A mixture of fault gouge and rubble taken out from a fault zone is used to prepare a S-RM (Soil-Rock Mixture) sample with rock block proportions of 20%, 30%, 40%, 50%, 60% and 70%, respectively. A GDS triaxial test system is used accordingly to measure the seepage characteristics of such samples under different loading and unloading confining pressures in order to determine the variation law of the permeability coefficient. The test results show that: (1) The permeability coefficient of the S-RM samples decreases as the pressure increases, and the decrease rate of this coefficient in the initial stage of confining pressure loading is obviously higher than in the semi-late period; (2) The permeability coefficient at different confining pressure levels presents a common trend as the rock block proportion is increased, i.e., it decreases first then it increases (the permeability coefficient of the sample with rock block proportion 40% being the smallest, 70% the largest); (3) In the stage of confining pressure unloading, the recovery degree of the permeability coefficient grows with the increase of rock block proportion (the recovery rate of S-RM sample with rock block proportion 70% reaches 50.2%); (4) In the stage of confining pressure loading and unloading, the sensitivity of the permeability coefficient to the rock block proportion displays the inverse “Z” variation rule (when rock block proportion reaches 60%, the sensitivity is highest); (5) In the stage of confining pressure loading, the relationship between the permeability coefficient and confining pressure can be described by an exponential relationship.
Among the water inrush accidents in mine roadway and stope, the water inrush accidents caused by faults account for 41% and 45%, respectively [
Soil-rock mixture is a special geological body between soil and rock mass [
During the permeability test, the soil samples taken from the fault fracture zone are made into soil-rock mixture plastic samples with different rock block proportion, and then the permeability coefficient under different confining pressure levels is measured by GDS triaxial test system to study the stress seepage coupling characteristics of soil-rock mixture in the fault fracture zone.
The natural moisture content of the test soil sample is about 9.5%, and the fault gouge matrix is gray white with strong cementation. The results of X-ray diffraction pattern and mineral relative content analysis showed that, the content of muscovite is 65.7%, quartz is 32.1%, and there is a small amount of kaolinite. The gravel in the fault zone is granite with clear edges and corners and regular shape.
The permeability test adopts the soil-rock mixture remolded sample with the diameter of 50 mm and the height of 100 mm. According to the calculation formula
Rock block proportion/% | |||||
---|---|---|---|---|---|
20 | 0.07 | 0.24 | 0.65 | 9.28 | 1.32 |
30 | 0.07 | 0.27 | 2.41 | 34.42 | 0.43 |
40 | 0.07 | 0.27 | 4.18 | 59.71 | 0.25 |
50 | 0.08 | 0.58 | 5.67 | 70.88 | 0.74 |
60 | 0.09 | 2.23 | 6.82 | 75.78 | 8.1 |
70 | 0.15 | 3.57 | 7.38 | 49.2 | 11.51 |
This paper mainly studies the influence of the rock block proportion on the seepage characteristics of the soil-rock mixture in the fault fracture zone, in order to make the density of the soil in the samples with different rock block proportion the same, before making samples, first carry out compaction test on soil samples with different rock block proportion, so as to obtain the compaction curve of the soil in the soil-rock mixture with different rock block proportion, as shown in
During sample preparation, the soil and block stone shall be mixed according to the determined soil rock mix proportion, and the initial mixing shall be uniform, and then the water shall be added to make the soil sample further uniform, the optimal water content is determined as 9% by the relationship curve of dry density and water content of the soil. Finally, the soil samples are loaded into the sample cylinder in three layers and compacted to the predetermined height. The soil-rock mixture at the real-time interface shall be roughened and a total of 6 samples shall be prepared according to the above steps. After the sample is prepared, it is cured in a protective box for 28 days to improve the probability of free water transforming into combined water in the sample and make the strength of the remolded sample close to the strength of the original soil-rock mixture. See
The permeability test of the soil-rock mixture is completed with the saturated unsaturated triaxial test system produced by GDS company, as shown in
Under natural conditions, the soil-rock mixture in the fault fracture zone is in a state of compaction due to the effect of different sizes of hydrostatic pressure, dynamic hydraulic pressure and structural stress, which has certain self stabilizing ability and storage of deformation energy. Under the influence of natural geological tectonic movement or disturbance of tunnel excavation, the equilibrium state of soil-rock mixture in fault fracture zone is destroyed and energy is released, which is in the state of stress relaxation. In this test, two loading methods, confining pressure loading and confining pressure unloading are used to characterize the two engineering states of the soil-rock mixture in the fault fracture zone. In order to simplify the test conditions and reduce the influence of other factors, it is set that the soil-rock mixture sample is only affected by confining pressure and seepage pressure, and no load is applied axially, the inlet water pressure is set as 0.08 MPa, the outlet is open to the atmosphere, and only the confining pressure changes. According to the purpose of this test, and to ensure that there is no water leakage between the outer wall of the soil-rock mixture sample and the latex sleeve during the test, the confining pressure is selected as 0.10, 0.12, 0.14, 0.16, 0.18 and 0.20 MPa six stress points respectively, and the seepage characteristics of the soil-rock mixture with different rock block proportion during the process of confining pressure loading and unloading are analyzed, respectively.
The permeability test of soil-rock mixture adopts the steady-state method [
According to the test results, the change curve of the permeability coefficient of the soil-rock mixture samples with different rock block proportion in the process of confining pressure loading and unloading is drawn, as shown in
The permeability coefficient of soil-rock mixture samples with different rock block proportion changes with the change of confining pressure level. In the confining pressure loading stage, the permeability coefficient decreases with the increase of confining pressure; In the confining pressure unloading stage, the permeability coefficient of the sample recovers to a certain extent with the decrease of confining pressure, and the change slope of the permeability coefficient in the confining pressure loading stage is significantly greater than that in the confining pressure unloading stage; Under the same confining pressure level, the permeability coefficient in confining pressure loading stage is greater than that in confining pressure unloading stage. The permeability coefficient of the soil-rock mixture sample is closely related to its porosity, under the action of confining pressure, the pores in the soil and between the soil and the block stone are gradually compressed, and the porosity decreases, therefore, in the confining pressure loading stage, the permeability coefficient of the sample decreases with the increase of confining pressure; In the unloading stage of confining pressure, with the release of confining pressure, the internal pores of the sample recover slightly, the connectivity becomes better and the porosity increases, so the permeability coefficient of the sample increases with the decrease of confining pressure; At the same time, in the confining pressure loading stage, plastic deformation occurs at weak positions such as soil and soil rock interface in the sample, and some closed seepage channels still cannot be reopened after the confining pressure is released, so the permeability coefficient of the sample in the unloading stage is small.
At the initial stage of confining pressure loading (0.10–0.14 MPa), the permeability coefficient of soil-rock mixture samples with different rock block proportion decreased rapidly, while at the middle and late stage of confining pressure loading (0.14–0.20 MPa), the permeability coefficient of samples decreased relatively slowly. Now, the reduction degree of permeability coefficient of soil-rock mixture samples with different rock block proportion at the initial stage of loading is described by the ratio of the difference of permeability coefficient of the sample when the confining pressure is loaded to 0.14 MPa (the difference between the permeability coefficient of the sample at any loading time and the permeability coefficient of the sample when the initial confining pressure is 0.10 MPa) to the difference of permeability coefficient of the sample when the confining pressure is loaded to 0.20 MPa, it also shows the influence of confining pressure on the permeability coefficient of soil-rock mixture. Through calculation, the reduction degree of permeability coefficient of soil-rock mixture samples with rock block proportion of 20%, 30%, 40%, 50%, 60% and 70% at the initial stage of confining pressure loading is 57%, 45%, 58%, 61%, 55% and 50%, respectively, therefore, the influence of confining pressure on the permeability coefficient of soil-rock mixture sample is very obvious. This is because at the initial stage of confining pressure loading, the soil-rock mixture sample is compressed and deformed, the soil compactness increases and the porosity decreases obviously, at the same time, the main seepage channel formed between block stones is blocked, resulting in the rapid decline of the permeability coefficient of the sample. In the middle and late stage of confining pressure loading, because the soil-rock mixture sample has been compacted in the early stage, under the continuous action of confining pressure, the internal spatial structure of the sample changes and the porosity further decreases, resulting in the closure of the remaining seepage channels, however, at this time, the increase of confining pressure has relatively little impact on the internal seepage channels of the sample, resulting in the slow reduction of the permeability coefficient of the sample. From the above analysis, it can be seen that the permeability coefficient of soil-rock mixture samples is strongly affected by confining pressure, and there are differences in permeability coefficients between samples with different rock block proportion.
Considering the influence of the rock block proportion on the permeability coefficient of the soil-rock mixture sample, the relationship curve between the permeability coefficient and the rock block proportion of the sample under different confining pressure levels at the loading and unloading stages is drawn in the same coordinate system for further analysis, as shown in
It can be seen from
As shown in
This paper mainly studies the influence of the rock block proportion on the seepage characteristics of soil-rock mixture, and now defines the sensitivity coefficient of the permeability coefficient of soil-rock mixture to the change of the rock block proportion,
The sensitivity coefficient
It can be seen from the
The reason for the above phenomenon is that when the rock block proportion is low (20%∼50%), the soil is dominant in the soil-rock mixture, the sample is dense and the porosity is low, at this time, the increase of the rock block proportion is conducive to the influence of the soil on the permeability coefficient of the sample, resulting in the decrease of the sensitivity of the permeability coefficient of the sample to the change of rock block proportion. The rock block proportion continues to increase (50%∼60%), and the soil-rock mixture sample is in the transition stage from soil dominated to block stone dominated, at this time, the small change of rock block proportion may cause a great change in the porosity of the sample, so the permeability coefficient of the sample is highly sensitive to the change of rock block proportion; When the rock block proportion increases to 70%, the block stone is in the dominant position in the soil-rock mixture sample, and the influence of its content change on the sample porosity is very limited, resulting in the gradual decline of the sensitivity of the sample permeability coefficient to the change of rock block proportion.
For the test results of the confining pressure and permeability coefficient of the soil-rock mixture samples with different rock block proportion in the process of confining pressure loading and unloading, all can be described by the function
In the formula,
In the process of confining pressure loading and unloading, the fitting curve and results of the relationship between the permeability coefficient and confining pressure of the earth rock mixture samples with different rock block proportion are shown in
Rock block proportion % | Loading stage | Degree of fitting | Unloading stage | Degree of fitting | ||
---|---|---|---|---|---|---|
20 | 11.18 | 6.91 | 98.63 | 4.09 | 1.72 | 96.62 |
30 | 9.38 | 6.51 | 98.47 | 3.62 | 1.68 | 93.49 |
40 | 7.55 | 5.96 | 96.94 | 3.56 | 2.01 | 92.92 |
50 | 8.76 | 6.41 | 97.35 | 4.47 | 2.88 | 97.75 |
60 | 10.73 | 6.02 | 98.05 | 6.29 | 3.40 | 91.18 |
70 | 10.95 | 5.41 | 99.27 | 6.93 | 3.26 | 93.07 |
It can be seen from
The permeability coefficient of soil-rock mixture samples with different rock block proportion decreases with the increase of confining pressure, and the decrease rate of permeability coefficient of samples is faster in the early stage of confining pressure loading, while the decrease rate of permeability coefficient of samples is relatively slow in the middle and late stage of confining pressure loading. In the stage of confining pressure loading and unloading, the permeability coefficient of soil-rock mixture samples under different confining pressure levels decreases first and then increases with the increase of rock block proportion, the permeability coefficient of samples with rock block proportion of 40% is the smallest and that of samples with rock block proportion of 70% is the largest. In the confining pressure unloading stage, the recovery degree of permeability coefficient of soil-rock mixture samples increases with the increase of rock block proportion, the recovery rate of permeability coefficient of samples with rock block proportion of 70% reaches 50.2%. In the stage of confining pressure loading and unloading, with the increase of rock block proportion, the sensitivity coefficient of permeability coefficient of soil-rock mixture to the change of rock block proportion shows a trend of first decreasing, then increasing and then decreasing, when the rock block proportion is 60%, the permeability coefficient of sample is the most sensitive to the change of rock block proportion. In the stage of confining pressure loading and unloading, the relationship between permeability coefficient and confining pressure can be described by exponential function.
The authors would like to thank the editors and the anonymous reviewers for their helpful and constructive comments.