A pile end post-grouting reinforcement structure of an overlying interstice stratum foundation pile and a construction method thereof

Abstract

The present application provides a kind of pile end post grouting reinforcement structure and construction method of overlying interstice stratum pile, the structure includes inflatable tube, grouting pipe, lower concrete layer, medium fine sand filling layer, upper concrete layer and steel plate;The method includes: drilling, grouting pipe is lowered, blocking rock layer interface, inflatable tube is lowered, hole sealing, inflation pressure grouting, stop inflation and pressure grouting;And it includes simultaneous pressure grouting and inflation, and the inflation pressure is higher than the pressure grouting pressure.The present application is combined with the method of high pressure inflation of overlying interstice stratum and high pressure grouting of underlying bearing stratum, and bearing stratum can be grouted and reinforced directionally under the action of pressure field difference.It is found that the single pile bearing capacity is significantly improved after using the method of the present application for reinforcement, the static pressure displacement curve develops smoothly, and the characteristic value of the bearing capacity meets the design requirements.The method described in the present application is scientific and reasonable, the process is simple, and good treatment effect is achieved.

Description

Technical Field

[0001] This invention belongs to the field of building construction, specifically relating to a post-grouting reinforcement structure and construction method for piles in overlying interstitial strata. Background Technology

[0002] Drilled piles, as a simple construction method with good bearing capacity, are widely used in various engineering projects. However, due to the influence of geological conditions, geological survey data, construction conditions, and human factors, the quality of drilled piles inevitably suffers from various defects and problems. For example, the embedment depth of end-bearing piles may not meet design requirements, and the pile tip may not be effectively embedded into the bearing stratum to a certain depth, resulting in a reduction in the single pile's bearing capacity and rendering it unusable. Simultaneously, due to inherent defects in the drilling process, problems such as pile tip sediment and surrounding mud cake are often unavoidable. Furthermore, structural disturbance and stress release of the stratum at the pile tip during construction can reduce the bearing capacity of the bearing stratum, leading to a significant decrease in end resistance and lateral resistance. To address the issues of insufficient embedment depth of end-bearing piles and reduced bearing stratum strength, various methods and technologies have been proposed and applied, achieving certain engineering treatment effects. Among these, post-grouting technology, as a method to increase the pile tip area and improve the unit area resistance of the pile tip, can significantly improve the pile's bearing capacity. However, post-grouting technology has experienced various grouting failures in different projects, making it impossible to guarantee the effectiveness of post-grouting. In cases where the bearing stratum at the pile tip is overlying with interstitial strata, the grout will flow towards the overlying strata with developed interstitial strata during grouting, i.e., the grout flows upward, which prevents the bearing stratum from being effectively grouted and fails to achieve the expected goal of subsequent grouting.

[0003] Furthermore, it is difficult to determine whether grouting has reached full saturation and final pressure, resulting in poor effectiveness of traditional post-grouting reinforcement techniques. Specifically, traditional post-grouting reinforcement involves drilling holes at equal intervals around the pile to the pile tip depth and then embedding grouting pipes for high-pressure grouting. To ensure reinforcement effectiveness, secondary grouting is used during post-grouting. However, in both grouting processes, the grout volume continuously increases while the grouting pressure remains stable, indicating that the full grout volume and final pressure of post-grouting have not been achieved. Under high-pressure grouting conditions, the grout fails to effectively fill the bearing layer, instead filling the overlying voids in the pile tip. The grout undergoes laminar flow in the overlying voids within the site area, causing the grout volume to continuously increase while the grouting pressure does not. Therefore, it is evident that traditional high-pressure grouting cannot effectively reinforce the pile foundation soil.

[0004] Therefore, for situations where there are overlying strata with gaps above the bearing stratum, there is a need in this field for a new post-grouting reinforcement structure and construction method. Summary of the Invention

[0005] This invention first provides a post-grouting reinforcement structure for a pile end of a foundation pile in an overlying interstitial stratum. The reinforcement structure includes an air-filling pipe (2), a grouting pipe (3), and a lower concrete layer (4), a medium-fine sand filling layer (5), and an upper concrete layer (6) located within a reinforcement grouting hole (1) around the foundation pile (04). It also includes a steel plate (7) positioned above the reinforcement grouting hole (1). The opening of the grouting pipe (3) is located within the bearing stratum (01), and the grouting pipe (3) is used for grouting from outside the reinforcement grouting hole (1) into the bearing stratum (01). The opening of the air-filling pipe (2) is higher than the lower concrete layer (4). 4) The top surface is lower than the top surface of the overlying interstitial stratum (02), and the air-filling pipe (2) is used to fill the overlying interstitial stratum (02) from outside the reinforcement grouting hole (1). The lower concrete layer (4), the medium and fine sand filling layer (5) and the upper concrete layer (6) are filled and arranged in the reinforcement grouting hole (1) from bottom to top. The steel plate (7) covers the upper concrete layer (6) before the concrete initially sets, so the steel plate (7) is fixed together with the upper concrete layer (6). The steel plate (7) has through holes for the grouting pipe and the air-filling pipe to pass through. After the grouting pipe passes through the steel plate, it is fully welded to the steel plate.

[0006] In this invention, the medium and fine sand filling layer (5) is filled with medium sand and / or fine sand.

[0007] In one specific embodiment, the thickness of the upper concrete layer (6) is 20-60cm, the steel plate is a circular steel plate (7), and its diameter is 10-40cm larger than the orifice diameter of the reinforcement grouting hole (1).

[0008] In one specific embodiment, the distance h1 between the grouting pipe opening and the pile end plane of the foundation pile (04) is greater than or equal to 30cm; the distance h2 between the top surface of the lower concrete layer (4) and the interface between the bearing layer (01) and the overlying interstitial stratum (02) is 10-50cm, preferably 20-30cm; the height h3 between the air inlet of the air inlet and the top surface of the lower concrete layer (4) is 5-20cm, preferably 8-12cm; and the distance h4 between the air inlet of the air inlet and the top surface of the overlying interstitial stratum (02) is more than 30cm.

[0009] This invention also provides a method for post-grouting reinforcement of pile ends in overlying interstitial strata, the method comprising the following steps: Step A, drilling: drilling reinforcement grouting holes (1) around the pile (04) using a drilling machine, the number of holes being one or more; Step B, lowering the grouting pipe: lowering the grouting pipe (3) into the reinforcement grouting hole (1), the pipe opening should reach the bottom of the hole and be firmly fixed, and the pipe opening should be lower than the pile end plane of the pile (04); Step C, sealing the rock interface: injecting concrete into the reinforcement grouting hole (1) to seal the rock interface in the hole; Step D, lowering the air inlet pipe: lowering the air inlet pipe into the air inlet pipe. The pipe (2) is lowered into the reinforcement grouting hole (1) and the air-filled pipe (2) is fixed to prevent floating. The opening of the air-filled pipe is higher than the top surface of the concrete poured in step C but lower than the top surface of the overlying interstitial stratum (02). Step E, sealing the hole: the reinforcement grouting hole (1) is sealed with filler. Step F, air-filled grouting: grouting and air-filling are carried out simultaneously, and the air-filling pressure is higher than the grouting pressure. The air-filling is the filling of high-pressure air into the overlying interstitial stratum (02), and the grouting is the grouting into the bearing layer (01). Step G, stopping air-filling and grouting: air-filling and grouting are stopped when the full grouting volume and final pressure are reached.

[0010] In this invention, the foundation pile is, for example, a bored cast-in-place pile, specifically, an end-bearing pile or a friction end-bearing pile.

[0011] In one specific implementation, in step A, the bottom of the reinforced grouting hole (1) reaches the bearing layer (01); and step A also includes a hole cleaning step, that is, using a grouting machine at low to high speed to pump clean water into the grouting pipe to discharge the rock debris, residue and muddy turbid water deposited in the reinforced grouting hole (1) from the outer wall of the pipe.

[0012] In one specific implementation, the distance h1 between the opening of the grouting pipe and the end plane of the pile (04) is greater than or equal to 30cm.

[0013] In one specific embodiment, the concrete poured is early-strength concrete, which forms a lower concrete layer (4) in the hole, and the distance h2 between the top surface of the poured lower concrete layer (4) and the interface between the bearing layer (01) and the overlying interstitial stratum (02) is 10 to 50 cm, preferably 20 to 30 cm.

[0014] In one specific embodiment, the height h3 of the inlet of the air-filled pipe above the top surface of the concrete poured in step C is 5 to 20 cm, preferably 8 to 12 cm; and the distance h4 of the inlet of the air-filled pipe below the top surface of the overlying interstitial stratum (02) is more than 30 cm; between step C and step D, there is also a grouting pipe pressure test step, and the pressure of the pressure test is 2 to 5 MPa.

[0015] In one specific embodiment, the filler includes medium-fine sand and concrete; and step E includes first filling the reinforced grouting hole (1) with medium-fine sand to form a medium-fine sand filling layer (5) in the hole, and then pouring concrete below the hole opening to form an upper concrete layer (6). The thickness of the upper concrete layer (6) is 20-60cm, and before the concrete initially sets, a steel plate (7) is covered at the hole opening to fix it together with the lower concrete layer (6). The steel plate (7) has a through hole for the grouting pipe and the air filling pipe to pass through, and the two are fully welded after the grouting pipe passes through the steel plate. Preferably, the steel plate is a circular steel plate (7), and its diameter is 10-40cm larger than the diameter of the hole opening.

[0016] In one specific implementation, the process includes first grouting for a period of time, preferably 1 to 5 minutes, and then injecting high-pressure air downwards from the air inlet pipe at a pressure greater than the grouting pressure by 0.05 to 0.2 MPa, and the grouting pressure is not less than 1.2 MPa.

[0017] In one specific implementation, an air-filling grouting step is carried out simultaneously in multiple reinforcement grouting holes (1) around the foundation pile, and preferably a process combining one-time injection and re-injection is used for grouting.

[0018] In one specific implementation, when the grouting pressure changes from low to high as grouting progresses and the air pressure remains stable or does not increase, the grouting qualification status is determined by comparing and verifying the grouting volume calculated in the design with the actual grouting volume.

[0019] In one specific implementation, the estimation formula for the grouting volume is as follows:

[0020]

[0021] Where: G - grouting volume (m³) 3 R - Grout diffusion radius (m); L - Grouting length from pile bottom to bottom of interstitial stratum (m); n - Porosity of soil around pile bottom; k - Absolute value of permeability coefficient of grouting soil layer around pile bottom (dimensionless); c - Grout filling coefficient, soft clay 0.3~0.4, fine sand 0.4~0.5, medium and coarse sand 0.5~0.7, gravel 0.7~1.0.

[0022] In this invention, after the reinforcement construction is completed, the steel plate (7) and the like are generally discarded, while the grouting pipe (3) and the air inlet pipe (2) are generally filled with grout before being discarded.

[0023] In this invention, the overlying interstitial strata (02) generally include pebble layers, coarse sand layers, and other types of rock layers. As long as the overall porosity of the strata is large, it can be called the overlying interstitial strata (02).

[0024] This invention offers at least the following advantages: By combining high-pressure air filling of the overlying interstitial strata with high-pressure grouting of the underlying bearing stratum, the bearing stratum can be directionally reinforced through grouting under the influence of a pressure field difference. Static load tests on pile foundations before and after reinforcement using the method described in this invention revealed that the pile foundations before treatment with the structure and method of this invention experienced rapid displacement development before reaching the design maximum ultimate bearing capacity, failing to meet design requirements. However, after reinforcement using the structure and method of this invention, the single pile bearing capacity significantly improved, the static pressure displacement curve developed smoothly, and the bearing capacity characteristic value met the design requirements. Engineering practice demonstrates that the method described in this invention is scientifically sound, simple in procedure, and achieves excellent treatment results. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the structure and method described in this invention.

[0026] Figure 2 The image shows the PS curve before grouting at the pile tip of the cast-in-place pile.

[0027] Figure 3 The image shows the PS curve after grouting at the pile tip of the cast-in-place pile.

[0028] Figure 4 This is a schematic diagram of the construction method described in this invention.

[0029] In the diagram: bearing layer 01, overlying interstitial stratum 02, fine sand layer 031, silty clay layer 032, saturated silt layer 033, silt layer 034, miscellaneous fill layer 035, ground surface 036, foundation pile 04, grouting hole 1 for reinforcement 2, air inlet pipe 2, grouting pipe 3, lower concrete layer 4, medium and fine sand filling layer 5, upper concrete layer 6, steel plate 7. Detailed Implementation

[0030] The post-grouting reinforcement construction method for pile ends in overlying void strata as described in this invention specifically includes the following steps:

[0031] 1) Drilling (or cleaning): Drill holes at equal intervals around the pile head in sequence using a drilling machine until the bottom of the hole is reached (determined based on the actual pile length on site). The bottom of the hole should reach the bearing layer of the foundation pile. If the bottom of the grouting hole does not reach the bearing layer, drilling must continue to the required depth. For holes where grouting was not successful in the early stage, clean the hole. Specifically, use a grouting machine at low to high speed to pump clean water into the pipe to remove rock cuttings, residues, muddy and turbid water deposited in the hole from the outer wall of the pipe.

[0032] Furthermore, the number of boreholes and the layout of grouting pipes and air-filling pipes include: the number of grouting pipes and air-filling pipes at the pile end should be set according to the pile diameter. For piles with a diameter not exceeding 800 mm, two pipes should be symmetrically set around the pile; for piles with a diameter greater than 800 mm but not exceeding 1600 mm, three pipes should be evenly set around the pile; for piles with a diameter greater than 1600 mm but not exceeding 2500 mm, four pipes should be evenly set around the pile; for piles with a diameter greater than 2500 mm, the number of grouting pipes and air-filling pipes should be increased and evenly arranged around the pile.

[0033] 2) Lowering the grouting pipe: Lower the high-pressure grouting pipe into the hole, ensuring its opening reaches the bottom of the hole and is securely fixed to prevent the pipe from floating during subsequent operations and grouting. The grouting pipe opening should penetrate the pile end plane and be at least 30cm below it. The grouting pipe can be connected using threaded connections or by welding with an external short sleeve. The connection should be tight and should not weld through the steel pipe or leave any gaps. The grouting pipe head can be fabricated using a one-way valve method.

[0034] 3) Sealing the rock strata interface: Inject early-strength concrete into the borehole so that the injection surface is about 20-30cm higher than the interface between the bearing stratum and the overlying interstitial stratum.

[0035] After sealing the rock interface and before lowering the air-filled pipe, a water pressure test is performed on the grouting pipe to prevent concrete from blocking the grouting pipe head. The relevant parameters of the grouting design can also be adjusted based on the results of the water pressure test. Specifically, the pressure of the water pressure test is generally 2–5 MPa, and the test continues until the grouting valve is opened. Intermittent water pressure tests are also performed afterward to prevent the successfully opened grouting holes from becoming blocked again.

[0036] 4) Lowering the inflation hose: Lower the high-pressure resistant inflation hose into the hole. The hose opening should be approximately 10cm above the top surface of the sealing concrete, and the opening should penetrate at least 30cm below the top surface of the overlying interstitial stratum. Secure the hose to prevent floating. The inflation hose head can be fabricated using a perforation and wrapping method. Both the grouting pipe and the inflation hose must be strictly sealed during installation to prevent grout and other debris from entering.

[0037] 5) Sealing the Hole: First, sprinkle medium-fine sand into the hole until it is about 40cm from the borehole opening, then pour concrete. Seal the hole opening with a circular steel plate, the diameter of which is 20cm larger than the opening. Pre-drill holes in the corresponding parts of the steel plate to pass through the grouting pipe and air inlet pipe, respectively. Fully weld the circular steel plate to the grouting pipe to seal the hole opening. The steel plate is placed before the concrete initially sets, so that it bonds to the concrete after hardening. The steel plate will not affect the grouting data, but it improves the overall airtightness of the reinforced structure.

[0038] 6) Air-filled grouting: Start and prepare relevant equipment and grouting fluid. Open the grouting valve and inject grout into the hole for about 2 minutes. Start the air inflator. The grouting pressure should not be less than 1.2 MPa. Inject high-pressure air into the hole at a pressure approximately 100 kPa higher than the grouting pressure. During construction, reinforce the grouting holes around the pile simultaneously. A combination of one-time injection and re-injection can be used for grouting. The grouting material should preferably be low-alkali silicate cement or low-alkali ordinary silicate cement with a strength grade of not less than 42.5. The water-cement ratio of the grout should be 0.5–0.8.

[0039] 7) Achieving full grout volume and final pressure: When the grouting pressure shows a pattern of increasing from low to high and the air pressure remains stable (especially without increasing), the grouting qualification status is determined by comparing the calculated grouting volume with the actual grouting volume. After the grouting is qualified, the grout is cured, and after passing the test, it is accepted.

[0040] The formula for estimating the grouting volume in the design calculation is as follows:

[0041]

[0042] Where: G - grouting volume (m³) 3 R - Grout diffusion radius (m); L - Grouting length from pile bottom to bottom of interstitial stratum (m); n - Porosity of soil around pile bottom; k - Absolute value of permeability coefficient of grouting soil layer around pile bottom (dimensionless); c - Grout filling coefficient, soft clay 0.3~0.4, fine sand 0.4~0.5, medium and coarse sand 0.5~0.7, gravel 0.7~1.0.

[0043] The following is a comparative analysis of the compressive bearing capacity results of cast-in-place concrete piles before and after reinforcement using the method described in this invention.

[0044] Specifically, in one embodiment of the present invention: a reconstruction project in Chengguan District, Lanzhou City, Gansu Province, uses cast-in-place concrete piles with diameters of 500mm and 800mm, without enlarged bases. The bearing stratum at the pile tip is a strongly weathered sandstone layer, penetrating at least 2m into the bearing stratum. During the construction of the cast-in-place concrete piles, the depth of the pile tip into the bearing stratum was too shallow, and the vertical compressive bearing capacity did not meet the design requirements. Therefore, it is necessary to reinforce the cast-in-place concrete piles by post-grouting at the pile tip to improve their compressive bearing capacity.

[0045] The bearing stratum 01 of this project is a strongly weathered sandstone layer, overlain by a gravel layer 02 with interstitial inclusions. For details of its stratigraphic structure, see [link to stratigraphic description]. Figure 1Above the pebble layer are, in sequence, a fine sand layer (031), a silty clay layer (032), a saturated silt layer (033), a silt layer (034), and a miscellaneous fill layer (035). Since the geological survey data indicated that the exploration points were not within the pile foundation construction area, borehole surveys were conducted in the surrounding geology. It was found that the depth of the pile tip embedded in the bearing stratum in the soil samples did not meet the design requirements. This project initially adopted the traditional post-grouting technique, which involves drilling holes at equal intervals around the pile to the pile tip depth and then burying grouting pipes for high-pressure grouting. To ensure the reinforcement effect, secondary grouting was used during the post-grouting process. However, during both grouting processes, the grout volume continuously increased while the grouting pressure remained stable, without any pressure rise, indicating that the full grout volume and final pressure of the post-grouting were not achieved. Field investigation revealed that under high-pressure grouting conditions, the grout failed to effectively fill the strongly weathered sandstone bearing stratum, instead filling the pebble stratum above the pile tip. Therefore, the high-pressure air-filled post-grouting technique described in this invention was adopted to solve this problem.

[0046] In accordance with current specifications, static load tests were conducted on the vertical compressive strength of individual piles before and after grouting reinforcement for the engineering piles of this project. The test method adopted was the slow sustained load method, with a maximum expected load of 3200kN. The load was divided into 10 levels, with each level being applied with equal magnitude.

[0047] Before using the method described in this invention, as shown in Table 1 and Figure 2 As shown, when pile No. 1 (pile diameter 800mm, pile length 10.92m) is loaded to a maximum of 1920kN, the load-settlement ps curve at the pile top shows a clear inflection point, with a steep drop. The pile top settlement (33.46mm) is more than 5 times the settlement under the previous load (6.04mm), and the total pile top settlement is 57.79mm. For pile No. 2 (pile diameter 800mm, pile length 10.85m), with a maximum load of 2240kN, the load-settlement ps curve at the pile top also shows a clear inflection point, with a steep drop. For pile segment 1, the settlement at the top of the pile (35.13 mm) is more than five times the settlement under the previous load (3.11 mm), and the total settlement at the top of the pile is 46.84 mm. For pile segment 3 (800 mm diameter, 11.65 m length), the load-settlement ps curve at the top of the pile shows a clear inflection point and a steep drop. The settlement at the top of the pile (33.44 mm) is more than five times the settlement under the previous load (3.64 mm), and the maximum load is 2560 kN, with a total settlement at the top of the pile of 48.92 mm. The settlement at the top of all three piles is more than five times the settlement under the previous load, and the total settlement at the top of the piles exceeds 40 mm, thus meeting the conditions for terminating the load. Therefore, under the conditions of natural water content and construction parameters, the characteristic values ​​of the single pile bearing capacity of the three engineering piles can be taken as 800kN, 960kN and 1120kN respectively, none of which meet the design requirement of 1600kN.

[0048] After using the method described in this invention, as shown in Table 1 and Figure 3As shown, under a maximum load of 3200kN, the ps curve of pile No. 4 (800mm diameter, 10.92m length) exhibits a gradual decrease without any obvious inflection point or steep drop. The loading test results for piles No. 5 (800mm diameter, 11.06m length) and No. 6 (800mm diameter, 11.59m length) are similar. The maximum load on all three piles is 3200kN, which is twice the design bearing capacity characteristic value. The characteristic value of the vertical compressive bearing capacity of the three piles after post-grouting reinforcement can be taken as half of the ultimate bearing capacity, i.e., 1600kN, meeting the design requirements and achieving the expected reinforcement effect.

[0049] Table 1 below shows the statistics of the maximum load and cumulative displacement of a single pile before and after post-grouting.

[0050] Table 1

[0051]

[0052] In this invention, to prevent the grout from flowing upwards into the overlying interstitial strata during high-pressure grouting, high-pressure air with a pressure greater than the grouting pressure is continuously injected above the grouting point through an air-filling pipe while high-pressure grouting is being performed through the grouting pipe. The high-pressure air enters the overlying interstitial strata (pebble layer), and because the strata above and below the interstitial strata (pebble layer) are relatively dense, the high-pressure air moves within the pebble layer, creating a pressure difference with the grouting pressure below it. This makes it difficult for the grout to flow upwards, instead directing it towards the reinforced bearing layer (strongly weathered sandstone layer), achieving the construction goal of reinforcing the soil at the pile tip. Specifically, during construction, air is typically injected and grout is injected simultaneously into each reinforcement grouting hole around the reinforced pile, forming a ring-shaped air pressure field, thereby improving the grouting effect.

[0053] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions and substitutions can be made without departing from the inventive concept, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A post-grouting reinforcement structure for pile ends in overlying interstitial strata, characterized in that, The reinforcement structure includes an air-filled pipe (2), a grouting pipe (3) located in the reinforcement grouting hole (1) around the foundation pile (04), a lower concrete layer (4), a medium-fine sand filling layer (5), and an upper concrete layer (6). It also includes a steel plate (7) positioned above the reinforcement grouting hole (1). The opening of the grouting pipe (3) is located in the bearing layer (01), and the grouting pipe (3) is used for grouting from outside the reinforcement grouting hole (1) into the bearing layer (01). The overlying interstitial stratum (02) is located above the bearing layer (01), and the opening of the air-filled pipe (2) is higher than that of the lower concrete layer (4). The top surface is lower than the top surface of the overlying interstitial stratum (02), and the air-filling pipe (2) is used to fill the overlying interstitial stratum (02) from outside the reinforcement grouting hole (1). The lower concrete layer (4), the medium and fine sand filling layer (5) and the upper concrete layer (6) are sequentially filled in the reinforcement grouting hole (1) from bottom to top. The steel plate (7) covers the upper concrete layer (6) before the concrete initially sets, so the steel plate (7) is fixed together with the upper concrete layer (6). The steel plate (7) has through holes for the grouting pipe and the air-filling pipe to pass through, and the grouting pipe is fully welded to the steel plate after passing through the steel plate.

2. The reinforced structure according to claim 1, characterized in that, The thickness of the upper concrete layer (6) is 20~60cm, and the steel plate is a circular steel plate (7) with a diameter 10~40cm larger than the diameter of the grouting hole (1).

3. The reinforced structure according to claim 1, characterized in that, The distance h1 between the grouting pipe opening and the pile end plane of the foundation pile (04) is greater than or equal to 30cm; the distance h2 between the top surface of the lower concrete layer (4) and the interface between the bearing layer (01) and the overlying interstitial stratum (02) is 20~30cm; the height h3 between the air inlet and the top surface of the lower concrete layer (4) is 8~12cm; and the distance h4 between the air inlet and the top surface of the overlying interstitial stratum (02) is more than 30cm.

4. A method for post-grouting reinforcement of pile ends in overlying voidy strata, characterized in that, The method includes the following steps: Step A, Drilling: Drill reinforcement grouting holes (1) around the foundation pile (04) using a drilling machine. The number of holes can be one or more. Step B, lowering the grouting pipe: lower the grouting pipe (3) into the reinforced grouting hole (1). The opening of the grouting pipe should reach the bottom of the hole and be fixed firmly. The opening of the grouting pipe should be lower than the pile end plane of the foundation pile (04). Step C, sealing the rock strata interface: pour concrete into the reinforced grouting hole (1) to seal the rock strata interface inside the hole; Step D, lowering the air inflator: lower the air inflator (2) into the reinforced grouting hole (1) and fix the air inflator (2) to prevent it from floating. The inlet of the air inflator is higher than the top surface of the concrete poured in step C and lower than the top surface of the overlying interstitial stratum (02). Step E, sealing the hole: Use filler to seal the reinforced grouting hole (1); Step F, air-filling grouting: includes grouting and air filling at the same time, with the air filling pressure being higher than the grouting pressure. The air filling is the filling of high-pressure air into the overlying interstitial stratum (02), and the grouting is the grouting into the bearing stratum (01). Step G, Stop air inflation and grouting: Stop air inflation and grouting when the full grouting volume and final pressure are reached.

5. The method according to claim 4, characterized in that, In step A, the bottom of the reinforced grouting hole (1) reaches the bearing layer (01); and step A also includes a hole cleaning step, that is, using a grouting machine at low to high speed to pump clean water into the grouting pipe to discharge the rock debris, residue and muddy turbid water deposited in the reinforced grouting hole (1) from the outer wall of the pipe; in step B, the distance h1 between the pipe opening of the grouting pipe and the pile end plane of the foundation pile (04) is greater than or equal to 30cm; in step C, the concrete poured is early-strength concrete, so that a lower concrete layer (4) is formed in the hole, and the distance h2 between the top surface of the poured lower concrete layer (4) and the interface between the bearing layer (01) and the overlying interstitial stratum (02) is 20~30cm.

6. The method according to claim 4, characterized in that, In step D, the height h3 of the air inlet above the top surface of the concrete poured in step C is 8~12cm; and the distance h4 of the air inlet below the top surface of the overlying interstitial stratum (02) is more than 30cm; between step C and step D, there is also a grouting pipe pressure test step, and the pressure of the pressure test is 2~5MPa.

7. The method according to claim 4, characterized in that, In step E, the filler includes medium and fine sand and concrete; and step E includes first filling the reinforced grouting hole (1) with medium and fine sand to form a medium and fine sand filling layer (5) in the hole, and then pouring concrete below the hole opening to form an upper concrete layer (6). The thickness of the upper concrete layer (6) is 20~60cm, and before the concrete initially sets, a steel plate (7) is covered at the hole opening so that the steel plate (7) is fixed together with the lower concrete layer (6). The steel plate (7) has a through hole for the grouting pipe and the air filling pipe to pass through, and the two are fully welded after the grouting pipe passes through the steel plate. The steel plate is a circular steel plate (7), and its diameter is 10~40cm larger than the diameter of the hole opening.

8. The method according to claim 4, characterized in that, Step F includes grouting for 1 to 5 minutes, followed by injecting high-pressure air from the air-filling pipe with a pressure greater than the grouting pressure by 0.05 to 0.2 MPa, and the grouting pressure is not less than 1.2 MPa; in step F, the air-filling and grouting steps are carried out simultaneously in multiple reinforcement grouting holes (1) around the foundation pile, and the grouting is carried out using a combination of one-time injection and re-injection.

9. The method according to claim 4, characterized in that, In step G, when the grouting pressure changes from low to high as grouting progresses and the air pressure remains stable or does not increase, the grouting qualification status is determined by comparing the design calculation grouting volume with the actual grouting volume.

10. The method according to claim 9, characterized in that, The formula for estimating the grouting volume in the design calculation is as follows: Where: G - grouting volume (m³); R - grout diffusion radius (m); L - grouting length from pile bottom to the bottom of the interstitial stratum (m); n - porosity of the soil around the pile bottom; k - absolute value of the permeability coefficient of the grouting soil layer around the pile bottom; c - grout filling coefficient, the values ​​of which are as follows: soft clay 0.3~0.4, fine sand 0.4~0.5, medium-coarse sand 0.5~0.7, gravel 0.7~1.0.

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