Gravel dynamic compaction replacement layered consolidation foundation enhancement method in permafrost region

Abstract

The invention discloses a permafrost region gravel dynamic compaction replacement layered consolidation foundation enhancement method, which relates to the technical field of foundation consolidation, and comprises the following steps: detection preparation work; planning a frozen soil area foundation construction plane area; a first pile hole construction point is selected, concrete pouring is carried out after piling, then a second pile hole construction point is selected, secondary concrete pouring is carried out, a cold medium is introduced into a first pile hole and a second pile hole, and a frozen pier is formed above the concrete column; multi-point compaction is conducted in the construction plane area, a compacted layer is obtained through a gravel dynamic compaction replacement layering method after leveling, and foundation construction is completed; the method is remarkable in effect of reinforcing the foundation in the permafrost region, short in construction period and low in cost, meanwhile, underground water can be prevented from making contact with the permafrost region, frost heaving is avoided, surface water can be prevented from invading the permafrost region in combination with steel plates and earthwork cloth, frost heaving is further avoided, meanwhile, heat preservation can be conducted on the interior of the permafrost region, and the service life of the permafrost region is prolonged. And the overall stability of the foundation is further improved.

Description

Technical Field

[0001] The invention belongs to the technical field of foundation reinforcement, and in particular relates to a method for reinforcing a foundation by layered reinforcement through crushed stone dynamic compaction and replacement in permafrost areas. Background Art

[0002] Permafrost refers to soil and rock with negative temperatures and ice. Due to the presence of ice and unfrozen water, its properties are extremely complex and temperature-sensitive. my country's permafrost area accounts for approximately 22.4% of its land area, making it the third largest country in the world in terms of permafrost. In permafrost areas, both construction and global warming can cause permafrost degradation, leading to thaw settlement and serious damage to the stability of projects in permafrost areas.

[0003] The specific method of gravel replacement layering by dynamic compaction is to backfill the ramming pit formed by dynamic compaction with coarse granular materials such as boulders and gravel, and then tamp it with a rammer. This process is repeated continuously to form a pier body, which is called a dynamic compaction replacement pier. This method has the advantages of significant reinforcement effect, short construction period and low cost.

[0004] Dynamic compaction is widely used to reinforce foundation soils with poor quality. It is easy to construct and has significant cost advantages. At present, dynamic compaction design still mainly estimates construction parameters based on engineers' experience or empirical formulas, and then determines them through on-site compaction tests. Empirical formulas are generally established based on engineering examples, model tests, or numerical calculation results. They quantitatively relate the size of the dynamic compaction reinforcement range (generally composed of the ramming pit depth, effective reinforcement depth, and lateral reinforcement range) and different construction parameters (ramming energy or impulse per hammer, hammer size, and number of hammer blows) under single-ramming conditions. This can facilitate the selection of construction parameters in dynamic compaction design. However, dynamic compaction construction has requirements for the size of the reinforcement range and the degree of soil density within the reinforcement range. Therefore, using only empirical formulas related to the size of the reinforcement range without considering the degree of soil density within the reinforcement range will lead to large errors in the estimated construction parameters, thereby reducing the efficiency of the test compaction and causing waste of construction costs. In addition, since the actual compaction construction site is composed of a series of compaction points, the multi-component parameters such as the spacing between compaction points need to be determined before the compaction construction. At the same time, since there is an overlapping area between adjacent compaction points and they are affected by the surrounding compaction points, the interaction between adjacent compaction points needs to be considered.

[0005] After research, the inventors have provided a method for strengthening the foundation in permafrost areas by using crushed stone dynamic compaction and replacement layering, based on the method of using crushed stone dynamic compaction and replacement layering. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for strengthening the foundation in permafrost areas by using crushed stone compaction replacement and layered reinforcement. This method has a significant reinforcement effect on the foundation in permafrost areas, a short construction period and low cost. At the same time, it can isolate the contact between groundwater and the permafrost area, avoiding the occurrence of frost heave. At the same time, combined with the use of steel plates and earthwork cloths, it can prevent surface water from invading the permafrost area, further avoiding frost heave while insulating the interior of the permafrost area, further improving the overall stability of the foundation.

[0007] To achieve the above object, the present invention provides the following technical solutions: A method for strengthening a foundation by layered reinforcement using crushed stone dynamic compaction and replacement in permafrost areas comprises the following steps: S1, detect the frozen soil depth d1 and groundwater depth d2 in the frozen soil area; S2. Plan the plane area for foundation construction in the frozen soil area and obtain the length and width data L×K; S3. Select the first pile hole construction point at the node of the equal length line and the equal width line in the construction plane area. After the first pile hole construction point is determined, pile driving construction is carried out. The pile driving depth is determined by the groundwater depth and stops when water emerges. S4. Pour a concrete column at the bottom of the pile hole to a height H1 below the frozen soil area. After pouring, insert steel bars along the middle of the concrete column, with the top of the steel bars extending above the concrete column to a height H2, where H2 < H1. S5, taking the node selected in step S3 as the center of the circle, selecting a second pile hole construction point, and performing piling construction after the second pile hole construction point is determined, with a piling depth of d1+H1; S6, after the piling is completed, the reaming drill bit is replaced to expand the bottom of the hole. After the bottom is expanded, a concrete column is poured along the first pile hole again, and a concrete column is poured along the second pile hole at the same time, with a pouring height of H1; S7, introducing a cold medium along the first pile hole and the second pile hole, the cold medium directly reaching the surface of the concrete column, and forming a frozen pier above the concrete column; S8. Stop the introduction of cold medium, carry out multi-point compaction in the construction plane area, level the surface of the construction plane area, and after leveling, use the crushed stone dynamic compaction replacement layering method to obtain the compacted layer to complete the foundation construction.

[0008] Preferably, in step S1, the frozen soil depth and groundwater depth are detected within the scope of foundation construction. Before detection, several detection points are planned with a spacing of 10-15m between detection points. The depth value of each detection point is counted to obtain the depth difference between two adjacent detection points. A bar graph is drawn using multiple depth differences, and multiple detection points where the changes tend to be gentle are selected.

[0009] Preferably, the specific construction plane area in step S2 includes multiple detection points in the flat area, and the area surrounded by the multiple detection points is used to draw a construction plane area with a length and width of L×K, and the construction plane area includes all the detection points.

[0010] Preferably, in step S3, the equal length line of the construction plane area extends along the length of the construction plane area, dividing the construction plane area into multiple areas of uniform width, and the width of the construction plane area divided by the equal length line is divided into 3-5 equal parts. The equal width line of the construction plane area extends along the length of the construction plane area, dividing the construction plane area into multiple areas of uniform length, and the length of the construction plane area divided by the equal width line is divided into 4-5 equal parts.

[0011] Preferably, in step S5, the second pile hole construction points are distributed along the outer circumference of the first pile hole construction point, with the first pile hole construction point as the center of the circle, and the second pile hole construction points are distributed at equal angles, and the interval angle between two adjacent second pile hole construction points is 30°-45°.

[0012] Preferably, the cold medium in step S7 is liquid nitrogen, and the cold medium is introduced and maintained at 15-25 cm above the concrete column for a duration of 20 minutes.

[0013] As a preferred solution of the present invention, it also includes laying earthwork cloth on the top of the compacted layer and inside the compacted layer.

[0014] Preferably, the following steps are included: S1, detect the frozen soil depth d1 and groundwater depth d2 in the frozen soil area; S2. Plan the plane area for foundation construction in the frozen soil area and obtain the length and width data L×K; S3. Select the first pile hole construction point at the node of the equal length line and the equal width line in the construction plane area. After the first pile hole construction point is determined, pile driving construction is carried out. The pile driving depth is determined by the groundwater depth and stops when water emerges. S4. Pour a concrete column at the bottom of the pile hole to a height H1 below the frozen soil area. After pouring, insert steel bars along the middle of the concrete column, with the top of the steel bars extending above the concrete column to a height H2, where H2 < H1. S5, taking the node selected in step S3 as the center of the circle, selecting a second pile hole construction point, and performing piling construction after the second pile hole construction point is determined, with a piling depth of d1+H1; S6, after the piling is completed, the reaming drill bit is replaced to expand the bottom of the hole. After the bottom is expanded, a concrete column is poured along the first pile hole again, and a concrete column is poured along the second pile hole at the same time, with a pouring height of H1; S7, introducing a cold medium along the first pile hole and the second pile hole, the cold medium directly reaching the surface of the concrete column, and forming a frozen pier above the concrete column; S8. Compact and level the frozen soil layer, and fill the cracks in the frozen soil layer caused by the pile hole driving process; S9. After compaction and leveling, fill the pit with sand and gravel and fully tamp it. After the pit is fully tamped and leveled, lay the earthwork cloth. S10. Spread fine soil on the gravel and tamp it fully. After the soil is fully tamped and leveled, lay the earthwork cloth again. S11. Repeat steps S9 and S10 2-3 times to obtain a compacted layer, and lay the last layer of earthwork cloth on the compacted layer to complete the foundation construction.

[0015] As a preferred solution of the present invention, it also includes installing steel sheet piles at the edge of the construction plane area. The steel sheet piles include several steel plates. The length of the steel plates is equal to the depth of the frozen soil layer. One side of the steel plates is coated with polyurethane waterproof coating, the other side is coated with thermal insulation coating, and the same side as the polyurethane waterproof coating is coated with corrosion-resistant coating.

[0016] Preferably, the following steps are included: S1, detect the frozen soil depth d1 and groundwater depth d2 in the frozen soil area; S2. Plan the plane area for foundation construction in the frozen soil area and obtain the length and width data L×K; S3. Select the first pile hole construction point at the node of the equal length line and the equal width line in the construction plane area. After the first pile hole construction point is determined, pile driving construction is carried out. The pile driving depth is determined by the groundwater depth and stops when water emerges. S4. Pour a concrete column at the bottom of the pile hole to a height H1 below the frozen soil area. After pouring, insert steel bars along the middle of the concrete column, with the top of the steel bars extending above the concrete column to a height H2, where H2 < H1. S5, taking the node selected in step S3 as the center of the circle, selecting a second pile hole construction point, and performing piling construction after the second pile hole construction point is determined, with a piling depth of d1+H1; S6, after the piling is completed, the reaming drill bit is replaced to expand the bottom of the hole. After the bottom is expanded, a concrete column is poured along the first pile hole again, and a concrete column is poured along the second pile hole at the same time, with a pouring height of H1; S7, introducing a cold medium along the first pile hole and the second pile hole, the cold medium directly reaching the surface of the concrete column, and forming a frozen pier above the concrete column; S8. Compact and level the frozen soil layer, and fill the cracks in the frozen soil layer caused by the pile hole driving process; S9. After compaction and leveling, fill the pit with sand and gravel and fully tamp it. After the pit is fully tamped and leveled, lay the earthwork cloth. S10. Spread fine soil on the gravel and tamp it fully. After the soil is fully tamped and leveled, lay the earthwork cloth again. S11, looping steps S9 and S10 2-3 times to obtain a compacted layer, and laying the last layer of earthwork cloth on the compacted layer; S12. Excavate a 10 cm wide pile trench 30 cm offset from the edge of the compacted layer; S13. Place a 10 mm thick steel plate inside the pile groove. Place the steel plate against the inner wall of the pile groove. Place several steel plates with their sides touching each other to form a complete construction plane area. S14. Backfill the pile trench to complete foundation construction.

[0017] In summary, the beneficial technical effects of the present invention are as follows: this method combines the crushed stone dynamic compaction and replacement layered construction method, and reduces the risk of frost heave from groundwater vapor based on the support of concrete layers and concrete columns; at the same time, based on the concrete and frozen soil that run through the cracks to form a reinforced frozen pier, during the crushed stone dynamic compaction and replacement construction method, earthwork cloth is continuously added, and the thermal insulation effect of the earthwork cloth is utilized to ensure that the temperature inside the frozen soil area is not lost, thereby enhancing the stability of the foundation support and improving the stability of the frozen soil area; at the same time, at the edge of the frozen soil area, a steel plate with the same height as the frozen soil area is installed, wherein the surface of the steel plate is compositely treated to improve the waterproof, thermal insulation and corrosion resistance of the steel plate, and in turn isolate the side of the frozen soil area from the external surface infiltration water, further improving the frost heave resistance of the frozen soil area inside the foundation, and thereby effectively improving the overall stability of the foundation in the frozen soil area. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification, but do not constitute a limitation of the present invention. In the accompanying drawings: Figure 1 This is a flow chart of a method for strengthening a foundation by layered reinforcement of crushed stone by dynamic compaction and replacement in permafrost areas according to this embodiment; Figure 2 This is a schematic cross-sectional view of a foundation construction method for layered reinforcement of a crushed stone compaction replacement foundation in permafrost areas according to this embodiment.

[0019] In the figure: 1. Compacted area; 2. First pile hole; 3. Second pile hole; 4. Steel plate. DETAILED DESCRIPTION

[0020] The present invention will be further described in detail below with reference to the accompanying drawings.

[0021] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. 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.

[0022] See also Figure 1-2 The present invention provides a technical solution: a method for strengthening the foundation by replacing the crushed stone with dynamic compaction in permafrost areas, comprising the following steps: S1, detect the frozen soil depth d1 and groundwater depth d2 in the frozen soil area; S2. Plan the plane area for foundation construction in the frozen soil area and obtain the length and width data L×K; S3. Select the first pile hole construction point at the node of the equal length line and the equal width line in the construction plane area. After the first pile hole construction point is determined, pile driving construction is carried out. The pile driving depth is determined by the groundwater depth and stops when water emerges. S4. Pour a concrete column at the bottom of the pile hole to a height H1 below the frozen soil area. After pouring, insert steel bars along the middle of the concrete column, with the top of the steel bars extending above the concrete column to a height H2, where H2 < H1. S5, taking the node selected in step S3 as the center of the circle, selecting a second pile hole construction point, and performing piling construction after the second pile hole construction point is determined, with a piling depth of d1+H1; S6, after the piling is completed, the reaming drill bit is replaced to expand the bottom of the hole. After the bottom is expanded, a concrete column is poured along the first pile hole again, and a concrete column is poured along the second pile hole at the same time, with a pouring height of H1; S7, introducing a cold medium along the first pile hole and the second pile hole, the cold medium directly reaching the surface of the concrete column, and forming a frozen pier above the concrete column; S8. Stop the introduction of cold medium, carry out multi-point compaction in the construction plane area, level the surface of the construction plane area, and after leveling, use the crushed stone dynamic compaction replacement layering method to obtain the compacted layer to complete the foundation construction.

[0023] Among them, in step S1, the frozen soil depth and groundwater depth are detected within the scope of foundation construction. Before detection, several detection points are planned with a spacing of 10m between detection points. The depth value of each detection point is counted to obtain the depth difference between two adjacent detection points. A bar graph is drawn using multiple depth differences, and multiple detection points where the changes tend to be flat are selected. The specific depth values are the values of the frozen soil depth and the groundwater depth. For the foundation construction area, the above method is used to select specific points of the foundation. After displaying on the bar graph, the detection points where the changes tend to be flat indicate that the frozen soil is uniform and the groundwater flow is stable, which is convenient for the reinforcement of the foundation after selection.

[0024] Specifically, the construction plane area includes all detection points in the flat area, and the area surrounded by all detection points inside the area is used to draw the construction plane area with a length and width of L×K.

[0025] In step S3, the equal-length line of the construction plane area extends along the length of the construction plane area, dividing the construction plane area into multiple areas of uniform width. The width of the construction plane area divided by the equal-length line is divided into three equal parts. The equal-width line of the construction plane area extends along the length of the construction plane area, dividing the construction plane area into multiple areas of uniform length. The equal-width line divides the length of the construction plane area into four equal parts.

[0026] In step S5, the second pile hole construction points are distributed along the outer circumference of the first pile hole construction point, with the first pile hole construction point as the center of the circle, and a number of second pile hole construction points are distributed at equal angles, and the interval angle between two adjacent second pile hole construction points is 45°. In this embodiment, the second pile hole construction points are selected according to the diameter of the first pile hole to ensure that the bottom of each second pile hole at an interval of 45° can reach the closest contact distance after being expanded. Then, after the first pile hole is reinforced with steel bars, it serves as the overall pile foundation support in the frozen soil area of the foundation. Combined with the concrete columns cast inside all the second pile holes and the concrete layer inside the expanded hole, a support layer can be formed at the bottom of the frozen soil area. At the same time, the concrete columns inside the second pile holes and the concrete columns inside the first pile holes can be covered together on the outside of the steel bars due to the penetration of the concrete layer to form a stable concrete support layer. At the same time, the concrete support layer can block the rising water vapor of groundwater to avoid frost heave in the frozen soil area.

[0027] Among them, due to the large rigidity of the permafrost layer, cracks exist between its inner wall and the bottom of the hole during the pile drilling process. Concrete is poured, and after pouring, a cold medium is used to seal it. In this embodiment, liquid nitrogen is used as the cold medium, and the cold medium is introduced and maintained 15 cm above the concrete column for 20 minutes. After the cold medium is introduced, a sealing plate is used 15 cm above the concrete column to avoid and keep the cold medium closed, so that the concrete and frozen soil running through the cracks form a reinforced frozen pier, thereby improving the support stability of the foundation.

[0028] In a specific implementation, it also includes laying earthwork cloth on the top of the compacted layer and inside the compacted layer.

[0029] The following steps are used for construction: S1, detect the frozen soil depth d1 and groundwater depth d2 in the frozen soil area; S2. Plan the plane area for foundation construction in the frozen soil area and obtain the length and width data L×K; S3. Select the first pile hole construction point at the node of the equal length line and the equal width line in the construction plane area. After the first pile hole construction point is determined, pile driving construction is carried out. The pile driving depth is determined by the groundwater depth and stops when water emerges. S4. Pour a concrete column at the bottom of the pile hole to a height H1 below the frozen soil area. After pouring, insert steel bars along the middle of the concrete column, with the top of the steel bars extending above the concrete column to a height H2, where H2 < H1. S5, taking the node selected in step S3 as the center of the circle, selecting a second pile hole construction point, and performing piling construction after the second pile hole construction point is determined, with a piling depth of d1+H1; S6, after the piling is completed, the reaming drill bit is replaced to expand the bottom of the hole. After the bottom is expanded, a concrete column is poured along the first pile hole again, and a concrete column is poured along the second pile hole at the same time, with a pouring height of H1; S7, introducing a cold medium along the first pile hole and the second pile hole, the cold medium directly reaching the surface of the concrete column, and forming a frozen pier above the concrete column; S8. Compact and level the frozen soil layer, and fill the cracks in the frozen soil layer caused by the pile hole driving process; S9. After compaction and leveling, fill the pit with sand and gravel and fully tamp it. After the pit is fully tamped and leveled, lay the earthwork cloth. S10. Spread fine soil on the gravel and tamp it fully. After the soil is fully tamped and leveled, lay the earthwork cloth again. S11. Repeat steps S9 and S10 2-3 times to obtain a compacted layer, and lay the last layer of earthwork cloth on the compacted layer to complete the foundation construction.

[0030] In the above embodiment, based on the support of the concrete layer and the concrete column, and based on the reinforced frozen pier formed by the concrete and frozen soil running through the cracks, earthwork cloth is continuously added during the gravel compaction replacement method, and the heat preservation effect of the earthwork cloth is utilized to ensure that the temperature inside the frozen soil area is not lost, thereby improving the stability of the frozen soil area while enhancing the stability of the foundation support.

[0031] Another embodiment also includes installing steel sheet piles at the edge of the construction plane area, the steel sheet piles include several steel plates, the length of the steel plates is equal to the depth of the permafrost layer, one side of the steel plates is coated with polyurethane waterproof coating, the other side is coated with thermal insulation coating, and the same side as the polyurethane waterproof coating is coated with corrosion-resistant coating.

[0032] Use the following steps to construct: S1, detect the frozen soil depth d1 and groundwater depth d2 in the frozen soil area; S2. Plan the plane area for foundation construction in the frozen soil area and obtain the length and width data L×K; S3. Select the first pile hole construction point at the node of the equal length line and the equal width line in the construction plane area. After the first pile hole construction point is determined, pile driving construction is carried out. The pile driving depth is determined by the groundwater depth and stops when water emerges. S4. Pour a concrete column at the bottom of the pile hole to a height H1 below the frozen soil area. After pouring, insert steel bars along the middle of the concrete column, with the top of the steel bars extending above the concrete column to a height H2, where H2 < H1. S5, taking the node selected in step S3 as the center of the circle, selecting a second pile hole construction point, and performing piling construction after the second pile hole construction point is determined, with a piling depth of d1+H1; S6, after the piling is completed, the reaming drill bit is replaced to expand the bottom of the hole. After the bottom is expanded, a concrete column is poured along the first pile hole again, and a concrete column is poured along the second pile hole at the same time, with a pouring height of H1; S7, introducing a cold medium along the first pile hole and the second pile hole, the cold medium directly reaching the surface of the concrete column, and forming a frozen pier above the concrete column; S8. Compact and level the frozen soil layer, and fill the cracks in the frozen soil layer caused by the pile hole driving process; S9. After compaction and leveling, fill the pit with sand and gravel and fully tamp it. After the pit is fully tamped and leveled, lay the earthwork cloth. S10. Spread fine soil on the gravel and tamp it fully. After the soil is fully tamped and leveled, lay the earthwork cloth again. S11, looping steps S9 and S10 2-3 times to obtain a compacted layer, and laying the last layer of earthwork cloth on the compacted layer; S12. Excavate a 10 cm wide pile trench 30 cm offset from the edge of the compacted layer; S13. Place a 10 mm thick steel plate inside the pile groove. Place the steel plate against the inner wall of the pile groove. Place several steel plates with their sides touching each other to form a complete construction plane area. S14. Backfill the pile trench to complete foundation construction.

[0033] In the above embodiment, based on the support of the concrete layer and the concrete column, and based on the reinforced frozen pier formed by the concrete and frozen soil running through the cracks, earthwork cloth is continuously added during the gravel compaction replacement method, and the heat preservation effect of the earthwork cloth is utilized to ensure that the temperature inside the frozen soil area is not lost, thereby enhancing the stability of the foundation support and improving the stability of the frozen soil area; at the same time, at the edge of the frozen soil area, a steel plate with the same height as the frozen soil area is installed, wherein the surface of the steel plate is compositely treated to improve the waterproof, heat-insulating and corrosion-resistant properties of the steel plate, and in turn isolate the side of the frozen soil area from the external surface seepage water, further improving the anti-frost heave performance of the frozen soil area inside the foundation, and thus effectively improving the overall stability of the foundation in the frozen soil area.

[0034] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus.

[0035] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for strengthening the foundation by layered reinforcement of crushed stone by dynamic compaction and replacement in permafrost areas, characterized in that: The following steps are involved: S1, detect the frozen soil depth d1 and groundwater depth d2 in the frozen soil area; S2. Plan the plane area for foundation construction in the frozen soil area and obtain the length and width data L×K; S3. Select the first pile hole construction point at the node of the equal length line and the equal width line in the construction plane area. After the first pile hole construction point is determined, pile driving construction is carried out. The pile driving depth is determined by the groundwater depth and stops when water emerges. S4. Pour a concrete column at the bottom of the pile hole to a height H1 below the frozen soil area. After pouring, insert steel bars along the middle of the concrete column, with the top of the steel bars extending above the concrete column to a height H2, where H2 < H1. S5, taking the node selected in step S3 as the center of the circle, selecting a second pile hole construction point, and performing piling construction after the second pile hole construction point is determined, with a piling depth of d1+H1; S6, after the piling is completed, the reaming drill bit is replaced to expand the bottom of the hole. After the bottom is expanded, a concrete column is poured along the first pile hole again, and a concrete column is poured along the second pile hole at the same time, with a pouring height of H1; S7, introducing a cold medium along the first pile hole and the second pile hole, the cold medium directly reaching the surface of the concrete column, and forming a frozen pier above the concrete column; S8. Stop the introduction of cold medium, carry out multi-point compaction in the construction plane area, level the surface of the construction plane area, and after leveling, use the crushed stone dynamic compaction replacement layering method to obtain the compacted layer to complete the foundation construction.

2. The method for strengthening the foundation by layered reinforcement of crushed stone by dynamic compaction and replacement in permafrost areas according to claim 1 is characterized in that: In the step S1, the depth of frozen soil and groundwater is detected within the scope of foundation construction. Before detection, a number of detection points are planned, and the detection point spacing is 10-15m. The depth value of each detection point is counted to obtain the depth difference between two adjacent detection points. A bar graph is drawn using the multiple depth differences, and multiple detection points where the changes tend to be flat are selected.

3. The method for strengthening the foundation by layered reinforcement of crushed stone by dynamic compaction and replacement in permafrost areas according to claim 2, characterized in that: The specific construction plane area in step S2 includes multiple detection points in the flat area. The area surrounded by the multiple detection points is used to draw a construction plane area with a length and width of L×K. The construction plane area includes all the detection points.

4. The method for strengthening the foundation by layered reinforcement of crushed stone by dynamic compaction and replacement in permafrost areas according to claim 1, characterized in that: In step S3, the equal length line of the construction plane area extends along the length of the construction plane area, dividing the construction plane area into multiple areas of uniform width, and the width of the construction plane area divided by the equal length line is divided into 3-5 equal parts. The equal width line of the construction plane area extends along the length of the construction plane area, dividing the construction plane area into multiple areas of uniform length, and the length of the construction plane area divided by the equal width line is divided into 4-5 equal parts.

5. The method for strengthening the foundation by layered reinforcement of crushed stone by dynamic compaction and replacement in permafrost areas according to claim 1, characterized in that: In step S5, the second pile hole construction points are distributed along the outer circumference of the first pile hole construction point, with the first pile hole construction point as the center of the circle, and the second pile hole construction points are distributed at equal angles, and the interval angle between two adjacent second pile hole construction points is 30°-45°.

6. The method for strengthening the foundation by layered reinforcement of crushed stone by dynamic compaction and replacement in permafrost areas according to claim 1, characterized in that: In step S7, the cold medium is liquid nitrogen, and the cold medium is introduced and maintained at 15-25 cm above the concrete column for 20 minutes.

7. The method for strengthening the foundation by layered reinforcement of crushed stone by dynamic compaction and replacement in permafrost areas according to claim 1, characterized in that: It also includes laying earthwork cloth on top of and within the compacted layer.

8. The method for strengthening the foundation by layered reinforcement of crushed stone by dynamic compaction and replacement in permafrost areas according to claim 7, characterized in that: The following steps are involved: S1, detect the frozen soil depth d1 and groundwater depth d2 in the frozen soil area; S2. Plan the plane area for foundation construction in the frozen soil area and obtain the length and width data L×K; S3. Select the first pile hole construction point at the node of the equal length line and the equal width line in the construction plane area. After the first pile hole construction point is determined, pile driving construction is carried out. The pile driving depth is determined by the groundwater depth and stops when water emerges. S4. Pour a concrete column at the bottom of the pile hole to a height H1 below the frozen soil area. After pouring, insert steel bars along the middle of the concrete column, with the top of the steel bars extending above the concrete column to a height H2, where H2 < H1. S5, taking the node selected in step S3 as the center of the circle, selecting a second pile hole construction point, and performing piling construction after the second pile hole construction point is determined, with a piling depth of d1+H1; S6, after the piling is completed, the reaming drill bit is replaced to expand the bottom of the hole. After the bottom is expanded, a concrete column is poured along the first pile hole again, and a concrete column is poured along the second pile hole at the same time, with a pouring height of H1; S7, introducing a cold medium along the first pile hole and the second pile hole, the cold medium directly reaching the surface of the concrete column, and forming a frozen pier above the concrete column; S8. Compact and level the frozen soil layer, and fill the cracks in the frozen soil layer caused by the pile hole driving process; S9. After compaction and leveling, fill the pit with sand and gravel and fully tamp it. After the pit is fully tamped and leveled, lay the earthwork cloth. S10. Spread fine soil on the gravel and tamp it fully. After the soil is fully tamped and leveled, lay the earthwork cloth again. S11. Repeat steps S9 and S10 2-3 times to obtain a compacted layer, and lay the last layer of earthwork cloth on the compacted layer to complete the foundation construction.

9. The method for strengthening the foundation by layered reinforcement of crushed stone by dynamic compaction and replacement in permafrost areas according to claim 1, characterized in that: It also includes installing steel sheet piles at the edge of the construction plane area. The steel sheet piles include several steel plates. The length of the steel plates is equal to the depth of the frozen soil layer. One side of the steel plates is coated with polyurethane waterproof coating, the other side is coated with thermal insulation coating, and the same side as the polyurethane waterproof coating is coated with corrosion-resistant coating.

10. The method for strengthening the foundation by layered reinforcement of crushed stone by dynamic compaction and replacement in permafrost areas according to claim 1, characterized in that: The following steps are involved: S1, detect the frozen soil depth d1 and groundwater depth d2 in the frozen soil area; S2. Plan the plane area for foundation construction in the frozen soil area and obtain the length and width data L×K; S3. Select the first pile hole construction point at the node of the equal length line and the equal width line in the construction plane area. After the first pile hole construction point is determined, pile driving construction is carried out. The pile driving depth is determined by the groundwater depth and stops when water emerges. S4. Pour a concrete column at the bottom of the pile hole to a height H1 below the frozen soil area. After pouring, insert steel bars along the middle of the concrete column, with the top of the steel bars extending above the concrete column to a height H2, where H2 < H1. S5, taking the node selected in step S3 as the center of the circle, selecting a second pile hole construction point, and performing piling construction after the second pile hole construction point is determined, with a piling depth of d1+H1; S6, after the piling is completed, the reaming drill bit is replaced to expand the bottom of the hole. After the bottom is expanded, a concrete column is poured along the first pile hole again, and a concrete column is poured along the second pile hole at the same time, with a pouring height of H1; S7, introducing a cold medium along the first pile hole and the second pile hole, the cold medium directly reaching the surface of the concrete column, and forming a frozen pier above the concrete column; S8. Compact and level the frozen soil layer, and fill the cracks in the frozen soil layer caused by the pile hole driving process; S9. After compaction and leveling, fill the pit with sand and gravel and fully tamp it. After the pit is fully tamped and leveled, lay the earthwork cloth. S10. Spread fine soil on the gravel and tamp it fully. After the soil is fully tamped and leveled, lay the earthwork cloth again. S11, looping steps S9 and S10 2-3 times to obtain a compacted layer, and laying the last layer of earthwork cloth on the compacted layer; S12. Excavate a 10 cm wide pile trench 30 cm offset from the edge of the compacted layer; S13. Place a 10 mm thick steel plate inside the pile groove. Place the steel plate against the inner wall of the pile groove. Place several steel plates with their sides touching each other to form a complete construction plane area. S14. Backfill the pile trench to complete foundation construction.

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