A method for grading filling of water-enriched stratum cave based on waste drilling mud

Abstract

The application relates to a water-enriched stratum cave grading filling method based on waste drilling mud, which comprises the following steps: drilling to make the obtained grouting hole penetrate through each layer of caves and extend to a preset depth below the bottom plate of the bottom layer of caves; implanting a steel casing into the grouting hole and making the bottom end of the steel casing be located on the rock layer; lowering a drill rod from the steel casing to the hole bottom of the grouting hole and connecting the drill rod to a grouting pipe; using the drill rod to fill filling materials into each layer of caves from bottom to top in a gravity static pressure mode until the filling materials return from the hole mouth of the grouting hole; taking out the drill rod, connecting the grouting pipe to the pipe mouth of the steel casing and filling the filling materials in a pressure filling mode by using a grouting pump until the filling pressure reaches a preset pressure or surface back slurry; and the filling materials comprise basic mud which is obtained by screening waste drilling mud. The cave treatment method can realize resource utilization of waste mud, adapt to a water-enriched environment and precisely control a filling range.

Description

Technical Field

[0001] This application relates to the field of karst cave treatment technology in geotechnical engineering, and in particular to a graded filling method for karst caves in water-rich strata based on waste borehole mud. Background Technology

[0002] In bridge pile foundation construction, the treatment of waste borehole mud and the filling of water-rich karst caves are two major technical challenges. Currently, the main methods for disposing of waste mud are off-site transportation and disposal or filter pressing into cakes. The former has the problems of high transportation costs and high risk of secondary pollution, while the latter has low sand separation efficiency and low water resource reuse rate, making it difficult to achieve resource utilization.

[0003] The conventional method for treating water-rich karst caves is to use sleeve valve pipe cement grouting combined with rubble backfilling, but it has the following drawbacks: (1) The grout is easily diluted by groundwater, which reduces the cementitious strength and the effective filling rate is less than 60%.

[0004] (2) Conventional cement-based grouts are prone to collapse under dynamic water conditions, while chemical grouts are costly and pose significant environmental risks.

[0005] (3) The grout spreads randomly along the cracks, making it difficult to accurately control the filling range and resulting in serious material waste.

[0006] (4) The existing drilling mud filling technology adopts the "top to bottom" grouting method, which has a poor effect on the treatment of interconnected karst caves, requires multiple hole cleanings, and the amount of grouting is uncontrollable. Summary of the Invention

[0007] This application provides a method for graded filling of karst caves in water-rich strata based on waste borehole mud, which can realize the resource utilization of waste mud, adapt to water-rich environments, and precisely control the filling range of karst cave treatment.

[0008] Firstly, a method for graded filling of karst caves in water-rich formations based on abandoned borehole mud is provided, comprising: Drill holes to ensure that the resulting grouting holes penetrate each layer of the karst cave and extend to a predetermined depth below the bottom slab of the lowest karst cave. A steel casing is inserted into the grouting hole, with the bottom end of the steel casing positioned in the rock layer; The drill rod is lowered from the steel casing to the bottom of the grouting hole and then connected to the grouting pipe. Using a drill rod, gravity static pressure is applied to inject filling material into each layer of the karst cave from bottom to top until the filling material returns from the opening of the grouting hole; Remove the drill rod, connect the grouting pipe to the opening of the steel casing, and use a grouting pump to inject the filling material using a pressure injection method until the injection pressure reaches the preset pressure or the grout returns to the ground surface. The filling material includes base mud, which is obtained by screening waste borehole mud.

[0009] In some embodiments, the base mud is the residue after screening out particles with a diameter >2mm from waste borehole mud, and the solid content of the base mud is 30-40%.

[0010] In some embodiments, the filler material further includes a curing agent slurry, wherein the mass ratio of the curing agent slurry to the base mud is 1:(3-5), and the initial viscosity of the filler material is 150-250 mPa. s.

[0011] In some embodiments, the curing agent slurry comprises, by weight, 40-50 parts of early-strength 42.5 ordinary Portland cement, 20-25 parts of slag, 5-8 parts of water glass, 10-15 parts of limestone and 5-10 parts of silica fume; The curing agent slurry is prepared by mixing 40-50 parts of early-strength 42.5 ordinary Portland cement, 20-25 parts of slag, 5-8 parts of water glass, 10-15 parts of limestone and 5-10 parts of silica fume evenly, and then stirring with water at a water-cement ratio of 0.5-0.6.

[0012] In some embodiments, the preset depth is 40–60 cm; And / or, when using gravity static pressure to inject filling material, the single injection height should be controlled between 3 and 5 m; And / or, when the filling material is injected by gravity static pressure, the outlet of the grouting pipe is buried 1 to 1.5 m below the liquid surface of the filling material; And / or, the preset pressure is 2 to 2.5 MPa; And / or, use a grouting pump to inject filling material using pressure grouting until the grouting pressure reaches the preset pressure or the grout returns to the surface, including: using a grouting pump for pressure grouting, with an initial pressure of 0.5 to 1 MPa, and increasing the pressure by 0.3 to 0.5 MPa every 5 to 10 minutes until the grouting pressure reaches the preset pressure or the grout returns to the surface.

[0013] In some embodiments, after the filling material is injected using a grouting pump via pressure grouting until the injection pressure reaches a preset pressure or grout returns to the surface, the method further includes: The soft plastic interlayers and fissures inside the karst cave were reinforced by targeted jet spraying.

[0014] In some embodiments, targeted reinforcement of soft plastic interlayers and fissures within karst caves is achieved through rotary jet grouting, including: The drill rod is inserted into the grouting hole and connected to the grouting pipe. A jet grouting machine is used to perform jet grouting at a pressure of 15-25 MPa and a lifting speed of 10-20 cm / min to specifically reinforce the soft plastic interlayer and fissure areas in each layer of the karst cave.

[0015] In some embodiments, after the filling material is injected using a grouting pump via pressure grouting until the injection pressure reaches a preset pressure or grout returns to the surface, the method further includes: Test holes were arranged around the perimeter of the pile foundation and between the grouting holes to collect core samples and confirm the filling effect.

[0016] In some embodiments, the number of detection holes is not less than 30% of the number of grouting holes, and at least one detection hole is arranged in each independent karst cave.

[0017] Secondly, a filling material for karst caves in water-rich strata is provided, the filling material comprising base mud and solidifying agent slurry, wherein the base mud is obtained by screening waste borehole mud; The mass ratio of the curing agent slurry to the base mud is 1:(3-5), and the initial viscosity of the filler material is 150-250 mPa. s; The curing agent slurry comprises, by weight, 40-50 parts of early-strength 42.5 ordinary Portland cement, 20-25 parts of slag, 5-8 parts of water glass, 10-15 parts of limestone and 5-10 parts of silica fume; The curing agent slurry is prepared by mixing 40-50 parts of early-strength 42.5 ordinary Portland cement, 20-25 parts of slag, 5-8 parts of water glass, 10-15 parts of limestone and 5-10 parts of silica fume evenly, and then stirring with water at a water-cement ratio of 0.5-0.6.

[0018] The beneficial effects of the technical solution provided in this application include: The filling method provided in this application utilizes the waste drilling mud generated during pile foundation construction to prepare specialized filling materials. It employs a "bottom-up graded gravity static pressure grouting + pressure grouting" technique, combined with static pressure venting and drainage, and pressure compaction filling. This prevents the filling material from being diluted or disintegrated by the karst cave water, achieving efficient filling of karst caves in water-rich strata. This application solves the problems of material waste, poor filling effect under water-rich conditions, and environmental pollution from waste mud in traditional karst cave treatment. It is particularly suitable for the precise filling of interconnected water-rich karst caves, offering significant economic and environmental benefits. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram of gravity static pressure injection of filling material provided in an embodiment of this application; Figure 2 This is a schematic diagram of the pressure injection method for injecting filling material according to an embodiment of this application; Figure 3 This is a schematic diagram of a rotary jet irrigation system provided in an embodiment of this application; Figure 4 This is a diagram showing the arrangement of detection holes provided in an embodiment of this application.

[0021] In the diagram: 1. Grouting hole; 2. Karst cave; 3. Steel casing; 4. Grouting pipe; 5. Drill rod; 6. Pile foundation perimeter; 7. Inspection hole; 10. Surface fill; 11. Sand layer; 12. Clay layer; 13. Rock layer; 14. Water accumulation in the karst cave; 15. Silt deposits in the karst cave; 16. Filling layer. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0023] See Figure 1 and Figure 2 As shown in the embodiments of this application, a graded filling method for karst caves in water-rich strata based on waste borehole mud is provided, which is particularly suitable for filling water-rich strata and interconnected karst caves in bridge pile foundation construction. The method includes the following steps: 101: Drill holes so that the resulting grouting holes 1 penetrate through each layer of cave 2 and extend to a predetermined depth below the bottom plate of the lowest cave 2.

[0024] See Figure 1 As shown, the original strata include the bottom rock layer 13, within which a cave 2 is formed. The cave 2 may have multiple layers distributed from bottom to top. Within the cave 2, cave deposits 15 (such as silt, gravel, etc.) and cave water 14 are naturally present.

[0025] Furthermore, the original strata may also include topsoil 10, sand layer 11, clay layer 12, etc., located above the rock layer 13.

[0026] Using a geological drilling rig, holes are drilled through drill rod 5 to form grouting holes 1, which penetrate through the karst caves 2 that need to be filled.

[0027] When drilling, the grouting hole 1 is made to penetrate to a preset depth below the bottom plate of the lowest karst cave 2. The purpose of introducing the preset depth is to ensure that the grouting depth of the karst cave has a certain redundancy to ensure the grouting effect. The preset depth can be set according to the actual grouting needs. For example, the preset depth is 40-60cm, and preferably, the preset depth is 50cm.

[0028] In addition, the aforementioned redundancy ensures that grouting can begin from the bottom, with the drill rod lowered to the bottom of the hole, and the grout injected to the bottom of the hole, while the water is squeezed out from the top of the hole. This also minimizes the possibility of the grout being diluted by water.

[0029] 102: Insert the steel sleeve 3 into the grouting hole 1, and place the bottom end of the steel sleeve 3 at the rock layer 13.

[0030] Because the rock layer 13 has high hardness, in order to facilitate construction, a steel sleeve 3 is inserted into the rock surface, with the bottom end of the steel sleeve 3 flush with the rock surface.

[0031] 103: Lower the drill rod 5 from the steel casing 3 to the bottom of the grouting hole 1, and connect the drill rod 5 to the grouting pipe 4.

[0032] The outer diameter of drill rod 5 is smaller than the inner diameter of steel casing 3, which creates a drainage and slurry return channel between drill rod 5 and steel casing 3, preparing for subsequent gravity static pressure.

[0033] 104: Using drill rod 5, gravity static pressure is used to inject filling material into each layer of karst cave 2 from bottom to top until the filling material returns from the opening of the grouting hole 1; the filling material includes basic mud, which is obtained by screening waste borehole mud.

[0034] See Figure 1 As shown, the filling material is poured into the grouting pipe 4 and flows along the drill rod 5 into the karst cave 2 under its own weight. The accumulated water 14 and gas in the karst cave 2 are discharged through the drainage and grout return channel. During the filling process, the drill rod 5 is lifted upwards, and the material is poured into each layer of the karst cave 2 from bottom to top, so that the accumulated water 14 and gas in each layer of the karst cave 2 are discharged through the drainage and grout return channel. The filling material forms a filling layer 16. Static pressure grouting ends when the filling material returns from the opening of the grouting hole 1.

[0035] The return of filling material from the grouting hole 1 refers to the continuous return of filling material. For example, the return of filling material may take longer than a certain period of time, which can be set according to the actual situation.

[0036] When using gravity static pressure to inject filling materials, the single injection height should be controlled between 3 and 5 meters.

[0037] When the filling material is injected using gravity static pressure, the outlet of the grouting pipe 4 is buried 1 to 1.5 m below the liquid surface of the filling material to prevent air from entering.

[0038] 105: Take out the drill rod 5, connect the grouting pipe 4 to the pipe opening of the steel casing 3, and use the grouting pump to inject the filling material by pressure injection until the injection pressure reaches the preset pressure or the grout returns to the ground surface.

[0039] See Figure 2 As shown, after static pressure grouting is completed, drill rod 5 can be removed, and the grouting pipe 4 and the steel casing 3 can be connected using an adapter ring to ensure a seal between them. Pressure grouting is then performed using a grouting pump. The pressure gauge is observed during the grouting process, and grouting continues until the grouting pressure reaches the preset pressure or grout returns to the surface. At this point, pressure grouting is stopped, and the filling is complete.

[0040] The adapter ring can be made using existing equipment.

[0041] The preset pressure can be set according to actual needs. For example, the preset pressure is 2 to 2.5 MPa.

[0042] During pressure grouting, pressure can be gradually increased until the pressure stabilizes. Specifically, a grouting pump is used for pressure grouting. The initial pressure is 0.5 to 1 MPa, and the pressure is increased by 0.3 to 0.5 MPa every 5 to 10 minutes until the grouting pressure reaches the preset pressure or the grout returns to the ground surface.

[0043] The filling method provided in this application utilizes the waste drilling mud generated during pile foundation construction to prepare specialized filling materials. It employs a "bottom-up graded gravity static pressure grouting + pressure grouting" technique, combined with static pressure venting and drainage, and pressure compaction filling. This prevents the filling material from being diluted or disintegrated by the karst cave water, achieving efficient filling of karst caves in water-rich strata. This application solves the problems of material waste, poor filling effect under water-rich conditions, and environmental pollution from waste mud in traditional karst cave treatment. It is particularly suitable for the precise filling of interconnected water-rich karst caves, offering significant economic and environmental benefits.

[0044] Furthermore, after injecting the filling material using a grouting pump via pressure grouting until the grouting pressure reaches the preset pressure or grout returns to the surface, the method further includes: The soft plastic interlayers and fissures within cavern 2 were reinforced by targeted jet grouting, specifically including: See Figure 3 As shown, the drill rod 5 is inserted into the grouting hole 1 and connected to the grouting pipe 4. A jet grouting machine is used to perform jet grouting at a pressure of 15-25 MPa and a lifting speed of 10-20 cm / min to specifically reinforce the soft plastic interlayer and fissure area in each layer of the karst cave 2.

[0045] This example combines static pressure venting and drainage, pressure compaction filling, and jet grouting targeted reinforcement techniques to achieve efficient filling of karst caves in water-rich strata.

[0046] Preferably, the nozzle diameter of the jet grouting drill is 2-4 mm, and the jet grouting fluid includes filler material and water, with a mass ratio of filler material to water of 1:(0.2-0.4), preferably 1:0.3. It is understood that if the filler material is too thick during jet grouting, it can be diluted with water to facilitate spraying.

[0047] Furthermore, after injecting the filling material using a grouting pump via pressure grouting until the grouting pressure reaches the preset pressure or grout returns to the surface, the method further includes: See Figure 4 As shown, inspection holes 7 are arranged on the outer perimeter 6 of the pile foundation and between the grouting holes 1 to take core samples to confirm the filling effect.

[0048] Specifically, judging the fill effect: 1) Core sampling from the inspection holes to determine whether the filling material in the karst cave has hardened; 2) Process the solidified soil sample and test its strength, which should be ≥1 MPa; 3) Observe the backflow of water during the core extraction process to determine the filling status of the karst cave by checking for leaks; 4) Whether the drill bit falls off during the core extraction process.

[0049] Geophysical exploration comparison and verification: The pile diameter range before and after grouting and filling was detected by micro-motion and other geophysical exploration methods. The changes in wave velocity in the karst cave area were compared and analyzed to determine the filling situation.

[0050] The number of detection holes 7 shall not be less than 30% of the number of grouting holes 1, and each independent karst cave 2 shall have at least one detection hole 7.

[0051] In this application, the base mud is the residue after screening out particles larger than 2mm from waste drilling mud, and the solid content of the base mud is 30-40%. The particle size is limited to within 2mm because the nozzle diameter of the equipment is 2-4mm, and particles larger than 2mm will clog the nozzle. Limiting the solid content is to control the density of the base mud; if the density is too low, the water content will be too high, which is not conducive to strength development, while if the density is too high, it will affect the mixing characteristics of the slurry.

[0052] The filler material also includes a curing agent slurry, wherein the mass ratio of the curing agent slurry to the base mud is 1:(3-5), and the initial viscosity of the filler material is 150-250 mPa. s.

[0053] Since the base slurry mainly contains soil and water, a curing agent slurry is added. After mixing with the curing agent slurry, it solidifies and hardens to form the required strength.

[0054] The initial viscosity is the viscosity of the base slurry and the hardener slurry after mixing and stirring. As the slurry solidifies and hardens over time, the viscosity gradually increases.

[0055] The initial viscosity is set in order to control the rheological properties of the slurry and ensure that it has suitable fluidity in the pipeline.

[0056] The curing agent slurry comprises, by weight, 40-50 parts of early-strength 42.5 ordinary Portland cement, 20-25 parts of slag, 5-8 parts of water glass, 10-15 parts of limestone and 5-10 parts of silica fume; The curing agent slurry is prepared by mixing 40-50 parts of early-strength 42.5 ordinary Portland cement, 20-25 parts of slag, 5-8 parts of water glass, 10-15 parts of limestone and 5-10 parts of silica fume evenly, and then stirring with water at a water-cement ratio of 0.5-0.6.

[0057] The filler material provided in this application has several advantages. First, after the curing agent slurry is added, the viscosity of the base slurry is higher than that of conventional cement slurry, resulting in better shear rheological properties. Second, the addition of active ingredients such as slag and silica fume increases the cohesive force within the filler material, making it easy to granulate and form granular clusters that are not easily dispersed. Third, the addition of water glass can increase the pH value of the filler material, increase the early reaction rate of the filler material, accelerate the setting of the filler material, and improve the water resistance of the filler material.

[0058] Example 1 The filling material comprises base mud and curing agent slurry, wherein the mass ratio of curing agent slurry to base mud is 1:3. The base mud is the residue remaining after screening out particles with a diameter >2mm from waste borehole mud.

[0059] The curing agent slurry comprises, by weight, 50 parts of early-strength 42.5 ordinary Portland cement, 25 parts of slag, 5 parts of water glass, 10 parts of limestone and 5 parts of silica fume; The curing agent slurry is prepared by mixing early-strength 42.5 ordinary silicate cement, slag, water glass, limestone and silica fume evenly, and then stirring with water at a water-cement ratio of 0.5.

[0060] Example 2 The only difference from Example 1 is that the filling material includes a base slurry and a curing agent slurry, with the mass ratio of the curing agent slurry to the base slurry being 1:4. By mass, the curing agent slurry comprises 45 parts of early-strength 42.5 ordinary Portland cement, 22.5 parts of slag, 6.5 parts of water glass, 15 parts of limestone, and 10 parts of silica fume, mixed with water at a water-cement ratio of 0.55.

[0061] Example 3 The only difference from Example 1 is that the filling material includes a base slurry and a curing agent slurry, with the mass ratio of the curing agent slurry to the base slurry being 1:4. The curing agent slurry comprises 40 parts of early-strength 42.5 ordinary Portland cement, 25 parts of slag, 8 parts of water glass, 15 parts of limestone, and 10 parts of silica fume, mixed with water at a water-cement ratio of 0.6.

[0062] Example 4 The only difference from Example 1 is that the filling material includes a base slurry and a curing agent slurry, with the mass ratio of the curing agent slurry to the base slurry being 1:5. The curing agent slurry comprises 50 parts of early-strength 42.5 ordinary Portland cement, 20 parts of slag, 8 parts of water glass, 15 parts of limestone, and 5 parts of silica fume, mixed with water at a water-cement ratio of 0.5.

[0063] Example 5 The only difference from Example 1 is that the filling material includes a base slurry and a curing agent slurry, with the mass ratio of the curing agent slurry to the base slurry being 1:4. The curing agent slurry comprises 50 parts of early-strength 42.5 ordinary Portland cement, 25 parts of slag, 8 parts of water glass, 10 parts of limestone, and 5 parts of silica fume, mixed with water at a water-cement ratio of 0.6.

[0064] Comparative Example 1: The only difference from Example 1 is that cement grout is used instead of curing agent grout. The filling material includes base mud and cement grout, and the mass ratio of cement grout to base mud is 1:3. The cement grout is prepared by mixing 100 parts of early-strength 42.5 ordinary Portland cement with water at a water-cement ratio of 0.5.

[0065] Comparative Example 2: The only difference from Example 1 is that no slag or silica fume is added.

[0066] Comparative Example 3: The only difference from Example 1 is that no water glass is added.

[0067] The examples and comparative examples were tested according to GB / T 43876-2024 "Determination of Viscosity of Cement Paste" and GB / T 8077-2012 "Test Method for Homogeneity of Concrete Admixtures". The test results are shown in the table below:

[0068] As can be seen from Examples 1 to 5 and Comparative Example 1, the filler material provided in this application has an initial viscosity of 184–217 mPa. The initial viscosity of cement slurry is relatively high, which results in good shear rheological properties. However, using cement slurry instead of curing agent slurry reduces its initial viscosity, thereby decreasing its shear rheological properties.

[0069] As can be seen from Example 1 and Comparative Example 2, without the addition of slag and silica fume, the initial viscosity of the filler material is greatly reduced. This indicates that the addition of active ingredients such as slag and silica fume makes the internal cohesion of the filler material strong, easy to granulate and form granular clusters, and not easy to disperse.

[0070] As can be seen from Example 1 and Comparative Example 3, the addition of water glass can increase the pH value of the filler material, increase the early reaction rate of the filler material, accelerate the coagulation of the filler material, and improve the water resistance of the filler material.

[0071] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for graded filling of karst caves in water-rich strata based on waste borehole mud, characterized in that, It includes: Drill holes so that the resulting grouting holes (1) penetrate through each layer of caverns (2) and extend to a predetermined depth below the bottom plate of the lowest cavern (2); Insert a steel sleeve (3) into the grouting hole (1) and place the bottom end of the steel sleeve (3) in the rock layer (13). The drill rod (5) is lowered from the steel casing (3) to the bottom of the grouting hole (1) and the drill rod (5) is connected to the grouting pipe (4); Using a drill rod (5), the filling material is injected into each layer of the karst cave (2) from bottom to top using a gravity static pressure method until the filling material returns from the opening of the grouting hole (1); Take out the drill rod (5), connect the grouting pipe (4) to the pipe opening of the steel casing (3), and use the grouting pump to inject the filling material by pressure injection until the injection pressure reaches the preset pressure or the surface grout returns. The filling material includes base mud, which is obtained by screening waste borehole mud.

2. The method for graded filling of karst caves in water-rich strata based on waste borehole mud as described in claim 1, characterized in that: The base mud is the residue after screening out particles with a diameter >2mm from waste drilling mud, and the solid content of the base mud is 30-40%.

3. The method for graded filling of karst caves in water-rich strata based on waste borehole mud as described in claim 1, characterized in that: The filler material also includes a curing agent slurry, wherein the mass ratio of the curing agent slurry to the base mud is 1:(3-5), and the initial viscosity of the filler material is 150-250 mPa. s.

4. The method for graded filling of karst caves in water-rich strata based on waste borehole mud as described in claim 3, characterized in that: The curing agent slurry comprises, by weight, 40-50 parts of early-strength 42.5 ordinary Portland cement, 20-25 parts of slag, 5-8 parts of water glass, 10-15 parts of limestone and 5-10 parts of silica fume; The curing agent slurry is prepared by mixing 40-50 parts of early-strength 42.5 ordinary Portland cement, 20-25 parts of slag, 5-8 parts of water glass, 10-15 parts of limestone and 5-10 parts of silica fume evenly, and then stirring with water at a water-cement ratio of 0.5-0.

6.

5. The method for graded filling of karst caves in water-rich strata based on waste borehole mud as described in claim 1, characterized in that: The preset depth is 40-60cm; And / or, when using gravity static pressure to inject filling material, the single injection height should be controlled between 3 and 5 m; And / or, when the filling material is injected by gravity static pressure, the outlet of the grouting pipe (4) is buried 1 to 1.5 m below the liquid surface of the filling material; And / or, the preset pressure is 2 to 2.5 MPa; And / or, use a grouting pump to inject filling material using pressure grouting until the grouting pressure reaches the preset pressure or the grout returns to the surface, including: using a grouting pump for pressure grouting, with an initial pressure of 0.5 to 1 MPa, and increasing the pressure by 0.3 to 0.5 MPa every 5 to 10 minutes until the grouting pressure reaches the preset pressure or the grout returns to the surface.

6. The method for graded filling of karst caves in water-rich strata based on waste borehole mud as described in claim 1, characterized in that, The method further includes injecting filling material using a grouting pump and pressure grouting until the injection pressure reaches the preset pressure or grout returns to the ground surface: The soft plastic interlayer and fissures inside the karst cave (2) are reinforced by targeted jet spraying.

7. The method for graded filling of karst caves in water-rich formations based on waste borehole mud as described in claim 6, characterized in that: The soft plastic interlayers and fissures inside the karst cave (2) are reinforced by targeted jet grouting, including: Insert the drill rod (5) into the grouting hole (1) and connect the grouting pipe (4). Use a jet grouting machine to perform jet grouting at a pressure of 15-25 MPa and a lifting speed of 10-20 cm / min to reinforce the soft plastic interlayer and fissure area in each layer of the karst cave (2).

8. The method for graded filling of karst caves in water-rich strata based on waste borehole mud as described in claim 1, characterized in that, The method further includes injecting filling material using a grouting pump and pressure grouting until the injection pressure reaches the preset pressure or grout returns to the ground surface: Test holes (7) are arranged on the outer perimeter (6) of the pile foundation and between the grouting holes (1) to take core samples to confirm the filling effect.

9. The method for graded filling of karst caves in water-rich strata based on waste borehole mud as described in claim 8, characterized in that: The number of the detection holes (7) shall not be less than 30% of the number of grouting holes (1), and each independent karst cave (2) shall have at least one detection hole (7).

10. A filling material for karst caves in water-rich strata, characterized in that: The filling material includes a base mud and a curing agent slurry, wherein the base mud is obtained by screening waste borehole mud. The mass ratio of the curing agent slurry to the base mud is 1:(3-5), and the initial viscosity of the filler material is 150-250 mPa. s; The curing agent slurry comprises, by weight, 40-50 parts of early-strength 42.5 ordinary Portland cement, 20-25 parts of slag, 5-8 parts of water glass, 10-15 parts of limestone and 5-10 parts of silica fume; The curing agent slurry is prepared by mixing 40-50 parts of early-strength 42.5 ordinary Portland cement, 20-25 parts of slag, 5-8 parts of water glass, 10-15 parts of limestone and 5-10 parts of silica fume evenly, and then stirring with water at a water-cement ratio of 0.5-0.6.

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