Soft soil foundation treatment method based on targeted fracturing crack-making and permeability-increasing

Abstract

The invention discloses a soft soil foundation treatment method based on targeted fracturing fracture-forming and permeability-increasing, which comprises the following steps: according to the type of soft soil and a target fracture-forming position, accurately regulating and controlling splitting and injection pressure, pressing an injection liquid containing a propping agent into a soil body by utilizing a water pressure splitting technology, and performing targeted fracture-forming on the soil bodies with different depths to form a single fracture with the length of 2-3m and the thickness of 2-3m; a plurality of horizontal stable seepage channels with the opening degree of 10-15 mm form a multi-layer drainage channel, the multi-layer drainage channel and a vertical drainage channel form a three-dimensional drainage system, then vacuum pumping is conducted by covering a sealing film, a negative pressure gradient is formed in a soil body, and drainage of pore water along a three-dimensional drainage network is accelerated. According to the method, splitting and injection pressure are accurately regulated and controlled, a horizontal crack drainage channel wide in extension range and stable is constructed at different depths of a soft soil layer in a targeted mode through the hydraulic pressure splitting technology, a three-dimensional drainage network is formed by the horizontal crack drainage channel and the vertical drainage plates, the passing ability of water flow in a low-permeability soil body is improved, and the solidification acceleration time can be shortened by 1 / 4-1 / 5.

Description

Technical Field

[0001] The present application relates to the technical field of soft soil foundation construction, and particularly relates to a soft soil foundation treatment method based on targeted fracturing and seam creation for permeability enhancement. Background Art

[0002] Soft soil layers are characterized by high water content, poor structure, high compressibility, and low permeability. Directly serving as an engineering foundation is extremely likely to cause structural instability. For the safety and stability of the project, it is necessary to consolidate the soft soil.

[0003] The drainage consolidation method is a commonly used soft soil foundation treatment method. It often applies external pressure by vacuum preloading, surcharge preloading, or vacuum combined with surcharge preloading, and inserts vertical drainage plates in the soft soil layer to optimize the drainage channel to accelerate the consolidation of the foundation. However, due to the extremely low permeability coefficient of soft soil, the consolidation period of the traditional drainage consolidation method is still generally greater than 3 months, which is difficult to meet the requirements of rapid construction. Adding horizontal drainage channels in the soft soil foundation and constructing a three-dimensional drainage system to accelerate the drainage speed of the foundation has become a development direction of foundation treatment.

[0004] Patent CN110847150B proposes a soft soil foundation treatment device and method. Using the principle of pneumatic splitting, a horizontal drainage channel is formed in the soft soil foundation through a gas-sand mixed transportation device and a drill pipe injection assembly to accelerate the drainage of the soft soil foundation. However, in actual operation, there are still certain deficiencies in the soft soil foundation treatment method in the above patent: First, the device injects drainage sand into the surrounding soil layer through pneumatic pressure. Due to the small pneumatic pressure, it has a certain applicability to the silty clay foundation with poor structure, and is not very applicable to the soft soil foundation composed of silty clay. Second, the earth pressure of the soil mass at the deep position is strong, and the splitting effect of this method on the deep soil mass is not good. The radial range of the horizontal drainage layer generated in the deep soil mass is limited, and the drainage effect on the deep soil mass is poor.

[0005] In view of the contradiction between the special engineering properties of soft soil and the construction requirements, how to break through the bottleneck of the consolidation period caused by poor permeability has become an urgent problem to be solved. Proposing a fast and efficient soft soil foundation treatment technology is an urgent need for the current market development. Summary of the Invention

[0006] In view of the above technical problems, the present invention proposes a soft soil foundation treatment method based on targeted fracturing and seam creation for permeability enhancement. The fracturing and seam creation rod is flexibly controlled based on the actual working conditions. According to the soft soil type and the target seam creation position, by precisely regulating the starting splitting and injection pressures, the injection liquid containing proppants is pressed into the soil mass using the water pressure splitting technology to create targeted seams in the soil mass at different depths, forming multiple horizontal stable seepage channels with a single seam length of 2 - 3m and an opening of 10 - 15mm, expanding the extension range of the horizontal drainage layer in the deep soil mass, improving the construction quality and drainage efficiency, and shortening the construction period.

[0007] To achieve the above technical objectives, the present invention adopts the following technical means:

[0008] A soft soil foundation treatment method based on targeted fracturing and creating cracks to enhance permeability, comprising the following steps:

[0009] S1. Construction preparation for foundation treatment: Level the site to the target elevation and lay an isolation layer on the surface of the leveled foundation.

[0010] S2. Positioning and marking for fracturing construction.

[0011] S3. Cracking and permeability enhancement network construction: Use the fracturing injection equipment to rotate and press the fracturing injection rod into the formation. After reaching the target position, turn on the jet liquid injection system, and construct horizontal cracks in the formation through the pressure pulse control method. Inject the fracturing injection liquid containing proppants into the cracks to form a horizontal stable seepage channel with a single crack length of 2 - 3 m and an opening of 10 - 15 mm. The pressure pulse control method is divided into the initial splitting stage and the injection stage. The initial splitting pressure P f is adjusted according to Equation (1):

[0012] P f = C1((3K0 - 1)(γ1h1 + γ'h2) + σ t ) + μ0 (1)

[0013] In the formula: P f is the initial splitting pressure, with the unit kPa; K0 is the coefficient of earth pressure at rest of the soil to be treated; γ1 is the unit weight of the soil above the groundwater level, with the unit kN / m 3 ; γ' is the buoyant unit weight of the soil; h1 is the thickness of the soil above the groundwater level, with the unit m; h2 is the thickness of the soil above the fracturing position below the groundwater level, with the unit m; σ t is the tensile strength of the soil, with the unit kPa; μ0 is the pore water pressure at the target soil layer, with the unit kPa; C1 is an empirical coefficient, determined according to the soil type, taking 12.3 - 13.5 for silty clay and 9.6 - 10.5 for silty clayey soil;

[0014] The pulsed jet pressure in the injection stage is determined according to Equation (2):

[0015] P y = C2[C1((3K0 - 1)(γ1h1 + γ'h2) + σ t ) + μ0] (2)

[0016] In the formula: P y is the pulsed jet pressure (kPa); C2 is an empirical coefficient, determined according to the soil type, taking 0.8 - 0.9 for silty clay and 0.55 - 0.65 for silty clayey soil;

[0017] The injection flow rate and time are adjusted according to the soil type. For silty clay, the injection flow rate is controlled at 30 - 40 L / min and the injection time is 5 min; for silty clay, the injection flow rate is controlled at 15 - 25 L / min and the injection time is 10 min.

[0018] After completing the crack formation work at the target depth, continue to press down the fracturing injection rod until the next target depth, and repeat the construction operation, thereby forming multiple horizontal cracks at different depths of the formation and constructing multiple drainage passages.

[0019] S4. Establishment of a three - dimensional drainage system: Vertically drive plastic drainage boards to the designed depth. The plastic drainage boards are inserted downward along the vertical holes formed by fracturing injection. Bury soil monitoring instruments, and lay medium - coarse sand cushions in layers on the surface of the isolation layer. At the same time, connect the exposed ends of the plastic drainage boards to the horizontal drainage pipes through three - way joints to form a three - dimensional drainage network.

[0020] S5. Vacuum preloading foundation treatment: Cover the sand cushion with a sealing film and pump out the vacuum to form a negative pressure gradient inside the soil body, accelerating the discharge of pore water along the three - dimensional drainage network.

[0021] Beneficial effects:

[0022] (1) Significantly increased the drainage path: By precisely regulating the initiation and injection pressures, the present invention uses the hydraulic fracturing technology to construct horizontal crack drainage passages with a wide extension range and stability at different depths of the soft soil layer. Together with the vertical drainage boards, a three - dimensional drainage network is formed, increasing the seepage drainage channels of the soft soil layer and improving the water flow permeability in low - permeability soil.

[0023] (2) Optimized the seepage field of soft soil: The seepage field caused by traditional single vertical drainage boards has significant "strong radial - weak vertical" non - uniform characteristics. The present invention optimizes the hydraulic gradient distribution by introducing horizontal drainage passages in the soft soil layer, strengthens the vertical seepage, and greatly improves the drainage efficiency of low - permeability soft soil.

[0024] (3) Reduced differential settlement of the soil body: Only concentrating drainage through multiple vertical drainage boards easily leads to uneven dissipation of excess pore water pressure, causing stress concentration or differential settlement of the soil body. The present invention constructs cracks in the soil body to introduce horizontal drainage passages, improves the pore water discharge rate of the soil body at the edge of the action range of the drainage boards, effectively improves the uneven dissipation of pore water pressure, reduces the differential settlement of the soil body, and improves the engineering quality.

[0025] (4) Weakens the "group board effect": In traditional multi-vertical drain board drainage, the "group board effect" is easily caused when multiple vertical drain boards drain water simultaneously. That is, due to unreasonable spacing of drain boards, the drainage paths of adjacent drain boards overlap, affecting their respective effective drainage ranges, or because the soil between drain boards drains poorly, forming a stagnant water area and reducing drainage efficiency. After constructing horizontal cracks in the soil body in the present invention, the traditional dot-matrix drainage is upgraded to area drainage, reducing the mutual influence of the seepage fields between drain boards, connecting the influence areas of adjacent drain boards as a whole, eliminating seepage blind areas, and significantly improving drainage efficiency.

[0026] (5) Overcomes the consolidation and seepage resistance around the board: During the drainage process of traditional vertical drain boards, the soil around the drain boards is easily pre-consolidated into a dense hardened cylinder due to rapid drainage, forming a seepage resistance area within a certain range around the drain boards. The large-area horizontal cracks introduced by fracturing in the present invention form multi-directional drainage channels, promoting the uniformity of soil consolidation and drainage. At the same time, it can cut off the continuity of the board-column structure, construct a stable high-permeability drainage path between the soft soil layer and the drain board, and effectively solve problems such as poor consolidation efficiency in the middle and late stages of vertical drain board consolidation and drainage.

[0027] (6) Targeted crack formation, flexible construction, and controllable quality: In the present invention, crack formation is carried out in the form of a crack-making rod probing downward, and the crack-making position can be targeted and controlled according to the actual engineering requirements; after crack formation is completed, the crack-making rod is drilled out and moved to the next position for work. There is no need to bury horizontal crack-making pipelines in the formation throughout the process, and the construction is flexible, economical, and efficient; the initial fracture pressure and injection pressure are accurately regulated according to the soil type and soil layer depth, with the characteristics of controllable horizontal crack-making quality, wide and stable drainage path extension range.

[0028] (7) Wide application range and high drainage efficiency for deep soil layers: The hydraulic-driven soft soil splitting technology adopted in the present invention is widely applicable to silty clay soft soil foundations or muddy clay soft soil foundations. At the same time, horizontal drainage channels can still be effectively constructed in deep soil layers.

[0029] In an alternative embodiment, in step S1, after the site is filled with sand and blown to the target elevation, it is leveled until the flatness reaches 5%. A geotextile is laid on the surface of the leveled foundation to form an isolation layer and enhance the shear strength of the ground surface.

[0030] Beneficial effects: Uniform load distribution, ensuring construction accuracy: Controlling the flatness ≤ 5% not only ensures that the fill sand load is evenly transmitted to the soft soil layer to avoid differential settlement, but also provides a uniform thickness foundation for the subsequent structural layer, guaranteeing construction accuracy and structural stability.

[0031] In an alternative embodiment, step S2 is specifically as follows: Set up a GPS reference station, perform construction coordinate conversion based on the CGCS2000 coordinate system, use the Beidou satellite navigation system for real-time kinematic positioning, input the designed point coordinates (X / Y / Z), navigate to the target point through the hand-held terminal, mark the fracturing injection points and the drainage board insertion points, and simultaneously record the measured coordinates.

[0032] Beneficial effects: Control construction accuracy and achieve efficient construction management: By accurately positioning the fracturing injection points and the drainage board insertion points through the Beidou navigation system, centimeter-level construction accuracy can be achieved, ensuring that the spacing between the fracturing injection points and the drainage boards is strictly consistent with the design drawings, avoiding problems such as misalignment of the drainage path and uneven consolidation caused by positioning deviation. At the same time, through digital construction real-time management and recording of coordinate data, construction efficiency is improved, providing accurate basis for later quality traceability.

[0033] In an alternative embodiment, in step S3, the proppant is quartz sand proppant with a diameter of 70 - 120 mesh, which is filled into the cracks along with the fracturing injection fluid.

[0034] Beneficial effects: Improve injection efficiency and resist crack closure: Selecting quartz sand proppant with a diameter of 70 - 120 mesh facilitates the fracturing fluid to carry the proppant and inject it into the horizontal cracks, preventing the closure of the horizontal fracturing cracks caused by the self-weight of the soil mass.

[0035] In an alternative embodiment, in step S2, the fracturing injection points are arranged in a square pattern, the spacing between the fracturing injection points is 4 - 5 m, and the insertion spacing of the drainage boards is half of the spacing between the fracturing injection points.

[0036] Beneficial effects: Reduce construction time consumption and improve drainage efficiency: The regular square layout simplifies the measurement and positioning through the orthogonal coordinate system, reduces the positioning time consumption, and improves the construction speed by reducing the frequency of equipment turning; based on the effective action range of the drainage boards, setting the drainage board spacing to half of the fracturing point spacing effectively eliminates the vacuum attenuation blind area and improves the drainage efficiency.

[0037] In an alternative embodiment, in step S4, the exposed length of the plastic drainage board is greater than 100 cm, and the exposed drainage board is wrapped with filter cloth to prevent sediment from entering; the verticality deviation during the insertion construction of the plastic drainage board is less than 1.5%, and the backhaul length is less than 50 cm;

[0038] The buried soil monitoring instruments include: settlement plates and vacuum pressure sensors, where,

[0039] The settlement plates are used to monitor the soil settlement in real time, are arranged at the center of the drainage board grid and the edge of the sealing film, with a layout spacing of 8 - 10 m, and should penetrate the soft soil layer and be arranged on the top of the hard bearing layer;

[0040] The vacuum pressure sensor is used to monitor the uniformity of the vacuum degree under the sealing film. It is arranged between the sealing film and the sand cushion layer, and at least one is arranged within the range of 500m 2 ~1000m 2 .

[0041] Beneficial effects: Digital construction to ensure construction quality: By using settlement plates and vacuum pressure sensors to monitor the soil settlement and vacuum degree during the vacuum preloading process, the construction effect can be grasped in real time, construction parameters can be adjusted in a timely manner, and construction quality can be ensured.

[0042] In an alternative embodiment, when the vacuum degree is less than 70 kPa, a pressurization operation is performed on the vacuum degree under the sealing film.

[0043] In an alternative embodiment, in step S5, a geotextile and a sealing film are sequentially laid on the sand cushion layer. The edge of the sealing film is buried in the sealing trench, and the drainage end of the horizontal drainage pipe passes through the sealing film and is connected to the vacuum pump unit; the vacuum pump unit is started to perform continuous vacuum pumping work, and the vacuum pump is shut down to terminate the operation when the soil consolidation degree reaches ≥90% or the daily average settlement amount <2 mm / d.

[0044] In an alternative embodiment, a vacuum check valve is provided between the vacuum pump and the drainage pipe. During construction, all vacuum pumps operate at full load for 24 hours, and spare vacuum pumps with a quantity of ≥10% of the total quantity are prepared for replacement when the vacuum pumps are damaged.

[0045] Beneficial effects: Strengthen the system stability and ensure the construction continuity: The vacuum check valve controls the vacuum degree fluctuation within ±3 kPa, ensuring the continuous and effective drainage of the drainage board. The full-load operation improves the vacuum transfer efficiency under the film and enhances the consolidation rate; the spare pump group enables timely switching in case of failure, avoiding a cliff-like drop in the vacuum degree. Description of the Drawings

[0046] Figure 1 is the construction schematic diagram of the fracturing and fracture creation of the present invention;

[0047] Figure 2 is the construction process diagram of the fracturing and fracture creation of the present invention;

[0048] Figure 3 is the top view layout diagram of the fracturing and fracture creation of the present invention;

[0049] Figure 4 is the front view of the drill pipe for fracturing and fracture creation of the present invention;

[0050] Figure 5 is the transverse cross-sectional view of the drill pipe for fracturing and fracture creation of the present invention;

[0051] Figure 6 is the construction drawing of the combined vacuum preloading and drainage for fracturing and fracture creation of the present invention;

[0052] Figure 7 Schematic diagram of the principle of the cavity expansion theory

[0053] Where: 1 mixing and preparation tank, 2 fracturing injection fluid, 3 jet pressure pump, 4 movable fracturing and slotting vehicle, 5 fracturing drilling rod, 6 geotextile, 7 horizontal fracture, 8 soft soil, 9 packer, 10 liquid injection port, 11 drill bit, 12 front-end drilling rod, 13 drilling connection rod, 14 liquid passage port, 15 gas injection port, 16 horizontal drainage pipeline, 17 sand cushion, 18 geotextile and sealing film, 19 sealing trench, 20 vacuum pump unit, 21 fracturing injection point, 22 drainage board Specific implementation method

[0054] Now, the present invention will be further described in detail with reference to the accompanying drawings. These drawings are all simplified schematic diagrams, only illustrating the construction method of the present invention in a schematic manner, so they only show the components related to the construction method

[0055] S1. Before construction, use mechanical equipment to remove surface debris, and blow the site to the target elevation through sand filling and then level it to a flatness of 5%

[0056] S2. Lay the geotextile 6 on the surface of the leveled foundation, and the specification of the geotextile is 150 g / m 2 .

[0057] S3. Start the stirrer of the mixing and preparation tank 1, and prepare the fracturing injection fluid 2 in the mixing and preparation tank 1. The fracturing injection fluid 2 contains quartz sand proppant with a diameter of 70 - 120 mesh, which can be filled into the fracturing crack 7 along with the fracturing injection fluid 2 to prevent the fracturing crack 7 from closing. During the entire fracturing injection and slotting period, the stirrer is always in the on state to ensure the uniformity of the fracturing injection fluid 2

[0058] S4. Assemble and debug the fracturing injection equipment, including connecting the mixing and preparation tank 1 with the jet pressure pump 3, connecting the jet pressure pump 3 with the movable fracturing and slotting vehicle 4, and installing the fracturing drilling rod 5 on the movable fracturing and slotting vehicle 4. After the equipment is installed, conduct a fracturing trial injection to ensure the normal operation of each equipment

[0059] S5. Set up a GPS reference station, perform construction coordinate conversion based on the CGCS2000 coordinate system, use the Beidou satellite navigation system for real-time dynamic positioning, input the designed point coordinates (X / Y / Z), navigate to the target point through the handset, mark the fracturing injection point 21 and the drainage board insertion point 22, and synchronously record the measured coordinates. As Figure 3 shown, the fracturing injection points 21 are arranged in a square, and the spacing between the fracturing injection points 21 is 4 m

[0060] S6. Rotate and press the fracturing injection rod 5 into the soft soil layer 8. After reaching the target position, open the gas injection system to expand the packer airbag 9 to isolate the fracturing section. After the setting pressure of the packer airbag reaches 2 MPa, start the jet pressure pump 3, construct horizontal fractures in the formation by the pressure pulse control method, and inject the fracturing injection fluid containing proppant into the fractures to form a horizontal stable seepage channel with a single fracture length of 2 - 3 m and an aperture of 10 - 15 mm. The pressure pulse control method is divided into the initiation stage and the injection stage. For under-consolidated soil, the hydraulic fracturing of the soil is more easily controlled by tensile failure. The soil fracturing process can be analyzed by the cavity expansion theory. Based on Figure 7 , within the elastic range, the radial and tangential stresses at a distance R from the hole center are as follows:

[0061]

[0062] In the formula, σ r is the radial stress; σ θ is the tangential stress; σ2 and σ3 are the initial stresses perpendicular to the borehole plane respectively, θ is the angle between the line connecting the calculation point to the borehole center and the horizontal line; R c is the initial radius of the hole, R is the distance from the calculation point to the borehole center; P i is the hole expansion pressure.

[0063] When θ is 0° or 180°, the radial stress σ r is the maximum, and the tangential stress σ θ is the minimum. At this time:

[0064] σ r = P i

[0065] σ θ = 3σ3 - σ2 - P i

[0066] Assume that the soil is a linear elastic material and is controlled by the tensile failure mechanism. When the effective principal stress of the soil becomes negative and exceeds the tensile strength of the soil, the soil undergoes tensile failure, that is:

[0067] σ θ = σ3′ = -σ t

[0068] In the formula, σ'3 is the effective minor principal stress of the soil, σ t is the tensile strength of the soil.

[0069] From this, the initiation pressure P f0 of the soil is:

[0070] P f0 = 3σ3 - σ2 + σ t

[0071] For saturated soil to be consolidated, σ2 and σ3 at a certain depth in the formation are as follows:

[0072] σ2 = (γ1h1 + γ'h2)

[0073] σ3 = K0(γ1h1 + γ'h2)

[0074] Where: K0 is the coefficient of earth pressure at rest of the soil to be treated; γ1 is the unit weight of the soil above the groundwater level, in kN / m 3 ; γ' is the buoyant unit weight of the soil; h1 is the thickness of the soil above the groundwater level, in m; h2 is the thickness of the soil above the fracturing position below the groundwater level, in m.

[0075] During hydraulic fracturing, due to the presence of pore water, different soil types will cause the dissipation of the hydraulic fracturing pressure. Therefore, an empirical coefficient C1 needs to be multiplied to meet the fracturing conditions. For silty clay, C1 ranges from 12.3 to 13.5, and for silty clay with mud, C1 ranges from 9.6 to 10.5; at the same time, the effective fracturing pressure of the soil should be the fracturing pressure after overcoming the pore water pressure μ0. Then, from the above, the initial fracturing pressure P f can be adjusted by the following formula:

[0076] P f = C1((3K0 - 1)(γ1h1 + γ'h2) + σ t ) + μ0

[0077] Where: P f is the initial fracturing pressure, in kPa; σ t is the tensile strength of the soil, in kPa; μ0 is the pore water pressure at the target soil layer, in kPa; C1 is the empirical coefficient, determined according to the soil type, 12.3 - 13.5 for silty clay, and 9.6 - 10.5 for silty clay with mud;

[0078] After the soil is fractured, the injection pressure required to inject the fracturing support fluid is less than the initial fracturing pressure. The injection pressure is multiplied by the empirical coefficient C2 on the basis of the initial fracturing pressure to obtain the pulsed jet pressure P y as shown in the following formula:

[0079] P y = C2[C1((3K0 - 1)(γ1h1 + γ'h2) + σ t ) + μ0]

[0080] Where: P y is the pulsed jet pressure (kPa); C2 is the empirical coefficient, determined according to the soil type, 0.8 - 0.9 for silty clay, and 0.55 - 0.65 for silty clay with mud;

[0081] The injection flow rate and time are adjusted according to the soil type. For silty clay, the injection flow rate is controlled at 30 - 40 L / min and the injection time is 5 min; for silty clay with silt, the injection flow rate is controlled at 15 - 25 L / min and the injection time is 10 min.

[0082] After completing the fracture creation work at the target depth, the gas in the packer bladder 9 is discharged to make the bladder contract, and the fracturing drilling rod 5 is further pressed down until the next target depth, and the construction operation is repeated, thereby forming multiple horizontal fractures at different depths of the formation and constructing multiple drainage passages. The fracturing drilling rod 5 is composed of a front-end drilling rod 12 and a drilling connecting rod 13. As Figure 4 shown, when the length of the fracturing drilling rod 5 is insufficient, the connection between the fracturing drilling rod 5 and the movable fracturing and fracture creating vehicle 4 can be disconnected first, a drilling connecting rod 13 is added behind the front-end drilling rod 12, and then the fracturing drilling rod 5 and the movable fracturing and fracture creating vehicle 4 are reconnected.

[0083] S7. Vertically drive the plastic drainage board 22 to the designed depth. The exposed length of the plastic drainage board 22 should be greater than 100 cm, and the exposed drainage board should be wrapped with a filter cloth to prevent sediment from entering. When the drainage board 22 is inserted, the insertion position and angle should be preferably Figure 3 as shown. The drainage board 22 is inserted downward along the vertical hole formed by fracturing injection. The verticality deviation during the insertion construction of the drainage board should be less than 1.5%, and the pull-back length should be less than 50 cm.

[0084] S8. Install soil monitoring instruments. The soil monitoring instruments include settlement plates and vacuum pressure sensors, which respectively monitor the settlement and vacuum degree of the soil in real time. The settlement plates are arranged at the center of the drainage board grid and the edge of the sealing film, with a layout spacing of 8 m, and penetrate through the soft soil layer and are arranged on the top of the hard bearing layer. The vacuum pressure sensors monitor the uniformity of the vacuum degree under the film and are arranged between the sealing film and the sand cushion layer. At least one is arranged within the range of 1000 m 2 of the range.

[0085] S9. Lay the sand cushion layer 17 in layers, and at the same time connect the exposed ends of the drainage boards 22 to the horizontal drainage pipeline 16 through tee joints to form a surface drainage network. The horizontal drainage pipeline uses a HDPE material drainage pipe, and the joints between the pipes should ensure the pipeline sealing through hot melt welding.

[0086] S10. Lay a geotextile and a polyvinyl chloride sealing film 18 on the sand cushion layer 17 in sequence, bury the edge of the polyvinyl chloride sealing film into the sealing groove 19 and compact it. The length buried into the sealing groove 19 is ≥1 m. The geotextile laid on the sand cushion layer 17 should be ≥250 g / m 2When the polyvinyl chloride sealing film is single-layer, its thickness should be ≥0.12 mm; when the polyvinyl chloride sealing film is double-layer, its thickness should be ≥0.08 mm. The polyvinyl chloride sealing films are welded by hot melting, and the joint strength should be ≥90% of the film body. When laying the film, the deformation requirements of the film caused by settlement should be considered.

[0087] S11. After the horizontal drainage pipe 16 exits the film, it is connected to the vacuum pump group 20. The vacuum pump group 20 is started for continuous vacuum pumping work to maintain the vacuum degree under the film >70 kPa. During the construction period, if the vacuum degree under the film is less than 70 kPa, the suction power of the vacuum pump is increased to raise the vacuum degree under the film to >70 kPa; when the soil consolidation degree measured by the settlement plate reaches ≥90% or the daily average settlement amount <2 mm / d, the vacuum pump group 20 is turned off to terminate the operation. During the construction period, all vacuum pumps operate at full load for 24 hours, and standby vacuum pumps with a quantity greater than or equal to 10% of the total quantity are prepared for replacement when the vacuum pumps are damaged.

Claims

1. A soft soil foundation treatment method based on targeted fracturing for creating fractures and enhancing permeability, characterized in that, It includes the following steps: S1. Construction preparation for foundation treatment: Level the site to the target elevation, and lay an isolation layer on the surface of the leveled foundation; S2. Positioning and marking for fracturing construction; S3. Fracture permeability enhancement network construction: Use the fracturing injection equipment to rotate and press the fracturing injection rod into the formation. After reaching the target position, turn on the jet liquid injection system, construct horizontal fractures in the formation through the pressure pulse control method, inject the fracturing injection fluid containing proppant into the fractures, and form a horizontal stable seepage channel with a single fracture length of 2 - 3 m and an aperture of 10 - 15 mm. The pressure pulse control method is divided into a fracture initiation stage and an injection stage, where the fracture initiation pressure P f is adjusted according to Equation (1): （1） Where: P f is the initial splitting pressure, in kPa; K0 is the coefficient of earth pressure at rest of the soil to be treated; is the unit weight of the soil above the groundwater level, in kN / m 3 ; is the buoyant unit weight of the soil; h1 is the thickness of the soil above the groundwater level, in m; h2 is the thickness of the soil above the fracturing position below the groundwater level, in m; is the tensile strength of the soil, in kPa; is the pore water pressure at the target soil layer, in kPa; C1 is an empirical coefficient determined according to the soil type, taking 12.3 - 13.5 for silty clay and 9.6 - 10.5 for silty clay with mud; The pulse jet pressure in the injection stage is determined according to Equation (2): （2） Where: P y is the pulsed jet pressure (kPa); C2 is an empirical coefficient, which is determined according to the soil type. For silty clay, it takes 0.8 - 0.9, and for silty clay with silt, it takes 0.55 - 0.65; The injection flow rate and time are adjusted according to the soil type. The injection flow rate for silty clay is controlled at 30 - 40 L / min, and the injection time is 5 min; The injection flow rate for silty clay is controlled at 15 - 25 L / min, and the injection time is 10 min; After completing the slotting work at the target depth, continue to lower the fracturing injection rod until the next target depth, and repeat the construction operation, thereby forming multiple horizontal fractures at different depths of the formation and constructing multiple drainage passages; S4. Construction of a three - dimensional drainage system: Vertically drive plastic drainage boards to the designed depth. The plastic drainage boards are inserted downward along the vertical holes formed by fracturing injection. Bury soil monitoring instruments, and lay medium - and coarse - sand cushions in layers on the surface of the isolation layer. At the same time, connect the exposed ends of the plastic drainage boards to the horizontal drainage pipes through three - way joints to form a three - dimensional drainage network; S5. Vacuum preloading foundation treatment: Cover the sand cushion with a sealing film and evacuate to form a negative pressure gradient inside the soil, accelerating the discharge of pore water along the three - dimensional drainage network.

2. The method for treating soft soil foundation based on targeted hydraulic fracturing for creating fractures and enhancing permeability according to claim 1, wherein In step S1, after blowing the site to the target elevation with sand filling and then leveling it to a flatness of 5%, lay geotextiles on the surface of the leveled foundation to form an isolation layer and enhance the shear strength of the ground surface.

3. The soft soil foundation treatment method based on targeted hydraulic fracturing for creating fractures and enhancing permeability according to claim 1, wherein Step S2 is specifically: Set up a GPS reference station, perform construction coordinate conversion based on the CGCS2000 coordinate system, use the Beidou satellite navigation system for real - time kinematic positioning, input the designed point coordinates (X / Y / Z), navigate to the target point through the handset, mark the fracturing injection points and the insertion points of the drainage boards, and simultaneously record the measured coordinates.

4. The method for treating soft soil foundation based on targeted hydraulic fracturing for creating fractures and enhancing permeability according to claim 1, characterized in that, In step S3, the proppant is quartz sand proppant with a diameter of 70 - 120 mesh, which is filled into the fractures along with the fracturing injection fluid to prevent the fracturing fractures from closing.

5. The soft soil foundation treatment method based on targeted hydraulic fracturing for creating fractures and enhancing permeability according to claim 1, characterized in that, In step S2, the fracturing injection points are arranged in a square pattern, with a spacing of 4 - 5 m between the fracturing injection points, and the insertion spacing of the drainage boards is half of the spacing between the fracturing injection points.

6. The soft soil foundation treatment method based on targeted hydraulic fracturing for creating fractures and enhancing permeability according to claim 1, characterized in that, In step S4, the exposed length of the plastic drainage board is greater than 100 cm, and the exposed drainage board is wrapped with a filter cloth to prevent sediment from entering; When inserting the plastic drainage board, the verticality deviation is less than 1.5%, and the back - pull length is less than 50 cm; The buried soil monitoring instruments include: settlement plates and vacuum pressure sensors, among which, The settlement plates are used to monitor the soil settlement in real - time, arranged at the center of the drainage board grid and the edge of the sealing film, with a spacing of 8 - 10 m, and should penetrate the soft soil layer and be arranged at the top of the hard bearing layer; The vacuum pressure sensor is used to monitor the vacuum degree under the sealing membrane and is arranged between the sealing membrane and the sand cushion layer, and at least one is arranged within the range of 2 500 m 2 to 1000 m.

7. The method for treating soft soil foundation based on targeted hydraulic fracturing to create fractures and enhance permeability according to claim 6, wherein, When the vacuum degree is less than 70 kPa, perform a pressurization operation on the vacuum degree under the sealing film.

8. The method for treating soft soil foundation based on targeted hydraulic fracturing for creating fractures and enhancing permeability according to claim 1, wherein In step S5, lay geotextiles and a sealing film on the sand cushion in sequence. The edge of the sealing film is buried in the sealing trench. The drainage end of the horizontal drainage pipe penetrates the sealing film and is connected to the vacuum pump unit; Turn on the vacuum pump unit for continuous evacuation work, and turn off the vacuum pump to terminate the operation when the soil consolidation degree reaches ≥90% or the daily average settlement is <2 mm / d.

9. The method for treating soft soil foundation based on targeted hydraulic fracturing for creating fractures and enhancing permeability according to claim 8, characterized in that, A vacuum check valve is installed between the vacuum pump and the drain pipe. During construction, all vacuum pumps operate at full load for 24 hours, and spare vacuum pumps with a quantity ≥ 10% of the total quantity are prepared for replacement in case of vacuum pump damage.

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