CN111501722B - Vacuum preloading and CFG (cement fly-ash gravel) long and short combined pile foundation treatment method - Google Patents

Abstract

The invention is suitable for the technical field of civil engineering, and provides a foundation treatment method of a vacuum preloading combined CFG (cement fly-ash gravel) long and short combined pile, which comprises the following steps: leveling the field; the method comprises the steps of preprocessing a deep soft soil foundation with high fill and no cover layer on the ground surface by adopting a vacuum preloading method, improving the shear strength of the surface layer of the foundation soil, correspondingly increasing the rigidity of the soil body, forming a hard shell layer on the surface layer to improve the stability of the surface soil, then processing by adopting CFG long and short composite piles, and reducing foundation settlement by adopting the cross section arrangement form of the long and short composite piles, adjusting the rigidity of the cross section, adapting the deformation of the foundation to the load distribution on the flexible foundation of the roadbed, and achieving the aims of ensuring the stability of the roadbed and controlling the settlement of the roadbed. The vacuum preloading method is adopted for pretreatment in advance, and then CFG long and short combined piles are used for treatment, and the combination of the two forms can quickly and comprehensively improve the strength of the foundation soil, reduce the settlement, ensure the safety of the high-fill roadbed and accelerate the construction progress.

Description

Vacuum preloading and CFG (cement fly-ash gravel) long and short combined pile foundation treatment method

Technical Field

The invention belongs to the technical field of civil engineering, and particularly relates to a foundation treatment method combining vacuum preloading and CFG (cement fly-ash gravel) long and short composite piles.

Background

With the continuous development of economy, the construction of railways and highways enters a rapid development period, and particularly, coastal regions have more gullies and thicker soft soil, most of the soft soil is soft plastic, even soft soil in a flowing plastic shape, which brings great difficulty to the construction of railways and highways. The foundation treatment mode is promoted to be continuously updated, a new treatment mode and new construction machines are continuously generated, and piles with higher rigidity, such as CFG piles, tubular piles, reinforced concrete piles and the like are generated.

On one hand, because the traditional composite foundation stability analysis method adopts a limit balance method, and the shear failure of a pile body and a soil body on a sliding surface is assumed, the stability detection method is suitable for discrete piles and flexible piles with lower shear strength, but for rigid piles such as CFG piles, tubular piles, reinforced concrete piles and the like, the method cannot reflect the failure mechanism of the pile body at different positions under a roadbed, so that the stability of the embankment is excessively estimated, and the safety problem is caused, therefore, the existing embankment stability analysis method has larger defects and cannot adapt to a new pile foundation treatment method with higher rigidity, on the other hand, for coastal flow molding and soft molding deep soft soil, because the resistance provided by the soil body is too small, the improvement of bending resistance and shear strength obtained by using stronger pile bodies is limited, and the soil body and the pile body have larger rigidity difference, the two are difficult to play a role together, the foundation destruction mode is different from the conventional flexible pile foundation destruction mode greatly, and various destruction modes such as pile soil turbulence, pile body inclination, pile body side shift, pile body bending destruction, pile body shearing and the like are generated.

SUMMARY OF THE PATENT FOR INVENTION

The invention provides a foundation treatment method combining vacuum preloading and CFG (cement fly-ash gravel) long and short composite piles, which is adopted on a design method, the transverse shear rigidity of foundation soil is increased by the vacuum preloading method, the possibility of damages such as soil turbulence around a pile, pile body inclination, pile body side shift and the like is eliminated, and the problem of overall stability is solved; and then, the vertical rigidity is adjusted by setting different pile lengths on the cross section through a CFG long and short combined pile method, so that the load is transferred to a deep layer, the soil body settlement and uneven settlement are reduced, and the soil body stability is enhanced.

The invention is realized in this way, a vacuum preloading unites CFG long and short composite pile foundation processing method, is used in the high fill foundation on the deep soft soil of flow molding, soft molding shape, characterized by, including the step:

step 1, leveling a field: ploughing and planting soil on the ground of the construction site, removing and leveling barriers;

step 2, paving a sand cushion layer: a sand cushion layer with the thickness of 0.3m is paved on the construction points;

step 3, arranging a drainage plate: arranging a plastic drainage plate at the construction point;

step 4, burying vacuumizing equipment: a monitoring instrument and a vacuum pump are installed at the construction point;

step 5, laying sealing protection equipment: after the vacuum equipment is connected into a system, excavating a sealing ditch, and laying sealing protection equipment such as geotextile, geomembrane and the like;

step 6, vacuumizing: pumping air to the construction points through the vacuum pump to pre-press the foundation, wherein the pre-pressing time is set to be 3 months;

step 7, driving a CFG pile: driving CFG long and short piles on the construction points, and pouring reinforced concrete pile caps after the bearing capacity of the CFG piles meets the requirements;

step 8, paving a gravel cushion layer: three layers of soil grids are clamped in the cushion layer;

step 9, filling a foundation: and filling the roadbed to the designed elevation, and carrying out corresponding observation.

Preferably, the cross section of the long and short combined piles in the CFG piles is arranged to be adaptive to the upper load distribution, the middle point of a roadbed slope is taken as a boundary point, different pile lengths are adopted on two sides, and the distance between two adjacent CFG piles is 3-5 times of the pile diameter.

Preferably, the CFG piles are divided into long piles and short piles, the long pile is arranged in the middle of the embankment, and the short piles are arranged on two sides of the embankment; the diameter of the CFG pile is 0.5m, the length of a long pile of the CFG pile is determined according to settlement and bearing capacity calculation, and the length is not less than 15 m; the short pile is only required to penetrate through the soft soil layer.

Preferably, the height of the gravel cushion is 0.5 m.

Preferably, the driving CFG pile step includes:

step 1, measuring and marking: marking the specific position of each pile on the construction point;

step 2, drilling: starting a motor to drill a hole;

step 3, pumping materials: pumping the material into the drill hole through a ground pump, vibrating and filling the material;

step 4, pile foundation detection: carrying out bearing capacity test and low-pressure-change detection on the CFG pile through a detection instrument;

step 5, mattress layer bedding: and after the pile foundation is detected to be qualified, paving a broken stone mattress layer on the pile foundation.

Preferably, the plastic drainage plates are arranged in a regular triangle, the plastic drainage plates are spaced at a distance of 0.8-1.5m, and the sand cushion layer laying comprises:

step 1: sequentially laying woven cloth, a Jingba cushion layer and geotextile on the construction point;

step 2: and pushing and filling a sand cushion layer on the geotextile.

Preferably, the step of evacuating includes:

step 1: trying to vacuumize the construction point;

and 2, step: collecting data of the construction points through the monitoring instrument, and checking whether air leakage exists or not;

and step 3: and formally pumping and prepressing the construction points, and recording vacuum degree data through the monitoring instrument.

Compared with the prior art, the invention has the beneficial effects that: the invention relates to a foundation treatment method combining vacuum preloading with long and short CFG (cement fly-ash gravel) composite piles, which is characterized in that deep soft soil with high fill and no covering layer (or thin covering layer) on the ground surface is pretreated in advance by adopting a vacuum preloading method, so that the shear strength of the surface layer of the foundation soil is improved, the rigidity of the soil body is correspondingly increased, a hard shell layer is formed on the surface layer, the stability of the surface layer is increased, the shear resistance of the soil is exerted to the maximum extent, the number of CFG piles is reduced, then the CFG long and short composite piles are adopted for treatment, the foundation settlement is reduced, the rigidity of the cross section is properly adjusted, the foundation deformation is adapted to the load distribution on the flexible foundation of a roadbed, and the aims of ensuring the stability of the roadbed and controlling the settlement of the roadbed are achieved. The treatment method increases the shear strength of the surface soil of the foundation through vacuum preloading, and improves the stability of the foundation; the long piles are arranged in the middle, the short piles are arranged on two sides, and the cross section of the long and short combined piles is arranged, so that the foundation settlement is reduced, the differential settlement is reduced, and the foundation treatment problem of high-fill and sea-phase soft soil is solved; the vacuum preloading method is adopted for pretreatment in advance and CFG long and short combined pile treatment, and the combination of the two forms can quickly and comprehensively improve the strength of foundation soil, reduce settlement, ensure the safety of the high-fill roadbed and accelerate the construction progress.

Drawings

FIG. 1 is a schematic view of a vacuum preloading foundation treatment structure of the present invention;

FIG. 2 is a schematic view of a foundation treatment structure of a vacuum preloading combined CFG long and short composite pile of the present invention;

FIG. 3 is a flow chart of the construction of the long and short composite piles combining vacuum preloading and CFG in the invention;

FIG. 4 is a flow chart of the CFG piling process of the present invention;

FIG. 5 is a design drawing of a vacuum preloading system according to the present invention.

In the figure: 1-short pile, 2-long pile, 3-gravel cushion (with geogrid), 4-vacuum pump, 5-sand cushion, 6-sealing membrane, 7-plastic drainage plate, 8-hand joint and 9-vacuum pipe network.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific examples described herein are intended to be illustrative only and are not intended to be limiting.

Experiment one: performing CFG pile construction method experiments in plastic-flowing and soft-plastic deep soft soil areas on the coastal areas;

firstly, leveling a field, and removing plowing soil and obstacles on an experimental ground surface; discharging the specific position of each pile according to the coordinates and marking to enable a drill rod of the long spiral drilling machine to be vertically aligned with the center of the pile position; starting a motor to drill a hole, and stopping the machine for waiting materials after the hole is drilled to a designed elevation; concrete is pumped to a feed inlet of a long spiral drilling machine by a ground pump and enters a drill rod, when the interior of the drill rod is filled with the concrete, the drill rod is pulled, and the concrete with a certain height always exists in the drill rod in the rod pulling process; when the drill rod is pulled out to be 0.5m above the designed pile top elevation, stopping pumping in the concrete; continuing pulling the rod to enable the residual concrete in the drill rod to flow into the pile hole, and after the pouring is finished, moving the drilling machine to the next pile for construction; manually matching with machinery to excavate soil between piles, and strictly prohibiting over-excavation; cutting off a pile head; after the concrete strength and the rest time meet the standard requirements, carrying out a bearing capacity test and low strain detection on the CFG pile; and (5) carrying out mattress layer construction after the pile foundation is detected to be qualified.

Through the experiment of the CFG pile construction method in the coastal flow plastic and soft plastic deep soft soil area, the test result is as follows: because the resistance provided by the soil body is too small, the bending resistance and shearing resistance of the pile body using the CFG pile are limited, the pile spacing has to be reduced, and the number of the CFG pile is increased. And along with the rigidity difference between the soil body and the pile body is larger and larger, the difference between the damage form of the foundation and the damage form of the traditional flexible pile foundation is larger, and a plurality of damage modes such as pile soil turbulence, pile body inclination, pile body lateral movement, pile body bending damage, pile body shearing and the like appear, instead of the shearing damage along the arc surface which is considered in the traditional method.

Experiment two: performing a vacuum preloading foundation treatment method experiment in a plastic-flowing and soft-plastic deep soft soil region on the coast;

sequentially laying woven cloth, a Jingba cushion layer and geotextile on the sludge at the test point; sequentially pushing and filling a sand cushion on the geotextile, drilling a drainage plate on the sand cushion to the bottom of the soft soil layer, laying a drainage channel, and burying a plate head of the drainage plate into the sand cushion; arranging a filter pipe and connecting the filter pipe with a drainage plate; digging a film pressing ditch along the periphery of the sand cushion layer, paving a sealing film, and installing a film discharging device before paving the sealing film. Arranging a membrane discharging device at the crossed position of the transverse filter tube and the longitudinal filter tube, wherein the connection between the membrane discharging device and the filter tube is required to be tight in the transverse direction and the longitudinal direction, a certain telescopic space is reserved to avoid uneven settlement damage, and after the filter tube and the membrane discharging device are connected, the filter tube and the membrane discharging device are fixedly buried by sand; installing a monitoring instrument and a vacuum pump, and pumping air in the sealing film through the vacuum pump to pre-press the foundation; and after the air exhaust prepressing reaches the unloading standard, the vacuum prepressing is unloaded, and the film pressing ditch is replaced and filled. Wherein, it includes to trade and fill the indentation membrane ditch: and draining accumulated water in the film pressing ditch, removing a soil layer in the film pressing ditch, filling soil in the film pressing ditch layer by layer, and compacting layer by layer.

The vacuum degree under the membrane of the on-site vacuum preloading test reaches more than 80kpa on average in a short time, the effective depth of the vacuum degree is larger than the buried depth of the drainage plate by about 2m, the physical and mechanical indexes after preloading are greatly improved, the soil body is basically improved to a soft plastic and plastic state from a flow plastic state, the water content is reduced, and the on-site cross plate test and the static sounding test also show that the shear strength of the soil is improved by 35-40% after vacuum preloading. And calculating according to the parameters of the soil body actually measured on site after vacuum preloading, wherein the roadbed stability and settlement with the filling height within 8m basically meet the requirements, when the filling height is more than 8m, the filling rate needs to be controlled, and after loads such as track laying, train operation and the like are added, the settlement rate after construction and in the early stage of operation do not meet the requirements.

The experiment of the vacuum preloading foundation treatment method is carried out in coastal plastic flowing and soft plastic deep soft soil areas, and the experimental result is as follows: the shear strength of the soft soil on the surface layer is improved quickly by vacuum preloading, the overall stability of the roadbed is improved, the added stress is increased along with the increase of the filling height (greater than 8 m), the settlement of the roadbed influences the depth, the filling rate is too large, the settlement of the center of the roadbed is still large, the stress is concentrated to the center of the road, the settlement rate is too large in the early period of post-construction settlement and operation, the standard requirement is not met, and therefore further settlement control measures need to be taken.

Experiment three: performing an actual work point experiment of a vacuum preloading and CFG long and short composite pile foundation treatment method on a Zhanjiang east island railway project work point in a plastic-flowing and soft-plastic deep soft soil region along the sea;

referring to fig. 1-5, on the basis of researching the existing soft soil foundation treatment mode, a foundation treatment idea of 'surface layer stability improvement and deep layer settlement adjustment' is provided, the foundation treatment idea is used for treating a foundation by combining a vacuum preloading method and a CFG (cement fly ash gravel) long and short combined pile method on a high-fill foundation on a flow-plastic and soft-plastic deep soft soil, and experiments I and II show that the shear strength improvement range of the soil treated by the vacuum preloading method is larger when the soil is softer, the reinforcement effect is more obvious, and the vacuum preloading method is more necessary to be adopted for pretreatment; for a high fill embankment larger than 8m, the requirement on the bearing capacity of the foundation is higher, and the softer the soil quality is, the larger the difference value between the bearing capacity and the actual bearing capacity of the soil layer is, which is required to be achieved by design, so that the foundation needs to be treated by adopting vacuum preloading and CFG combined piles, and the invention provides a technical scheme that:

specific examples are as follows: the method selects DDK27+500-DDK27+800 sections as experimental construction points in the plastic-flow and soft-plastic deep soft soil region along the sea of the Shanjiang east island railway, and comprises the following steps:

step 1, leveling a field: soil planting and obstacles are removed from the ground of a construction point, and the construction ground is filled and leveled by equipment;

step 2, setting a sand cushion layer 5: paving a sand cushion layer 5 on the original mud surface of the construction point; sequentially laying woven cloth, a Jingba mat layer and geotextile on the leveled original mud surface, wherein the number of the material laying layers can be properly adjusted according to the soft condition of the on-site soil, and a sand mat layer 5 with the thickness of 0.3m is laid on a construction point;

step 3, arranging a drainage plate: arranging a plastic drainage plate 7 at a construction point; drainage channel that drain bar and filter tube connection formed is whole vacuum system's aorta, the filter tube is laid and the filter tube is in direct relation with whole system's evacuation effect with the connection quality direct relation of drain bar, for guaranteeing the smooth and easy of aorta, at first, beat and establish high performance plastics drain bar 7 to soft soil layer bottom, the drain bar is regular triangle and arranges, 7 interval distances of plastics drain bar are 1.0m, beat and establish the drain bar in-process and aim at and avoid appearing the mud pumping, phenomenons such as board head pollution, plastics drain bar 7 is connected with vacuum pipe network 9 through hand type joint 8.

Step 4, burying vacuumizing equipment: a monitoring instrument and a vacuum pump 4 are installed at the construction point;

step 5, after the vacuum equipment is connected into a system, digging a sealing ditch, laying sealing protection equipment such as geotextile, geomembrane and the like;

step 6, vacuumizing: pumping air to the construction points through the vacuum pump 4 to pre-press the foundation, wherein the pre-pressing time is set to be 3 months; the pre-compaction ground of bleeding includes: vacuumizing in a trial mode, and checking whether air leakage exists or not through a monitoring instrument; and formally exhausting and prepressing the foundation, and recording the vacuum degree. The arrangement of monitoring instruments such as a pore water pressure instrument, a vacuum degree measuring head and the like is required to be carried out before the filter tube is bound so as to avoid influencing the laying of a sealing film 6, a vacuum pump 4 is arranged along the boundary of a reinforced area, a rubber sealing gasket is adopted to be added with a sealing adhesive, the vacuum pump 4 and the exposed parts of a communicating pipeline and the like thereof need to be subjected to anticorrosion and antirust measures, debugging and quality inspection are carried out after installation, air extraction preparation is carried out, air extraction is carried out before formal vacuum extraction, a construction manager needs to check the operation condition of each pump and the sealing condition of a film during the air extraction to check whether the air leakage condition exists or not, glue is used for adhesion and repair in time, and water is covered in time after the detection and repair are finished; in the stage of vacuum test, the vacuum pressure under the film is required to be more than 80Kpa, if the vacuum pressure is lower than the value, the reason is required to be found immediately, the positions with air leakage are carefully checked on the film surface and the sealing ditch, the parameters such as the vacuum pressure, the sedimentation amount and the like are observed from the beginning of the vacuum test, when the vacuum degree reaches 85Kpa, the inspection is carried out in time, after the inspection is qualified, the water covering is carried out, the normal air suction is carried out, the operation lasts for 3 months all the time, the requirement that the vacuum degree under the film is required to be more than 85KPa stably, and after the unloading standard is met, the air suction and the unloading are stopped, and the corresponding effect inspection is carried out;

step 7, driving a CFG pile: driving a CFG pile on the construction point;

setting coordinates, arranging CFG piles in a square shape, enabling the interval between two adjacent CFG piles to be 2.0m, discharging the specific position of each pile according to the coordinates, marking, measuring the verticality of the hole by adopting a method of hanging a ball on a drill frame, or adopting a verticality regulator carried by a drilling machine to ensure the verticality of a drill rod, wherein the diameter of each CFG pile is 0.5m, a long pile 2 is arranged in the middle of the cross section of a roadbed, CFG piles with short piles 1 on two sides are arranged, starting a motor to drill in, filling concrete into the piles by adopting a ground pump, and performing a composite foundation load test and low-strain pile body integrity test on the CFG piles after judging that the concrete strength of the CFG piles reaches 70% through a standard maintenance test block compressive strength test;

step 8, paving a gravel cushion layer: pushing and filling a gravel cushion layer 3 on the pile top through small equipment, burying three layers of geogrids in the gravel cushion layer 3, wherein the paving and filling height is 0.5 m;

step 9, filling the roadbed: and filling the roadbed on the gravel cushion layer 3 according to the design, filling the roadbed to the designed elevation, and carrying out corresponding observation.

The implementation result of the project is as follows: the vacuum preloading and long and short combined pile foundation treatment is tried for the first time in the project of Zhanjiang east island railway, the shear strength of the surface layer flow-plastic and soft-plastic marine soft soil can be rapidly improved through the vacuum preloading, the stability of the foundation is improved, the effect of the soil is exerted to the maximum extent, and the number of CFG piles is reduced; by arranging the long piles 2 in the middle and the short piles 1 on two sides, the cross section arrangement form of the long and short combined piles is adapted to the upper load distribution, so that the shear resistance of surface soil is further improved, foundation settlement is reduced, and differential settlement is reduced; the combination of the two forms can quickly and comprehensively improve the strength of the foundation soil, reduce the settlement, ensure the safety of the high fill roadbed and accelerate the construction progress; the design idea of high fill and sea phase soft soil foundation treatment is summarized by combining a foundation treatment method of CFG long and short composite piles with vacuum preloading, namely: the surface layer is improved and stabilized, and the deep layer is used for regulating the sedimentation.

The direct-discharging vacuum preloading method is adopted, the whole construction period is controlled to be 6 months, the requirement of the construction period is met, meanwhile, the pile length on two sides is shortened due to the optimized layout of the piles, the stability of the surface layer is improved by the vacuum preloading method, the pile spacing is increased, and therefore, the investment is saved by about 15% compared with that of the equal-length piles.

The above description is only a preferred embodiment of the present invention, and should not be taken as limiting the invention, and any modifications, equivalents, improvements, etc. made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims (5)

1. A foundation treatment method combining vacuum preloading with CFG long and short composite piles is used for high-fill foundations on deep soft soil with flow molding and soft molding, and is characterized by comprising the following steps:

step 1, leveling a field: ploughing and planting soil on the ground of a construction site, removing barriers and leveling;

step 2, paving a sand cushion layer: laying a sand cushion layer with the thickness of 0.3m on a construction point;

step 3, arranging a drainage plate: arranging a plastic drainage plate at the construction point;

step 4, burying vacuumizing equipment: a monitoring instrument and a vacuum pump are installed at the construction point;

step 5, laying sealing protection equipment: after the vacuum equipment is connected into a system, excavating a sealing ditch, and laying geotextile and geomembrane sealing protection equipment;

step 6, vacuumizing: pumping air to the construction points through the vacuum pump to pre-press the foundation, wherein the pre-pressing time is set to be 3 months;

step 7, driving a CFG pile: driving CFG long and short piles on the construction points, and pouring reinforced concrete pile caps after the bearing capacity of the CFG piles meets the requirements;

step 8, paving a gravel cushion layer: three layers of soil grids are clamped in the cushion layer;

step 9, filling a foundation: filling the roadbed to a designed elevation, and carrying out corresponding observation;

the cross section arrangement of long and short combined piles in the CFG piles is adapted to the upper load distribution, the middle point of a roadbed slope is taken as a boundary point, different pile lengths are adopted on two sides, and the distance between two adjacent CFG piles is 3-5 times of the pile diameter;

the CFG piles are divided into long piles and short piles, the long pile is arranged in the middle of the embankment, and the short piles are arranged on two sides of the embankment; the diameter of the CFG pile is 0.5m, the length of the long pile of the CFG pile is determined according to settlement and bearing capacity calculation, and the length is not less than 15 m; the short pile is long enough to penetrate through the soft soil layer;

the surface layer of the foundation soil is pretreated in advance by adopting a vacuum preloading method, so that the shear strength of the surface layer of the foundation soil is improved, the rigidity of the soil body is correspondingly increased, and a hard shell layer is formed on the surface layer, so that the stability of the surface layer is improved, and the shear resistance of the soil is exerted to the maximum extent;

the cross section arrangement form of the long and short composite piles is adapted to the upper load distribution, so that the shear resistance of surface soil is further improved;

vertical rigidity is adjusted by arranging different pile lengths on the cross section, so that load is transferred to a deep layer, soil body settlement and uneven settlement are reduced, and soil body stability is enhanced.

2. The foundation treatment method of the vacuum preloading combined CFG long and short combined pile as claimed in claim 1, wherein: the height of the gravel cushion layer is 0.5 m.

3. The foundation treatment method of the vacuum preloading combined CFG long and short combined pile as claimed in claim 1, wherein: the step of driving the CFG pile comprises the following steps:

step 1, measuring and marking: marking the specific position of each pile on the construction point;

step 2, drilling: starting a motor to drill;

step 3, pumping materials: pumping the material into the drill hole through a ground pump, vibrating and filling the material;

step 4, pile foundation detection: carrying out bearing capacity test and low-pressure-change detection on the CFG pile through a detection instrument;

step 5, mattress layer bedding: and after the pile foundation is detected to be qualified, paving a broken stone mattress layer on the pile foundation.

4. The foundation treatment method of the vacuum preloading combined CFG long and short combined pile as claimed in claim 1, wherein: arranging the plastic drainage plates in a regular triangle, wherein the spacing distance between the plastic drainage plates is 0.8-1.5m, and the sand cushion laying process comprises the following steps:

step 1: sequentially laying woven cloth, a Jingba cushion layer and geotextile on the construction point;

and 2, step: and pushing and filling a sand cushion layer on the geotextile.

5. The foundation treatment method of the vacuum preloading combined CFG long and short combined pile as claimed in claim 1, wherein: the step of evacuating includes:

step 1: trial vacuumizing the construction point;

step 2: collecting data of the construction points through the monitoring instrument, and checking whether air leakage exists or not;

and step 3: and formally pumping and prepressing the construction points, and recording vacuum degree data through the monitoring instrument.

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