion, thereby improving scour resistance to meet performance criteria. The relationship between suspended solids content and flowability can be accurately described by a quadratic polynomial with a fitting coefficient R² of 0.977.