Method for judging permeation type and permeation failure of binary structure embankment foundation

Abstract

The invention discloses a penetration type and penetration failure judgment method of a binary structure embankment foundation. The method comprises the following steps that it is determined that an embankment foundation is composed of a silty-fine sand layer located on the surface layer and a sand gravel layer located on the lower layer; assuming that the silty-fine sand layer has a critical thickness, and when the actual thickness of the silty-fine sand layer is smaller than the critical thickness, calculating a first critical water level H1 of the embankment foundation; when the actual thickness of the silty-fine sand layer is larger than the critical thickness, the second critical water level H2 of the embankment foundation is obtained; comparing the H1 with the H2, and judging whether the seepage control key layer is a sand gravel layer or a silty-fine sand layer; when the seepage control key layer is a sand gravel layer or a silty-fine sand layer, the embankment foundation seepage failure judgment method is provided. According to the method, whether the key layer of seepage deformation development is the silty-fine sand layer or the sand gravel layer can be rapidly determined, meanwhile, the embankment foundation seepage failure judgment method is provided for different seepage deformation key layers, and technical support is provided for flood season seepage failure emergency rescue and post-flood seepage failure treatment.

Description

Technical Field

[0001] The invention relates to the technical field of dike seepage, and in particular to a method for judging the seepage type and seepage damage of a binary structure dike foundation. Background Art

[0002] In levee projects, when the levee body is composed of relatively impermeable soil, if the levee foundation is sandy soil, the levee foundation is prone to seepage damage during the flood season. If the levee foundation is a single fine sand structure, the seepage damage process can be summarized as the fine sand horizontally contacting and scouring along the bottom of the levee, and the scouring damage continues to develop toward the foot of the levee and the outer river, and finally forms a seepage channel that runs through the upstream and downstream, and the levee collapses; if the levee foundation is a single gravel structure, the seepage damage process can be summarized as the gravel horizontally piping along the bottom of the levee, and the piping damage continues to develop toward the foot of the levee and the outer river, and finally forms a seepage channel that runs through the upstream and downstream, and the levee collapses.

[0003] A consensus has been reached on the seepage failure mechanism and failure type of the above two types of structural embankments, which is convenient for guiding engineering and technical personnel to carry out seepage control design.

[0004] However, in the prior art, for a binary structure embankment foundation composed of a surface layer of fine sand and a lower layer of gravel, whether the seepage deformation occurs in the fine sand layer or the gravel layer, and whether the key layer for controlling the development of the seepage deformation is the fine sand layer or the gravel layer, have not been effectively solved, resulting in the seepage control measures taken by engineering and technical personnel for the binary structure embankment foundation composed of fine sand and gravel being not very targeted, causing hidden dangers to the seepage safety of the embankment foundation. Summary of the invention

[0005] In view of the shortcomings of the prior art, the present invention proposes a method for judging the permeability type and permeability damage of a binary structure embankment foundation, which can quickly judge whether the permeability deformation occurs in the silt sand layer or the gravel layer, and determine whether the key layer for the development of permeability deformation is the silt sand layer or the gravel layer, and provide a method for judging the permeability damage of the embankment foundation, providing technical support for the rescue of permeability damage during the flood season and the control of permeability damage after the flood.

[0006] In order to achieve the above-mentioned purpose, the present invention is designed to provide a method for judging the permeability type and permeability damage of a binary structure embankment foundation, which is particularly characterized in that it includes the following steps:

[0007] S1) Determine that the embankment foundation is composed of a silty sand layer at the surface and a sandy gravel layer at the bottom;

[0008] S2) Assuming that there is a critical thickness of the silt-fine sand layer, when the actual thickness of the silt-fine sand layer is less than the critical thickness, the silt-fine sand layer undergoes a horizontally evolving vertical flow deformation, and at the same time, the gravel surface layer undergoes a horizontal pipe flow deformation, and the first critical water level of the embankment foundation is obtained; when the actual thickness of the silt-fine sand layer is greater than the critical thickness, the silt-fine sand layer undergoes a shallow horizontally evolving vertical flow deformation, and at the same time, the gravel surface layer does not undergo a seepage deformation, and the second critical water level of the embankment foundation is obtained;

[0009] The first critical water level of the embankment foundation is expressed by the following formula

[0010]

[0011] In the formula,

[0012] H1 represents the critical water level of the first embankment foundation,

[0013] G s represents the density of the gravel layer relative to water,

[0014] n1 represents the porosity of the gravel layer,

[0015] d5 represents the particle size value corresponding to the cumulative distribution percentage of sand and gravel reaching 5%.

[0016] d 20 It indicates the particle size value corresponding to the cumulative distribution percentage of 20% in sand and gravel.

[0017] L represents the shortest distance between the water inlet and the center of the infiltration outlet;

[0018] The second critical water level of the embankment foundation is expressed by the following formula

[0019]

[0020] In the formula,

[0021] H2 represents the critical water level of the second embankment foundation.

[0022] γ'1 is the floating density of the gravel layer,

[0023] n1 represents the porosity of the gravel layer,

[0024] h1 represents the actual thickness of the gravel layer.

[0025] γ w Indicates the weight of water.

[0026] γ'2 represents the floating density of the fine sand layer,

[0027] n2 represents the porosity of the fine sand layer,

[0028] h2 represents the actual thickness of the fine sand layer;

[0029] S3) Compare the sizes of H1 and H2. If H2>H1, the fine sand layer undergoes horizontal vertical flow deformation, and the surface of the gravel layer undergoes horizontal piping deformation. The key layer for permeability control is the gravel layer. If H2>H1, the fine sand layer undergoes horizontal vertical flow deformation, and the surface of the gravel layer undergoes horizontal piping deformation.

[0030] S4) When the seepage deformation develops to the point where the upstream and downstream seepage channels are connected, the embankment foundation will be damaged by seepage; for H2>H1, when the sum of the allowable loss head of each section of the vertical channel and the horizontal channel ∑Δh1 is greater than the total head H, the embankment foundation will not be damaged by seepage, otherwise, seepage damage will occur, that is,

[0031] ∑Δh1=J x L+J z h2>H

[0032] Where:

[0033] ∑Δh1 is the sum of the allowable loss heads of the vertical channel and the horizontal channel,

[0034] H is the water level difference between the upstream and downstream of the dike,

[0035] J x is the allowable hydraulic gradient for the gravel layer.

[0036] L is the width of the embankment bottom,

[0037] J z Allowable hydraulic gradient for vertical flow of fine sand layer;

[0038] h2 is the actual thickness of the fine sand layer;

[0039] For H2

[0040] ∑Δh2=J z h2>H

[0041] In the formula,

[0042] ∑Δh2 is the sum of the allowable loss heads in the vertical channel,

[0043] H is the water level difference between the upstream and downstream of the dike,

[0044] J z The allowable hydraulic gradient for vertical flow of fine sand layer is:

[0045] h2 is the actual thickness of the silt-fine sand layer. ​​

[0046] Furthermore, in S1), an impermeable embankment is provided on the upper part of the embankment foundation, and the permeability coefficient of the fine sand layer in the embankment foundation is smaller than the permeability coefficient of the gravel layer.

[0047] Furthermore, in S2), the particle size of the fine sand in the fine sand layer ranges from 0.1 to 2 mm, and the particle size of the gravel in the gravel layer ranges from 0.5 to 10 mm.

[0048] Furthermore, in S2), when piping occurs, the maximum value of the shortest distance L between the water inlet and the center of the infiltration outlet is the width of the embankment bottom.

[0049] Furthermore, in S3), when the key layer for permeability control is a gravel layer, as long as the horizontal piping deformation of the gravel layer does not further develop to the outer river, the embankment foundation will not suffer from permeability damage; when the key layer for permeability control is a silt sand layer, as long as the horizontal flow soil deformation of the surface layer of the silt sand does not further develop to the outer river, the embankment foundation will not suffer from permeability damage.

[0050] Furthermore, in S3), when the key layer for permeability control is a gravel layer, the anti-seepage measures need to penetrate into the gravel layer to inhibit the further development of the horizontal deformation of the gravel layer towards piping; when the key layer for permeability control is a silt-fine sand layer, the anti-seepage measures need to penetrate into the surface layer of the silt-fine sand to inhibit the further development of the horizontal deformation of the silt-fine sand layer towards soil flow.

[0051] Furthermore, in S3), when the key layer for permeability control is a gravel layer, the anti-seepage measure is to set a vertical anti-seepage wall inside the gravel layer; when the key layer for permeability control is a fine sand layer, the anti-seepage measure is to set a vertical anti-seepage wall on the surface of the fine sand.

[0052] The advantages of the present invention are:

[0053] 1. When the embankment foundation is a binary structure consisting of a surface silt-fine sand layer and a lower gravel layer, the present invention simply and clearly determines whether the embankment foundation seepage deformation occurs in the silt-fine sand layer or the gravel layer according to the respective permeability coefficients of the silt-fine sand and the gravel, the thickness of the soil layer and other factors, and whether the key layer for controlling the development of seepage deformation is the silt-fine sand layer or the gravel layer, thereby facilitating engineering and technical personnel to take seepage control measures for the binary structure embankment foundation consisting of silt-fine sand and gravel, and realizing the pertinence, reliability and safety of the seepage control measures;

[0054] 2. The present invention proposes a method for judging seepage damage of embankment foundation when the key seepage control layer is a gravel layer or a silt-fine sand layer, which can provide technical support for the emergency rescue of seepage damage of the binary structure embankment foundation composed of silt-fine sand and gravel during the flood season and the treatment of seepage damage after the flood season, and has important theoretical and engineering practical significance;

[0055] The method for judging the permeability type and permeability damage of the binary structure embankment foundation of the present invention can quickly judge whether the permeability deformation occurs in the silt sand layer or the gravel layer, and determine whether the key layer for the development of the permeability deformation is the silt sand layer or the gravel layer. At the same time, a method for judging the permeability damage of the embankment foundation is proposed for different key layers of permeability deformation, which can provide technical support for the rescue of permeability damage during the flood season and the control of permeability damage after the flood. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 It is a cross-sectional view of the dual-structure embankment foundation in the present invention along the upstream and downstream directions;

[0057] Figure 2 It is a flow chart of the present invention. DETAILED DESCRIPTION

[0058] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0059] In the description of the present invention, it is necessary to understand that the terms "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the invention.

[0060] like Figure 2 As shown, the method for judging the seepage type and seepage damage of the binary structure embankment foundation of the present invention includes the following steps:

[0061] S1) Determine that the embankment foundation is composed of a silt-fine sand layer at the surface and a gravel-gravel layer at the bottom. Specifically, an impermeable embankment is arranged on the upper part of the embankment foundation, and the permeability coefficient of the silt-fine sand layer in the embankment foundation is less than the permeability coefficient of the gravel-gravel layer.

[0062] like Figure 1 As shown in the figure, since the permeability coefficient of silt sand is smaller than that of gravel, the confined water head in the gravel layer is very large. When the thickness of the silt sand layer is thin, the silt sand layer is penetrated due to the large confined water pressure in the lower gravel layer, and vertical soil flow occurs. At this time, the hydraulic gradient along the surface of the gravel is the largest, and the surface of the gravel layer is the preferred development direction of seepage deformation. Horizontal piping occurs in the gravel, and the embankment foundation failure mainly advances to the outside of the embankment in the form of horizontal soil flow of silt sand.

[0063] When the thickness of the silt sand layer is large, although the pressure head in the gravel layer is large, the silt sand layer is not easily penetrated by the pressure water due to its thickness, and the silt sand layer will not undergo vertical soil flow damage. At this time, the silt sand layer mainly undergoes vertical soil flow deformation in the horizontal direction under the bottom of the embankment, and the gravel layer does not undergo seepage deformation.

[0064] Therefore, based on the above analysis, for the binary structure embankment foundation of silt-fine sand + gravel, the thickness of the silt-fine sand layer determines the occurrence mechanism and type of seepage failure of the embankment foundation.

[0065] S2) Assuming that there is a critical thickness of the silt sand layer, when the actual thickness of the silt sand layer is less than the critical thickness, the silt sand layer undergoes a horizontally evolving vertical flow deformation, and at the same time, the gravel surface layer undergoes a horizontal piping deformation, and the first critical water level of the embankment foundation is obtained; when the actual thickness of the silt sand layer is greater than the critical thickness, the silt sand layer undergoes a shallow horizontally evolving vertical flow deformation, and at the same time, the gravel surface layer does not undergo a seepage deformation, and the second critical water level of the embankment foundation is obtained.

[0066] The first critical water level of the embankment foundation is expressed by the following formula

[0067]

[0068] In the formula,

[0069] H1 represents the critical water level of the first embankment foundation,

[0070] G s represents the density of the gravel layer relative to water,

[0071] n1 represents the porosity of the gravel layer,

[0072] d5 represents the particle size value corresponding to the cumulative distribution percentage of sand and gravel reaching 5%.

[0073] d 20 It indicates the particle size value corresponding to the cumulative distribution percentage of 20% in sand and gravel.

[0074] L represents the shortest distance between the water inlet and the center of the infiltration outlet.

[0075] The second critical water level of the embankment foundation is expressed by the following formula

[0076]

[0077] In the formula,

[0078] H2 represents the critical water level of the second embankment foundation.

[0079] γ'1 is the floating density of the gravel layer,

[0080] n1 represents the porosity of the gravel layer,

[0081] h1 represents the actual thickness of the gravel layer.

[0082] γ w Indicates the weight of water.

[0083] γ'2 represents the floating density of the fine sand layer,

[0084] n2 represents the porosity of the fine sand layer,

[0085] h2 represents the actual thickness of the fine sand layer.

[0086] Specifically, the particle size of the fine sand in the fine sand layer ranges from 0.1 to 2 mm, and the particle size of the gravel in the gravel layer ranges from 0.5 to 10 mm.

[0087] Specifically, when piping occurs, the maximum value of the shortest distance L between the water inlet and the center of the infiltration outlet is the embankment bottom width. The shortest distance L between the water inlet and the center of the infiltration outlet is obtained by measurement.

[0088] S3) Compare the sizes of H1 and H2. If H2>H1, the fine sand layer undergoes horizontal vertical flow deformation, and the surface of the gravel layer undergoes horizontal piping deformation. The key layer for permeability control is the gravel layer. If H2>H1, the fine sand layer undergoes horizontal vertical flow deformation, and the surface of the gravel layer undergoes horizontal piping deformation.

[0089] According to the above S2)~S3), the type of seepage deformation and the key layer of seepage control of the binary structure embankment foundation can be quickly and easily determined. However, the seepage deformation of the embankment foundation does not mean that the embankment foundation has been damaged by seepage. Only when the seepage deformation develops to the point where the upstream and downstream seepage channels are connected, will the embankment foundation be damaged by seepage.

[0090] Therefore, when the key layer for permeability control is the gravel layer, as long as the horizontal piping deformation of the gravel layer does not further develop to the outer river, the embankment foundation will not suffer from permeability damage; when the key layer for permeability control is the silt sand layer, as long as the horizontal flow soil deformation of the surface layer of the silt sand does not further develop to the outer river, the embankment foundation will not suffer from permeability damage.

[0091] ​When the embankment foundation is a binary structure consisting of an upper silt-fine sand layer and a lower gravel layer, the present invention simply and clearly determines whether the embankment foundation seepage deformation occurs in the silt-fine sand layer or the gravel layer according to the respective permeability coefficients of the silt-fine sand and the gravel, the soil layer thickness and other factors. At the same time, when the key permeability control layer is a gravel layer or a silt-fine sand layer, a method for judging the seepage damage of the embankment foundation is proposed, so that it is convenient for engineering and technical personnel to take seepage control measures for the binary structure embankment foundation of silt-fine sand and gravel, and realize the pertinence, reliability and safety of the seepage control measures, and provide technical support for the flood season seepage damage rescue and post-flood seepage damage control of the binary structure embankment foundation composed of silt-fine sand and gravel, which has important theoretical and engineering practical significance.

[0092] The present invention will be further described below in conjunction with specific embodiments.

[0093] Example 1: Take the results of Experiment 3 provided in the literature (Wang Shuang, Chen Jiansheng, Zhou Peng. Experimental study on the effect of fine sand layer thickness on piping in three-layer embankment foundation [J]. Rock and Soil Mechanics, 2015, 36(10): 2847-2854.) as an example. The thickness of the gravel is 22 cm, n2 = 0.21, G s2 =2.65,ρ d2 =2.10g / cm 3 ; Colored sand thickness 2cm, n1 = 0.4, G s1 =2.62,ρ d1 =1.58g / cm 3 According to the original text, L = 60cm, and the formula for the first critical water level of the embankment foundation is used to obtain H1 = 49.0cm, which is consistent with the fact that the infiltration channel has been connected when the upstream water level is 46.5cm. According to the formula for the second critical water level of the embankment foundation, H2 = 23.9cm.

[0094] This conclusion is consistent with the description of Experiment 3 in the literature: "The channel is mainly formed on the surface of the colored sand layer, that is, shallow pipe burst damage, and the damage form is mainly horizontal soil flow damage."

[0095] Example 2: Take the experimental results provided in the literature (Mao Haitao, Wang Zhengcheng, Wang Xiaoju, et al. Experimental study on pipe burst failure of overlying sand layer of embankment foundation containing shallow permeable layer [J]. Civil Engineering, Architecture and Environmental Engineering, 2018, 40(3): 73-80.) as an example. The thickness of gravel and pebbles is 15 cm, n1 = 0.42, G s1 =2.65, d5=0.48mm, d 20 =2.8mm,ρ d1 =1.98g / cm 3 , k1=5.18×10 -1 cm / s; sandy soil thickness 8-20 cm, n2 = 0.33, G s2 ​=2.65,ρ d2 =2.42g / cm 3 , k2=5.07×10 -3 cm / s. According to the original text, L = 75cm, H1 = 15.7cm is obtained from the formula for the first critical water level of the embankment foundation, and H2 = 18.8cm is obtained from the formula for the second critical water level of the embankment foundation. Since H2>H1, vertical soil flow occurs in the sand layer, and horizontal pipe flow damage occurs on the surface of the gravel layer.

[0096] This conclusion is consistent with the conclusion of the literature experiment that "the thin overlying sand layer is easily "broken through", and only coarse particles remain in the sand near the surface of the sand and gravel layer, and the sand skeleton particles are connected to the sand and gravel layer to form a preferential seepage channel."

[0097] Specifically, when the key layer for permeability control is a gravel layer, the anti-seepage measures need to penetrate into the gravel layer to inhibit the further development of horizontal deformation of the gravel layer towards piping; when the key layer for permeability control is a silt-fine sand layer, the anti-seepage measures need to penetrate into the surface layer of the silt-fine sand to inhibit the further development of horizontal deformation of the silt-fine sand layer towards soil flow.

[0098] Preferably, when the key layer for permeability control is a gravel layer, the anti-seepage measure is to set a vertical anti-seepage wall inside the gravel layer; when the key layer for permeability control is a silt-fine sand layer, the anti-seepage measure is to set a vertical anti-seepage wall on the surface of the silt-fine sand.

[0099] S4) When the seepage deformation develops to the point where the upstream and downstream seepage channels are connected, the embankment foundation will be damaged by seepage; for H2>H1, when the sum of the allowable loss head of each section of the vertical channel and the horizontal channel ∑Δh1 is greater than the total head H, the embankment foundation will not be damaged by seepage, otherwise, seepage damage will occur, that is,

[0100] ∑Δh1=J x L+J z h2>H

[0101] Where:

[0102] ∑Δh1 is the sum of the allowable loss heads of the vertical channel and the horizontal channel,

[0103] H is the water level difference between the upstream and downstream of the dike,

[0104] J x is the horizontal allowable hydraulic gradient of the gravel layer, which can be taken as 0.1.

[0105] L is the width of the embankment bottom,

[0106] J z The allowable hydraulic gradient for vertical flow of fine sand layer can be 0.5.

[0107] h2 is the actual thickness of the fine sand layer;

[0108] For H2

[0109] ∑Δh2=J z h2>H

[0110] In the formula,

[0111] ∑Δh2 is the sum of the allowable loss heads in the vertical channel,

[0112] H is the water level difference between the upstream and downstream of the dike,

[0113] J z The allowable hydraulic gradient for vertical flow of fine sand layer can be 0.5.

[0114] h2 is the actual thickness of the silt-fine sand layer.

[0115] The method for judging the permeability type and permeability damage of the binary structure embankment foundation of the present invention can quickly judge whether the permeability deformation occurs in the silt sand layer or the gravel layer, and determine whether the key layer for the development of the permeability deformation is the silt sand layer or the gravel layer. At the same time, a method for judging the permeability damage of the embankment foundation is proposed for different key layers of permeability deformation, which can provide technical support for the rescue of permeability damage during the flood season and the control of permeability damage after the flood.

[0116] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.​

Claims

1. A method for judging the seepage type and seepage damage of a binary structure embankment foundation, characterized in that: It includes the following steps: S1) Determine that the dike foundation consists of a fine silt and fine sand layer on the surface and a gravel layer on the lower layer; S2) Assume that there is a critical thickness for the fine silt and fine sand layer. When the actual thickness of the fine silt and fine sand layer is less than the critical thickness, vertical flow soil deformation with horizontal evolution occurs in the fine silt and fine sand layer, and at the same time, piping deformation in the horizontal direction occurs on the surface of the gravel layer, and the first critical water level of the dike foundation is obtained; when the actual thickness of the fine silt and fine sand layer is greater than the critical thickness, shallow vertical flow soil deformation with horizontal evolution occurs in the fine silt and fine sand layer, and at the same time, no seepage deformation occurs on the surface of the gravel layer, and the second critical water level of the dike foundation is obtained; The first critical water level of the dike foundation is expressed by the following formula In the formula, H1 represents the first critical water level of the dike foundation, G s represents the density of the gravel layer relative to water, n1 represents the porosity of the gravel layer, d5 represents the particle size value corresponding to when the cumulative distribution percentage in the gravel reaches 5%, d 20 It indicates the particle size value corresponding to the cumulative distribution percentage of 20% in sand and gravel. L represents the shortest distance between the water inlet and the center of the seepage outlet; The second critical water level of the dike foundation is expressed by the following formula In the formula, H2 represents the second critical water level of the dike foundation, γ'1 is the buoyant unit weight of the gravel layer, n1 represents the porosity of the gravel layer, h1 represents the actual thickness of the gravel layer, γ w Indicates the weight of water. γ'2 represents the buoyant unit weight of the fine silt and fine sand layer, n2 represents the porosity of the fine silt and fine sand layer, h2 represents the actual thickness of the fine silt and fine sand layer; S3) Compare the magnitudes of H1 and H2. If H2 > H1, then vertical flow soil deformation with horizontal evolution occurs in the fine silt and fine sand layer, and horizontal piping deformation occurs on the surface of the gravel layer, and the key layer for seepage control is the gravel layer; if H2 < H1, then vertical flow soil deformation with horizontal evolution occurs on the surface of the fine silt and fine sand layer, and no seepage deformation occurs in the gravel layer, and the key layer for seepage control is the fine silt and fine sand layer; if H2 = H1, then h2 is the critical thickness of the fine silt and fine sand layer; S4) When the seepage deformation develops to the point where the upstream and downstream seepage channels are connected, seepage failure of the dike foundation will occur. For H2 > H1, when the sum of the allowable head losses ∑Δh1 of each section of the vertical channel and the horizontal channel is greater than the total head H, seepage failure of the dike foundation will not occur, otherwise, seepage failure will occur, that is ∑Δh1=J x L+J z h2>H In the formula: ∑Δh1 is the sum of the allowable head losses of the vertical channel and the horizontal channel, H is the water level difference between the upstream and downstream of the dike, J x is the allowable hydraulic gradient for the gravel layer. L is the bottom width of the dike, J z Allowable hydraulic gradient for vertical flow of fine sand layer; h2 is the actual thickness of the fine silt and fine sand layer; For H2 < H1, when the sum of the allowable head losses ∑Δh2 of the vertical channel is greater than the total head, seepage failure of the dike foundation will not occur, otherwise, seepage failure will occur, that is ∑Δh2=J z h2>H In the formula, ∑Δh2 is the sum of the allowable head losses of the vertical channel, H is the water level difference between the upstream and downstream of the dike, J z The allowable hydraulic gradient for vertical flow of fine sand layer is: h2 is the actual thickness of the fine silt and fine sand layer.

2. The method for judging the seepage type and seepage damage of a dual-structure embankment foundation according to claim 1, characterized in that: In S1), an impervious dike is provided on the upper part of the dike foundation, and the permeability coefficient of the fine silt and fine sand layer in the dike foundation is less than that of the gravel layer.

3. The method for determining the type of seepage and seepage damage of a dual-structure embankment according to claim 2, characterized in that: In S2), the particle size range of the fine silt and fine sand in the fine silt and fine sand layer is 0.1 - 2 mm, and the particle size range of the gravel in the gravel layer is 0.5 - 10 mm.

4. The method for determining the seepage type and seepage damage of a dual-structure embankment foundation according to claim 3, characterized in that: In S2), when piping occurs, the maximum value of the shortest distance L between the water inlet and the center of the seepage outlet is the bottom width of the dike.

5. The method for determining the seepage type and seepage damage of a dual-structure embankment foundation according to claim 1, characterized in that: In S3), when the key permeability control layer is the gravel layer, as long as the horizontal piping deformation of the gravel layer does not further develop to the outer river, the embankment foundation will not suffer from permeability damage; when the key permeability control layer is the silt-fine sand layer, as long as the horizontal flow soil deformation of the silt-fine sand surface layer does not further develop to the outer river, the embankment foundation will not suffer from permeability damage.

6. The method for determining the seepage type and seepage damage of a dual-structure embankment according to claim 5, characterized in that: In S3), when the key layer for permeability control is the gravel layer, the anti-permeability measures need to penetrate into the gravel layer to inhibit the further development of the horizontal deformation of the gravel layer towards piping; when the key layer for permeability control is the silt-fine sand layer, the anti-permeability measures need to penetrate into the surface layer of the silt-fine sand to inhibit the further development of the horizontal deformation of the silt-fine sand layer towards flowing soil.

7. The method for determining the seepage type and seepage damage of a dual-structure embankment according to claim 6, characterized in that: In S3), when the key layer for permeability control is a gravel layer, the anti-permeability measure is to set a vertical anti-permeability wall inside the gravel layer; when the key layer for permeability control is a silt-fine sand layer, the anti-permeability measure is to set a vertical anti-permeability wall on the surface of the silt-fine sand.

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