Study on the Permeability Characteristics of Slurry-like Mud Treated by Physicochemical Composite Method

Abstract

The disposal of waste slurry in engineering construction and water environment remediation has become increasingly prominent. The physicochemical composite method integrating flocculation, solidification, and precompression has emerged as an efficient treatment approach, yet the permeability characteristics of slurry reinforced by this method remain insufficiently understood. This paper takes the high-moisture-content sludge generated from lake dredging projects reinforced by the physicochemical composite method as the research objective. Through permeability tests, the permeability characteristics of the physicochemical composite-modified slurry under different factors are tested, and its permeability characteristics are quantified through fitting methods. The research results show that the permeability coefficient decreases with the extension of curing time, decreases with the increase in curing agent dosage, increases with the increase in initial moisture content, and decreases with the increase in pre-stress.

1. Introduction

During the ecological dredg8ing process of rivers, lakes, and reservoirs, a large amount of high-moisture slurry is produced [1,2]. According to extensive investigations and research statistics, China generates dredged sludge in the order of hundreds of millions of cubic meters per year [3]. The inherent characteristics of hydraulic dredging result in the sludge having unfavorable engineering properties such as naturally high moisture content, strong compressibility, extremely low shear strength, and small permeability coefficient. These properties lead to large land occupation, long treatment duration, and difficulty in direct resource utilization at the terminal stage. How to economically, reasonably, and efficiently dispose of such a massive volume of dredged sludge has become a major challenge in the engineering field.

To improve solidification efficiency and address the challenges of dredged sludge treatment, Zhang et al.’s [4,5] research team proposed an integrated vacuum preloading–focculation–solidiffcation combined method (VP-FSCM) method. This approach enhances the permeability coefficient of sludge through flocculation, followed by a significant reduction in water content under vacuum preloading, thereby achieving improved solidification strength. As an emerging dredged sludge treatment technology, the VP-FSCM has effectively advanced the resource utilization of dredged sludge. It not only resolves the disposal issues of massive volumes of dredged sludge but also enables “waste-treating-waste” and “turning waste into treasure,” providing ideal fill materials for embankments and other construction projects. Several domestic and international scholars have validated through laboratory and field tests that flocculation–solidification combined modified sludge (FSCM) is feasible as fill material [6,7]. After modification with solidifiers, flocculants, and other additives, the permeability characteristics of the soil undergo significant changes. As a key parameter in the modification process, clarifying the permeability performance of modified soil is of great importance.

In the current literature, numerous scholars have conducted a series of permeability tests to study the permeability performance of flocculated and solidified dredged sediment, providing a theoretical basis for optimizing sediment modification technology and improving the resource utilization efficiency of modified soil. Numerous studies have shown that after adding flocculants to dredged sludge, fine particles form flocs under the flocculation effect, thereby improving the permeability of the sludge. Xu et al. [8] focused on the influence of flocculant types on the permeability characteristics of dredged sediment. The research results showed that calcium hydroxide could significantly improve the permeability of dredged sediment; the permeability coefficients of soil samples treated with anionic polyacrylamide and ferric chloride were similar to that of the original sediment, thus their impact on the permeability of dredged sediment was not significant. Huang [9] found that the addition of lime and polyacrylamide could enhance the permeability of dredged sediment, which was beneficial for accelerating drainage and consolidation of the sediment under vacuum preloading in the later stage, thereby significantly reducing the construction period. In the combination of 1% lime + polyacrylamide, there was an optimal dosage of polyacrylamide (0.1% for anionic polyacrylamide, 0.1% for cationic polyacrylamide, and 0.125% for non-ionic polyacrylamide). Cai et al. [10] proposed a composite technology combining flocculation dewatering and vacuum preloading (i.e., flocculation-vacuum preloading technology) and studied its operational procedures and the permeability performance of modified sediment. The results showed that the flocculation-vacuum preloading technology could fundamentally reduce the risk of sediment blockage around drainage pipes, thereby significantly improving the permeability of modified soil. The above studies mainly focused on pure flocculated sediment, which is different from the properties of mixed sediment modified by the flocculation–solidification combined treatment.

Research on the permeability of chemically stabilized soils has demonstrated that, under identical void ratios, stabilized soils exhibit significantly lower permeability compared to untreated soils. This phenomenon clearly indicates that stabilizers can effectively reduce soil permeability [11]. Li et al. [12] conducted a study on the consolidation and permeability characteristics of marine-dredged slurry treated with a composite system of carbide slag and granulated blast furnace slag. The results showed that the consolidation coefficient of the treated slurry was 30–100 times higher than that of untreated slurry due to the increased permeability coefficient. During the consolidation test, the permeability of the treated slurry decreased sharply during the initial compression of the sample and then decreased at a lower rate with the reduction of void ratio, showing a bilinear correlation between permeability and void ratio. Wang et al. [13] investigated the permeability characteristics of dredged sludge treated with a combination of magnesium oxychloride cement and industrial solid waste. The research results showed that the formation of amorphous calcium silicate hydrate (C-S-H) gel, phase 5 and phase 3, could create a stronger interlocking network, resulting in magnesium oxychloride cement, which is an industrial solid waste stabilized slurry exhibiting higher water resistance and lower permeability coefficient. Cui et al. [14] systematically analyzed the factors influencing the permeability of stabilized dredged soils at a microscopic level using scanning electron microscopy (SEM). Their findings revealed that the permeability coefficient of stabilized dredged soils is not governed by a single factor but rather by the synergistic effects of the intrinsic permeability of the original soil and the stabilization mechanism of the curing agent. Liu et al. [15] observed that the permeability coefficient of stabilized soils decreases markedly with increasing stabilizer content and extended curing periods. Notably, during early curing stages or at low stabilizer dosages, even minor variations in these parameters can lead to significant changes in the permeability coefficient. Li et al. [16] conducted falling-head permeability tests on Yellow River alluvial silt treated with cement and industrial byproduct-based stabilizers. Their results showed that intense chemical reactions occur during the initial stabilization phase where rapidly formed hydration products densely fill soil pores, resulting in tighter particle packing and a substantial reduction in permeability. It should be noted, however, that these studies involved prolonged curing periods and did not account for the influence of stabilizers on permeability in the presence of flocculants.

This paper focuses on the early permeability characteristics of dredged sludge modified by the FSCM. Through permeability tests, it systematically analyzes the evolution mechanism of the early permeability coefficient of physicochemical composite-modified sludge under different conditions including stabilizer dosage, equivalent initial water content, curing time, and preloading stress. This study summarizes the functional relationships between various factors and the permeability coefficient, providing theoretical support for relevant engineering practices.

2. Materials and Methods

Test Materials and Protocol

(1)

Test Materials

The high-water-content dredged sludge used in this study was collected from the dredging project site at Jingyue Lake, Zhongxiang City, and Hubei Province, China, exhibiting a grayish black appearance. After retrieval, the fundamental physical property indices of the sludge were determined in accordance with the “Standard for Soil Test Methods” (GB/T 50123-2019) [17], with the specific measurement results presented in

Table 1. Basic physical property indices of Zhongxiang fluid mud samples.

Sample Category	Liquid Limit/%	Plastic Limit/%	Plasticity Index	Loss on Ignition (LOI)/%	Specific Gravity of Soil Particles
	51.12	26.18	24.94	4.41	2.69

. The analysis of the test results identified the Zhongxiang sludge as high-liquid-limit clay (CH). Furthermore, particle size analysis of the Zhongxiang sludge samples was conducted using a laser particle size analyzer reveals that the Zhongxiang sludge samples contain clay particles (diameter < 0.005 mm): 44.88%, silt particles (diameter 0.005–0.075 mm): 54.80%, and sand particles (diameter > 0.075 mm): 0.32%.

The composite curing agent is uniformly mixed and prepared at a mass ratio of 1:1 using ordinary Portland cement (OPC) and ground granulated blast furnace slag (GGBS) as shown in

Table 2. The chemical composition of cement and blast furnace slag.

Material Type	Mass Fraction of Chemical Composition/%
	CaO	SiO2	Al2O3	Fe2O3	MgO	K2O	Na2O
OPC	57.25	23.54	6.60	3.25	1.88	0.71	0.15
GGBS	0.17	55.06	44.12	0.76	0.06	0.55	0.06

. The composite flocculant used consists of the inorganic flocculant quicklime (CaO) and the organic polymer flocculant anionic polyacrylamide (APAM).

(2)

Sample Preparation Method

To ensure the quality control of the specimens, the following sample preparation procedures shall be strictly followed, where the materials and apparatus used are shown in Figure 1:

a. Preparation of fluid mud slurry: Based on the measured natural water content of the fluid mud, calculate the required water content of the fluid mud slurry for specimens with the specified equivalent water content using Equation (1) below. Then, determine the additional amount of water to be added to the original mud accordingly. Subsequently, process the mixture using a specific mixing procedure (1 min manual stirring + 5 min mechanical stirring at 120 r/min) to obtain the fluid mud slurry meeting the required water content (as Figure 2a shows).

$$m={\displaystyle \frac{m_{\mathrm{M}}}{1+w}}\times W-{\displaystyle \frac{m_{\mathrm{M}}}{1+w}}\times w-m_{\mathrm{wAPAM}}$$

(1)

In the equation,

m is the mass of water to be added for preparing the fluid mud slurry (g), and

m_M is the mass of the fluid mud (g), which can be weighed according to the following experimental requirements:

w is the water content of the fluid mud (%), measured as 110%;

W is the equivalent initial water content (%);

m_wAPAM is the mass of water required for preparing the APAM solution (g), with its specific value given in Equation (2).

b. Incorporation of Cement-GGBS Composite Binder: First, accurately weigh ground granulated blast furnace slag (GGBS) and ordinary Portland cement (OPC) according to the predetermined binder dosage. Subsequently, rapidly transfer both materials into a beaker and homogenize using glass rod manual mixing to obtain a uniform cement-GGBS composite binder. Add the composite binder into the fluid mud slurry prepared in Step a, then execute a specified mixing sequence (1 min mechanical mixing at 120 rpm + 1 min manual mixing + 5 min mechanical mixing at 120 rpm) to produce a homogeneous binder–mud slurry mixture (as Figure 2b shows).

c. Flocculant Addition: Prepare a 0.2% anionic polyacrylamide (APAM) solution by dissolving APAM powder completely using a 2.5 L mini mixer at 120 rpm for 30–60 min. Precisely weigh the required masses of CaO particles and APAM solution, then gradually introduce them into the binder–mud slurry mixture from Step 5. Employ the 2.5 L mini mixer (60 rpm) for 3–4 min until clearly visible macro-flocs form, at which point mixing is immediately terminated. The resulting product constitutes a homogenized flocculant-binder–mud slurry system (as Figure 2c,d shows).

$$m_{\mathrm{wAPAM}}={\displaystyle \frac{m_{\mathrm{M}}}{1+w}}\times W_{\mathrm{F}}\times 500$$

(2)

In the equation,

m_wAPAM is the mass of water required for preparing the anionic polyacrylamide (APAM) solution (g);

m_M is the mass of the fluid mud (g);

w is the water content of the fluid mud (%), taken as 110%;

W_F is the dosage of APAM (%), taken as 0.125%.

d. Using a sample preparation method similar to consolidation testing, the mixed slurry is slowly poured into the cutting ring of the low-pressure consolidometer while being vibrated. It is then placed into a laboratory-customized low-pressure consolidometer for stepwise preloading and curing to obtain the specimen required for permeability testing.

e. Finally, the constant-head permeability test is conducted using the TKA-STC series fully automatic permeameter–consolidometer (as shown in Figure 3), which consists of a confining pressure permeation line, back-pressure seepage line, permeameter cell, base plate, loading rod, confining pressure valve, and back-pressure valve, to determine the permeability coefficient of the specimen.

During the permeability test, the pressure difference between the permeation pressure and back pressure was set at 20 kPa, thus yielding the calculation formula for the specimen’s permeability coefficient:

$$k={\displaystyle \frac{QL}{3.06\times \left(20+9.8L\right)\times t}}\times {10}^4$$

(3)

In the formula,

k= permeability coefficient (m/s);

Q = seepage flow volume during time t (m³);

L = seepage path length (i.e., specimen height) (m);

t = seepage duration (s).

Three experimental groups (A, B, and C) were established, comprising a total of 36 test conditions. The specific parameters are summarized in

Table 3. Test conditions.

Grouping	Test Conditions	W/%	C/%	WF/%	WC/%	P/kPa	T/h
A	W180C7.5P30	180	7.5	0.125	1.5	30	6, 24, 48, 72
	W180C10P30		10
	W180C12.5P30		12.5
	W180C15P30		15
B	W120C10P30	120	10	0.125	1.5	30	6, 24, 48, 72
	W150C10P30	150
	W180C10P30	180
	W210C10P30	210
C	W180C10P30	180	10	0.125	1.5	30	6, 24, 48, 72
	W180C10P60					60
	W180C10P90					90

Note: The equivalent initial water content (W), binder dosage (C), APAM dosage (W_F), and quicklime (CaO) dosage (W_C) are respectively defined as the mass ratios of water, binder, APAM, and CaO to soil particles in the mixed slurry; P denotes preconsolidation stress; T represents curing time.

:

Group A: Binder dosage levels of 7.5%, 10%, 12.5%, and 15%;

Group B: Equivalent initial water content values of 120%, 150%, 180%, and 210%;

Group C: Preconsolidation pressure settings of 30 kPa, 60 kPa, and 90 kPa.

All three groups shared the same curing time conditions (6 h, 24 h, 48 h, and 72 h) as the common controlled variable.

3. Results

3.1. Early Permeability Characteristics of Fluid Mud Modified by FSCM

3.1.1. Effect of Binder Dosage on Permeability Coefficient

Figure 4 shows the relationship curves between the permeability coefficient (k) of fluid mud modified by the physicochemical composite method and the binder dosage (C) under different curing times (T), when the equivalent initial water content (W) is 180% and the preconsolidation stress (P) is 30 kPa. As can be seen from the figure, the permeability coefficient of the specimen gradually decreases with the continuous increase of the binder dosage, reaching its minimum value when the binder dosage attains 15%.

Further analysis of the trend in Figure 5 reveals a significant pattern: the permeability coefficient of physicochemical composite-modified fluid mud exhibits a “rapid initial followed by gradual” decreasing trend with increasing binder dosage. This experimental finding is consistent with the conclusions drawn by Wu Yihan in their study on the impermeability characteristics of cement-stabilized soil-sand mixtures. Taking the 6-h curing condition as an example, at low binder dosage levels (7.5–10%), each incremental increase in binder content resulted in a sharp reduction in the specimen’s permeability coefficient (total decrease of 0.93 × 10⁻⁷ cm/s within this range). At high binder dosage levels (12.5–15%), further increases in binder content led to a slower rate of permeability reduction (total decrease of 0.48 × 10⁻⁷ cm/s within this range, approximately only 50% of the reduction observed at low dosage levels). The data in Figure 5 demonstrate that the specimen’s permeability coefficient consistently fluctuates within the same order of magnitude (10⁻⁷ cm/s), indicating that the physicochemical composite-modified fluid mud possesses relatively low permeability. Therefore, in practical engineering applications, increasing the binder dosage can effectively reduce the early-stage permeability of modified fluid mud, though it should be noted that this method has limited effectiveness in further permeability reduction.

Subsequently, we conducted a numerical investigation into the variation patterns of the permeability coefficient under different binder dosages. Specifically, following the trend shown in Figure 5, we performed a regression analysis using the exponential relationship expressed in Equation (4), with detailed results presented in Figure 5 and

Table 4. Fitting parameters of permeability coefficient for modified fluid mud under different binder dosages.

Working Condition	a1	Error Range	a2	Error Range	a3	Error Range	R2
W180T06P30	9.32	±0.82	8.51	±2.69	2.38	±0.81	0.998
W180T24P30	9.27	±1.12	8.39	±3.451	2.38	±1.03	0.997
W180T48P30	9.04	±1.16	8.64	±4.25	2.31	±1.25	0.996
W180T72P30	9.06	±1.36	8.43	±4.39	2.36	±1.28	0.996

. From a data representation perspective, Equation (4) demonstrates excellent goodness-of-fit (R² > 0.99), indicating that this mathematical expression can accurately describe the relationship between the permeability coefficient of physicochemical composite-modified fluid mud and the binder dosage.

$$k=a_1\times \exp (-{\displaystyle \frac{C}{a_2}})+a_3$$

(4)

In the equation,

k = permeability coefficient (10⁻⁷ cm/s);

C = binder dosage (%);

a₁, a₂, and a₃ = fitting parameters.

3.1.2. Effect of Equivalent Initial Water Content on Permeability Coefficient

Figure 6 shows the relationship curves between the permeability coefficient (k) of physicochemical composite-modified fluid mud and the equivalent initial water content (W) under different curing times (T), when the binder dosage (C) is 10% and the preconsolidation stress (P) is 30 kPa.

As evident from the overall trend in Figure 6, the specimen’s permeability coefficient increases continuously with rising equivalent initial water content. In other words, increasing the equivalent initial water content enhances the permeability performance of physicochemical composite-modified fluid mud. This experimental result agrees with the findings of Wang et al. [18].

Subsequently, we further investigate the influence patterns of equivalent initial water content on the permeability coefficient of physicochemical composite-modified fluid mud. The specific methodology involves conducting systematic regression analysis of experimental results using the mathematical model shown in Equation (5), with analytical results presented in

Table 5. Fitting parameters of permeability coefficient for modified fluid mud under different equivalent initial water contents.

Working Condition	b1	Error Range	b2	Error Range	b3	Error Range	R2
T06C10P30	−7.93	±1.38	195.90	±178.29	8.51	±3.13	0.996
T24C10P30	−7.98	±1.13	194.22	±148.97	8.44	±2.64	0.997
T48C10P30	−7.94	±0.35	170.64	±77.73	8.01	±1.39	0.999
T72C10P30	−7.95	±0.27	153.17	±52.99	7.67	±0.95	0.999

and Figure 7. For practical engineering applications, Equation (5) can be employed to preliminarily predict the permeability coefficient under the following conditions: binder dosage of 10%, preconsolidation stress of 30 kPa, specified curing times (T = 6 h, 24 h, 48 h, 72 h), and any test condition with an arbitrary equivalent initial water content.

$$k=b_1\times \exp (-{\displaystyle \frac{W}{b_2}})+b_3$$

(5)

In the equation,

k = permeability coefficient (10⁻⁷ cm/s);

W = equivalent initial water content (%);

b₁, b₂, and b₃ = fitting parameters.

3.1.3. Effect of Curing Time on Permeability Coefficient

Figure 6 shows the relationship curves between the permeability coefficient (k) of physicochemical composite-modified fluid mud and curing time (T) when the binder dosage (C) is 7.5%, 10%, 12.5%, and 15%, respectively. As can be observed from Figure 8, the influence of curing time on the specimen’s permeability coefficient follows similar patterns, all exhibiting time-dependent characteristics. Specifically, as the curing time progressively increases, the permeability coefficient of the physicochemical composite-modified fluid mud continuously decreases, reaching its minimum value at 72 h of curing. This phenomenon aligns with the conclusions drawn by numerous scholars [19,20] in their studies on the permeability characteristics of stabilized soils.

To gain a more detailed understanding of the aforementioned properties, consider the following experimental conditions: an equivalent initial water content of 180%, a curing agent content of 10%, and a pre-stress of 30 kPa. When the curing time is extended from 6 h to 24 h, the reduction in the permeability coefficient of the sample is 1.13%, and when the curing time is extended from 24 h to 48 h, the decrease in the permeability coefficient of the sample is 0.57%. Further extending the curing time from 48 h to 72 h results in a decrease of only 0.38% in the permeability coefficient of the sample. The data indicate that although the permeability coefficient of the physically and chemically modified slurry decreases with increasing curing time, showing a trend of “rapid decrease followed by slow decrease,” the overall change is relatively small [11].

Based on the shape of the relationship curve shown in Figure 9, the exponential function shown in Equation (6) is selected as the analytical model. Figure 9 demonstrates the fitting effect of Equation (6) on the experimental data, and Table 5 lists the corresponding parameter fitting values. As observed in Figure 7, the experimental data points are closely distributed around the fitted curve, indicating good fitting performance. Analyzing the data in

Table 6. Fitting parameters of permeability coefficient for modified fluid mud under different curing times.

Working Condition	c1	Error Range	c2	Error Range	c3	Error Range	R2
W180C7.5P30	0.24	±0.01	32.55	±2.96	6.03	±0.01	0.999
W180C10P30	0.15	±0.01	27.10	±5.37	5.18	±0.01	0.997
W180C12.5P30	0.19	±0.01	25.50	±6.14	4.33	±0.01	0.995
W180C15P30	0.11	±0.01	19.68	±2.58	3.92	±0.01	0.998

, the R² values for the fitted curve of Equation (6) are all greater than 0.99 under different curing agent dosages, proving that the exponential function can effectively describe the relationship between the permeability coefficient of physically and chemically modified sludge and curing time during the early stage.

$$k=c_1\times \exp (-{\displaystyle \frac{T}{c_2}})+c_3$$

(6)

In the equation,

k = permeability coefficient (10⁻⁷ cm/s);

T = curing time (h);

c₁, c₂, and c₃ = fitting parameters.

3.1.4. Effect of Preconsolidation Pressure on Permeability Coefficient

Figure 10 clearly shows the relationship curves between the permeability coefficient k of the physically and chemically modified slurry and the preload stress P at curing durations T of 6 h, 24 h, 48 h, and 72 h. As shown by the curve patterns in Figure 10, within a certain range of preload stress (30–90 kPa), as the preload stress gradually increases, the permeability coefficient of the physically and chemically modified slurry exhibits a decreasing trend that is “slow at first and then rapid” under different curing times. This phenomenon indicates that increasing the preload stress can reduce the permeability performance of the physically and chemically modified slurry, providing an important theoretical basis and data foundation for related engineering practices.

To gain a more detailed understanding of the aforementioned changes, we consider a specific case with an equivalent initial moisture content of 180%, a curing time of 6 h, and a curing agent content of 10%. When the pre-stress was increased from 30 kPa to 60 kPa, the permeability coefficient of the physically and chemically modified slurry decreased by 0.56 × 10⁻⁷ cm/s; When the pre-compression stress was increased from 60 kPa to 90 kPa, the permeability coefficient of the physically and chemically modified slurry decreased by 1.37 × 10⁻⁷ cm/s. Compared to the pre-stress change range from 30 kPa to 60 kPa, the reduction in permeability coefficient of the physically and chemically modified slurry in the range from 60 kPa to 90 kPa reached as high as 145%, nearly 1.5 times the reduction.

Next, we will conduct an in-depth study on the influence of preload stress on the permeability coefficient of physically and chemically modified slurry. Specifically, based on the relationship curve shown in Figure 10, we will select a suitable regression formula (see Formula (7)) to perform data fitting. Figure 11 shows the fitting effect of the exponential function shown in Equation (7) on the experimental data. As can be seen from the figure, the experimental data points closely align with the fitted curve, indicating good fitting performance. Table 6 lists the numerical values of the corresponding fitting parameters in detail. All correlation coefficients R² reached 1, proving that Equation (7) can accurately describe the intrinsic relationship between the early permeability coefficient of chemically modified sludge and pre-compression stress.

Under specific conditions where the curing agent content is 10% and the equivalent initial moisture content is 180%, the general Formulas (7) can be used to conduct preliminary permeability coefficient predictions for specified curing times (T = 6, 24, 48, or 72 h) and any preload stress test conditions. This prediction function enables relevant personnel to anticipate the permeability coefficient of chemically modified slurry under different conditions, thereby enhancing the predictability of the chemical modification method in engineering applications.

$$k=d_1\times \exp (-{\displaystyle \frac{P}{d_2}})+d_3$$

(7)

In the equation,

k = permeability coefficient (10⁻⁷ cm/s);

P = preconsolidation pressure (kPa);

d₁, d₂, and d₃ = fitting parameters.

Table 7. Fitting parameters of permeability coefficient for modified fluid mud under different preconsolidation pressures.

Working Condition	d1	Error Range	d2	Error Range	d3	Error Range	R2
W180T06C10	−0.16	0	−33.92	0	5.70	0	1
W180T24C10	−0.20	0	−35.96	0	5.70	0	1
W180T48C10	−0.23	0	−37.74	0	5.72	0	1
W180T72C10	−0.28	0	−40.20	0	5.77	0	1

shows the fitting parameters of permeability coefficient for modified fluid mud under different preconsolidation pressures. Among these parameters, R² = 1, indicating excellent fitting performance and high prediction accuracy.

4. Limitation

(1) The relatively short study period of this paper is mainly because this research focuses on the permeability characteristics under the coupled effect of consolidation and solidification, with the scope concentrated on the early stage after the application of this method—specifically within approximately three days. When the time period is longer, the solidification effect becomes dominant. Investigating long-term permeability characteristics is of great significance, but it is not the research focus of this paper.

(2) The model proposed in this paper is more suitable for the experimental conditions of this study. If applied to other scenarios, this model should be revised as appropriate.

5. Conclusions

This study investigated the early permeability characteristics of physically and chemically modified slurry under different curing agent dosages, equivalent initial moisture content, curing times, and pre-compression stresses through indoor permeability tests at constant head. The main conclusions are as follows:

(1) Early permeability characteristics of chemically modified slurry: When the curing time is within the range of 6 to 72 h, the permeability performance of the modified slurry exhibits time-dependent characteristics, i.e., the permeability coefficient of the modified slurry decreases in an exponential manner with prolonged curing time, following a “rapid initial decline followed by a gradual decrease” trend.

(2) Under conditions where the curing agent content is within the range of 7.5% to 15%, increasing the curing agent content leads to a decrease in the permeability coefficient of the modified slurry, with the trend also showing a rapid decrease initially followed by a slower decrease.

(3) When the equivalent initial moisture content is within the range of 120% to 210%, the permeability coefficient of the modified slurry increases continuously as the equivalent initial moisture content continues to rise.

(4) When the pre-compression stress is within the range of 30 to 90 kPa, as the pre-compression stress increases, the permeability coefficient of the modified slurry gradually decreases, and the rate of decrease continues to increase.

(5) Empirical formula for the permeability coefficient of physically and chemically modified slurry: A series of permeability coefficient prediction equations for physically and chemically modified slurry have been established, thereby enhancing the predictability of physically and chemically modified slurry engineering applications. The permeability performance of chemically modified slurry can be reduced by increasing the amount of solidifiers, lowering the equivalent initial moisture content, extending the curing time, or increasing the preload stress. However, since the permeability coefficient remains within the same order of magnitude (10⁻⁷ cm/s), these measures have certain limitations in reducing permeability performance.