# Sample Rocks Tests and Slope Stability Analysis of a Mine Waste Dump

Authors: Ping Zou; Ximo Zhao; Zhonghua Meng; Aibing Li; Zhengyu Liu; Wanjie Hu
Journal: Advances in Civil Engineering
 (2018)

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## Abstract

The safety and stability of waste dump are vital influencing factors to the mine sustainability and mine employees. Based on a real mine project in a certain open-pit mine waste dump in Tibet, the in situ test on waste rocks from waste dump, including measurements of density, water content, rock size, and natural repose angle, was conducted. Afterwards, these sample waste rocks, of which grain size is less than 5 cm, were selected for indoor large-scale shear test under natural and saturated conditions. By using some engineering methods, the physical and mechanical parameters of waste rocks layer were then determined accordingly. MIDAS-GTS/NX has the advantage of pre-processing modeling. FLAC3D has good computational and analytical capabilities. The process of dump accumulation is simulated numerically. According to the calculation results of FLAC3D, the distribution of stress, displacement and plastic zone in the dump is obtained. FOS (factor of safety) for each analytical step in this model was then calculated through the strength reduction method. The limit equilibrium method is used for waste dump stability analysis considering three states: only applied gravity, applied gravity and rainfall, and applied gravity and underground water. The results from this analysis show that the waste dump is stable. The potential failure modes of waste dump mainly consist of the “combined sliding mode” which has circular sliding in upper side and broken line sliding that cuts through gravel-soil layer into heavily weathered layer in the bottom. This paper documents some of the procedures and approaches utilized for waste dump life-of-mine design analysis. It provides reference for further waste dump optimization.


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## Body

## 1. Introduction

Mine stockpile also called mine waste dump is vital to the open-pit mine exploitation. So called by its name, mine waste dump is primarily utilized to storage the overburden and waste rock from open-pit mines [1]. The safety and stability of waste dump refers to the mine sustainability and mine employees and should be paid enough attention. There are many factors contributing to the stability of a mine waste dump, including physical and chemical composition of the waste rock, the dumping technologies being used, engineering conditions of the landscape, hydrogeological condition, and other related parameters.At present, a lot of research has been done on mine dump at home and abroad. It mainly includes using test and back analysis method to obtain rock mechanics parameters. The stability is determined by limit equilibrium analysis, numerical simulation analysis and simulation test. Cho and Song [2] studied the dumping behavior of dump slope and natural slope under dump. Linear sensors are installed at the top of the slope of the dump site to monitor the pile-up behavior of the dump site. Turer and Turer [3] used the two methods to determine the weight of the unit and the waste of the shear strength parameters to analyze a slope stability map. In another example, Adamczy et al. [4] introduce the stability of garbage sandstone open slope and choose six sections to analyze the stability of slope. Behera et al. [5] analyzed the stability of open-pit coal mine dump in Odisha area based on different geotechnical parameters and mineralogical composition. Verma et al. [6] analyzed the stability of existing dump by the analytical method. The properties of the material in the dump are measured in the laboratory, such as cohesive force, internal friction angle, permeability, volume density, particle density, particle size distribution, and field water content. In a very dry and humid environment, the stability of dump slope is simulated. Kainthola et al. [7] used the western coalfield limited, Nagpur, India, as an example. The shear strength reduction technique has been applied to achieve the desired factor of safety using a two-dimensional finite element code. Zhou et al. [8] have studied the effects, from inner drainage parameter variation of the northern slope of Haerwusu Open-pit Coal Mine, on its imbalance stress and slop stability. Cheng et al. [9] analyzed the slope stability of an open-pit mine under the combined effect of waste dump and blasting and hence estimated and verified the required minimum distance between mine and waste dump accordingly. Dong [10] invented a method in evaluating the stability of long-bench-waste-dump under heavy rainfall and applied this in Longyan Iron Mine Waste Dump in Fujian. Huang et al. [11] have determined the creeping-cracking failure and cracking-sliding failure for Jinduicheng Open-pit long-bench-waste-dump, analyzed the stability of this kind of waste dump under natural state, blasting vibration state, and seismic state, and finally provided some countermeasures in case of failure. The stability of rock slope is also related to different types of rock [12, 13]. The slope stability comparison between those only applied static load and those count for the extra impacts from earthquake [14–16]. The microstructure of the rock-soil in the dump has significant influence on the stability of dump [13, 17–25].This article, based on a real mine project in a certain open-pit mine waste dump in Tibet, demonstrates the stress, displacement, and plastic zone distribution, and variation directions refer to the overall stacking process of this waste dump, as well as the possible failure mode, by conducting in situ tests and indoor rock experiments on the waste rocks from dump and establishing 3D simulation models. Thus, it provides reference for further waste dump optimization.


## 2. Overview of the Waste Dump

### 2.1. Engineering Geological Condition

The hornfels waste dump is valley type, with natural slope 10–20°, two side slopes 30–40°; the boundary altitudes in east, north, and west are 5105 m.The exposed bedrock is beneficial to the stability of dump site. The exposure strata are mainly residual gravel, flood-accumulated rocks, strongly weathered limestone, and medium weathered limestone; the slopes are shallowly covered by Quaternary diluvial remnant gravels. The waste rocks stacked on dump are mainly slate, hornfels, limestone, granite porphyry, and Quaternary topsoil. There is no indication of negative geological development, such as landslide or collapse, or any cracking formation.


### 2.2. Hydrogeologic Condition

The topography of the waste dump is complex with a number of crossed valleys; its catchment area is 1.4 km2 and volume is 10800 × 104 m3. The flow of surface water varies greatly with the seasonal precipitation. During the rainy season (mainly during June till September), the seasonal flood can be easily formed in gully and hence causes negative effect on waste dump. The atmospheric precipitation can possibly form surface flows, which is finally collected in gully. The stable phreatic surface is at the depth of 0.1 to 1.0 m, mainly supplied by atmospheric precipitation and bedrock fissure leakage. The groundwater is less likely to be stored beneath these steeply slopes. The fractures developed in strong weathered limestone are filled well but with poor connections. For medium weathered limestone, the fissure is comparably developed, and mass rock body is more complete with poor permeability, which is the natural interlayer.


### 2.3. Overview of Design

The dumping process consists of waste rock transportation, dumping by self-discharging vehicles, and supplementing by auxiliary bulldozer. Multibench dumping starts from bottom to top and finally piles up to 5105 m with an overall heap height of 580 m and every bench height is 30 m. Set those benches be 5075 m, 5045 m, 5015 m, 4985 m, 4955 m, 4925 m, 4895 m, 4865 m, 4835 m, 4805 m, 4775 m, 4745 m, 4715 m, 4685 m, 4655 m, 4625 m, and 4595 m with the bench width of 30 m. The slope of waste dump is 1 : 1.75 and capability volume of which is 16674 × 104 m3.


## 2.1. Engineering Geological Condition

The hornfels waste dump is valley type, with natural slope 10–20°, two side slopes 30–40°; the boundary altitudes in east, north, and west are 5105 m.The exposed bedrock is beneficial to the stability of dump site. The exposure strata are mainly residual gravel, flood-accumulated rocks, strongly weathered limestone, and medium weathered limestone; the slopes are shallowly covered by Quaternary diluvial remnant gravels. The waste rocks stacked on dump are mainly slate, hornfels, limestone, granite porphyry, and Quaternary topsoil. There is no indication of negative geological development, such as landslide or collapse, or any cracking formation.


## 2.2. Hydrogeologic Condition

The topography of the waste dump is complex with a number of crossed valleys; its catchment area is 1.4 km2 and volume is 10800 × 104 m3. The flow of surface water varies greatly with the seasonal precipitation. During the rainy season (mainly during June till September), the seasonal flood can be easily formed in gully and hence causes negative effect on waste dump. The atmospheric precipitation can possibly form surface flows, which is finally collected in gully. The stable phreatic surface is at the depth of 0.1 to 1.0 m, mainly supplied by atmospheric precipitation and bedrock fissure leakage. The groundwater is less likely to be stored beneath these steeply slopes. The fractures developed in strong weathered limestone are filled well but with poor connections. For medium weathered limestone, the fissure is comparably developed, and mass rock body is more complete with poor permeability, which is the natural interlayer.


## 2.3. Overview of Design

The dumping process consists of waste rock transportation, dumping by self-discharging vehicles, and supplementing by auxiliary bulldozer. Multibench dumping starts from bottom to top and finally piles up to 5105 m with an overall heap height of 580 m and every bench height is 30 m. Set those benches be 5075 m, 5045 m, 5015 m, 4985 m, 4955 m, 4925 m, 4895 m, 4865 m, 4835 m, 4805 m, 4775 m, 4745 m, 4715 m, 4685 m, 4655 m, 4625 m, and 4595 m with the bench width of 30 m. The slope of waste dump is 1 : 1.75 and capability volume of which is 16674 × 104 m3.


## 3. In Situ Test on Waste Rocks

The in situ tests on waste rocks from waste dump include density, water content, rock size, and natural repose angle measurements. The rock size can greatly change the mechanical properties and stability of waste dump. Different sizes of rock were distributed to a certain height of slope automatically through sliding movement after dumps. Larger size of rocks lie regularly on the bottom while those smaller remain closer to the top; it is rare to see bigger size rock on the upper slope. At present, there are three normal methods being used to measure the rock size; they are screening, direct measurement, and photographic image analysis, which can be complement and verification of each other during experiments.The process of in situ waste rock test can be seen in Figure1.Figure 1
Flow chart of in situ tests on sample waste rocks.(1)
Sample pit digging: all these sample pits were dug by our research team members. The waste rocks extracted from the pit were collected on the prepared plastic sheeting. After the sample pit digging was completely done, another plastic sheeting was covered on top of it for obtaining the pit volume by measuring the volume of same amount of water filled in the hole, as well as prevention of further pit collapse due to gravity. A total of 6 pits named from J1 to J2 and N1 to N3 were dug. The sample pit J1 can be seen in Figure2.
(2)
Sample sieving: constrained by the maximum size of sample chosen for indoor large-scale direct shear test and waste rock composition measurement, the sieving meshes were eventually selected for 2 cm∗ 2 cm, 5 cm ∗ 5 cm, 10 cm ∗ 10 cm, 20 cm ∗ 20 cm, and 40 cm ∗ 40 cm, respectively. The waste rocks with grain size 2 cm ≤ d < 5 cm from J1 are shown in Figure 3.
(3)
The grain size of big rock with diameter over 20 cm can be measured directly by steel tape.
(4)
After sieving or direct measurement of waste rocks, an electronic scale (100 kg) was used to weigh those sample rocks. Finally, those samples with grain size less than 5 cm were bagged into prepared woven bags for large-scale direct shear test. Total amount of sample rocks that were collected from Hornfels Waste Dump weigh more than 1000 kg.
(5)
Measuring the sample pit volume: we filled this pit with water; the volume of those can be measured much easier when reloading it into a container. Tapes were used to prevent any water leakage. The water filled in was sourced from a mine sprinkler.
(6)
Water content testing: a bag of waste rocks from each sample pit (totally 6 bags) was randomly chosen for water content tests.
(7)
Measuring natural repose angle: a straight plank and compass were used in this measurement. The plank, which is about 2.5 m in length, was attached closely along slope, so the dipping angle of this plank can be measured through compass. Thus, totally 30 natural repose angles were obtained.
(8)
Statistical analysis of waste rocks from sample pits: in Figure4 the curve represents the composition of waste rocks with different grain sizes in the selected 6 sample pits. The detail parameters obtained from tests are displayed in Table 1.Figure 2
Sample pit J1 and waste rocks digged from it.Figure 3
Sieved sample waste rocks with grain size 2 cm ≤d < 5 cm.Figure 4
Grain size distribution curve of waste rocks. (a) Sample Pit J1. (b) Sample Pit J2. (c) Sample Pit J3. (d) Sample Pit N1. (e) Sample Pit N2. (f) Sample Pit N3.
(a)
(b)
(c)
(d)
(e)
(f)Table 1
Parameters of waste rocks measured from in situ survey.
Sample pit
Volume (L)
Grain size distribution
Total weight (kg)
Max grain size (cm)
Natural density (g/cm³)
Natural water content (%)
Natural repose angle (°)
d < 2 cm
2 cm ≤d < 5 cm
5 cm ≤d < 10 cm
10 cm ≤d < 20 cm
20 cm ≤d < 40 cm
Weight (kg)
Percentage (%)
Weight (kg)
Percentage (%)
Weight (kg)
Percentage (%)
Weight (kg)
Percentage (%)
Weight (kg)
Percentage (%)
J-1
231.3
161.50
29.51
202.86
37.06
111.28
20.33
39.68
7.25
32.02
5.85
547.34
37
2.37
5.26
37
J-2
180.3
104.36
27.18
127.10
33.11
64.76
16.87
59.00
15.37
28.70
7.48
383.92
31
2.13
3.14
J-3
148.2
95.02
32.44
97.50
33.29
41.54
14.18
41.54
14.18
7.86
2.68
292.92
31
1.98
3.68
N-1
123.9
137.64
46.19
91.44
30.69
31.34
10.52
29.34
9.85
8.22
2.76
297.98
28
2.41
6.38
N-2
140.5
176.52
57.04
94.08
30.40
21.28
6.88
17.56
5.67
-
-
309.44
19
2.20
3.29
N-3
148.68
112.48
34.48
126.50
38.77
60.32
18.49
16.08
4.93
10.88
3.33
326.26
29
2.19
2.89


## 4. Indoor Direct Shear Test for Waste Rocks from Dump Site

The indoor direct shear tests for waste rocks from dump site were conducted by utilizing strain-controllable direct shear device developed by Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. This device can shear the sample with grain size no more than 5 cm diameters and guarantee that the physical and mechanical parameters obtained from tests is reliable and practicable.


### 4.1. Test Samples

The test samples are the waste rocks with grain size less than 5 cm in diameter from pits J1, J2, N2, and N3. The detail parameters of selected samples list are shown in Tables2 and 3. Due to the limitation of test device, additional works must be done to deal with the waste rocks with grain size equal to or even bigger than 5 cm, which exceeds the maximum allowable test sample size, normally by using the replacement method.Table 2
Sampling and weight.
Grain size (cm)
J-1 bag no.
Weight (kg)
Grain size (cm)
N-2 bag no.
Weight (kg)
d < 2
#1
44.54
d < 2
#1
46.10
#2
58.50
#2
49.22
#3
58.46
#3
52.96
#4
28.24
2 ≤d < 5
#1
48.40
2 ≤d < 5
#1
42.20
#2
63.84
#2
51.88
#3
55.34
#4
35.28
Total weight (kg)
364.36
Total weight (kg)
270.60
J-2
N-3
d < 2
#1
55.16
d < 2
#1
60.44
#2
49.20
#2
52.04
2 ≤d < 5
#1
45.20
2 ≤d < 5
#1
37.60
#2
41.12
#2
47.38
#3
40.78
#3
41.52
Total weight (kg)
231.46
Total weight (kg)
238.98Table 3
Bulk density and water content of waste rocks.
Sample pit no.
Natural density (g/cm3)
Natural water content (%)
J-1
2.37
5.26%
J-2
2.13
3.14%
N-2
2.20
3.29%
N-3
2.19
2.89%


### 4.2. Test Process and Results

In the laboratory, mixing the sample rocks with different grain sizes from the same pit was firstly carried out to measure the mixture moisture content; then water accordingly until it reaches natural state, mix them fully again, and the direct shear box is treated. Another test should be conducted when those waste rocks are in saturated state by just adding adequate water into the box until those absorb sufficient water.Applying various testing loads (Table4) in shear tests, the parameters such as displacement-stress curves and shear stress-positive stress curve can be seen in Figure 5; other test results are shown in Table 5.Table 4
Loading test results.
Pit J1 (natural)
Pit J1 (saturated)
Pit J2 (natural)
Pit J2 (saturated)
Pit J3 (natural)
Pit J3 (saturated)
Normal stress (kPa)
Peak shear stress (kPa)
Normal stress (kPa)
Peak shear stress (kPa)
Normal stress (kPa)
Peak shear stress (kPa)
Normal stress (kPa)
Peak shear stress (kPa)
Normal stress (kPa)
Peak shear stress (kPa)
Normal stress (kPa)
Peak shear stress (kPa)
201
211
207
238
202
223
200
196
193
237
198
229
404
421
410
418
222
242
449
433
401
475
405
428
614
580
614
500
404
393
652
599
652
712
652
712
842
794
842
707
728
642
842
724
842
907
842
813Figure 5
The relationship curve representing test results for sample waste rocks. (a) Shear stress-displacement curve for Pit J1 (natural). (b) Peak shear stress-normal stress curve for Pit J1 (natural). (c) Shear stress-displacement curve for Pit J1 (saturated). (d) Peak shear stress-normal stress curve for Pit J1 (saturated). (e) Shear stress-displacement curve for Pit J2 (natural). (f) Peak shear stress-normal stress curve for Pit J2 (natural). (g) Shear stress-displacement curve for Pit N2 (natural). (h) Peak shear stress-normal stress curve for Pit N2 (natural). (i) Shear stress-displacement curve for Pit N3 (natural). (j) Peak shear stress-normal stress curve for PitN3 (natural). (k) Shear stress-displacement curve for Pit N3 (saturated) (l) Peak shear stress-normal stress curve for Pit N3 (saturated).
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
(k)
(l)Table 5
Test results of sample waste rock.
Sample rocks from
Cohesion (kPa)
Internal friction angle (°)
Water content (%)
Pit J1 (natural)
40.74
41.8
5.26%
Pit J1 (saturated)
92.01
35.6
11.12%
Pit J2 (natural)
66.22
38.4
3.14%
Pit N2 (natural)
45.41
39.56
3.29%
Pit N1 (natural)
58.00
44.6
2.89%
Pit N3 (saturated)
54.00
42.4
8.07%


### 4.3. Test Data Process

Based on the field survey for waste dump and shear test results, the internal friction angle and cohesion of waste rocks in different heights along slopes can be calculated. The shear test results of fine particles (c and φ) and the distribution of fine particles in different horizons have been obtained. Through the analysis of five kinds of fine particle composition, the proportion of fine particle and large particle waste rock can be calculated. The formula for calculating the cohesion and internal friction angle of waste rocks in a certain height is shown in equations (1) and (2), and the calculation results are shown in Table 6.(1)chi=cMhi×aMhi(2)tanφhi=tanφk−tanφk−tanφMhiaMhiwhere chi is the cohesion of waste rocks in crest slope (height hi), cMhi is cohesion of fine waste rocks in crest slope (height hi), aMhi is percentage of fine waste rocks (height hi), φk is the internal friction angle of big rocks, which is equal to natural repose angle, and φMhi is the internal friction angle of fine waste rock (height hi).Table 6
Calculation of physical and mechanical parameters of waste rocks.
Waste rocks from
For waste rocks of which grain size < 5 cm
Percentage of those waste rocks of which grain size < 5 cm (%)
Natural repose angle
Derived overall waste rocks in comprehensive grain size
c (kPa)
φ (°)
c (kPa)
φ (°)
Pit J1 (natural)
40.74
41.8
66.57
37°
27.12
40.3
Pit J1 (saturated)
92.01
35.6
61.25
37.0
Pit J2 (natural)
66.22
38.4
60.29
39.92
37.9
Pit N2 (natural)
45.41
39.56
87.45
39.71
39.2
Pit N1 (natural)
58.00
44.6
73.25
42.48
42.7
Pit N3 (saturated)
54.00
42.4
39.55
37.0


## 4.1. Test Samples

The test samples are the waste rocks with grain size less than 5 cm in diameter from pits J1, J2, N2, and N3. The detail parameters of selected samples list are shown in Tables2 and 3. Due to the limitation of test device, additional works must be done to deal with the waste rocks with grain size equal to or even bigger than 5 cm, which exceeds the maximum allowable test sample size, normally by using the replacement method.Table 2
Sampling and weight.
Grain size (cm)
J-1 bag no.
Weight (kg)
Grain size (cm)
N-2 bag no.
Weight (kg)
d < 2
#1
44.54
d < 2
#1
46.10
#2
58.50
#2
49.22
#3
58.46
#3
52.96
#4
28.24
2 ≤d < 5
#1
48.40
2 ≤d < 5
#1
42.20
#2
63.84
#2
51.88
#3
55.34
#4
35.28
Total weight (kg)
364.36
Total weight (kg)
270.60
J-2
N-3
d < 2
#1
55.16
d < 2
#1
60.44
#2
49.20
#2
52.04
2 ≤d < 5
#1
45.20
2 ≤d < 5
#1
37.60
#2
41.12
#2
47.38
#3
40.78
#3
41.52
Total weight (kg)
231.46
Total weight (kg)
238.98Table 3
Bulk density and water content of waste rocks.
Sample pit no.
Natural density (g/cm3)
Natural water content (%)
J-1
2.37
5.26%
J-2
2.13
3.14%
N-2
2.20
3.29%
N-3
2.19
2.89%


## 4.2. Test Process and Results

In the laboratory, mixing the sample rocks with different grain sizes from the same pit was firstly carried out to measure the mixture moisture content; then water accordingly until it reaches natural state, mix them fully again, and the direct shear box is treated. Another test should be conducted when those waste rocks are in saturated state by just adding adequate water into the box until those absorb sufficient water.Applying various testing loads (Table4) in shear tests, the parameters such as displacement-stress curves and shear stress-positive stress curve can be seen in Figure 5; other test results are shown in Table 5.Table 4
Loading test results.
Pit J1 (natural)
Pit J1 (saturated)
Pit J2 (natural)
Pit J2 (saturated)
Pit J3 (natural)
Pit J3 (saturated)
Normal stress (kPa)
Peak shear stress (kPa)
Normal stress (kPa)
Peak shear stress (kPa)
Normal stress (kPa)
Peak shear stress (kPa)
Normal stress (kPa)
Peak shear stress (kPa)
Normal stress (kPa)
Peak shear stress (kPa)
Normal stress (kPa)
Peak shear stress (kPa)
201
211
207
238
202
223
200
196
193
237
198
229
404
421
410
418
222
242
449
433
401
475
405
428
614
580
614
500
404
393
652
599
652
712
652
712
842
794
842
707
728
642
842
724
842
907
842
813Figure 5
The relationship curve representing test results for sample waste rocks. (a) Shear stress-displacement curve for Pit J1 (natural). (b) Peak shear stress-normal stress curve for Pit J1 (natural). (c) Shear stress-displacement curve for Pit J1 (saturated). (d) Peak shear stress-normal stress curve for Pit J1 (saturated). (e) Shear stress-displacement curve for Pit J2 (natural). (f) Peak shear stress-normal stress curve for Pit J2 (natural). (g) Shear stress-displacement curve for Pit N2 (natural). (h) Peak shear stress-normal stress curve for Pit N2 (natural). (i) Shear stress-displacement curve for Pit N3 (natural). (j) Peak shear stress-normal stress curve for PitN3 (natural). (k) Shear stress-displacement curve for Pit N3 (saturated) (l) Peak shear stress-normal stress curve for Pit N3 (saturated).
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
(k)
(l)Table 5
Test results of sample waste rock.
Sample rocks from
Cohesion (kPa)
Internal friction angle (°)
Water content (%)
Pit J1 (natural)
40.74
41.8
5.26%
Pit J1 (saturated)
92.01
35.6
11.12%
Pit J2 (natural)
66.22
38.4
3.14%
Pit N2 (natural)
45.41
39.56
3.29%
Pit N1 (natural)
58.00
44.6
2.89%
Pit N3 (saturated)
54.00
42.4
8.07%


## 4.3. Test Data Process

Based on the field survey for waste dump and shear test results, the internal friction angle and cohesion of waste rocks in different heights along slopes can be calculated. The shear test results of fine particles (c and φ) and the distribution of fine particles in different horizons have been obtained. Through the analysis of five kinds of fine particle composition, the proportion of fine particle and large particle waste rock can be calculated. The formula for calculating the cohesion and internal friction angle of waste rocks in a certain height is shown in equations (1) and (2), and the calculation results are shown in Table 6.(1)chi=cMhi×aMhi(2)tanφhi=tanφk−tanφk−tanφMhiaMhiwhere chi is the cohesion of waste rocks in crest slope (height hi), cMhi is cohesion of fine waste rocks in crest slope (height hi), aMhi is percentage of fine waste rocks (height hi), φk is the internal friction angle of big rocks, which is equal to natural repose angle, and φMhi is the internal friction angle of fine waste rock (height hi).Table 6
Calculation of physical and mechanical parameters of waste rocks.
Waste rocks from
For waste rocks of which grain size < 5 cm
Percentage of those waste rocks of which grain size < 5 cm (%)
Natural repose angle
Derived overall waste rocks in comprehensive grain size
c (kPa)
φ (°)
c (kPa)
φ (°)
Pit J1 (natural)
40.74
41.8
66.57
37°
27.12
40.3
Pit J1 (saturated)
92.01
35.6
61.25
37.0
Pit J2 (natural)
66.22
38.4
60.29
39.92
37.9
Pit N2 (natural)
45.41
39.56
87.45
39.71
39.2
Pit N1 (natural)
58.00
44.6
73.25
42.48
42.7
Pit N3 (saturated)
54.00
42.4
39.55
37.0


## 5. 3D Numerical Simulation Analysis for Hornfels Waste Dump

### 5.1. Modeling

The computing package FLAC3D has strong computational function and simulation analysis capability, and it has been widely recognized in the world. Refer to domestic and international FLAC3D complex 3D engineering modeling method, the basic thought can be roughly summarized by using other professional 3D modeling software or finite element analysis software to build complex 3D engineering model and then load this complex model into FLAC3D for analysis and calculation. The Mohr-Coulomb strength criterion is adopted. The constraint boundary is adopted around the model, and the free boundary is used on the empty surface. This method can greatly improve the modeling efficiency, as well as save the modeling time and guarantee the authenticity and accuracy [26]. In this article, a method combining GTS NX and FLAC3D is proved to be highly efficiency in building a 3D finite model. Firstly, GTS NX was used to complete 3D geometric modeling for Hornfels Waste Dump, tetrahedron grid model was generated, and then this model was transformed into ∗. FLAC3D format which can be loaded into FLAC3D, using import grid function to accomplish the FLAC3D hornfels mine modeling. The final model can be seen in Figure 6; different colors represent different rock mass.Figure 6
3D numerical analysis model of waste dump. (a) Waste dump before dumping process. (b) Waste rocks scheduled to be dumped. (c) The waste dump after dumping process.
(a)
(b)
(c)The range of the model isx = 3200 m to 5350 m (2150 m in total) in east-west direction, y = 550 m to 3900 m (3450 m in total) in north-south direction, z = 4400 m to the surface; there are totally 94063 nodes and 504907 elements being generated during modeling.


### 5.2. Rock Mass Mechanics Parameters

Refer to geology in Hornfels Waste Dump, there are 4 kinds of mechanics medium being considered for modeling: they are waste rocks, gravel and soil, heavily weathered rock layer, and medium weathered rock layer. After a comprehensive selection among in situ investigation, the physics and mechanics test, the rock mass quality evaluation and engineering analogy, and the rock mechanics parameters are summarized as shown in Table7.Table 7
Physical and mechanical parameters of the sample rocks from waste dump.
Lithology
Densityγ (g/cm3)
Cohesionc (kPa)
Inner friction angleφ (°)
Compressive strengthσcm (MPa)
Tensile strengthσct (MPa)
Elasticity modulusEm (MPa)
Poisson’s ratioμ
Heavily weathered rock layer
2.56
320
28
7.00
0.90
3400
0.23
Medium weathered rock layer
2.66
650
30
14.20
2.03
7341
0.23
Natural gravel and soil
2.10
40
28
—
—
200
0.30
Saturated gravel and soil
2.20
20
16
—
—
130
0.30
Natural waste rocks
2.21
35
35
—
—
800
0.27
Saturated waste rocks
2.33
25
30
—
—
550
0.27


### 5.3. Analysis Process

In order to simulate the piled up process of Hornfels Waste Dump, as well as to quantify surface and waste rock deformation, totally 8 analysis steps were designed and simulated accordingly for this stacking process, which can be seen in Figure7 and Table 8.Figure 7
Schematic diagram of the numerical simulation for waste dump.Table 8
Analytical steps of 3D numerical simulation for waste dump.
Step no.
Stacking introduction
Stacking height
Stacking time (year)
1
Calculate the initial stress field
2
4865 m
2015
3
4885 m
2016
4
4895 m
2017
5
4907 m
2018
6
4965 m
2023
7
5045 m
2028
8
5105 m
2036


### 5.4. Analysis Results

In order to observe the internal stress, strain, displacement, and plastic zone of the model, a section view was selected right in the middle of the stacking models (Figure8).Figure 8
Section view of principle stress isoline at step 8. (a) Isoline graph of maximum principle stress. (b) Isoline graph of minimum principle stress.
(a)
(b)


#### 5.4.1. Stress Distribution

Figure9 shows maximum and minimum principal stresses of stacking step 8. The key stress in the accumulation process increases with the depth. In the area near the slope, the principal stress contour of the slope is approximately parallel to the surface of the slope. The results also show that principle stress has been lightly disturbed by stacking process within the area near the contact surface of waste rock and gravel soil. There is no stress concentration during the overall stacking process; stresses are mainly compressive stress while only a small area of tensile stress is produced inside the waste rock layer (Table 9).Figure 9
Isoline graph of displacement at step 8. (a) The 3D view of displacement isoline. (b) The section view of displacement isoline.
(a)
(b)Table 9
Maximum tensile stress of each step in the numerical simulation for waste dump.
Step no.
2
3
4
5
6
7
8
Max tensile Stress (MPa)
0.17697
0.17702
0.17737
0.17701
0.17518
0.19035
0.20828


#### 5.4.2. Displacement

Figure9 shows the top view and section view of displacement of step 8. The maximum displacements of all 8 steps can be seen in Figure 10 and Table 10. The key displacement changes in the accumulation process can be summarized as the displacement contours distributed in the waste rock layer. The maximum displacement points are also distributed in this region. Displacement gets bigger with increasing stacking height, and the displacement in Z direction always bigger than those in other directions; the maximum displacement points of stacking step 2 to step 7 appear in bench 4865 m or inside the waste rock layer; the maximum displacement point of step 8 locates inside the waste rock layer near 4965 m; there are two to three big displacement areas formed since step 7 and gradually gathered, having put some negative impacts to the stability of waste dump.Figure 10
Displacement of each analytical step.Table 10
Maximum displacement (absolute value) of each step in numerical simulation.
Step no.
Displacement (m)
Displacement in directionX
Displacement in directionY
Displacement in directionZ
Total displacement
2
0.113
−0.099
−0.37
0.38
3
0.122
−0.105
−0.398
0.409
4
0.127
−0.11
−0.41
0.423
5
0.131
−0.115
−0.421
0.434
6
0.197
−0.198
−0.662
0.686
7
0.211
−0.232
−0.71
0.746
8
0.215
−0.248
−0.802
0.815


#### 5.4.3. Safety Factor and Shear Strain Increment Analysis

The strength subtraction calculation function of FLAC3D was utilized in determining safety factors of waste dump slopes. The safety factors and shear strain distribution and changes in step 8 can be seen in Figure 11, and the safety factors of all 8 steps are shown in Figure 12. The key displacement changes during stacking process can be summarized as safety factor gets smaller with increasing stacking height (from 1.73 of step 2 to 1.63 of step 8); however, the decrease rate tends to be smaller (from 1.2% of step 3 to 0.6% of step 8). Luckily, these factors are all greater than the allowable safety factors (according to Chinese specification for design of nonferrous metal mining dump GB 50421–2007). The safety factor is more appropriate to be 1.15 to 1.30 when sliding mode of waste dump slope is circular sliding, plane sliding, or broken line sliding, depending on the safety regulations [27]. The overall waste dump is stable.Figure 11
FOS and Contour of shear strain increment at step 8. (a) FOS and Contour of shear strain increment. (b) Section view of Contour of shear strain increment and FOS.
(a)
(b)Figure 12
FOS of each analysis step.The potential failure modes of waste dump mainly consist of the “combined sliding mode” that has circular sliding in upper side and broken line sliding which cuts through gravel-soil layer into heavily weathered layer in the bottom. The simulated sliding curve in each step cut through different waste dump benches in different elevations, which can be seen in Table11.Table 11
FOS and number of potential failing bench at each step.
Step no.
2
3
4
5
6
7
8
Safety factor
1.73
1.71
1.68
1.68
1.64
1.64
1.63
Height of potential failing bench
4865 m
4885 m
4895 m
4907 m
4775 m
4745 m
4745 m
Number of potential failing benches
1
2
2
3
4
3
3


#### 5.4.4. Plastic Zone Analysis

Figure13 shows the distribution of plastic zone in step 8. The tensile strength and shear-plastic zones, formed during stacking process in each step, are mainly distributed inside waste rock layer, gravel-soil layer, and heavily weathered layer near slope toe.Figure 13
Plastic zone distribution of step 8. (a) 3D view of plastic zone. (b) Section view of plastic zone.
(a)
(b)


## 5.1. Modeling

The computing package FLAC3D has strong computational function and simulation analysis capability, and it has been widely recognized in the world. Refer to domestic and international FLAC3D complex 3D engineering modeling method, the basic thought can be roughly summarized by using other professional 3D modeling software or finite element analysis software to build complex 3D engineering model and then load this complex model into FLAC3D for analysis and calculation. The Mohr-Coulomb strength criterion is adopted. The constraint boundary is adopted around the model, and the free boundary is used on the empty surface. This method can greatly improve the modeling efficiency, as well as save the modeling time and guarantee the authenticity and accuracy [26]. In this article, a method combining GTS NX and FLAC3D is proved to be highly efficiency in building a 3D finite model. Firstly, GTS NX was used to complete 3D geometric modeling for Hornfels Waste Dump, tetrahedron grid model was generated, and then this model was transformed into ∗. FLAC3D format which can be loaded into FLAC3D, using import grid function to accomplish the FLAC3D hornfels mine modeling. The final model can be seen in Figure 6; different colors represent different rock mass.Figure 6
3D numerical analysis model of waste dump. (a) Waste dump before dumping process. (b) Waste rocks scheduled to be dumped. (c) The waste dump after dumping process.
(a)
(b)
(c)The range of the model isx = 3200 m to 5350 m (2150 m in total) in east-west direction, y = 550 m to 3900 m (3450 m in total) in north-south direction, z = 4400 m to the surface; there are totally 94063 nodes and 504907 elements being generated during modeling.


## 5.2. Rock Mass Mechanics Parameters

Refer to geology in Hornfels Waste Dump, there are 4 kinds of mechanics medium being considered for modeling: they are waste rocks, gravel and soil, heavily weathered rock layer, and medium weathered rock layer. After a comprehensive selection among in situ investigation, the physics and mechanics test, the rock mass quality evaluation and engineering analogy, and the rock mechanics parameters are summarized as shown in Table7.Table 7
Physical and mechanical parameters of the sample rocks from waste dump.
Lithology
Densityγ (g/cm3)
Cohesionc (kPa)
Inner friction angleφ (°)
Compressive strengthσcm (MPa)
Tensile strengthσct (MPa)
Elasticity modulusEm (MPa)
Poisson’s ratioμ
Heavily weathered rock layer
2.56
320
28
7.00
0.90
3400
0.23
Medium weathered rock layer
2.66
650
30
14.20
2.03
7341
0.23
Natural gravel and soil
2.10
40
28
—
—
200
0.30
Saturated gravel and soil
2.20
20
16
—
—
130
0.30
Natural waste rocks
2.21
35
35
—
—
800
0.27
Saturated waste rocks
2.33
25
30
—
—
550
0.27


## 5.3. Analysis Process

In order to simulate the piled up process of Hornfels Waste Dump, as well as to quantify surface and waste rock deformation, totally 8 analysis steps were designed and simulated accordingly for this stacking process, which can be seen in Figure7 and Table 8.Figure 7
Schematic diagram of the numerical simulation for waste dump.Table 8
Analytical steps of 3D numerical simulation for waste dump.
Step no.
Stacking introduction
Stacking height
Stacking time (year)
1
Calculate the initial stress field
2
4865 m
2015
3
4885 m
2016
4
4895 m
2017
5
4907 m
2018
6
4965 m
2023
7
5045 m
2028
8
5105 m
2036


## 5.4. Analysis Results

In order to observe the internal stress, strain, displacement, and plastic zone of the model, a section view was selected right in the middle of the stacking models (Figure8).Figure 8
Section view of principle stress isoline at step 8. (a) Isoline graph of maximum principle stress. (b) Isoline graph of minimum principle stress.
(a)
(b)


### 5.4.1. Stress Distribution

Figure9 shows maximum and minimum principal stresses of stacking step 8. The key stress in the accumulation process increases with the depth. In the area near the slope, the principal stress contour of the slope is approximately parallel to the surface of the slope. The results also show that principle stress has been lightly disturbed by stacking process within the area near the contact surface of waste rock and gravel soil. There is no stress concentration during the overall stacking process; stresses are mainly compressive stress while only a small area of tensile stress is produced inside the waste rock layer (Table 9).Figure 9
Isoline graph of displacement at step 8. (a) The 3D view of displacement isoline. (b) The section view of displacement isoline.
(a)
(b)Table 9
Maximum tensile stress of each step in the numerical simulation for waste dump.
Step no.
2
3
4
5
6
7
8
Max tensile Stress (MPa)
0.17697
0.17702
0.17737
0.17701
0.17518
0.19035
0.20828


### 5.4.2. Displacement

Figure9 shows the top view and section view of displacement of step 8. The maximum displacements of all 8 steps can be seen in Figure 10 and Table 10. The key displacement changes in the accumulation process can be summarized as the displacement contours distributed in the waste rock layer. The maximum displacement points are also distributed in this region. Displacement gets bigger with increasing stacking height, and the displacement in Z direction always bigger than those in other directions; the maximum displacement points of stacking step 2 to step 7 appear in bench 4865 m or inside the waste rock layer; the maximum displacement point of step 8 locates inside the waste rock layer near 4965 m; there are two to three big displacement areas formed since step 7 and gradually gathered, having put some negative impacts to the stability of waste dump.Figure 10
Displacement of each analytical step.Table 10
Maximum displacement (absolute value) of each step in numerical simulation.
Step no.
Displacement (m)
Displacement in directionX
Displacement in directionY
Displacement in directionZ
Total displacement
2
0.113
−0.099
−0.37
0.38
3
0.122
−0.105
−0.398
0.409
4
0.127
−0.11
−0.41
0.423
5
0.131
−0.115
−0.421
0.434
6
0.197
−0.198
−0.662
0.686
7
0.211
−0.232
−0.71
0.746
8
0.215
−0.248
−0.802
0.815


### 5.4.3. Safety Factor and Shear Strain Increment Analysis

The strength subtraction calculation function of FLAC3D was utilized in determining safety factors of waste dump slopes. The safety factors and shear strain distribution and changes in step 8 can be seen in Figure 11, and the safety factors of all 8 steps are shown in Figure 12. The key displacement changes during stacking process can be summarized as safety factor gets smaller with increasing stacking height (from 1.73 of step 2 to 1.63 of step 8); however, the decrease rate tends to be smaller (from 1.2% of step 3 to 0.6% of step 8). Luckily, these factors are all greater than the allowable safety factors (according to Chinese specification for design of nonferrous metal mining dump GB 50421–2007). The safety factor is more appropriate to be 1.15 to 1.30 when sliding mode of waste dump slope is circular sliding, plane sliding, or broken line sliding, depending on the safety regulations [27]. The overall waste dump is stable.Figure 11
FOS and Contour of shear strain increment at step 8. (a) FOS and Contour of shear strain increment. (b) Section view of Contour of shear strain increment and FOS.
(a)
(b)Figure 12
FOS of each analysis step.The potential failure modes of waste dump mainly consist of the “combined sliding mode” that has circular sliding in upper side and broken line sliding which cuts through gravel-soil layer into heavily weathered layer in the bottom. The simulated sliding curve in each step cut through different waste dump benches in different elevations, which can be seen in Table11.Table 11
FOS and number of potential failing bench at each step.
Step no.
2
3
4
5
6
7
8
Safety factor
1.73
1.71
1.68
1.68
1.64
1.64
1.63
Height of potential failing bench
4865 m
4885 m
4895 m
4907 m
4775 m
4745 m
4745 m
Number of potential failing benches
1
2
2
3
4
3
3


### 5.4.4. Plastic Zone Analysis

Figure13 shows the distribution of plastic zone in step 8. The tensile strength and shear-plastic zones, formed during stacking process in each step, are mainly distributed inside waste rock layer, gravel-soil layer, and heavily weathered layer near slope toe.Figure 13
Plastic zone distribution of step 8. (a) 3D view of plastic zone. (b) Section view of plastic zone.
(a)
(b)


## 5.4.1. Stress Distribution

Figure9 shows maximum and minimum principal stresses of stacking step 8. The key stress in the accumulation process increases with the depth. In the area near the slope, the principal stress contour of the slope is approximately parallel to the surface of the slope. The results also show that principle stress has been lightly disturbed by stacking process within the area near the contact surface of waste rock and gravel soil. There is no stress concentration during the overall stacking process; stresses are mainly compressive stress while only a small area of tensile stress is produced inside the waste rock layer (Table 9).Figure 9
Isoline graph of displacement at step 8. (a) The 3D view of displacement isoline. (b) The section view of displacement isoline.
(a)
(b)Table 9
Maximum tensile stress of each step in the numerical simulation for waste dump.
Step no.
2
3
4
5
6
7
8
Max tensile Stress (MPa)
0.17697
0.17702
0.17737
0.17701
0.17518
0.19035
0.20828


## 5.4.2. Displacement

Figure9 shows the top view and section view of displacement of step 8. The maximum displacements of all 8 steps can be seen in Figure 10 and Table 10. The key displacement changes in the accumulation process can be summarized as the displacement contours distributed in the waste rock layer. The maximum displacement points are also distributed in this region. Displacement gets bigger with increasing stacking height, and the displacement in Z direction always bigger than those in other directions; the maximum displacement points of stacking step 2 to step 7 appear in bench 4865 m or inside the waste rock layer; the maximum displacement point of step 8 locates inside the waste rock layer near 4965 m; there are two to three big displacement areas formed since step 7 and gradually gathered, having put some negative impacts to the stability of waste dump.Figure 10
Displacement of each analytical step.Table 10
Maximum displacement (absolute value) of each step in numerical simulation.
Step no.
Displacement (m)
Displacement in directionX
Displacement in directionY
Displacement in directionZ
Total displacement
2
0.113
−0.099
−0.37
0.38
3
0.122
−0.105
−0.398
0.409
4
0.127
−0.11
−0.41
0.423
5
0.131
−0.115
−0.421
0.434
6
0.197
−0.198
−0.662
0.686
7
0.211
−0.232
−0.71
0.746
8
0.215
−0.248
−0.802
0.815


## 5.4.3. Safety Factor and Shear Strain Increment Analysis

The strength subtraction calculation function of FLAC3D was utilized in determining safety factors of waste dump slopes. The safety factors and shear strain distribution and changes in step 8 can be seen in Figure 11, and the safety factors of all 8 steps are shown in Figure 12. The key displacement changes during stacking process can be summarized as safety factor gets smaller with increasing stacking height (from 1.73 of step 2 to 1.63 of step 8); however, the decrease rate tends to be smaller (from 1.2% of step 3 to 0.6% of step 8). Luckily, these factors are all greater than the allowable safety factors (according to Chinese specification for design of nonferrous metal mining dump GB 50421–2007). The safety factor is more appropriate to be 1.15 to 1.30 when sliding mode of waste dump slope is circular sliding, plane sliding, or broken line sliding, depending on the safety regulations [27]. The overall waste dump is stable.Figure 11
FOS and Contour of shear strain increment at step 8. (a) FOS and Contour of shear strain increment. (b) Section view of Contour of shear strain increment and FOS.
(a)
(b)Figure 12
FOS of each analysis step.The potential failure modes of waste dump mainly consist of the “combined sliding mode” that has circular sliding in upper side and broken line sliding which cuts through gravel-soil layer into heavily weathered layer in the bottom. The simulated sliding curve in each step cut through different waste dump benches in different elevations, which can be seen in Table11.Table 11
FOS and number of potential failing bench at each step.
Step no.
2
3
4
5
6
7
8
Safety factor
1.73
1.71
1.68
1.68
1.64
1.64
1.63
Height of potential failing bench
4865 m
4885 m
4895 m
4907 m
4775 m
4745 m
4745 m
Number of potential failing benches
1
2
2
3
4
3
3


## 5.4.4. Plastic Zone Analysis

Figure13 shows the distribution of plastic zone in step 8. The tensile strength and shear-plastic zones, formed during stacking process in each step, are mainly distributed inside waste rock layer, gravel-soil layer, and heavily weathered layer near slope toe.Figure 13
Plastic zone distribution of step 8. (a) 3D view of plastic zone. (b) Section view of plastic zone.
(a)
(b)


## 6. Limit Equilibrium Analysis of Hornfels Waste Dump

### 6.1. Analysis Method

The utilized limit equilibrium methods for Hornfels Waste Dump are the Bishop method that satisfies overall moment equilibrium about the center of the circular trial surface [28] and the Morgen-Prince method that satisfies overall moment equilibrium about arbitrary shape surface [29].


### 6.2. The Classification of Sliding Mode and Sliding Surface of Waste Dump

Landslide failure modes of dump are mainly divided into three types: internal landslide of dump, landslide along the interface between waste rock pile and foundation, and landslide along the weak layer of foundation of dump. The possible sliding modes in Hornfels Waste Dump are as follows:(i)
Sliding occurs inside waste rock layer. The waste rocks are mainly consisting of slate, hornfels, skarn, marble (or limestone), granite porphyry, and small amounts of Quaternary soil. The dumping, using self-discharging vehicles to transport waste rocks and supplementing by auxiliary bulldozer is less likely to form weak intercalations. The internal medium inside waste rock layer is relatively homogeneous (its mechanical property is dominated by friction). Therefore, the potential sliding mode of waste rock layer inside waste dump is circular sliding or other smooth-surface sliding.(ii)
Sliding along contact surface between waste rock layer and foundation layer. When the friction strength between waste rock layer and foundation is less than the shear strength of waste rock layer inside dumping site, sliding can be easily formed along this contact surface where the dipping angle of foundation is comparably big or the strength of contact is weak. The foundation of Hornfels Waste Dump and South Pit Waste Dump are mainly covered by Quaternary gravel and soil, of which strength is weak. The waste dump has a long and steep slope, and waste rock layer is thick. The potential failure mode of waste dump, regarding 3D simulation, is “combined sliding mode” which has circular sliding in upper side and broken line sliding that cuts through gravel-soil layer into heavily weathered layer in the bottom. Therefore, the waste dump is associated with a sliding threat along contact between waste dump layer and foundation.(iii)
Sliding occurs in some weak intercalated part inside foundation layer. If there are some relatively weak formations or weak intercalated inside foundation, due to their weak strength or low bearing capacity, it is easy to form foundation subsidence during the dumping process or in rainfall circumstance or impacted by the other factors. The subsidence ranges and scales vary in different parts of waste dump and some parts of foundation arise by inner stress, which can subsequently cause landslides along these weak formations or intercalated inside foundation. The waste rocks in waste dump mainly consists of slate, hornfels, skarn, marble (or limestone), and granite porphyry. No unstable geological formations exist in waste dump. Therefore, it seems impossible to slide in this particular mode.To sum up, this limit equilibrium analysis shows that the main sliding modes of waste dump are sliding occuring inside waste rock layer and sliding along contact surface between waste rock layer and foundation layer.


### 6.3. Assumed or Utilized Parameters

(1)
Parameters of rock massThe parameters of rock mass can be seen in Table7.(2)
The impact of earthquakes and waterThe seismic peak acceleration is 0.15 g, and seismic intensity of the mine is classified into level VII. The phreatic surface, in the raining reason, is about 50 meters above the surface of foundation.(3)
Allowable safety factors under different conditions(I) Consider gravity, the allowable safety factor (K) = 1.25(II) Consider combined impacts of gravity and seismic, the allowable safety factor (K) = 1.05(III) Consider combined impacts of gravity and steady seepage of groundwater, the allowable safety factor (K) = 1.05


### 6.4. Analysis Results

The calculation results of the safety factors of typical sections are shown in Table12. The comparison of safety factors of each year under different conditions is shown in Figure 14. From Table 12 and Figure 14, the following can be summarized: (1)
Considering all three conditions each year, the slope is stable only except when it reached the year 2018 that the slope may suffer from foundation contact sliding under condition III.(2)
Under condition I and II, slope safety factor is predicted to increase gradually from 2018 to 2036 and since then remain unchanged.(3)
The impact of seismic, on circular sliding, can reduce safety factor by 20.10% to 20.47%, while on contact-between-foundation-and-waste-rock-layer sliding can reduce FOS by 18.54 to 19.85%. This impact can decrease overall waste dump stability by 20.32% to 20.49%.(4)
The impact of steady groundwater seepage, on circular sliding, can reduce safety factor by 10.38% to 25.22%, while on contact-between-foundation-and-waste-rock-layer sliding can reduce FOS by 14.03 to 22.98%. This impact can decrease overall waste dump stability by 20.32% to 20.49%.Table 12
Calculation results using limit equilibrium method.
Year
Conditions
FOS of circular sliding (simplified bishop)
FOS of sliding contact between foundation and waste rock layer (Morgenstern–Price)
FOS of partial slope
FOS of overall slope
2015
I
1.408
1.402
1.491
II
1.120
1.115
1.198
III
1.144
1.189
1.102
2016
I
1.404
1.401
1.481
II
1.119
1.114
1.194
III
1.250
1.188
1.059
2017
I
1.407
1.401
1.490
II
1.119
1.114
1.198
III
1.261
1.079
1.146
2018
I
1.410
1.400
1.511
II
1.122
1.114
1.211
III
1.135
1.130
1.033
2023
I
1.479
1.447
1.578
II
1.180
1.153
1.272
III
1.106
1.244
1.122
2028
I
1.462
1.455
1.676
II
1.168
1.157
1.364
III
1.160
1.247
1.149
2036
I
1.468
1.455
1.769
II
1.173
1.150
1.441
III
1.134
1.231
1.147Figure 14
FOS comparisons for Hornfels Waste Dump by years and conditions. (a) FOS comparison of partial slope for circular sliding. (b) FOS comparison of overall slope for circular sliding. (c) FOS comparison of sliding along contact between foundation and waste rock layer.
(a)
(b)
(c)


## 6.1. Analysis Method

The utilized limit equilibrium methods for Hornfels Waste Dump are the Bishop method that satisfies overall moment equilibrium about the center of the circular trial surface [28] and the Morgen-Prince method that satisfies overall moment equilibrium about arbitrary shape surface [29].


## 6.2. The Classification of Sliding Mode and Sliding Surface of Waste Dump

Landslide failure modes of dump are mainly divided into three types: internal landslide of dump, landslide along the interface between waste rock pile and foundation, and landslide along the weak layer of foundation of dump. The possible sliding modes in Hornfels Waste Dump are as follows:(i)
Sliding occurs inside waste rock layer. The waste rocks are mainly consisting of slate, hornfels, skarn, marble (or limestone), granite porphyry, and small amounts of Quaternary soil. The dumping, using self-discharging vehicles to transport waste rocks and supplementing by auxiliary bulldozer is less likely to form weak intercalations. The internal medium inside waste rock layer is relatively homogeneous (its mechanical property is dominated by friction). Therefore, the potential sliding mode of waste rock layer inside waste dump is circular sliding or other smooth-surface sliding.(ii)
Sliding along contact surface between waste rock layer and foundation layer. When the friction strength between waste rock layer and foundation is less than the shear strength of waste rock layer inside dumping site, sliding can be easily formed along this contact surface where the dipping angle of foundation is comparably big or the strength of contact is weak. The foundation of Hornfels Waste Dump and South Pit Waste Dump are mainly covered by Quaternary gravel and soil, of which strength is weak. The waste dump has a long and steep slope, and waste rock layer is thick. The potential failure mode of waste dump, regarding 3D simulation, is “combined sliding mode” which has circular sliding in upper side and broken line sliding that cuts through gravel-soil layer into heavily weathered layer in the bottom. Therefore, the waste dump is associated with a sliding threat along contact between waste dump layer and foundation.(iii)
Sliding occurs in some weak intercalated part inside foundation layer. If there are some relatively weak formations or weak intercalated inside foundation, due to their weak strength or low bearing capacity, it is easy to form foundation subsidence during the dumping process or in rainfall circumstance or impacted by the other factors. The subsidence ranges and scales vary in different parts of waste dump and some parts of foundation arise by inner stress, which can subsequently cause landslides along these weak formations or intercalated inside foundation. The waste rocks in waste dump mainly consists of slate, hornfels, skarn, marble (or limestone), and granite porphyry. No unstable geological formations exist in waste dump. Therefore, it seems impossible to slide in this particular mode.To sum up, this limit equilibrium analysis shows that the main sliding modes of waste dump are sliding occuring inside waste rock layer and sliding along contact surface between waste rock layer and foundation layer.


## 6.3. Assumed or Utilized Parameters

(1)
Parameters of rock massThe parameters of rock mass can be seen in Table7.(2)
The impact of earthquakes and waterThe seismic peak acceleration is 0.15 g, and seismic intensity of the mine is classified into level VII. The phreatic surface, in the raining reason, is about 50 meters above the surface of foundation.(3)
Allowable safety factors under different conditions(I) Consider gravity, the allowable safety factor (K) = 1.25(II) Consider combined impacts of gravity and seismic, the allowable safety factor (K) = 1.05(III) Consider combined impacts of gravity and steady seepage of groundwater, the allowable safety factor (K) = 1.05


## 6.4. Analysis Results

The calculation results of the safety factors of typical sections are shown in Table12. The comparison of safety factors of each year under different conditions is shown in Figure 14. From Table 12 and Figure 14, the following can be summarized: (1)
Considering all three conditions each year, the slope is stable only except when it reached the year 2018 that the slope may suffer from foundation contact sliding under condition III.(2)
Under condition I and II, slope safety factor is predicted to increase gradually from 2018 to 2036 and since then remain unchanged.(3)
The impact of seismic, on circular sliding, can reduce safety factor by 20.10% to 20.47%, while on contact-between-foundation-and-waste-rock-layer sliding can reduce FOS by 18.54 to 19.85%. This impact can decrease overall waste dump stability by 20.32% to 20.49%.(4)
The impact of steady groundwater seepage, on circular sliding, can reduce safety factor by 10.38% to 25.22%, while on contact-between-foundation-and-waste-rock-layer sliding can reduce FOS by 14.03 to 22.98%. This impact can decrease overall waste dump stability by 20.32% to 20.49%.Table 12
Calculation results using limit equilibrium method.
Year
Conditions
FOS of circular sliding (simplified bishop)
FOS of sliding contact between foundation and waste rock layer (Morgenstern–Price)
FOS of partial slope
FOS of overall slope
2015
I
1.408
1.402
1.491
II
1.120
1.115
1.198
III
1.144
1.189
1.102
2016
I
1.404
1.401
1.481
II
1.119
1.114
1.194
III
1.250
1.188
1.059
2017
I
1.407
1.401
1.490
II
1.119
1.114
1.198
III
1.261
1.079
1.146
2018
I
1.410
1.400
1.511
II
1.122
1.114
1.211
III
1.135
1.130
1.033
2023
I
1.479
1.447
1.578
II
1.180
1.153
1.272
III
1.106
1.244
1.122
2028
I
1.462
1.455
1.676
II
1.168
1.157
1.364
III
1.160
1.247
1.149
2036
I
1.468
1.455
1.769
II
1.173
1.150
1.441
III
1.134
1.231
1.147Figure 14
FOS comparisons for Hornfels Waste Dump by years and conditions. (a) FOS comparison of partial slope for circular sliding. (b) FOS comparison of overall slope for circular sliding. (c) FOS comparison of sliding along contact between foundation and waste rock layer.
(a)
(b)
(c)


## 7. Discussion

(1)
The mechanics parameters, initially obtained from tests on the sample waste rocks of which grain size is less than 5 cm, were then derived utilizing empirical formula. This parameter determining process is ideal and could be better improved or optimized by using large-scale direct shear test devices in the future.
(2)
The waste rock from waste dump is noncontinuum; however, the mechanical analysis software, FLAC3D, is only capable to analyse continuum mechanics. The analysis process considered all waste rocks as a whole, that neglected the size effects and interaction forces between each rock. Better approach is required for break through of the analytical method.
(3)
The equivalent static load method was utilized for limit equilibrium analysis considering earthquake effect; but in a real mine, earthquake damage is associated with dynamic process and it needs further analysis.


## 8. Conclusions

In this paper, the tests were conducted for the waste dump in an open-pit mine in Tibet, including in situ survey and laboratory large-scale direct shear test, as well as some 3D numerical simulations for the overall stacking process. The following conclusions are made:(1)
Through in situ survey, the bulk density, water content, grain size, and composition of waste rock, natural repose angle and other related parameters were obtained. The sample waste rocks in natural and saturated state were selected for large-scale direct shear test, and shear strength of fine rocks was calculated consequently. The physical and mechanical parameters of waste rocks layer were then determined accordingly.(2)
There is no stress concentration during the overall stacking process; stresses are mainly compressive stress while only a small area of tensile stress is produced inside the waste rock layer. Stress is well distributed. The displacement isoline populated inside the waste rock layers, while the maximum displacement point locates in this area as well. Displacement gets bigger with increasing stacking height, and the displacement inZ direction is always bigger than these in other directions. The tensile strength and shear-plastic zones, formed during stacking process in each step, are mainly distributed inside waste rock layer, gravel-soil layer, and heavily weathered layer near slope toe.(3)
The limit equilibrium method is used for waste dump stability analysis considering three states: only applied gravity, applied gravity and rainfall, and applied gravity and underground water. The results from this analysis show that the waste dump is stable.(4)
The potential failure modes of waste dump mainly consist of the “combined sliding mode” that has circular sliding in upper side and broken line sliding which cuts through gravel-soil layer into heavily weathered layer in the bottom. The potential failures are mainly distributed in benches near the slope toe. Due to the thick layer of waste rock layer at the bottom of dumping site, according to the failure mode and the plastic zone analysis results, it is recommended to completely remove the gravels, soils, and overburden before dumping waste rocks.



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*Source: 6835709-2018-12-27.xml*