Rapid evaluation method for water holding capacity of rock and soil medium

Abstract

The invention discloses a rapid evaluation method for the water holding capacity of a rock-soil medium. The rapid evaluation method comprises the following steps: acquiring the relationship between the suction force and the aperture of the rock-soil medium; establishing a rock-soil medium aperture probability density distribution function by measuring the aperture density distribution characteristics of the rock-soil medium; obtaining the relationship between the moisture content of the rock-soil medium and the pore size distribution; the relation between the rock-soil medium suction force and the pore diameter and the relation between the rock-soil medium water content and pore diameter distribution are combined, the relation between the rock-soil medium water content and the suction force is established with the pore diameter as a bridge, and therefore the water holding curve of the rock-soil medium is obtained. According to the method, the water holding capacity of the rock-soil medium is evaluated from the micro-pore angle, a statistical model considering the pore nonuniformity of the rock-soil medium is provided, the saturation degree is deduced from the pore diameter of the statistical model, the capillary water holding mechanism of liquid water and the adsorption water holding mechanism of vaporous water are considered, and the water holding capacity of the rock-soil medium is evaluated. And the evaluation method better conforms to the real water holding characteristic of the rock-soil medium.

Description

Technical Field

[0001] This invention relates to the fields of energy extraction and storage, pollutant migration and ecological restoration, and in particular to a rapid evaluation method for the water-holding capacity of soil and rock media. Background Technology

[0002] The assessment of water-holding capacity in soil and rock media plays a crucial role in energy extraction and storage, pollutant migration, and ecological restoration. For example, in shale gas and coalbed methane extraction, moisture affects fracture conductivity, gas desorption efficiency, and fracturing fluid flowback effectiveness; in geothermal energy utilization, water content determines the thermal conductivity and heat capacity of the formation and facilitates fluid circulation and heat extraction; in carbon sequestration and hydrogen storage, rock formations with high water-holding capacity can serve as low-permeability capping layers, enhancing storage security; simultaneously, water-holding capacity also affects soil moisture environment and pollutant migration processes in ecological restoration, making it a key factor in various energy development and environmental protection processes.

[0003] Currently, the evaluation of the water-holding capacity of soil and rock media mainly relies on borehole sampling and indoor suction and water content tests. Soil-water characteristic curves are obtained based on empirical formulas such as the Van Genuchten model and the Frendlund & Xing model. However, these evaluation methods are generally costly, time-consuming, have discrete data, limited application scope, and lack corresponding physical meaning. They are also unable to evaluate the various water-holding mechanisms of liquid water and gaseous water. Summary of the Invention

[0004] To address the problems of high cost, long time consumption, discrete data, limited application scope, lack of corresponding physical meaning, and inability to evaluate various water-holding mechanisms of liquid and gaseous water in existing methods for evaluating the water-holding capacity of soil and rock media, this invention provides a rapid evaluation method for the water-holding capacity of soil and rock media, aiming to solve the above-mentioned problems and defects.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A rapid evaluation method for the water-holding capacity of soil and rock media includes the following steps:

[0007] S1. Obtain the relationship between suction and pore size of soil and rock media, including the relationship between capillary suction and large pore size above 50 nm, and the relationship between adsorption suction and mesoscopic pore size of 2 to 50 nm.

[0008] S2. By measuring the pore size density distribution characteristics of the soil and rock medium, establish the pore size probability density distribution function of the soil and rock medium;

[0009] S3. Obtain the relationship between the water content and pore size distribution of the soil and rock medium, including the relationship between the water content and pore size distribution of liquid water and the relationship between the water content and pore size distribution of gaseous water;

[0010] The relationship between the water content of the liquid water and the pore size distribution is as follows:

[0011]

[0012] The relationship between the water content of gaseous water and the pore size distribution is as follows:

[0013]

[0014] In the formula, θ c Let θ be the water content of liquid water. a Let r be the water content of gaseous water and r be the pore size. f(r) is the probability density distribution function of pore size in soil and rock media; These represent the maximum and minimum pore sizes in soil and rock media, respectively. This represents the minimum mesoscopic pore size in soil and rock media.

[0015] S4. Combine the relationship between suction and pore size of the soil and rock medium obtained in step S1 with the relationship between water content and pore size distribution of the soil and rock medium obtained in step S3, and establish the relationship between water content and suction of the soil and rock medium with pore size as the bridge to obtain the water holding curve of the soil and rock medium.

[0016] The relationship between the water content of liquid water and the suction force is as follows:

[0017]

[0018] The relationship between the water content of gaseous water and suction is as follows:

[0019]

[0020] In the formula, ψ c For capillary attraction, ψ a For adsorption suction, These represent the maximum and minimum values ​​of capillary suction in soil and rock media, respectively. f(ψ) represents the maximum adsorption suction force in the soil and rock medium. c ), f(ψ a All of these are probability density distribution functions of pore size in soil and rock media.

[0021] Furthermore, in step S1, the relationship between the capillary suction and the large pore size of 50 nm or more is as follows:

[0022]

[0023] In the formula, ψ c For capillary suction, T m For surface tension, α mIt represents the contact angle.

[0024] Further, in step S1, the relationship between the adsorption force and the mesopore size between 2 and 50 nm is as follows:

[0025]

[0026] In the formula, ψ a For adsorption attraction, ψ sorp For adsorption potential energy, ψ vdw For van der Waals force, ψ ele electrostatic potential, ψ osm For the osmotic potential, ψ hyd Let x be the hydration potential, and x be the distance between the water molecule and the soil particle, where:

[0027]

[0028] In the formula, ψ hyd0 λ is the water content on the surface of soil and rock particles. s The decay length of the surface hydration component is 0.2-1.0 nm; A H Let ε0 be the Hamek constant, L and t be the width and thickness of the clay particles, respectively; ε0 be the vacuum volume fraction, and εt be the lattice volume fraction. r Where is the dielectric constant, E(x) is the electric field strength, c0 is the salt concentration, and N is the electric field strength. A K is Avogadro's constant. B Where is Boltzmann's constant, T is the absolute temperature, e is the electron charge number, and V is the electron charge. edl (x) represents the electric potential intensity;

[0029] Among them, V edl0 The surface potential;

[0030] Where k is the thickness of the electrical double layer, α is the surface potential function.

[0031] Further, in step S2, the pore size probability density distribution function of the soil and rock medium is:

[0032]

[0033] In the formula, f(r) represents the differential intrusion amount corresponding to pore size r, and N is the main pore size r. i The quantity, R i The main pore diameter r i The corresponding pore volume fraction; m i The main pore diameter r i The corresponding unitless aperture spectral number is 0. <m i <1, Represents the main pore diameter r i The corresponding maximum pore radius.

[0034] Compared with the prior art, the present invention has the following advantages:

[0035] This invention evaluates the water-holding capacity of soil and rock media from the perspective of micropores and proposes a statistical model that considers the non-uniformity of pores in soil and rock media. It provides a beneficial supplement to the statistical model in terms of pore size derivation of saturation. By incorporating the influence of adsorption potential energy, it improves the prediction of the high-adsorption segment of water-holding capacity, and the model is simple and convenient. This invention differs from other empirical methods or water-holding curve methods obtained by fractal theory. In this model, by incorporating a probability density distribution function, the discrete pore size distribution curve and the water-holding curve become continuous. It considers various pore distributions in soil and rock media, with clear physical meaning and simple parameters. It also considers the capillary water-holding mechanism of liquid water and the adsorption water-holding mechanism of gaseous water, which is more consistent with the actual water-holding characteristics of soil and rock media. Attached Figure Description

[0036] Figure 1 This is a flowchart of a rapid evaluation method for the water-holding capacity of soil and rock media according to an embodiment of the present invention;

[0037] Figure 2 This is a schematic diagram of fitting the probability density distribution function of pore size in soil and rock media according to an embodiment of the present invention;

[0038] Figure 3 This is a comparison chart of the soil-water characteristic curves of the soil and rock medium derived from the embodiments of the present invention and the measured curves. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0040] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] It should be understood that, when used in this specification and the appended claims, the terms "comprising" and "including" indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0042] It should also be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0043] It should also be further understood that the term "and / or" as used in this specification and the appended claims refers to any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0044] refer to Figure 1 A rapid evaluation method for the water-holding capacity of soil and rock media includes the following steps:

[0045] S1. Obtaining the relationship between capillary suction and large pore size in soil and rock media.

[0046] In soil and rock media, pores with a diameter greater than 50 nm are called large pores. Such pores are classified as capillary pores. The relationship between capillary pore size and capillary suction can be obtained using the Yang-Laplace equation:

[0047]

[0048] In the formula, ψ c For capillary suction, T m For surface tension, α m θ is the contact angle, and r is the aperture.

[0049] S2. Obtain the relationship between the adsorption suction of soil and rock media and the mesopore size between 2 and 50 nm.

[0050] In soil and rock media, pore sizes between 20 and 50 nm are called mesopore sizes. Such pores belong to capillary pores. The relationship between adsorption pore size and adsorption attraction can be obtained using the physical equation of adsorption potential energy:

[0051] Adsorption potential energy is expressed as a function of soil and rock properties and spatial coordinates. The attractive force between the soil / rock medium and water molecules can be generated through several physical mechanisms, namely van der Waals attraction, electric double layer, osmotic potential, and hydration potential. Therefore, adsorption potential energy can be described by four components:

[0052] ψ sorp (x)=ψ hyd (x)+ψ vdw (x)+ψ ele (x)+ψ osm (x)

[0053] Where, ψ sorp For adsorption potential energy, ψ vdw For van der Waals force, ψele electrostatic potential, ψ osm For the osmotic potential, ψ hyd Let x be the hydration potential, and x be the distance between the water molecule and the soil particle.

[0054] If the adsorption potential energy at a certain pore size is taken as the average adsorption potential energy, then the relationship between the adsorption attraction and the pore size is as follows:

[0055]

[0056] Where, ψ a It is the adsorption force;

[0057] The magnitudes of each adsorption potential energy component satisfy the following formulas:

[0058]

[0059] Where, ψ hyd0 λ is the water component on the particle surface. s The decay length of the surface hydration component is 0.2–1.0 nm.

[0060]

[0061] Among them, A H Let be the Hamek constant, and L and t be the width and thickness of the clay particle, respectively.

[0062]

[0063] Where ε0 is the vacuum permittivity, ε r Let E(x) be the dielectric constant and E(x) be the electric field strength.

[0064]

[0065] Where c0 is the salt concentration, e is the electron charge, and k B Where is Boltzmann's constant, T is absolute temperature, and V is... edl0 N is the surface potential. A It is Avogadro's constant;

[0066]

[0067] Among them, electric potential intensity Electrical double layer thickness function of surface potential

[0068] Based on a review of the literature, the parameters of each component of the above-mentioned adsorption force are shown in the table below:

[0069] Table 1: Parameters of Adsorption Force Components

[0070]

[0071] S3. By measuring the pore size density distribution characteristics of discrete soil and rock media, a continuous soil and rock media pore size probability density distribution function f(r) is established.

[0072] The example of this invention uses loess from Heifangtai. A saturated sample with a density of 1.35 is used for MIP testing. A prepared saturated soil block sample with a diameter of 1 cm and a height of 1.5 cm is placed in liquid nitrogen to instantly freeze the water-containing sample below the freezing point. Then, it is placed in a low-temperature vacuum dryer to convert the water in the soil sample into water vapor in a vacuum. The water vapor is then removed by the condenser in the dryer. The dried sample is then subjected to MIP mercury intrusion porosimetry to process the data and calculate the soil pore size density distribution.

[0073] To enhance the accuracy of the statistical model, the discrete pore size distribution measured experimentally is represented using a continuous mathematical model. Generally, the pore size distribution measured in soil is multimodal and irregular in shape, making it difficult to describe using a suitable probability density function. Existing methods often use basic probability density functions such as normal distribution and chi-square distribution for simple and general summaries of soil pore size distribution, which are often not very accurate and cannot represent multimodal distributions. Therefore, to accurately describe the pore size distribution of soil, this embodiment of the invention adopts a general analytical expression for pore size distribution based on probability theory, decomposing the multimodal pore size distribution into a multimodal mathematical probability density function. The established probability density distribution function for soil and rock media pore size is as follows:

[0074]

[0075] The aperture distribution function f(r) is used to represent the continuous function of the measured Differential Intrusion, which can be obtained from the Differential Intrusion-Pore size Diameter. V i and V tot These represent the main aperture r. i The corresponding pore volume and total pore volume, therefore R i Represents the main aperture r i The corresponding pore volume fraction; m i (0 <m i <1) represents the main aperture r i The dimensionless pore size spectral number corresponding to the pore size; Represents the main aperture r i The maximum pore radius corresponding to the pore size; N represents the main pore diameter r i Quantity;

[0076] Express the pore size *r* on a logarithmic scale, and the pore size density function in dimensionless form. Then we have:

[0077] ∫0 +∞ ω(r)dlogr=1

[0078] Here, ω(r) is defined as a dimensionless aperture density function. Therefore, the relationship between f(r) and ω(r) can be obtained:

[0079] ω(r)=(ln10)rf(r)

[0080] Therefore, we have:

[0081]

[0082] Using the dimensionless form ω(r), the cumulative pore size distribution curve of multimodal soils can be described by the following equation:

[0083]

[0084] Using a simple mathematical transformation:

[0085]

[0086] Where s * If is the variable representing the reciprocal of the aperture diameter r, then:

[0087]

[0088] in r represents the cumulative volume corresponding to each aperture. j The aperture size can be obtained from the aperture distribution curve;

[0089] The parameters of the aforementioned pore size probability density distribution function f(r) for soil and rock media can be determined by the cumulative pore volume measured using the above formula. This allows the experimentally measured discrete pore size distribution to be expressed using a continuous mathematical model. After parameter calibration, the fitting parameters for the pore size probability density distribution function f(r) are obtained.

[0090] Combining the adsorption pore size and the capillary pore size yields the pore size density distribution across the entire pore size range, with the following parameters:

[0091] Table 2: Parameter values ​​of the aperture probability density distribution function in the examples

[0092]

[0093] The fitted image of the pore size probability density distribution function f(r) of the soil and rock medium established in this embodiment is as follows: Figure 2 As shown.

[0094] S4. Obtain the relationship between water content and pore size distribution in soil and rock media.

[0095] Because the inherent pore size of soil and rock media is discontinuous, the water-holding curve obtained by directly using statistical models in the capillary pore size range will have a large error compared with the actual measured water-holding curve. Therefore, it is necessary to establish a new water-holding model that takes into account the inhomogeneity of soil and rock media.

[0096] The internal pore structure of soil and rock media consists of many large pores surrounded by smaller pores. This pore structure, due to its non-uniformity, affects the water-holding capacity of the soil and rock media. Since capillary pores are relatively large, during the drying and water loss process of the soil and rock media, these capillary pores can retain moisture even under suction forces exceeding the corresponding suction strength, thus affecting the water-holding capacity of the soil and rock media.

[0097] During the drying process of soil and rock media, under a given suction force, the water in the pores of the soil and rock media can be considered to consist of two parts. The first part is the water in the pores of the soil and rock media corresponding to the suction force, and the water content of this part can be expressed as:

[0098]

[0099] The second part is the water content in the pores of the soil and rock medium with a pore size equal to or smaller than the corresponding radius. The water content of this part can be expressed as:

[0100]

[0101] Where A(r) is the total probability of pores with a diameter r in the interval (0,r) when the matrix suction is s during the drying process;

[0102] The total probability of blockage in this part of the capillary pores can be expressed as:

[0103]

[0104] Where f(r) is the aperture probability density distribution function.

[0105] Corrected capillary liquid water content after taking into account soil porosity heterogeneity:

[0106]

[0107] For pore gaseous water caused by adsorption potential energy, soil pore heterogeneity is not considered, and its water content is expressed as:

[0108]

[0109] In the formula, θ cLet θ be the water content of liquid water. a This refers to the water content of gaseous water. These represent the maximum and minimum pore sizes in soil and rock media, respectively. This represents the minimum mesoscopic pore size in soil and rock media.

[0110] S5. Combining the relationship between suction and pore size in soil and rock media and the relationship between water content and pore size distribution in soil and rock media, the relationship between water content and suction in soil and rock media is established using pore size as a bridge, thereby obtaining the water holding curve of soil and rock media:

[0111] The capillary water retention mechanism of liquid water can be represented as:

[0112]

[0113] The adsorption and water retention mechanism of gaseous water can be represented as follows:

[0114]

[0115] In the formula, ψ c For capillary attraction, ψ a For adsorption suction, These represent the maximum and minimum values ​​of capillary suction in soil and rock media, respectively. f(ψ) represents the maximum adsorption suction force in the soil and rock medium. c ), f(ψ a All of these are aperture probability density distribution functions.

[0116] By establishing the relationship between water content θ and suction ψ, the water holding curve of the soil and rock medium is obtained.

[0117] To verify the model's correctness, soil-water characteristic curve experiments were conducted on soil samples. To ensure conformity with the model, the experiment was divided into two parts: capillary suction and adsorption suction. The capillary suction experiment used a pressure membrane apparatus to measure the soil-water characteristic curve at the capillary suction stage. The clay plate in the pressure chamber needed to be saturated before use. The clay plate was connected to a water outlet pipe, and deionized water was added to the pressure chamber through the clay plate. The top cover of the pressure chamber was sealed with bolts. A pressure of 100 kPa was applied to the pressure chamber using a pressure controller, forcing the deionized water into the clay plate. Continuous water flow from the outlet indicated that the clay plate was saturated. After saturation, each saturated ring sample was placed on the clay plate in the pressure chamber, connected to the water outlet pipe, and the top cover was sealed. Pressure was applied in stages (40 kPa-800 kPa) using the pressure controller. When no more water flowed from the pressure membrane apparatus outlet, it indicated that the suction of the soil sample in the pressure chamber had reached equilibrium with that pressure level. The top cover was then opened, and the mass of each ring sample was measured. The next pressure level was then applied. After the final pressure is balanced with the soil sample, the ring sampler is removed and its mass is measured. The ring sampler is then dried in an oven to obtain the soil sample mass and moisture content corresponding to the final pressure.

[0118] The saturated salt solution method for determining the soil-water characteristic curve during the adsorption suction stage involves placing saturated ring samplers, numbered sequentially, into glass jars containing saturated K₂SO₄, KNO₃, Na₂CO₃, KCl, NaBr, K₂CO₃, CH₃CO₃, and NaOH solutions. The jar lid and body are sealed with Vaseline to maintain humidity balance. The mass change of each ring sample is measured periodically using a precision balance. When the average daily mass change is less than 0.001 g, the humidity in the soil sample is considered to have reached equilibrium with the humidity of the saturated salt solution. The ring samples are then removed and dried in an oven to obtain the mass moisture content of each soil sample.

[0119] By combining the soil-water characteristic curve data of the capillary suction stage measured by the pressure membrane method and the soil-water characteristic curve data of the adsorption suction stage measured by the saturated salt solution method, a soil-water characteristic curve with a wide suction range for the soil sample can be obtained. The final result is as follows: Figure 3 The diagram shown is a comparison of the soil-water characteristic curve derived from the statistical model theory of pore size distribution and the measured curve in this embodiment of the invention.

[0120] from Figure 3 As can be seen, the predicted SWCC exhibits a double-decline pattern, with two decreasing segments at 10–100 kPa and 100–1000 kPa, respectively. The slope of the lower suction decreasing segment is significantly steeper than that of the higher suction segment. Similarly, the PSD curve shows two peaks, with the main peak in the large pore size segment being much higher than that in the small pore size segment. Apart from these two decreasing segments, the rest of the SWCC is relatively flat, and these two main peaks or slopes also indicate the bimodal form of the curve. The main peak of the PSD curve is caused by intergranular porosity, which constitutes the majority of the loess porosity. The higher content of aggregates due to the large amount of clay particles leads to an increase in intragranular porosity, resulting in a secondary peak in small pores. This provides a rapid evaluation method for the water-holding capacity of soil and rock media from a microscopic porosity perspective.

[0121] This invention, in its embodiments, links pore size distribution with pore size probability density function, capillary suction, and adsorption suction, enabling a more accurate description of the water-holding curve. This allows for the calculation of soil-water characteristic curves based on pore size distribution considering multiple suction forces. Its key feature is that it obtains soil-water characteristic curves for pore sizes that simultaneously consider capillary suction and adsorption suction. Furthermore, because this model uses a continuous mathematical model rather than a few discrete points to represent the soil-water characteristic curves of unsaturated soil, this method not only fills the theoretical gap in deriving soil-water characteristic curves from pore size distribution but can also be further extended to predict and calculate permeability coefficients.

[0122] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for rapid evaluation of water holding capacity of geotechnical medium, characterized in that, The method comprises the following steps: S1, obtaining the relationship between the rock-soil medium suction and the pore diameter, including the relationship between the capillary suction and the pore diameter above 50 nm, and the relationship between the adsorption suction and the mesopore diameter between 2 nm and 50 nm; S2, establishing the pore diameter probability density distribution function of the rock-soil medium by measuring the pore diameter density distribution characteristics of the rock-soil medium; S3, obtaining the relationship between the water content of the rock-soil medium and the pore diameter distribution, including the relationship between the liquid water content and the pore diameter distribution, and the relationship between the gaseous water content and the pore diameter distribution; The relationship between the liquid water content and the pore diameter distribution is: The relationship between the gaseous water content and the pore diameter distribution is: In the formula, θ c is the liquid water content, θ a is the gaseous water content, r is the pore diameter, f(r) is the pore size probability density distribution function of the rock-soil medium; respectively, the maximum and minimum of the large pore size in the rock-soil medium; is the minimum of the mesopore size in the rock-soil medium; S4, combining the relationship between the rock-soil medium suction and the pore diameter obtained in step S1 and the relationship between the water content of the rock-soil medium and the pore diameter distribution obtained in step S3 to establish the relationship between the water content of the rock-soil medium and the suction with the pore diameter as the bridge, and obtaining the water retention curve of the rock-soil medium; The relationship between the liquid water content and the suction is: The relationship between the gaseous water content and the suction is: In the formula, ψ c is the capillary suction, ψ a is the adsorption suction, are the maximum and minimum values of the capillary suction in the rock-soil medium, respectively, is the maximum value of the adsorption suction in the rock-soil medium, f(ψ c ) and f(ψ a ) are both the pore size probability density distribution functions of the rock-soil medium.

2. The method for rapid evaluation of water holding capacity of geotechnical medium according to claim 1, characterized in that, In step S1, the relationship between the capillary suction and the pore diameter above 50 nm is: where ψ c is the capillary suction, T m is the surface tension, and α m is the contact angle.

3. The method for rapid evaluation of water holding capacity of geotechnical medium according to claim 2, characterized in that, In step S1, the relationship between the adsorption suction and the mesopore diameter between 2 nm and 50 nm is: where ψ a is the adsorption attraction, ψ sorp is the adsorption potential, ψ vdw is the van der Waals potential, ψ ele is the electrostatic potential, ψ osm is the osmotic potential, ψ hyd is the hydration potential, and x is the distance between a water molecule and a soil particle, where: where ψ hyd0 is the hydration component of the rock-soil particle surface, λ s is the decay length of the surface hydration component, with a value of 0.2-1.0 nm; A H is the Hamaker constant, L and t are the width and thickness of the clay particle, respectively; ε0 is the vacuum permittivity, ε r is the dielectric constant, E(x) is the electric field strength; c0 is the salt concentration, N A is the Avogadro constant, k B is the Boltzmann constant, T is the absolute temperature, e is the electronic charge number, V edl (x) is the potential strength; where V edl0 is the surface potential; wherein K is the electrical double layer thickness, a is the surface potential function, 4. The method for rapid evaluation of water holding capacity of geotechnical medium according to claim 3, characterized in that, In step S2, the pore diameter probability density distribution function of the rock-soil medium is: where f(r) represents the differential intrusion corresponding to the pore diameter r, N is the number of the main pore diameters r i , R i is the pore volume fraction corresponding to the main pore diameter r i ; m i is the pore volume fraction corresponding to the main pore diameter r i ; 0 < m i < 1, R i represents the maximum pore radius corresponding to the main pore diameter r.

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