CN112850544B - A composite piston jack for load boxes - Google Patents

Abstract

The invention relates to a single-liquid-path composite piston jack for a load box, which is mainly used in a foundation pile self-balancing static load test. A composite piston jack for a load box comprises a cylinder body and a piston, wherein a top plate of the cylinder body is connected with a top plate of the load box, a through hole for allowing the piston to move up and down is formed in a bottom plate of the cylinder body, and a liquid inlet is formed in the side wall of the cylinder body; the piston comprises a convex piston and a concave piston, the convex piston comprises a piston plate and a piston column, the piston plate is positioned at the top of the piston column, a bulge is arranged at the top of the piston plate, the piston column is inserted into the inner cavity of the concave piston, a channel is formed in the convex piston and used for communicating the concave piston with a liquid inlet, a liquid outlet is formed in the side wall of the concave piston, and a bottom plate of the concave piston is connected with a bottom plate of a load box. The invention has the following advantages: 1. realizing small-area large-tonnage pressurizing detection; 2. the pile body strength and the integrity of the foundation pile are ensured; 3. the processing difficulty is reduced; 4. after static load test pile is realized, the loading liquid is discharged first, and grouting is then carried out.

Description

A composite piston jack for load boxes

Technical Field

The invention relates to a single-fluid-path composite piston jack for a load box, which is mainly used in a self-balancing static load test of a foundation pile.

Background Art

In the self-balancing static load test of pile foundation, a load box is used, and the most important component in the load box is the jack.

When the current jack is pressurized, the thrust of the small-area jack is limited, and the small-area jack cannot meet the requirements of large-tonnage pressurization testing.

Moreover, currently, jacks generally have only one injection pipe, so that after the test, the space between the cylinder and the piston of the load box is filled with liquid that never solidifies, resulting in displacement of the pile under the vertical pressure state of the project after the self-balancing static load test of the foundation pile due to the liquid in the load box cylinder being squeezed out. Although some load boxes can achieve drainage and grouting through two pipes, since the two pipes are at the same elevation on the upper side of the cylinder, during grouting, only the liquid squeezed above the liquid outlet elevation can be discharged, and the liquid below the liquid outlet elevation cannot be completely discharged, resulting in the injected slurry being diluted by the retained liquid, thereby reducing the concentration of the slurry, so that the final setting strength of the slurry is seriously reduced, and it cannot meet the reinforcement requirements.

Summary of the invention

The object of the present invention is to provide a composite piston jack for a load box, so that the thrust can be increased under the condition of the same jack bottom area, so as to realize the pressurization detection of small area and large tonnage.

In order to achieve the above-mentioned purpose, the present invention adopts the following technical scheme: a composite piston jack for a load box, comprising a cylinder body and a piston, wherein a cylinder body top plate is connected to a load box top plate, a through hole for accommodating the piston to move up and down is provided on a cylinder body bottom plate, and a liquid inlet is provided on a cylinder body side wall; the piston comprises a convex piston and a concave piston, the convex piston comprises a piston plate and a piston column, the piston plate is located at the top of the piston column and is adapted to the inner cavity of the cylinder body, a protrusion is provided on the top of the piston plate for separating the piston plate from the cylinder body top plate, the piston column is inserted into the inner cavity of the concave piston, a channel is provided on the convex piston for connecting the concave piston with the liquid inlet, a limiting portion is provided on the top of the concave piston, the limiting portion extends radially outward along the concave piston and is adapted to the inner cavity of the cylinder body, when the concave piston moves downward, it is limited by the cylinder body bottom plate to prevent the concave piston from moving downward out of the cylinder body, a liquid outlet is provided on the side wall of the concave piston, the liquid outlet is located below the cylinder body bottom plate, and the bottom plate of the concave piston is connected to the bottom plate of the load box.

Preferably, a reaction beam is installed in the cylinder body, and the reaction beam is arranged around the inner cavity of the cylinder body to limit the piston plate and the limiting part at the upper part and the lower part thereof respectively.

Preferably, the bottom of the piston column contacts the bottom of the inner cavity of the concave piston. When liquid is injected, the pressure of the liquid on the convex piston is transmitted to the bottom surface of the concave piston through the piston column, thereby achieving the superposition and compounding of the pressures of the convex piston and the concave piston.

Preferably, the inlet of the channel on the convex piston is located at the axis of the piston plate, and the outlet is located at the side wall of the piston column.

Preferably, a top column is installed between the reaction beam and the top plate of the cylinder body, and the top column passes through a through hole opened at a corresponding position of the piston plate to further limit the upward movement of the reaction beam.

Preferably, the piston plate is made of metal, and the protrusion is a magnet adsorbed on the piston plate.

Preferably, the cylinder top plate, the cylinder bottom plate and the cylinder body are assembled in an assembled manner.

Preferably, the piston plate is detachably connected to the piston column.

Preferably, it also includes a hollow connecting rod for connecting the piston plate and the piston column, the outer diameter of the hollow connecting rod is adapted to the channel between the piston plate and the piston column, and the interior of the hollow connecting rod is hollow and communicated with the channel on the piston column.

Working principle: During liquid injection, the liquid flows into the concave piston below through the channel in the middle of the convex piston; when the liquid fills the concave piston, it continues to be pressurized. At this time, the pressure at the convex piston plate is equal to the pressure in the concave piston groove, that is, the concave piston is pressurized while the convex piston is pressurized; therefore, the pressure of the liquid on the convex piston is transmitted to the bottom surface of the concave piston through the convex piston column, thereby realizing the superposition and compounding of the pressures of the two pistons. The pressure value is increased while the unit diameter remains unchanged, so it can be used for small area and large tonnage pressure detection; a cylindrical piston plate + magnet combination is used, that is, the piston plate is processed into a plane, the material is metal, and the magnet is adsorbed on the piston plate to form a convex platform. Since the position of the magnet is movable, it can avoid the liquid channel in the middle of the convex piston, so that the liquid channel can be kept unobstructed during liquid injection.

Compared with the prior art, the present invention has the following advantages:

1. The convex piston and the concave piston are combined into an integrated composite piston. When injecting liquid, the liquid flows into the concave piston below through the channel in the middle of the convex piston. The entire liquid path forms an integrated whole, the pressure is equal and can be superimposed and composited, which can increase the thrust under the same bottom area of the jack, and realize the pressure test of small area and large tonnage;

2. It can ensure the slurry quality and improve the filling rate while grouting, so that the reinforced load box and the engineering pile form a continuous whole by reinforcing the load box through grouting, ensuring the strength and integrity of the foundation pile body, so that the load on the upper part of the load box can be effectively transferred to the pile body below the load box;

3. The jack can be split into multiple parts and processed separately, which reduces the production difficulty, reduces the production cost and improves the production efficiency;

4. The convex platform is formed by combining a cylindrical piston plate and a magnet, which can reduce the processing difficulty and cost. At the same time, since the position of the magnet is movable, the liquid channel in the middle of the piston can be avoided, so that the liquid channel can be kept unobstructed during injection;

5. The convex piston plate and the convex piston column are connected together through the convex piston hollow connecting rod, thereby improving the original one-time processed piston into a piston assembled from multiple parts, reducing the processing difficulty and the production cost;

6. Through the two pipes, one for inlet and one for outlet, the loading liquid can be discharged first and then grouting can be carried out after the static load test, so as to avoid the situation where the cylinder and piston of the single-tube load box are filled with never-solidifying loading liquid after the test, and the pile is squeezed out of the liquid under the vertical load of the project, causing the foundation pile to move downward.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the structure of an embodiment of the present invention;

FIG2 is a schematic diagram of the structure of a cylinder body;

FIG3 is a schematic diagram of the structure of the cylinder body, wherein (a) is a three-dimensional diagram of the cylinder body, and (b) is an axial cross-sectional diagram of the cylinder body along the liquid inlet;

FIG4 is a schematic diagram of a cylinder top plate, wherein (a) is a three-dimensional view of the cylinder top plate, and (b) is an axial cross-sectional view of the cylinder top plate;

FIG5 is a schematic diagram of the structure of a convex piston;

FIG6 is a schematic diagram of the structure of the convex piston plate in FIG5 , wherein (a) is a three-dimensional view of the convex piston plate, and (b) is a cross-sectional view of the convex piston plate 1 - 1 ;

FIG. 7 is a schematic diagram of the structure of the convex piston column in FIG. 5 , wherein (a) is a three-dimensional view of the convex piston column, and (b) is an axial cross-sectional view of the convex piston column;

FIG8 is an enlarged view of the convex piston hollow connecting rod in FIG5 , wherein (a) is a stereoscopic view of the convex piston hollow connecting rod, and (b) is an axial cross-sectional view of the convex piston hollow connecting rod;

Figure 9 is an enlarged view of the top column;

FIG10 is an enlarged view of the reaction beam, wherein (a) is a three-dimensional view of the reaction beam, and (b) is an axial cross-sectional view of the reaction beam;

FIG11 is a schematic diagram of a concave piston structure, wherein (a) is a three-dimensional view of the concave piston, and (b) is an axial cross-sectional view of the concave piston at the liquid outlet;

Figure 12 is an enlarged view of the cylinder bottom plate;

FIG13 is an assembly flow chart of a product according to an embodiment of the present invention, including (a)-(g);

FIG14 is an application flow chart of an embodiment of the present invention, including (a)-(g);

In the figure, 1, cylinder body; 1-1, cylinder body; 1-2, cylinder body top plate; 1-1-1, opening; 1-1-2, upper end step; 1-1-3, middle end step; 1-1-4, liquid inlet; 1-1-5, thread; 1-2-1, upper end; 1-2-2, lower end; 1-2-3, sealing groove and sealing ring; 1-2-4, thread;

2. Convex piston; 2-1. Convex piston plate; 2-2. Convex piston column; 2-3. Convex piston hollow connecting rod; 2-4. Magnet; 2-1-1. Sealing groove and sealing ring; 2-1-2. Top column channel; 2-1-3. Convex piston hollow connecting rod channel; 2-2-1. Convex piston hollow connecting rod channel; 2-2-2. Liquid channel; 2-3-1. Liquid channel; 2-3-2. Sealing groove and sealing ring;

3. Top column; 3-1. Sealing groove and sealing ring;

4. Reaction beam; 4-1. Sealing groove and sealing ring; 4-2. Piston channel;

5. Concave piston; 5-1. Step limit structure; 5-2. Liquid outlet; 5-3. Thread;

6. Cylinder bottom plate; 6-1. Thread; 6-2. Sealing groove and sealing ring.

DETAILED DESCRIPTION

It should be noted that in this embodiment, the directional words "upper", "lower", "top", "bottom", "inside", "outside", etc. are described according to the drawings and do not constitute a limitation to the present invention; in addition, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, they can be directly connected or indirectly connected through an intermediate medium. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

The present invention is further described in detail below in conjunction with Figures 1-14: A composite piston jack for a load box, as shown in Figure 1, comprises a cylinder body 1, a convex piston 2, a top column 3, a reaction beam 4, a concave piston 5 and a cylinder body bottom plate 6; the cylinder body 1 is composed of a cylinder body 1-1 and a cylinder body top plate 1-2, the cylinder body top plate 1-2 is connected to the load box top plate, a through hole for accommodating the concave piston 5 to move up and down is opened on the cylinder body bottom plate 6, and a liquid inlet 1-1-4 is opened on the side wall of the cylinder body 1; the piston comprises a convex piston 2 and a concave piston 5, the convex piston 2 comprises a convex piston plate 2-1, a convex piston column 2-2, a convex piston hollow connecting rod 2-3 and a magnet 2-4, the convex piston plate 2-1 is located at the top of the convex piston column 2-2, and is adapted to the inner cavity of the cylinder body 1, and the magnet 2-4 is located at its top for separating the convex piston plate 2-1 The convex piston column 2-2 is adapted to the inner cavity of the concave piston 5 and the cylinder top plate 1-2. The convex piston hollow connecting rod 2-3 is used to connect the convex piston plate 2-1 and the convex piston column 2-2. The interior thereof is hollow. Channels are opened at corresponding positions of the convex piston plate 2-1 and the convex piston column 2-2. The convex piston hollow connecting rod 2-3 is connected to the liquid inlet 1-1-4, the channel and the concave piston 5. A stepped limiting structure 5-1 is provided on the top of the concave piston 5. The stepped limiting structure 5-1 extends radially outward along the concave piston 5 and is adapted to the inner cavity of the cylinder 1. When the concave piston 5 moves downward, it is limited by the cylinder bottom plate 6 to prevent the concave piston 5 from moving downward out of the cylinder. A liquid outlet 5-2 is opened on the side wall of the concave piston 5. The liquid outlet 5-2 is located below the cylinder bottom plate 6. The bottom plate of the concave piston 5 is connected to the bottom plate of the load box.

In this embodiment, the specific structures are all produced and assembled in an assembled manner, that is, the jack is split into multiple components and produced and processed separately.

As shown in FIG2-3, an opening 1-1-1 is provided at the top of the cylinder body 1-1, and the radial dimension of the opening is smaller than the radial dimension of the inner cavity of the cylinder body 1-1, so that an upper step 1-1-2 is formed between the edge of the opening 1-1-1 and the inner wall of the cylinder body, and a middle step 1-1-3 is provided near the middle of the cylinder body 1-1, and the radial dimension of the inner cavity of the cylinder body above the middle step 1-1-3 is smaller than the radial dimension of the inner cavity of the cylinder body below it, and a liquid inlet 1-1-4 provided on the side wall of the cylinder body 1 is close to the upper step 1-1-2, and an internal thread 1-1-5 is provided at the bottom of the inner cavity of the cylinder body, and the screw thread 1-1-5 is provided at the bottom of the inner cavity of the cylinder body. The groove 1-1-5 is adapted to the external thread of the cylinder bottom plate 6; as shown in Figure 4, the upper end 1-2-1 of the cylinder top plate 1-2 is adapted to the opening 1-1-1, the lower end 1-2-2 is adapted to the inner cavity of the cylinder body, the transition section between the upper end 1-2-1 and the lower end 1-2-2 is adapted to the upper end step 1-1-2, and a sealing groove and a sealing ring 1-2-3 are provided on the circumference of the upper end 1-2-1 for sealing the cylinder top plate 1-2 and the cylinder opening 1-1-1, and an internal thread 1-2-4 is provided at the axis of the upper end 1-2-1, so as to be fixed to the load box top plate by screws or bolts adapted thereto.

As shown in FIG5 , the convex piston 2 is composed of a convex piston plate 2-1, a convex piston column 2-2, a convex piston hollow connecting rod 2-3 and a magnet 2-4; as shown in FIG6 , the convex piston plate 2-1 is a cylindrical flat plate, and a plurality of sealing grooves and sealing rings 2-1-1 are provided along the circumference of the cylinder body to seal the gap between the convex piston plate 2-1 and the inner wall of the cylinder body, and at least two top column channels 2-1-2 are provided along the axial direction thereof. In this embodiment, the top column channels 2-1- There are four of them, which are evenly distributed along the circumference. A convex piston hollow connecting rod channel 2-1-3 is opened at the axis of the convex piston plate 2-1, wherein the top column channel 2-1-2 is adapted to the top column 3, and the convex piston hollow connecting rod channel 2-1-3 is adapted to the convex piston hollow connecting rod 2-3; when injecting liquid into the cylinder from the side of the cylinder body, it is necessary to process a cut on the piston plate to make the piston plate a convex platform, thereby ensuring that when injecting liquid from the side of the cylinder body , the liquid can enter the cavity and push the piston downward. In this embodiment, in order to reduce the difficulty and cost of processing, the convex piston plate 2-1 is made of metal, and the protrusion is made of magnet 2-4. In this embodiment, the magnet 2-4 is adsorbed on the top of the convex piston plate 2-1 to form a convex platform. At the same time, since the position of the magnet 2-4 is movable, it can avoid the liquid channel in the middle of the piston, so that the liquid channel is kept unobstructed during injection; as shown in Figure 7, a convex piston hollow connecting rod channel 2-2-1 is opened at the axis of the convex piston column 2-2, and the convex piston hollow connecting rod channel 2-2-1 is connected and opposite to the convex piston hollow connecting rod channel 2-1-3. The channel formed by the two is adapted to the convex piston hollow connecting rod 2-3, and a number of liquid channels 2-2-2 are opened along the convex piston hollow connecting rod channel 2-2-1 to the side wall of the convex piston column. The liquid channels 2-2-2 have the same structure and are symmetrical; as shown in Figure 8, The convex piston hollow connecting rod 2-3 is hollow inside and serves as a liquid channel 2-3-1 for connecting the liquid inlet 1-1-4, the liquid channel 2-2-2 and the concave piston 5. A sealing groove and a sealing ring 2-3-2 are arranged along the circumference of the convex piston hollow connecting rod 2-3 to seal the gap between the convex piston hollow connecting rod 2-3 and the convex piston hollow connecting rod channel.

As shown in FIG9 , the top column 3 is a rod-shaped member, and a plurality of sealing grooves and sealing rings 3-1 are provided along its circumference. The plurality of sealing grooves and sealing rings 3-1 are distributed up and down. In the present embodiment, there are two sealing grooves and sealing rings 3-1, which are respectively close to the upper and lower parts of the top column channel 2-1-2, thereby sealing the gap between the top column 3 and the convex piston plate 2-1.

As shown in Figure 10, the reaction beam 4 is convex and is arranged around the inner cavity of the cylinder body 1. The interior of the reaction beam 4 is a piston channel 4-2. The piston channel 4-2 is adapted to the convex piston column 10, and its outer circumferential shape is adapted to the middle-end step 1-1-3. A sealing groove and a sealing ring 4-1 are provided on the outer circumference of the reaction beam and the circumference of the piston channel to seal the gap between the inner wall of the cylinder body above the middle-end step 1-1-3 and the reaction beam 4, and the gap between the piston channel 4-2 and the convex piston column 2-2; the reaction beam 4 limits the convex piston 2 and the concave piston 5 to the upper and lower parts of the cylinder body, respectively.

In this embodiment, since the cylinder body 1 and the cylinder body top plate 1-2 are detachable structures, one end of the top column 3 abuts against the top of the reaction beam, and the other end passes through the top column channel 2-1-2 and abuts against the bottom of the cylinder body top plate 1-2. It can not only provide support for the cylinder body top plate 1-2, but also limit the upward movement of the reaction beam. At the same time, it can also provide guidance for the convex piston 2 during the movement of the convex piston 2.

As shown in Figure 11, the stepped limit structure 5-1 on the concave piston 5 is adapted to the cylinder bottom plate 6. When the concave piston 5 moves downward, it is limited by the cylinder bottom plate 6 to prevent the concave piston 5 from moving downward out of the cylinder body 1. The starting position of the liquid outlet 5-2 is below the bottom of the cylinder body 1-1 and close to the bottom of the concave piston 5. The bottom plate of the concave piston 5 is connected to the bottom plate of the load box.

As shown in FIG12 , a thread 6-1 is provided along the circumference of the cylinder bottom plate 6, and the thread 6-1 is matched with the thread 1-1-5. A sealing groove and a sealing ring 6-2 are provided on the axial through hole to seal the gap between the concave piston 5 and the cylinder bottom plate 6.

As shown in Figure 13, the assembly process is:

1. Assemble the convex piston 2: Assemble the convex piston plate 2-1, the convex piston column 2-2, and the convex piston hollow connecting rod 2-3, and adsorb the magnet 2-4 on the top of the convex piston plate, as shown in FIG13 (a);

2. Assemble the cylinder body 1: Push the cylinder body top plate 1-2 in from the bottom until it fits tightly with the cylinder body, as shown in FIG13(b);

3. Push the convex piston 2 from the bottom of the cylinder 1 until it contacts the top plate 1-2 of the cylinder, as shown in FIG13(c);

4. Push the top column 3 from the bottom, through the top column channel 2-1-2 on the convex piston plate 2-1, until it presses against the cylinder top plate 2-1, as shown in FIG13 (d);

5. Push the reaction beam 4 in from the bottom until it fits tightly with the middle step 1-1-3 of the cylinder body 1-1, as shown in FIG13(e);

6. Push the concave piston 5 into the cylinder 1 from below, as shown in FIG13( f );

7. Install the cylinder base plate 6 from below and tighten it, as shown in Figure 13 (g).

As shown in Figure 14, the workflow is:

1. Liquid injection and pressurization

1. The liquid outlet 5-2 of the concave piston 5 is closed, and the liquid is injected into the liquid inlet 1-1-4 of the cylinder body. The liquid flows into the concave piston 5 along the hollow in the hollow connecting rod of the convex piston and the liquid channel in the convex piston column until the concave piston 5 is filled. At this stage, the piston does not move, as shown in FIG. 14 (a).

2. Continue to inject liquid and pressurize it, and the liquid pushes the convex piston 2 to move downward; at the same time, the liquid continues to flow into the concave piston along the hollow core in the hollow connecting rod 2-3 of the convex piston and the liquid channel in the convex piston column 2-2, which also pushes the concave piston to move downward, as shown in Figure 14 (b);

3. Continue to inject liquid and pressurize until the self-balancing static load pile test termination conditions required by the design requirements or test specifications are met, as shown in Figure 14 (c);

2. Inflation and Discharge

1. After the liquid injection is completed, the gas is inflated and the liquid is discharged. During the gas inlet 1-1-4 of the cylinder body and the liquid outlet 5-2 of the concave piston are not closed, the gas is filled into the cylinder body from top to bottom, and the liquid is squeezed out from the liquid outlet of the concave piston, as shown in Figure 14 (d);

2. Continue to inflate until no more liquid flows out of the liquid outlet of the concave piston, indicating that the liquid in the cylinder has been exhausted, and stop inflating, as shown in Figure 14 (e);

3. Grouting

1. When the liquid outlet of the concave piston stops discharging liquid, grouting begins. During grouting, the liquid inlet of the cylinder body and the liquid outlet of the concave piston are not sealed, and the slurry is filled into the cylinder body from top to bottom, and the squeezed gas is discharged from the liquid outlet of the concave piston, as shown in Figure 14 (f);

2. Continue grouting until slurry overflows from the outlet of the concave piston, as shown in Figure 14 (g).

The above embodiments are only preferred implementations of the present invention and do not constitute limitations of the present invention. In the present invention, some structures may also be produced and assembled in an assembled manner. It should be understood by those skilled in the art that any extension, deformation, equivalent replacement, etc. made without departing from the principles of the present invention should be included in the protection scope of the present invention.

Claims (4)

1. A composite piston jack for a load box, comprising a cylinder and a piston, characterized in that: a cylinder top plate is connected to a load box top plate, a through hole is provided on a cylinder bottom plate for accommodating the piston to move up and down, and a liquid inlet is provided on a side wall of the cylinder; the piston comprises a convex piston and a concave piston, the convex piston comprises a piston plate and a piston column, the piston plate is located at the top of the piston column and is adapted to the inner cavity of the cylinder body, a protrusion is provided on the top of the piston plate for separating the piston plate and the cylinder top plate, the piston column is inserted into the inner cavity of the concave piston, a channel is provided on the convex piston for connecting the concave piston with the liquid inlet, a limiting portion is provided on the top of the concave piston, the limiting portion extends radially outwardly along the concave piston and is adapted to the inner cavity of the cylinder body, when the concave piston moves downward, it is limited by the cylinder bottom plate to prevent the concave piston from moving downward out of the cylinder, a liquid outlet is provided on the side wall of the concave piston, the liquid outlet is located below the cylinder bottom plate, and the bottom plate of the concave piston is connected to the bottom plate of the load box; A reaction beam is installed in the cylinder body, and the reaction beam is arranged around the inner cavity of the cylinder body, and the piston plate and the limit part are respectively limited at the upper part and the lower part thereof; The inlet of the channel on the convex piston is located at the axis of the piston plate, and the outlet is located at the side wall of the piston column; A top column is installed between the reaction beam and the top plate of the cylinder body, and the top column passes through a through hole opened at a corresponding position of the piston plate to further limit the upward movement of the reaction beam; The piston plate is made of metal, and the protrusions are magnets that are attached to the piston plate; The cylinder top plate, cylinder bottom plate and cylinder body are assembled in an assembled manner; The workflow of liquid injection and pressurization is as follows: (1) The liquid outlet of the concave piston is closed, and liquid is injected into the liquid inlet of the cylinder body. The liquid flows into the concave piston along the hollow core in the hollow connecting rod of the convex piston and the liquid channel in the convex piston column until the concave piston is filled. At this stage, the piston does not move. (2) Liquid is continuously injected and pressurized, and the liquid pushes the convex piston downward. At the same time, the liquid continues to flow into the concave piston along the hollow core in the hollow connecting rod of the convex piston and the liquid channel in the convex piston column, which also pushes the concave piston downward. (3) Continue to inject liquid and increase pressure until the test termination conditions of the self-balancing static load pile test pile are met and meet the design requirements or test specifications. 2. The composite piston jack for a load box according to claim 1 is characterized in that the bottom of the piston column contacts the bottom of the inner cavity of the concave piston. When liquid is injected, the pressure of the liquid on the convex piston is transmitted to the bottom surface of the concave piston through the piston column, thereby realizing the superposition and compounding of the pressure of the convex piston and the concave piston. 3. The composite piston jack for a load box according to claim 1, wherein the piston plate and the piston column are detachably connected. 4. The composite piston jack for a load box according to claim 3 is characterized in that it also includes a hollow connecting rod for connecting the piston plate and the piston column, the outer diameter of the hollow connecting rod is adapted to the channel between the piston plate and the piston column, and the interior of the hollow connecting rod is hollow and connected to the channel on the piston column.