Method for determining reasonable overlying strata thickness of subsea tunnel in multi-construction-method combined mode

Abstract

The invention relates to a method for determining the reasonable overlying strata thickness of a subsea tunnel in a multi-construction-method combined mode. The method comprises the following steps that the minimum overlying strata thickness in a construction mileage interval where different construction methods are located is obtained; the set construction parameters in the multi-hole parallel mode are used for correcting the minimum overlying strata thicknesses corresponding to the different construction methods, and the final minimum overlying strata thicknesses of the different construction methods are obtained; determining the upper and lower boundaries of the longitudinal section of the tunnel according to the maximum overlying strata thickness of the construction mileage interval where different construction methods are located and the obtained final minimum overlying strata thickness; the gradient range of each section of the tunnel is designed according to the upper and lower boundaries of the longitudinal section of the tunnel, the optimal overlying strata thickness of the subsea tunnel is obtained according to the obtained gradient range and the gradient-construction cost calculation model, and the construction cost of the subsea tunnel is reduced to the maximum extent by adopting the method.

Description

Technical Field

[0001] The present invention relates to the technical field of geotechnical engineering, and particularly relates to a method for determining a reasonable overburden rock thickness of a subsea tunnel under a multi-construction method combination mode. Background Technique

[0002] The statements herein only provide background techniques related to the present invention, and do not necessarily constitute prior art.

[0003] Compared with land tunnels, subsea tunnels have their unique design key points at the beginning of design. Among them, determining the minimum rock cover thickness of a subsea tunnel is one of the key technologies. Due to the unique use scenario of a subsea tunnel, after the cross-sea line is roughly determined, the minimum thickness of the rock mass above the proposed tunnel is closely related to the economy and safety of tunnel construction. If the cover thickness is too thin, the tunnel construction working face will be unstable and the risk of sudden water inrush will increase, resulting in an increase in construction costs; while if the cover is too thick, the length and burial depth of the subsea tunnel will increase, which will not only increase the construction cost, but also increase the costs of pumping drainage and vehicle climbing during the operation period.

[0004] Certain achievements have been made in the existing research on the calculation of the minimum overburden rock thickness of subsea tunnels, such as methods for determining the minimum burial depth of underwater tunnels, methods for determining the minimum overburden soil layer thickness of shallowly covered underwater shield tunnels, etc. However, in the above methods, the determination of the overburden rock thickness is mostly based on the safety angle. However, the minimum overburden rock thickness of a tunnel does not necessarily mean the minimization of the overall tunnel cost. When reducing the overburden rock thickness, the overall cost measurement still needs to consider the additional costs such as excavation plugging and grouting reinforcement during the construction period brought about by the reduction of the overburden rock thickness, and the cost savings of pumping drainage during the operation period after the elevation reduction. At the same time, in the multi-tunnel parallel mode, the deformation of the surrounding rock between tunnels affects each other. When the subsequent tunnel is blasted and excavated, the surrounding rock-support system of the previous tunnel will be subjected to the blasting strong dynamic disturbance effect of several superpositions. Especially in the case of small spacing, the surrounding rock-support system of the previous tunnel often undergoes secondary deformation or even damage; in addition, the multiple blasting excavation effects will promote the generation of secondary cracks in the surrounding rock between the parallel tunnels and the mutual penetration of the main cracks. In the subsea tunnel environment, it is more likely to exacerbate the water-hydrodynamic connection between the parallel tunnels and the seepage flow rate of fissure water, resulting in the phenomenon of "delayed" water outflow often occurring in the supported sections of the previous tunnel, which causes the on-site construction to generate grouting water blocking and surrounding rock reinforcement work outside the design, seriously affecting the construction organization and project progress of the subsequent processes, and also bringing great challenges to the control of the overall water inflow of the tunnel. The reason is that the current longitudinal section design of subsea tunnels does not fully consider the water-hydrodynamic connection under the multi-tunnel parallel mode and the complex seepage evolution law of the composite water blocking system under this mode.

[0005] It can be seen that aiming at the scientific determination of the overburden rock thickness of multi-tunnel parallel extra-long subsea tunnels under the multi-construction method combination mode, how to carry out the coordinated measurement of economy and safety while ensuring safe construction is still a difficult problem to be solved at present. Summary of the Invention

[0006] Aiming at the deficiencies of the existing technology, the purpose of the present invention is to provide a method for determining the reasonable overburden thickness of a submarine tunnel under a multi-construction method combination mode, which can ensure the safe construction of a multi-tunnel parallel submarine tunnel under the multi-construction method combination mode and has good economic benefits at the same time.

[0007] To achieve the above purpose, the present invention is realized through the following technical solutions:

[0008] An embodiment of the present invention provides a method for determining the reasonable overburden thickness of a submarine tunnel under a multi-construction method combination mode, including the following steps:

[0009] Obtain the minimum overburden thickness within the construction mileage section where different construction methods are located;

[0010] Use the set construction parameters in the multi-tunnel parallel mode to correct the minimum overburden thickness corresponding to different construction methods, and obtain the final minimum overburden thickness of different construction methods;

[0011] Determine the upper and lower limits of the tunnel longitudinal section according to the maximum overburden thickness within the construction mileage section where different construction methods are located and the obtained final minimum overburden thickness;

[0012] Design the slope range of each section of the tunnel according to the upper and lower limits of the tunnel longitudinal section, and obtain the optimal overburden thickness of the submarine tunnel by combining the obtained slope range with the slope-construction cost calculation model.

[0013] Optionally, the construction methods are the drill and blast method and the shield tunneling method. The submarine tunnel is excavated from one side by the drill and blast method and from the other side by the shield tunneling method.

[0014] Optionally, according to the costs during the engineering construction period and operation period, comprehensively considering the grouting cost, waterproof and drainage cost of the drill and blast method during the construction period, the drainage, segment lining transportation cost of the shield tunneling method, the drainage cost and vehicle energy consumption during the operation period, and combining with the actual construction material cost to obtain the slope-construction cost calculation model.

[0015] Optionally, use the minimum displacement method to obtain the minimum overburden thickness within the construction mileage section where the drill and blast method is located.

[0016] Optionally, use the mechanical equilibrium method to obtain the minimum overburden thickness within the construction mileage section where the shield tunneling method is located.

[0017] Optionally, a correction index is obtained based on the cross-sectional area of the advanced tunnel excavation, the lateral spacing between the advanced tunnel and the two tunnels to be excavated on both sides, the number of enlarged cross-section intervals of the tunnel to be excavated, the axial length of the enlarged chamber, the influence coefficient of the multi-tunnel parallel excavation on the disturbance range of the overlying rock stratum, and the influence coefficient of the enlarged cross-section excavation of the tunnel on the disturbance range of the overlying rock stratum. The minimum overburden thickness of different construction methods is corrected according to the correction index.

[0018] Optionally, the influence coefficient of the multi-tunnel parallel excavation on the disturbance range of the overlying rock stratum is obtained according to the tunnel diameter, the lateral spacing between the advanced tunnel and the two tunnels to be excavated on both sides, and the rock mass correction coefficient;

[0019] The influence coefficient of the enlarged cross-section excavation of the tunnel on the disturbance range of the overlying rock stratum is obtained according to the excavation area of the tunnel to be excavated, the difference between the enlarged cross-section and the conventional cross-section area, and the rock mass correction coefficient.

[0020] Optionally, for each section of the undersea tunnel construction, multiple set gradients are selected within the gradient range of the construction mileage interval where different construction methods are located, and multiple construction gradient schemes are obtained. For each construction gradient scheme, the construction cost is obtained using the gradient-construction cost calculation model, and the construction gradient scheme with the least construction cost is selected as the best construction gradient scheme. Then, the best overburden thickness of the undersea tunnel is obtained according to the best construction gradient scheme.

[0021] Optionally, the gradient range of each section of the tunnel is controlled within 2%-3%.

[0022] Optionally, the engineering geological and hydrogeological information of the tunnel location area is obtained, and the construction mileage interval of different construction methods is determined according to the obtained engineering geological and hydrogeological information.

[0023] The beneficial effects of the present invention are as follows:

[0024] The method for determining the overburden thickness of the undersea tunnel of the present invention corrects the minimum overburden thickness of the mileage interval where different construction methods are located by using the set construction parameters in the multi-tunnel parallel mode, and then determines the upper and lower bounds of the longitudinal section of the tunnel. Considering the specific working conditions in the multi-tunnel parallel mode, the determined overburden thickness meets the construction safety during the multi-tunnel parallel mode construction. At the same time, after determining the gradient range of each section of the tunnel, the best gradient of each section is obtained by combining the gradient-construction cost calculation model. Considering the construction difficulty and construction cost that may be brought by different longitudinal section gradients under the combined construction method, and based on the floating of the cost expenditures under different longitudinal section gradients, a reasonable minimum overburden thickness is determined, which minimizes the construction cost to the greatest extent, has good economic benefits, and minimizes the overall construction cost of the tunnel. Description of the Drawings

[0025] The attached drawings forming a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention.

[0026] Figure 1 is the flowchart of the method in Embodiment 1 of the present invention;

[0027] Figure 2 is the schematic diagram of the upper and lower boundaries of the longitudinal section of the tunnel determined in Embodiment 1 of the present invention; Detailed implementation manners

[0028] Embodiment 1

[0029] This embodiment provides a method for determining the reasonable overburden thickness of a submarine tunnel under a multi-construction method combination mode, as Figure 1 shown, including the following steps:

[0030] Step 1: Obtain the engineering geological and hydrogeological information of the tunnel location area, and determine the construction section mileage intervals of different construction methods according to the obtained engineering geological and hydrogeological information.

[0031] Specifically, it includes the following steps:

[0032] Step 1.1: Understand the engineering background of the proposed extra-long submarine tunnel, analyze the survey data of the site hydrogeological conditions, and obtain the relevant parameters and information of the engineering geology and hydrogeology in the tunnel site area.

[0033] Step 1.2: According to the survey data and engineering hydrogeological parameters and data, preliminarily determine the tunnel plane alignment, the construction section mileage intervals of the drill and blast method and the shield tunneling method.

[0034] In this embodiment, Steps 1.1 - 1.2 are to collect the geological literature materials of the tunnel alignment area, conduct detailed engineering geological mapping, engineering geological geophysical prospecting and engineering geological drilling, investigate the topographic and geomorphic features, geological structures, meteorological conditions, rock properties, distribution of adverse geology and hydrogeological elements in the tunnel alignment area, and conduct in-situ experiments and laboratory experiments, including pumping and water pressure tests, in-hole wave velocity tests, uniaxial compressive tests and triaxial shear tests, to obtain the physical and mechanical parameters and permeability parameters of the rock formation. Obtain the corresponding soil mechanics and hydraulics parameters.

[0035] Based on the survey and experimental results of Steps 1.1 and 1.2, determine the applicable excavation methods for each excavation face of the tunnel.

[0036] In this embodiment, the multi-construction method includes the drill and blast method and the shield tunneling method. The tunnel is excavated from one side by the drill and blast method and from the other side by the shield tunneling method.

[0037] Step 2: Obtain the minimum overburden thickness within the construction mileage intervals of different construction methods.

[0038] In this embodiment, taking the safety of the project main body and construction safety as the main factors, the minimum overburden thickness within the construction mileage intervals of the drill and blast method and the shield tunneling method is preliminarily calculated.

[0039] Among them, for the excavation by the drill and blast method, factors such as the stability of the tunnel surrounding rock, rock fracture damage, excavation blasting vibration effect, in-situ stress, seismic load, grouting reinforcement measures, and tunnel construction safety are mainly considered, and the minimum overburden thickness H 1 is obtained; for the excavation by the shield tunneling method, factors such as the stability of the tunnel excavation face and meeting the anti-floating stability of the tunnel are mainly considered; the minimum overburden thickness H 2 is obtained.

[0040] For the excavation by the drill and blast method, the minimum displacement method is used to solve the minimum overburden thickness H 1 of the drill and blast method, specifically:

[0041] Grade II surrounding rock: h r = 0.037A + 5.192ln(h w ) - 11.296

[0042] Grade III surrounding rock: h r = 0.037A + 5.321ln(h w ) - 11.447

[0043] Grade IV surrounding rock: h r = 0.038A + 5.591ln(h w ) - 11.88

[0044] Among them: h r is the thickness of the overlying layer, that is, the minimum overburden thickness H 1 ;

[0045] A is the tunnel excavation area;

[0046] h w is the water depth, which can be obtained through Step 1.

[0047] In actual construction, due to the broken rock mass, low strength, developed joints and fissures of Grade V surrounding rock, and poor overall self-stabilizing ability, in order to ensure construction safety, it is generally not directly excavated. Instead, pre-reinforcement and water-blocking measures such as advanced bolts, ducts, advanced curtain grouting, and even freezing method are first used to improve the self-stabilizing and anti-seepage ability of the surrounding rock, and then excavation is carried out. After the advanced support of Grade V surrounding rock, its physical and mechanical parameters can basically reach the level of Grade IV. Therefore, only the relevant calculations for Grade II-IV surrounding rock are carried out.

[0048] For shield tunneling method excavation, considering the factors of overburden balance and maximum grouting pressure of the tunnel, the minimum overburden thickness H of the shield tunneling method is solved by the mechanical equilibrium method 2 , specifically:

[0049]

[0050] Among them, π is the pi;

[0051] γ g is the unit weight of the shield tail slurry of the proposed underwater shield tunnel;

[0052] γ s is the saturated unit weight of the soil, obtained through Step 1;

[0053] γ w is the unit weight of groundwater, obtained through Step 1;

[0054] R t is the radius of the proposed underwater shield tunnel;

[0055] t t is the thickness of the segment of the proposed underwater shield tunnel;

[0056] γ c is the unit weight of the segment of the proposed underwater shield tunnel.

[0057] h LL i.e., the minimum overburden thickness H of the shield tunneling method 2

[0058] Step 3: Use the set construction parameters in the multi-tunnel parallel mode to correct the minimum overburden thickness of the mileage intervals where different construction methods are located, and obtain the final minimum overburden thickness of different construction methods in the corresponding construction mileage intervals.

[0059] Specifically:

[0060] The set construction parameters include the excavation cross-sectional area of the advanced tunnel, the number of enlarged cross-sectional intervals of the tunnel to be excavated, the difference in area between the enlarged cross-section and the conventional cross-section, the lateral distance between the advanced tunnel and the two tunnels to be excavated on both sides, the axial length of the enlarged chamber of the tunnel to be excavated, the influence coefficient of the multi-tunnel parallel excavation of the tunnel on the disturbance range of the overlying rock formation, and the influence coefficient of the excavation of the enlarged cross-section of the tunnel on the disturbance range of the overlying rock formation.

[0061] First, determine the first correction index and the second correction index:

[0062] The first correction index:

[0063] The second correction index:

[0064] For the drill and blast method, the corrected final minimum overburden thickness is:

[0065] H 1修正 = H 1 + Δh 1 + Δh 2

[0066] For the shield tunneling method, the minimum overburden thickness in the middle after correction is:

[0067] H 2修正 = H 2 + Δh 1 + Δh 2

[0068] Δh 1 is the corrected thickness value for the influence of parallel tunnel construction on the stability of the overlying surrounding rock;

[0069] Δh 2 is the corrected thickness value for the influence of the enlarged cross-section in the tunnel on the stability of the overlying surrounding rock;

[0070] A is the excavation area of the tunnel to be excavated;

[0071] A FW is the excavation cross-section area of the advanced tunnel;

[0072] n is the number of intervals of the enlarged cross-section of the tunnel to be excavated;

[0073] ΔA is the difference in area between the enlarged cross-section and the conventional cross-section of the tunnel to be excavated;

[0074] L 1 is the lateral distance between the advancing direction of the advanced tunnel and the tunnel to be excavated on the left;

[0075] L 2 is the lateral distance between the advancing direction of the advanced tunnel and the tunnel to be excavated on the right;

[0076] L 3 is the axial length of the enlarged chamber of the tunnel to be excavated;

[0077] α 1 is the influence coefficient of the disturbance range of the overlying rock strata caused by the parallel excavation of multiple tunnels, where:

[0078]

[0079] D is the tunnel diameter of the tunnel to be excavated, γ is the rock mass correction coefficient (0.2 - 0.3 for hard rock, 0.5 - 0.7 for soft rock), and m is the empirical exponent (generally 1.0 - 1.5);

[0080] α 2 is the influence coefficient of the disturbance range of the overlying rock strata caused by the excavation of the enlarged cross-section of the tunnel, where:

[0081]

[0082] Step 4: Figure 2 As shown, the upper and lower limits of the tunnel longitudinal section are determined according to the maximum overburden thickness of the construction mileage intervals of different construction methods and the final minimum overburden thickness obtained;

[0083] In this embodiment, the minimum overburden thickness corrected by the drilling and blasting method and the shield tunneling method can obtain the upper limit of the slope, that is, the upper limit of the tunnel longitudinal section, and the maximum overburden thickness of the drilling and blasting method and the shield tunneling method are obtained according to the existing construction design method, and then the lower limit of the tunnel longitudinal section is obtained. The method for obtaining the lower limit of the tunnel longitudinal section can be obtained by using the existing technology, which will not be described in detail here.

[0084] Step 5: Based on the upper and lower boundaries of the tunnel longitudinal section obtained in step 4, the slope range of each section of the tunnel is designed. For each section, the slope range is controlled within the recommended range of 2%-3%. Combined with the lithology distribution of the strata surveyed in the early stage and the thickness of the overburden of each excavation section under safe conditions, the approximate direction range of the tunnel longitudinal section is drawn. Considering the ventilation efficiency of the tunnel, the slope range is controlled not to exceed the recommended range.

[0085] The design method of the slope range of each section can be designed using existing technology and will not be described in detail here.

[0086] Step 5: Obtain the optimal undersea tunnel overburden thickness based on the obtained slope range and the slope-construction cost calculation model.

[0087] In this embodiment, the safety construction cost E of the engineering construction period (grouting, plugging and reinforcement, etc.) of the above longitudinal section is considered. 1 and maintenance cost during operation period (system pumping and drainage, etc.) 2 , the grouting reinforcement cost Δe of a special point or unit length increase in the longitudinal section under the drilling and blasting construction mode during the comprehensive construction period 1-D , the unit height savings in pumping and drainage costs Δe during the operation period 2-S At the same time, the cost of joint waterproofing at a special point in the longitudinal section of the shield tunneling method during the construction period or the cost of increased height per unit length drop and overpressure drainage cost Δe 1-S , the difficulty of underground connection, the increased drainage cost per unit height during the operation period Δe 2-S , the drainage cost and ventilation cost during the operation period are combined with the actual material cost of the project to obtain the slope-construction cost calculation model.

[0088] The analysis steps are as follows:

[0089] (5.1) Drainage during the construction period. Mechanical drainage is mostly adopted for subsea tunnels. The cost of foundation waterproofing for subsea tunnels accounts for 5%-15% of the total construction cost, including construction materials, construction expenses, etc. Additional costs will be incurred as the tunnel burial depth increases. Generally speaking, for every 1% increase in the longitudinal slope of a subsea tunnel, the drainage cost may increase by 5% to 10%. The specific amount is determined in combination with the actual project situation;

[0090] (5.2) Grouting for water plugging and reinforcement during the construction period. A firm waterproof barrier can be formed through grouting in subsea tunnels to prevent the penetration of groundwater and seawater. In this invention, twice the tunnel excavation area is taken as the grouting area. Combining with the grouting material cost used in the actual project, the increase or decrease in the tunnel mileage caused by the slope change is calculated, and finally the change in the grouting cost is obtained;

[0091] (5.3) Segment transportation. The cost of shield segment transportation accounts for a considerable proportion of the overall construction cost. Generally, the cost of shield segment transportation may account for 10% to 30% of the overall tunnel construction cost. During the construction of a subsea tunnel, the change in the tunnel longitudinal section directly affects the increase in the transportation distance of the segment lining. Combining with the actual project cost of segment lining transportation, the transportation cost required per unit distance is counted;

[0092] (5.4) Vehicle traffic during the operation period. Vehicles need to consume more power to overcome the influence of gravitational potential energy when climbing, while brakes may need to be used to control the speed when going downhill, resulting in the loss of braking energy. These additional energy consumptions will increase the operation cost of the vehicles. Combining with the daily traffic volume of the tunnel and the composition of passing vehicles in the previous research, the possible passing quantities of various types of vehicles and the fuel consumption per unit distance are counted, and the specific cost is calculated in combination with the design life;

[0093] (5.5) Drainage during the operation period. Different from (5.1), more stable and long-term drainage facilities are required during the operation period, and regular maintenance and management are needed to ensure the normal operation of the drainage system. For relatively shallow burial depths, a simple drainage system may only be needed to handle the infiltration of rainwater and groundwater, while for deeper burial depths, a more powerful drainage system may be required to cope with higher water pressure and larger drainage volumes. The cost generated needs to refer to the specific project materials to obtain a more accurate figure.

[0094] Based on the analysis of the above five items with relatively high costs, considering the costs during the project construction period and operation period, comprehensively considering the grouting cost, waterproof and drainage cost during the construction period by the drill and blast method, the drainage cost and the cost of segment lining transportation by the shield tunneling method, the drainage cost and vehicle energy consumption during the operation period, and combining with the actual material cost of the project, the following calculation formula is deduced and summarized:

[0095]

[0096] Among them: E is the total cost;

[0097] n is the total number of different slope sections of the tunnel;

[0098] d i is the horizontal distance of the i-th section;

[0099] y i is the initial slope value of the i-th section;

[0100] is the slope adjustment value of the i-th section;

[0101] e 排1 is the drainage cost required to reduce the unit elevation during the tunnel construction period, which can be obtained in advance;

[0102] e 排2 is the drainage cost required to reduce the unit elevation during the tunnel operation period, which can be obtained in advance;

[0103] A i is the excavation area of the tunnel;

[0104] e 注 is the cost of the grouting material per unit volume, which can be obtained in advance;

[0105] k is the total number of different slope sections of the shield section;

[0106] e 运 is the transportation cost per unit distance of the segment lining, which can be obtained in advance;

[0107] e 车 is the energy consumption cost per unit distance of the vehicle during the operation period, which can be obtained in advance.

[0108] For each section, select a set number of slope values within the determined slope range. The set number of slope values can be obtained by adjusting different amplitudes from an initial slope value. Integrate all sections to obtain slope schemes for multiple tunnels. Substitute the slope values corresponding to each slope scheme into the slope-construction cost calculation model to obtain the total construction cost of each slope scheme. Select the slope scheme with the lowest cost as the optimal slope scheme. Through the optimal slope scheme, a reasonable longitudinal section of the tunnel can be obtained, and then the optimal reasonable overburden thickness of the subsea tunnel can be obtained.

[0109] Using the determination method of this embodiment, on the premise of the minimum overburden thickness of the traditional subsea tunnel drilling and blasting and shield machine tunneling construction methods, comprehensively considering the construction difficulty and construction cost that may be brought by different longitudinal section slopes under the drilling and blasting - shield machine tunneling combined construction method, and determining the reasonable minimum overburden thickness according to the floating of various cost expenditures under different longitudinal section slopes. At the same time, use the set construction parameters in the multi-tunnel parallel mode to correct the minimum overburden thickness of the mileage intervals where different construction methods are located, ensuring the safety during multi-tunnel parallel construction.

[0110] The above are only the preferred embodiments of the present application and are not intended to limit the present application. For those skilled in the art, various modifications and changes can be made to the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included within the protection scope of the present application.

Claims

1. A method for determining the reasonable overburden thickness of a submarine tunnel under a multi-method combination mode, characterized in that: The following steps are involved: Obtain the minimum overburden thickness within the construction mileage interval of different construction methods; The minimum overburden thickness corresponding to different construction methods is corrected by using the set construction parameters in the multi-hole parallel mode to obtain the final minimum overburden thickness of different construction methods; Determine the upper and lower limits of the tunnel longitudinal section based on the maximum overburden thickness of the construction mileage intervals of different construction methods and the final minimum overburden thickness obtained; The slope range of each section of the tunnel is designed according to the upper and lower limits of the tunnel longitudinal section. The optimal undersea tunnel cover thickness is obtained based on the obtained slope range and the slope-construction cost calculation model.

2. The method for determining the reasonable overburden thickness of a submarine tunnel under a multi-method combination mode as claimed in claim 1, characterized in that: The construction methods described are drilling and blasting and shield tunneling. The submarine tunnel is excavated from one side using the drilling and blasting method and from the other side using the shield tunneling method.

3. The method for determining the reasonable overburden thickness of a submarine tunnel under a multi-method combination mode as claimed in claim 2, characterized in that: The slope-construction cost calculation model is obtained based on the costs of the project construction and operation periods, the drilling and blasting grouting costs, drainage costs during the comprehensive construction period, shield tunneling drainage, segment lining transportation costs, operation period drainage costs, vehicle energy consumption, and the actual construction material costs.

4. The method for determining the reasonable overburden thickness of a submarine tunnel under a multi-method combination mode as claimed in claim 2, characterized in that: The minimum displacement method is used to obtain the minimum overburden thickness in the construction mileage interval of the drilling and blasting method.

5. The method for determining the reasonable overburden thickness of a submarine tunnel under a multi-method combination mode as claimed in claim 2, characterized in that: The minimum overburden thickness in the construction mileage interval of the shield tunneling method is obtained by using mechanical balance.

6. The method for determining the reasonable overburden thickness of a submarine tunnel under a multi-method combination mode as claimed in claim 1, characterized in that: Correction indicators are obtained based on the cross-sectional area of ​​the advance tunnel, the lateral distance between the advance tunnel and the tunnels to be excavated on both sides, the number of expanded cross-sectional intervals of the tunnel to be excavated, the axial length of the expanded cavern, the influence coefficient of the disturbance range of the overlying strata caused by the parallel excavation of multiple tunnels, and the influence coefficient of the disturbance range of the expanded cross-sectional excavation of the tunnel on the overlying strata, and the minimum overburden thickness of different construction methods is corrected according to the correction indicators.

7. The method for determining the reasonable overburden thickness of a submarine tunnel in a multi-method combination mode according to claim 6, characterized in that: The influence coefficient of the disturbance range of the overlying rock strata caused by the parallel excavation of multiple tunnels is obtained according to the tunnel diameter, the lateral distance between the preceding tunnel and the tunnels to be excavated on both sides, and the rock mass correction coefficient; The influence coefficient of the disturbance range of the tunnel expansion section excavation on the overlying rock strata is obtained according to the excavation area of ​​the tunnel to be excavated, the difference between the expanded section and the conventional section, and the rock correction coefficient.

8. The method for determining the reasonable overburden thickness of a submarine tunnel under a multi-method combination mode as claimed in claim 1, characterized in that: For each section of the undersea tunnel construction, multiple set slopes are selected within the slope range of the construction mileage interval where different construction methods are obtained to obtain multiple construction slope schemes. For each construction slope scheme, the construction cost is obtained using the slope-construction cost calculation model, and the construction slope scheme with the least construction cost is selected as the optimal construction slope scheme, and then the optimal undersea tunnel cover thickness is obtained based on the optimal construction slope scheme.

9. The method for determining the reasonable overburden thickness of a submarine tunnel under a multi-method combination mode as claimed in claim 1, characterized in that: The first slope range of the construction mileage interval of different construction methods is determined according to the maximum overburden thickness of the construction mileage interval of different construction methods and the final minimum overburden thickness obtained. The final slope range is the overlapping part of the first slope range and the second slope range, and the second slope range is 2%-3%.

10. The method for determining the reasonable overburden thickness of a submarine tunnel in a multi-method combination mode according to claim 1, characterized in that: Obtain engineering geological and hydrogeological information of the tunnel site selection area, and determine the mileage intervals of construction sections for different construction methods based on the acquired engineering geological and hydrogeological information.

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