Study on the stability of iron tailings dam improved by curing agent

Fine-grained tailings exhibit poor permeability, prolonged consolidation time, low mechanical strength, and difficulty in dissipating excess pore water pressure. Tailings disposal often encounters challenges such as dam construction difficulties, inadequate seepage drainage from the dam body, gentle slopes on sedimentation areas, and poor stability. Starting from the basic physical properties of fine-grained tailings and taking a tailings pond in Sichuan Province as the engineering background, The curing test was carried out on the mixed tailings when the proportion of fine-grained tailings was 60%, and the optimal proportion of cementitious materials was studied when the mixed tailings were solidified for 28 days under the proportion of fine-grained tailings. Under the optimal proportion of cementitious materials, the mixed tailings when the proportion of fine-grained tailings was 60% were solidified to improve their strength, and the stability of the modified mixed tailings dam was explored. So as to provide reference for the utilization of fine tailings.

1 Introduction

With the promotion of fine-grained mineral processing technology, the particle size of tailings discharged from mineral processing plants is becoming finer and finer, which easily forms unfavorable sedimentary layers with high water content, large porosity, strong compressibility, and low permeability. This leads to unfavorable engineering geological phenomena such as long sedimentation stabilization time, large settlement, and low sedimentation strength index, resulting in instability and failure of tailings dams (Yang et al., 2021). However, due to the special sedimentary stratification characteristics of fine-grained tailings, after entering the reservoir, the tailings pond has a short dry beach length, small slope, and soft beach surface, which is not conducive to dam construction. Under the dual constraints of technology and policies, dam construction with fine-grained tailings is an inevitable research topic. Therefore, scholars have carried out extensive research on this.

Zhong Changyun et al. used fly ash to replace part of cement to make cementitious materials, and solved the problem of low early strength of fly ash paste by chemically activating the activity of fly ash (Zhong et al., 2017); Wu Aixiang et al. analyzed the hydration reaction mechanism of the solidified body. In the early stage of hydration, the calcium-rich phase in the slag glass and high-calcium lime interact to form Ca(OH)₂, which then reacts with the silicon-rich phase to form C-S-H gel (Wu et al., 2017). With the deepening of the hydration reaction, the structure of the sample becomes more compact; Lan Wentao et al. determined the optimal formula for semi-hydrated phosphogypsum filling through orthogonal experiments, and identified the main factors affecting the compressive strength in different curing ages (Lan et al., 2019); Hua Shaoguang et al. solidified tailings with slag-based cementitious materials, and measured the compressive strength and heavy metal leaching rate of test blocks at various ages (Hua et al., 2020; Wang et al., 2025); Desogus P et al. studied the stabilization/solidification of lead-zinc mine tailing waste with potassium dihydrogen phosphate and ferric chloride hexahydrate, and verified the possibility of solidification using potassium dihydrogen phosphate alone and in combination with ferric chloride hexahydrate (Desogus et al., 2013); To address the issues of the low pozzolanic activity and high pollution potential of red mud (RM), Huang J. J. et al. utilized different industrial solid wastes to synergistically enhance the physicochemical properties of red mud-based filling materials (Huang et al., 2025); Kundu S. et al. used copper mine tailings to partially replace cement to prepare concrete, and measured the unconfined compressive strength of concrete mixtures (Kundu et al., 2016); Nusri S. et al. used geopolymerization to solve the problem of oil sand tailings reclamation, and geopolymers improve tailings strength through bonding properties (Nusri et al., 2016); Kiventerä J. et al. used calcium sulfoaluminate-silicate cement to stabilize gold mine tailings, and through hydration reaction, tobermorite and monosulfate were produced, which had a good solidification effect when the tailings content was 50% of the binder material (Kiventerä et al., 2019); Bah A. et al. determined the influence of each component of cementitious materials on the compressive strength of test blocks, ensuring that feasible and effective solidification of mines can be achieved with the participation of fly ash (Bah et al., 2022). Figureueroa A. et al. believed that the increase in the percentage of fine particles in tailings may lead to a decrease in permeability, shear strength, and cyclic resistance, thereby affecting the stability of the dam (Figueroa et al., 2015); Jenni Kiventera et al. studied the chemical stability of polymers synthesized from metakaolin and alkali-activated blast furnace slag, and verified that elements such as Cr, Cu, Ni, Zn, and Mn in gold tailings can be completely fixed by the polymer under alkali activation conditions (Kiventera et al., 2018); Nierwinski H P et al. compared coarse-grained gold tailings and fine-grained bauxite tailings, and clarified the influence of increased load on the reduction of the seepage coefficient of the bottom layer (Nierwinski et al., 2019). Alsharedah, Y A analyzed the stability of an upstream tailings dam considering its staged construction. A two-dimensional nonlinear finite element model was developed using the program Plaxis 2-D to investigate the potential for stabilizing the tailings dam by using emulsified polymer and a mixture composed of cement kiln dust (CKD) and re-cycled gypsum (B) (Alsharedah et al., 2023). Consoli N C analyzed the mechanical behavior of reconstituted state gold tailings specimens, in contrast to artificially cemented gold mine tailings specimens, considering the use of small and large amounts of Portland cement under a similar high void ratio. The influence of Portland cement content in the stabilization of gold tailings specimens molded with their in situ void ratio was evaluated through saturated undrained triaxial compression tests carried out under small confining pressures (Consoli et al., 2023). Liang L introduced the comprehensive analysis method of power transformation into the analytic hierarchy process to study the safety and stability of tailings dams, thereby establishing a tailings dam failure risk model (Liang et al., 2017). Liu J X employed centrifugal model tests for tailings dam overtopping failures to investigate the evolution patterns of breach size and discharge rate under the action of sand-laden overtopping flows (Liu et al., 2022).

Therefore, this study investigates the safety and stability of embankments constructed using solidified and modified mixed tailings with varying fine-grained tailings content. Using a tailings pond in Sichuan Province as the engineering context, the research employs a combined approach of field sampling, geotechnical testing, solidification modification experiments, small-scale embankment model tests, and numerical simulation.

2 Cementitious material proportioning test

2.1 Particle characteristic analysis

In order to more accurately reflect the particle size distribution of tailings stored in engineering practice, 3 tailings samples were taken from different positions on the dry beach, uniformly mixed and analyzed. The dried samples were subjected to three screening experiments, each using 100 g of tailings. The particle content of tailings in each screening experiment is shown in Table 1, and the cumulative curve of tailings particle size gradation is drawn as shown in Figure 1. In the cumulative gradation curve, the characteristic particle sizes of the 3 groups of samples are marked: d10 (effective particle size), d30, d50 (average particle size), d60 (control particle size).

Table 1

Table 1. Tailings particle content composition.

Figure 1

Figure 1. Tailings particle accumulation curve.

By consulting and referring to relevant studies, it was finally determined to use blast furnace slag, fly ash, quicklime, and NaOH as cementitious materials for curing agent proportioning experiments. The orthogonal test method was used to design the experiment, and indoor solidification performance tests were carried out to explore the influence of the proportion of blast furnace slag, fly ash, quicklime, and NaOH in cementitious materials on the strength of specimens. Finally, the optimal proportion parameters of cementitious materials were determined through regression analysis.

2.2 Cementitious material proportioning test

In this test, the cement-sand ratio was set to 1:10, the content of fine tailings in the mixed tailings was 60%, and the concentration of mixed tailings slurry was 65%. The orthogonal design method was used, and the L9 (3³) orthogonal table was adopted in the cementitious material proportioning design, involving three factors, including fly ash content (A), quicklime content (B), and NaOH content (C). The blast furnace slag content was adjusted to achieve the specified cement-sand ratio. The factor levels of the orthogonal experiment are shown in Table 2.

Table 2

Table 2. Orthogonal experiment factor level.

According to the orthogonal experiment design table, a total of 9 groups of experiments were carried out. The basic steps of the experiment were: weighing, mixing, grouting, curing, and unconfined compressive strength test (3 days, 7 days, and 28 days). Axial strain and axial stress were calculated using Equations 1–3:

$\text{Axial strain}{\mathrm{\epsilon }}_1:{\mathrm{\epsilon }}_1=\frac{\Delta \mathrm{h}}{{\mathrm{h}}_0}\times 100(1)$

$\text{Average cross}-\text{sectional area}{\mathrm{A}}_{\mathrm{a}}:{\mathrm{A}}_{\mathrm{a}}=\frac{{\mathrm{A}}_0}{1-0.01{\mathrm{\epsilon }}_1}(2)$

$\text{Axial stress}\mathrm{\sigma }:\mathrm{\sigma }=\frac{\text{CR}}{{\mathrm{A}}_{\mathrm{a}}}\times 10(3)$

Where: σ - axial stress, kPa; C - dynamometer calibration coefficient, N/0.01 mm; R - dynamometer reading, 0.01 mm; A_a - area of the sample during shearing, cm².

The solidification experiment process is shown in Figure 2 After curing the test blocks for 3 days, 7 days, and 28 days respectively, the unconfined compressive strength was measured with instruments, and the results of unconfined compressive strength are shown in Table 3.

Figure 2

Figure 2. Curing experiment process: (a) Tailings weighing, (b) Tailings mixing ratio, (c) Cementitious material weighing, (d) Material mixing, (e) Grouting, (f) Demolding, (g) Oven, (h) Specimen curing.

Table 3

Table 3. Unconfined compressive strength results.

2.3 Particle characteristic analysis

Range analysis was performed on the unconfined compressive strength test results of the test blocks after different curing periods to determine the influence degree of each factor on the unconfined compressive strength, and the optimal solidification parameter combination when the test blocks reach the maximum compressive strength in different curing periods was obtained. The SPSS software was used to perform analysis of variance on the unconfined compressive strength results of the test blocks after curing for 3 days, 7 days, and 28 days to verify whether the significance of each factor is consistent with the results obtained by range analysis.

2.3.1 Range analysis

Range is used to reflect the influence degree of each cementitious material on the unconfined compressive strength. The range analysis results of unconfined compressive strength at each curing age are shown in Table 4. It can be seen from the table that the order of influence factors on the unconfined compressive strength of the test blocks after 3 days curing is: quicklime > fly ash > NaOH, that is, quicklime has the most significant influence, followed by fly ash and NaOH; the order of influence factors on the unconfined compressive strength after 7 days curing is: quicklime > NaOH > fly ash, that is, quicklime has the most significant influence, followed by NaOH and fly ash; the order of influence factors on the unconfined compressive strength after 28 days curing is: fly ash > NaOH > quicklime, that is, fly ash has the most significant influence, followed by NaOH and quicklime. It can be concluded from the analysis that the cementitious material with the greatest influence on the early strength of the test blocks is quicklime, while the most significant influence on the later strength of the test blocks is fly ash. According to the results of range analysis, the optimal cementitious material proportion combination for the test blocks after 3 days curing is A1B2C3, that is: fly ash content is 10%, quicklime content is 8%, and NaOH content is 8%; the optimal cementitious material combination after 7 days curing is A2B2C2, that is: fly ash content is 15%, quicklime content is 8%, and NaOH content is 5%; the optimal cementitious material combination after 28 days curing is A2B2C2, that is: fly ash content is 15%, quicklime content is 8%, and NaOH content is 5%.

Table 4

Table 4. Results of range analysis.

2.3.2 Analysis of variance

Compared with range analysis, variance analysis can analyze the influence degree of different factors on data errors (Lan et al., 2019). The SPSS analysis software was used to perform variance analysis on the unconfined compressive strength results of the test blocks after curing for 3 days, 7 days, and the results of the analysis of variance are shown in Table 5.

Table 5

Table 5. Analysis of variance results.

2.4 Strength performance regression model

Combining orthogonal test and regression analysis can establish an effective mathematical model and find the optimal experimental scheme. Therefore, through the analysis of the range analysis and variance analysis of the above orthogonal experiments, it can be concluded that quicklime is the main factor affecting the early strength of the test blocks, while fly ash is the main factor affecting the later strength of the test blocks, and NaOH, as an alkali activator, plays an important role in the whole process of solidification (early, middle, and late stages). According to the relationship between the content of each component of cementitious materials and the unconfined compressive strength, a multivariate nonlinear regression model for the compressive strength of the test blocks is proposed, the defined model formula is shown in Equation 4.

$y=a_0+a_1{x}_1+{a_{2}x}_2+a_3{x}_3-a_4{x}_1{x}_2-a_5{x}_1{x}_3-{a_{6}x}_2{x}_3+a_7{x}_1^2-{a_{8}x}_2^2-{a_{9}x}_3^2(4)$

The nlinfit function was used for nonlinear least squares fitting to find the regression coefficients of the multivariate nonlinear equations of the test blocks at each curing age. Substituting the obtained coefficients into the defined model equations yields the multivariate nonlinear regression Equations 5–7 for different curing ages.

1. Regression model of unconfined compressive strength of test blocks at 3 days:

$\begin{array}{ll}\hfill y& =0.359+0.0191x_1+0.0775x_2+0.1215x_3-0.0004x_1{x}_2\hfill \\ \hfill & -0.0089x_1{x}_3-0.0084x_2{x}_3+0.0002x_1^2-0.0059x_2^2-0.0039x_3^2\hfill \end{array}(5)$

2. Regression model of unconfined compressive strength of test blocks at 7 days:

$\begin{array}{ll}\hfill y& =-1.4457+0.0748x_1+0.4472x_2+0.7833x_3+0.0114x_1{x}_2\hfill \\ \hfill & -0.012x_1{x}_3-0.0058x_2{x}_3-0.0039x_1^2-0.028x_2^2-0.0563x_3^2\hfill \end{array}(6)$

3. Regression model of unconfined compressive strength of test blocks at 28 days:

$\begin{array}{ll}\hfill y& =-7.4223+0.2139x_1+1.2568x_2+2.5683x_3+0.0453x_1{x}_2\hfill \\ \hfill & +0.0299x_1{x}_3-0.1261x_2{x}_3-0.0201x_1^2+0.081x_2^2-0.1853x_3^2\hfill \end{array}(7)$

Using Origin software, the relationship diagrams of the influence of each component of cementitious materials alone and their interactions on the unconfined compressive strength of the test blocks after curing for 3 days, 7 days, and 28 days were drawn, as shown in Figure 3.

Figure 3

Figure 3. The influence of fly ash, quick lime and NaOH on the compressive strength of test block alone and by their interaction (3 d): (a) Fly ash, (b) quick lime, (c) NaOH.

It can be seen from Figure 3a that the unconfined compressive strength of the test blocks after 3 days curing increases with the decrease of fly ash content, and first increases and then decreases with the increase of quicklime content; this is because during the 3 days curing process, the quicklime in the cementitious materials reacts with blast furnace slag to produce hydration reaction, while the fly ash participating in the hydration reaction in the early stage is very little or even none; it can be seen from Figure 3b that the unconfined compressive strength of the test blocks after 3 days curing increases with the decrease of fly ash content, and increases with the increase of NaOH content; during the 3 days curing process, NaOH reacts with water to release “OH⁻” ions, forming an alkaline environment, thereby further promoting hydrolysis; it can be seen from Figure 3c that the unconfined compressive strength of the test blocks after 3 days curing first increases and then decreases with the increase of quicklime content, and increases with the increase of NaOH content.

It can be seen from Figure 4a that the unconfined compressive strength of the test blocks after 7 days curing first increases and then slightly decreases with the increase of fly ash content, but the change range is small; and the unconfined compressive strength of the test blocks first increases and then decreases with the increase of quicklime content; it can be seen from Figure 4b that the unconfined compressive strength of the test blocks after 7 days curing first increases and then decreases with the increase of fly ash content, and first decreases and then increases with the gradual increase of NaOH content; it can be seen from Figure 4c that the unconfined compressive strength of the test blocks after 7 days curing first increases and then decreases with the increase of quicklime content, and also first increases and then decreases with the increase of NaOH content.

Figure 4

Figure 4. The influence of fly ash, quick lime and NaOH on the compressive strength of test block alone and by their interaction (7 d): (a) Fly ash, (b) quick lime, (c) NaOH.

It can be seen from Figure 5a that the unconfined compressive strength of the test blocks after 28 days curing first increases and then slightly decreases with the increase of fly ash content; at the same time, the compressive strength of the test blocks also first increases and then decreases with the increase of quicklime content, but the change range is small; it can be seen from Figure 5b that the unconfined compressive strength of the test blocks after 28 days curing first increases and then decreases with the increase of fly ash content, and also first increases and then decreases with the increase of NaOH content; it can be seen from Figure 5c that the unconfined compressive strength of the test blocks after 28 days curing first increases and then decreases with the increase of quicklime content, but the overall change range is small, and first increases and then decreases with the increase of NaOH content.

Figure 5

Figure 5. The influence of fly ash, quick lime and NaOH on the compressive strength of test block alone and by their interaction (28 d): (a) Fly ash, (b) quick lime, (c) NaOH.

Through the analysis of Figure 3, it can be known that in the 3 days curing process after the test blocks are solidified, the main cementitious materials participating in the hydration reaction are quicklime and NaOH; in the 7 days curing process shown in Figure 4, the fly ash in the cementitious materials begins to gradually participate in the hydration reaction, but its effect on the solidification and modification of the test blocks is small, and the main roles are quicklime and NaOH in the cementitious materials; in the 28 days curing process shown in Figure 5, fly ash is completely involved in the hydration reaction, and the overall compressive strength of the test blocks is greatly affected by fly ash. That is, the contents of the three cementitious materials (fly ash, quicklime, and NaOH) all have an impact on the unconfined compressive strength of the test blocks. The early strength of the test blocks is not high, and the main cementitious materials participating in the hydration reaction are quicklime and NaOH; the strength of the test blocks in the middle and later stages is significantly higher than that in the early stage, and the main cementitious material participating in the hydration reaction is fly ash.

3 Effect of fine tailings content on the strength properties of consolidated materials

Different fine tailings contents refer to the proportion of fine tailings (after screening) in the mixed tailings. In order to make better use of fine tailings, keeping the cement-sand ratio and mixed tailings concentration unchanged, under the optimal cementitious material proportion condition for the mixed tailings with a fine tailings proportion of 60% after 28 days curing, the proportion of fine tailings was increased. Through experiments, the mechanical parameters such as compressive strength, cohesion, internal friction angle, and permeability coefficient of the mixed tailings after solidification with different fine tailings proportions were determined, and the maximum fine tailings proportion that can meet the strength requirements of the tailings dam under the optimal cementitious material proportion was obtained, so as to better utilize fine-grained tailings.

3.1 Unconfined compressive strength test

Due to the high specific surface area of fine-grained tailings, they easily absorb water, resulting in low compressive strength. By adding curing agents, the tailings particles can be enveloped by cementitious substances generated by hydration reaction, squeezing out water, playing a filling and bonding role, and forming a cemented body with certain strength, thereby improving the compressive strength. Under the optimal cementitious material proportion condition for the mixed tailings with a fine tailings proportion of 60% after 28 days curing, that is, fly ash content is 15%, quicklime content is 8%, and NaOH content is 5%, the unconfined compressive strength of the test blocks of mixed tailings with different fine tailings proportions after 28 days curing was measured. The compressive strength of the test blocks with different fine tailings proportions is shown in Figure 6.

Figure 6

Figure 6. Relationship between the proportion of fine tailings and compressive strength (28 d).

It can be seen from Figure 6 that under the optimal cementitious material combination condition, with the increase of the proportion of fine tailings in the mixed tailings, the compressive strength of the test blocks increases, but the increase range of the compressive strength of the test blocks becomes smaller. Because smaller soil particles are more likely to form agglomerate structures, resulting in larger pores, lower unit volume mass, smaller relative density, and larger porosity (Zhang et al., 2021), therefore, the anti-dispersion ability of the test blocks will be significantly enhanced with the increase of clay content. That is, the compressive strength of the test blocks increases with the increase of the proportion of fine tailings in the mixed tailings.

3.2 Direct shear test

Under the optimal cementitious combination of the mixed tailings with a fine tailings proportion of 60% after 28 days curing, direct shear tests were carried out on the test blocks after 28 days curing with different fine tailings proportions. The relationship between shear displacement and shear stress of mixed tailings with different fine tailings proportions is shown in Figure 7.

Figure 7

Figure 7. Shear displacement and shear stress curves of mixed tailings with different proportion of fine tailings: (a) Shear displacement-shear stress relationship curve (60%), (b) Shear displacement-shear stress relationship curve (65%), (c) Shear displacement-shear stress relationship curve (70%), (d) Shear displacement-shear stress relationship curve (75%).

It can be seen from Figure 7 that the shear stress of the mixed tailings with different fine tailings proportions increases with the increase of shear displacement; comparing the test blocks with different fine tailings proportions, it is found that the shear stress at the same shear displacement decreases with the increase of the proportion of fine tailings in the mixed tailings. Under the conditions of 100 kPa and 200 kPa, with the increase of the proportion of fine tailings in the mixed tailings, the shear displacement when the shear stress reaches the stable value gradually decreases, and the shear stress when reaching stability gradually decreases. This is because: with the increase of fine tailings content, the internal friction angle gradually decreases, while the cohesion increases. High clay content (smaller particles) will lead to a stronger structural connection between particles and stronger cohesion; while the surface of tailings sand particles with high clay content is smooth, and the friction between particles decreases, thus reducing the internal friction angle.

3.3 Permeability test

After the tailings test blocks are solidified, their permeability coefficient can be obtained through permeability experiments. The samples used in this experiment are mixed tailings with a fine tailings proportion exceeding 50%, which belong to the category of fine-grained soil. Therefore, the variable head permeability test was used to measure the permeability coefficient. The permeability experiments were carried out on the test blocks of mixed tailings with different fine tailings proportions after 28 days curing, and the permeability coefficients of the test blocks are shown in Table 6.

Table 6

Table 6. Permeability coefficient of samples with different proportion of fine tailings.

It can be seen from Table 6 that after solidification of the mixed tailings test blocks with different fine tailings proportions, the average permeability coefficient is basically between 1.57 × 10⁻⁵ cm/s and 2.82 × 10⁻⁵ cm/s, and the permeability coefficient decreases with the continuous increase of the proportion of fine tailings in the mixed tailings. This is because with the increase of fine tailings content, the pores between tailings become smaller and smaller. At the same time, due to the occurrence of hydration reaction, the originally loose fine tailings are agglomerated to form cemented particles with larger diameter and certain strength, which further reduces the porosity of tailings, thus increasing the overall compactness and causing the overall permeability coefficient to decrease with the increase of the proportion of fine tailings.

4 Stability analysis of damming fine-grained tailings after solidification modification

4.1 Dam model design and stacking

The initial dam height of the tailings reservoir design is 20.5 m, the height of the accumulation dam is 44.0 m, the total dam height is 64.5 m, the total reservoir capacity is 8.260 million m3, and the effective reservoir capacity is 7.108 million m3, which belongs to the third-class reservoir. The dam height of the tailings pond accumulation dam is 26.0 m, and the total dam height is 46.5 m. In order to observe the damming effect of adding curing agent, an indoor small damming model is established after the tailings reservoir is scaled according to the similar model. By comparing the changes of saturation line and dam body before and after damming of mixed tailings with fine tailings accounting for 60%, the feasibility of adding curing agent to dam is demonstrated. According to the actual situation of the tailings pond, the model similarity theory such as geometric similarity, motion similarity and dynamic similarity is comprehensively considered, and the model is finally determined to be created according to the scale of 1: 1,000. The plan layout of the tailings is shown in Figure 8, and the parameters before and after the scale of the tailings pond are shown in Table 7.

Figure 8

Figure 8. Horizontal layout of tailings pond.

Table 7

Table 7. Relevant parameters before and after scaling model experiment.

According to the investigation of the surrounding terrain in the survey data of the tailings reservoir, the surrounding terrain of the tailings reservoir was built on the model site according to the scale of 1: 1,000, and five saturation line monitoring points were set up in the dam model experiment. PVC pipes with a diameter of 20 mm were used to lay some of the pipes in the reservoir, laying them horizontally and vertically on the ground to achieve effective water discharge and monitor changes in the saturation line. As shown in Figure 9.

Figure 9

Figure 9. Dam model design and stacking: (a) Outline of the model, (b) Topographic map of the surrounding area of the tailings pond, (c) Pipe laying of infiltration line, (d) Laying of the observation tube of the infiltration line.

4.2 Damming test of mixed tailings when the proportion of fine tailings is 60%

The mixed tailings with a fine tailings ratio of 60% were used to build dams before and after solidification to observe the damming effect, and then the proportion of fine tailings in the mixed tailings was gradually increased, and the mixed tailings with a fine tailings ratio of 60% were used. The optimal curing conditions for curing 28 days were solidified until the dam body could not meet the damming conditions of the tailings reservoir, so as to find the optimal curing conditions for curing 28 days when the mixed tailings with a fine tailings ratio of 60% could meet the maximum fine tailings ratio of the damming conditions of the tailings reservoir.

After completing the terrain construction, the dam model test is carried out. By using the mixed tailings before and after solidification when the proportion of fine tailings is 60%, the water level change in the saturation line monitoring pipe and the seepage, deformation and cracking of the dam surface are observed, and the feasibility of adding curing agent to fine tailings for dam construction is evaluated. The specific steps are as follows: The initial dam of gravel accumulation is adopted, with a height of 4.1 cm laying geotextiles inside the initial dam for filtering the tailings in the reservoir; the upstream damming method is adopted to discharge the prepared tailings slurry step by step in front of the dam. The uniformly stirred tailings slurry is discharged into the reservoir, and the water flow impact will form an impact fan and an impact pit, which will form a dry beach after precipitation. A total of 13 sub-dams were built, each of which is 0.4 cm high and 0.8 cm wide, and the total height of the accumulation dam is 5.2 cm. After the completion of each sub-dam ore drawing, the ore drawing is carried out at the next level; after the completion of the construction of the dam, the water level changes in the five groups of saturation line monitoring pipes were recorded. By monitoring the water level changes in the reservoir, the situation of the dam before and after curing was compared.

4.2.1 Changes of water level line

By observing the water level line transparent tube and comparing it with the scale line, the data of the water level line height is read every 10 min. The data records of the water level before and after the solidification of the mixed tailings when the proportion of fine tailings is 60% are shown in Table 8, Table 9.

Table 8

Table 8. Change of water level at each monitoring point before adding curing agent (60% of fine tailings).

Table 9

Table 9. Change of water level at each monitoring point after adding curing agent (60% of fine tailings).

Before adding the solidifying agent to the mixed tailings with a fine tailings content of 60%, a graph showing the relationship between monitoring time and water level changes was plotted based on the water level variations in each monitoring pipe, as shown in Figure 10. Observation of Figure 10a reveals that water levels at monitoring points J1-J5 generally decreased with increasing monitoring time. Specifically, J1 exhibited significant fluctuations between 0 and 40 min, followed by smaller fluctuations between 40 and 120 min. Monitoring points J2-J5 showed substantial changes between 0 and 30 min, with changes stabilizing between 30 and 120 min. To investigate the relationship between water level decline rate and monitoring duration, the decline rates for the first 30 min were calculated for each monitoring point, yielding the relationship diagram shown in Figure 10b. This diagram reveals that during the 0–10 min interval, decline rates rapidly increased at all monitoring points except J1, with J4 exhibiting the fastest rate. Between 10 and 20 min, rates continued to increase at all points except J3. During the 20–30 min interval, only monitoring points J1 and J2 exhibited increasing rates. Overall, within 30 min, the water level decline rates at J1 and J2 increased with time, while those at J3, J4, and J5 first increased and then decreased. This variation resulted from the differing locations of each monitoring point within the tailings pond, leading to significant differences in water level height and rate of change among the points.

Figure 10

Figure 10. Relationship between monitoring time and water level height and descent rate (prior to adding solidifying agent when fine tailings constitute 60% of the mixture): (a) Relationship between monitoring time and water level height, (b) Relationship between monitoring time and water level decline rate.

After adding a solidifying agent to mixed tailings with 60% fine tailings content, a graph depicting the relationship between monitoring time and water level changes was plotted based on water level variations in each monitoring tube, as shown in Figure 11. Figure 11a reveals that water levels at all monitoring points generally decreased as monitoring time increased. Compared to the condition without solidifying agent, the addition of the solidifying agent increased the cohesive force between tailings particles, reduced inter-particle voids, and made the tailings more compact. Consequently, the rate of water level rise slowed at each monitoring point over time. Overall, after 90 min, the water levels at J1 and J2 began to stabilize, while those at J3, J4, and J5 continued to decline. During the entire monitoring period, the largest proportion of water level decline occurred within the first 0–30 min at all monitoring points. To investigate the relationship between monitoring time and water level decline rate, the rates for the first 30 min at each monitoring point were calculated and plotted against monitoring time. As shown in Figure 11b, the decline rate at all monitoring points increased sharply during the 0–10 min segment, with J5 exhibiting the highest rate. Between 10 and 20 min, the decline rates at J1 and J3 continued to increase, while those at J2 and J4 remained largely unchanged; J5’s rate began to decrease. During the 20–30 min segment, the rates at all monitoring points showed a downward trend. Compared to the earlier phase, the rate of decline in the latter segment becomes more gradual and stabilizes toward a constant value. This occurs because the addition of the solidifying agent causes the tailings particles to bond together, increasing the tailings’ compactness. This reduces the available flow channels and prolongs the time required for water to pass through the tailings, resulting in a slower rate of decline in the water level.

Figure 11

Figure 11. Relationship between monitoring time and water level height and descent rate (after adding solidifying agent when fine tailings account for 60%): (a) Relationship between monitoring time and water level height, (b) Relationship between monitoring time and water level decline rate.

4.2.2 Changes in dam surface conditions

Adding a solidifying agent during dam construction enhances the dam’s compressive and shear strength. The addition of the solidifying agent increases the density of the tailings, leading to a rise in reservoir water levels and prolonging drainage time. Simultaneously, the solidifier generates cementitious materials through hydration reactions, promoting greater aggregation between tailings particles. To evaluate the dam construction performance of mixed tailings with 60% fine tailings after solidification, observations were conducted on surface changes at three time points: 1 day, 3 days, and 7 days post-solidification. The observation diagrams are shown in Figure 12.

Figure 12

Figure 12. Observation of dam surface in different time periods: (a) 3d, (b) 7d, (c) 28d.

As shown in Figure 12, no deformation or cracking of the dam body was observed during the 3-day, 7-day, and 28-day monitoring periods. However, at 7 days, water seepage was detected from the initial dam location to the midpoint of the embankment dam. By 28 days, seepage occurred from the initial dam all the way to the crest of the embankment dam, with a significant increase in seepage volume. This phenomenon primarily resulted from two factors: firstly, the influence of the solidifying agent materials used. In this experiment, the cementitious materials comprised blast furnace slag, fly ash, quicklime, and NaOH. Upon mixing, these materials underwent hydration reactions to form cementitious substances with certain strength, encapsulating the tailings particles and enhancing the cohesive force between them. On the other hand, the scale-down effect of the dam model also contributed to this phenomenon.

4.3 Numerical simulation and results analysis of dam construction effect after curing modification

A combined approach of physical model experiments and numerical simulations was employed to validate the feasibility of constructing dams using modified mixed tailings with varying proportions of fine-grained tailings. Under the optimal curing conditions for mixed tailings at 60% fine-grained tailings content after 28 days of curing, scaled-down physical model tests were conducted to replicate the actual dam construction process. The water table elevation at different locations was recorded to reflect changes in the water table within the dammed reservoir after curing. Concurrently, deformation and cracking of the tailings dam body were observed. Subsequently, GeoStudio simulation software was employed to analyze the stability of dam structures constructed with solidified mixed tailings containing 60% fine-grained tailings before and after solidification, as well as with higher fine-grained tailings proportions. This analysis determined the safety stability factors for dam construction using solidified mixed tailings under various fine-grained tailings proportions.

4.3.1 Establishment of numerical computation models

Creating a slope model in Geostudio primarily involves the following steps: model setup, defining material properties, setting the groundwater level, specifying appropriate shear entry and exit ranges, and solving the model. The specific model setup process is shown in Figure 13.

Figure 13

Figure 13. Establishment of Numerical Computation Models: (a) Sectional model of tailings pond, (b) Material assignment diagram under the SEEP/W analysis module, (c) Mapping of groundwater levels, (d) Stratigraphic assignment diagram of tailings pond model, (e) The result of the tailings pond model.

After establishing the material boundaries, the material properties must be defined. The parameters for each material after adding the curing agent in this experiment are shown in Table 10.

Table 10

Table 10. Properties of various materials (after adding curing agent).

4.4 Analysis of numerical results

According to the “China ground motion parameter zoning map” (GB18306-2015), the seismic intensity of the area where the tailings pond is located is 7°, the peak acceleration value of the earthquake is 0.15 g, the annual average rainfall is 801.6 mm, and the annual maximum rainfall is 1,006.9 mm.

The operation conditions under three working conditions are considered respectively. One is that the accumulation layer is in the normal state, which is called the normal condition; the second is to consider the variation law of the saturation line in the tailings pond under the condition of the maximum annual rainfall, which is called the maximum rainfall situation; the third is the working state of the tailings reservoir in the event of an earthquake, known as the earthquake situation.

The tailings pond belongs to the third-class reservoir, and the safety standard corresponding to the dump level specified in the ' Non-ferrous Metal Mine Dump Design Standard ' specification is shown in Table 11. Relevant specifications point out that the safety factor limit can be appropriately reduced when considering seismic factors. According to the ' Design Standard for Dumping Site of Non-ferrous Metal Mines ' (GB50421-2018), the calculated safety factor results can refer to Table 12.

Table 11

Table 11. Overall safety and stability standards for dumps.

Table 12

Table 12. Criteria for judging the stability safety factor k of the dump slope.

According to the ' Tailings Reservoir Safety Regulations ' (GB39496-2020), the Janbu method and the simplified Bishop method are used to analyze the stability of the mixed tailings dam after solidification when the proportion of fine tailings is 60% under the current dam height. The stability calculation results are shown in Table 13. The calculation and analysis results of the corresponding minimum safety factor and the position of the slip surface of the section are shown in Figures 14, 15.

Table 13

Table 13. The stability calculation results of 60% fine tailings are summarized.

Figure 14

Figure 14. Calculation results of Janbu method when the proportion of fine tailings is 60 %: (a) normal, (b) rainfall, (c) earthquake.

Figure 15

Figure 15. Calculation results of Bishop method when the proportion of fine tailings is 60 %: (a) normal, (b) rainfall, (c) earthquake.

The numerical simulation calculation and analysis of the stability of the mixed tailings dam tailings reservoir under different fine tailings ratios are carried out. The calculation and analysis results show that under normal working conditions, rainfall conditions and earthquake conditions, the safety factor of the mixed tailings dam tailings reservoir under the fine tailings ratio of 60% is higher than the specification requirements, which can meet the requirements of the relevant specifications, that is, when the fine tailings ratio is 60%, the mixed tailings dam tailings reservoir is in a safe and stable state after solidification. By observing the summary table of stability calculation results, it can be found that under the same working condition, whether it is normal working condition, rainfall working condition or earthquake working condition, the safety factor calculated by stability analysis decreases with the increase of the proportion of fine tailings.

5 Discussion

In this paper, the proportioning experiment of cementitious materials was studied from four aspects: selection of cementitious materials, experimental design, result analysis, and establishment of regression model. The optimal cementitious material combination for the mixed tailings with a fine tailings proportion of 60% after 28 days curing was obtained. Under this cementitious material combination, the mixed tailings with different fine tailings proportions were solidified. Through the study of unconfined compressive strength test, direct shear test, and variable head permeability test, the performance changes of the test blocks after solidification were comprehensively analyzed. Through the experiment of small dam construction model, the difference of dam stability after solidification of mixed tailings under different proportions of fine tailings is explored. After dam construction, the change of water level in the pipe and the deformation and cracking of dam surface are monitored by observing the saturation line of tailings reservoir, and the stability of tailings reservoir is evaluated by Geostudio numerical simulation software.

6 Conclusion

This study investigates the solidification of mixed tailings containing 60% fine-grained tailings, starting from the fundamental physical properties of fine-grained tailings. It determines the optimal proportion of cementitious materials for solidifying mixed tailings at 28 days under this fine-grained tailings ratio. The stability of modified mixed tailings for dam construction is explored, providing reference for utilizing fine-grained tailings. The main research findings are as follows:

1. By measuring the unconfined compressive strength of mixed tailings solidified at 3 days, 7 days, and 28 days with 60% fine tailings content, the influence of each component in the cementitious material at different curing ages was determined. At 3 days, quicklime had the most significant effect, followed by fly ash and NaOH; at 7 days, quicklime again had the most significant effect, followed by NaOH and fly ash; At 28 days, fly ash exerted the most significant influence, followed by NaOH and quicklime.

2. Through strength performance regression analysis, a multivariate nonlinear regression model was obtained for mixed tailings solidified at different curing ages when the fine tailings content was 60%. The optimal cementitious material mix ratio for solidified mixed tailings at 28 days with 60% fine tailings content was determined: 15% fly ash, 8% quicklime, and 5% NaOH. The study analyzed the influence of varying fine tailings proportions on the strength properties of the solidified composite tailings. Key mechanical parameters of the cured specimens were determined, including compressive strength, cohesion, internal friction angle, and permeability coefficient.

3. The deformation and cracking of the dam body were not found in the observation process of 3 days, 7 days and 28 days after the curing of the mixed tailings with 60% fine tailings. The Janbu method and the simplified Bishop method were used to analyze the stability of the mixed tailings dam body under the optimal curing condition of the mixed tailings curing 28 days when the fine tailings accounted for 60%. The calculation results show that the stability calculation results of the mixed tailings with 60% fine tailings meet the standard values regardless of the normal working conditions, rainfall conditions or earthquake conditions.

Data availability statement

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/supplementary material.

Ethics statement

Ethical approval was not required for the studies involving humans because this manuscript does not involve ethical research. The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

Author contributions

SL: Data curation, Supervision, Investigation, Conceptualization, Project administration, Writing – original draft. GW: Funding acquisition, Writing – original draft, Validation, Conceptualization. MW: Writing – review and editing. XD: Writing – review and editing, Validation, Project administration. BZ: Methodology, Funding acquisition, Writing – review and editing. JY: Validation, Funding acquisition, Resources, Writing – review and editing. YZ: Validation, Funding acquisition, Resources, Writing – review and editing. ZP: Writing – review and editing, Software, Visualization. JP: Software, Writing – review and editing, Formal Analysis, Visualization. RL: Formal Analysis, Writing – review and editing, Supervision. ML: Writing – review and editing, Resources.

Funding

The author(s) declare that financial support was received for the research and/or publication of this article. This research was funded by the National Engineering and Technology Research Center for Development &Utilization of Phosphate Resources, grant number NECP2025-10; the Central Guidance of Local Science and Technology Development Fund, grant number 202407AC110019; the Innovation Center of Phosphorus Resource, Yunnan Province, grant number 202305AK340002, the Pilot Project of Ministerial-Provincial Collaboration under the Ministry of Natural Resources (No.2023ZRBSHZ009), the Geological Research Project of Guizhou Bureau of Geology and Mineral Exploration and Development (No. Qian Dikuang Kehe [2024] 17), and the Youth Guidance Project of Basic Research Program of Guizhou Provincial Department of Science and Technology (No. Qian Kehe Basic QN [2025] 452).

Conflict of interest

Authors SL, MW, and JP were employed by Yunnan Phosphate Chemical Group Co., Ltd. Authors RL and ML were employed by Kunming Engineering and Research Institute of Nonferrous Metallurgy Co., Ltd.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The reviewer ST declared a past co-authorship with the author GW to the handling editor.

Generative AI statement

The author(s) declare that no Generative AI was used in the creation of this manuscript.

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