After completing the index selection of rock burst tendency in western Hubei, the field ground stress test and indoor test of western Hubei phosphate mine were carried out to obtain the evaluation index parameters.
3.1 Ground stress test
The earth stress test adopts the hydraulic fracture-induced ground stress measurement system independently developed by Wuhan Institute of Geotechnical technology of China, Chinese Academy of Sciences. The field test diagram is shown in Fig. 1. Hydrofracture is a direct method to inject high pressure water into the sealed rock according to the pressure data recorded during water injection fracture.
The tangential stress of the measured lead direct stress, horizontal stress and maximum horizontal main stress direction is calculated by formula (1).
$${\sigma }_{\theta }={\sigma }_{1}+{\sigma }_{2}-2\left({\sigma }_{1}-{\sigma }_{2}\right)cos2\theta$$
1
Where \({\sigma }_{1}\) is the horizontal stress; \({\sigma }_{2}\) is the lead direct stress; \(\theta\) is the direction of the maximum horizontal primary stress. The calculation results are shown in Table 2, the height of the roadway is about 4m, and its influence on tangential stress can be ignored. Therefore, the top floor of the roadway is regarded as the same buried depth.
Table 2
Tangential stress index calculation
Mine name
|
Rock type
|
Embedded depth/(m)
|
Tangential stress /(MPa)
|
Single axis compressive strength/(MPa)
|
The tangential stress index T
|
Shanshuya
|
Silicone dolomite
|
450
|
30.98
|
109
|
0.28
|
Phosphorus block rock sandwich cloud rock
|
66
|
0.47
|
Fine crystal cloud rock
|
86
|
0.36
|
Houping
|
Dolomite
|
800
|
55.07
|
68
|
0.81
|
Cillaceous strip phosphorous rock
|
63
|
0.87
|
Phosphorous strip with mudstone
|
152
|
0.36
|
Wawu
|
(Including phosphorus strip) dolomite
|
580
|
37.6
|
117
|
0.32
|
Collophane
|
109
|
0.34
|
Phosphoric mudstone
|
67
|
0.56
|
Dongda
|
Cillaceous dolite containing phosphorus
|
750
|
48.62
|
84
|
0.58
|
Phosphorite
|
79
|
0.62
|
Biopelite
|
74
|
0.66
|
The ratio of the tangential stress of surrounding rock to the compressive strength of rock indicates the tangential stress index of rock burst tendency T. Tangential stress index T < 0.3, no rock burst; 0.3 < T < 0.5, weak rock burst; 0.5 < T < 0.7, medium rock burst; T༞0.7, strong rock burst (Cai et al. 2001). As can be seen from the above table, the greater the depth of burial, the higher the propensity to rock explosion, all types of lithology have rock explosion risk, dolomite and mudstone strips of phosphorite at the highest propensity to rock explosion, but its considerations are not comprehensive enough, can only be used as a reference.
3.2 Rock mass integrity coefficient test
The integrity coefficient of rock mass refers to the square of the ratio of the longitudinal wave velocity of rock mass to the longitudinal wave velocity of rock mass(Cai et al. 2019). Its expression is as follows:
$${K}_{v}=({\frac{{V}_{pm}}{{V}_{pr}})}^{2}$$
2
Where \({V}_{pm}\) is the longitudinal wave velocity of rock mass, and \({V}_{pr}\) is the longitudinal wave velocity of rock. When calculating the integrity coefficient, the rock mass wave velocity is tested by the roof and the fresh and complete rock samples. As shown in Fig. 2, the acoustic wave velocity of ZBL-U5200 non-metallic ultrasonic detector can be measured by recording the time used to pass the rock mass and using the length of the rock mass divided by the time.
After completing the rock wave velocity measurement, the core machine is used to cored the rock. As shown in Fig. 3, the core machine measures the removed rock with the vernier caliper, and then conducts the sound wave test. Then by measuring the time spent through the rock, dividing the length by the time to obtain the rock longitudinal wave velocity.
After measuring the longitudinal wave velocity of different phosphate ores and top and bottom plates in western Hubei, the rock mass integrity coefficient Kv calculated by formula (2) is shown in Table 3.
Table 3
Integrity coefficient of phosphate ore rock mass in western Hubei
Mine name
|
Sample number
|
Rock type
|
\({{V}}_{{p}{m}}\)/(m/s)
|
\({{V}}_{{p}{r}}/\)(m/s)
|
Kv
|
Shanshuya
|
Roof-SSY
|
Silicone dolomite
|
5132
|
5880
|
0.76
|
Sample ore-SSY
|
Phosphorus block rock sandwich cloud rock
|
5012
|
6353
|
0.62
|
Floor -SSY
|
Fine crystal cloud rock
|
5211
|
5919
|
0.48
|
Houping
|
Roof-HP
|
Dolomite
|
5861
|
6230
|
0.88
|
Sample ore-HP
|
Cillaceous strip phosphorous rock
|
4826
|
5596
|
0.74
|
Floor-HP
|
Phosphorous strip with mudstone
|
4681
|
6687
|
0.49
|
Wawu
|
Roof-WW
|
(Including phosphorus strip) dolomite
|
6114
|
6725
|
0.83
|
Sample ore-WW
|
Collophane
|
4698
|
5733
|
0.67
|
Floor-WW
|
Phosphoric mudstone
|
5864
|
6857
|
0.73
|
Dongda
|
Roof-DD
|
Cillaceous dolite containing phosphorus
|
6059
|
6119
|
0.98
|
Sample ore-DD
|
Phosphorite
|
4056
|
4959
|
0.67
|
Floor-DD
|
Biopelite
|
4857
|
5077
|
0.92
|
According to GB50218-94 (1994)of Engineering Rock Mass Classification Standard, the rock mass is complete when KV>0.75; when 0.55 < KV<0.75, the rock mass is relatively complete. According to Table 3 of the rock mass integrity coefficient, the rock rock are basically complete and relatively complete, and the rock is easier to store elastic strain energy.
3.3 Uniaxial compression and splitting test
After completing the acoustic test, in order to obtain the compressive strength and tensile strength of the rock, the uniaxial compression and fracture test obtained the rock tendency index parameters of the standard rock specimen. The standard specimen is shown in Fig. 4.
The rock uniaxial compressive strength is measured under load on the computer-hydraulic servo universal testing machine. The test instrument is shown in Fig. 5a, the uniaxial compression test is shown in Fig. 5b, and the rock tensile strength is measured by the Brazilian splitting test, as shown in Fig. 5c.
Through the uniaxial compression and Brazilian splitting test of rock taken from Shanshuya, Houping, Wawu and Dongda phosphate ore, the uniaxial compressive strength and tensile strength of rock are measured as shown in Table 4.
Table 4. Compressive pressure and tensile strength of phosphate ore in western Hubei
|
Mine name
|
Sample number
|
Rock type
|
Compression strength/(MPa)
|
Tensile strength /(MPa)
|
Fragile coefficient B
|
Shanshuya
|
Roof-SSY
|
Silicone dolomite
|
109
|
4.59
|
23.75
|
Sample ore-SSY
|
Phosphorus block rock sandwich cloud rock
|
66
|
3.38
|
19.23
|
Floor -SSY
|
Fine crystal cloud rock
|
86
|
6.08
|
14.14
|
Houping
|
Roof-HP
|
Dolomite
|
68
|
4.01
|
16.96
|
Sample ore-HP
|
Cillaceous strip phosphorous rock
|
63
|
2.16
|
29.17
|
Floor-HP
|
Phosphorous strip with mudstone
|
152
|
4.12
|
36.89
|
Wawu
|
Roof-WW
|
(Including phosphorus strip) dolomite
|
117
|
4.74
|
24.68
|
Sample ore-WW
|
Collophane
|
109
|
2.23
|
48.88
|
Floor-WW
|
Phosphoric mudstone
|
67
|
1.31
|
51.15
|
Dongda
|
Roof-DD
|
Cillaceous dolite containing phosphorus
|
84
|
3.53
|
23.80
|
Sample ore-DD
|
Phosphorite
|
79
|
3.47
|
22.77
|
Floor-DD
|
Biopelite
|
74
|
1.61
|
46.0
|
Combined with the compressive strength and tensile strength, the fragility coefficient B value is calculated by its ratio, B > 40, no rock burst; 26.7 < B < 40, weak rock burst; 14.5 < B < 26.7, medium rock burst; B < 14.5, strong rock burst(Wang et al. 2013). As can be seen from the above table, rock explosions mainly occur in dolomite and mineral sample phosphate rock.