Damping grouting design method for shield tunnel in composite stratum

Abstract

The invention discloses a damping grouting design method for a shield tunnel in a composite stratum. The damping grouting design method comprises the steps that a beam-spring structure numerical model is established; determining the rigidity of a grouting layer erection spring according to the traditional grouting slurry to obtain a resistance coefficient of a formation-grouting layer combination; the safety performance of the duct pieces and the joints is calculated and analyzed, and displacement and internal force calculation results of the shield tunnel in the composite stratum are obtained; if the calculation result meets the safety regulation, it is indicated that the anti-seismic requirement of the shield tunnel can be met by selecting the traditional slurry material for grouting; if the calculation result does not meet the safety regulation, determining the rigidity difference according to the typical section, dynamically adjusting the resistance coefficient parameter of the stratum-grouting layer combination until the safety regulation is met, calculating the difference value at the moment, and determining the optimal difference value; calculating the stiffness of the erection spring according to the adjusted value, and determining the elastic modulus of the grouting material according to a stiffness formula of the erection spring; and determining the type of the grouting material according to the elastic modulus of the grouting material.

Description

Technical Field

[0001] The invention belongs to the field of shock-absorbing construction design of underground engineering, and in particular relates to a shock-absorbing grouting design method for a shield tunnel under a complex stratum. Background Art

[0002] During the actual construction process, shield tunnels inevitably pass through high-intensity earthquake zones, where earthquake damage is severe and post-earthquake repair is difficult. Under the action of earthquakes, tunnels move with the surrounding strata. Their safety mainly depends on the deformation of the surrounding strata and their ability to resist deformation. Especially in complex strata environments, the propagation speed and stiffness of seismic waves in different strata vary greatly. Therefore, the internal forces and displacements generated by the structure at places where geological conditions suddenly change are large, making the structure more susceptible to damage. Therefore, improving the seismic resistance of shield tunnels in complex strata is an important part of the shield tunnel design process. There are three main ways to design shield tunnels for seismic resistance and shock absorption, as follows:

[0003] (1) Strengthen the stratum through grouting and other means to improve its anti-deformation ability. However, in actual engineering, grouting reinforcement of strata is often costly, labor-intensive, and the reinforcement quality is unstable.

[0004] (2) For the seismic design of shield tunnels in heterogeneous medium strata, seismic resistant components can be added to the weak points, or flexible segment rings or steel segment rings can be used to adapt to the displacement of the surrounding strata through their own deformation, thereby reducing the internal forces of the structure caused by earthquakes. However, this requires complex design technology, and such components are often only applicable to specific stratum combination conditions. When the stratum combination conditions change, such components are difficult to use again. In addition, such construction technology is relatively complex and has poor universality, which increases the shield tunnel construction design process and construction costs.

[0005] (3) Related studies and examples have shown that setting up a seismic isolation layer can effectively reduce the damage to shield tunnels during earthquakes. By using the key process of grouting behind the shield tunnel wall, asphalt-based grouting materials are used to form a softer seismic isolation layer between the stratum and the tunnel to prevent the displacement and force of stratum deformation from being transmitted to the pipe segments, thereby protecting the tunnel structure. Related studies have proven the feasibility of this operation. However, for more complex sites, there is no systematic design solution for the grouting parameter values. Summary of the invention

[0006] In order to solve the technical problems existing in the prior art, the purpose of the present invention is to provide an energy-absorbing and shock-absorbing structure grouting design method suitable for complex strata in response to the shock absorption needs of shield tunnels, so that it is more suitable for the protection of tunnel structures in earthquakes in complex strata, while improving the feasibility of the structure.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A composite stratum shield tunnel shock absorption grouting design method comprises the following steps:

[0009] Step 1: Obtain relevant parameters of the shield tunnel in the composite stratum; select a typical section of the shield tunnel in a representative complex stratum;

[0010] Step 2: Establish a numerical model of the beam-spring structure; determine the stratum structure to determine the stiffness of the stratum spring, and determine the stiffness of the grouting layer frame spring according to the traditional grouting slurry to obtain the combined resistance coefficient of the stratum-grouting layer;

[0011] Step 3: Apply the combined resistance coefficient of the stratum-grouting layer and the seismic force to the beam-spring structure numerical model, calculate and analyze the safety performance of the segments and joints, obtain the displacement and internal force calculation results of the shield tunnel in the composite stratum, and perform strength verification;

[0012] Step 4: If the calculation result meets the safety requirements, it means that the selection of traditional grouting materials for grouting can meet the seismic requirements of the shield tunnel, and the traditional grouting slurry can be used for grouting construction; if the calculation result does not meet the safety requirements, the stiffness difference is determined based on the typical section, and the parameters of the combined resistance coefficient of the stratum-grouting layer are dynamically adjusted. Step (3) is repeated until the safety requirements are met. At this time, the difference between the combined resistance coefficient of the stratum-grouting layer before and after the adjustment is calculated to determine the optimal difference between the two; the spring stiffness of the grouting layer is calculated based on the adjusted combined resistance coefficient of the stratum-grouting layer, and then the elastic modulus of the grouting material is determined based on the spring stiffness formula;

[0013] Step 5: Determine the type of grouting material based on the elastic modulus of the grouting material.

[0014] As a further technical solution, step 2 simulates the shield tunnel as a homogeneous circular ring beam, the annular joint can be simulated by elastic hinges, and the surrounding rock and soil can be simulated by stratum springs; the grouting layer frame springs are connected in series with the stratum springs to simulate the grouting layer and soil constraints.

[0015] As a further technical solution, the formation spring stiffness of different formations is determined by the formula:

[0016] k 1i =K 1i Ld

[0017] Where:

[0018] k 1i —formation spring stiffness (N / m);

[0019] K 1i —Base coefficient (N / m 3 );

[0020] L—central spring spacing of foundation (m);

[0021] d—Calculated length of the stratum along the longitudinal direction of the underground structure (m).

[0022] As a further technical solution, the stiffness calculation formula of the grouting layer frame spring is as follows:

[0023] k 2i =EI

[0024] E—elastic modulus;

[0025] I—Intersection moment of inertia.

[0026] As a further technical solution, the calculation formula of the stratum-grouting layer combined resistance coefficient is as follows:

[0027]

[0028] k 1i — formation spring stiffness;

[0029] k 2i —Spring stiffness of the grouting layer frame.

[0030] As a further technical solution, when the static method is used for calculation, the seismic effects include the seismic inertia force of the lining's own weight, the seismic inertia force of the overlying soil columns, and the seismic lateral earth pressure increment.

[0031] As a further technical solution, the lining self-weight seismic inertia force includes the segment self-weight horizontal and vertical seismic inertia forces;

[0032] E ih =A h m is =C i C s Am is

[0033] E iv =K v E ih =K v C i C s Am is

[0034] C i —Seismic importance coefficient;

[0035] C s —Site seismic peak acceleration adjustment factor;

[0036] A—peak value of horizontal basic ground acceleration;

[0037] m is—The mass of the tunnel segment at the calculation point (kg);

[0038] k v —The ratio of the vertical peak acceleration to the horizontal peak acceleration.

[0039] As a further technical solution, the seismic inertia force of the overlying soil column includes the horizontal seismic force of the overlying soil column and the vertical seismic force of the overlying soil column; the horizontal seismic force of the overlying soil column is:

[0040] F ih =A h Q i / g

[0041] The vertical seismic force of the overlying soil column is:

[0042] F iv =K v A h Q i / g

[0043] A h —Horizontal design ground motion peak acceleration

[0044] g—acceleration due to gravity;

[0045] k v —Ratio of vertical peak acceleration to horizontal peak acceleration;

[0046] Q i —vertical earth pressure of overlying soil column;

[0047] As a further technical solution, the lateral earth pressure increment during an earthquake includes the inner earth pressure increment and the outer earth pressure increment:

[0048] The inner earth pressure increment is:

[0049] Δe 1i =C i C s γh 1i (λ 1 -λ)

[0050] The outer side earth pressure increment is:

[0051] Δe 2i =C i C s γh 2i (λ 2 -λ′)(9)

[0052] λ, λ′—inner and outer side constant pressure coefficients;

[0053] h 1i 、h2i —The distance from any point i inside or outside the segment to the ground surface (m).

[0054] C i —Seismic importance coefficient;

[0055] C s —Site adjustment factor;

[0056] γ—surrounding rock mass;

[0057] λ 1 , 2 —Lateral pressure coefficient during internal and external earthquakes.

[0058] As a further technical solution, in step 3, when performing strength verification, the opening and displacement of the shield segment joints should not exceed the design allowable values, the displacement of the axial steel bars at the expansion joints should be less than the yield displacement, the rotation angle at the expansion joints should be less than the yield rotation angle, and the joint opening should be less than the allowable opening of the ductile sealing pad.

[0059] Compared with the prior art, the shield tunnel shock absorption design method of the present invention has the following advantages:

[0060] 1. Based on the stiffness coupling theory, the present invention proposes a shield tunnel construction design method, and proposes a basis for selecting grouting materials based on the existing shield tunnel construction process, making the most of the existing process, greatly reducing the cost of process improvement, and quantitatively analyzing the seismic performance of the grouting layer, providing a theoretical basis for the shock absorption effect of the shield tunnel.

[0061] 2. The seismic measures of the present invention are based on the grouting reinforcement method. During construction, slurry that meets the safety factor method is selected for grouting. Compared with the use of special segments and seismic-resistant structures, the design method of this scheme has a wider range of applications and can be adapted to complex strata with multiple layers of different strata combinations, effectively reducing construction costs and meeting the needs of reducing costs and increasing efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 Flow chart of the shock absorbing grouting design of the present invention;

[0063] Figure 2 A simplified diagram of the stratum distribution at a typical section of the shield tunnel of the present invention;

[0064] Figure 3 A simplified diagram of the load-structure model of the present invention;

[0065] Figure 4 A schematic diagram of the grouting layer frame spring and the formation spring connected in series in the present invention;

[0066] Figure 5 The combined resistance coefficient adjustment flow chart proposed by the present invention;

[0067] In the figure: 1-first stratum; 2-shield tunnel; 3-second stratum; 4-third stratum; 5-grouting layer; 6-spring for building the second stratum; 7-equivalent circular ring beam of shield tunnel; 8-spring for building the third stratum; 9-equivalent elastic hinge of annular joint; 10-segment deadweight; 11-inner soil pressure increment; 12-outer soil pressure increment; 13-seismic inertia force of upper soil column; 14-spring for building the grouting layer; 15-stratum spring; 16-spring for building the first stratum. DETAILED DESCRIPTION

[0068] The present invention is described below in conjunction with the accompanying drawings, specifically as follows:

[0069] (1) Conduct detailed geological surveys at the construction site to collect information on the stratigraphic structure, surrounding rock grade, elastic modulus of different strata, site seismic fortification intensity, and segment elastic modulus of the shield tunnel area.

[0070] (2) According to the design data, a typical section of the shield tunnel under a representative complex stratum is selected (typical section method: section at the joint at the demarcation point; section at the place where the spring stiffness difference of the stratum is large; section at the place where the terrain is undulating) to determine the stratum distribution at the typical section. Taking a complex stratum with three different stratum combinations as an example, the stratum distribution diagram is shown in the figure below; it includes the first stratum 1, the second stratum 3, and the third stratum 4; in the three strata, the positions of the shield tunnel 2 and the grouting layer 5 in the three strata are as follows: Figure 2 As shown;

[0071] A numerical model of a beam-spring structure is established, and the tunnel structure is equivalent to a homogeneous circular beam for simulation. Figure 3 In (shield tunnel equivalent circular ring beam 7), the ring joint can be simulated by elastic hinge (see Figure 3 The annular joint equivalent elastic hinge 9 in the beam-spring model is simulated by springs (see the second stratum frame spring 6, the first stratum frame spring 16, and the third stratum frame spring 8 for details). Figure 3 shown.

[0072] (3) There are grouting layers 5 and soil constraints around the tunnel, so the construction spring is simulated by connecting the grouting layer construction spring 14 and the ground spring 15 in series, as shown in the schematic diagram. Figure 4 shown.

[0073] For the stratum spring, due to the complex stratum conditions where the tunnel is located, the stiffness of different strata varies greatly, and the spring stiffness established is also quite different. The spring stiffness of different strata is determined by the formula:

[0074] k 1i =K 1i Ld (1)

[0075] Where:

[0076] k 1i —formation spring stiffness (N / m);

[0077] K 1i —Base coefficient (N / m 3 ), can be taken according to the current national standard "Code for Geotechnical Engineering Investigation of Urban Rail Transit" GB50307;

[0078] L—central spring spacing of foundation (m);

[0079] d—Calculated length of the stratum along the longitudinal direction of the underground structure (m).

[0080] For the grouting layer spring, the calculation formula for the spring stiffness is as follows:

[0081] k 2i =EI (2)

[0082] E—elastic modulus

[0083] I—Cross-sectional moment of inertia

[0084] According to the stiffness coupling effect, the stratum spring stiffness and the grouting layer spring stiffness constitute the stratum-grouting layer combined resistance coefficient, which is used as an indicator for the seismic verification of the beam-spring model. The calculation formula of the stratum-grouting layer combined resistance coefficient is shown in (1):

[0085]

[0086] k 1i — Formation spring stiffness Spring stiffness

[0087] k 2i —Spring stiffness of grouting layer

[0088] When using the static method for calculation, the earthquake action should include the earthquake inertia force of the lining self-weight, the earthquake inertia force of the overlying soil column, and the earthquake lateral earth pressure increment:

[0089] Furthermore, the lining self-weight seismic inertia force includes the lining self-weight horizontal seismic inertia force and the vertical seismic inertia force. The specific calculation formulas are as follows (4) and (5):

[0090] E ih =A h m is =C i C s Am is (4)

[0091] E iv =K v E ih =K v Ci C s Am is (5)

[0092] E ih —Horizontal seismic inertia force of lining deadweight;

[0093] E iv —Vertical seismic inertia force of lining deadweight;

[0094] C i —Seismic importance coefficient;

[0095] C s —Site seismic peak acceleration adjustment factor;

[0096] A—peak value of horizontal basic ground acceleration;

[0097] m is —The mass of the tunnel segment at the calculation point (kg);

[0098] k v —Ratio of vertical peak acceleration to horizontal peak acceleration

[0099] In the calculation of the seismic inertia force of the overlying soil column, it is assumed that the seismic inertia force of the overlying soil column acts on the center of mass of the soil column unit. When calculating the internal force of the structure, since the force does not act directly on the structure, it is necessary to use the translation theorem of force to transform the seismic inertia force of the overlying soil column to the structure, and then apply the transformed nodal force and added point bending moment to the structure.

[0100] The horizontal seismic force of the overlying soil column is as follows:

[0101] F ih =A h Q i / g (6)

[0102] The vertical seismic force of the overlying soil column is as follows:

[0103] F iv =K v A h Q i / g (7)

[0104] A h —Horizontal design ground motion peak acceleration

[0105] g—acceleration due to gravity;

[0106] k v —Ratio of vertical peak acceleration to horizontal peak acceleration;

[0107] Q i—vertical earth pressure of overlying soil column;

[0108] The lateral earth pressure increment during an earthquake should be calculated according to the following formula and applied in an anti-symmetrical manner, and the solution is as follows:

[0109] 1) The increment of the inner earth pressure is as follows:

[0110] Δe 1i =C i C s γh 1i (λ 1 -λ) (8)

[0111] 2) The increment of the outer soil pressure is as follows:

[0112] Δe 2i =C i C s γh 2i (λ 2 -λ′)(9)

[0113] Where: λ, λ′—inner and outer side constant pressure coefficients;

[0114] h 1i 、h 2i —The distance from any point i inside or outside the segment to the ground surface (m).

[0115] C i —Seismic importance coefficient;

[0116] C s —Site adjustment factor;

[0117] γ—surrounding rock mass;

[0118] λ 1 , 2 —Lateral pressure coefficient during internal and external earthquakes.

[0119] (4) The above parameters and numerical models are imported into numerical calculation software, where the grouting slurry parameters are preferentially assumed to be traditional slurry parameters. According to the solution steps of the static method, the seismic action is equivalent to a static load and applied to the beam-spring model. The safety performance of the segments and joints is calculated and analyzed by numerical calculation software, and the displacement and internal force calculation results of the shield tunnel in the composite stratum are obtained.

[0120] According to the specification "Code for Seismic Design of Highway Tunnels" JTG / T2232-01—2019, the safety factor method is used for strength verification, and the structural strength should comply with the provisions of the following formula.

[0121]

[0122] S()—action effect function related to the load acting on the structure;

[0123] R()—structural resistance effect function related to the strength of structural materials and the geometric dimensions of components;

[0124] F r —Combination value of loads acting on the structure;

[0125] f—strength value of the material;

[0126] α k —Geometric parameter values ​​of the structure;

[0127] C—limit constraint value of the structure;

[0128] γ 0 —Component working condition coefficient, the value is 1.0;

[0129] γ 1 —Additional structural safety factor, the value is 1.0;

[0130] γ m —Partial coefficient of action on the structure

[0131] γ f —Material performance partial coefficient

[0132] According to the code, the deformation calculation under earthquake combination should comply with the following formula

[0133] S q ”C(11)

[0134] S q —Effect value of earthquake action combination

[0135] C—Corresponding limits specified in the design for deformation, displacement, etc.

[0136] The opening and displacement of shield segment joints should not exceed the design allowable values, the displacement of the axial steel bars (bolts) at the expansion joints should be less than the yield displacement, the rotation angle at the expansion joints should be less than the yield rotation angle, the joint opening should be ≤2mm (rock formations or with important buildings (structures) nearby), or ≤4mm (large-section shield tunnels or located in soft soil formations), and should be less than the allowable opening of the ductile sealing pad.

[0137] (5) Determine whether the traditional grout selected in step (4) meets the seismic safety requirements of the shield tunnel; if the calculation results meet the above safety requirements, it means that the selection of traditional grout materials for grouting can meet the seismic requirements of the shield tunnel, and the traditional grouting grout can be used for grouting construction; if the calculation results do not meet the above safety requirements, the stiffness difference is determined based on the typical section, and the combined resistance coefficient K of the stratum-grouting layer is dynamically adjusted. iParameters, and then repeat step (4) until the safety regulations are met (import calculation software to obtain the results of internal stress displacement), then calculate the K before and after adjustment i The difference between them is used to determine the optimal difference between the two. i Use formula (3) to calculate the spring stiffness k 2i , and then determine the elastic modulus E of the grouting material according to the spring stiffness formula (2). For specific steps, refer to Figure 5 ;

[0138] (6) According to the slurry elastic modulus E inverted in step (5), determine the type of grouting material. Some grouting material selections are shown in Table 1 below:

[0139] Table 1 Elastic modulus of grouting materials

[0140]

[0141] Based on the stiffness coupling theory, the present invention innovatively proposes a shield tunnel construction design method, and proposes a basis for selecting grouting materials based on the existing shield tunnel construction technology. It makes maximum use of the existing technology, greatly reduces the cost of process improvement, and quantitatively analyzes the seismic performance of the grouting layer, providing a theoretical basis for the shock absorption effect of the shield tunnel.

[0142] 2 The seismic measures in this design are based on the grouting reinforcement method. During construction, grouting is performed using a slurry that meets the safety factor method. Compared with the use of special segments and seismic-resistant structures, this design method has a wider range of applications and can be adapted to complex strata with multiple layers of different strata, effectively reducing construction costs and meeting the needs of reducing costs and increasing efficiency.

Claims

1. A design method for shock-absorbing grouting of composite stratum shield tunnels, characterized in that: The following steps are involved: Step 1: Obtain relevant parameters of the composite stratum shield tunnel; Select a typical section of a shield tunnel located under representative complex strata; Step 2: Establish a numerical model of the beam-spring structure; determine the stratum structure to determine the stiffness of the stratum spring, and determine the stiffness of the grouting layer frame spring according to the traditional grouting slurry to obtain the combined resistance coefficient of the stratum-grouting layer; Step 3: Apply the combined resistance coefficient of the stratum-grouting layer and the seismic force to the beam-spring structure numerical model, calculate and analyze the safety performance of the segments and joints, obtain the displacement and internal force calculation results of the shield tunnel in the composite stratum, and perform strength verification; Step 4: If the calculation result meets the safety requirements, it means that the selection of traditional grouting materials for grouting can meet the seismic requirements of the shield tunnel, and the traditional grouting slurry can be used for grouting construction; if the calculation result does not meet the safety requirements, the stiffness difference is determined based on the typical section, and the parameters of the combined resistance coefficient of the stratum-grouting layer are dynamically adjusted. Step (3) is repeated until the safety requirements are met. At this time, the difference between the combined resistance coefficient of the stratum-grouting layer before and after the adjustment is calculated to determine the optimal difference between the two; the spring stiffness of the grouting layer is calculated based on the adjusted combined resistance coefficient of the stratum-grouting layer, and then the elastic modulus of the grouting material is determined based on the spring stiffness formula; Step 5: Determine the type of grouting material based on the elastic modulus of the grouting material.

2. The composite stratum shield tunnel shock absorption grouting design method according to claim 1, characterized in that: In step 2, the shield tunnel is simulated as a homogeneous circular ring beam, the annular joint can be simulated by elastic hinges, and the surrounding rock and soil are simulated by stratum springs; the grouting layer frame spring is connected in series with the stratum spring to simulate the grouting layer and soil constraints.

3. The composite stratum shield tunnel shock absorption grouting design method according to claim 2 is characterized in that: The formation spring stiffness of different formations is determined by the formula: k 1i =K 1i Ld Where: k 1i — formation spring stiffness; K 1i —base coefficient; L—central spring spacing of foundation; d—The calculated length of the stratum along the longitudinal direction of the underground structure.

4. The composite stratum shield tunnel shock absorption grouting design method according to claim 2, characterized in that: The stiffness k of the grouting layer spring 2i The calculation formula is as follows: k 2i =NO E—elastic modulus; I—Intersection moment of inertia.

5. The composite stratum shield tunnel shock absorption grouting design method according to claim 1, characterized in that: The combined resistance coefficient of the stratum-grouting layer K i The calculation formula is as follows: k 1i — formation spring stiffness; k 2i —Spring stiffness of the grouting layer frame.

6. The composite stratum shield tunnel shock absorption grouting design method according to claim 1, characterized in that: When the static method is used for calculation, the seismic effects include the seismic inertia force of the lining's own weight, the seismic inertia force of the overlying soil columns, and the seismic lateral earth pressure increment.

7. The composite stratum shield tunnel shock absorption grouting design method according to claim 6, characterized in that: The lining self-weight seismic inertia force includes the lining self-weight horizontal seismic inertia force and the lining self-weight vertical seismic inertia force; E ih =A h m is =C i C s Am is E iv =K v E ih =K v C i C s Am is E ih —Horizontal seismic inertia force of lining self-weight; E iv —Vertical seismic inertia force of lining deadweight; C i —Seismic importance coefficient; C s —Site seismic peak acceleration adjustment factor; A—peak value of horizontal basic ground acceleration; m is —Quality of the calculated points of the tunnel segments; K v —The ratio of the vertical peak acceleration to the horizontal peak acceleration.

8. The composite stratum shield tunnel shock absorption grouting design method according to claim 6, characterized in that: The seismic inertia force of the overlying soil column includes the horizontal seismic force of the overlying soil column and the vertical seismic force of the overlying soil column; the horizontal seismic force F ih for: F ih =A h Q i / g Vertical seismic force F of the overlying soil column iv for: F iv =K v A h Q i / g A h —Horizontal design ground motion peak acceleration g—acceleration due to gravity; k v —Ratio of vertical peak acceleration to horizontal peak acceleration; Q i —Vertical earth pressure of overlying soil column.

9. The composite stratum shield tunnel shock absorption grouting design method according to claim 8, characterized in that: The lateral earth pressure increment during an earthquake includes the inner earth pressure increment and the outer earth pressure increment: The inner earth pressure increment is: No 1i =C i C s c h 1i (λ1-λ) (8) The outer side earth pressure increment is: No 2i =C i C s c h 2i (λ2-λ′)(9) λ, λ′—inner and outer side constant pressure coefficients; h 1i 、h 2i —The distance from any point i inside or outside the segment to the ground surface; C i —Seismic importance coefficient; C s —Site adjustment factor; γ—surrounding rock mass; λ1, λ2—lateral pressure coefficients during internal and external earthquakes.

10. The composite stratum shield tunnel shock absorption grouting design method according to claim 1, characterized in that: In step 3, when performing strength verification, the opening and displacement of the shield segment joints should not exceed the design allowable values, the displacement of the axial steel bars at the expansion joint should be less than the yield displacement, the rotation angle at the expansion joint should be less than the yield rotation angle, and the joint opening should be less than the allowable opening of the ductile sealing pad.

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