Construction method for reinforcing super-soft ground foundation by sand-free cushion vacuum preloading combined with hard shell layer

Abstract

The application discloses a construction method of a sand-free cushion vacuum preloading combined hard shell layer reinforced super-soft soil foundation, which comprises the following steps: dividing a target super-soft soil foundation into a plurality of foundation units, digging a foundation pit in the foundation units, and filling a hard shell layer region in the foundation pit, and the density of the hard shell layer region is less than the density of the target super-soft soil foundation below the foundation pit, so that the hard shell layer region bears the operation equipment for digging and filling the next foundation unit; the operation equipment continues to dig and fill the next foundation unit in the hard shell layer region of the current foundation unit, so that the next foundation unit forms a hard shell layer region, and the process is sequentially continued until a hard shell layer region completely covering the target super-soft soil foundation is formed; and the sand-free cushion vacuum preloading operation is performed on the super-soft soil foundation with the formed hard shell layer region until the current target super-soft soil foundation is consolidated and settled to a preset elevation. The application avoids the problem of air leakage, saves the engineering cost, and improves the engineering safety factor.

Description

Technical Field

[0001] This invention relates to the field of foundation reinforcement technology, specifically a construction method for reinforcing ultra-soft soil foundations using a combination of sand-free cushion layer vacuum preloading and hard shell layer. Background Technology

[0002] In recent years, with the rapid development of my country's economy, the contradiction of land scarcity in coastal areas has become increasingly prominent. Under these circumstances, a wave of land reclamation has swept across cities. The foundation formed by land reclamation is deep fill soil mainly composed of clay. Most coastal cities in my country have a large amount of silty soil, while land reclamation mostly uses dredged ultra-soft soil, which contains 30%-50% clay particles with a particle size of less than 0.005mm. It generally has characteristics such as high water content, large void ratio, low strength, high compressibility, low permeability, and rheological properties. Therefore, this ultra-soft soil foundation cannot meet the basic conditions required for engineering construction, and the consolidation of ultra-soft soil under its own weight can take several years or even longer.

[0003] To enable ultra-soft soil foundations to achieve sufficient strength for engineering construction within a short period, researchers have proposed the sand-free cushion vacuum preloading method. The sand-free cushion vacuum preloading method shares the same reinforcement principle as the traditional vacuum preloading method: both utilize atmospheric pressure to create negative pressure on the ground surface, which diffuses to the surrounding soil through vertical drainage systems within the foundation, reducing pore water pressure, increasing effective stress, and enhancing soil strength, thereby reinforcing the foundation. The biggest difference between the sand-free cushion vacuum preloading method and conventional vacuum preloading technology lies in the process: the former eliminates the need for a horizontal drainage sand cushion layer. Instead, it uses denser spacing of horizontal drainage filter pipes directly connected to drainage boards to rapidly transfer negative pressure from the ground surface to the vertical drainage boards. This avoids safety hazards such as the inability to lay the sand cushion layer, slurry buildup contaminating the sand cushion layer, and even the collapse of machinery. Although the sand-free cushion vacuum preloading method has been widely used, it still has many limitations, such as frequent air leakage of the sealing membrane, inability to bring in large equipment, and significant construction difficulties.

[0004] A hard crust is a layer of harder soil distributed on top of soft soil. Compared to the underlying soft soil foundation, it has higher density, compressibility, and shear strength, and possesses a certain degree of stiffness, enabling it to bear some external loads. The hard crust significantly affects the bearing capacity and deformation of the underlying soft soil layer, mainly through stress diffusion, sealing, and settlement lag effects. Stress diffusion and sealing both positively impact the bearing capacity of the foundation, increasing the bearing capacity of the original soft soil and weak foundation. However, sealing can cause excess pore pressure in the underlying layer, and the area affected is relatively large. The settlement lag effect causes the settlement of the underlying layer to be asynchronous with that of the hard crust, resulting in slower overall settlement, which is detrimental to the foundation. Furthermore, hard crusts can be divided into natural hard crusts and artificial hard crusts. The latter is typically formed by chemically modifying and reinforcing the surface layer of the weak foundation and then backfilling it; it forms once the surface layer has gained strength.

[0005] However, for ultra-soft soils, due to their high water content, low strength, large void ratio, and low density, the addition of conventional modifiers reduces the water content and void ratio while increasing the density. After backfilling, the soil sinks into the underlying layer and cannot form an overlying hard shell. Therefore, conventional methods are not suitable for ultra-soft soil foundations. Currently, no technology has been found that combines vacuum preloading of sand cushion layers with a hard shell layer for ultra-soft soil foundation construction. Furthermore, no existing technology has been found that can accelerate the reinforcement process of ultra-soft soil foundations, thereby shortening the consolidation time. In particular, compared to the traditional ultra-soft soil, which requires several years or even longer to consolidate under its own weight, the consolidation time is significantly shortened, enabling the formation of strength sufficient for engineering construction in a short time. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a construction method for reinforcing ultra-soft soil foundations using a combination of sand-free cushion layer vacuum preloading and hard shell layer, which can solve the problems described in the background art.

[0007] The technical solution to achieve the objective of this invention is: a construction method for reinforcing ultra-soft soil foundations with a sand-free cushion layer, vacuum preloading, and a hard shell layer, comprising the following steps:

[0008] Step S1: Divide the target ultra-soft soil foundation into several foundation units, excavate foundation pits within each foundation unit, and fill the foundation pits with a hard crust layer. The density of the hard crust layer is less than the density of the target ultra-soft soil foundation below the foundation pit, so that the hard crust layer can support the equipment used for excavating and filling the next foundation unit.

[0009] The working equipment continues to excavate and fill the hard shell layer area of ​​the current foundation unit to form a hard shell layer area for the next foundation unit, and so on, until a hard shell layer area completely covers the target ultra-soft soil foundation is formed.

[0010] Step S2: Perform vacuum preloading with a sand-free cushion layer on the ultra-soft soil foundation in the region where the hard crust layer has been formed, until the current target ultra-soft soil foundation has consolidated and settled to the preset elevation.

[0011] Furthermore, after step S1 and before step S2, the formed hard shell layer region is left to stand for a certain period of time before a sand-free pad vacuum pre-compression operation is performed.

[0012] Furthermore, the specific implementation process of step S1 includes the following steps:

[0013] Step 1: Excavate a foundation pit of a predetermined size in the target ultra-soft soil foundation, and process the excavated ultra-soft soil material after it has been left to stand to obtain modified ultra-soft soil material. The processing includes one or more of the following: reducing the water content of the ultra-soft soil material, increasing the strength of the ultra-soft soil material, and reducing the density of the ultra-soft soil material.

[0014] Step 2: Backfill a portion of the modified ultra-soft soil into the foundation pit and compact it, thereby forming a first modified soil layer of a certain thickness in the foundation pit;

[0015] Step 3: Above the first modified soil layer in the foundation pit, continue to lay a foamed concrete layer with a thickness of b;

[0016] Step 4: Backfill the remaining modified ultra-soft soil material into the foundation pit, and after compaction, form a second modified soil layer on top of the foamed concrete layer. This will form a hard shell layer area, including the ultra-soft soil foundation below the foundation pit and the first modified soil layer, foamed concrete layer, and second modified soil layer in the foundation pit.

[0017] Furthermore, in step 1, the pre-defined foundation pit is a cuboid foundation pit, a circular foundation pit, a rhomboid foundation pit, or an irregularly shaped foundation pit, and the depth of the foundation pit is a meters.

[0018] Furthermore, in step 1, the excavated ultra-soft soil material is placed around the target ultra-soft soil foundation and left to stand for a period of time to allow the moisture inside the ultra-soft soil material to evaporate. The duration of the standing time is at least such that there is no obvious free water flow on the surface of the ultra-soft soil material.

[0019] Furthermore, the treatment includes adding lime, modifier and ESP particles in a preset ratio to the settled ultra-soft soil material, stirring evenly and letting it stand for a certain period of time. The purpose of adding lime is to reduce the moisture content of the ultra-soft soil material. The modifier includes cement and fly ash, which is used to increase the strength of the ultra-soft soil material. The purpose of EPS particles is to reduce the density of the ultra-soft soil material and prevent the ultra-soft soil material from settling during subsequent backfilling.

[0020] Furthermore, the content ratio of lime, modifier, and ESP particles was determined experimentally. The conditions for determining this ratio were: an unconfined compressive strength greater than 0.6 MPa after 2 days indoors and a density of less than 1.3 g / cm³ for the modified ultra-soft soil material. 3 The mixing ratio at this time is taken as the preset mixing ratio.

[0021] Furthermore, the thickness of the first modified soil layer is c times the depth a of the foundation pit, 0 < c < 1, 0 < b < a, and the thickness of the first modified soil layer + b < the depth of the foundation pit. The purpose of laying a foamed concrete layer of thickness b is to prevent the subsequent hard shell structure from having excessively large pores due to the small impact energy of the operating equipment, and to prevent air and water leakage during the subsequent vacuum preloading operation of the sand-free cushion layer.

[0022] Furthermore, the excavated ultra-soft soil is modified and then backfilled into the foundation pit. The surface of the bottom wall of the foundation pit is filled with ultra-modified soft soil to prevent some parts of the bottom wall of the foundation pit from being exposed. The filling continues towards the pit opening to form a first modified soil layer of a certain thickness.

[0023] Furthermore, the specific implementation process of step S2 includes the following steps:

[0024] Step 5: Dig several recesses of a certain size downwards in the hard shell area. The recesses are arranged at intervals along the extension direction of the hard shell area and distributed throughout the hard shell area. The recesses pass through the entire hard shell area. Drainage boards are buried in the round holes. One drainage board is laid in each round hole. The drainage boards extend into the ultra-soft soil foundation below the foundation pit. The recesses and drainage boards are set longitudinally.

[0025] Step 6: Lay horizontal filter pipes on the surface of the hard shell area and connect the drainage board to the horizontal filter pipes. The horizontal filter pipes are also connected to the vacuum pressure system, and the connection between the drainage board and the horizontal filter pipes is sealed.

[0026] Step 7: Fill and seal the concave holes with foamed concrete. After the foamed concrete has reached a certain strength, turn on the vacuum pressure system until the current target ultra-soft soil foundation has consolidated and settled to the preset elevation.

[0027] The beneficial effects of the present invention are as follows: 1. The present invention can use small equipment to treat soft soil foundations and form an overlying hard shell layer on the foundation surface. The hard shell layer itself has a certain strength and the density of the hard shell layer is less than that of soft soil. When the hard shell layer is under stress, the underlying ultra-soft soil foundation provides support for it, so that the foundation has a certain bearing capacity and also provides conditions for subsequent equipment to enter the site, thereby avoiding the need for large-scale construction equipment and a lot of time in traditional operation methods.

[0028] 2. This invention uses a hard shell layer to replace the sealing membrane in the traditional vacuum pre-compression method, which effectively avoids the problem of air leakage in the sealing membrane during the project. At the same time, after the vacuum pre-compression is completed, there is no need to recycle the sealing membrane, which saves a certain amount of manpower and material resources, and also avoids prolonging the solidification time due to air leakage.

[0029] 3. Compared with the traditional sand-free cushion vacuum preloading method, the sand-free cushion vacuum preloading combined with hard shell layer reinforcement of ultra-soft soil foundation construction method of the present invention does not require secondary treatment for construction sites with low bearing capacity requirements. The hard shell layer formed by this method can directly bear the load of the upper structure, which saves engineering costs to a certain extent.

[0030] 4. Compared with traditional artificial hard shell layers, the construction method of super-soft soil foundation reinforcement by vacuum preloading combined with hard shell layer of the present invention can realize the construction of overlying hard shell layer of super-soft soil foundation. At the same time, the vacuum preloading stage promotes the dissipation of excess pore pressure in the underlying layer, improves the safety factor of the project, and reduces the consolidation time. Attached Figure Description

[0031] Figure 1 A schematic diagram illustrating the construction status of the hard crust layer area in the target ultra-soft soil foundation;

[0032] Figure 2 In order to be in Figure 1 A schematic diagram of the vacuum preloading construction process based on the above.

[0033] In the figure, 1-first modified soil layer, 2-foamed concrete layer, 3-second modified soil layer, 4-ultra-soft soil foundation, 5-concave hole, 6-drainage board, 7-horizontal filter pipe, 8-vacuum pressure system, 101-hard shell layer area. Detailed Implementation

[0034] The present invention will be further described below with reference to the accompanying drawings and specific embodiments:

[0035] like Figures 1-2 As shown, a construction method for reinforcing ultra-soft soil foundations with a sand-free cushion layer, vacuum preloading, and a hard shell layer includes the following steps:

[0036] Step 1: Excavate a foundation pit of a preset size in the target ultra-soft soil foundation 4, and process the excavated ultra-soft soil material after it has been left to stand to obtain modified ultra-soft soil material. The processing includes one or more of the following: reducing the water content of the ultra-soft soil material, increasing the strength of the ultra-soft soil material, and reducing the density of the ultra-soft soil material.

[0037] In this step, the pre-defined foundation pit is a rectangular pit with a length of 5m, a width of 5m, and a depth of 2m. In practical applications, other shapes can also be excavated, such as circular, rhomboid, or irregularly shaped foundation pits. The size of the foundation pit can be adjusted according to actual needs, for example, excavating to a deeper or shallower depth. A small excavator can be used to excavate the foundation pit on the target ultra-soft soil foundation 4.

[0038] The excavated soft soil material can be placed around the target soft soil foundation 4 and left to stand for a period of time. The purpose of standing is to let the moisture in the soft soil material dry out, so that at least there is no obvious free water flow on the surface of the soft soil material.

[0039] The treatment involves adding lime, a modifier, and ESP particles in a preset ratio to the settled ultra-soft soil material, stirring thoroughly, and then allowing it to stand for a certain period, for example, one day. The purpose of adding lime is to reduce the moisture content of the ultra-soft soil material. The modifier, including cement and fly ash, aims to increase the strength of the ultra-soft soil material. The ESP particles aim to reduce the density of the ultra-soft soil material, preventing subsidence during subsequent backfilling. The ratio of lime, modifier, and ESP particles can be determined experimentally, for example, by achieving an unconfined compressive strength greater than 0.6 MPa and a density of less than 1.3 g / cm³ in a 2-day indoor test. 3 The mixing ratio at that time is taken as the preset mixing ratio.

[0040] Step 2: Backfill a portion of the modified ultra-soft soil into the foundation pit and compact it, thereby forming a first modified soil layer 1 of a certain thickness in the foundation pit. The first modified soil layer 1 fills the entire bottom wall of the foundation pit, and the thickness of the first modified soil layer 1 is adjusted according to the actual situation. That is, the excavated ultra-soft soil is modified and then backfilled into the foundation pit, and the surface of the bottom wall of the foundation pit is filled with ultra-modified soft soil to avoid any part of the bottom wall of the foundation pit being exposed. The filling continues towards the direction closer to the pit opening, thereby forming a first modified soil layer 1 of a certain thickness.

[0041] In one optional implementation, the thickness x of the first modified soil layer 1 is a times the depth c of the foundation pit, where 0 < a < 1. In this embodiment, a = 0.5, that is, the thickness x of the first modified soil layer 1 is half the depth c of the foundation pit, i.e., x = 0.5c. For example, if the depth of the foundation pit is 2m, then the thickness of the first modified soil layer 1 is 1m. That is, the first modified soil layer 1 is formed by filling half the volume of the foundation pit with modified ultra-soft soil material 2, meaning that not all of the modified ultra-soft soil material is filled into the foundation pit, and a portion of the modified ultra-soft soil material is still placed around the target ultra-soft soil foundation 4.

[0042] Step 3: Above the first modified soil layer 1 in the foundation pit, continue to lay a foamed concrete layer 2 with a thickness of b, where 0 < b < a and the thickness of the first modified soil layer 1 + b < the depth of the foundation pit. In this embodiment, b ≥ 0.3m. The purpose of laying the foamed concrete layer 2 is to prevent the subsequent hard shell structure from having excessively large pores due to the relatively small impact energy of the working equipment (e.g., a small excavator), and to prevent air and water leakage during vacuum preloading.

[0043] Step 4: Backfill the remaining modified ultra-soft soil material into the foundation pit, and after compaction, form a second modified soil layer 3 above the foamed concrete layer 2, thereby forming a hard shell layer area 101 together with the ultra-soft soil foundation 4 below the foundation pit and the first modified soil layer 1, foamed concrete layer 2 and second modified soil layer 3 in the foundation pit.

[0044] In this step, since the first modified soil layer 1 and the second modified soil layer 3 have been compacted, and a foamed concrete layer 2 has been added, the surface of the second modified soil layer 3 is flush with the pit opening. That is, the surface of the second modified soil layer 3 is flush with the surface of the original target ultra-soft soil foundation 4. In other words, in step 3, the thickness of the foamed concrete layer 2 also needs to take into account the height of the remaining modified ultra-soft soil material after compaction, so that the final surface of the second modified soil layer 3 can be flush with the pit opening. Of course, in actual use, the surface of the second modified soil layer 3 may be lower or higher than the pit opening, meaning the surface of the second modified soil layer 3 may be lower or higher than the surface of the original target ultra-soft soil foundation 4.

[0045] In an optional implementation, the target ultra-soft soil foundation 4 is divided into several foundation units, and a foundation pit is excavated in each foundation unit. After step 4 is completed in the current foundation unit, steps 1-4 are repeated starting from the current foundation unit, thereby forming a hard shell layer region 101 on each foundation unit, thus forming a hard shell layer region 101 covering the entire target ultra-soft soil foundation 4.

[0046] By adopting zonal construction of each foundation unit, the hard shell area 101 formed by the previous foundation unit can be made to have load-bearing capacity, and can support working equipment (such as a small excavator) on the hard shell area 101 of the current foundation unit to carry out work on the next foundation unit, thus meeting the requirements for equipment entry and providing conditions for the subsequent entry of vacuum preloading equipment.

[0047] Step 5: After the formed hard crust region 101 has been left to stand for a certain period of time, for example, 3 days, several recessed holes 5 of a certain size are excavated downwards in the hard crust region 101. These recessed holes 5 are spaced apart along the extension direction of the hard crust region 101, penetrating the entire hard crust region 101, that is, excavating downwards until the bottom wall of the second modified soil layer 3 is formed. For example, a circular hole with a diameter of 15cm and a depth penetrating the entire hard crust region 101 is excavated. Drainage boards 6 are buried in the circular holes, one drainage board 6 in each circular hole, extending into the ultra-soft soil foundation 4 below the foundation pit. Both the recessed holes 5 and the drainage boards 6 are arranged longitudinally.

[0048] Among them, a recessed hole 5 can be opened in the hard shell region 101 by drilling equipment or other equipment.

[0049] Step 6: Lay a horizontal filter pipe 7 on the surface of the hard shell region 101, and connect the drainage plate 6 to the horizontal filter pipe 7. The horizontal filter pipe 7 is also connected to the vacuum pressure system 8. Seal the connection between the drainage plate 6 and the horizontal filter pipe 7.

[0050] Step 7: Fill and seal the concave hole 5 with foamed concrete. After the foamed concrete has reached a certain strength, turn on the vacuum pressure system 8 until the current target ultra-soft soil foundation 4 has consolidated and settled to the preset elevation.

[0051] In an optional implementation, after step 7 is completed, i.e., after the vacuum preloading stage is finished, because the hard shell region 101 itself has high strength and its density is less than that of the ultra-soft soil foundation 4, when a load is applied to the hard shell region 101, the ultra-soft soil foundation 4 provides support to the hard shell region 101, giving the hard shell region 101 a certain bearing capacity. Generally speaking, for construction sites where the foundation bearing capacity requirement is less than 0.5 MPa, secondary treatment is not required, and the site can be put directly into operation.

[0052] Compared with the prior art, the present invention achieves the following effects:

[0053] 1. This invention can use small equipment to treat soft soil foundations and form an overlying hard shell layer on the foundation surface. The hard shell layer itself has a certain strength and its density is less than that of soft soil. When the hard shell layer is under stress, the underlying ultra-soft soil foundation 4 provides support for it, giving the foundation a certain bearing capacity and providing conditions for subsequent equipment to enter the site. This avoids the need for large-scale construction equipment and the time wasted in traditional operation methods.

[0054] 2. This invention uses a hard shell layer to replace the sealing membrane in the traditional vacuum pre-compression method, which effectively avoids the problem of air leakage in the sealing membrane during the project. At the same time, after the vacuum pre-compression is completed, there is no need to recycle the sealing membrane, which saves a certain amount of manpower and material resources, and also avoids prolonging the solidification time due to air leakage.

[0055] 3. Compared with the traditional sand-free cushion vacuum preloading method, the sand-free cushion vacuum preloading combined with hard shell layer reinforcement of ultra-soft soil foundation of the present invention can be used for construction sites with low bearing capacity requirements without secondary treatment. The hard shell layer formed by this method can directly bear the load of the upper structure, which saves engineering costs to a certain extent.

[0056] 4. Compared with the traditional artificial hard shell layer, the construction method of the present invention for reinforcing ultra-soft soil foundation 4 by vacuum preloading combined with hard shell layer without sand cushion can realize the construction of the overlying hard shell layer of ultra-soft soil foundation 4. At the same time, the vacuum preloading stage promotes the dissipation of excess pore pressure in the underlying layer, improves the safety factor of the project, and reduces the consolidation time.

[0057] The embodiments disclosed in this specification are merely illustrative of one aspect of the invention, and the scope of protection of the invention is not limited to these embodiments. Any other functionally equivalent embodiments fall within the scope of protection of the invention. Those skilled in the art can make various other corresponding changes and modifications based on the technical solutions and concepts described above, and all such changes and modifications should fall within the scope of protection of the claims of this invention.

Claims

1. A construction method for reinforcing ultra-soft soil foundations with a sand-free cushion layer, vacuum preloading, and a hard shell layer, characterized in that... Includes the following steps: Step S1: Divide the target ultra-soft soil foundation into several foundation units, excavate foundation pits within each foundation unit, and fill the foundation pits with a hard crust layer region. The density of the hard crust layer region is less than the density of the target ultra-soft soil foundation below the foundation pit, so that the hard crust layer region has the capacity to support the work equipment used for excavating and filling the next foundation unit. The work equipment continues to excavate and fill the hard crust region of the current foundation unit into the next foundation unit, so that the next foundation unit forms a hard crust region. This process continues until a hard crust region completely covers the target ultra-soft soil foundation. The specific implementation process of step S1 includes the following steps: Step 1: Excavate a foundation pit of a predetermined size in the target ultra-soft soil foundation, and process the excavated ultra-soft soil material after it has been left to stand to obtain modified ultra-soft soil material. The processing includes one or more of the following: reducing the water content of the ultra-soft soil material, increasing the strength of the ultra-soft soil material, and reducing the density of the ultra-soft soil material. Step 2: Backfill a portion of the modified ultra-soft soil into the foundation pit and compact it, thereby forming a first modified soil layer of a certain thickness in the foundation pit; Step 3: Above the first modified soil layer in the foundation pit, continue to lay a foamed concrete layer with a thickness of b; Step 4: Backfill the remaining modified ultra-soft soil material into the foundation pit, and after compaction, form a second modified soil layer on top of the foamed concrete layer, thereby forming a hard shell layer area including the ultra-soft soil foundation below the foundation pit and the first modified soil layer, foamed concrete layer, and second modified soil layer in the foundation pit. Step S2: Perform vacuum preloading with a sand-free cushion layer on the ultra-soft soil foundation where a hard crust layer has been formed, until the current target ultra-soft soil foundation has consolidated and settled to the preset elevation.

2. The construction method for reinforcing ultra-soft soil foundations with a sand-free cushion layer, vacuum preloading, and a hard shell layer according to claim 1, is characterized in that... After step S1 and before step S2, the process also includes allowing the formed hard shell layer area to stand for a certain period of time before performing a sand-free pad vacuum pre-compression operation.

3. The construction method for reinforcing ultra-soft soil foundations with a sand-free cushion layer, vacuum preloading, and a combined hard shell layer according to claim 1, is characterized in that... In step 1, the pre-defined foundation pit is a cuboid foundation pit with a depth of a meters.

4. The construction method for reinforcing ultra-soft soil foundations with a sand-free cushion layer, vacuum preloading, and a hard shell layer according to claim 3, is characterized in that... In step 1, the excavated soft soil material is placed around the target soft soil foundation and left to stand for a period of time to allow the moisture in the soft soil material to evaporate. The duration of the standing time is at least until there is no obvious free water flow on the surface of the soft soil material.

5. The construction method for reinforcing ultra-soft soil foundations with a sand-free cushion layer, vacuum preloading, and a combined hard shell layer according to claim 4, is characterized in that... The process includes adding lime, modifier, and EPS particles in a preset ratio to the settled ultra-soft soil material, stirring evenly, and then letting it stand for a certain period of time. The purpose of adding lime is to reduce the moisture content of the ultra-soft soil material. The modifier includes cement and fly ash, which is used to increase the strength of the ultra-soft soil material. The purpose of EPS particles is to reduce the density of the ultra-soft soil material and prevent the ultra-soft soil material from settling during subsequent backfilling.

6. The construction method for reinforcing ultra-soft soil foundations with a sand-free cushion layer, vacuum preloading, and a combined hard shell layer according to claim 5, is characterized in that... The content ratio of lime, modifier, and EPS particles was determined experimentally. The conditions for determining this ratio were: an unconfined compressive strength greater than 0.6 MPa after 2 days indoors and a density of less than 1.3 g / cm³ for the modified ultra-soft soil. 3 The mixing ratio at this point is taken as the preset mixing ratio.

7. The construction method for reinforcing ultra-soft soil foundations with a sand-free cushion layer, vacuum preloading, and a combined hard shell layer according to claim 6, is characterized in that... The thickness of the first modified soil layer is c times the depth a of the foundation pit, where 0 < c < 1, 0 < b < a, and the thickness of the first modified soil layer + b < the depth of the foundation pit.

8. The construction method for reinforcing ultra-soft soil foundations with a sand-free cushion layer, vacuum preloading, and a combined hard shell layer according to claim 7, is characterized in that... The excavated soft soil is modified and then backfilled into the foundation pit. The surface of the bottom wall of the foundation pit is filled with modified soft soil to prevent any part of the bottom wall from being exposed. The filling continues towards the pit opening to form a first modified soil layer of a certain thickness.

9. The construction method for reinforcing ultra-soft soil foundations with a sand-free cushion layer, vacuum preloading, and a combined hard shell layer according to claim 8, is characterized in that... The specific implementation process of step S2 includes the following steps: Step 5: Dig several recesses of a certain size downwards in the hard shell area. The recesses are arranged at intervals along the extension direction of the hard shell area and are distributed throughout the hard shell area. The recesses pass through the entire hard shell area. Drainage boards are buried in the recesses. One drainage board is laid in each recess. The drainage boards extend into the ultra-soft soil foundation below the foundation pit. Both the recesses and the drainage boards are set longitudinally. Step 6: Lay horizontal filter pipes on the surface of the hard shell area and connect the drainage board to the horizontal filter pipes. The horizontal filter pipes are also connected to the vacuum pressure system, and the connection between the drainage board and the horizontal filter pipes is sealed. Step 7: Fill and seal the concave holes with foamed concrete. After the foamed concrete has reached a certain strength, turn on the vacuum pressure system until the current target ultra-soft soil foundation has consolidated and settled to the preset elevation.

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