Research on rock immersion time scale and deterioration prediction model under the action of acidic solution

Abstract:

In order to study the influence of water-rock interaction on the deterioration of rock mass, the soaking effect of the sample in a solution with a low pH value is not obvious, the soaking time is long, and the Time immersion has a greater impact on the results. Taking the sandstone on the bank slope of the Three Gorges Reservoir area as the research object, based on the erosion effect of chemical solutions with different pH values ????on the rock, the issue of the immersion time scale of the rock and the strength model after immersion damage were developed. Research. Using the amount of H substance as an indicator, a high-concentration sulfuric acid solution is used to simulate the reaction speed of a low-concentration solution in a short period of time and the chemical erosion effect of a low-concentration solution in a longer period of time. At the same time, the influence of the immersion time period on the sandstone samples was analyzed, and the relationship between the sandstone corrosion immersion depth and the immersion time was obtained. The results show that when the number of fittings reaches 4th order, the fitting curve has basically coincided with the data points, indicating that it is feasible to use the reaction of high-concentration sulfuric acid solution to simulate the reaction of low-concentration acid solution and rock mass; for lower pH value, the changes in the amount of H substance calculated on an hourly basis appear to lack regularity; as the immersion time increases, the H reaction surface gradually deepens from the surface to the inside, and the immersion path is also continuously lengthened, resulting in a gradual increase in the corrosion immersion rate. slow. The accuracy verification analysis of the strength parameters of sandstone on the rock deterioration damage model shows that the prediction model can better reflect the changing trend of the strength of the rock mass after deterioration.

Keywords:

Deterioration; chemical solution; immersion; time scale; damage model;

About the author:

Zhou Jifang (1980—), male, senior engineer, Ph.D., mainly engaged in water conservancy, hydropower and geotechnical engineering research. E-mail: zhoujifang@ylhdc.com.cn;

Funding:

National Natural Science Foundation of China (U1765206);

Citation:

Zhou Jifang． Research on rock immersion time scale and deterioration prediction model under the action of acidic solution [J]. Water Conservancy and Hydropower Technology (Chinese and English), 2021, 52(8): 162-171.

ZHOU Jifang. Study on prediction model for rock immersion time scale and deterioration under effect of acidic solution［J］. Water Resources and Hydropower Engineering, 2021, 52(8): 162-171.

Surface or groundwater and other chemicals During the migration process, the solution continuously undergoes physical, mechanical and chemical interactions with the surrounding rock mass, which weakens the bonding ability between mineral particles in the surrounding rock mass and changes the material composition and pore structure. Under this long-term water-rock interaction, the rock mass The further expansion of defects within the body will lead to the gradual deterioration of the physical and mechanical properties of the rock, bringing huge hidden dangers to the long-term stability of the surrounding rock and slopes.

In recent years, many scholars at home and abroad have conducted a series of studies on the environmental damage to rock immersion from the perspective of chemical mechanisms. KUVA et al. used chemical solutions to soak rocks for different times, and combined X-ray tomography and SEM electron microscopy to find out the micro-pore structure of the rock, which provides a basis for further analysis of the microscopic immersion damage of rocks; TIWARI et al. used fluid penetration of different solutions Soil, the relationship between the shear residual strength of deteriorated soil, chemical composition, and plastic deformation was obtained. The research method can provide a reference for the immersion damage of rock samples. Many scholars have also conducted a large number of immersion damage studies on the physical and mechanical properties and response mechanisms of rocks after immersion, and many results provide an important research basis for the development of rock mechanics.

Among them: Fu Yan, Liu Xinrong, etc. used uniaxial, triaxial compression and electron microscope scanning to analyze sandstones with acidic dry-wet cycles, and concluded that the deterioration of cohesion c due to acidic dry-wet cycles is greater than that of internal friction. This result provides a theoretical basis for slope management; Wang Lunan, Yu Jin et al. tested the physical properties and microstructure of siltstone after soaking in different chemical solutions and freeze-thaw cycles, and obtained the chemical The long-term attenuation law of siltstone strength under the coupling effect of corrosion and frost heave load, and established a prediction model that can reflect the nonlinear attenuation characteristics of siltstone strength; Wang Sijing et al. used the large mode and small mode theory of water-rock reaction, It explains why severe deformation and destruction of rock masses in nature often occur during heavy rain or human engineering activities. Previous researchers have studied the microscopic and macroscopic changes in rocks after immersion through electron microscopy and mechanics. The results have important research value and provide effective means for studying rock immersion damage.

In terms of chemical damage model research, Li Ning et al. used solutions with different pH values ??to perform immersion damage on calcareous cemented feldspathic sandstone, and studied the weakening of the mechanical properties of sandstone containing calcareous cement in an acidic environment. problem, and established a sandstone damage model from the perspective of chemical kinetics. Huo Runke et al. studied the physical and chemical properties of acid-corroded sandstone through long-term accelerated corrosion indoors, and established an acid-rock reaction kinetic model to provide a reference for engineering durability evaluation in acidic environments. By performing triaxial creep on damaged red sandstone, Wang Wei et al. proposed a sandstone rheological damage constitutive model that considers water chemistry based on the Kelvin model. This model can well explain the linear and linear processes in the rock rheology process. nonlinear characteristics. Deng Huafeng [21] established a sandstone damage constitutive model segmentally based on the characteristics of the triaxial compressive stress-strain curve of sandstone during the water-rock interaction process, describing the mechanical properties of sandstone at different stages. The research results are stable for long-term deformation of the reservoir bank slope. Analysis provides a reference. At present, although there have been many results on the impact of chemical solution immersion on rock mechanical properties and related research, most of them focus on the impact of chemical solution immersion damage on rock mechanical properties and the study of damage constitutive models. However, there are few studies on the interference of the time effect of immersion damage on the process and results, which cannot fully reflect the impact of the time scale of immersion in different concentrations of chemical solutions on rock degradation, especially the impact of the length of immersion time on data accuracy.

In order to solve the problems of insignificant immersion effect of samples in solutions with lower pH value and greater impact on the results during long-term immersion, this paper studies the immersion of sandstone in sulfuric acid solutions with different pH values. The reaction speed of low-concentration sulfuric acid solution in a short period of time (characterized by the change in the amount of H substance) and the chemical erosion effect of low-concentration solution in a longer period of time, and verified the feasibility of the method, summarized the effect of acidic solution Understand the time scale effect of rock immersion, establish the functional relationship between the sandstone corrosion immersion depth and the immersion time, and the relationship model between the elastic modulus and the immersion time within a period of time after the sandstone immersion damage, and use the strength parameters of the sandstone to accurately predict the model Conduct verification analysis.

1.1 Instruments

Taking the RMT-150C rock mechanics system developed by the Wuhan Institute of Rock and Soil Mechanics, Chinese Academy of Sciences as the main platform (see Figure 1), this equipment can perform uniaxial compression, Triaxial compression, direct shearing and indirect stretching (Brazilian method) are all controlled by servo control. This article mainly uses this platform for uniaxial compression. The LC-pHB-1M digital display pH meter (as shown in Figure 2) is used to measure the pH values ??of four acidic solutions before and after the rock is soaked. It is a pH meter with dual display of temperature and pH controlled by a single-chip microcomputer. It is suitable for industry, mining, and research. The laboratory of the institute takes samples to measure the acidity (pH) value of the aqueous solution and measures the electrode potential (mV) value.

1.2 Rock sample properties and solution preparation

Taking the sandstone collected from the reservoir bank slope of the Badong part of the Three Gorges Reservoir area as the research object, the rock samples were processed into 54 mm along the same direction. For a 100 mm cylinder, the rock sample diameter error is 0.3 mm, and the end surface unevenness error is 0.05 mm. The rock is relatively dense and contact-cemented (higher particle content and lower cement content). The clastic materials in the rock are mainly quartz, dolomite, feldspar, rock debris and biotite. The grain size is between 0.1 and 0.3 mm, and the sand grains are mainly 0.15 and 0.2 mm. It belongs to medium-fine grained sandstone. Its single polarization and cross polarization are shown in Figure 3.

In order to shorten the time and eliminate the interference of other factors, a sulfuric acid solution with a lower pH value is used for rock soaking. The initial solution pH is pH=1, 2, 3, and 4 respectively, and the soaking period is 1 h and 12 h is the period for measurement, and the measured values ??include the instant pH value of the solution and the diameter of the sample. The main reasons for using 12 h as a period during the soaking period are: (1) When the measurement period is small, the difference in measured data is not obvious, making it difficult to compare and analyze effective data; (2) The data density with a 12 h period is sufficient to reflect the H substance Quantity changes over time.

The water-rock chemistry in the Three Gorges Reservoir area mainly changes the surface characteristics of slope rocks through chemical dissolution. The soaking solution used is a sulfuric acid solution with a low pH value. The amount of consumed H material is used as By establishing the conversion standard and establishing the corresponding relationship between the two, the time scale can be used to convert between the two, that is, the time required for each under the condition of consuming the same amount of H substance. At the same time, soaking can effectively shorten the required time and obtain a curve covering pH=0-7 in a short time. By simulating a long time in a short time, it can not only save time, but also effectively reduce temperature changes and vibrations in the process. The influence of external factors such as disturbance on the test results makes the results more accurate and reliable.

2.1 Measurement ideas

(1) List a relative time scale. The amount of hydrogen ion substances consumed per day in a solution with pH=1 is γPH=1, and the amount of hydrogen ion substances consumed per day in a solution with pH=4 is γPH=4. The time scale between the two is λ= γPH=1/ γPH=4, that is, the reaction effect of a rock sample soaked in a pH=1 solution for one day is equivalent to the reaction effect of a rock sample soaked in a pH=4 solution for λ days.

(2) Compare the changes in the amounts of different H substances at different pH values ??in the same reaction curve. The ratio of the changes in the amounts of different H substances is the time scale ratio. The selection of the measurement period and the determination of the number of measurement points are key.

2.2 Selection of measurement period and determination of the number of measurement points

2.2.1 Taking 1 h as the measurement period

As shown in Figure 4, each two data The measurement interval between points is 1 h, and polynomial fitting is used. It can be seen from Figure 4: (1) When the number of fittings reaches 4th order, the fitting curve has basically coincided with the data points, and the fitting coefficient of determination R2 reaches 0.95. The fitting reliability is very high, indicating that high concentration of acid is used. It is feasible to simulate the reaction of low-concentration acid solution and rock mass through the reaction of the solution; (2) For lower pH values, the changes in the amount of H substances calculated on an hourly basis appear to lack regularity. As it goes deeper, the sulfuric acid solution with higher concentration reacts with dolomite, calcite and other substances in the sandstone sample to form water. After the sulfuric acid solution with higher concentration is ionized, it forms stable hydrated H with water, which causes The changes in the amount of H substances at lower pH values ??lack regularity.

Therefore, the measurement time period and measurement period must be selected according to actual conditions. Next, we will consider increasing the measurement time period so that disturbances caused by other factors will not obscure the amount of H substances at different pH values. The evolution pattern of changes over time.

2.2.2 Taking 12 hours as the measurement period

In the fitting process, values ??greater than 0 are first removed. Based on the observation of the fitting curve, in order to meet the objective rules: As the pH value increases, that is, as the H concentration in the solution decreases, the amount of change in the amount of H species will decrease.

Polynomial fitting—the value is too large, and the rounding error during fitting is too large, so the following uses polynomial fitting and logarithm first and then linear fitting (see Figure 5 and Figure 6), compare the two analysis effect.

It can be seen from Figure 5 and Figure 6: (1) If the measurement period is hours, it is advisable to use a solution with pH=4 or a higher pH value. The fitting curve at pH=3 is relative to pHgt. ;5 When predicting a solution, the change in the amount of H material obtained is opposite to the actual (qualified rule). When the pH is lower, it is difficult to obtain a curve consistent with the identified rule through fitting. (2) Using a higher concentration can simulate the reaction rate of low concentration (characterized by the change in the amount of H substance) in a shorter time and the chemical erosion effect of low concentration solutions in a longer period of time. The closer the concentration of the two solutions is, the better the simulation effect will be. The greater the concentration difference will bring certain errors in the simulation, which is related to the disturbance of other factors in the reaction and the premise assumptions of this simulation.

2.3 Comprehensive comparative analysis of calculation and prediction of different pH values

According to the above time scale method, the corresponding pH prediction values ??were obtained for the measured values ??of 4 different pH values, and a comprehensive comparison was made. analysis (see Figure 7). It can be seen from Figure 7 that this prediction method is reliable and reasonable under certain conditions. When the pH is 2, 3, and 4, the predicted value and the measured value are in good agreement. Among them, the measured value of pH=4 and the predicted value corresponding to pH=3 The difference between the two is only 0.14, which means that the prediction accuracy of this method is very high at lower concentrations; however, the predicted value at pH=1 deviates greatly from the actual measured value, with the maximum difference being 1.59, indicating that this method is used in high-concentration acid solutions. The accuracy of forecasts decreases. This also confirms the conclusion that the greater the concentration difference, the greater the prediction deviation.

It is assumed that the secondary mineral clay particles produced during the reaction will not block the existing through-holes, thus reducing the reaction speed. In actual reactions, the reduction in reaction surface dS caused by this blockage is very small relative to the larger surface area S of the specimen, that is,

Therefore, this assumption is true. Through the analysis of the above content, it can be concluded that the amount of H substance consumed by the reaction rate within the selected time period is proportional to time, that is,

In the formula, t represents time; C is a constant.

The above time scale relationship can not only be applied in acid solutions, but also can be applied to the immersion of rock masses in alkaline solutions or other chemical reagents after some approximate processing. For acid solutions, this method is suitable for situations where the main factor in rock quality deterioration during the immersion process of the rock mass is the reaction between the minerals of the rock mass and the hydrogen ions in the solution.

3.1 Analysis of Soaking Phenomenon

The sandstone used in the test is medium-fine-grained sandstone. The substances that react with the sulfuric acid solution during the soaking process are mainly calcite and dolomite. The reaction equation As follows

After the reaction between sandstone and sulfuric acid solution, the products adhere to the surface and slow down the reaction speed. Because the reaction products are mostly "porous" and "lamellar" (see Figure 8 and Figure 9), so that the solution can still react further with the internal rock mass. As the reaction proceeds, the pH value of the solution gradually decreases, indicating that the reaction proceeds gradually from the surface to the inside, and the reaction speed gradually attenuates. .

During the process of acid immersion, the connection state between mineral particles is usually changed through erosion. If some mineral particles, especially cements, are mainly composed of carbonate, the above-mentioned cementation type will change from filling cementation to contact cementation, or the cement will deteriorate or disappear.

If the mineral composition of the rock mass is mainly carbonate or is highly soluble in water (such as calcareous sandstone), the overall collapse and granulation of the specimen will occur after immersion erosion, achieving a rapid transformation from rock to soil.

3.2 Calculation of sandstone corrosion immersion depth

During the test, the diameter D of the immersed sample was measured regularly with a vernier caliper. During each measurement, 3 or 4 measuring points were selected for measurement, and then Calculate the average value: D is the sum of the diameters of 4 measurements, and D/4 is the corrosion immersion depth corresponding to the 4 measurement periods. Through fitting analysis of the data, it can be found that the change of corrosion immersion depth with time conforms to the following formula

In the formula, a and b are coefficients, which are parameters determined by the rock properties, temperature and other factors. For this sample, a = 0.391 and b = 0.154.

Figure 10 shows the change curve of corrosion immersion depth with time. It can be seen that the corrosion depth increases rapidly in the early stage of immersion, and as the immersion time increases, the growth of corrosion immersion depth gradually slows down. This is because: (1) as the immersion time increases, the amount of H substance in the solution gradually decreases, and the pH value of the solution gradually increases, causing the corrosion immersion rate to gradually slow down; (2) before the end of the chemical reaction, the reaction surface Gradually deepening from the surface to the inside, the H immersion path is also continuously lengthened, making the corrosion immersion depth gradually slow down.

4.1 Division of chemical damage model

Based on the analysis of the phenomenon, the rock mass is divided into three regions (see Figure 11).

4.1.1 Shedding area

The shedding area is the surface layer of the specimen, which is detached from the specimen after erosion (automatically detaches or peels off after being subjected to slight external force), and no longer acts as a load-bearing skeleton. . Assume that all substances in this region that can react with hydrogen ions have reacted.

Determination of the volume of the fallen rock sample: corresponding to the pH value test per unit time, use a vernier caliper to measure the diameter of the specimen, and measure the change in diameter of the specimen? The change pattern with time, which can be obtained after conversion The variation pattern of rock sample shedding volume with time.

4.1.2 Progressive damage zone

The progressive damage zone is adjacent to the shedding zone, but the cement is only partially dissolved. This area can be used as part of the load-bearing frame. If the specimen is small, has many holes, and has a large damage rate connected to the surface, it is considered that the progressive damage area of ??the specimen runs through the core of the specimen, and the specimen only consists of two parts: the shedding zone and the progressive damage zone. The chemical damage in this area is gradual, and the farther away from the reaction surface, the smaller the damage. The consumption of hydrogen ions in this area is

In the formula, H0 is the number of hydrogen ions contained in the solution measured and calculated with a pH meter before the specimen is immersed in the reaction; Ht is when the specimen is soaked for t. Use a pH meter to measure and calculate the number of hydrogen ions contained in the resulting solution.

The volume of dissolved cement in this area is

4.1.3 Damage-free zone

The damage-free zone does not cause any chemical damage and is an integral part of the load-bearing skeleton. one. If the specimen is large enough or the surface product structure of the specimen is dense (such as a limestone specimen), which can prevent further reaction of hydrogen ions with the internal rock mass, then there may be an area inside the specimen that is not corroded by the acid solution. Called the lossless zone.

4.2 Prediction of model elastic modulus after immersion

The elastic modulus of rock mass is closely related to the rock mass components, internal structure and stress state. Hypothesis: rock samples are regarded as two-phase materials, composed of rock matrix and pores; the elastic modulus of the sample is positively correlated with the matrix bearing area; chemical corrosion results in a decrease in the matrix bearing area and an increase in the sample porosity , so the elastic modulus of the sample will become smaller as the porosity increases.

Define the percentage of residual damage modulus as the relative value of the elastic modulus before and after the rock is soaked and damaged, that is,

In the formula, Et is the elastic modulus of the sandstone after damage; E0 is the undamaged sandstone of elastic mold.

Zhao Yang equated rocks into a combination of parts with different properties. After theoretical derivation, he concluded that the relationship between the relative elastic modulus value and porosity is

In the formula, nρ is Sandstone porosity sandstone sample; χ is relative elastic modulus.

Soaked in a sulfuric acid solution with a certain concentration, the H in the solution reacts with the cement in the sample, causing the porosity of the sandstone sample to increase; at the same time, as the soaking time increases, the sandstone The immersion corrosion process of the sample gradually deepens from the surface to the inside (see Figure 12), and the corresponding load-bearing cross-sectional area of ??the corrosion zone changes with time as follows

In the formula, R (0) is the sandstone sample without Radius before damage; R(t) is the corrosion immersion depth of the sandstone sample.

Because the pore volume of the sample after immersion is proportional to the volume of the acid corrosion shedding zone hA(t), where h is the height of the sample, then there is

where, γ It is the product of a proportional constant and the height h of the specimen, and is obtained by fitting the regression equation to the obtained data.

Putting formula (8) into formula (6), we get

Formula (15) approximately expresses the relationship between the elastic modulus of the sample and time under the action of sulfuric acid solution.

Calculation example: For a sulfuric acid solution with pH=2, the height of the sandstone sample is h=100 mm, R(0)=27 mm, γ=10, and the initial elastic modulus E0=41.5 GPa. Find Obtain a1=5.75 10-6, a2=-7.94 10-4, a3=-3.13 10-5, b1=3.13 10-5, b2=-4.33 10-3, b3=-1.71 10-4, convert the above coefficients The corresponding degradation Et value can be calculated by substituting the soaking time into equation (15). The corresponding calculation prediction results are compared with the measured data as shown in Figure 13.

It can be seen from Figure 13 that the calculated values ??are generally higher than the measured values. The lower the measured final value, the greater the proportion. This is mainly because the deterioration effect of water on the rock mass is not considered in the calculation formula. However, the changing trends are roughly the same, indicating that the model has certain rationality and applicability. In the early stage of immersion corrosion, the strength of the sandstone sample decreased rapidly. As the immersion time increased, the strength of the sandstone gradually remained stable.

5 Conclusions

(1) A measurement method is proposed to use the change in the amount of H substance to characterize the time scale of chemical reaction in rock mass, and it is verified that using a higher concentration can The feasibility of simulating the reaction speed of low concentration in a short period of time and the chemical erosion effect of low concentration solution in a longer period of time.

(2) Use the time scale λ= γPH=1/ γPH=4 method to predict each other for solutions of different concentrations, and compare and analyze the predicted values ??and actual measured values ??to obtain: the closer the concentrations of the two solutions The better the simulation effect, the greater the difference will bring certain errors in the simulation, which is related to the disturbance of other factors in the reaction and the premise assumptions of this simulation.

(3) Analyze the morphology of the sandstone surface after soaking and point out that the reaction products between sandstone and sulfuric acid solution are mostly "porous" and "lamellar" structures, and the products can be attached to the surface. It plays the role of slowing down the reaction speed and provides a basis for the establishment of reaction partitions and equations in chemical damage of rock samples.

(4) The damaged rock samples were divided into areas - shedding area, progressive damage area and non-damage area, and a prediction model of elastic modulus after immersion was established. Through calculation and analysis of the prediction model, it was concluded that: after immersion In the early stage of corrosion, the strength of the sandstone sample decreased rapidly. As the immersion time increased, the strength of the sandstone gradually remained stable.

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