1. A method for preparing a sample for artificially simulating discontinuous fractures is characterized by comprising the following steps:

(1) taking cement, sand and talcum powder in a ratio of 1:2:5, uniformly stirring, adding a saturated sodium glutamate solution with a mass corresponding to a water-cement ratio of 1:2, fully stirring, adding sodium glutamate crystals with a mass corresponding to the calculated porosity, uniformly stirring, and loading the mixture into a sample size mould required by a single-shaft and three-shaft test in a layering manner;

(2) after the test piece is maintained in air for 28 days, placing the test piece into a flat-bottom metal container, adding water until the upper surface of the test piece is 8-10 cm, heating the metal container until the water body is boiled, continuing to heat for 2-3 minutes, turning the test piece upside down after the water body in the container is cooled to room temperature, supplementing clear water into the container and heating again when the liquid level in the container is greatly reduced in the heating process, taking out the test piece from the water and placing the test piece into a vacuum saturated container after the water body is boiled and cooled to room temperature again, injecting clear water into the saturated container in advance, controlling the negative pressure to be 0.8MPa, carrying out vacuum saturation for 48 hours, and repeating the water treatment process for three times;

(3) and taking out the water body in the last vacuum saturated container, filtering the water body through filter paper, filtering out solid impurities, pouring the water body into a clean vessel, heating and evaporating the water body in the container, considering that water passing cracks which are uniformly distributed and have uniform geometric dimension are formed in the test piece if no crystals are separated out, and repeating the water treatment stage operation if a large number of crystals are separated out.

2. The method for preparing the artificial simulation discontinuous crack sample as claimed in claim 1, wherein in the step (1), the sample is loaded after the upper surface is scraped before the loading is compacted in a layering manner, a standard cylindrical sample is radially and additionally provided with a throat hoop for constraint, the periphery of a rectangular sample is fixed by a steel plate, and the upper surface of the sample is covered with the steel plate to prevent the upper surface from bulging.

3. The method for preparing the artificial simulated discontinuous crack specimen as claimed in claim 1, wherein the specimen in the step (2) is always below the liquid level during the inversion process.

4. The method for preparing a manually simulated discontinuous crack specimen as claimed in claim 1, wherein the step (2) comprises cleaning the inner wall of the flat-bottomed metal container after each finishing operation of the flat-bottomed metal container and the vacuum saturation container.

Background

Fractured rock mass is a complex medium common in numerous geotechnical engineering, having material properties intermediate between solid and fluid. In actual underground engineering, rock mass is in a complex geological environment with the coupling effect of a seepage field and a stress field, so the mechanical deformation characteristic of fractured rock mass, the coupling problem of the interaction of the rock mass and underground water seepage and the creep problem of the fractured rock mass under the seepage are always emphasized by the rock mechanics and engineering boundary at home and abroad.

Horii (Horii H, Nemat-Nasser S, 1985.Compression-induced micro crack growth in crack defects. journal of geological reaction, 90 (B4): 3105. sub. 3125) based on the theory of linear elastic fracture mechanics, the crack initiation, propagation and through failure mechanisms of the prefabricated open fracture test piece under uniaxial Compression were studied and analyzed. According to the method, experimental research is carried out on a surface containing prefabricated open single cracks and open multi-crack surfaces by Ashby (Ashby M F, halam S D, the failure of crack contained in stress states, acta metal, 1986,34 (3): 497-5 510), crack initiation, expansion and through mechanisms are analyzed in a comparison mode based on a linear elastic fracture mechanics theory and combined with the experimental results, and a related mechanics theory model is established according to the crack initiation, expansion and through mechanisms. According to the method, a Weibull distribution and energy conservation principle is adopted by the Zhang Yuan (Zhang Yuan. research on mechanical properties and fracture evolution process of the sandstone in the prefabricated fracture, university of Henan's marble, 2019), a Lemaitre hypothesis is utilized, a prefabricated fracture sandstone damage constitutive model is deduced, and the effectiveness of the model is verified.

The method comprises the steps of sampling a large rock drill core on site, sealing the rock sample in a rock transfer box by using a plastic freshness protection package, conveying the rock sample back to a laboratory, performing double-sided sampling cutting on the original rock by using an SPQJ-300 type double-blade rock cutting machine, fixing the sandstone core on an SHM-200 type double-end-face rock grinding machine, finely grinding the cross sections at two ends, ensuring that a sample meets the specified requirements in GB/T50266-2013 rock engineering test method standard (GB/T50266-2013), finally preparing a standard specification test piece with the diameter of 50mm and the height of 100mm, and performing a mechanical test, wherein the top surface flatness of the upper end and the lower end of the test piece is +/-0.1 mm.

And then selecting the test piece, removing the test piece with obvious defects and cracks on the surface, carrying out ultrasonic detection on the processed test piece by using a GTJ-U820 nonmetal ultrasonic tester, removing the test block with low sound velocity, and selecting the test piece with consistent wave velocity to carry out a mechanical test, wherein the sample is 2.65-2.75 km/s. However, the above method has (1) a difficulty in that the molding process of the test piece is complicated; (2) inconvenient material taking and transportation and high price.

Disclosure of Invention

Based on the defects of the prior art, the technical problem to be solved by the invention is to provide a method for preparing a sample by manually simulating discontinuous fractures, so that the sizes of the prefabricated fractures are distributed in the internal space of the sample as uniformly and randomly as possible, the authenticity of the sample is restored to the maximum extent, and the forming process and the prefabricated fracture manufacturing process of the sample are convenient and fast; convenient material taking and reasonable price.

In order to solve the technical problems, the invention is realized by the following technical scheme: the invention provides a method for preparing a sample for artificially simulating a discontinuous crack, which comprises the following steps:

(1) taking cement, sand and talcum powder in a ratio of 1:2:5, uniformly stirring, adding a saturated sodium glutamate solution with a mass corresponding to a water-cement ratio of 1:2, fully stirring, adding sodium glutamate crystals with a mass corresponding to the calculated porosity, uniformly stirring, and loading the mixture into a sample size mould required by a single-shaft and three-shaft test in a layering manner;

(2) after the test piece is maintained in air for 28 days, placing the test piece into a flat-bottom metal container, adding water until the upper surface of the test piece is 8-10 cm, heating the metal container until the water body is boiled, continuing to heat for 2-3 minutes, turning the test piece upside down after the water body in the container is cooled to room temperature, supplementing clear water into the container and heating again when the liquid level in the container is greatly reduced in the heating process, taking out the test piece from the water and placing the test piece into a vacuum saturated container after the water body is boiled and cooled to room temperature again, injecting clear water into the saturated container in advance, controlling the negative pressure to be 0.8MPa, carrying out vacuum saturation for 48 hours, and repeating the water treatment process for three times;

(3) and taking out the water body in the last vacuum saturated container, filtering the water body through filter paper, filtering out solid impurities, pouring the water body into a clean vessel, heating and evaporating the water body in the container, considering that water passing cracks which are uniformly distributed and have uniform geometric dimension are formed in the test piece if no crystals are separated out, and repeating the water treatment stage operation if a large number of crystals are separated out.

Preferably, in the step (1), before the materials are compacted in a layered mode, the materials are loaded after the upper surface of the materials is scraped, the standard cylindrical sample is restrained by a throat hoop in the radial direction, the periphery of the rectangular sample is fixed through a steel plate, and the upper surface of the sample is covered with the steel plate to prevent the upper surface from bulging.

Preferably, the test piece in the step (2) is always below the liquid level in the inversion process.

Preferably, in the step (2), after the operation of the flat-bottom metal container and the vacuum saturation container is finished each time, the inner wall of the container is cleaned.

Compared with the prior art, the technical scheme of the invention has the following beneficial effects: (1) the main physical and mechanical properties are close to real rocks or have certain similarity; (2) the physical and mechanical properties are stable, and the influence of external environment change is not easy to occur; (3) the molding process and the prefabricated crack manufacturing process of the test piece are convenient and fast; (4) convenient material taking and reasonable price. The specificity of the sodium glutamate crystals is applied to be added into the test piece and separated out to obtain the uniform and randomly distributed fracture test piece.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings of the embodiments will be briefly described below.

FIG. 1 is a diagram of internal fractures of surrounding rocks of a sandstone section under a drilling peeping instrument;

FIG. 2 is a diagram of in situ retrieval of virgin rock;

FIG. 3 is a water cut test chart;

FIG. 4 is a solid composition diagram of an orthogonal test of similar siltstone materials, wherein, (a) the diameter of the siltstone is less than or equal to 0.075mm, (b)325 Portland cement, (c) 800 meshes of gypsum, (d) 800 meshes of talcum powder, and (e) bentonite (dried and powdered);

fig. 5 is a graph of the solubility of sodium glutamate.

Detailed Description

Other aspects, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which form a part of this specification, and which illustrate, by way of example, the principles of the invention. In the referenced drawings, the same or similar components in different drawings are denoted by the same reference numerals.

The method for preparing the artificial simulation discontinuous fracture sample comprises the following steps: a drilling peeping instrument is utilized to observe the internal fractures and water containing conditions of surrounding rocks of the sandstone section of the main inclined shaft of the Hongqing beam coal mine, as shown in figure 1. Analysis shows that the objective existence of adverse factors such as water content of the surrounding rock of the sandstone section, crack development, weakness of the surrounding rock and the like. The samples were taken from the site of the excavation of the working section as shown in fig. 2, and the samples were returned to the laboratory to measure the parameters such as density, water content, and component content of the rock.

The density of rock refers to the weight of the rock per unit volume. The natural density of a rock is the ratio of mass to volume of the rock at its natural moisture content.

The rock gravities were calculated by the following formula and the test recordings are shown in table 1.

In the formula: m-weight of natural hydrous rock, kg;

gamma-the natural gravity of the rock, kN/m³；

V-volume of rock, cm³。

TABLE 1 field rock sample Density measurement

FIG. 3 is a water content test chart, in which the natural water content of a sample is calculated as follows:

in the formula: m is_s-natural mass, g;

m_d-dry mass, g.

Omega-water content%

The water content test results are shown in Table 2.

TABLE 2 field Water content test results for rock samples

Based on the characteristic that the weakly cemented soft rock is easy to disintegrate, break and deform after the rock sample is softened in water, the composition of the rock stratum mainly comprises quartz, potash feldspar, albite and clay minerals which are mainly montmorillonite and kaolinite and are measured by utilizing a polarizer, a high-power microscope and a weakly cemented soft rock tissue structure which is given by a scanning electron microscope and analyzing through X-ray diffraction, and the composition proportion of the clay minerals is shown in table 3. The rock sample quartz and feldspar have the content of 67.2 percent and the clay mineral content of 32.6 percent, and the rock sample quartz and feldspar accord with the argillaceous siltstone mineral content, namely the clay content is 50-25 percent and the silt content is 70-50 percent.

TABLE 3 analysis results of rock mineral composition by X-ray diffraction analysis

The chemical components of the argillaceous siltstone are analyzed on the basis of the research of the existing similar materials, the main materials of the siltstone similar materials are quartz sand, 32.5 ordinary portland cement, tap water (saturated sodium glutamate solution is adopted in the similar material proportion test for ensuring the stability of the subsequent prefabricated fracture sample), and the like, and the quartz sandThe sand grain diameter is less than or equal to 0.075 mm. In order to more accurately and scientifically determine the similar proportion meeting the research, an orthogonal design method is adopted to design the proportion of similar conglomerate-like materials, the main consideration factors are the water-cement ratio (mass ratio of water to cement), the sand-cement ratio (mass ratio of sand to cement), the mud-cement ratio (mass ratio of impurity components to cement) and the solid components of similar material tests of argillaceous types (talc powder, gypsum and bentonite are dried and then pulverized by a 300-mesh screen), which are shown in figure 4, and the test adopts an orthogonal design scheme L9 (3) with 4 factors and 3 levels⁴) The details of the orthogonal design level of the similar siltstone material are shown in table 4.

TABLE 4 Quadrature design level of similar siltstone materials

Test level	Water to glue ratio	Sand to glue ratio	Clay to rubber ratio	Argillaceous material
					1	1:1.5	2.0:1	1:1.5	Talcum powder
2	1:2.0	2.5:1	1:1.0	Gypsum plaster
					3	1:2.5	3.0:1	1:0.5	Bentonite clay

Orthogonal design of similar material proportion:

using orthogonal design Assistant software, the design levels of the factors in Table 4 were adjusted to the orthogonal design Table L9 (3)⁴) The arrangement is carried out, 9 groups of proportioning schemes are calculated, and the specific proportioning scheme is shown in table 5.

Orthogonal proportioning scheme for similar materials of siltstone in Table 5

Similar material proportioning test results:

4 groups of comparative test pieces are made for each proportioning scheme. The manufactured and maintained test piece is subjected to a conventional physical mechanical property test, and simultaneously, the parameter test results of each group are subjected to range analysis, so that the change rule of the existing influence factors can be met to a certain extent, and the physical mechanical parameter test result of the similar siltstone material is directly given, and is shown in table 6 in detail.

TABLE 6 orthogonal test similar material physical and mechanical parameters of siltstone

And comparing and analyzing the obtained physical and mechanical parameters of the argillaceous siltstone rock, so that the permeability coefficient of the argillaceous siltstone is used as a main similar parameter of the similar siltstone material. Permeability coefficient of original rock sample 5.65X 10^-6cm/s, the permeability coefficients of similar materials obtained by analyzing 9 proportioning schemes in the table 6 from the aspect of scientific counting method, and the permeability coefficients obtained by only two proportioning schemes of L-3 and L-5 are in the order of magnitude (multiplied by 10)^-6) Basically consistent with each other, the rest have almost magnitude difference, but the difference between the L-3 bentonite scheme and the original rock is larger, and each physical and mechanical parameter of the material with the similar proportion between the original rock sample and the L-5 is shown in the table 7.

TABLE 7 physical and mechanical parameters of similar materials of original rock and siltstone-like rock

The physical mechanical parameters in table 7 above were subjected to calculation of the similar parameter ratio (the ratio of the original rock to the similar material), and the calculation results are shown in table 8.

TABLE 8 similar parameter ratio of similar materials of virgin rock and siltstone-like rock

As can be seen from the analysis of table 8,C_E≈C_c≈C_σc≈C_σt1, all meet the set similarity criterion. Comprehensively considering, simulating siltstone surrounding rock containing underground water fracture development in the main inclined shaft of the Hongqing beam coal mine by adopting an L-5 orthogonal proportioning scheme, wherein the similar material proportion of the siltstone is as follows: cement, sand, water, talcum powder (2: 5:1: 4), powder sand with particle size less than 0.075mm, 325-class silicate cement and saturated sodium glutamate solution.

The invention adds a certain solid admixture to a sample of a similar material, and then replaces the solid admixture through related processing means to enable the space volume occupied by the solid admixture in the sample preparation and compaction process to become a space crack which influences the propagation speed of ultrasonic waves in the test piece. Through multi-field consultation and extensive literature reading, the solubility of the crystal is utilized to realize the replacement of the occupied volume of the prefabricated crack. After relevant reference, the cross section diameter of the crystal of the sodium glutamate crystal is about 0.15-1 mm, the length of the crystal is about 6-8 mm, the geometric dimensions are uniform and consistent, the length-diameter ratio required by the prefabricated crack is met, and the basic physical characteristics of the crystal are shown in a table 9.

TABLE 9 basic physical Properties of sodium glutamate

At the same time, the crystal solubility increased rapidly with increasing water temperature, as shown in the solubility curve of fig. 5, for which two considerations were added during the operation of preparing the preformed fracture: firstly, after solid components of similar materials are uniformly stirred, pure water is replaced by saturated sodium glutamate solution at normal temperature, and volume loss in the compaction process caused by dissolution of crystals and water in mortar after water is added and stirred is reduced as much as possible; secondly, the influence of the temperature on the solubility of the crystal is utilized as much as possible when the crack volume occupied by the crystal is replaced, and the influence is used as a judgment basis for judging whether the crystal is completely replaced into the crack.

The process of manufacturing the crack is divided into a preparation and maintenance stage of a sample, a prefabricated crack water treatment stage, a prefabricated overflow crack forming detection stage and a crack content and integrity engineering detection index conversion stage, and the preparation method comprises the following steps:

(1) the proportion of L-5 determined by the above test: and (3) uniformly stirring cement, sand and talcum powder in a ratio of 1:2.5:2, adding a sodium glutamate solution with a mass corresponding to a water-cement ratio of 1:2, fully stirring, adding sodium glutamate crystals with a mass corresponding to the calculated porosity, and uniformly stirring. The sample size mould required by the single-shaft and three-shaft test is loaded in a layering mode, the upper surface of the sample is scraped and loaded after the material is loaded in a layering mode, the standard cylindrical sample is radially additionally provided with throat hoop restraint, the rectangular sample is fixed through steel plates all around, and the upper surface of the sample is covered with the steel plates to prevent the upper surface from bulging.

(2) After the test piece is maintained in air for 28 days, the test piece is placed in a flat-bottom metal container, water is added until the upper surface of the test piece is 8-10 cm, then the metal container is heated until the water body is boiled, the heating is continued for 2-3 minutes, the test piece is turned upside down after the water body in the container is cooled to room temperature, the test piece is always below the liquid level in the inversion process, when the liquid level in the container is greatly reduced in the heating process, clear water is additionally injected into the container and is heated again, after the water body is boiled and cooled to room temperature again, the test piece is taken out of the water and is placed in a vacuum saturated container prepared in advance, clear water is injected into the saturated container in advance, the negative pressure is controlled to be 0.8MPa, the vacuum is saturated for 48 hours, the water treatment process is repeated for three times, and the inner wall of the container is cleaned after the flat-bottom metal container and the vacuum saturated container are required to be operated each time.

(3) And in the water passing crack forming detection stage, the water body in the last vacuum saturation device is taken out, filtered by filter paper, poured into a clean vessel to be heated and evaporated after solid impurities are filtered out, water in the vessel is considered to be formed in the test piece if no crystal is separated out, the water passing crack with uniform distribution and more uniform geometric dimension is considered to be formed in the test piece, and the water treatment stage operation is repeated if a large amount of crystals are separated out.

While the foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.