A framework to determine soil-water retention relation for mine wastes and its applications in emergency risk assessment

Tailing dams and waste dumps formed by the accumulation of mine wastes are usually at long-term unsaturated state under evaporation and consolidated drainage conditions. Soil-water retention relation is one of the key constitutive relations to analyze the seepage processes in tailing dams and waste dumps. In this study, typical coarseand fine-grained tailings from a metallurgical mine were chosen to study the soil-water retention characteristics of the tailing samples. To begin with, the relationship between volumetric water content and suction of the tailing samples was experimentally measured, and typical soil-water characteristic curves (SWCCs) (i.e., Gardner, van Genuchten and Fredlund–Xing curves) were applied to fit the experimental data. After that, four empirical models to estimate the parameters in SWCCs (i.e., Aubertin-1998 model, Aubertin-2003 model, Vanapalli-2005 model and Chin-2010 model) were tested, and the Vanapalli-2005 model was the best-fit model for the tailing samples. Furthermore, this study proposes a generalized emergency risk assessment soil-water retention characteristics model for tailing dams and waste dumps, and a framework for the quick estimation of parameters in the SWCC is proposed as well. The recommended soil-water retention characteristics model and the related parameters can be used to predict water levels in tailing dams and waste dumps, which are very helpful for emergency risk assessment under rainstorm or flooding conditions.


INTRODUCTION
Mine wastes are usually stored in two forms: tailing dam and waste dump. In the past 20 years, the number of accidents related to tailing dams and waste dumps substantially increases around the world (e.g., Wang et al. a; Armstrong et al. ). Seepage failure is one of the most important failure types to cause accidents (e.g., Rico et al. ; Wang et al. a, b, ). When a tailing dam is constructed, fine tailing particles need to be mixed with waste rocks from waste dumps before the fine particles can be used to construct the tailing dam. As a result, the cross-scale granular particles coexist in the construction of a tailing dam. In addition, a tailing dam is usually constructed in a ravine, where the upstream catchment area is large. As a result, the complicated seepage in cross-scale granular materials needs to be considered in the hydraulic analysis of the upstream catchment area.
Tailing sands and waste rocks are both typical granular materials with heterogeneity (e.g., Rico et al. ). These materials have internal fabrics, in which the skeleton is formed by large particles, and fine particles are filled in the spaces between large particles. In the coarse-graindominated layer, the rock fragments make up the skeleton of the packing materials, while in the fine-grain-dominated layer, the skeleton is formed by fine particles.
The packing state and mechanical behavior of tailing dams and waste dumps are greatly affected by hydraulic environments. For example, the solid skeleton can be destroyed by the seepage force under rainfall and watertable fluctuation conditions. Furthermore, the frictional force among the particles may decrease during a seepage flow, which can trigger particle migration and particle rearrangement. Field investigations show that a large area of mine wastes in front of a tailing dam is in an unsaturated state under alternative drying and wetting cycles, and the fully saturated and dry states in waste dumps and tailing dams are rare (e.g., Azam et al. ). Hence, the unsaturated characteristics of mine wastes need to be considered in the assessment of hydraulic flow and stability of tailing dams and waste dumps. A soil-water characteristic curve (SWCC) describes the relationship between water content and matric suction, and the SWCC is an important constitutive relation to analyze unsaturated flow and slope stability (Chen & Wei ; Chen et al. ). A SWCC can also be related to unsaturated permeability coefficient and shear strength (e.g., Rassam & Williams ; Lu & Likos ).
The suction and water content sensors are often used to measure the SWCCs of soils in the field. In the laboratory, the axis transition technique is widely used to determine the SWCCs of unsaturated soils (e.g., Fredlund & Rahadjo ). Based on the experimental data, some SWCC models have been developed, such as Gardner's model, van Genuchten (VG)'s model and Fredlund-Xing (FX)'s model (Gardner ; van Genuchten ; Fredlund & Xing ). Furthermore, alternative approaches to determine the SWCC have also been proposed if there are very limited measured data.
For example, the probability function of the particle size distribution is widely used to obtain the SWCC model. In this model, the characteristic parameters of the particle size are used to determine the SWCC parameters (e.g., Aubertin Compared to natural soils, tailings are artificial soil that has different characteristics from natural soil (e.g., Kossoff et al. ; Xu et al. ). Compared to the sandy soil, the particle size of the grinding tailing soil is much smaller than the one in the natural sands. However, compared to the clayey soils, the bonding behavior of tailings is much weaker due to a lack of clay minerals in tailing soils. Hence, the soilwater retention behavior is expected to be different from the natural soils. Meanwhile, there are two different scales of solid particles in the waste dump: fine-grained soils and coarse-grained rocks (Azam et al. ; Xu et al. ).
There are many large pores in the waste rocks due to the insufficient filling of the fine particles. As a result, the preferential flow and erosion behavior can easily occur for such crossscale materials. Hence, the unsaturated hydraulic characteristics of the mine wastes need to be determined and simulated (e.g., Aubertin et al. ; Azam et al. ). However, there are very limited SWCC data of tailing dams and waste dumps reported in the literature, perhaps due to the time-consuming data acquisition processes. In summary, an appropriate SWCC model for mine tailings and waste rocks has not been developed, and the suitability of existing SWCC models for mine tailings and waste dumps has not been explored. Hence, it is difficult to accurately analyze the evolution of the seepage processes and mechanical behaviors in tailing dams and waste dumps under the changes of water content. It is necessary to explore the suitability of the existing SWCC models for simulating the soil-water retention behavior of the mine wastes. Also, it is important to determine the advantages and disadvantages of the existing SWCC models so as to capture the soil-water retention characteristics of the mine wastes and assess the unsaturated flow and stability behavior of the mine wastes. In addition, to avoid catastrophic mine waste collapse risk, an emergency planning needs to be done during the construction of tailing dams and waste dumps, which includes assessing the site stability and advising prevention measures (e.g., Kossoff et al. ). In many cases, there are very limited SWCC data for emergency planning. As a result, it is particularly important to evaluate the seepage flow and site stability of the constructions by using the predicted SWCC model with very limited experimental data.
To explore the above issues related to the SWCC, the SWCCs were first measured using the coarse-and finegrained tailing samples from a metallurgical mine, respectively. The effects of grain size and compaction on the SWCCs were then explored. The reasons for the discrepancy between measured data and predicted curves were discussed based on the comparison of measured data and fitting SWCCs with three different SWCC models. Furthermore, the existing SWCC models were also used to reproduce the measured data of the cross-scale waste rocks in the literature.
The suitability of the developed SWCC models was also explored. At last, a method to determine the appropriate SWCC model was given according to the partition of particle size in a mine waste site. The recommended range of the model parameters was also determined for the purpose of emergency assessment. To validate the reasonability of the SWCC recommendation, a simulated example for unsaturated seepage in a tailing dam was done using the measured and suggested SWCCs. Two key parameters in hydraulic analysis, i.e., the phreatic line and pore pressure, were chosen to validate the developed method in this paper.

EXPERIMENTAL
In this paper, seven representative tailing sand samples from metallurgical iron mines were used for the experiments. The samples were taken from the tailings of two iron mines in Shanxi Province, China. The sandy tailings were obtained from magnetic separation at one site. There are two types of tailings from the site, including the tailing coarse sand (>50% of the total mass with a particle size larger than 0.5 mm) and the tailing silty sand (>50% of the total mass with a particle size larger than 0.074 mm). At another site, the flotation process was used to separate the minerals and tailings. The tailings are tailing silty clay, in which <50% of the total mass is with a particle size larger than 0.074 mm, and its plasticity index is 10-17. Table 1 lists the particle size distribution and characteristic parameters of the samples. The particle distribution curve of the sample is shown in Figure 1. From Table 1, the effective particle size D 10 and median particle size D 50 of the coarse-grained tailing samples (C-1, C-2 and C-3) are larger than 10 times of the fine-grained tailing samples (F-1, F-2, F-3 and F-4).

Material properties of tailings
The influence of dry density on the SWCC of the tailing samples was experimentally explored in this section. A total of seven groups of samples were prepared for coarse-and fine-grained tailings, which include the samples with the in situ dry density of 1.7 × 10 3 kg/m 3 and different dry densities, as shown in Table 2.

Measured SWCCs of tailings
Combined with the axis translation technique, the pressure plate extractor equipment (from Soil Moisture Ltd) was used to determine the SWCCs of the tailing samples. The ceramic plate with high air-entry values of 3 and 5 bars was used for coarse-and fine-grained samples in the experiments, respectively. The sample preparation procedures were as follows: first, the dried sample of a certain quantity was weighted, and a small amount of water was sprayed on the sample and was well mixed according to the initial water content. Then, the wet sample was put into a cutting ring (7.0 cm in diameter and 5.2 cm in height) with several layers according to the given dry density. The cutting ring with the sample was fixed into a soil sample saturator.
After the saturator was put in a saturation tank for vacuuming for about 4 h, the deionized water was injected into the tank to saturate the tailing sample.  The ceramic plate of the pressure plate equipment was also saturated with deionized water by the similar method.
The saturated samples were weighted and put on the ceramic plate. The equipment was sealed, and the drying experiment started. The applied steps of matric suction are given in Tables 2 and 3 for coarse-and fine-grained tailings. The equilibrium standard is that the change of the flow-out water content within 24 h is less than 0.2%. After the equilibrium condition was reached at each matric suction, the samples were weighed and the changes of the moisture content of the samples were calculated. The maximum applied matric suction in the experiments was 280 kPa for the coarsegrained tailings and 310 kPa for the fine-grained tailings.
After the experiment was finished, the samples were taken out and dried in an oven to determine the final water content of the samples. The dried water content was used to validate the measured water content at each matric suction condition.

Measured SWCCs of waste rocks
Unlike fine-grained tailings, waste rocks in waste dumps belong to coarse-grained soil. Figure 2 shows the particle size distribution of waste-rock samples in the dump. The particle size distribution characteristics are shown in Table 3. The gradations of samples are well except for the sample W-3, which has a gap-graded particle distribution.
Compared to the tailings, there is one order of magnitude

ASSESSMENTS OF THE MEASURED SWCCS
Measured SWCC data of tailings The volumetric water content of coarse-grained tailing samples under different matric suctions measured by the pressure plate test is shown in Table 3, and the data of the fine-grained tailings are shown in Table 4. As shown in Tables 3 and 4, the air-entry value of the samples increases with the finer particle size both for coarse-grained tailings (C-1, C-2 and C-3) and fine-grained tailings (F-1, F-2, F-3 and F-4).
In the analysis of engineering seepage, there are commonly used soil-water characteristic models, such as Gardner model, VG model and FX model. For the VG model: where θ, θ r and θ s represent the volumetric moisture content, residual volumetric moisture content and saturated volumetric moisture content, respectively. ψ is the matrix suction, and a, m and n are model fitting parameters.  The FX model is described by the following equations: where e is the natural base; a, m and n are model fitting parameters; and ψ r is the residual suction.
The Gardner model is similar to the VG model, using the following equation: where a and k are model fitting parameters.
The least-squares method is used to fit the experimental data when the three SWCC models are used. The model parameters are shown in Table 5. The determination coefficients R 2 for the Gardner model, the VG    (2) the fitting accuracy can be met no matter whether the tailings are coarse or fine, and the fitting accuracy is satisfactory given different tailing densities. Therefore, in practice, on the basis of calibrating the model fitting parameters, the aforementioned SWCC models can be chosen for the seepage analysis of tailings.
Given the fact that the determination coefficient R 2 of all fitting models meets the requirements, the following analysis and discussion are based only on the fitting curves from the FX model, and the analysis and discussion are expected to be applicable to other models as well.

SWCC characteristics of tailings
Relationship between volumetric water content and matric suction Figure 3(a) shows a comparison between the measured and fitted tailing SWCCs using the FX model with different particle sizes and the same dry density (2.0 × 10 3 kg/m 3 ). It can be seen that the relationship between the volumetric water content and the matrix suction of the three samples had the standard S-type for coarse-grained tailings. Due to the difference in particle gradation, the air-entry value and the residual water content of the samples were different.
The air-entry value and the residual water content of the sample C-3 were the largest, while the air-entry value and the residual water content for the sample C-1 were the smallest. Table 1 shows that the air-entry value and the residual moisture content of the samples were perhaps related to the content with the particle size of 0.25-0.075 mm. Among the three samples, C-3 had the largest content (65.1%), and C-1 had the smallest content (13.8%). As the content of the 0.25-0.075 mm particle group increased, the air-entry value, water-entry value and residual volume water content all increased on a semi-logarithmic coordinate. Chen & Uchimura () showed that the air-entry value and residual water content were related to the effective particle size D 10 of the soil. However, this study shows that D 10 had no correlation with air-entry value and residual Table 5 | Parameters for different SWCC models Model Fitting parameters moisture content (C-1, C-2 and C-3 were almost the same).
Interestingly, from the analysis of the particle size distribution characteristics in Table 1, the air-entry value and residual water content were closely related to the unevenness coefficient C u (C u ¼ D 60 /D 10 ) of the three samples, and the C u values were 14.60, 5.942 and 2.981 for the three samples, respectively.
The water retention capacity of a soil is not only related to the particle size distribution but also directly related to the pore size and pore distribution in the soil. On the one hand, when the soil material is given, the matric suction during the drying processes of the soil depends on the size and number of pores in the soil. According to the Young-Laplace equation, the matric suction is related to the equivalent pore size. Under the given matric suction, the pores smaller than the equivalent pore size are filled with water, and pores larger than the equivalent pore size lose water. On the other hand, the size of the soil particle is related to the mineral compositions of the soil. The matric suction is larger under the same water content when the particle becomes finer, and the contact angle is smaller. Our test confirmed the aforementioned implications from the Young-Laplace equation: the mean particle diameter D 50 of C-1, C-2 and C-3 decreased in sequence, as shown in Table 1. The matric suction of the samples sequentially increased under the same water content condition. Furthermore, the measured and fitted SWCCs of the fine-grained tailings with the same dry density are given in Figure 3(b).
The trend of the SWCCs for the fine-grained tailings is similar to the one for the coarse-grained tailings.
It is worth noting that the residual water content of different fine tailing samples is close, and the statistical D 10 is also close. Compared to the fine-grained tailing, the slope of the SWCC curve becomes steeper when the particle changes from coarse to fine. Under the same matric suction force, the water content of the sample increases with the decrease of the particle size, showing an enhanced water retention capacity. From the perspective of the stability of the tailing dam, the pore pressure cannot dissipate in time in finegrained tailings, which decreases the strength of solid skeleton and increases the safety risk during dam construction.
To avoid this risk, waste dumps are usually used to build dams, such as Yuanjiacun Iron Mine (Wang et al. , ).

Effect of dry density on SWCCs
Under certain conditions of gradation and soil properties, the dry density characterizes the compactness of the sample. For the tailing dam, the dry density is controlled by the dam construction technology (swirl damming or layered rolling) and is also affected by the self-weight consolidation. For the dump, the density is affected by the self-weight consolidation.
However, the orientation of internal stratification waste is affected by particle separation during material falling in the dumping of waste rock. Under this situation, the permeability of the waste rock is usually anisotropic. also show that the residual water content increases for coarse-grained tailings, and the residual water content reaches the same value for fine-grained tailings. Compared to C-2, the increase in the air-entry value and the residual water content of C-3 are more obvious. Based on the microscopic mechanism, the fine particle content has a great effect on the water retention characteristics of the soil.
The water retention capacity can increase when the fine particle content increases in soil. Among the three coarsegrained samples, the fine particle content of the sample C-3 is the highest, as shown in Figure 1. Furthermore, the difference in residual water content for fine-grained tailings with increasing dry density is limited. The results show that there is a threshold pore size (lower limit). The residual water content is no longer affected by the density when the threshold pore size reaches during a drying process.

SWCC characteristics of dumping materials
This section explores the suitability of the existing SWCC models to describe the soil-water characteristics of waste rocks with substantially varied grain sizes. The three SWCC models are used to fit the measured data from Azam et al.
(). The key parameters for the SWCC of waste rocks are listed in Table 6. The fitting parameters from different SWCC models are listed in Table 7. Except for a low determination coefficient for the sample W-5 by the Gardner model, the determination coefficient by the three models is greater than 0.96. The comparison between the measured and fitted SWCCs by the FX model is shown in Figure 5. The results show that the fitting SWCC models are suitable to describe the soil-water characteristics of the waste rock.
The SWCC shape of the waste-rock sample is different from the single S-shape of the tailing sample. The SWCC curve includes inverted L-shape, nearly straight line, and normal L-shape. Unlike the SWCC of tailings, the change in the SWCC shape of waste rocks is not closely related to the particle size characteristic parameters, such as particle size and unevenness coefficient.
The bulk of the waste dump spans two scales of fine-      areas under the effect of the transport machinery often appear on the top of the dump. The density and porosity change in different depths and areas due to the special technique on the tailing dam and waste dump. Therefore, the divisional and layered characteristics in the particle size should be considered in the seepage analysis under the damming process, which has great important effects on the establishment and selection of SWCC models.
In summary, the fitting SWCC models developed for natural soils are suitable for describing the soil-water retention behavior of mine wastes, including tailings and waste rocks in dumps. Our experimental studies reveal that the difference in dry density and the content of key granule groups substantially affect the characteristics of the SWCCs. Therefore, in the analysis of seepage stability, the effects of the different dry density and the content of the particle group on the SWCC should be considered caused by consolidation age in different areas of the tailing pond and particle sorting. It is recommended that the spatio-temporal characteristics need to be considered in the simulation strategy. In the horizontal direction, the model can be developed based on the divisional regions according to the particle size.
In the vertical direction, the SWCC values are assigned in layers based on the stacking age of the tailings.

PREDICTION OF SWCC PARAMETERS
where n is the porosity; S c depends on the capillary force; S a depends on the adhesion; h co represents the height of capillary rise of water, expressed in centimeter head; ψ 90 is the estimated air-entry value; e is the porosity ratio; D 10 (mm) is the particle size value with a mass fraction less than 10%; b is an empirical value of the estimated air-entry value, and an empirical value of 4.0 mm 2 is used. ψ 0 is 10 8 cm water column. ψ r is 1.5 × 10 4 cm water column, which is an empirical value. a and m are two empirical values as well, and this study uses an a value of 0.006 and an m value of 0.05.

Aubertin-2003 model
To reflect the difference in unsaturated properties of granular soils with a liquid limit of less than 30% and clayey plastic soils with a liquid limit of more than 30%, Aubertin  (6) and (8) in the Aubertin-2003 model: where 〈〉 is an operator and 〈x〉 ¼ 0:5(x þ jxj). ψ n is the parameter for dimensionless, if h co expresses in centimeters of water column, then ψ n ¼ 1. a c is an empirical parameter, which takes 0.01 for granular soil and 7 × 10 À4 for clayey plastic soil. m is an empirical parameter, which takes 1=C u for granular soil and 3 × 10 À5 for clayey plastic soil. h co and ψ r are related to the type of soil. For granular soils, Equations (13) and (14) are used for calculating h co and ψ r ; for clayey plastic soil, Equations (15) and (16) where ρ s is the particle density, and w L is the liquid limit expressed as a percentage.

Vanapalli-Catana-2005 model
The where d e is the characteristic particle size in millimeters. Δg i is the mass fraction of particles in a specific particle size range. d g i is the largest particle size in a certain particle size range. d d i is the smallest particle size within a certain particle size range. x is the fitting parameter.

Chin-2010 model
The Chin-2010 model also uses the form of the FX model, n ¼ x ψ r ¼ 100 kPa (28) where D 50 is the particle size value whose mass fraction is less than 50%, and x is the fitting parameter.
Theoretically, the more physical indicators used in the model, and the predicted model can give much closer predicted curves to the measured data. Table 8  cators, but the calculation of its characteristic particle size requires the entire particle curve. The actual information contained is much larger than other models, plus a fitting point; in fact, the most used the physical property indexes.

Suitability evaluation of the prediction models
Evaluation of the prediction models for the tailings The particle size distribution characteristics (Table 1) show that there are two types of tested samples: coarse-grained tailings (C-1 to C-3 and F-1) and fine-grained tailings (F-2 to F-4).
For C-1 to C-3 and F-1, the model adopts the equation suitable for granular soil, because C-1 to C-3 and F-1 are granular soils.
The comparison between the experimental and predicted results of the SWCCs of different samples is shown in Figure 6.
Overall, the predicted values of the one-point models are close to the experimental data. However, the prediction results from the Aubertin-1998 andAubertin-2003  , and the discrepancy may be related to the particle size curve. The threshold particle size (e.g., 4.75 mm) separating the waste rock in the dump and the coarse-grained tailings contribute to the fact that the SWCC prediction does not apply to the gravel part in the particle size distribution curve. As a result, only the fine particle part is needed to obtain the key parameters of the SWCC curve, such as C u , e and d e . In summary, the influence of the relative abundance of the two-particle groups on the overall hydraulic properties of the dump materials needs to be considered. Chin-2010 (P 200 > 30%) √ Figure 6 | Measured and predicted SWCCs for sample C-1 with a dry density of 2.0 g/cm 3 (a), sample C-2 with a dry density of 1.7 g/cm 3 (b), sample C-2 with a dry density of 2.0 g/cm 3 (c), sample C-3 with a dry density of 1.8 g/cm 3 (d) and sample C-3 with a dry density of 2.0 g/cm 3 (e).

Discussion of the limitations of the prediction models
Limitations caused by statistical samples   Table 9, the porosity of this sample is 0.637, D 10 is 0.06 mm and D 60 is 0.3 mm. It can be seen that the statistical samples applicable to the above two models are quite different from the samples tested here. The D 10 of 15 samples used for the Aubertin-1998 model is lower by one order of magnitude than the samples in this paper. In theory, the finer the sample particles, the pore size is smaller in the sample, which may be the reason the existing prediction models generally estimate the low air-entry values.
A comparison between the experimental value from Rassam & Williams () and the predicted value from Aubertin et al. () shows that the predicted air-entry value is close in the two studies, but the predicted slope of the curve in the descending section is greater than that of the measured curve, as shown in Figure 8. In short, if the information on the particle size distribution of a given sample is limited, the prediction models cannot perform well in predicting the SWCCs for similar types of samples.

Limitations on assumed particle shape and mineral compositions
Some empirical coefficients in the models imply some assumptions for the sample particle shape and minerals.
For example, Equation (12) where σ w is the surface tension of water, β w is the contact angle, γ w is the weight of water, α is the shape factor of the particles and D H is the equivalent particle size that keeps the pore surface area of the sample unchanged.
When simplified, α takes 10 and β w takes 0 degree, which assumes that the particles are all quartz. However, the Note: These parameters are similar to those of the sample C-2 with a dry density of 1.7 g/cm 3 . Rassam & Williams (1999) and the predicted SWCC based on the model in Aubertin et al. (2003).

Figure 8 | Comparison between the measured SWCC from
tailings are the product of manual crushing rather than natural weathering, so the tailing particles are very rough (Maqsoud et al. ). In addition, the shape factor may be different from the empirical value. The XRD analysis of the samples in this paper shows that the quartz content is only one-third, as shown in Table 10. Hence, the contact angle is not zero.
Taking C-2 with a dry density of 1.7 × 10 3 kg/m 3 as an example, by keeping the contact angle unchanged and changing the shape factor, the prediction results of the Aubertin-2003 model are obtained and shown in Figure 9. When the shape factor is close to 15, the air-entry value is closer to the measured value. If the increase in contact angle is considered, the shape factor should continue to increase.
Therefore, particle shape and mineral compositions are two important reasons to cause the deviation of the predicted value from the measured value.
Limitations caused by sample description indicators Figure 10 shows the prediction results of the Aubertin-2003 model for samples C-2 and C-3 with different dry densities.
The prediction results fail to reflect the change of the SWCC caused by the difference in dry density, which does not agree with the measured SWCC data in Figure 4. The The Aubertin-1998 model also has the same issue. Both the two models lack suitable indicators to reflect the change of the dry density in tailings, which represents a change of the pore structure. Therefore, there is a large deviation between the prediction results and the measured data for Aubertin-1998 andAubertin-2003

Discussion on the suitability of the prediction models
To further verify the suitability of the prediction models, the predicted models are used to predict the measured data in Wen et al. (), as shown in Figure 11. Figure 11 shows Figure 9 | Effect of the shape factor on the predicted SWCCs. that (1) for the tailing medium sand and tailing fine sand, the predictions from Aubertin-1998 andAubertin-2003  (3) At present, the prediction model established completely based on the particle size distribution curve still cannot consider the impact of pore structure changes on the SWCC

SWCC MODEL FOR THE EMERGENCY RISK ASSESSMENT OF MINE WASTES
Tailing dams and dumps come with a potential risk of collapse in mines. Different from hydraulic earth-rock dams and slope engineering with relatively fixed structural Therefore, the SWCC model can be determined according to the particle size of the sedimentary tailings.
In the horizontal direction, two large areas are set, namely (1) dam body (coarse-grained tailings) and (2) deposited fine-grained tailings. From the crest of the dam, sandy, silty and clayey tailings may be distributed in sequence.
According to the particle size, it is further divided into different areas: tailing gravel sand (2 mm); tailing coarse sand (0.5-2 mm); tailing medium sand (0.25-0.5 mm); tailing fine sand (0.074-0.25 mm); tailing silty sand (<0.074 mm); tailing silt; tailing silty clay and tailing clay. In the vertical direction, based on self-weight stress and consolidation age, the different dry densities are set based on the stratification.
For the dumping site, Wang et al. (b) found that gravity separation results in inclined stratification characteristics and the reverse sequence layers with the upper coarsegrained particles and the lower fine-grained particles.
Under this situation, the hydraulic properties on the down and vertical slope directions show strong anisotropy.

Divisional assignment of SWCC parameters
The SWCC model for emergency risk assessment adopts a divisional assignment method.
Steps of this method are as follows: (1)

Influence on seepage characteristics
To further verify the validity of the recommended parameters in the emergency model in Table 11, a simulation   Table 12. It is clear that the recommended soilwater retention characteristics model and the related parameters can be used to predict water levels in tailing dams and waste dumps, which is very helpful for emergency risk assessment under rainstorm or flooding conditions.

SUMMARY AND CONCLUSIONS
In this study, representative coarse-and fine-grained tailings from a metallurgical mine were used to experimentally determine the soil-water retention characteristics of the  tailings. Three fitting SWCC models were used to explore the effects of particle size and compactness on the soilwater retention behavior. The suitability of the SWCC prediction models is compared and discussed in detail. Given the need of rapidly determining the SWCC parameters for emergency risk assessment under rainstorm or flooding conditions, the possibility of replacing the measured SWCCs with the predicted SWCC model is explored. The main conclusions are as follows: 1. The measured SWCC of coarse-and fine-grained tailings and waste dumps can be fit well using Gardner's, VG's and FX-SWCC's models, respectively.
2. For the cross-particle scale mine waste, tailing mud, tailing sand and dumping materials, the Vanapalli-Catana-2005 model can achieve an acceptable accuracy for the prediction of SWCC parameters. By comparing the results from different prediction models, the applicability of the SWCC model is substantially affected by sample statistics used in model development. The assumptions of particle shape and mineral compositions in the model have a great influence on the calculation of the air-entry value. The effect of pore structure changes on the SWCC cannot be included in the existing models due to a lack of physical parameters to reflect the pore structure.
3. The effect of the threshold particle size (d thres ) that dis-