High-efficiency preloading treatment method for soft soil foundation based on air lift combined with vacuum

Abstract

An efficient preloading treatment method and device for soft soil foundation based on gas lift combined with vacuum is developed. Gas of a certain pressure and flow rate is introduced from the bottom of the drain board, so that the positive pressure gas flows upward along the core of the drain board, and during its flow, the water in the core of the board is entrained and carried upward out of the drain board. After the water is drained, the vacuum is only transmitted in the gas environment and can be transmitted to the bottom of the board without attenuation. Only simple improvements need to be made to the drain board of the traditional vacuum preloading device system. A joint is connected to the top and bottom of the drain board, the upper end is connected to the vacuum pump through a vacuum pipeline, and the lower end is connected to the air compressor through a pressure gas pipeline. When the vacuum is stable at ‑80 to ‑90 kPa, the air compressor is intermittently turned on for gas injection, and the gas lift principle is applied to empty the water stored in the drain board, so as to achieve the purpose of promoting drainage and the transmission of vacuum pressure to the deep without attenuation. The present invention has low modification difficulty for traditional equipment and is easy to install and maintain for new equipment. Therefore, it can be widely promoted to various engineering practices.

Description

Technical Field

[0001] The present invention relates to the field of geotechnical engineering, and in particular to a high-efficiency preloading treatment method for deep soft soil based on air lift combined with vacuum. Background Art

[0002] As an efficient and economical method for treating soft foundations and ultra-soft soil foundations for land reclamation, vacuum preloading technology is widely used in coastal and riverside areas. During construction, plastic drainage boards are usually installed in the soil, and then geomembranes are laid on the soil surface to form a closed space. Under the action of the vacuum pump, the vacuum pressure under the membrane (up to 85kPa in engineering) is transmitted radially along the drainage board into the soil to form a stable pressure field, which promotes seepage consolidation of the soft soil. After the water in the soil enters the drainage channel, it is pumped out of the foundation from bottom to top, ultimately achieving the effect of promoting soil drainage and consolidation and improving soil strength.

[0003] As can be seen from the above description, the seepage consolidation process of soil is largely controlled by vacuum pressure, and its magnitude and distribution control the soil seepage consolidation rate and the ultimate reinforcement effect. However, existing research and engineering practice have found that due to the continuous flow of water within the drain plate, the vacuum degree has a significant attenuation effect along the drain plate (for example, the vacuum negative pressure decreases by 5 to 10 kPa for every 1 m of drain plate depth). This effect has a relatively limited adverse impact on shallow soft soil vacuum preloading (treatment depth within 6 m), but when the treatment object is deep soft soil (treatment depth of more than 10 m), it will greatly affect the treatment effect of deep soil. Therefore, there is an urgent need for a treatment method that can effectively improve the vacuum preloading effect of deep soft soil.

[0004] Gas lift is often used in mining and dredging projects to vertically lift solids and liquids (CN110630226A, CN105775752A). The method is to inject compressed air at the bottom of the lifting channel to reduce the fluid density at the inlet, and under the action of the pressure difference, drive the target (for example, slag or silt) to move vertically upward. The top outlet of a conventional gas lift lifting channel is generally at atmospheric pressure, and the pressure difference is formed only by the high-pressure gas injected at the bottom of the lifting channel to complete the lifting of a large amount of solids and liquids. During the vacuum preloading process, the top outlet of the channel is a vacuum negative pressure, and the pressure difference between the top and bottom of the lifting channel is even greater. Therefore, the method of gas lift combined with vacuum to empty the water stored in the drain plate is significantly different from the above-mentioned conventional gas lift method. In addition, the lifting channel of gas lift combined with vacuum is a permeable vertical channel (for example, a prefabricated drain plate or a bagged sand well, whose side walls are permeable), which is significantly different from the sealed pipe (for example, a steel pipe, a flexible riser, etc.) of the conventional gas lift method.

[0005] Taking into account the aforementioned characteristics of vacuum preloading and gas lift, when negative vacuum pressure is applied throughout the top section, positively pressured gas flows upward along the drain panel core, entraining water within the panel core and ultimately emptying the panel. Immediately after the gas lift ends, the panel is filled with gas, but the continued suction of the vacuum pump at the top of the channel rapidly draws the gas out. Because the channel sidewalls are permeable, surrounding liquid re-accumulates within the channel, but this process is very slow due to the low permeability of the soft soil. Consequently, after the gas lift ends, the vacuum level within the channel can be transferred from the top of the panel to the bottom with virtually no attenuation. Clearly, the combination of gas lift and vacuum can effectively increase the vacuum consolidation pressure throughout the entire depth of the drain panel, significantly consolidating the surrounding soil and effectively addressing the problem of vertical attenuation of the vacuum level within the panel. Currently, there are no patents or papers related to gas lift combined with vacuum preloading for soft soil foundations.

[0006] In the soft soil foundation treatment method, patents CN104895046A, CN106677156A and CN109653186A propose a method of disturbing soft soil using high-pressure gas or high-pressure aerosol. The high-pressure disturbance medium is sprayed into the soil through a special conduit, and a spiral or conical disturbance surface / body is formed in the soil under pressure, increasing the horizontal drainage channel to improve the soil permeability coefficient. The paper "Model Experimental Study on the Air Lift Dewatering Effect of Active Drainage Consolidation Method" (DOI: 10.16285 / j.rsm.2020.1645) records that when the above patents are implemented, the high-pressure gas / aerosol will play a cutting role in the soil, and will be discharged upward through the vertical drainage channel, quickly bringing the water in it to the ground, playing a role similar to the air lift effect in mining and dredging projects. It should be pointed out that the air lift dewatering effect in the above paper is an additional effect produced when high-pressure disturbance is implemented. The above three patents and papers mainly use high-pressure media to cut and disturb the soil, rather than directly ventilating the drainage channel to implement gas lift, and do not pay attention to the effect of significantly improving the vacuum consolidation pressure after gas lift. The above three patents require more equipment. In addition to conventional vacuum preloading related equipment, high-pressure rotary jet equipment is also required, and additional medium pipelines with rotary jet holes need to be set up in the site to be treated. The gas lift combined with vacuum efficient preloading treatment method for soft soil foundation proposed in this patent directly injects positive pressure gas to the bottom of the drainage channel, empties the water in the channel at one time, and creates a high vacuum environment with no attenuation from top to bottom, thereby effectively improving the soil consolidation effect in the full depth range of the drainage channel, which is significantly different from the above three patents (CN104895046A, CN106677156A and CN109653186A).

[0007] Drain boards themselves are vertical drainage channels with excellent flowability. When surrounded by soft clay with poor permeability, injecting high-pressure gas directly into the drain board allows it to function as a lifting conduit in gas lift, making it feasible to implement gas lift methods within the soil. Based on this, the present invention proposes a highly efficient preloading treatment method for soft soil foundations based on gas lift combined with vacuum. This method introduces gas at a certain pressure and flow rate into the bottom of the drain board. Water stored in the core is lifted by the positive pressure of the gas at the bottom and pulled by the negative pressure of the vacuum at the top, allowing water stored in the core to be quickly removed along the entire length of the board, prioritizing the gas lift effect of the high-pressure gas. When there is no flowing water in the drain board, a vacuum pump connected to the drain board head continuously extracts gas from the core, allowing the vacuum to be transmitted from the board head to the bottom with almost no attenuation, significantly improving the boundary conditions for consolidation and drainage. Compared to existing patents, the main purpose of the present invention is to facilitate the transmission of vacuum pressure deeper into the soil. To achieve the gas lift effect by injecting gas into the drain board, it is only necessary to connect the drain board to a pneumatic pipeline. Since the drainage board can transmit the vacuum pressure to the deep part, even if the drainage depth is limited, it will improve the boundary conditions in the deep part.

[0008] The present invention has carried out indoor model tests to prove the above discussion. The test results show that there is a difference in pressure between the top and bottom of the drainage plate before gas lift. After the water stored in the plate is emptied by gas lift at one time, the vacuum degree at the bottom rises sharply and is basically the same as that at the top. Under this condition, the maximum vacuum pressure along the depth in the core of the plate is the top. As the water in the soil continues to seep back into the core of the plate, the vacuum degree in the plate is restored to the state before gas lift. In the process of re-collecting water after emptying, the seepage velocity in the soil improves and rises under the high vacuum pressure without attenuation, so the seepage consolidation in the soil is significantly accelerated after gas injection. The pore pressure in the soil will drop significantly. This shows that using gas lift to empty the drainage plate can allow the vacuum pressure to be transmitted downward to the bottom without attenuation, thereby accelerating the rate of seepage consolidation of soft soil.

[0009] Experiments have shown that according to the working principle of air lift, by injecting an appropriate amount of compressed air continuously into the drain board, it is possible to empty the water in the drain board, promote the efficient downward transmission of vacuum pressure, and effectively increase the rate of consolidation and settlement of soft soil foundations. The method of the present invention only requires the addition of an interface for connecting to an external air compressor at the bottom of the drain board, which is simple and easy to install. Air lift can be completed in a very short time, the operating pressure of the mechanical equipment is low, and the operating cost is low. It is an easy and reliable construction method with both engineering and economic benefits. Summary of the Invention

[0010] The first aspect of the present invention provides a method for preloading soft soil foundations based on air lift combined with vacuum, which is used for drainage preloading treatment of high-water content fluid mud, sludge foundations, and deep soft foundations. The method is characterized by comprising the following steps:

[0011] (1) The gas lift principle is applied to vacuum preloading. Gas of a certain pressure and flow rate is introduced from the bottom of the drain board, so that the positive pressure gas flows upward along the core of the drain board, and in the process of its flow, the water in the core of the board is carried upward and out of the drain board;

[0012] (2) When the gas lift has completely removed the water in the core of the board, it stops and the core of the board is in a gas-filled state. The vacuum pump connected to the head of the drain plate continuously extracts the gas in the core of the board, so that the gas in the core of the board can be quickly extracted, so that the vacuum can be transmitted from the head of the board to the bottom of the board with almost no attenuation;

[0013] (3) Achieve high vacuum along the entire length of the drainage board, so that the soil within the depth range of the drainage board can be under high vacuum consolidation pressure, promoting drainage and consolidation of deep soil.

[0014] (4) After one gas lift is completed, under the vacuum pressure of the plate core, the pore water in the soil continuously accumulates in the plate core through the drainage plate filter membrane, and the water level in the plate core gradually rises until it is filled with water again. At this time, gas lift is performed again to bring all the water in the plate core out of the plate again, thus starting a new round of high vacuum drainage consolidation within the entire length of the drainage plate.

[0015] Furthermore, in step (1), when the bottom of the drainage board is ventilated and lifted, the vacuum pump connected to the head of the drainage board is still continuously extracting gas, and the water stored in the board core is lifted by the positive pressure of the gas at the bottom and pulled by the negative pressure of the vacuum at the top, so that the water stored in the board core within the entire length of the drainage board can be quickly brought out.

[0016] Furthermore, in steps (3) and (4), after the water is drained, the vacuum is transmitted only in the gas environment and is transmitted to the bottom of the plate without attenuation; the boundary conditions of soil consolidation are improved due to the efficient transmission of the vacuum, the pressure difference between the pore pressure in the soil and the pressure in the drainage plate is increased, the water seepage rate in the soil is accelerated, and the efficiency of vacuum preloading treatment of soft soil is improved.

[0017] A second aspect of the present invention provides an engineering implementation method for a soft soil foundation efficient preloading treatment method based on air lift combined with vacuum, comprising the following steps:

[0018] Step 1: Modify the drainage board and connect hand-shaped joints at both ends;

[0019] Step 2: Connect the pressure gas pipeline to the bottom joint and drive it into the soft soil foundation together with the drainage board. The end of the pipeline is connected to the ground to form a gas lift positive pressure pipeline;

[0020] Step 3: Connect the vacuum pipe to the drain board through the top joint, and connect the pipe end on the ground to form a vacuum negative pressure pipe;

[0021] Step 4: Lay geotextile and geomembrane for sealing;

[0022] Step 5: Connect the compressed gas pipeline and the vacuum pipeline to the air compressor and the vacuum pump respectively;

[0023] Step 6: Start the equipment for construction. When the vacuum degree stabilizes at -80 to -90 kPa, start the air compressor intermittently for air injection and air lift.

[0024] 5. The engineering implementation method according to claim 4, characterized in that the bottom interface of the drainage board is connected to the air compressor through a pneumatic pipe to form an air lift positive pressure gas passage.

[0025] 6. The engineering implementation method according to claim 5, characterized in that the bottom interface of the drainage board and the pneumatic pipe are connected before the drainage board is inserted, ensuring that the gas injection path is maintained before and after the drainage board is inserted into the soft soil.

[0026] 7. The engineering implementation method as described in claim 5 is characterized in that: in this method, gas lift positive pressure gas passage is used for gas lift operation multiple times to promote the discharge of water stored in the core of the drainage board, and the gas lift operation interval is 6 hours / time to 48 hours / time. The gas lift time interval is selected based on the water level rising speed in the drainage board after the previous gas lift, the vacuum gradient formation speed between the top and bottom of the drainage board, and the change in the pore water pressure of the soil around the drainage board.

[0027] The beneficial effects of the present invention are:

[0028] (1) Applying the principle of gas lift, a new method of vacuum preloading is provided. There are two main beneficial effects in promoting the consolidation of soft soil: 1) Promoting drainage. By lifting the positive pressure gas at the bottom and pulling the negative pressure at the top, the positive pressure gas flows upward along the core of the drainage board. During the flow, the water in the core of the board is carried upward out of the drainage board, accelerating the speed of draining the stored water; 2) Improving the consolidation efficiency of the drainage board at all depths. When the gas injection pressure and flow rate reach a certain value, the water stored in the drainage board can be emptied in a relatively short time. Under the condition of no flowing water, the vacuum degree can be transmitted to the deep part along the drainage board without attenuation, so that the pressure difference between the pore pressure in the soil and the pressure inside the drainage board is further increased. The deeper the drainage board is installed, the more obvious the effect of promoting consolidation is. Therefore, this method is particularly suitable for the consolidation treatment of deep soft soil.

[0029] (2) Compared with the traditional vacuum preloading method, the present invention requires fewer changes to the drainage board and is easy to implement. The newly added equipment is located outside the soil, which is easy to install and maintain, and is conducive to promotion in engineering practice.

[0030] (3) The indoor test device provided can not only simulate on-site construction conditions, but also provide a window for observing the flow pattern within the plate. It can qualitatively and quantitatively analyze the drainage consolidation efficiency, and can be used to explore the drainage mechanism, obtain the optimal gas injection flow rate, gas injection pressure and gas injection frequency, and then optimize the construction plan.

[0031] (4) A geomembrane is used to seal the soft soil in the test device, and the vacuum generating system is first turned on to pump water. After water begins to enter the air-water separation bottle, the pressure gas system is turned on, and the compressed air is used to quickly remove the water in the drainage board using the air lift principle. After the liquid is discharged, the pressure gas system is turned off, and the vacuum pressure at the board head can be efficiently transmitted downward. During the treatment process, the pressure gas system is opened intermittently, and the vacuum pressure range and size of the deep soft soil are improved, which can significantly improve the consolidation and drainage efficiency of the soft soil. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a structural schematic diagram of the testing device of the present invention.

[0033] Figure 2 It is a structural schematic diagram of the drainage plate and the hand-shaped structure in the testing device of the present invention.

[0034] Figure 3 It is a flow chart of the testing method of the present invention.

[0035] Figure 4 This is a diagram showing the flow pattern changes within the drainage board when the present invention is implemented.

[0036] Figure 5 It is a schematic diagram of the change of water output during a single implementation of the present invention.

[0037] Figure 6 It is a schematic diagram of the change of water output when the present invention is implemented.

[0038] Figure 7 It is a schematic diagram of the changes in the vacuum degree in the plate and the pore pressure in the soil when the present invention is implemented.

[0039] Figure 8 It is a schematic diagram of the overall structure of the on-site construction of the present invention.

[0040] Explanation of the accompanying symbols: 1. Model box; 2. Water outlet; 3. Air inlet; 4. Drain board; 5. Flange ring; 6. Geomembrane; 7. Through hole; 8. Pore pressure sensor; 9. Precision electronic scale; 10. Air-water separation bottle; 11. Vacuum gauge; 12. Vacuum pump; 13. Check valve; 14. Ball valve; 15. Pressure gauge; 16. Pressure regulating valve; 17. Air compressor; 18. Industrial camera; 19. Drain board shaped internal structure; 20. Drain board filter membrane; 21. Hand-shaped joint; 22. Drain board; 23. Joint; 24. Air compressor; 25. Pressure gas pipeline; 26. Vacuum pump; 27. Vacuum pipeline; 28. Geotextile and geomembrane. DETAILED DESCRIPTION

[0041] The following will clearly and completely describe the technical solution of the present invention in conjunction with the accompanying drawings. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0042] In the description of the present invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer" and the like, indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely to facilitate and simplify the description of the present invention. They are not intended to indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation, and are therefore not to be construed as limitations on the present invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.

[0043] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they may refer to fixed connections, detachable connections, or integral connections; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention in specific contexts.

[0044] Example 1

[0045] An efficient preloading treatment method for soft soil foundation based on air lift combined with vacuum is used for drainage preloading treatment of high-water content fluid mud, sludge foundation and deep soft foundation, including the following steps:

[0046] (1) The gas lift principle is applied to vacuum preloading. Gas of a certain pressure and flow rate is introduced from the bottom of the drain board, so that the positive pressure gas flows upward along the core of the drain board, and in the process of its flow, the water in the core of the board is carried upward and out of the drain board;

[0047] (2) When the gas lift has completely removed the water in the core of the board, it stops and the core of the board is in a gas-filled state. The vacuum pump connected to the head of the drain plate continuously extracts the gas in the core of the board, so that the gas in the core of the board can be quickly extracted, so that the vacuum can be transmitted from the head of the board to the bottom of the board with almost no attenuation;

[0048] (3) Achieve high vacuum along the entire length of the drainage board, so that the soil within the depth range of the drainage board can be under high vacuum consolidation pressure, promoting drainage and consolidation of deep soil.

[0049] (4) After one gas lift is completed, under the vacuum pressure of the plate core, the pore water in the soil continuously accumulates in the plate core through the drainage plate filter membrane, and the water level in the plate core gradually rises until it is filled with water again. At this time, gas lift is performed again to bring all the water in the plate core out of the plate again, thus starting a new round of high vacuum drainage consolidation within the entire length of the drainage plate.

[0050] In the step (1), when the bottom of the drain board is ventilated and lifted, the vacuum pump connected to the head of the drain board is still continuously extracting gas. The water stored in the core of the board is lifted by the positive pressure of the gas at the bottom and pulled by the negative pressure of the vacuum at the top. The water stored in the core of the board within the entire length of the drain board can be quickly brought out. In the steps (3) and (4), after the water is drained, the vacuum is only transmitted in the gas environment and can be transmitted to the bottom of the board without attenuation. The boundary conditions of soil consolidation have been significantly improved due to the efficient transmission of vacuum. The pressure difference between the pore pressure in the soil and the pressure in the drain board is increased, which can accelerate the seepage rate of water in the soil and effectively improve the efficiency of vacuum preloading treatment of soft soil.

[0051] This method applies the principle of gas lift to the vacuum preloading process. Gas at a specific pressure and flow rate is introduced from the bottom of the drain panel, causing the positively pressurized gas to flow upward along the panel core. This flow entrains water within the panel core and carries it upward and out of the panel. After the water is drained, the vacuum is transmitted only within the gaseous environment, reaching the panel bottom without attenuation. Laboratory model testing has shown that gas lift significantly increases drainage speed, allowing the panel to be emptied in a shorter time, while maintaining a similar vacuum level at the panel bottom as at the panel top. This efficient transfer of vacuum significantly improves the boundary conditions for soil consolidation. The increased differential between the pore pressure in the soil and the pressure within the panel accelerates water seepage, effectively improving the efficiency of vacuum preloading in soft soils. Application of this method on-site requires only simple modifications to the drain panel of a conventional vacuum preloading system. The drain panel is connected to a connector at both the top and bottom. The upper end is connected to a vacuum pump via a vacuum line, while the lower end is connected to an air compressor via a pressurized gas line. When the vacuum degree stabilizes at -80 to -90 kPa, the air compressor is turned on intermittently to inject gas, and the air lift principle is used to clear the water in the drainage board to achieve the purpose of promoting drainage and transferring the vacuum pressure to the deep without attenuation.

[0052] It should be noted that once the gas pressure and flow rate reach a certain value, the drainage rate of water in the core will be greatly increased, and the drainage board will be emptied in a shorter time. Therefore, the air compressor for gas injection can be turned off after each drainage, requiring less energy. Furthermore, the length of the pressurized gas path is not limited by the depth of the foundation, and the air compressor can be placed on the surface for operation and maintenance.

[0053] Example 2

[0054] Refer to the attached Figure 1-2, this embodiment provides a test device based on the method described in Embodiment 1, including a transparent model box 1, drainage plates 4, a vacuum generating system, and a pressure gas system;

[0055] Before adopting the method of the present invention, the drainage plate 4 needs to be made first; the H-shaped internal structure drainage plate with water inlet on both sides used in the project is simplified to a U-shaped internal structure with water inlet on one side. The U-shaped structure 19 of the drainage plate is made of transparent material, and the open side is adhered to the drainage plate filter membrane 20. The side of the drainage plate 4 covered with the filter membrane 20 is called the permeable side, and the side of the drainage plate 4 opposite to the filter membrane 20 is called the transparent side. The transparent side is pasted on the inner wall of the model box with glass glue to provide a window for observing the fluid change inside the drainage plate 4. Both ends of the drainage plate 4 are connected to the hand-type joint 21. The hand-type joint 21 at the upper end is connected to the vacuum generating system, and the hand-type joint 21 at the lower end is connected to the pressure gas system. The internal dimensions of the model box are 50 cm × 30 cm × 100 cm. The transparent side of the drainage plate 4 is vertically close to the inner wall of the model box to provide an observation window. The soft soil to be treated is filled into the box, and then covered with a geomembrane 6. The geomembrane 6 is fixed on the top of the model box 1 through a flange ring 5. The flange ring 5 is connected to the top of the model box 1 by bolts to provide a sealed environment.

[0056] The vacuum generating system includes a pore pressure sensor 8, a gas-water separation bottle 10, a vacuum gauge 11, and a vacuum pump 12; there is a water outlet 2 on the upper wall surface of the model box 1. The hand-type joint 19 at the upper end of the drainage plate 4 is connected to a first steel wire water pipe. The first steel wire water pipe passes through the water outlet 2 and is connected to a vacuum hose. The vacuum hose is connected to the upper side of the gas-water separation bottle 8; the pore pressure sensor 8 is placed into the soil body and the drainage plate through a through hole 7 on the model box 1 to record the pressure change during the vacuum pumping and gas injection cycles; there is a vacuum gauge 11 on the top surface of the gas-water separation bottle 10, and the top surface of the gas-water separation bottle 10 is also connected to the vacuum pump 12 through a vacuum hose; the gas-water separation bottle 10 is placed on a precision electronic scale 9 to record the change of the consolidated drainage volume in the model box over time.

[0057] The pressure gas system includes a check valve 13, a ball valve 14, a pressure gauge 15, a pressure regulating valve 16, an air compressor 17, and an industrial camera 18; there is an air inlet 3 on the bottom surface of the model box 1. The hand-type joint 21 at the lower end of the drainage plate 4 is connected to a second steel wire water pipe; the second steel wire water pipe passes through the air inlet 3 and is connected to the check valve 13. The other end of the check valve 13 is sequentially connected to the ball valve 14, the pressure gauge 15, the pressure regulating valve 16, and the air compressor 17 through a pneumatic tube; the pressure regulating valve 16 and the pressure gauge 15 control the pressure and flow rate of the gas output by the air compressor. The ball valve 14 and the check valve 13 control the gas input and prevent backflow; the bottom of the industrial camera 18 is supported by a tripod. The industrial camera is directly aligned with the central axis of the transparent side of the drainage plate 4 to observe and record the change of the flow pattern inside the plate during air-lift. The precision electronic scale for recording the water output and the industrial camera for recording the fluid change are used to qualitatively and quantitatively analyze the effect of air-lift vacuum preloading.

[0058] When in use, first install the vertical plastic drainage board 4 on the inner wall of the model box 1; connect the pressure gas system and the vacuum generation system; fill the soil; lay the geomembrane 6; seal the model box; start the vacuum generation system; and intermittently start the pressure gas system.

[0059] First, activate the vacuum generating system. Once the drain panel 4 is filled with water, open the ball valve 14 connected to the pressure gas system, start the air compressor 17, and inject pressurized gas into the bottom of the drain panel. The gas injection rate and pressure can be adjusted appropriately based on the pressure gauge 15 and the flow pattern within the drain panel. Once all the water in the drain panel has been drained, close the pressure gas system, and the entire drain panel will maintain maximum vacuum pressure along its entire depth. Under pressure, the water in the soil will quickly converge into the drain panel. By intermittently activating the pressure gas system, drainage is promoted and the vacuum pressure is transmitted to the deeper layer without attenuation.

[0060] The air-water separator bottle 10 in the vacuum system is placed on a high-precision electronic scale 9 to measure the water output over a specified period of time. A pore pressure sensor 8 is inserted into the soil and drainage plate through a through-hole 7 in the model box 1 to record pressure changes during the vacuum and gas injection cycles. The changes in pressure and water output with ventilation, combined with fluid changes captured by an industrial camera, are used to determine the effectiveness of the proposed method. Furthermore, the consolidation and drainage efficiency of the soft soil within the box is analyzed, enabling qualitative and quantitative testing of the method.

[0061] Example 3

[0062] Since the volume of soft foundation treated by vacuum preloading is generally large and the cost is high, the above-mentioned practical engineering operation steps have not yet been implemented in actual projects. Therefore, an indoor test is designed to explore the feasibility of this method. Figure 3 This embodiment provides a testing method based on the device described in the second embodiment, comprising the following steps:

[0063] Step 1: Before vacuuming, adjust the pressure and flow of the incoming air; adjust the air pressure and flow output by the air compressor through the pressure regulating valve 16 and the pressure gauge 15, and then close the air compressor 17 and the ball valve 14;

[0064] Step 2: Turn on the vacuum pump and start vacuuming. You can observe that water continues to enter the drain plate 4 through the filter membrane 20. After water begins to enter the air-water separator bottle 10, turn on the air compressor 17 and the ball valve 14 to use the air lift principle to remove the water in the drain plate 4.

[0065] Step 3: After the drain plate 4 is emptied, the pressure gas system is turned off to maintain the vacuum state;

[0066] Step 4: Repeat the intermittent opening and closing of the pressure gas system until the soft soil consolidation reaches the expected state; during the whole process, the pore pressure sensor 8, the precision electronic scale 9 and the industrial camera 18 are used to record the pressure, water output and flow pattern changes.

[0067] Observed that there is such a thing in the drainage board Figure 4 As shown in the gas-liquid two-phase flow, as the water is continuously discharged, the gas content in the plate increases, and the flow pattern changes from mainly slug flow to mainly annular flow. Figure 5 The weight change in the air-water separator bottle before and after a ventilation cycle indicates that the water in the plate is quickly drained, effectively improving drainage efficiency. After the water is cleared, the pressure gas system is turned off, completing the ventilation cycle.

[0068] The soft soil is vacuum preloaded under the repeated start-stop gas injection state, and the water output changes as follows Figure 6 shown. Figure 6 It can be seen that each time the air is injected, the water in the board can be discharged smoothly. After the air injection is turned off, it takes a certain amount of time for the water in the soil to enter the board and refill the center of the board. Figure 7 It is the change in the vacuum degree in the plate and the pore water pressure in the soil. After the vacuum is started, the vacuum value at the top remains at around -70kPa, and the vacuum value at the bottom is about -65kPa, indicating that there is a pressure difference at both ends of the drainage plate. After the water is discharged by gas injection, the vacuum degree at the bottom rises sharply, which is basically the same as the top. Under this condition, the maximum vacuum pressure at the top is maintained along the depth in the core of the plate. As the water in the soil is continuously re-injected into the plate, the vacuum degree in the plate returns to the state before gas injection. In the process of re-collecting water after emptying, the seepage rate in the soil increases due to the improvement of the boundary conditions, so the pore pressure in the soil will drop significantly after gas injection. This shows that the use of gas lift to empty the drainage plate can allow the vacuum pressure to be transmitted downward to the bottom without attenuation, and accelerate the rate of seepage consolidation of soft soil. Compared with the traditional vacuum preloading in which the vacuum pressure in the plate is attenuated, the method of the present invention can greatly improve the boundary conditions for the consolidation of deep soil, and the consolidation drainage efficiency will be greatly improved.

[0069] From the above implementation steps, it can be seen that the present invention has successfully applied the gas lift principle to the vacuum preloading indoor model test and achieved ideal results. Therefore, a similar design to the above can be used to transform the on-site construction equipment and develop a new on-site construction method. It is worth mentioning that the use of the same Figure 1 Similar pressure gas systems and connection methods can be used to achieve on-site construction. The power and operating time of each device need to be adjusted according to actual conditions.

[0070] Example 4

[0071] The efficiency of gas lift methods is influenced by the injection pressure, injection flow rate, and the resulting two-phase flow pattern. To conduct indoor testing and study its mechanism and operational details, the present invention provides a highly efficient deep soft soil vacuum preloading device based on gas lift, tailored to field conditions. Referring to the testing method described in Example 3, i.e., the aforementioned indoor test procedure, the present invention has designed an on-site method.

[0072] The overall structure of the on-site construction is as follows Figure 8 As shown, the operation process includes the following steps:

[0073] Step 1: Modify the drainage board and connect hand-shaped joints at both ends;

[0074] Step 2: Connect the pressure gas pipeline to the bottom joint and drive it into the soft soil foundation together with the drainage board. The end of the pipeline is connected to the ground to form a gas lift positive pressure pipeline;

[0075] Step 3: Connect the vacuum pipe to the drain board through the top joint, and connect the pipe end on the ground to form a vacuum negative pressure pipe;

[0076] Step 4: Lay geotextile and geomembrane for sealing;

[0077] Step 5: Connect the compressed gas pipeline and the vacuum pipeline to the air compressor and the vacuum pump respectively;

[0078] Step 6: Start the equipment for construction. When the vacuum degree stabilizes at -80 to -90 kPa, start the air compressor intermittently for air injection and air lift.

[0079] The pressure gas system is composed of an air compressor and a pneumatic pipe. The bottom interface of the drainage plate can be connected to the air compressor through the pneumatic pipe to form an air lift positive pressure gas passage.

[0080] The bottom interface and the pneumatic pipe of the drainage board are connected before the drainage board is inserted, ensuring that the gas injection passage is maintained before and after the drainage board is inserted into the soft soil.

[0081] During the vacuum preloading construction process, the air lift system can be used for air lift operations multiple times to promote the discharge of water stored in the core of the drainage board. The air lift operation interval ranges from 6 hours / time to 48 hours / time. The selection of the air lift time interval is related to the water level rising speed in the drainage board after the previous air lift, the vacuum gradient formation speed between the top and bottom of the drainage board, and the change in the pore water pressure of the soil around the drainage board.

[0082] It should be noted that once the gas pressure and flow rate reach a certain value, drainage speed will be greatly increased, and the drain plate will be emptied in a shorter time. Therefore, the air compressor for gas injection can be turned off after each drainage is completed, requiring less energy. Furthermore, the length of the pressurized gas path is not limited by the depth of the soft foundation soil, and the air compressor can be placed on the surface for operation and maintenance.

[0083] This invention proposes a new vacuum preloading method based on the principle of gas lift. Currently, no process combines the two to reinforce soft foundations. The proposed device also provides a way to observe the flow pattern within the drainage plate and control the injection pressure and flow rate, enabling better analysis of the method's effectiveness.

[0084] The contents described in the embodiments of this specification are merely an enumeration of the implementation forms of the inventive concept. The scope of protection of the present invention should not be regarded as limited to the specific forms described in the embodiments. The scope of protection of the present invention also extends to equivalent technical means that can be conceived by those skilled in the art based on the inventive concept.

Claims

1. An efficient preloading treatment method for soft soil foundation based on air lift combined with vacuum, used for drainage preloading treatment of high-water content fluid mud, sludge foundation and deep soft foundation, characterized by: The following steps are involved: (1) Of the upper and lower ends of the drain plate, only the upper end of the drain plate is connected to the vacuum generating system, and the lower end of the drain plate is only connected to the pressure gas system; (2) Applying the gas lift principle to vacuum preloading, a certain pressure and flow rate of gas is introduced from the bottom of the drain board, so that the positive pressure gas flows upward along the core of the drain board, and in the process of its flow, the water in the core of the board is carried upward and out of the drain board; (3) When the gas lift has completely taken out the water in the core of the board, it stops and the core of the board is in a gas-filled state. The vacuum pump connected to the head of the drain plate continuously extracts the gas in the core of the board, so that the gas in the core of the board can be quickly extracted, so that the vacuum can be transmitted from the head of the board to the bottom of the board with almost no attenuation; (4) Achieve high vacuum over the entire length of the drainage board, so that the soil within the depth range of the drainage board can be under high vacuum consolidation pressure, promoting drainage and consolidation of deep soil; (5) After one gas lift is completed, under the vacuum pressure of the plate core, the pore water in the soil penetrates the filter membrane of the drainage board and continuously accumulates in the plate core. The water level in the plate core gradually rises until it is filled with water again. At this time, gas lift is performed again to bring all the water in the plate core out of the plate again, thus starting a new round of high vacuum drainage consolidation within the full length of the drainage board; In step (2), when the bottom of the drain board is ventilated and lifted, the vacuum pump connected to the head of the drain board is still continuously extracting gas, and the water stored in the board core is lifted by the positive pressure of the gas at the bottom and pulled by the negative pressure of the vacuum at the top, and the water stored in the board core within the entire length of the drain board is quickly brought out.

2. According to the method for efficiently preloading a soft soil foundation based on air lift combined with vacuum in claim 1, in said steps (3) and (4), after the water is drained, the vacuum is transmitted only in the gas environment and is transmitted to the bottom of the plate without attenuation; the boundary conditions of soil consolidation are improved due to the efficient transmission of the vacuum, the pressure difference between the pore pressure in the soil and the pressure in the drainage plate is increased, the water seepage rate in the soil is accelerated, and the efficiency of vacuum preloading treatment of soft soil is improved.

3. An engineering implementation method for the high-efficiency preloading treatment method for soft soil foundation based on air lift combined with vacuum according to claim 1, characterized in that: The following steps are involved: Step 1: Modify the drainage board and connect hand-shaped joints at both ends; Step 2: Connect the pressure gas pipeline to the bottom joint and drive it into the soft soil foundation together with the drainage board. The end of the pipeline is connected to the ground to form a gas lift positive pressure pipeline; Step 3: Connect the vacuum pipe to the drain board through the top joint, and connect the pipe end on the ground to form a vacuum negative pressure pipe; Step 4: Lay geotextile and geomembrane for sealing; Step 5: Connect the compressed gas pipeline and the vacuum pipeline to the air compressor and the vacuum pump respectively; Step 6: Start the equipment for construction. When the vacuum degree stabilizes at -80 to -90 kPa, start the air compressor intermittently for air injection and air lift.

4. The engineering implementation method according to claim 3, characterized in that: Connect the bottom interface of the drain plate to the air compressor through a pneumatic tube to form an air lift positive pressure gas passage.

5. The engineering implementation method according to claim 4, characterized in that: The bottom interface and the pneumatic pipe of the drainage board are connected before the drainage board is inserted, ensuring that the gas injection passage is maintained before and after the drainage board is inserted into the soft soil.

6. The engineering implementation method according to claim 5, characterized in that: In this method, gas lift positive pressure gas passages are used multiple times to perform gas lift operations to promote the discharge of water stored in the core of the drainage board. The gas lift operation interval is 6 hours / time to 48 hours / time. The selection of the gas lift time interval is related to the water level rising speed in the drainage board after the previous gas lift, the vacuum gradient formation speed between the top and bottom of the drainage board, and the change in the pore water pressure of the soil around the drainage board.

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