step S1, collecting rock joint samples;

step S2, acquiring point cloud data of the rock joint samples with 3D laser scanning;

step S3, preprocessing the acquired point cloud data to obtain preprocessed data;

step S4, conducting indoor mechanical tests on the rock joint samples to determine rock mechanics parameters;

step S5, calculating, with the preprocessed data, joint roughness coefficients (JRCs) under different intervals obtained from statistical roughness parameters;

step S6, fitting different relational curves according to a scatter plot between the different intervals and the JRCs to obtain a fitting equation;

step S7, reversely calculating a JRC of each of the rock joint samples with the rock mechanics parameters to obtain a reversely calculated JRC;

step S8, substituting the reversely calculated JRC in the step S7 into the fitting equation obtained in the step S6 to determine a reasonable sampling interval for roughness evaluation;

wherein the rock mechanics parameters comprise a normal stress σ, a peak shear strength τ_p, a basic frictional angle φ_b, and a joint compressive strength (JCS);

wherein the step S5 specifically comprises:

step S51, evaluating roughness of any two-dimensional (2D) profile of the rock joint samples with a roughness parameter SD_i, wherein an interval between selected 2D profiles is the same as a sampling interval of a point cloud for the rock joint samples, and the roughness parameter is calculated by:

wherein z_j+1 and z_j are respectively a Z-coordinate of a (j+1)th point and a Z-coordinate of a jth point in a given 2D profile, y_j+1 and y_j are respectively a Y-coordinate of the (j+1)th point and a Y-coordinate of the jth point, N is a total number of points in the any 2D profile of the rock joint samples, L is a profile length, and i_ave is an average dip angle;

step S52, calculating a JRC^2D of a single 2D profile according to the roughness parameter SD_i;

step S53, averaging JRCs of all 2D profiles of a given rock joint sample to determine a JRC^3D of a rock joint surface:

wherein, M is a total number of the 2D profiles of the given rock joint sample, and i is an ith 2D profile;

step S54, drawing the scatter plot between sampling intervals and the JRCs;

wherein the step S6 specifically comprises:

step S61, constructing the relational curves between the sampling intervals and the JRCs, comprising a first exponential type, a second exponential type and a Logistic type;

step S62, observing each of the relational curves, proceeding with a step S63 when the relational curve is the first exponential type, proceeding with a step S64 when the relational curve is the second exponential type, and proceeding with a step S65 when the relational curve is the Logistic type;

step S63, defining the fitting equation as y=A₁×e^−x/t₁+y₀ or a second-order form y=A₁×e^(−x/t₁)+A₂×e^(−x/t₂)+y₀, wherein, A₁ is an attenuation coefficient, and controls a rate and a direction in attenuation or growth, t₁ is a time scale in the attenuation or the growth and is a constant term, y₀ is an offset and a constant term, and determines a position of a curve below or above an x-axis;

step S64, performing fitting by:

wherein, the two stages of the equation are growth and attenuation, respectively, and y₀, A_d, A_g, t_g, x_c, and t_d are undetermined coefficients;

step S65, performing fitting by:

wherein, A₁ is an upper limit of a Logistic function, A₂ is a lower limit of the Logistic function, p is a shape parameter, and x₀ is a midpoint parameter of the Logistic function;

wherein the reversely calculated JRC in the step S7 is specifically given by:

wherein, τ is the peak shear strength of the rock joint sample obtained in the tests.