1. The method for analyzing the stability of the carbonaceous mudstone side slope based on the dry-wet circulation effect is characterized by comprising the following steps of:

acquiring axial strain of a carbon mudstone sample subjected to dry and wet circulation under different axial pressures, and obtaining dry and wet circulation times, and axial pressure values and axial strain values corresponding to the dry and wet circulation times;

obtaining a first change rule of the axial strain of the carbonaceous mudstone sample under a first condition of different dry and wet cycle times and the same axial pressure value based on the axial pressure value, the dry and wet cycle times and the axial strain value, wherein the first change rule of the axial strain is used for expressing a first influence of the dry and wet cycle on the axial strain;

obtaining a second change rule of the axial strain of the carbonaceous mudstone sample under a second condition of the same dry-wet cycle number and different axial pressure values based on the axial pressure value, the dry-wet cycle number and the axial strain value, wherein the second change rule of the axial strain is used for expressing a second influence of the axial pressure on the axial strain;

and acquiring instantaneous stage strain, attenuation stage strain, steady stage strain and acceleration stage strain of the carbonaceous mudstone sample based on the axial strain first change rule and the axial strain second change rule to obtain viscoelastic plastic creep of the carbonaceous mudstone sample for analyzing the stability of the carbonaceous mudstone side slope.

2. The method for analyzing the stability of the carbonaceous mudstone slope under the action of the dry-wet cycle according to claim 1,

and in the process of collecting the axial strain, applying different axial pressures in a graded incremental loading mode, wherein the graded incremental loading mode comprises the steps of setting the confining pressure to be 2MPa, setting the first-stage loading strength to be 14.26MPa, setting the graded loading increment to be 3MPa, keeping the loading stress of each stage for 48 hours, and when the axial strain value is less than 0.001/d, carrying out next-stage loading until the carbonaceous mudstone sample is damaged.

3. The method for analyzing the stability of the carbonaceous mudstone slope under the action of the dry-wet cycle according to claim 2,

in the process of obtaining the dry-wet cycle number, the dry-wet cycle number at least comprises 0 time, 3 times, 6 times and 9 times.

4. The method for analyzing the stability of the carbonaceous mudstone slope under the action of the dry-wet cycle according to claim 3,

and in the process of collecting the axial strain value, collecting the axial strain value of the carbonaceous mudstone sample after creep deformation for 20 hours.

5. The method for analyzing the stability of the carbonaceous mudstone slope under the action of the dry-wet cycle according to claim 4,

in the course of obtaining the viscoelastic creep of the carbonaceous mudstone sample,

acquiring instantaneous strains of the carbonaceous mudstone sample under the first condition and the second condition respectively according to the instantaneous stage strains;

acquiring the creep of the carbonaceous mudstone sample in the attenuation stage under the first condition and the second condition according to the strain in the attenuation stage;

according to the strain of the steady stage, creep of the carbonaceous mudstone sample in the steady stage under the first condition and the second condition is obtained;

acquiring the creep of the carbonaceous mudstone sample in the damage stage under the first condition and the second condition according to the strain in the attenuation stage, the strain in the steady stage and the strain in the acceleration stage;

and obtaining the viscoelastic-plastic creep according to the instantaneous strain, the creep in the attenuation stage, the creep in the steady stage and the creep in the damage stage.

6. The method for analyzing the stability of the carbonaceous mudstone slope under the action of the dry-wet cycle according to claim 5,

obtaining viscoelastic creep and first viscoelastic creep of the viscoelastic creep according to the instantaneous strain, the creep in the attenuation stage, the creep in the steady stage and the creep in the damage stage;

and obtaining the long-term strength of the carbon mudstone sample according to the creep finish time, the creep rate start time and the creep rate of the first viscoplastic creep and the second viscoplastic creep of the creep in the attenuation stage, and analyzing the stability of the carbon mudstone side slope.

7. A system for analyzing the stability of a carbonaceous mudstone side slope based on the action of dry and wet cycles is characterized by comprising,

the data acquisition module is used for acquiring axial strain of the carbonaceous mudstone sample subjected to dry and wet circulation under different axial pressures to obtain dry and wet circulation times and axial pressure values and axial strain values corresponding to the dry and wet circulation times;

the first law analysis module is used for obtaining a first change law of axial strain of the carbonaceous mudstone sample under the first condition of different dry and wet cycle times and the same axial pressure value based on the axial pressure value, the dry and wet cycle times and the axial strain value, and the first change law of axial strain is used for expressing the first influence of the dry and wet cycle on the axial strain;

the second law analysis module is used for obtaining a second change law of axial strain of the carbon mudstone sample under a second condition of the same dry-wet cycle number and different axial pressure values based on the axial pressure value, the dry-wet cycle number and the axial strain value, and the second change law of axial strain is used for expressing a second influence of the axial pressure on the axial strain;

the slope stability analysis module is used for acquiring instantaneous phase strain, attenuation phase strain, steady phase strain and acceleration phase strain of the carbon mudstone sample based on the axial strain first change rule and the axial strain second change rule to obtain viscoelastic-plastic creep of the carbon mudstone sample and analyze the stability of the carbon mudstone slope;

the first storage module is used for storing first data generated by the system, and the first data at least comprises dry-wet cycle times, an axial pressure value, an axial strain first change rule, an axial strain second change rule, instantaneous phase strain, attenuation phase strain, steady phase strain and acceleration phase strain;

and the first communication module is used for the carbon mudstone slope stability analysis system to perform data interaction.

8. The system for analyzing the stability of the carbonaceous mudstone slope under the action of the dry-wet cycle according to claim 7, further comprising a first system applied to the equipment for monitoring the carbonaceous mudstone slope, comprising,

the sensor module is used for acquiring the physical conditions of the carbonaceous mudstone side slope, wherein the physical conditions at least comprise stress conditions, temperature and humidity;

the second communication module is used for the first system to perform data interaction with the system;

the first early warning module is used for obtaining a first early warning signal according to the first data provided by the system and the physical condition;

and the first display module is used for displaying the physical condition and the first early warning signal.

9. The system for analyzing stability of carbon mudstone slopes based on dry and wet cycles of claim 8, further comprising a second system applied to a mobile device, comprising,

the second display module is used for displaying the first early warning signal and the first data;

and the third communication module is used for the second system to respectively perform data interaction with the system and the first system.

10. The system for analyzing the stability of the carbon mudstone side slope based on the dry-wet cycle of claim 9, further comprising a third system applied to the cloud server, comprising,

the second storage module is used for storing the physical condition, the first data and second data generated by the third system according to the physical condition and the first data;

the fourth communication module is used for the third system to respectively perform data interaction with the system, the first system and the second system;

the data processing module is used for obtaining a creep curve graph and early warning information according to the physical condition and the first data, wherein the creep curve graph is used for representing a creep curve of the carbonaceous mudstone side slope, and the early warning information is used for representing the trend of the carbonaceous mudstone side slope in dangerous conditions;

the second early warning module generates a second early warning signal according to the early warning information;

the first display module and the second display module are further used for displaying the second early warning signal.

Background

The carbonaceous mudstone has the characteristics of water absorption disintegration and softening, and the dry-wet cycle action is to intensify the weathering disintegration of the carbonaceous mudstone, so that the side slope rock mass damage caused by the disintegration seriously influences the long-term stability of the side slope. The long-term strength and creep damage characteristics of the carbonaceous mudstone under the action of dry and wet circulation are researched, and the method has important significance for correctly evaluating the stability of the damaged rock mass side slope. Most of the previous researches focus on the research of the creep property of soft rock, but the long-term strength of the carbonaceous mudstone under the action of dry and wet cycles is not sufficiently researched. A method for accurately determining the long-term strength of the carbonaceous mudstone under the action of dry-wet circulation is urgently needed, and the evaluation requirement on the stability of the damaged rock mass side slope is met.

Disclosure of Invention

In order to solve the problems, the invention provides a method for analyzing the stability of a carbonaceous mudstone side slope based on the action of dry and wet circulation, which comprises the following steps:

acquiring axial strain of the carbonaceous mudstone sample subjected to dry and wet circulation under different axial pressures to obtain dry and wet circulation times and axial pressure values and axial strain values corresponding to the dry and wet circulation times;

obtaining a first change rule of the axial strain of the carbonaceous mudstone sample under a first condition of different dry and wet cycle times and the same axial pressure value based on the axial pressure value, the dry and wet cycle times and the axial strain value, wherein the first change rule of the axial strain is used for expressing a first influence of the dry and wet cycle on the axial strain;

obtaining a second change rule of the axial strain of the carbonaceous mudstone sample under a second condition of the same dry-wet cycle number and different axial pressure values based on the axial pressure value, the dry-wet cycle number and the axial strain value, wherein the second change rule of the axial strain is used for expressing a second influence of the axial pressure on the axial strain;

and acquiring instantaneous phase strain, attenuation phase strain, steady phase strain and acceleration phase strain of the carbonaceous mudstone sample based on the first axial strain change rule and the second axial strain change rule to obtain viscoelastic plastic creep of the carbonaceous mudstone sample for analyzing the stability of the carbonaceous mudstone side slope.

Preferably, in the process of acquiring the axial strain, different axial pressures are applied through a graded increment loading mode, wherein the graded increment loading mode comprises the steps of setting the confining pressure to be 2MPa, setting the first-stage loading strength to be 14.26MPa, setting the graded loading increment to be 3MPa, continuing the loading stress of each stage for 48 hours, and when the axial strain value is less than 0.001/d, carrying out next-stage loading until the carbonaceous mudstone sample is damaged.

Preferably, in the process of obtaining the number of dry-wet cycles, the number of dry-wet cycles at least includes 0, 3, 6 and 9.

Preferably, in the process of acquiring the axial strain value, the axial strain value of the carbonaceous mudstone sample after creep for 20 hours is acquired.

Preferably, during viscoelastic creep of the carbonaceous mudstone sample,

according to the transient stage strain, acquiring the transient strain of the carbonaceous mudstone sample under a first condition and a second condition respectively;

acquiring the creep of the carbonaceous mudstone sample in the attenuation stage under a first condition and a second condition according to the strain in the attenuation stage;

acquiring creep deformation of the carbonaceous mudstone sample in the steady stage under a first condition and a second condition according to the strain in the steady stage;

acquiring the creep of the carbonaceous mudstone sample in the damage stage under the first condition and the second condition according to the strain in the attenuation stage, the strain in the steady stage and the strain in the acceleration stage;

viscoelastic-plastic creep is obtained according to instantaneous strain, creep in the attenuation stage, creep in the steady stage and creep in the destruction stage.

Preferably, the viscoelastic creep and the first viscoelastic creep of the viscoelastic-plastic creep are obtained according to the instantaneous strain, the creep in the decay stage, the creep in the steady stage and the creep in the damage stage;

and obtaining the long-term strength of the carbon mudstone sample according to the creep finish time, the creep rate start time and the creep rate of the first viscoplastic creep and the second viscoplastic creep of the creep in the attenuation stage, and analyzing the stability of the carbon mudstone side slope.

A system for analyzing the stability of a carbonaceous mudstone side slope based on the action of dry and wet cycles comprises,

the data acquisition module is used for acquiring axial strain of the carbonaceous mudstone sample subjected to dry and wet circulation under different axial pressures to obtain dry and wet circulation times and axial pressure values and axial strain values corresponding to the dry and wet circulation times;

the first rule analysis module is used for obtaining a first change rule of axial strain of the carbon mudstone sample under the first condition of different dry and wet cycle times and the same axial pressure value based on the axial pressure value, the dry and wet cycle times and the axial strain value, and the first change rule of axial strain is used for expressing the first influence of the dry and wet cycle on the axial strain;

the second law analysis module is used for obtaining a second change law of axial strain of the carbon mudstone sample under a second condition of the same dry-wet cycle number and different axial pressure values based on the axial pressure value, the dry-wet cycle number and the axial strain value, and the second change law of axial strain is used for expressing a second influence of the axial pressure on the axial strain;

the side slope stability analysis module is used for acquiring instantaneous stage strain, attenuation stage strain, steady stage strain and acceleration stage strain of the carbon mudstone sample based on the axial strain first change rule and the axial strain second change rule to obtain viscoelastic creep of the carbon mudstone sample and analyze the side slope stability of the carbon mudstone;

the first storage module is used for storing first data generated by the system, and the first data at least comprises dry-wet cycle times, an axial pressure value, an axial strain first change rule, an axial strain second change rule, instantaneous stage strain, attenuation stage strain, steady stage strain and acceleration stage strain;

and the first communication module is used for the system to carry out data interaction.

Preferably, the system further comprises a first system applied to the carbon mudstone slope monitoring device, comprising,

the sensor module is used for acquiring the physical conditions of the carbonaceous mudstone side slope, wherein the physical conditions at least comprise stress conditions, temperature and humidity;

the second communication module is used for the data interaction between the first system and the system;

the first early warning module is used for obtaining a first early warning signal according to first data and physical conditions provided by the system;

the first display module is used for displaying the physical condition and the first early warning signal.

Preferably, the system further comprises a second system for use in a mobile device, comprising,

the second display module is used for displaying the first early warning signal and the first data;

and the third communication module is used for the second system to respectively carry out data interaction with the system and the first system.

Preferably, the system further comprises a third system applied to the cloud server, including,

the second storage module is used for storing the physical condition, the first data and second data generated by the third system according to the physical condition and the first data;

the fourth communication module is used for the data interaction of the third system with the system, the first system and the second system respectively;

the data processing module is used for obtaining a creep curve graph and early warning information according to the physical condition and the first data, wherein the creep curve graph is used for representing a creep curve of the carbon mudstone side slope, and the early warning information is used for representing the trend of the carbon mudstone side slope in dangerous conditions;

the second early warning module generates a second early warning signal according to the early warning information;

the first display module and the second display module are also used for displaying a second early warning signal.

The invention discloses the following technical effects:

according to the invention, through analyzing the creep characteristic of the carbonaceous mudstone under the action of dry-wet circulation, the creep of the carbonaceous mudstone only shows viscoelasticity under the action of low stress, and the creep of the carbonaceous mudstone shows viscoelastoplasticity characteristics under the action of high stress; the rock visco-elastic and visco-plastic creep increases non-linearly with time and stress; the steady-state viscoplastic creep rate increases exponentially with stress level. As the number of dry and wet cycles increases, the transient strain, decay phase creep and steady phase creep increase, and the creep rupture stress and elastic modulus decrease.

The invention provides the long-term strength of the carbonaceous mudstone under the action of dry-wet cycle by taking the threshold stress of the steady-state viscoplasticity creep rate as the threshold stress, and the obtained long-term strength of the carbonaceous mudstone with dry and wet times of 0, 3, 6 and 9 is 26.47MPa, 23.13MPa, 20.52MPa and 18.95MPa respectively. Comparative analysis of steady-state viscoplastic creep rate method, isochronal curve method and transition creep method found: compared with a transition creep method and an isochronal curve method, the viscoplastic creep method provided by the invention can determine the long-term strength of the rock more accurately, is less influenced by subjective factors, and can provide a reference for determining the long-term strength of the soft rock under the action of dry and wet cycles.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings described below are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without creative efforts.

FIG. 1 is a flow chart of a method according to the present invention;

FIG. 2 is a block diagram of the system of the present invention;

FIG. 3 is a triaxial creep curve of carbonaceous mudstone under the action of a dry-wet cycle according to an embodiment of the present invention;

FIG. 4 is a graph illustrating the overall process of rock creep according to an embodiment of the present invention;

FIG. 5 is a creep curve of a stage 7 loading of a carbonaceous mudstone sample during a 6 cycle of dry and wet cycles according to an embodiment of the present invention;

FIG. 6 is a graded loading creep curve according to an embodiment of the present invention;

fig. 7 is a graph showing the change of long-term strength of carbonaceous mudstone according to the number of dry and wet cycles in the embodiment of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be described clearly and completely with reference to the accompanying drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The carbonaceous mudstone has the characteristics of water absorption disintegration and softening, and the dry-wet cycle action is to intensify the weathering disintegration of the carbonaceous mudstone, so that the side slope rock mass damage caused by the disintegration seriously influences the long-term stability of the side slope. The long-term strength and creep damage characteristics of the carbonaceous mudstone under the action of dry and wet circulation are researched, and the method has important significance for correctly evaluating the stability of the damaged rock mass side slope. Most of the previous researches focus on the research of the creep property of soft rock, but the long-term strength of the carbonaceous mudstone under the action of dry and wet cycles is not sufficiently researched. Therefore, a triaxial compression creep test of the carbonaceous mudstone under the action of dry and wet cycles is developed, the creep characteristic of the carbonaceous mudstone is researched, a method for evaluating the long-term strength of the carbonaceous mudstone under the action of the dry and wet cycles is provided, and the change rule of the long-term strength of the carbonaceous mudstone under the action of the dry and wet cycles is researched.

As shown in fig. 1-7, the invention provides a method for analyzing the stability of a carbonaceous mudstone slope based on the action of dry and wet cycles, which comprises the following steps:

acquiring axial strain of the carbonaceous mudstone sample subjected to dry and wet circulation under different axial pressures to obtain dry and wet circulation times and axial pressure values and axial strain values corresponding to the dry and wet circulation times;

obtaining a first change rule of the axial strain of the carbonaceous mudstone sample under a first condition of different dry and wet cycle times and the same axial pressure value based on the axial pressure value, the dry and wet cycle times and the axial strain value, wherein the first change rule of the axial strain is used for expressing a first influence of the dry and wet cycle on the axial strain;

obtaining a second change rule of the axial strain of the carbonaceous mudstone sample under a second condition of the same dry-wet cycle number and different axial pressure values based on the axial pressure value, the dry-wet cycle number and the axial strain value, wherein the second change rule of the axial strain is used for expressing a second influence of the axial pressure on the axial strain;

and acquiring instantaneous phase strain, attenuation phase strain, steady phase strain and acceleration phase strain of the carbonaceous mudstone sample based on the first axial strain change rule and the second axial strain change rule to obtain viscoelastic plastic creep of the carbonaceous mudstone sample for analyzing the stability of the carbonaceous mudstone side slope.

In the process of collecting axial strain, different axial pressures are applied in a graded incremental loading mode, wherein the graded incremental loading mode comprises the steps of setting the confining pressure to be 2MPa, setting the first-stage loading strength to be 14.26MPa, setting the graded loading increment to be 3MPa, keeping the loading stress of each stage for 48 hours, and when the axial strain value is less than 0.001/d, carrying out next-stage loading until the carbonaceous mudstone sample is damaged.

In the process of obtaining the dry-wet cycle number, the dry-wet cycle number at least comprises 0 time, 3 times, 6 times and 9 times.

And in the process of acquiring the axial strain value, acquiring the axial strain value of the carbonaceous mudstone sample after creep deformation for 20 hours.

In the process of obtaining viscoelastic-plastic creep of the carbonaceous mudstone sample, obtaining instantaneous strain of the carbonaceous mudstone sample under a first condition and a second condition respectively according to the instantaneous phase strain; acquiring the creep of the carbonaceous mudstone sample in the attenuation stage under a first condition and a second condition according to the strain in the attenuation stage; acquiring creep deformation of the carbonaceous mudstone sample in the steady stage under a first condition and a second condition according to the strain in the steady stage; acquiring the creep of the carbonaceous mudstone sample in the damage stage under a first condition and a second condition according to the strain in the attenuation stage, the strain in the steady stage and the strain in the acceleration stage; viscoelastic-plastic creep is obtained according to instantaneous strain, creep in the attenuation stage, creep in the steady stage and creep in the destruction stage.

Obtaining viscoelastic creep and first viscoelastic creep of viscoelastic-plastic creep according to instantaneous strain, creep in an attenuation stage, creep in a steady stage and creep in a destruction stage; and obtaining the long-term strength of the carbon mudstone sample according to the creep finish time, the creep rate start time and the creep rate of the first viscoplastic creep and the second viscoplastic creep of the creep at the attenuation stage, and analyzing the stability of the carbon mudstone side slope.

A system for analyzing the stability of a carbonaceous mudstone side slope based on the action of dry and wet cycles comprises,

the data acquisition module is used for acquiring axial strain of the carbonaceous mudstone sample subjected to dry and wet circulation under different axial pressures to obtain dry and wet circulation times and axial pressure values and axial strain values corresponding to the dry and wet circulation times;

the first rule analysis module is used for obtaining a first change rule of axial strain of the carbon mudstone sample under the first condition of different dry and wet cycle times and the same axial pressure value based on the axial pressure value, the dry and wet cycle times and the axial strain value, and the first change rule of axial strain is used for expressing the first influence of the dry and wet cycle on the axial strain;

the second law analysis module is used for obtaining a second change law of axial strain of the carbon mudstone sample under a second condition of the same dry-wet cycle number and different axial pressure values based on the axial pressure value, the dry-wet cycle number and the axial strain value, and the second change law of axial strain is used for expressing a second influence of the axial pressure on the axial strain;

the side slope stability analysis module is used for acquiring instantaneous stage strain, attenuation stage strain, steady stage strain and acceleration stage strain of the carbon mudstone sample based on the axial strain first change rule and the axial strain second change rule to obtain viscoelastic creep of the carbon mudstone sample and analyze the side slope stability of the carbon mudstone;

the first storage module is used for storing first data generated by the system, and the first data at least comprises dry-wet cycle times, an axial pressure value, an axial strain first change rule, an axial strain second change rule, instantaneous stage strain, attenuation stage strain, steady stage strain and acceleration stage strain;

and the first communication module is used for the system to carry out data interaction.

The system also comprises a first system applied to the carbon mudstone slope monitoring equipment, which comprises,

the sensor module is used for acquiring the physical conditions of the carbonaceous mudstone side slope, wherein the physical conditions at least comprise stress conditions, temperature and humidity;

the second communication module is used for the data interaction between the first system and the system;

the first early warning module is used for obtaining a first early warning signal according to first data and physical conditions provided by the system;

the first display module is used for displaying the physical condition and the first early warning signal.

Preferably, the system further comprises a second system for use in a mobile device, comprising,

the second display module is used for displaying the first early warning signal and the first data;

and the third communication module is used for the second system to respectively carry out data interaction with the system and the first system.

The system also includes a third system for application to the cloud server, including,

the second storage module is used for storing the physical condition, the first data and second data generated by the third system according to the physical condition and the first data;

the fourth communication module is used for the data interaction of the third system with the system, the first system and the second system respectively;

the data processing module is used for obtaining a creep curve graph and early warning information according to the physical condition and the first data, wherein the creep curve graph is used for representing a creep curve of the carbon mudstone side slope, and the early warning information is used for representing the trend of the carbon mudstone side slope in dangerous conditions;

the second early warning module generates a second early warning signal according to the early warning information;

the first display module and the second display module are also used for displaying a second early warning signal.

Example 1: the invention carries out technical verification through a test method, and the specific test process is as follows:

the triaxial compression creep test of the carbonaceous mudstone under the action of dry-wet circulation:

carrying out a triaxial compression rheological test on the sample which completes the dry-wet cycle, wherein a triaxial rheological tester is RLW-2000 and a deformation measuring device thereof; and (3) adopting a graded increment loading mode, setting confining pressure to be 2MPa, designing the first-stage loading of a creep test to be 40% of uniaxial compressive strength, namely 14.26MPa, designing the graded loading increment to be 3MPa, keeping the loading stress of each stage for 48h, and then carrying out next-stage loading when the axial strain value is less than 0.001/d until the rock sample is damaged.

Analyzing the results of the triaxial compression creep test:

the results of the triaxial compression rheological test of the carbonaceous mudstone under the action of the dry-wet cycle are shown in figure 3. As can be seen from fig. 3: the axial strain of the carbonaceous mudstone is increased along with the increase of the dry and wet cycle times and stress; under the same axial pressure, the strain of the carbonaceous mudstone is increased along with the increase of the dry and wet cycle times; under the action of the axial pressure of 17.26MPa, the creep axial strain of the carbonaceous mudstone samples with the dry-wet cycles of 0, 3, 6 and 9 times is respectively 0.606 multiplied by 10^-3、0.958×10^-3、1.168×10^-3And 1.340X 10^-3Average increase in axial strain per wet-dry cycle of 9.75X 10^-5。

Under the action of the same dry-wet cycle times, the strain of the carbonaceous mudstone is increased along with the increase of the axial pressure; the carbon mudstone is loaded for 1-7 levels in dry and wet cycle for 6 times, and the axial strain is 1.168 multiplied by 10 when creep deformation is 20 hours^-3、1.468×10^-3、1.786×10^-3、2.144×10^-3、2.538×10^-3、 2.978×10^-3And 3.364X 10^-3Mean axial strain increase per stage loading of 3.34 x 10^-4。

The whole process curve of rock creep is shown in figure 4. Rock strain from transient phase strain₀Strain epsilon of attenuation stage₁Strain epsilon in stationary phase₂And strain in acceleration phase ∈₃And (4) forming. The calculation can be made as follows:

ε＝ε₀+ε₁+ε₂+ε₃ (5.1)

carbon with 6 times of dry and wet circulation and 32.26MPa of axial pressureThe creep curve of a shale sample is taken as an example. The carbon mudstone generates instantaneous strain when being loaded, then enters an attenuation creep stage, the strain is increased along with the time, the creep rate is gradually reduced, the accumulated strain in the attenuation stage can be determined by the intersection point of the tangent line of the curve in the steady creep stage and the ordinate, and the creep in the attenuation stage is epsilon₁＝2.46×10^-4The stage of the attenuation creep of 9.33h enters the stage of the steady creep, and the stage of the steady creep can obtain epsilon by subtracting the strain of the attenuation stage from the strain of the starting point of the acceleration creep₂＝1.67×10^-4After 39.33h, an accelerated creep phase begins to appear, which becomes ε₃＝3.21×10^-4。

Similarly, the creep of the carbonaceous mudstone at various stages under the action of the dry-wet cycle was determined according to the method described above and is shown in table 1. From table 1, it can be seen that: the carbon mudstone samples with the dry-wet cycle of 0, 3, 6 and 9 times only creep in the attenuation stage under the action of the first 3 stages of loading, and the steady creep stage appears in the 4 th loading; the carbon mudstone samples subjected to dry and wet cycles of 0 and 3 are subjected to creep rupture during the loading of the 8 th level, and the carbon mudstone samples subjected to dry and wet cycles of 6 and 9 are subjected to creep rupture during the loading of the 7 th level. The method is characterized in that a carbon mudstone sample generates more microcracks and micropores under the action of dry-wet circulation, part of the microcracks and the micropores are pressed and closed during the first 3-level loading, rock particles are kept stable under the constraint of friction force due to smaller stress of the rock, and the creep rate is gradually reduced and then tends to be stable; along with the increase of stress, the microcracks and the pores expand towards the periphery, the constant flow resistance of the granules is reduced, the granules start to directionally flow, and the rock has a constant creep stage; when the stress is further increased, the cracks in the rock are continuously expanded to form through cracks, the granules flow at an accelerated speed, and finally creep failure occurs. As the rock is damaged due to the interaction of water and rocks in the process of the dry-wet cycle, the rock damage is aggravated along with the increase of the times of the dry-wet cycle, and the creep rupture load of the carbonaceous mudstone sample in the dry-wet cycle for 6 times and 9 times is reduced.

The instantaneous strain in each phase of strain of the carbonaceous mudstone under the action of each dry-wet cycle is the largest and accounts for 77.2-87.8% of the total strain, which indicates that most of the strain of the carbonaceous mudstone is generated instantaneously. At lower stress levels, the creep of the carbonaceous mudstone is mainly concentrated in the attenuating creep phase, whereas the creep of the carbonaceous mudstone under destructive stress is mainly generated in the accelerating creep phase. This is because the micro-cracks and micro-pores inside the rock are loaded and complete the compression deformation in a short time, so that the strain at the attenuation stage is large; and along with the increase of the load, the original micro cracks in the rock expand and new cracks develop, the directional flow speed of rock particles is accelerated, and the rock generates larger deformation in the accelerated creep stage.

TABLE 1

Transient strain analysis:

instantaneous strain of the carbonaceous mudstone under the action of different dry and wet cycle times. Under the action of the same dry-wet cycle times, the instantaneous strain of the carbonaceous mudstone is increased along with the increase of the axial pressure; the carbon mudstone sample subjected to dry and wet cycles for 6 times has 60.8 percent and 181.5 percent of transient strain increase at the axial pressure of 20.26MPa and 29.26MPa compared with the transient strain of 14.26MPa respectively. Under the same axial pressure, the instantaneous strain of the carbonaceous mudstone is increased along with the increase of the dry and wet cycle times; under the action of the axial pressure of 20.26MPa, the transient strains of 3, 6 and 9 times of dry-wet cycles are respectively increased by 9.91 percent, 22.31 percent and 35.54 percent compared with the transient strains of 0 time of the dry-wet cycles. The method is characterized in that the carbonaceous mudstone is subjected to various geological actions in the diagenesis process, a large number of micro cracks and micro pores which are randomly distributed exist in the rock, hydrophilic minerals of the carbonaceous mudstone are dissolved and corroded under the action of dry and wet circulation, cracks are developed under the action of water and rock in the dry and wet circulation process, the effective bearing area of the rock is further reduced, and therefore large instantaneous strain is generated under the action of load.

Decay phase creep analysis:

and (3) creep deformation of the carbonaceous mudstone in the creep attenuation stage under the action of the dry-wet cycle. Dry matterCreep of the carbon mudstone in the attenuation stage under the action of wet circulation generally increases along with the increase of the dry and wet circulation times; under the action of axial pressure of 14.26MPa, 20.26MPa and 29.26MPa, the creep of the carbon mudstone sample increases 1.479 x 10 times in the decay stage of each dry-wet cycle on average^-5、5.451×10^-5And 0.883X 10^-5(ii) a The creep of the first 3-level loading (14.26, 17.26 and 20.26MPa) attenuation stage is linearly increased along with the increase of the dry-wet cycle times, and the creep of the 4-7-level loading (23.26, 26.26, 29.26, 32.26 and 35.26MPa) attenuation stage is regularly reduced along with the increase of the dry-wet cycle times, so that the microcracks and micropores in the carbonaceous mudstone develop along the weak part of the cementing body under the action of the dry-wet cycle, the rock microcracks and micropores develop rapidly when the axial pressure is higher, the stress of the infinitesimal body in the rock is more uneven, and the creep growth difference of the rock under the action of the load is more obvious.

Creep analysis at the steady state:

creep deformation of the carbonaceous mudstone sample in a steady creep stage under the action of the dry-wet cycle. Creep deformation of the carbonaceous mudstone at the steady-state stage is increased along with the increase of the dry-wet cycle times, the volatility is enhanced, and the increase amplitude of the steady creep deformation of the carbonaceous mudstone after 6 dry-wet cycles is increased, and the reason may be that: the weak part of the carbon shale microstructure is damaged under the action of water force under the action of dry and wet circulation, so that the microcracks and pores of the rock are developed, and the creep of the rock at the steady stage is increased under the action of axial pressure; meanwhile, as microcracks and pores develop irregularly, the creep volatility of the rock is increased.

And (3) creep analysis in a failure stage:

creep curves of the carbon mudstone samples in the damage stage with different dry and wet cycle times. The creep curves of the two groups of failure stress phases have the same change rule and have three obvious creep phases (an attenuated creep phase, a steady creep phase and an accelerated creep phase), wherein the time spent in the attenuated creep phase and the accelerated creep phase is shorter. The duration of the carbonaceous mudstone damage stage is reduced along with the increase of the dry-wet cycle times, and the rate of the steady creep stage is increased; the total duration of the dry-wet cycle is reduced by 15.33h than the total duration of the dry-wet cycle by 9 times, and the steady creep rate is reduced from 3.875X 10^-6Increase to 1.51110^-5The increase is 3.9 times. The decay creep decreases, the steady creep and the accelerated creep increase as the number of dry and wet cycles increases during the failure stress phase. The damping creep, the steady creep and the accelerated creep of the dry-wet cycle of 9 times are respectively increased by 8.0 multiplied by 10 than those of the dry-wet cycle of 6 times^-5、1.2×10^-4And 2.1X 10^-4。

TABLE 2

Long-term strength of carbonaceous mudstone under action of dry-wet circulation

Rock long term strength is the maximum stress at which the rock can remain stable under long term loading. When the stress is larger than the long-term strength, the internal cracks of the rock are expanded and communicated, so that the rock is subjected to creep rupture. When the rock stress is less than the creep fracture toughness, the internal pores and cracks of the rock are compressed and closed, and the rock is kept stable after creeping for a certain time; when the stress is greater than the creep fracture toughness, the rock particles generate plastic flow, the crack tips continuously expand, converge and run through, and finally the rock generates creep damage. Thus, rock creep fracture toughness can be considered to be the long term strength of the rock. The creep fracture toughness of the rock is closely related to the viscoplastic creep of the rock, when the stress is less than the creep fracture toughness, the internal pores of the rock are compressed and closed, and the rock undergoes viscoelastic creep; when the stress is greater than the creep rupture toughness, plastic damage occurs at the tip of the crack, the crack propagates, and viscoelastic-plastic creep occurs in the rock. Rock failure must occur over a considerable period of time as long as the rock strain remains at a steady viscoplastic creep rate and increases. To this end, it is proposed herein to use the threshold stress of steady-state viscoplastic creep (creep rupture toughness) as the long-term strength of the rock.

Viscoelastic creep analysis

The rock fractional loading creep curve is shown in figure 5. As can be seen from fig. 5: at low stress levels, the rock undergoes a transient elastic strain epsilon under stress₀Then the creep rate is gradually reduced, after a period of creep, the creep rate of the rock is reduced to zero, and the rock at the stageOnly viscoelastic deformation epsilon of stone_ceRock creep can be expressed as:

ε＝ε₀+ε_ce (5.2)

at high stress levels, the rock is first elastically strained₀Then the creep rate is gradually reduced to a stable non-zero rate, and the viscoelastic creep of the rock occurs at the stage, and the rock creep can be expressed as:

ε＝ε₀+ε_ce+ε_cp (5.3)

ε₀、ε_ce、ε_cpinstantaneous strain, viscoelastic creep and viscoelastic creep, respectively.

The creep characteristics of the rock are researched by separating the visco-elastic creep and the visco-plastic creep of the rock, the visco-elastic creep of the rock is known to be a function related to stress according to an element theory, and the visco-plastic creep of the rock can be separated by establishing a functional relation between the visco-elastic creep and the stress of the rock at a low stress level and substituting in a formula (5.3).

Viscoelastic creep analysis:

and (3) calculating the viscoelastic creep of the first 4-stage loading under the action of different dry and wet cycle times of the carbonaceous mudstone according to the formula (5.2): the viscoelastic creep of the rock increases with time, but the creep rate gradually decreases and finally remains stable, and the stable value thereof increases with the increase of the axial pressure. Viscoelastic creep for rock can be described by well-established element theory and empirical models, and a Kelvin model is used to fit the viscoelastic creep, and the model equation is shown in formula (5.4), and the fitting parameters are shown in table 3.

From table 3, it can be seen that: viscoelastic creep model parameters of carbonaceous mudstone under the action of different dry and wet cycle timesAndis a parameter related to stress, and the function relation of the visco-elastic creep model parameter obtained by least square fitting is as follows:

the parameters a, b, c, d and e relate to the wet and dry cycle and their values are shown in Table 4.

TABLE 3

TABLE 4

And (3) substituting 5-8-grade stress into the formulas (5.5) and (5.6) to obtain the viscoelastic creep of the rock at the high-stress stage. Therefore, the viscoelastic creep change laws in the high and low stress stages are similar and are increased along with the increase of the axial pressure, but the creep rate is gradually reduced, and finally, a stable value is kept.

Viscoplastic creep analysis:

the viscoelastic creep can be separated from the total strain by substituting the viscoelastic creep calculation result into formula (5.4). Viscoplastic creep of carbonaceous mudstones under the action of dry-wet cycles: when the carbon mudstone samples subjected to dry and wet cycles for 0 and 3 times are subjected to axial pressure of 26.26-32.26 MPa and the carbon mudstone samples subjected to dry and wet cycles for 6 and 9 times are subjected to axial pressure of 26.26-29.26 MPa, the viscous plastic creep of the carbon mudstone increases along with the increase of time, but the increasing rate is gradually reduced, and finally the stable rate is kept to increase; when the carbon mudstone samples subjected to dry-wet cycle for 0 time and 3 times are subjected to axial pressure of 35.26MPa and the carbon mudstone samples subjected to dry-wet cycle for 6 time and 9 times are subjected to axial pressure of 32.26MPa, the viscous plastic creep keeps transient stable growth, and then the accelerated growth occurs, and finally the damage occurs. The reason is that the rock plastically flows under the action of larger stress, so that the cementing state of a particle structure is damaged, and the internal cracks of the rock are stably expanded; when the stress exceeds the fracture toughness of the rock, the cracks are communicated with each other to form macroscopic cracks, the deformation of the rock is increased rapidly, and the accelerated creep failure occurs. In addition, the dry-wet circulation action greatly increases the pores and cracks of the rock, reduces the effective stress area of the rock, and reduces the stress and time required for mutual communication among the cracks.

Long-term strength analysis:

steady-state viscoplastic creep method:

the viscoelastic creep analysis of the carbonaceous mudstone discovers that: when the stress of the carbonaceous mudstone is lower, the creep rate is reduced to zero only after viscoelastic creep or transient viscoplastic deformation, and the rock can be kept stable all the time; when the stress exceeds creep fracture toughness, the rock generates stable viscoplastic creep growth or accelerated growth, cracks gradually expand and are communicated with each other to form macrocracks along with plastic flow of the particles, and finally the rock generates deformation damage. It can be seen that the threshold stress for steady-state visco-plastic creep of the rock is the creep rupture toughness. For this purpose, the long-term strength of the rock can be determined by establishing a steady-state viscoplastic creep rate as a function of stress. And (3) a viscoplastic creep curve of the carbonaceous mudstone at a steady-state creep stage under the action of different dry and wet cycles.

The steady-state viscoplastic creep has a good linear relationship with time, and can be expressed as:

t₂end time of steady-state viscoplastic creep, t₁Is the onset time of the viscoplastic creep steady-state rate, epsilon'_ceTo attenuate the visco-plastic creep at the creep stage,is the steady state viscoplastic creep rate. Steady state viscoplastic creep rate and stressAs a function of (c). Steady state viscoplastic creep increases exponentially with increasing axial pressure and can be expressed as:

the threshold stress of the viscoplastic creep is substituted into the formula (5.8) to obtain the long-term strength of the carbonaceous mudstone with the dry-wet cycle of 0, 3, 6 and 9 times of 26.47MPa, 23.13MPa, 20.52MPa and 18.95MPa respectively.

Isochronal curve method:

the isochronal curve method is characterized in that strain of the same time under different loading stress is drawn into an isochronal line, and the inflection point of an isochronal curve cluster is used as the long-term strength of the rock. And (3) forming an isochronal curve cluster of the creep of the carbonaceous mudstone under the action of different dry and wet cycle times: as the dry-wet cycle times increase, the isochronous curve clusters of the carbonaceous mudstones gradually become sparse; under the same dry-wet cycle frequency, the carbon shale isochronal curve cluster changes from dense to sparse along with the increase of the axial pressure and gradually deviates to a strain axis, but the isochronal curve cluster has no obvious inflection point. According to the curve change rule of the isochronal curve, the long-term strength of the carbonaceous mudstone of 0, 3, 6 and 9 times of dry-wet cycle is judged to be 32.26MPa, 29.26MPa, 26.26MPa and 26.26MPa respectively.

Transition creep method:

the transition creep method considers the maximum load stress with a steady state creep rate of zero, i.e. the long-term strength of the rock. Using epsilon_ce＝A[B-exp(Ct)]And ε_cpDt-the rock creep visco-elastic creep and visco-plastic creep were fitted. Creep curves of the carbonaceous mudstones loaded at different levels under the action of different dry and wet cycles are shown in the table 5. From table 5, it can be seen that: the transition creep method can only determine the approximate range of the long-term strength of the rock, and the ranges of the long-term strength of the carbonaceous mudstone with the dry-wet cycles of 0, 3, 6 and 9 are respectively 26.26-29.26 MPa, 23.26-26.26 MPa, 20.26-23.26 MPa and 17.26-20.26 MPa.

TABLE 5

The long-term strength of the carbonaceous mudstone under the action of the dry-wet cycle obtained by the three methods is summarized in table 6. Three methods for solving the long-term strength of the rock are compared to obtain the following results: the inflection point of the isochronal curve method is not clear, and the determination of the long-term strength of the rock through subjective estimation tends to cause larger errors; the transition creep method determines that the long-term strength of the rock can only determine an approximate interval range, and cannot accurately determine a specific numerical value of the long-term strength of the rock; the steady-state viscoplasticity creep rate method provided by the invention can accurately determine the long-term strength of the rock by establishing a functional relation between the steady-state viscoplasticity creep rate and the stress, and the determination of the long-term strength of the rock by the method is reasonable within the interval range of the long-term strength of the rock determined by the transition creep method. It follows that the steady-state viscoplastic creep rate can be used as a method of determining the long-term strength of a rock.

TABLE 6

The long-term strength of the carbonaceous mudstone under the action of the dry-wet cycle is determined by using a steady-state viscoplastic creep rate method, and is shown in figure 7. As can be seen from fig. 7: the long-term strength of carbonaceous mudstones decreases with increasing number of wet and dry cycles, but the rate of decrease gradually decreases. The method is characterized in that the carbonaceous mudstone minerals are corroded under the action of dry-wet circulation, the crack expansion causes damage deterioration of the rock structure, soluble minerals on the surface of the rock are gradually corroded along with the dry-wet circulation, the contact difficulty of water and the soluble substances is increased, meanwhile, the energy at the tip of the crack is gradually released along with the crack expansion, the crack expansion kinetic energy and potential energy are reduced, and the degradation damage rate of the carbonaceous mudstone is reduced. The relationship between the dry-wet cycle times and the long-term intensity is obtained by least square fitting:

σ_∞＝-3.021ln(1+n)+26.78 (5.9)

the following is derived from equation (5.9): after the carbonaceous mudstone undergoes 5113 dry-wet cycles, the long-term strength is reduced to 0MPa, and at the moment, the bearing capacity of the rock is lost.

It should be noted that: like reference numbers and letters refer to like items in the following figures, and thus once an item is defined in one figure, it need not be further defined and explained in subsequent figures, and moreover, the terms "first", "second", "third", etc. are used merely to distinguish one description from another and are not to be construed as indicating or implying relative importance.

Finally, it should be noted that: the above-mentioned embodiments are only specific embodiments of the present invention, which are used for illustrating the technical solutions of the present invention and not for limiting the same, and the protection scope of the present invention is not limited thereto, although the present invention is described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: any person skilled in the art can modify or easily conceive the technical solutions described in the foregoing embodiments or equivalent substitutes for some technical features within the technical scope of the present disclosure; such modifications, changes, or substitutions do not depart from the spirit and scope of the present invention in its spirit and scope. Are intended to be covered by the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.