Analysis method for determining surrounding rock-support stabilization time based on viscoelastic-plastic model

Abstract

The invention discloses an analysis method for determining surrounding rock-support stabilization time based on a viscoelastic-plastic model, and the method comprises the steps: determining geological parameters of surrounding rock through on-site tunnel face photographing and geological investigation, and obtaining creep mechanical characteristic parameters of the surrounding rock through an indoor test; preliminarily determining an excavation and support scheme of the tunnel; the standing time of the surrounding rock and the surrounding rock-structure stabilization time of the preliminarily determined excavation and support scheme are calculated according to a tunnel viscoelastic-plastic model and a longitudinal deformation curve; judging whether the stabilization time of the surrounding rock-support structure meets the time required by the secondary lining installation safety step pitch or not, if so, carrying out excavation construction according to a designed excavation and support scheme, and if not, adjusting the support scheme and recalculating to determine the stabilization time for investigating the section surrounding rock. According to the method, the creep mechanical effect of the surrounding rock is fully considered, and evaluation of the joint tunnel stabilization time is more specific, scientific and quantitative.

Description

Technical Field

[0001] The present invention relates to the technical field of tunnel construction, and in particular to an analysis method for determining surrounding rock-support stabilization time based on a viscoelastic-plastic model. Background Art

[0002] In tunnel engineering, the stability time is clearly defined as the period of time during which a tunnel can support itself without significant deformation or collapse of the surrounding rock mass, in the absence of any additional supporting structures. Determining this time point is crucial for engineers because it is directly related to the distance that can be safely advanced during tunnel excavation—that is, the maximum amount of excavation before support measures are required. The length of the stability time has a significant impact on the tunnel excavation process. It not only determines the excavation cycle, but also affects the choice of support methods and specific excavation techniques. Previous studies have explored in depth the relationship between tunnel stability, its standing time, and tunnel span. Based on extensive field data collection and analysis, a tunnel standing time prediction chart based on the rock quality rating system (RMR) has been further developed to provide engineers with a practical reference tool.

[0003] However, these studies still have the following shortcomings:

[0004] 1. Due to the limitations of analytical methods, there is still a lack of comprehensive and in-depth understanding of the creep mechanical properties of the surrounding rock for the standing time of tunnels excavated in weak rock masses, and the influence of this key factor has not been considered.

[0005] 2. Existing research lacks the long-term stability time of the support structure after installation, as well as the impact of short-term standing time on the long-term stability time.

[0006] 3. The three-dimensional numerical simulation takes a long time to calculate and is not suitable for quickly judging the stability and timeliness of the surrounding rock. Summary of the Invention

[0007] In order to address the deficiencies in the above-mentioned prior art, the present invention provides an analytical method for determining the surrounding rock-support stabilization time based on a viscoelastic-plastic model. This method provides a more specific, scientific and quantitative evaluation of the tunnel stabilization time.

[0008] In order to achieve the above technical objectives, the technical solution adopted by the present invention is:

[0009] An analytical method for determining surrounding rock-support stability time based on a viscoelastic-plastic model is characterized by comprising the following steps:

[0010] S1. Determine the geological parameters of the surrounding rock through on-site tunnel face photography and geological surveys, and obtain the creep mechanical properties of the surrounding rock through indoor tests;

[0011] S2. Determine the geological classification of the surrounding rock based on the geological parameters of the surrounding rock obtained in S1, and preliminarily determine the tunnel excavation and support plan based on the classification results and in combination with the excavation and support plan of the previous cycle;

[0012] S3. Based on the support data obtained in S2, by determining the allowable deformation of the surrounding rock, the standing time of the surrounding rock and the stabilization time of the surrounding rock-support structure of the preliminarily determined excavation and support scheme are calculated according to the tunnel viscoelastic-plastic model and the longitudinal deformation curve;

[0013] S4. Determine whether the stabilization time of the surrounding rock-support structure meets the time required for the safe step distance of the secondary lining installation. If so, proceed with excavation construction according to the designed excavation and support plan. If not, repeat steps S2 to S3, adjust the support plan, and recalculate until the stabilization time requirement is met before proceeding with tunnel excavation.

[0014] S5. After tunnel excavation, on-site deformation monitoring is carried out. By fitting the on-site deformation results with the predicted deformation curve, the surrounding rock parameters of the next cycle are reversely analyzed and corrected, so that the prediction of the surrounding rock-support stabilization time is gradually accurate in an area with similar geological conditions.

[0015] As a preferred technical solution, in S1, the geological parameters of the surrounding rock include burial depth, ground stress state, surrounding rock integrity, groundwater state and structural surface occurrence, and the creep mechanical characteristic parameters of the surrounding rock include uniaxial compressive strength, Maxwell elastic modulus G in the creep constitutive model, M , Maxwell viscosity coefficient H M , Kelvin bulk elastic modulus G K , Kelvin bulk viscosity H K , accelerated creep viscosity coefficient H P , material coefficient k, geotechnical creep coefficient n, damage coefficient α, ξ, the creep constitutive model is:

[0016]

[0017] Where t is time in hours, S ij is the deviatoric stress, S s It is long-term strength.

[0018] As a preferred technical solution, in S1, the ground stress state is reflected by the surrounding rock stress ratio; the surrounding rock integrity is divided into complete, relatively complete, relatively broken, broken, and extremely broken; the groundwater state is divided into wet or dripping water, rain-like or linear flow water, and gushing water; the structural surface attitude includes the inclination and dip of the rock mass structural surface; the BQ value of the surrounding rock is determined based on the obtained surrounding rock geological parameters, and the geological strength index GSI of the surrounding rock is determined based on the BQ value. The calculation formula is as follows:

[0019]

[0020] The material parameters m, s and the expansion angle ψ are calculated using the Geostrength Index GSI:

[0021]

[0022] where m i is the material constant of intact rock, D is the factor reflecting the degree of rock disturbance, is the internal friction angle of the rock.

[0023] As a preferred technical solution, in S1, the creep mechanical parameters of the rock are obtained by collecting rock samples on site and processing them into specimens with a diameter of 50 mm and a length of 100 mm, conducting creep mechanical tests, and fitting the creep model through the test results using 1stop software.

[0024] As a preferred technical solution, in S2, the excavation and support scheme of the corresponding section is queried through the design drawings, and the excavation and support scheme of this cycle is preliminarily adjusted in combination with the geological classification of the surrounding rock and the excavation and support scheme of the previous cycle. The excavation and support scheme mainly includes: excavation method, primary support number, primary support thickness, steel arch number, steel arch spacing, anchor length, anchor diameter, and anchor spacing; the support structure characteristic curve is calculated based on the given excavation and support scheme. The characteristic curve expression of a single support structure unit is:

[0025] P=k s u 0≤u≤u max

[0026] Where: u max is the maximum radial displacement of the supporting structure;

[0027] Select support units for initial support of tunnel engineering, including but not limited to steel arch, shotcrete and anchor support, and provide calculation methods for characteristic curves of single support structure units and combined support structure systems based on their relevant support characteristics;

[0028] The total stiffness of the combined support structure system is calculated in parallel with the structural stiffness of each single support structure unit, and is based on the minimum deformation that a single support unit structure can withstand. The characteristic curve expression of the combined support system is:

[0029]

[0030] P total =k total u min

[0031] Where: umin is the minimum deformation in the combined support structure; P tatal Support reaction force provided for the combined support structure; k total is the stiffness of the combined support structure.

[0032] As a preferred technical solution, in S3, the calculation formula of the tunnel viscoelastic-plastic model is:

[0033] The displacement deformation in the viscoelastic-plastic zone is:

[0034]

[0035] Where W(t), f1(r), f2(r), and f3(r) are:

[0036]

[0037] σ c is the uniaxial compressive strength of intact rock, r is the distance to the tunnel center, R p is the distance between the elastic-plastic interface and the tunnel center; k ψ is the expansion coefficient, and k ψ =(1+sinψ) / (1-sinψ);

[0038] The longitudinal deformation curve is calculated using the following formula:

[0039]

[0040] Where X is the normalized distance from the tunnel face; U f is the normalized convergence point of the tunnel working face; U ∞ represents the normalized maximum convergence far behind the tunnel face; α and β are monotonic functions of the normalized distance X, and X ≥ 0 behind the tunnel face. In front of the palm face, X<0, β(X)=exp(X);

[0041] Combining the longitudinal deformation curve with the time-dependent deformation, the time-dependent displacement of the surrounding rock deformation is obtained as follows:

[0042]

[0043] As a preferred technical solution, in S3, the standing time of the tunnel surrounding rock is solved, including the following two cases:

[0044] ① At a distance l≥l0 far enough from the tunnel surface, the displacement u l (t) shall not exceed the critical value u l * (t), that is:

[0045]

[0046] At this time, the time-dependent deformation of the surrounding rock is minimally affected by the longitudinal space, and the time-dependent stability of the surrounding rock is obtained using the following formula:

[0047] F(t′,u rp (t′))=0

[0048] Where t' is the standing time of the tunnel surrounding rock;

[0049] ②. At a certain distance (l≥l0), displacement u l (t) reaches the allowable value u l ’ (t), the surrounding rock-support structure stabilization time is determined according to the following formula:

[0050] F(t′,u rt (t′))=0

[0051] Where t' is the standing time of the tunnel surrounding rock.

[0052] As a preferred technical solution, in S3, the method for solving the stability time of the tunnel surrounding rock-support structure is:

[0053] After the tunnel face is excavated, the time-dependent deformation of the surrounding rock consists of two parts: the deformation of the unsupported section and the deformation of the supported section. The total displacement of the time-dependent deformation considering the effect of the support structure is:

[0054] u z,t (t)=u1(t0)+u2(t)

[0055] Where u1(t0) is the deformation of the unsupported section, and u2(t) is the deformation of the supported section, which is obtained by incorporating the corresponding support reaction force into the time-dependent displacement formula of surrounding rock deformation;

[0056] The stabilization time of the surrounding rock-support structure considering the support effect is:

[0057] F(t * ,u z,t (t * ))=0

[0058] where t * is the stabilization time of the tunnel surrounding rock-support structure.

[0059] As a preferred technical solution, in S4, the safe step distance for the secondary lining installation refers to the distance between the secondary lining and the face. By determining the construction speed of the secondary lining, the required stabilization time of the surrounding rock-support structure of the section can be determined. If the calculated stabilization time of the surrounding rock-support structure is less than the required stabilization time, it is necessary to improve the excavation and support plan. If it is greater than the required stabilization time, on-site construction can be carried out.

[0060] As an optimal technical solution, after determining the geological parameters of the surrounding rock and the excavation and support scheme parameters, a deformation prediction curve of the surrounding rock deformation over time is calculated. After tunnel excavation, the surrounding rock deformation is monitored. The creep mechanical parameters of the surrounding rock are optimized using the particle swarm algorithm and the two deformation curves. The surrounding rock parameters of the next cycle are back-analyzed and corrected, achieving a gradually accurate prediction of the surrounding rock-support stability time in an area with similar geological conditions.

[0061] The beneficial effects of the present invention are:

[0062] The results calculated by the analysis method of the present invention fully consider the creep mechanical effect of the surrounding rock, and can quantitatively calculate the self-stabilization time of the surrounding rock in the prepared support section, as well as the overall stabilization time of the surrounding rock-support structure after support. It can quantitatively provide the support timing of the initial support and the safe step distance of the secondary lining structure, solving the problem that the support timing at the current stage on site relies on the subjective judgment of engineering personnel.

[0063] The results calculated by the analysis method of the present invention use the distance between the section to be supported and the tunnel face as a parameter, and provide the specific stabilization time of the section and the deformation process of the surrounding rock, which is convenient for application in actual engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0065] Figure 1 is a flow chart of the steps of the analytical method of the present invention;

[0066] Figure 2 Schematic diagram of the viscoelastic-plastic calculation model of the present invention;

[0067] Figure 3 This is a deformation diagram of the LDP curve considering the influence of aging of the present invention;

[0068] Figure 4 This is a schematic diagram of the two-stage stability time considering the influence of the support structure of the present invention;

[0069] Figure 5 This is an example of the present invention: deformation monitoring data of the DK501+050 section of Leye Tunnel. DETAILED DESCRIPTION

[0070] In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. The components of the embodiments of the present application generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the application for protection, but merely represents the selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0071] This example provides an analytical method for determining the surrounding rock-support stability time based on the viscoelastic-plastic model. Figure 1 As shown in the flow chart, the main steps are as follows:

[0072] S1. Determine the geological parameters of the surrounding rock through on-site tunnel face photography and geological surveys, and obtain the creep mechanical properties of the surrounding rock through indoor tests.

[0073] The geological parameters of the surrounding rock include burial depth, ground stress state, surrounding rock integrity, groundwater state and structural surface occurrence.

[0074] The creep mechanical characteristic parameters include uniaxial compressive strength, Maxwell elastic modulus G in the creep constitutive model, M , Maxwell viscosity coefficient H M , Kelvin bulk elastic modulus G K , Kelvin bulk viscosity H K , accelerated creep viscosity coefficient H P , material coefficient k, geotechnical creep coefficient n, damage coefficient α, ξ. The constitutive model of creep is:

[0075]

[0076] Where t is time in hours, S ij is the deviatoric stress, S s It is long-term strength.

[0077] The creep mechanical parameters of rock are obtained by collecting rock samples on site and processing them into specimens with a diameter of 50 mm and a length of 100 mm. Creep mechanical tests are carried out and the creep model is fitted using 1stop software based on the test results.

[0078] Furthermore, the in-situ stress state is mainly reflected by the surrounding rock stress ratio; the surrounding rock integrity is divided into complete, relatively complete, relatively broken, broken, and extremely broken; the groundwater state is divided into wet or dripping water, rain-like or linear flow water, and gushing water; the structural surface attitude includes the inclination and dip of the rock mass structural surface; the BQ value of the surrounding rock is determined based on the above-obtained surrounding rock geological content. The BQ value is calculated using the following formula:

[0079] The surrounding rock geomechanical parameters are obtained using the [BQ] method, a modification of the BQ method. The BQ method is the only rock classification standard in China that applies to all rock types. The BQ score depends on the rock hardness and rock mass integrity. The BQ has been changed to [BQ], taking into account factors such as the influence of groundwater, the orientation of weak zones associated with excavation, and the initial stress state. Different surrounding rock grades in the [BQ] system correspond to different [BQ] scores and rock mass qualities. This is shown in Table 1. This table reflects the fact that higher [BQ] values indicate better surrounding rock quality. The basic quality index correction value [BQ] is then calculated using the following formula.

[0080] BQ=90+R c +250K v (1)

[0081] R c Indicates the uniaxial compressive strength of rock, K v It is a quantitative indicator of rock integrity.

[0082] [BQ] = BQ - 100 (k1 + k2 + k3) (2)

[0083] k1, k2 and k3 represent the groundwater conditions, the direction of the weak zone related to the excavation and the in-situ stress conditions.

[0084] Table 1 BQ classification of rock mass

[0085]

[0086] Equation (21) provides ranges for k1, k2, and k3 under different geological conditions. However, it is difficult to determine the range of [BQ] for different surrounding rock grades. Therefore, k1 and k2 are taken as average values. Due to the dry tunnel face in the DK501+540 to +086 section of the Leye Tunnel, k3 can be taken as 0. The values of the correction coefficients are shown in Table 2.

[0087] Table 2 Correction coefficients

[0088] Surrounding rock grade Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ k1 0.1 0.15 0.3 0.55 0.7 k2 0.3 0.3 0.3 0.3 0.3 K3 0 0 0 0 0

[0089] The relationship between GSI and [BQ] can be obtained through conversion using RMR as an intermediate transfer medium.

[0090] The relationship equation is

[0091] BQ=6.0943RMR+80.786 (3)

[0092] According to the literature, the relationship between GSI and RMR can be expressed as (23)

[0093]

[0094] Substituting formula (23) into formula (22), the relationship between GSI and BQ can be obtained as follows:

[0095]

[0096] Substituting Equation (24) into Equation (21) and combining the influence coefficient, the GSI range of tunnels with different surrounding rock grades can be obtained, as shown in Table 3.

[0097] Table 3 GSI range

[0098] Surrounding rock grade Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ [BQ] >550 550-451 450-351 350-251 ≤250 GSI >77 77-67 66-54 53-39 ≤38

[0099] The material parameters m, s and the expansion angle ψ are calculated using the Geostrength Index GSI:

[0100]

[0101] where m i is the material constant of intact rock, D is the factor reflecting the degree of rock disturbance, is the internal friction angle of the rock.

[0102] S2. Determine the geological classification of the surrounding rock based on the geological parameters of the surrounding rock obtained in S1, and preliminarily determine the tunnel excavation and support plan based on the classification results and in combination with the excavation and support plan of the previous cycle.

[0103] First, the excavation and support scheme for the corresponding section is checked through the design drawings. The excavation and support scheme for this cycle is initially adjusted based on the geological classification of the surrounding rock and the excavation and support scheme from the previous cycle. The excavation and support scheme mainly includes the following: excavation method, primary support number, primary support thickness, steel arch number, steel arch spacing, anchor length, anchor diameter, and anchor spacing. The support structure characteristic curve is calculated based on the given excavation and support scheme. The characteristic curve of a single support structure unit is usually expressed as:

[0104] P=ks u 0≤u≤u max (26)

[0105] Where: u max is the maximum radial displacement of the supporting structure, k s is the total stiffness of the supporting structure.

[0106] The common support units for initial support in tunnel engineering, such as steel arch, shotcrete and anchor support, are selected. Based on their relevant support characteristics, the characteristic curve calculation methods for single support structure unit and combined support structure system are given respectively. The specific support stiffness is:

[0107] (1) Steel arch support unit

[0108] Steel arch stiffness K set for

[0109]

[0110] Where: E set is the elastic modulus of the steel arch material; d is the longitudinal arrangement spacing of the steel arch; A set is the cross-sectional area of the steel arch; h set is the cross-sectional height of the steel arch; r i is the tunnel radius.

[0111] The maximum support force of the steel arch is

[0112]

[0113] Where: P set,max is the maximum support force of the steel arch; σ st,y is the yield stress of the steel arch material.

[0114] The maximum deformation of the steel arch (or the maximum radial displacement produced in the elastic stage) is

[0115]

[0116] Where: u set,max is the maximum allowable displacement of the steel arch, u set,ini This is the displacement during installation of the steel arch.

[0117] Therefore, the characteristic curve expression of the steel arch support unit is:

[0118] P set =k set △u set 0≤P set ≤P set,max (10)

[0119] Where: P setis the steel arch support force; Δu set is the radial deformation of the steel arch.

[0120] (2) Shotcrete support unit

[0121] When the thickness of shotcrete is greater than 4% of the tunnel radius, the shotcrete can be calculated by assuming it is an elastic thick-walled cylinder. The stiffness of the shotcrete is

[0122]

[0123] Where: k shot is the support stiffness of the shotcrete structure; E con and μ con are the elastic modulus and Poisson's ratio of shotcrete respectively; t shot is the thickness of shotcrete; r i is the tunnel radius.

[0124] Ultimate bearing capacity of shotcrete P shot,max for

[0125]

[0126] Where: σ con The initial support strength is 23 MPa, which is the uniaxial compressive strength of sprayed concrete material.

[0127] Maximum deformation of shotcrete Δu shot,max (or the maximum radial displacement generated in the elastic stage) is

[0128]

[0129] Where: u shot,max is the maximum allowable displacement of shotcrete; u shot,ini It refers to the displacement during the construction of shotcrete.

[0130] Therefore, the characteristic curve of the shotcrete support structure is

[0131] P shot =k shot △u shot 0≤P shot ≤P shot,max (14)

[0132] Where: k shot , P shot , Δu shot They are shotcrete stiffness, support reaction force and deformation respectively.

[0133] (3) Anchor support unit

[0134] According to the calculation formula of anchor stiffness given by Hoke et al.

[0135]

[0136] Where: k bolt is the anchor support stiffness; S e is the circumferential spacing of anchor rods; S l is the longitudinal spacing of anchor rods; L bolt is the length of the anchor rod; d bolt is the diameter of the anchor rod; E st is the elastic modulus of the anchor material; Q bolt is the stress-deformation constant of the anchoring end of the anchor rod, which can be obtained according to the Hoek literature.

[0137] Maximum support force of anchor bolt P bolt,max for

[0138]

[0139] Where: T bolt It is the ultimate failure load in the anchor pull-out test.

[0140] Therefore, the maximum deformation of the anchor rod Δu can be obtained bolt,max (or the maximum radial displacement generated in the elastic stage) is

[0141]

[0142] Where: u bolt,max is the maximum allowable displacement of the anchor rod; u bolt,ini Allowable displacement during anchor construction.

[0143] Therefore, the characteristic curve expression of the anchor support structure unit is:

[0144] P bolt =k bolt △u bolt 0≤P bolt ≤P bolt,max (18)

[0145] Where: k bolt , P bolt , Δu bolt They are the stiffness, support reaction force and deformation of the anchor structure respectively.

[0146] Because multiple support structures often act on the surrounding rock during on-site tunnel construction, the initial support is a combined support structure system formed by multiple support structures such as anchors, steel arches, and shotcrete. The total stiffness of the combined support structure system is calculated in parallel using the stiffness of each individual support structure unit, and is based on the minimum deformation that a single support structure unit can withstand. If this deformation is exceeded, the support structure system will enter a plastic state. Therefore, the characteristic curve expression of the combined support structure system is:

[0147]

[0148] P total =k total u min (40)

[0149] Where: u min is the minimum deformation in the combined support structure; P tatal Support reaction force provided for the combined support structure; k total is the stiffness of the combined support structure.

[0150] S3. Based on the support data obtained in S2, by determining the allowable deformation of the surrounding rock, the standing time of the surrounding rock and the surrounding rock-structure stability time of the preliminarily determined excavation and support scheme are calculated according to the tunnel viscoelastic-plastic model and the longitudinal deformation curve.

[0151] The allowable deformation of the surrounding rock is determined based on the design results and subsequent monitoring results. The schematic diagram of the viscoelastic-plastic calculation model is shown in the figure. Figure 2 , the calculation formula of the tunnel viscoelastic-plastic model is:

[0152] The displacement deformation in the viscoelastic-plastic zone is:

[0153]

[0154] Where W(t), f1(r), f2(r), and f3(r) are:

[0155]

[0156] σ c is the uniaxial (unconstrained) compressive strength of intact rock, r is the distance to the tunnel center, R p is the distance between the elastic-plastic interface and the tunnel center; k ψ is the expansion coefficient, and k ψ =(1+sinψ) / (1-sinψ).

[0157] The longitudinal surrounding rock response curve is calculated using the following formula:

[0158]

[0159] where X (=x / R0, x is positive relative to the excavation direction) is the normalized distance from the tunnel face; U f (=U a (X=0)) is the normalized convergence point of the tunnel working face X=0; U ∞ (=U a (X=∞)) represents the normalized maximum convergence far behind the tunnel face; α and β are both monotonic functions of the normalized distance X and some other parameters, which are selected as shown in Table 4 below.

[0160] Table 4 Values of α and β

[0161]

[0162] The longitudinal deformation curve shows the deformation ratio of the surrounding rock at different distances from the tunnel face, such as Figure 3 Combining it with the time-dependent deformation, that is, substituting formula (46) into formula (41), the time-dependent displacement of the surrounding rock deformation can be obtained as follows:

[0163]

[0164] The solution to the standing time of the rock around the tunnel is divided into the following two cases:

[0165] ①, at a distance far enough from the tunnel surface (l ≥ l0), the displacement u l (t) shall not exceed the critical value u l * (t),

[0166] Right now:

[0167]

[0168] At this time, the time-dependent deformation of the surrounding rock is minimally affected by the longitudinal space, so the time-dependent stability of the surrounding rock can be obtained by using formula (41):

[0169]

[0170] Where t' is the standing time of the tunnel surrounding rock.

[0171] ②. At a certain distance (l≥l0), displacement u l (t) reaches the allowable value u l '(t), the surrounding rock-support structure stabilization time can be determined according to formula (47) as follows:

[0172] F(t′,u rt (t′))=0 (50)

[0173] Where t' is the standing time of the tunnel surrounding rock.

[0174] The method for solving the stability time of the tunnel surrounding rock-support structure is as follows:

[0175] In terms of relative stability time, the construction time of the support structure is similar, and the construction time is t0. When t≤t0, the support reaction force p i When the support structure is installed, according to the calculation method of the combined support structure characteristic curve, the support reaction force p i Increase to P total , the surrounding rock deformation gradually becomes stable. After the tunnel face is excavated, the time-dependent deformation of the surrounding rock mainly consists of two parts, namely the deformation of the unsupported section of the surrounding rock and the deformation of the supported section, such as Figure 5 Therefore, the total displacement of the time-dependent deformation considering the effect of the supporting structure is:

[0176] u z,t (t)=u1(t0)+u2(t) (51)

[0177] where u1(t0) is the deformation of the unsupported section, and u2(t) is the deformation of the supported section; these are obtained by incorporating the corresponding support reaction force into the time-dependent displacement formula (47) for surrounding rock deformation.

[0178] Combining formulas (48-50), the stability time of the surrounding rock-support structure considering the support effect can be obtained as follows:

[0179] F(t * ,u z,t (t * ))=0 (52)

[0180] where t * is the stabilization time of the tunnel surrounding rock-support structure.

[0181] S4. Inspect whether the stabilization time of the surrounding rock-support structure meets the time required for the safe step distance of the secondary lining installation. If so, excavation construction can be carried out according to the designed excavation and support plan. If it does not meet the requirements, the support plan needs to be adjusted and recalculated until the stabilization time requirements are met before tunnel excavation.

[0182] The safe step distance for secondary lining installation refers to the distance between the secondary lining and the tunnel face. By determining the construction speed of the secondary lining, the required stabilization time of the surrounding rock-support structure of the section can be determined. If the calculated surrounding rock-initial support stabilization time is less than the required stabilization time, the excavation and support plan needs to be improved. If it is greater than the required stabilization time, on-site construction can be carried out.

[0183] S5. After tunnel excavation, on-site deformation monitoring is required. By fitting the on-site deformation results with the predicted deformation curve, the mechanical parameters of the surrounding rock can be further adjusted, so that the prediction of the surrounding rock-support stabilization time can be gradually accurate in an area with similar geological conditions.

[0184] After determining the surrounding rock geological parameters and the excavation and support scheme parameters, the deformation prediction curve of the surrounding rock deformation over time can be calculated using formula (47). After tunnel excavation, the surrounding rock deformation monitoring curve can be obtained. The creep mechanical parameters of the surrounding rock can be further optimized through the particle swarm algorithm and the two deformation curves, thereby achieving a gradually accurate prediction of the surrounding rock-support stability time in an area with similar geological conditions. Specific embodiment:

[0186] Based on the above analysis, Leye Tunnel is selected as an example for demonstration, and DK501+050 section is selected for on-site theoretical verification. The tunnel section is buried at a depth of 263m, and the corresponding ground stress σ c is 5.78MPa. In addition, the uniaxial compressive strength R of the surrounding rock was obtained through on-site point load test. c The pressure is 27.25 MPa. The tunnel section spans 14.86 meters and is 12.54 meters high. Face photographs and on-site geological sketches provide a brief geological description of the section: the lithology is muddy sandstone, sandstone interbedded with mudstone, with a brownish-red, brownish-red, and gray coloration. It is slightly weathered, with the rock formations trending slightly to the right of the line, at an angle of approximately 35°. The mudstone is a weakly expansive rock that softens and disintegrates easily upon contact with water, with smooth structural surfaces. The muddy sandstone is relatively soft; the sandstone, developed on the left and right sides of the face, is hard. Joints and fissures are well developed, with average bonding, and the rock mass is relatively fragmented. There is no water at the face.

[0187] The surrounding rock mass integrity index (Kv) at DK501+050 is 0.55. The surrounding rock mass's [BQ] and GSI values can be determined using formulas (22-25), as shown in Table 5. Based on Table 5, the surrounding rock mass is preliminarily classified as Class V.

[0188] Table 5 Values of surrounding rock geological indicators

[0189] Pile number BQ value K1 K2 K3 [BQ] GSI DK501+050 254.75 0.55 0.3 0 169.75 19.15

[0190] Other Hoek-Brown criterion parameters, including the material constant mi for intact rock, are calculated using Equation (54) proposed by Cui et al. For tunnels, D, reflecting the degree of rock mass disturbance, is taken as 0.5, and the surrounding rock parameters are determined using Equation (25). The creep mechanical parameters of the tunnel surrounding rock are based on experimental results, as shown in Table 5.

[0191] m i =0.7375GSI 0.7586 (19)

[0192] The main structure of the Leye Tunnel utilizes a composite lining system consisting of primary support, geotextile, waterproof sheet, and secondary lining. The primary support for Class III surrounding rock utilizes anchor bolts, steel mesh, and shotcrete, while steel arches are added for Class IV and V surrounding rock. The secondary lining utilizes stamped concrete, with reinforcement placed within the secondary lining for Class V surrounding rock. The support parameters are shown in Table 6.

[0193] Table 6 Physical parameters of initial support structure

[0194]

[0195] The surrounding rock grade of the DK501+050 section of Leye Tunnel is defined as Grade V. Grade V support parameters are selected. In addition to the support physical parameters given in Table 6, the basic mechanical parameters of the support structure are shown in Table 7.

[0196] Table 7 Mechanical parameters of initial support structure

[0197]

[0198] Based on the surrounding rock geological parameters and support parameters given above, the analysis method used in this application calculates that the surrounding rock standing time under unsupported conditions is only 4.97 hours, which does not meet the requirements of initial support construction and requires advanced reinforcement measures. When the surrounding rock parameters increase by 10%, the corresponding surrounding rock standing time can be increased to 15.13 hours; when the surrounding rock parameters increase by 15%, the corresponding surrounding rock standing time can be increased to 106.38 hours, fully meeting the requirements of initial support structure construction.

[0199] Based on a 10% increase in surrounding rock parameters, the support time is 8 hours, and the secondary lining support distance is 40 meters. Under Level IV surrounding rock support conditions, the surrounding rock-support structure stability time is maintained at 24.32 hours, which does not meet construction requirements. Under Level V surrounding rock support conditions, the surrounding rock-support structure stability time is maintained at 47.61 hours, which still does not meet construction requirements.

[0200] Based on a 15% increase in surrounding rock parameters, the support time is 8 hours and the secondary lining support distance is 40m. Under the IV level surrounding rock support condition, the surrounding rock-support structure stability time can be maintained for 1765 hours, 2.45 months. According to the statistical analysis results of the stability time of the Chongqing-Kunming High-Speed Railway Tunnel, the traditional drilling and blasting excavation method is used, and the stability of 2.45 months meets the construction requirements. Under the V level surrounding rock support condition, the surrounding rock-support stability time can be maintained for 4100 hours, 5.69 months. When the secondary lining support distance is increased to 140m, the surrounding rock support stability time can be maintained for 2272 hours, 3.15 months, which still meets the construction requirements.

[0201] From the above, it can be seen that for the DK501+050 section, advanced surrounding rock support should be strengthened, and the surrounding rock parameters need to be increased to more than 15%. When using Level IV surrounding rock support parameters, the safe distance from the secondary lining to the tunnel face should be controlled within 40m. When using Level V surrounding rock support parameters, the safe distance from the secondary lining to the tunnel face can be increased to 140m.

[0202] The construction was carried out on site using the IV level surrounding rock support parameters and the tunnel deformation was monitored by a total station. The monitoring results are as follows: Figure 5 shown.

[0203] Comparative analysis shows that, using a 150mm control line as the control line for the reserved deformation, the stabilization time of the surrounding rock-support structure is 11 to 20 days, far short of the estimated 2.45 months. This indicates an overestimation of the improvement in surrounding rock parameters achieved through advanced support measures. Re-evaluation should be conducted based on monitoring results. For an 11.5% increase in surrounding rock parameters, with a support time of 8 hours and a secondary lining distance of 40m from the tunnel face, the stabilization time of the surrounding rock-support structure is 15.26 days, consistent with field monitoring results. For an 11.5% increase in surrounding rock parameters, using Grade V support, the stabilization time of the surrounding rock-support structure is 22.6 days. Therefore, the support grade can be improved to ensure tunnel construction safety.

[0204] Of course, the present invention may have many other embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art may make various corresponding changes and modifications based on the present invention, but these corresponding changes and modifications should all fall within the scope of protection of the claims attached to the present invention.

Claims

1. An analytical method for determining surrounding rock-support stability time based on a viscoelastic-plastic model, characterized by the following steps: include: S1. Determine the geological parameters of the surrounding rock through on-site tunnel face photography and geological surveys, and obtain the creep mechanical properties of the surrounding rock through indoor tests; S2. Determine the geological classification of the surrounding rock based on the geological parameters of the surrounding rock obtained in S1, and preliminarily determine the tunnel excavation and support plan based on the classification results and in combination with the excavation and support plan of the previous cycle; S3. Based on the support data obtained in S2, by determining the allowable deformation of the surrounding rock, the standing time of the surrounding rock and the stabilization time of the surrounding rock-support structure of the preliminarily determined excavation and support scheme are calculated according to the tunnel viscoelastic-plastic model and the longitudinal deformation curve; S4. Determine whether the stabilization time of the surrounding rock-support structure meets the time required for the safe step distance of the secondary lining installation. If so, proceed with excavation construction according to the designed excavation and support plan. If not, repeat steps S2 to S3, adjust the support plan, and recalculate until the stabilization time requirement is met before proceeding with tunnel excavation. S5. After tunnel excavation, on-site deformation monitoring is carried out. By fitting the on-site deformation results with the predicted deformation curve, the surrounding rock parameters of the next cycle are reversely analyzed and corrected, so that the prediction of the surrounding rock-support stabilization time is gradually accurate in an area with similar geological conditions.

2. The analytical method for determining surrounding rock-support stability time based on a viscoelastic-plastic model according to claim 1, characterized in that: In S1, the geological parameters of the surrounding rock include burial depth, ground stress state, surrounding rock integrity, groundwater state and structural surface occurrence. The creep mechanical characteristic parameters of the surrounding rock include uniaxial compressive strength, Maxwell elastic modulus G in the creep constitutive model, and the creep mechanical characteristic parameters of the surrounding rock. M , Maxwell viscosity coefficient H M , Kelvin bulk elastic modulus G K , Kelvin bulk viscosity H K , accelerated creep viscosity coefficient H P , material coefficient k, geotechnical creep coefficient n, damage coefficient α, ξ, the creep constitutive model is: Where t is time in hours, S ij is the deviatoric stress, S s It is long-term strength.

3. The analytical method for determining surrounding rock-support stability time based on a viscoelastic-plastic model according to claim 1, characterized in that: In S1, the in-situ stress state is reflected by the surrounding rock stress ratio; the surrounding rock integrity is divided into complete, relatively complete, relatively broken, broken, and extremely broken; the groundwater state is divided into wet or dripping water, rain-like or linear flow water, and gushing water; the structural surface attitude includes the inclination and dip angle of the rock mass structural surface; the BQ value of the surrounding rock is determined based on the obtained surrounding rock geological parameters, and the geological strength index (GSI) of the surrounding rock is determined based on the BQ value. The calculation formula is as follows: The material parameters m, s and the expansion angle ψ are calculated using the Geostrength Index GSI: where m i is the material constant of intact rock, D is the factor reflecting the degree of rock disturbance, is the internal friction angle of the rock.

4. The analytical method for determining surrounding rock-support stability time based on a viscoelastic-plastic model according to claim 1, characterized in that: In S1, the creep mechanical parameters of rock were obtained by collecting rock samples on site and processing them into specimens with a diameter of 50 mm and a length of 100 mm. Creep mechanical tests were carried out and the creep model was fitted using 1stop software based on the test results.

5. The analytical method for determining surrounding rock-support stability time based on a viscoelastic-plastic model according to claim 1, characterized in that: In S2, the excavation and support scheme for the corresponding section is queried through the design drawings. The excavation and support scheme for this cycle is preliminarily adjusted based on the geological classification of the surrounding rock and the excavation and support scheme of the previous cycle. The excavation and support scheme mainly includes: excavation method, primary support number, primary support thickness, steel arch number, steel arch spacing, anchor length, anchor diameter, and anchor spacing; the support structure characteristic curve is calculated based on the given excavation and support scheme. The characteristic curve expression of a single support structure unit is: P=k s u 0≤u≤u max Where: u max is the maximum radial displacement of the supporting structure; Select support units for initial support of tunnel engineering, including but not limited to steel arch, shotcrete and anchor support, and provide calculation methods for characteristic curves of single support structure units and combined support structure systems based on their relevant support characteristics; The total stiffness of the combined support structure system is calculated in parallel with the structural stiffness of each single support structure unit, and is based on the minimum deformation that a single support unit structure can withstand. The characteristic curve expression of the combined support system is: P total =k total u min Where: u min is the minimum deformation in the combined support structure; P tatal Support reaction force provided for the combined support structure; k total is the stiffness of the combined support structure.

6. The analytical method for determining surrounding rock-support stability time based on a viscoelastic-plastic model according to claim 1, characterized in that: In S3, the calculation formula of the tunnel viscoelastic-plastic model is: The displacement deformation in the viscoelastic-plastic zone is: Where W(t), f1(r), f2(r), and f3(r) are: σ c is the uniaxial compressive strength of intact rock, r is the distance to the tunnel center, R p is the distance between the elastic-plastic interface and the tunnel center; k ψ is the expansion coefficient, and k ψ =(1+sinψ) / (1-sinψ); The longitudinal deformation curve is calculated using the following formula: Where X is the normalized distance from the tunnel face; U f is the normalized convergence point of the tunnel working face; U ∞ represents the normalized maximum convergence far behind the tunnel face; α and β are monotonic functions of the normalized distance X, and X ≥ 0 behind the tunnel face. In front of the palm face, X<0, β(X)=exp(X); Combining the longitudinal deformation curve with the time-dependent deformation, the time-dependent displacement of the surrounding rock deformation is obtained as follows:

7. The analytical method for determining surrounding rock-support stability time based on a viscoelastic-plastic model according to claim 1, characterized in that: In S3, the standing time of the tunnel surrounding rock is solved, including the following two cases: ① At a distance l≥l0 far enough from the tunnel surface, the displacement u l (t) shall not exceed the critical value u l * (t), that is: At this time, the time-dependent deformation of the surrounding rock is minimally affected by the longitudinal space, and the time-dependent stability of the surrounding rock is obtained using the following formula: F(t′,u rp (t′))=0 Where t' is the standing time of the tunnel surrounding rock; ②. At a certain distance (l≥l0), displacement u l (t) reaches the allowable value u l '(t), the surrounding rock-support structure stabilization time is determined according to the following formula: F(t′,u rt (t′))=0 Where t' is the standing time of the tunnel surrounding rock.

8. The analytical method for determining surrounding rock-support stability time based on a viscoelastic-plastic model according to claim 7, characterized in that: In S3, the method for solving the stability time of the tunnel surrounding rock-support structure is: After the tunnel face is excavated, the time-dependent deformation of the surrounding rock consists of two parts: the deformation of the unsupported section and the deformation of the supported section. The total displacement of the time-dependent deformation considering the effect of the support structure is: u z,t (t)=u1(t0)+u2(t) Where u1(t0) is the deformation of the unsupported section, and u2(t) is the deformation of the supported section, which is obtained by incorporating the corresponding support reaction force into the time-dependent displacement formula of surrounding rock deformation; The stabilization time of the surrounding rock-support structure considering the support effect is: F(t * ,u z,t (t * ))=0 where t * is the stabilization time of the tunnel surrounding rock-support structure.

9. The analytical method for determining surrounding rock-support stability time based on a viscoelastic-plastic model according to claim 1, characterized in that: In S4, the safe step distance for the secondary lining installation refers to the distance between the secondary lining and the tunnel face. By determining the construction speed of the secondary lining, the required stabilization time of the surrounding rock-support structure of the section can be determined. If the calculated stabilization time of the surrounding rock-support structure is less than the required stabilization time, it is necessary to improve the excavation and support plan. If it is greater than the required stabilization time, on-site construction can be carried out.

10. The analytical method for determining surrounding rock-support stability time based on a viscoelastic-plastic model according to claim 1, characterized in that: After determining the geological parameters of the surrounding rock and the parameters of the excavation and support schemes, a deformation prediction curve showing the surrounding rock deformation changing with time is calculated. After tunnel excavation, the surrounding rock deformation is monitored. The creep mechanical parameters of the surrounding rock are optimized using a particle swarm algorithm and two deformation curves. The surrounding rock parameters of the next cycle are then back-analyzed and corrected, achieving a gradually more accurate prediction of the surrounding rock-support stabilization time in an area with similar geological conditions.

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