3.1 Graded loading creep curve of fractured gangues in goaf
The creep compaction test of the crushed gangue specimens in the mining airspace caved zone was conducted in accordance with the graded loading test program and test steps in section 2.1. The graded loading creep curves of the crushed gangue in the caved zone were obtained, as illustrated in Fig 6.
As seen by Fig 6 and 7:
(1) There are clear creep and stage change features in the shattered gangue in the mining airspace caved zone. Crushed gangue experiences three stages of creep compaction deformation: rapid (shown as the AB section in Figure 7), attenuation (shown as the BC section in Figure 7), and stable (shown as the CD section in Figure 7). The strain in the AB section increases quickly as axial stress increases, the creep strain rate in the BC section slows down with time, and the strain in the CD section tends to stabilize over time. Furthermore, certain nonlinear features are displayed by the creep curves. The final strain of the various lithology specimens exhibits a positive correlation with the axial load; the creep curve begins to decay following the transient deformation stage and moves into the decay creep stage; under the same level of axial stress, the creep characteristics of the various lithology fracture zones exhibit variation.
(2)When loading is graded, the deformation of the gangue samples that correspond to each loading level gradually diminishes as the loading level rises. There are two types of stable creep deformation: when the crushed gangue samples are loading at a low stress level, the creep deformation is essentially maintained at a constant value, meaning the deformation rate is zero; when the crushed gangue samples are loading at a high stress level, the creep deformation increases and the deformation rate is not zero in the stable creep stage.
(3)Because the boundary of the axial loading system of the crushed gangue in the caved zone is set as a side limit in order to accurately reflect the stress state of the crushed gangue in the field, the instantaneous deformation of gangue samples with different lithologies is different under the same stress level, and the creep curve of the crushed gangue samples in the goaf did not show an accelerated creep stage. More than 70% of the overall axial displacement deformation under the graded loading condition was produced by the crushed gangue samples during the fast deformation stage.
3.2 Creep curve clusters of samples with different gangue types in goaf under graded loading conditions
As seen in Fig 8, the creep curve clusters of crushed gangue samples with various lithologies in the caved zone under graded loading circumstances were obtained by processing the creep test data.
Creep compaction deformation indexes of crushed gangue samples with different single and combined lithologies in the mining area under graded loading are shown in Tab 4.
Tab. 4 Creep deformation index of crushed gangue in goaf caved zone under step loading stress level
Lithology
|
Loading stress level /MPa
|
Instantaneous strain
|
Creep strain
|
Sandstone
|
15
|
0.230
|
0.362
|
18
|
0.445
|
0.770
|
21
|
0.342
|
0.709
|
24
|
0.278
|
0.469
|
Sandstone-sandy mudstone (7/3)
|
15
|
0.314
|
0.488
|
18
|
0.470
|
0.788
|
21
|
0.361
|
0.727
|
24
|
0.301
|
0.475
|
Sandstone-sandy mudstone (5/5)
|
15
|
0.380
|
0.627
|
18
|
0.483
|
0.807
|
21
|
0.416
|
0.731
|
24
|
0.312
|
0.552
|
Sandy mudstone
|
15
|
0.418
|
0.688
|
18
|
0.517
|
0.859
|
21
|
0.433
|
0.755
|
24
|
0.323
|
0.601
|
Mudstone-sandy mudstone (5/5)
|
15
|
0.427
|
0.730
|
18
|
0.596
|
0.878
|
21
|
0.457
|
0.759
|
24
|
0.332
|
0.642
|
Mudstone-sandy mudstone (7/3)
|
15
|
0.460
|
0.747
|
18
|
0.696
|
0.916
|
21
|
0.497
|
0.833
|
24
|
0.365
|
0.706
|
Mudstone
|
15
|
0.584
|
0.812
|
18
|
0.712
|
0.924
|
21
|
0.537
|
0.886
|
24
|
0.413
|
0.725
|
The examination of Fig. 8 and Tab 4 demonstrates this:
(1) The instantaneous strains of the crushed gangue samples in the caved zone of the goaf under three single lithologies and four combinations of lithologies are ranked from largest to smallest, respectively, mudstone, mudstone-sandy mudstone (7/3), mudstone-sandy mudstone (5/5), sandy mudstone , sandstone-sandy mudstone (5/5), sandstone-sandy mudstone (7/3), and sandstone. These strains are measured under the same loading stress level.
(2) The results of the creep compaction test indicate that the crushed sandstone samples have the strongest deformation resistance in the caved zone of the goaf. This is because the sandstone itself has a high compressive strength, which reduces creep deformation in the later stages of the test; the crushed mudstone samples have the weakest deformation resistance because their microstructure clearly demonstrates the development of their own pores and fissures and because their overall integrity is poor, which results in lower strength. Its strength is less than that of other single and mixed lithology samples during the creep compaction test, and further block slippage, rotation, and fragmentation take place, leading to greater creep deformation in the end. Between crushed sandstone and crushed mudstone comes crushed sandy mudstone.
Tab 5 and 6 display the different strain rates of change in the different lithologies of crushed gangue samples in the goaf.
Tab. 5 Scheme of sample preparation of instantaneous compaction
Stress level
|
Sandstone
|
Sandstone-sandy mudstone(7/3)
|
Sandstone-sandy mudstone(5/5)
|
Sandy mudstone
|
Mudstone-sandy mudstone(5/5)
|
Mudstone-sandy mudstone(7/3)
|
Mudstone
|
18MPa
|
1.932
|
1.495
|
1.272
|
1.237
|
1.396
|
1.202
|
1.218
|
21MPa
|
1.487
|
1.148
|
1.096
|
1.034
|
1.069
|
1.040
|
0.919
|
24MPa
|
1.208
|
0.958
|
0.820
|
0.772
|
0.777
|
0.880
|
0.707
|
Tab. 6 Scheme of sample preparation of creep compaction
Stress level
|
Sandstone
|
Sandstone-sandy mudstone(7/3)
|
Sandstone-sandy mudstone(5/5)
|
Sandy mudstone
|
Mudstone-sandy mudstone(5/5)
|
Mudstone-sandy mudstone(7/3)
|
Mudstone
|
18MPa
|
2.123
|
1.615
|
1.287
|
1.249
|
1.202
|
1.227
|
1.138
|
21MPa
|
1.955
|
1.492
|
1.167
|
1.098
|
1.040
|
1.116
|
1.091
|
24MPa
|
1.294
|
0.973
|
0.880
|
0.874
|
0.880
|
0.945
|
0.893
|
(1) When the loading stress level increases for shattered gangue specimens with the same lithology in the caved zone, the specimens' instantaneous strain rises and subsequently falls, with the first and second levels rising and the third and fourth levels falling and eventually stabilizing. Consider the crushed sandstone specimen as an example, At different loading stress levels—from 15 MPa to 18 MPa, 21 MPa, and 24 MPa—the instantaneous strain increases to 1.932, 1.487, and 1.208 times that of 15 MPa. Similarly, at different loading stress levels—from 15 MPa to 18 MPa, 21 MPa, and 24 MPa—the creep strain increases to 2.123, 1.955, and 1.294 times at 15 MPa. Growth rate decreases as the stress loading level increases, suggesting a gradual compacting of the inter-block pores.
(2) When the stress level is 21MPa, the crushed sandstone, sandstone-sandy mudstone (7/3), sandstone-sandy mudstone (5/5), sandy mudstone, mudstone-sandy mudstone (5/5), mudstone-sandy mudstone (7/3), and mudstone have transient strain change rates of 148.7%, 114.8%, 109.6%, 104%, 111.6%, and 91.9%, respectively; and creep strain change rates of 195.5%, 149.2%, 116.7%, 109.8%, 104%, 111.6%, and 109.1%, respectively. Both the instantaneous strain and the creep strain change rate gradually decrease as the gangue strength gets softer.
(3) Different lithologies and stress levels in crushed gangue samples result in varying creep deformation stabilization times. When the stress level is the same, the creep deformation stabilization time of gangue samples with the same lithology decreases gradually; when the stress level is the same, the creep deformation stabilization time of crushed gangue samples with different lithologies decreases gradually as the lithological strength increases.
3.3 Mechanism of bearing compression creep response of gangue samples in goaf
Fig 9 shows a schematic depiction of the creep compaction process of shattered gangue samples of various lithologies.
As Fig 9 and the preceding analysis demonstrate:
(1) creep compaction primarily consists of the compaction-dense process, the crushing and reorganization process, and the stable compaction process in the three compaction processes of rapid deformation, attenuation creep deformation, and stable creep deformation of the crushed gangue in the caved zone. The crushed gangue has a looser structure and more void space between the blocks before it is strained (Fig 9a).
(2) The internal block of the crushed gangue produces large deformation during the process of rapid compaction, and this deformation primarily accounts for the overall deformation. Inter-block compacting, fracturing, angle grinding, and crushing processes are the main causes of this deformation. When the applied stress is high enough to cause the gangue's occlusion to be destroyed, crushed small blocks will once more fill the large block between the gaps. This stage is primarily for the block as a whole cracking and block between the reorganization of the filler (Fig 9b, c).
(3) With the axial stress stabilized at a certain level into the creep deformation. The pore space between the crushed gangue samples block is gradually compressed as the stress level rises, the skeleton of each other rotating occlusion. Stage: The crushed block gradually solidifies and forms a more complete skeleton bearing structure (Fig 9d) as the creep deformation of the gangue samples gradually decreases and tends to stabilize.
3.4 Construction of creep prediction model for fractured rock in mining area
Based on the test of creep compaction characteristics of broken rocks with different lithologies in the collapse zone, the creep strain of broken rock samples with different lithologies in Fig. 8 is fitted to the loading time, and it is concluded that the creep deformation of broken rock samples with different lithologies, ε, and axial compression time, t, satisfy the exponential decay function, and the related fitting parameters and the degree of fit are shown in Tab 7.
Tab. 7 Related parameters of creep deformation curve fitting of broken rock in goaf under different axial stress levels
Crushed gangue sample number
|
Relevant parameters
|
Axial stress level /15MPa
|
Axial stress level /18MPa
|
Axial stress level /21MPa
|
Axial stress level /24MPa
|
CS-15-25
|
a
|
0.21916
|
0.46031
|
0.48889
|
0.33237
|
b
|
0.3573
|
0.41464
|
0.33288
|
0.34922
|
c
|
0.33824
|
1.30525
|
2.40494
|
3.41813
|
CSN-15-25
|
a
|
0.45521
|
0.47868
|
0.44918
|
0.43572
|
b
|
0.46961
|
0.24799
|
0.28167
|
0.70936
|
c
|
0.58954
|
1.87534
|
3.09397
|
3.97173
|
CN-15-25
|
a
|
0.57687
|
0.56839
|
0.53207
|
0.42928
|
b
|
0.65794
|
0.33485
|
0.30286
|
0.2567
|
c
|
0.76511
|
2.36935
|
3.8404
|
4.94682
|
CSNS(55)-15-25
|
a
|
0.43209
|
0.4805
|
0.41252
|
0.30342
|
b
|
0.40353
|
0.29777
|
0.25298
|
0.22589
|
c
|
0.53346
|
1.78461
|
2.94004
|
3.8244
|
CSNS(37)-15-25
|
a
|
0.49491
|
0.55579
|
0.39429
|
0.31186
|
b
|
0.5539
|
0.32302
|
0.21377
|
0.15863
|
c
|
0.65854
|
2.21997
|
3.6118
|
4.64202
|
CSNN(55)-15-25
|
a
|
0.47644
|
0.42501
|
0.42194
|
0.35402
|
b
|
0.46993
|
0.25603
|
0.28008
|
0.22631
|
c
|
0.61664
|
2.1675
|
3.36695
|
4.40685
|
CSNN(37)-15-25
|
a
|
0.49491
|
0.55579
|
0.39429
|
0.31186
|
b
|
0.5539
|
0.32302
|
0.21377
|
0.15863
|
c
|
0.65854
|
2.21997
|
3.61118
|
4.64202
|
It can be seen that the relationship between creep strain ε and creep time t of crushed gangue in the caved zone of the goaf can be described by Eq(3-1):
where t is the loading time, εc is the crushed gangue's axial creep strain, and a, b, and c are the pertinent fitting equation parameters that rely on the lithology of the crushed gangue and the axial stress in the goaf.
The creep strain-time prediction model for the crushed rock in the longwall mining zone's collapse zone is represented by this equation. It is evident from Fig 8 and Tab 7 that the creep prediction model of Eq. (3-1) has a high fitting accuracy and that all of the fitting correlation coefficients are larger than 0.8.