Enclosed low-pressure consolidation grouting method

Abstract

This invention proposes a confined low-pressure consolidation grouting method, which includes forming a plurality of confined piles around at least one foundation pile using a high-pressure single-pipe grouting method. These confined piles surround the at least one foundation pile and are interconnected, thereby forming a confined zone between the at least one foundation pile and the confined piles. A cement grout is then injected into the confined zone using a low-pressure permeation grouting method until the soil is fully saturated. This method confines and consolidates the friction pile from the ground surface to the effective friction depth, transforming the soft plastic soil into a harder, brittle soil. This results in high-strength shear force and prevents negative surface friction from occurring under stress, thus increasing the bearing capacity of the friction pile. Furthermore, it eliminates the need for design changes such as thickening or lengthening the pile or altering the pile foundation location.

Description

[Technical Field]

[0001] This invention relates to a technology for the application of geological improvement methods in geotechnical engineering, and in particular to a confined low-pressure consolidation grouting method for increasing the bearing capacity of friction piles in soft soil. [Previous Technology]

[0002] Generally speaking, in riverine areas, near-shore areas, or mudstone deposits, the underlying soil layers are loose sand, soft clay, and sediment, which are considered relatively soft soil. In civil engineering, when the depth of a foundation pile does not reach the bearing layer required for a standard penetration test (SPT) of N≥50, the pile is called a friction pile. The depth at which the pile exerts its gripping force with the soil is called the effective friction depth of the pile, which is 15 to 20 times the pile diameter. If the friction pile is too deep, the gripping force cannot be fully exerted at the entire pile depth. Therefore, when friction piles are installed in the aforementioned relatively soft soil, the friction pile will sink and displace downwards under load, causing the soft soil next to the friction pile to sink. The soft soil generates a downward adhesion force on the friction pile (this adhesion force is called negative skin friction force), which will offset the upward positive skin friction force of the harder soil layer below and the friction pile moving in the opposite direction (providing the bearing capacity of the friction pile), resulting in insufficient bearing capacity of the friction pile, and consequently causing the friction pile to fail the pile load test. Therefore, when foundation pile engineering must be carried out on soft ground, the designed friction piles often fail the pile load test because the bedrock is very deep, which is a headache for the engineers.

[0003] Generally speaking, engineers may solve the problem of pile failure in load tests by changing the design of the piles, such as increasing the pile diameter or length, or even changing the pile location (i.e., relocation); however, some projects have budget constraints, schedule pressures, or even land restrictions, and cannot make arbitrary changes; therefore, a new solution is urgently needed to overcome the dilemma of insufficient bearing capacity of soft ground faced by the aforementioned friction piles. [Summary of the Invention]

[0004] The main objective of this invention is to propose a grouting method that can increase the surface friction of the soil around the friction pile within the effective friction depth, thereby improving the bearing capacity of the friction pile.

[0005] To achieve the aforementioned objective, the confined low-pressure consolidation grouting method proposed in this invention comprises forming a plurality of confined piles around at least one foundation pile using a high-pressure single-pipe grouting method. These confined piles surround the at least one foundation pile and are connected to each other, thereby forming a confined zone between the at least one foundation pile and the confined piles. A cement grout is injected into the confined zone using a low-pressure permeation grouting method until it is saturated. The at least one foundation pile has a diameter length, the net distance between each confined pile and its nearest at least one foundation pile is between 1.5 and 1.75 times the diameter length, the interval between the grouting positions of two adjacent confined piles is 0.5 times the diameter length, and the diameter of each confined pile is greater than or equal to 0.5 times the diameter length plus 5 cm.

[0006] Therefore, the advantages of the present invention are that it can confine and solidify the soft soil layer from the ground to the effective friction depth of the friction pile, so that the soil next to the friction pile changes from soft plastic soil to harder brittle soil. This can prevent the friction pile from generating negative surface friction force under stress, allow the soil next to the friction pile to exert high-strength gripping shear force, and increase the bearing capacity of the friction pile, thereby improving the problem of insufficient bearing capacity of the friction pile and avoiding the need to change the design or change the pile foundation location. In addition, the present invention can also be implemented around existing bridge foundations and piles, thereby improving the geology around the bridge foundations and piles and increasing the bearing capacity of the piles.

[0007] As described above, the confined low-pressure consolidation grouting method further includes compacting soil around the at least one pile before forming the plurality of confined piles by the high-pressure single-pipe grouting method.

[0008] As described above, the confined low-pressure consolidation grouting method, wherein the number of the at least one pile is multiple, and the net distance between any two piles is between 3 and 3.5 times the diameter length.

[0009] As described above, the distance between the grouting depth of each confined pile and the ground surface is between 15 and 20 times the diameter length.

[0010] As described above, the confined low-pressure consolidation grouting method, wherein the grouting pressure of the high-pressure single-pipe grouting method is greater than or equal to 180 kg / cm2; and the grout setting time is between 8 seconds and 12 seconds.

[0011] As described above, in the confined low-pressure consolidation grouting method, the drill rod rotation speed in the high-pressure single-pipe grouting method is greater than or equal to 21 rpm, and the drill rod pull-out speed is less than or equal to 5 cm / 20 sec.

[0012] As described above, the confined low-pressure consolidation grouting method, wherein the 28-day uniaxial compressive strength of such confined piles is greater than or equal to 15 kg / cm2.

[0013] As described above, the low-pressure consolidation grouting method for confinement is wherein the starting pressure of the low-pressure permeation grouting method is 5 kg / cm2.

[0014] As described above, the confined low-pressure consolidation grouting method, wherein the ratio of water to cement in the cement grout is 1:1.

[0015] As described above, the confined low-pressure consolidation grouting method, wherein the uniaxial compressive strength of the confined area after consolidation is greater than or equal to 15 kg / cm2 after 28 days.

Implementation Method

[0016] First, please refer to Figure 1. The present invention is a confined low-pressure consolidation grouting method. Its first implementation process includes the following steps as shown in Figure 1: S1: A plurality of confined piles 20 are formed around at least one foundation pile 10 by a high-pressure single-pipe grouting method. The confined piles 20 surround the at least one foundation pile 10 and are connected to each other, thereby forming a confined area 30 between the at least one foundation pile 10 and the confined piles 20; S2: A cement grout is injected into the confined area 30 by a low-pressure permeation grouting method until it is saturated with grout.

[0017] The steps of the present invention are described in detail below.

[0018] In step S1, please first refer to the first embodiment shown in Figures 2 and 3. In the first embodiment, the technology of the present invention is implemented on a single foundation pile 10. As shown in Figure 3, the foundation pile 10 does not contact the deeper bearing layer B, so the foundation pile 10 is a friction pile and has a diameter length (hereinafter, the diameter length is represented by d). Multiple grouting positions are selected around the foundation pile 10. These grouting positions surround the foundation pile 10 and maintain a distance from the foundation pile 10. Then, a confined pile 20 is formed by grouting the soil at each grouting position using a high-pressure single-pipe grouting method. The aforementioned high-pressure single-pipe grouting method is the high-pressure CCP (Chemical Churning Pile) method, but it is not limited to this. For example, high-pressure jet mixing pile methods such as JGP (Jumbo-jet Special Grouting Pile), JSP, or SJM (Superjet Midi) can be used. Specifically, the confined pile 20 is gradually formed by grouting the soil at each grouting position from deep to the surface. Furthermore, since the CCP method is a well-known technique in this field, it will not be described again here.

[0019] The construction parameters for the aforementioned high-pressure CCP method include: grouting pressure greater than or equal to 180 kg / cm², and grout setting time between 10 and 12 seconds; drill rod rotation speed greater than or equal to 21 rpm, and drill rod pull-out speed less than or equal to 5 cm every 20 seconds. Based on these construction parameters, the formed retaining piles 20 are less prone to breakage. Furthermore, the 28-day uniaxial compressive strength of each retaining pile 20 must be greater than or equal to 15 kg / cm², i.e., the soil standard penetration test value N must be greater than or equal to 120. The conversion between the 28-day uniaxial compressive strength (qu) and the soil standard penetration test value N can be obtained using the semi-empirical formula for soil uniaxial compressive strength proposed by Terzaghi & Peck, as follows:

[0020] In addition, in this embodiment, the grout used for grouting includes cement, water glass and water, but is not limited to these, and the proportion of each component can be adjusted as needed; if the confinement pile 20 requires greater strength, a plasticizer can be added to the cement grout.

[0021] The net distance between each of the formed retaining piles 20 and the foundation pile 10 is between 1.5d and 1.75d, and the interval between the grouting positions of two adjacent retaining piles 20 is 0.5d. In this embodiment, the diameter of each retaining pile 20 is equal to 0.5d plus 5 cm, thereby having an overlap 21 with a width of 5 cm between two adjacent retaining piles 20, and the two adjacent retaining piles 20 are connected to each other, but not limited thereto; in other embodiments, the diameter of each retaining pile 20 may be greater than 0.5d plus 5 cm. A plurality of retaining piles 20 surround the foundation pile 10 in the aforementioned manner to form a ring structure, enclosing the foundation pile 10 and the surrounding soil.

[0022] In this embodiment, since the part of the soil around the foundation pile 10 that can exert a gripping shear force on the foundation pile 10 (i.e., the effective friction depth of the foundation pile 10) is usually 15 to 20 times the diameter of the foundation pile 10, the distance between the grouting depth of each cofferdam 20 and the ground surface is between 15d and 20d. Specifically, when forming each cofferdam 20, the effective friction depth of the foundation pile 10 can be used as the starting point for grouting, and grouting can be gradually carried out towards the ground surface, but this is not a limitation. If there is a relatively hard soil with a soil standard penetration test value N greater than 15 between the depth of the cofferdam 20 grouting position and the ground surface, which is less than 15d to 20d, then grouting can also start from the relatively hard soil and carry out the cofferdam 20 towards the ground surface.

[0023] After grouting is completed, a confinement zone 30 will be formed between each confinement pile 20 and the foundation pile 10. Specifically, the range of the confinement zone 30 is a three-dimensional space between each confinement pile 20 and the foundation pile 10 and between the bottom of the confinement pile 20 and the ground.

[0024] In addition, before proceeding to step S1, a trial injection can be carried out next to the base. After three days, samples can be taken. If the Rock Quality Designation (RQD) of the sampled results is greater than or equal to 90%, then full injection can be carried out.

[0025] Next, in step S2, cement grout is injected into the soil within the confinement area 30 using a low-pressure permeation grouting method, controlling the flow rate, pressure and time until the soil is fully saturated; in this embodiment, the initial injection pressure is 5 kg / cm2, and the ratio of cement to water in the cement grout is 1:1, but it is not limited to this.

[0026] Specifically, in this embodiment, the initial grouting pressure in step S2 is 5 kg / cm². During permeation grouting, the grouting pressure and grouting volume need to be controlled. After the cement grout is output, the grouting pressure will decrease while the grouting volume increases. When the grout is about to fill the pores of the soil within the confinement zone 30, the grouting pressure will return. After filling, the pressure will return to approximately the same as the initial grouting pressure. Grouting is stopped after holding the pressure for about 10 minutes to prevent cracking of the joints of the adjacent confinement piles 20, which would cause grout leakage. In addition, the 28-day uniaxial compressive strength of the consolidated soil within the confinement zone 30 must be greater than or equal to 15 kg / cm², that is, the soil standard penetration test value N must be greater than or equal to 120.

[0027] Next, please refer to Figures 4 and 5 for the second embodiment of the present invention, which is a case of double foundation piles 10. The construction method of the second embodiment is generally the same as that of the first embodiment, but the net distance between the two foundation piles 10 is greater than 3d, preferably between 3d and 3.5d in this embodiment, and the net distance between each confinement pile 20 and the nearest foundation pile 10 is between 1.5d and 1.75d.

[0028] Please refer to Figures 6 and 7, which show the third embodiment of the present invention, which is in the form of three foundation piles 10. The construction method of the third embodiment is the same as that of the second embodiment. In the third embodiment, the net distance between any two foundation piles 10 is between 3d and 3.5d. Please refer to Figures 8 and 9, which show the fourth embodiment of the present invention, which is in the form of a group of foundation piles 10. The construction method of the fourth embodiment is the same as that of the second embodiment. Multiple foundation piles 10 are arranged in the form of a square array. In the length and width directions of the square array, the net distance between any two adjacent foundation piles 10 is between 3d and 3.5d, but it is not limited to this. As long as the distance between each foundation pile 10 and its adjacent foundation pile 10 is greater than 3d.

[0029] By maintaining an interval of 3d to 3.5d between the two foundation piles 10, the soil around the foundation piles 10 can be prevented from interfering with each other when the shear force is exerted due to the distance between the two foundation piles 10 being too close, thus reducing the strength.

[0030] The advantages of the present invention will be described below through a comparison of bearing capacity and soil adhesion.

[0031] First, the ultimate bearing capacity of a pile includes the friction force of the pile skin and the resistance of the pile tip, that is: where, the calculation formula is as follows: In the above formula, is the stress of the pile skin adhering to the soil, L is the depth of the pile into the soil, and is the skin area of ​​the pile per meter of length, that is, the pile diameter d multiplied by pi multiplied by one meter of length.

[0032] After grouting according to the present invention, the total diameter (D) of the confinement area and the confinement pile is the pile diameter d plus twice the net distance between the pile and the confinement pile plus twice the diameter of the confinement pile. That is, it can be seen that the total diameter D of the confinement area and the confinement pile is at least approximately equal to 5 times the pile diameter d. Therefore, the surface friction force provided by the confinement area and the confinement pile is at least 4 times greater than the original pile surface friction force.

[0033] In addition, the relationship between the short-term shear strength (τ) of the soil and the standard penetration test value (N) of the soil is as follows: From the above relationship, it can be seen that if the geological conditions of the foundation for the pile construction are soft clay, that is, the soil standard penetration test value N is not greater than 4, then the shear strength of the soil in the foundation is 0.25 kg / cm2, that is, 2.5 t / m2; and as mentioned above, the 28-day uniaxial compressive strength (fc') of the soil after consolidation in the confinement area after grouting of the present invention is greater than or equal to 15 kg / cm2, and due to the shear strength of concrete, it can be calculated that its shear strength is at least about 2.05 kg / cm2; since 2.05 ÷ 0.25 = 8.21, it can be seen that the adhesion of the soil of the foundation after grouting of the present invention to a single pile is at least 8.21 times that of the original.

[0034] Please refer to Figures 10 to 13, which are schematic diagrams of the confined low-pressure consolidation grouting method of the present invention applied to the foundation pile 10A of an existing bridge C. Here, only the foundation pile 10A of bridge C is used as an example for illustration, but it is not limited thereto. The present invention can be applied to the geological improvement around other building structures.

[0035] As shown in Figure 10, the present invention is implemented on the existing foundation C1 and pile 10A of bridge C to improve the defects of bridge C. As mentioned above, if the bearing capacity of the pile 10A of bridge C is insufficient, the bearing capacity can be increased by at least 5 times by the present invention.

[0036] As shown in Figure 14, the steps of the second implementation process of the confined low-pressure consolidation grouting method of the present invention are largely the same as those of the first implementation process. The main difference between the two is that the second implementation process further includes a step S0 as shown in Figure 10: compacting soil around at least one foundation pile 10A. Specifically, in the second implementation process, soil is first piled up and covered around the exposed foundation and foundation piles of the bridge, and after compaction, a soil layer 40A is formed. Then, a plurality of confined piles 20A are formed in the soil layer 40A, and a confined area 30A is formed between the plurality of confined piles 20A. Cement grout is then injected into the confined area 30A using a low-pressure permeation grouting method until the grout is fully saturated. In this way, the foundation C1 of the bridge C can be repaired and the bearing capacity of the foundation piles 10A can be increased.

[0037] Next, as shown in Figure 11, the foundation C1 and pile 10A of bridge C lose stability due to exposure, forming a dangerous bridge that is susceptible to earthquake failure. In this case, soil can be added first, followed by grouting, thereby repairing and increasing the bearing capacity of pile 10A. Specifically, when implementing the confinement-type low-pressure consolidation grouting method of the present invention on pile 10A in this situation, the second implementation process shown in Figure 14 can be referred to.

[0038] Next, as shown in Figure 12, when the groundwater level W0 drops to groundwater level W1 due to groundwater extraction, causing the foundation C1 and foundation pile 10A of bridge C to suffer from subsidence, the confined low-pressure consolidation grouting method of the present invention can be used to grout to below the extracted groundwater level W1, which can mitigate or avoid the hazards of ground subsidence and differential subsidence of the bridge caused by large-area water extraction under the bridge.

[0039] Next, as shown in Figure 13, the foundation pile 10A of bridge C is a friction pile. The soft soil in the strata between 0 meters above ground (GL0.0m) and 20 meters below ground (GL-20m), or below the groundwater level W0, is susceptible to ground liquefaction during an earthquake. In other words, foundation pile 10A is located in the seismic liquefaction potential zone of loose sand and sediment below the groundwater level W0. Therefore, if ground liquefaction occurs during an earthquake, the foundation C1 and foundation pile 10A of bridge C may lose their bearing capacity, causing significant damage. Therefore, the confined low-pressure consolidation grouting method of this invention can be applied to the soft soil in the aforementioned strata between 0 meters above ground and 20 meters below ground, or below the groundwater level, to prevent foundation pile 10A from losing its bearing capacity due to ground liquefaction during an earthquake.

[0040] In summary, if the ground at the construction site is soft and the bedrock of the bearing layer is too deep, preventing the foundation pile 10 from contacting and forming a friction pile, the confinement pile 20 of the present invention can be used to confine the soft soil around the foundation pile 10 and solidify it by low-pressure permeable grouting, transforming the soft soil into brittle soil. This increases the shear force on the foundation pile 10 and reduces the negative surface friction, further increasing the bearing capacity of the foundation pile 10. This eliminates the need to modify the design of the foundation pile 10 by means of thickening or lengthening, and also eliminates the need for relocation. When the construction site area is small, making it impossible to increase the size of the foundation pile 10, the present invention can solve the problem of insufficient bearing capacity of the foundation pile 10. Furthermore, the present invention can also be implemented around existing bridge foundations and foundation piles, thereby improving the geology around the bridge foundation and foundation piles, increasing the bearing capacity of the foundation piles, and preventing the bridge from losing its bearing capacity due to ground liquefaction. [Simplified Explanation of the Diagram]

[0041] Figure 1 is a schematic diagram of the first embodiment of the present invention. Figure 2 is a top view of the first embodiment of the present invention. Figure 3 is a side sectional view of the first embodiment of the present invention. Figure 4 is a top view of the second embodiment of the present invention. Figure 5 is a side sectional view of the second embodiment of the present invention. Figure 6 is a top view of the third embodiment of the present invention. Figure 7 is a side sectional view of the third embodiment of the present invention. Figure 8 is a top view of the fourth embodiment of the present invention. Figure 9 is a side sectional view of the fourth embodiment of the present invention. Figures 10 to 13 are schematic diagrams of the present invention applied to the foundation piles of existing bridges. Figure 14 is a schematic diagram of the second embodiment of the present invention.

Claims

1. A confined low-pressure consolidation grouting method, comprising: forming a plurality of confined piles around at least one foundation pile using a high-pressure single-pipe grouting method, the confined piles surrounding the at least one foundation pile and being interconnected, thereby forming a confined zone between the at least one foundation pile and the confined piles; injecting a cement grout into the confined zone using a low-pressure penetrating grouting method until saturation; wherein, The at least one foundation pile has a diameter length, the net distance between each of the retaining piles and the nearest at least one foundation pile is between 1.5 and 1.75 times the diameter length, the interval between the grouting positions of two adjacent retaining piles is 0.5 times the diameter length, and the diameter of each retaining pile is greater than or equal to 0.5 times the diameter length plus 5 centimeters.

2. The confinement-type low-pressure consolidation grouting method as described in claim 1, wherein, Before forming the plurality of confined piles around the at least one pile using the high-pressure single-pipe grouting method, the method further includes: compacting the soil around the at least one pile.

3. The confinement-type low-pressure consolidation grouting method as described in claim 1, wherein: The number of the at least one pile is multiple, and the net distance between any two piles is between 3 and 3.5 times the diameter length.

4. The confinement-type low-pressure consolidation grouting method as described in claim 3, wherein: The distance between the grouting depth of each of the confinement piles and the ground surface is between 15 and 20 times the diameter length.

5. The confinement-type low-pressure consolidation grouting method as described in claim 1 or 2, wherein, The grouting pressure of this high-pressure single-pipe grouting method is greater than or equal to 180 kg / cm2; the grout setting time is between 8 and 12 seconds.

6. The confinement-type low-pressure consolidation grouting method as described in claim 5, wherein, In this high-pressure single-pipe grouting method, the drill rod rotation speed is greater than or equal to 21 rpm, and the drill rod pull-out speed is less than or equal to 5 cm / 20 sec.

7. The confinement-type low-pressure consolidation grouting method as described in claim 6, wherein: The 28-day uniaxial compressive strength of these confinement piles is greater than or equal to 15 kg / cm2.

8. The confinement-type low-pressure consolidation grouting method as described in claim 1 or 2, wherein: The starting pressure for this low-pressure permeation grouting method is 5 kg / cm².

9. The confinement-type low-pressure consolidation grouting method as described in claim 8, wherein: The ratio of water to cement in this cement slurry is 1:

1.

10. The confinement-type low-pressure consolidation grouting method as described in claim 9, wherein: The uniaxial compressive strength of the confined zone after 28 days is greater than or equal to 15 kg / cm2.

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