Saline-alkali soil concealed pipe drainage system and construction method

Abstract

The invention discloses a saline-alkali soil concealed pipe drainage system and a construction method, and belongs to the technical field of saline-alkali soil treatment. Comprising a main pipe and branch pipes which are arranged in a fishbone shape. The peripheries of the main pipe and the branch pipes are coated with the filter wrapping layer, and the grading filter material layer is backfilled in a groove around the filter wrapping layer; the main drainage ditch is connected with the water outlet end of the main pipe, the cross section of the main drainage ditch is trapezoidal, and a composite lining structure composed of concrete plate buckling bricks and geotechnical cloth is lined in the main drainage ditch; an anti-salt-return isolation layer; the fishbone-shaped arrangement and the sealing joints are adopted, the water flow resistance of the system is small, and the continuous and effective drainage capacity is improved. The underground concealed pipe structure avoids the problems of collapse and land occupation of the open channel, and the maintenance cost per unit area is greatly reduced. Due to the double-layer geotechnical cloth and the three-stage gradient filter material, particle size grading filtration is achieved, and it is guaranteed that the permeability coefficient attenuation rate of the system is extremely low. And pipe distribution parameters are dynamically adjusted based on real-time detection of soil conductivity, so that the pipes are distributed accurately corresponding to the saline-alkali enrichment layer.

Description

Technical Field

[0001] This invention relates to the field of saline-alkali land management technology, and in particular to a subsurface drainage system and construction method for saline-alkali land. Background Technology

[0002] Current drainage methods for saline-alkali land mainly employ open channel drainage and vertical well desalination technologies. Open channel drainage suffers from problems such as large land occupation, susceptibility to collapse, and high maintenance costs; vertical well desalination requires significant investment and is only suitable for areas with high groundwater levels. Currently emerging underground pipe drainage technology involves burying permeable pipes, but this method suffers from issues such as filter media caking and rapid decline in drainage efficiency.

[0003] For example, the filter layer of traditional underground drainage systems is easily clogged by fine particles and fails within 3-5 years; leakage and damage are prone to occur at the connection between the drainage pipe and the main ditch; the differences in regional soil texture are not taken into account, and the design of the burial depth and spacing of the drainage pipe is unreasonable; and there is a lack of structural design to prevent secondary soil salinization. Summary of the Invention

[0004] The purpose of this invention is to solve the problems of easy caking of filter media, easy leakage at joints, unreasonable pipe layout parameters, and lack of anti-salinization mechanism in the existing technology, and to propose a buried pipe drainage system and construction method for saline-alkali land.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A buried pipe drainage system and construction method for saline-alkali land, including Main pipe, wherein the main pipe is a PVC corrugated pipe with an opening ratio of 15-20%; Multiple branch pipes are connected to the main pipe at an angle of 45-60°, forming a fishbone-like arrangement; A sealing structure is provided at the connection node between the branch pipe and the main pipe, and the sealing structure is a rubber sealing ring with a Shore hardness of 60±5. A filter wrapping layer is wrapped around the outer periphery of the main pipe and the branch pipe. The filter wrapping layer includes a lower geotextile and an upper geotextile from the inside to the outside. The pore size of the lower geotextile is less than 0.5 mm, the pore size of the upper geotextile is less than 0.1 mm, and the joints are welded by hot melt welding. A graded filter media layer is backfilled in the trench around the filter wrapping layer. The graded filter media layer, from bottom to top, includes a 5cm thick 5-10mm gravel layer, a 3cm thick 2-5mm coarse sand layer, and a 2cm thick 0.5-1mm fine sand layer. The main drainage ditch is connected to the outlet end of the main pipe. The cross-section of the main drainage ditch is trapezoidal, and its inner lining is a composite lining structure composed of concrete slabs, snap bricks, and geotextile. The overlap width of the geotextile is not less than 20cm. The anti-salinity isolation layer is laid 15cm below the ground surface and is formed by mixing soil with 20-30% medium sand.

[0006] In some embodiments, the slope ratio of the main drainage ditch is 1:1.

[0007] In some embodiments, the geotextile in the filter wrapping layer is a 200g / m² long-filament geotextile.

[0008] In some embodiments, the geotextile in the filter wrapping layer may be replaced with a natural fiber woven layer.

[0009] In some embodiments, the natural fiber woven layer is a palm fiber woven layer.

[0010] In some embodiments, the concrete slab snap-fit ​​bricks in the composite lining structure of the main drainage ditch can be replaced with eco-friendly concrete lining.

[0011] In some embodiments, the medium sand in the anti-salinization isolation layer can be replaced with rice husk charcoal, and the mixing ratio of the rice husk charcoal to the soil is 1:5.

[0012] In some embodiments, in the composite lining structure of the main drainage ditch, a layer of bentonite waterproofing blanket is also provided between the concrete slab snap-fit ​​bricks and the geotextile to prevent capillary water from rising.

[0013] A construction method for a buried pipe drainage system in saline-alkali land includes the following steps: S1. Excavation and trenching in saline-alkali land: Use a laser level to level the construction area and calibrate the ground slope to 0.1-0.3%; measure the soil electrical conductivity, and dynamically determine the trench depth to be 0.8-1.2m and the spacing to be 20-30m based on the measurement results, and then carry out excavation; S2. Pipeline laying and connection: PVC corrugated pipes with an opening ratio of 15-20% are laid as main pipes in the trench; branch pipes are connected to the main pipes at an angle of 45-60° in a fishbone pattern, and rubber sealing rings with a Shore hardness of 60±5 are installed at the connection nodes. S3. Geotextile wrapping: The main pipe and branch pipe are wrapped with double-layer geotextile, wherein the lower geotextile close to the pipe has a pore size of less than 0.5mm and the upper geotextile on the outside has a pore size of less than 0.1mm, and the joints of the geotextile are heat-fused welded. S4. Filter material backfilling: Backfill the filter material in layers around the pipe wrapped with geotextile. First, backfill with 5cm thick 5-10mm gravel, then backfill with 3cm thick 2-5mm coarse sand, and finally backfill with 2cm thick 0.5-1mm fine sand. S5. Construction of main drainage ditch: Excavate and trim the main drainage ditch to a trapezoidal cross-section, and use concrete slabs, snap bricks and geotextile for composite lining construction, ensuring that the overlap width of the geotextile is not less than 20cm, and properly connect it with the outlet end of the main pipe. S6. Construction of anti-salinization layer: Mix 20-30% medium sand into the soil layer 15cm below the surface and mix evenly to form an anti-salinization isolation layer.

[0014] In some embodiments, in step S5, the concrete slab clip-on bricks can be replaced with eco-concrete for lining construction. In step S6, the medium sand can be replaced with rice husk charcoal and mixed with the soil at a ratio of 1:5.

[0015] Compared with the prior art, the present invention provides a buried pipe drainage system and construction method for saline-alkali land, which has the following beneficial effects.

[0016] 1. This invention, through the adoption of a fishbone-shaped arrangement and sealed nodes, reduces system flow resistance and increases continuous and effective drainage capacity. Simultaneously, the underground pipe structure avoids the collapse and land occupation problems associated with open channels, significantly reducing maintenance costs per unit area.

[0017] 2. This invention achieves particle size classification filtration through a composite filtration structure consisting of double-layer geotextile and three-stage gradient filter media, namely gravel, coarse sand and fine sand. It has an automatic backwashing function, effectively prevents caking, and ensures that the system has an extremely low permeability coefficient decay rate within five years.

[0018] 3. The sand-mixed or rice husk charcoal anti-salinization isolation layer below the ground surface of this invention works in conjunction with the seepage-proof lining structure of the main ditch to effectively block capillary water rise and lateral seepage, and has the function of regulating seasonal salt fluctuations, thereby keeping the EC value of the topsoil stable in the long term.

[0019] 4. The various components of the drainage system of the present invention, such as prefabricated corrugated pipes, snap-fit ​​bricks, and graded filter media, are easy to obtain and standardized to install, and have modular construction characteristics, which makes the overall project progress significantly faster than traditional methods.

[0020] Other advantages, objectives and features of the invention will be set forth in part in the description which follows; and in part will be apparent to those skilled in the art upon examination of the following description; or may be learned from practice of the invention. Attached Figure Description

[0021] Figure 1 This is a flowchart illustrating the construction process of this invention.

[0022] Figure 2 The corrugated pipe backfill section of the present invention Figure 1 .

[0023] Figure 3 The corrugated pipe backfill section of the present invention Figure 2 . Detailed Implementation

[0024] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0025] Reference Figure 1-3 A buried pipe drainage system for saline-alkali land, comprising: Main pipe, wherein the main pipe is a PVC corrugated pipe with an opening ratio of 15-20%; Multiple branch pipes are connected to the main pipe at an angle of 45-60° to form a fishbone-like arrangement, which reduces the water flow resistance of the drainage system.

[0026] A sealing structure is provided at the connection node between the branch pipe and the main pipe. The sealing structure is a rubber sealing ring with a Shore hardness of 60±5 to reduce leakage at the connection node between the branch pipe and the main pipe.

[0027] A filter wrapping layer, covering the outer perimeter of the main pipe and branch pipes, comprises a lower geotextile layer and an upper geotextile layer from the inside out. The pore size of the lower geotextile layer is less than 0.5 mm, and the pore size of the upper geotextile layer is less than 0.1 mm, with the seams welded by heat fusion. The geotextile can be selected as a 200 g / m² long-filament geotextile. Alternatively, the geotextile in the filter wrapping layer can be replaced with a natural fiber woven layer. The natural fiber woven layer is preferably a palm fiber woven layer.

[0028] A graded filter media layer is backfilled in the trench surrounding the filter wrapping layer. From bottom to top, the graded filter media layer comprises a 5cm thick layer of 5-10mm gravel, a 3cm thick layer of 2-5mm coarse sand, and a 2cm thick layer of 0.5-1mm fine sand. Through the composite structure of a double-layer geotextile and a three-layer gradient filter media, the different particle sizes of the filter media are used for graded filtration to improve the anti-clogging effect.

[0029] The main drainage ditch connects to the outlet of the main pipe. The main drainage ditch has a trapezoidal cross-section and is lined with a composite lining structure consisting of concrete slabs, interlocking bricks, and geotextile fabric, with the geotextile fabric overlapping at least 20cm. The slope ratio of the main drainage ditch is 1:1. The concrete slabs and interlocking bricks in the composite lining structure of the main drainage ditch can be replaced with eco-friendly concrete lining.

[0030] The anti-salinization isolation layer is laid 15cm below the ground surface and is formed by mixing soil with 20-30% medium sand. The medium sand can be replaced with rice husk charcoal, and the mixing ratio of rice husk charcoal to soil is 1:5.

[0031] As a preferred embodiment, an additional layer of bentonite waterproofing blanket can be added as a core seepage-proof layer to the composite lining structure of the main drainage ditch, which consists of concrete slabs, interlocking bricks, and geotextile. This creates a synergistic composite lining system with enhanced seepage prevention capabilities, working together with the upper concrete slab and the lower geotextile. This synergistic structure significantly enhances the ditch's seepage prevention capacity, effectively hindering capillary rise and lateral seepage, thereby preventing external salts from flowing back into the improved area with the water and reducing salt backflow in the test area.

[0032] By using a trapezoidal cross-section concrete slab with snap-fit ​​brick lining and an overlap of ≥20cm geotextile composite structure, the lining structure's erosion resistance is increased by 3 times, and its service life reaches 15 years.

[0033] The present invention has the following technical effects: The drainage system, with its herringbone layout and sealed nodes, exhibits low flow resistance and increased continuous and effective drainage capacity. Simultaneously, the underground pipe structure avoids the collapse and land occupation issues associated with open channels, significantly reducing maintenance costs per unit area.

[0034] The filtration system, through a composite filtration structure consisting of double-layer geotextile and three-stage gradient filter media, namely gravel, coarse sand and fine sand, achieves particle size classification filtration, has an automatic backwashing function, effectively prevents caking, and ensures that the system has an extremely low permeability coefficient decay rate within five years.

[0035] Dynamic pipe layout, based on real-time detection of soil conductivity, dynamically adjusts pipe layout parameters, namely depth and spacing, ensuring that the pipes are precisely positioned to correspond to the saline-alkali enrichment layer. This ability to adapt to soil salinity distribution greatly improves desalination efficiency and saves energy consumption in desalination operations.

[0036] The anti-salinization structure, with its sand-mixed or rice husk charcoal anti-salinization isolation layer below the surface and its seepage-proof lining structure in the main ditch, works in synergy to effectively block capillary water rise and lateral seepage, and has the function of regulating seasonal salt fluctuations, thereby keeping the EC value of the topsoil stable in the long term.

[0037] The various components of the system, such as prefabricated corrugated pipes, snap-fit ​​bricks, and graded filter media, are easy to obtain and standardized to install, and have modular construction characteristics, which makes the overall project progress significantly faster than traditional methods.

[0038] Example 1 This embodiment is based on the above-mentioned underground drainage system for saline-alkali land, and provides a construction method for an underground drainage system for saline-alkali land that has been tested and verified in Linxi County, Chifeng City. This embodiment was based on soil survey, and the target area is moderately saline-alkali soil with a field water holding capacity of 23%.

[0039] The specific steps are as follows: S1. Excavation and trenching in saline-alkali land: Use a laser level to calibrate the ground slope to 0.3%. Based on soil survey data, determine the spacing between underground pipes to be 20 meters and the burial depth to be 1.0 meter. Excavate trenches with a bottom width of 0.3 meters and a slope ratio of 1:0.5.

[0040] S11. Topsoil treatment and sand layer construction: The 0.4-meter-thick topsoil above the excavated trench is stripped and temporarily stored. A 0.05-meter-thick layer of fine, medium and coarse mixed sand is evenly spread on the stripped ground. After the land is leveled, a 0.12-meter-thick layer of filter sand with gravel diameter of 3-5 centimeters is then filled in.

[0041] S2. Pipeline Laying and Connection: First, lay a 0.1-meter-thick, 3-5 cm-diameter gravel bedding layer at the bottom of the trench. Then, lay PE double-wall corrugated pipes as underground drainage pipes. The inner diameter is 19.5 cm, the outer diameter is 22 cm, and the perforation rate is 15-20%, using irregular drilling with 8 mm diameter holes. Branch pipes and main pipes are arranged in a herringbone pattern and connected at a 45° angle. Rubber sealing rings with a Shore hardness of 60 are used to seal the joints.

[0042] S3. Geotextile wrapping: Use 200g / m² geotextile to wrap the pipe in two layers. The lower layer of geotextile close to the pipe has a pore size of <0.5mm, and the upper layer of geotextile on the outside has a pore size of <0.1mm. The joints are heat-fused and welded.

[0043] S4. Filter media backfilling: Then, backfill the filter media around the wrapped pipe: first backfill with 0.12-meter-thick gravel with a diameter of 3-5 centimeters until it covers the top of the pipe.

[0044] S41. Topsoil backfilling and isolation layer construction: The 0.4-meter-thick topsoil stripped in step S11 is mixed evenly with 30% medium sand and backfilled to form an anti-salinization isolation layer.

[0045] S5. Construction of the main drainage ditch: Excavate the main drainage ditch with a trapezoidal cross-section and a slope ratio of 1:1. Use concrete slabs, interlocking bricks, and geotextile for composite lining, with the geotextile overlapping 20 cm wide, and properly connect it to the outlet end of the underground pipe system.

[0046] S6. Sand layer covering: A 5cm thick layer of sand is laid on top of the anti-salt return isolation layer.

[0047] Example 2 This embodiment provides a construction method for a buried pipe drainage system in saline-alkali land. Measurements in this embodiment show that the soil electrical conductivity in the area is significantly high, indicating severely saline-alkali clay. This embodiment is suitable for areas with high groundwater levels or more severe salinization, by reducing the spacing and increasing the burial depth to enhance drainage and salt removal effects.

[0048] The specific steps are as follows: S1. Excavation and trenching in saline-alkali land: Calibrate the slope to 0.2% using a laser level. Dynamically adjust the spacing of underground pipes to 15 meters and the burial depth to 1.2 meters. Excavate trenches with a bottom width of 0.3 meters and a slope ratio of 1:0.5.

[0049] S11. Topsoil treatment and sand layer construction: The 0.4-meter-thick topsoil above the excavated trench is stripped and temporarily stored. A 0.05-meter-thick layer of fine, medium and coarse mixed sand is evenly spread on the stripped ground. After the land is leveled, a 0.12-meter-thick layer of filter sand with gravel diameter of 3-5 centimeters is then filled in.

[0050] S2. Pipeline Laying and Connection: First, lay a 0.1-meter-thick, 3-5 cm-diameter gravel bedding layer at the bottom of the trench. Then, lay PE double-wall corrugated pipes as underground drainage pipes. The inner diameter is 19.5 cm, the outer diameter is 22 cm, and the perforation rate is 15-20%, using irregular drilling with 8 mm diameter holes. Branch pipes and main pipes are arranged in a herringbone pattern and connected at a 45° angle. Rubber sealing rings with a Shore hardness of 60 are used to seal the joints.

[0051] S3. Geotextile wrapping: Use 200g / m² geotextile to wrap the pipe in two layers. The lower layer of geotextile close to the pipe has a pore size of <0.5mm, and the upper layer of geotextile on the outside has a pore size of <0.1mm. The joints are heat-fused and welded.

[0052] S4. Filter media backfilling: Then, backfill the filter media around the wrapped pipe: first backfill with 0.12-meter-thick gravel with a diameter of 3-5 centimeters until it covers the top of the pipe.

[0053] S41. Topsoil backfilling and isolation layer construction: The 0.4-meter-thick topsoil stripped in step S11 is mixed evenly with 35% medium sand and backfilled to form an anti-salinization isolation layer.

[0054] S5. Construction of the main drainage ditch: Excavate the main drainage ditch with a trapezoidal cross-section and a slope ratio of 1:1. Use concrete slabs, interlocking bricks, and geotextile for composite lining, with the geotextile overlapping 20 cm wide, and properly connect it to the outlet end of the underground pipe system.

[0055] S6. Sand layer covering: A 5cm thick layer of sand is laid on top of the anti-salt return isolation layer.

[0056] In step S41, 35% medium sand is added to enhance the anti-saltation effect.

[0057] Example 3 This embodiment provides a construction method for a buried pipe drainage system in saline-alkali land. Measurements in this embodiment show that the soil conductivity in this area is low, indicating it is slightly saline-alkali sandy loam. This embodiment is suitable for areas with mild salinity or limited budgets, by appropriately increasing the spacing to reduce material and construction costs.

[0058] The specific steps are as follows: S1. Excavation and trenching in saline-alkali land: Calibrate the slope to 0.3% using a laser level. Dynamically adjust the spacing of underground pipes to 30 meters and the burial depth to 0.9 meters. Excavate trenches with a bottom width of 0.3 meters and a slope ratio of 1:0.5.

[0059] S11. Topsoil treatment and sand layer construction: The 0.4-meter-thick topsoil above the excavated trench is stripped and temporarily stored. A 0.05-meter-thick layer of fine, medium and coarse mixed sand is evenly spread on the stripped ground. After the land is leveled, a 0.12-meter-thick layer of filter sand with gravel diameter of 3-5 centimeters is then filled in.

[0060] S2. Pipeline Laying and Connection: First, lay a 0.1-meter-thick, 3-5 cm-diameter gravel bedding layer at the bottom of the trench. Then, lay PE double-wall corrugated pipes as underground drainage pipes. The inner diameter is 19.5 cm, the outer diameter is 22 cm, and the perforation rate is 15-20%, using irregular drilling with 8 mm diameter holes. Branch pipes and main pipes are arranged in a herringbone pattern and connected at a 45° angle. Rubber sealing rings with a Shore hardness of 60 are used to seal the joints.

[0061] S3. Geotextile wrapping: Use 200g / m² geotextile to wrap the pipe in two layers. The lower layer of geotextile close to the pipe has a pore size of <0.5mm, and the upper layer of geotextile on the outside has a pore size of <0.1mm. The joints are heat-fused and welded.

[0062] S4. Filter media backfilling: Then, backfill the filter media around the wrapped pipe: first backfill with 0.12-meter-thick gravel with a diameter of 3-5 centimeters until it covers the top of the pipe.

[0063] S41. Topsoil backfilling and isolation layer construction: Mix the 0.4-meter-thick topsoil stripped in step S11 with 20% medium sand evenly and backfill to form an anti-salinization isolation layer.

[0064] S5. Construction of the main drainage ditch: Excavate the main drainage ditch with a trapezoidal cross-section and a slope ratio of 1:1. Use concrete slabs, interlocking bricks, and geotextile for composite lining, with the geotextile overlapping 20 cm wide, and properly connect it to the outlet end of the underground pipe system.

[0065] S6. Sand layer covering: A 5cm thick layer of sand is laid on top of the anti-salt return isolation layer.

[0066] To further reduce costs, in step S2 above, a double-walled corrugated pipe with an outer diameter of 22cm is selected for the drainage pipe.

[0067] In step S41, the medium sand can be replaced with rice husk charcoal and mixed with the soil at a ratio of 1:5.

[0068] In step S5, the main drainage ditch is lined with eco-friendly concrete to balance cost and ecological benefits.

[0069] The above-mentioned embodiment 1 uses real-time soil conductivity detection data to dynamically adjust and determine the pipe laying parameters of 20m spacing and 1.0m burial depth, so that the pipe laying position accurately corresponds to the salt and alkali enrichment layer, significantly improving the salt removal efficiency and achieving the optimization goal of water and energy saving.

[0070] Example 2, based on real-time soil conductivity monitoring data, dynamically adjusts and determines the pipe layout parameters to a spacing of 15m and a burial depth of 1.2m. This ensures the pipe placement precisely corresponds to the heavily saline-alkali enriched layer, significantly enhancing desalination efficiency and effectively controlling the groundwater level. As a high-density drainage enhancement implementation method, this scheme allows for faster drainage initiation and a greater desalination capacity per unit area, effectively addressing more severe waterlogging and salinization problems. However, the corresponding cost is slightly increased.

[0071] Example 3, based on real-time soil conductivity monitoring data, dynamically adjusts and determines the pipe layout parameters to a spacing of 30m and a burial depth of 0.9m. This ensures that the pipe placement meets salt drainage requirements while minimizing material and construction costs, demonstrating the method's economic adaptability. This is an economical wide-spacing implementation method suitable for the initial stages of mild saline-alkali land improvement or large-scale promotion.

[0072] Example 1 is the standard and optimal implementation method. Under this combination of parameters, the system has the highest drainage and salt removal efficiency, which can stabilize the soil moisture content in the topsoil layer at about 80% of the field capacity, and meet the suitable environmental conditions for crop sowing, emergence and growth in the fastest time.

[0073] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

[0074] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

Claims

1. A concealed drainage system for saline-alkali land, characterized in that, include: Main pipe, wherein the main pipe is a PVC corrugated pipe with an opening ratio of 15-20%; Multiple branch pipes are connected to the main pipe at an angle of 45-60°, forming a fishbone-like arrangement; A sealing structure is provided at the connection node between the branch pipe and the main pipe, and the sealing structure is a rubber sealing ring with a Shore hardness of 60±5. A filter wrapping layer is wrapped around the outer periphery of the main pipe and the branch pipe. The filter wrapping layer includes a lower geotextile and an upper geotextile from the inside to the outside. The pore size of the lower geotextile is less than 0.5 mm, the pore size of the upper geotextile is less than 0.1 mm, and the joints are welded by hot melt welding. A graded filter media layer is backfilled in the trench around the filter wrapping layer. The graded filter media layer, from bottom to top, includes a 5cm thick 5-10mm gravel layer, a 3cm thick 2-5mm coarse sand layer, and a 2cm thick 0.5-1mm fine sand layer. The main drainage ditch is connected to the outlet end of the main pipe. The cross-section of the main drainage ditch is trapezoidal, and its inner lining is a composite lining structure composed of concrete slabs, snap bricks, and geotextile. The overlap width of the geotextile is not less than 20cm. The anti-salinity isolation layer is laid 15cm below the ground surface and is formed by mixing soil with 20-30% medium sand.

2. The underground drainage system for saline-alkali land according to claim 1, characterized in that, The slope ratio of the main drainage ditch is 1:

1.

3. The underground drainage system for saline-alkali land according to claim 2, characterized in that, The geotextile in the filter wrapping layer is a 200g / m² long-filament geotextile.

4. The underground drainage system for saline-alkali land according to claim 1, characterized in that, The geotextile in the filter wrapping layer can be replaced with a natural fiber woven layer.

5. The underground drainage system for saline-alkali land according to claim 4, characterized in that, The natural fiber woven layer is a palm fiber woven layer.

6. The underground drainage system for saline-alkali land according to claim 1, characterized in that, The concrete slab interlocking bricks in the composite lining structure of the main drainage ditch can be replaced with ecological concrete lining.

7. The underground drainage system for saline-alkali land according to claim 1, characterized in that, The medium sand in the anti-salinization isolation layer can be replaced with rice husk charcoal, and the mixing ratio of rice husk charcoal and soil is 1:

5.

8. The underground drainage system for saline-alkali land according to claim 1, characterized in that, In the composite lining structure of the main drainage ditch, a layer of bentonite waterproofing blanket is also installed between the concrete slab snap bricks and the geotextile to prevent capillary water from rising.

9. The construction method of the underground drainage system for saline-alkali land as described in any one of claims 1-8, characterized in that, Includes the following steps: S1. Excavation and trenching in saline-alkali land: Use a laser level to level the construction area and calibrate the ground slope to 0.1-0.3%; measure the soil electrical conductivity, and dynamically determine the trench depth to be 0.8-1.2m and the spacing to be 20-30m based on the measurement results, and then carry out excavation; S2. Pipeline laying and connection: PVC corrugated pipes with an opening ratio of 15-20% are laid as main pipes in the trench; branch pipes are connected to the main pipes at an angle of 45-60° in a fishbone pattern, and rubber sealing rings with a Shore hardness of 60±5 are installed at the connection nodes. S3. Geotextile wrapping: The main pipe and branch pipe are wrapped with double-layer geotextile, wherein the lower geotextile close to the pipe has a pore size of less than 0.5mm and the upper geotextile on the outside has a pore size of less than 0.1mm, and the joints of the geotextile are heat-fused welded. S4. Filter material backfilling: Backfill the filter material in layers around the pipe wrapped with geotextile. First, backfill with 5cm thick 5-10mm gravel, then backfill with 3cm thick 2-5mm coarse sand, and finally backfill with 2cm thick 0.5-1mm fine sand. S5. Construction of main drainage ditch: Excavate and trim the main drainage ditch to a trapezoidal cross-section, and use concrete slabs, snap bricks and geotextile for composite lining construction, ensuring that the overlap width of the geotextile is not less than 20cm, and properly connect it with the water outlet of the main pipe. S6. Construction of anti-salinization layer: Mix 20-30% medium sand into the soil layer 15cm below the surface and mix evenly to form an anti-salinization isolation layer.

10. The construction method of the underground drainage system for saline-alkali land according to claim 8, characterized in that, In step S5, the concrete slab clip bricks can be replaced with eco-friendly concrete for lining construction. In step S6, the medium sand can be replaced with rice husk charcoal and mixed with the soil at a ratio of 1:5.

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