CN117905050A - Karst area bored pile foundation construction method - Google Patents

Abstract

The application provides a construction method of a bored pile foundation in a karst region, which comprises the following steps: s1, measuring and lofting, S2, manufacturing and burying a steel pile casing, S3, positioning a drilling machine, S4, drilling a hole by the drilling machine, S5, positioning and measuring a karst cave, S6, drilling a hole by the drilling machine, S7, clearing and checking, S8, manufacturing, transporting and installing a reinforcement cage, S9, a downcomer, S10, clearing a hole secondarily, S11 and pouring underwater concrete. The construction method of the bored pile foundation in the karst area efficiently solves the problem that the concrete overfilling amount is difficult to control because the karst cave size cannot be judged in the pile foundation, and has the advantages of high pile forming quality, small influence on natural environment, high green construction level and greatly shortened construction period.

Description

A method for constructing bored pile foundation in karst areas

Technical Field

The present application relates to the technical field of bored piles, and in particular to a bored pile foundation construction method in karst areas.

Background technique

Bored cast-in-place piles are currently the most widely used type of pile foundation, with a high degree of mechanization, large single pile bearing capacity and strong pile body rigidity.

When encountering a karst cave during drilling construction, stone flakes, concrete, clay, etc. are often thrown into the cave to form a good protective wall together with the karst filling. However, this method still has the following problems:

In the construction of bored pile foundations in karst areas, it is impossible to judge the size of the cave, which makes it difficult to control the over-injection of concrete, resulting in poor pile quality, a great impact on the natural environment, and a low level of green construction.

Summary of the invention

The present application provides a bored pile foundation construction method in karst areas, which effectively solves the problem of difficulty in controlling the over-injection of concrete due to the inability to determine the size of the karst cave in the pile foundation. The pile quality is high, the impact on the natural environment is small, the green construction level is high, and the construction period is greatly shortened.

The present application provides a bored pile foundation construction method in karst areas, comprising: S1, surveying and setting out, using instruments to accurately locate the position of the pile hole, pulling cross-line nails according to the pile positioning points to place four control piles, and using the four control piles as a reference to control the buried position of the steel casing and the accurate positioning of the drilling rig; S2, steel casing production and burying, using steel plates to roll and shape the steel casing, so that the inner diameter of the steel casing is 0.2 to 0.4 m larger than the designed pile diameter; after the steel casing is buried, the top of the steel casing is controlled to be 0.3 m higher than the original ground for drill bit positioning and pile hole protection; S3, drilling rig positioning, when the drilling rig is in place, check in advance whether the performance status of the drilling rig is good to ensure that the drilling rig works normally; accurately determine the position of the drilling rig through the measured pile position, and ensure that the drilling rig is stable, and manually roughly level the drill rod to ensure that it is basically vertical, Use the automatic control system to adjust the drill rod to keep it in a vertical state; S4, the drill rig drills a hole, sets up a mud pool on site, and then carries out drilling operations; S5, cave positioning and measurement, preliminary positioning is carried out according to the location of the cave in the geological survey report and the drilling depth, and the ground mud is blocked from flowing into the hole; when drilling to the specific location of the cave, the mud surface in the hole will drop rapidly. At this time, the hole depth position can be approximated to the top elevation of the side wall cave. According to the height of the mud surface drop, the minimum capacity of the cave is calculated; then, according to the elevation of the cave bottom shown in the geological survey report and the drilled rock sample, the approximate bottom elevation of the cave is determined, and the cross-sectional size of the cave on the side wall of the hole is approximately determined; S6, the drill rig drills to the final hole; S7, hole cleaning and acceptance, wet hole cleaning adopts the slurry replacement method to clean the hole, and it is necessary to pay attention to keeping the hole when cleaning the hole Internal water level; the dry hole sediment is cleaned with a flat-head drill bit after settling; S8, production, transportation and installation of steel cage. If the depth of the cave exceeds 4m or the volume of the cave cannot be determined, a quick-closing net is fixed on the outer side of the steel cage corresponding to the cave side wall. The size of the quick-closing net is larger than the size of the cave entrance; S9, lowering the guide tube. When installing the guide tube, measure the actual length of the guide tube section by section and number them in sequence, keep records, and check that there is no broken thread at both ends of each section of the guide tube; S10, secondary hole cleaning. After the guide tube is lowered into place, the sediment at the bottom of the pile foundation is immediately detected. If the sediment thickness does not meet the design requirements, air lift reverse circulation is used for secondary hole cleaning. After the hole cleaning is completed and the inspection is qualified, the mud suction elbow is immediately removed; S11, underwater concrete pouring.

In the optional technical solution of the above-mentioned bored pile foundation construction method in karst areas, in step S2, stiffening rings are welded to the periphery of the top and bottom ports of the steel casing.

In the above optional technical solution of the bored pile foundation construction method in karst areas, in step S3, a rotary drilling rig is used for drilling.

In the above-mentioned optional technical scheme of the bored pile foundation construction method in karst areas, in step S4, for rock and soil layers with good adhesion, a dry or clean water drilling process is adopted without using mud wall protection, and the gravel and soil within 0.5m of the hole mouth are cleaned; for loose and easily collapsed strata or the bottom layer with groundwater distribution, a static mud wall drilling process is adopted.

In the above-mentioned optional technical solution of the bored pile foundation construction method in karst areas, in step S4, the penetration speed is controlled according to the geological conditions during the drilling process: when drilling from a hard stratum to a soft stratum, the drilling speed is appropriately accelerated; when the soft stratum becomes a hard stratum, the penetration speed is slowed down; in strata that are prone to shrinkage, the number of hole sweeps is appropriately increased to prevent shrinkage; high-speed drilling is used for hard plastic layers to improve drilling efficiency; for sand layers, slow drilling is used with a slow speed and the mud density and viscosity are appropriately increased.

In the above-mentioned optional technical solution of the bored pile foundation construction method in karst areas, in step S7, the hole cleaning is performed twice. The first hole cleaning is performed after the drilling depth reaches the designed depth, and the second hole cleaning is performed after the steel cage is installed in place and before the guide tube is lowered.

In the optional technical solution of the bored pile foundation construction method in karst areas described above, in step S8, in order to prevent the steel cage from being deformed during transportation and hoisting, the hoisting points are reinforced and the stiffening bars are encrypted.

In the above-mentioned optional technical solution of the bored pile foundation construction method in karst areas, in step S8, a quick-closing net with folded edges at the top and bottom is used, and after the steel cage is lowered into the hole, the folded edges are made close to the hole wall; a quick-closing net with a top and bottom at least 50 cm larger than the size of the side wall cave is used; the quick-closing net is tied with U-shaped steel bars, and the U-shaped steel bars are denser, and the U-shaped steel bars are evenly arranged along the quick-closing net.

In the above-mentioned optional technical solution of the bored pile foundation construction method in karst areas, in step S11, the concrete delivery pipeline and container are sprinkled with water to moisten them before pouring concrete.

The present application provides a method for constructing bored pile foundations in karst areas, wherein step S8 is a key step. A quick-closing net is fixedly wrapped around the outer side of a steel cage corresponding to a side wall of a karst cave, wherein the size of the quick-closing net is larger than the size of the cave opening. This effectively solves the problem of difficulty in controlling the amount of over-pouring of concrete due to the inability to judge the size of the cave in the pile foundation. The piles are of high quality, have little impact on the natural environment, have a high level of green construction, and greatly shorten the construction period.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

FIG1 is a flow chart of a bored pile foundation construction method in a karst area provided by an embodiment of the present application;

Figure 2 is a diagram showing the steps for determining the location and size of a cave;

FIG3 is a schematic diagram of the installation of a steel cage with a quick-closing mesh;

FIG. 4 is a schematic diagram of the concrete pouring process.

The above drawings have shown clear embodiments of the present application, which will be described in more detail later. These drawings and text descriptions are not intended to limit the scope of the present application in any way, but to illustrate the concept of the present application to those skilled in the art by referring to specific embodiments.

Detailed ways

Exemplary embodiments will be described in detail herein, examples of which are shown in the accompanying drawings. When the following description refers to the drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The implementations described in the following exemplary embodiments do not represent all implementations consistent with the present application. Instead, they are merely examples of devices and methods consistent with some aspects of the present application as detailed in the appended claims.

As described in the background art, bored piles are currently the most widely used type of pile foundation, which has a high degree of mechanization, a large single pile bearing capacity, and a strong pile body rigidity.

When encountering a karst cave during drilling construction, stone flakes, concrete, clay, etc. are often thrown into the cave to form a good protective wall together with the karst filling. However, this method still has the following problems:

In the construction of bored pile foundations in karst areas, it is impossible to judge the size of the cave, which makes it difficult to control the over-injection of concrete, resulting in poor pile quality, a great impact on the natural environment, and a low level of green construction.

In response to the above-mentioned technical problems, an embodiment of the present application provides a method for constructing bored pile foundations in karst areas, by fixing and wrapping a quick-closing net on the outer side of the steel cage corresponding to the side wall of the cave, thereby blocking the leakage of the cave, thereby solving the technical problems that the size of the cave cannot be judged during the construction of bored pile foundations in karst areas, resulting in difficulty in controlling the over-pouring of concrete, poor pile quality, great impact on the natural environment, and low level of green construction.

The technical solution of the present application and how the technical solution of the present application solves the above-mentioned technical problems are described in detail below with specific embodiments. The following specific embodiments can be combined with each other, and the same or similar concepts or processes may not be repeated in some embodiments. The embodiments of the present application will be described below in conjunction with Figures 1-4.

The present application provides a method for constructing a bored pile foundation in a karst area, comprising the following steps:

S1. Measurement and layout. Please refer to FIG1. Use an instrument to accurately locate the position of the pile hole. In this embodiment, a total station can be used. Pull crosshairs and place four control piles according to the pile positioning points. The four control piles are used as a reference to control the buried position of the steel casing and the accurate positioning of the drilling rig. The control piles need to be well protected to prevent them from being disturbed during the construction process.

S2. Steel casing production and burial, please refer to Figure 1, steel casing is made of rolled steel plate. Specifically, the steel casing can be made of 6mm steel plate, so that the inner diameter of the steel casing is 0.2m~0.4m larger than the designed pile diameter; stiffening rings are welded on the outer periphery of the top and bottom ports of the steel casing; when installing the steel casing, the drill operator uses the reamer to expand the pile hole, and then presses the steel casing into the designed elevation with a high-torque drill bit. Before and after the casing is pressed in, the vertical position of the steel casing is adjusted by a precise level against the steel casing; after the steel casing is buried, the top of the steel casing is controlled to be 0.3m higher than the original ground for drill bit positioning and pile hole protection.

S3. The drilling rig is in place. Please refer to FIG. 1 . Specifically, in this embodiment, a rotary drilling rig can be used for drilling. When the drilling rig is in place, check in advance whether the performance of the drilling rig is good to ensure that the drilling rig works normally. The position of the drilling rig is accurately determined by measuring the pile position, and the drilling rig is ensured to be stable. After roughly leveling manually to ensure that the drill rod is basically vertical, the automatic control system is used to adjust the drill rod to maintain a vertical state.

S4. The drilling rig drills into a hole. Please refer to Figure 1. A mud pool is set up on site, and then the drilling operation is carried out.

① Mud preparation

For rock and soil layers with good adhesion, dry or clean water drilling technology is adopted without mud wall protection, and the gravel and soil within 0.5m of the hole mouth are cleaned up; for loose and easily collapsed strata or the bottom layer with groundwater distribution, static mud wall drilling technology is adopted.

The mud pool includes a sedimentation tank for slurry return and a mud storage tank, which is generally 1.5 to 2.0 times the volume of the borehole. Plastic sheets should be laid on the bottom and around the mud pool or other sealing measures should be taken to prevent the mud from flowing out.

Specifically, there are two types of equipment for preparing mud: mud mixer and hydraulic mixer. When using clay powder to make mud, hydraulic mixer is preferred; when using bentonite to make mud, mud mixer is preferred.

After slurry making, all performance indicators are tested. During the drilling process, the mud density and sand content are checked at any time, and the mud test record form is filled in. Attention is paid to geological changes at any time, and the mud performance indicators are adjusted at any time according to the changes in geological conditions to ensure that all indicators of the mud meet the specifications.

A recovery mud pool is set up at the drilling construction site to recycle the wall protection mud. After sedimentation and purification, the mud is transported to the slurry storage pool, where it is further processed (adding appropriate amount of soda ash and CMC to improve mud properties) and reused after passing the test.

②Drilling operation

When drilling, the weight and pressure of the drill bucket are used as the drilling power. The short bar column of the one-time footage shows the current drilling depth of the drill bit, the long bar column dynamically shows the movement position of the drill bit, and the number of the hole depth shows the total depth of the hole. When the drill bucket is squeezed and filled with drill cuttings, it is lifted out of the ground, and the rotary operating handle is operated to turn the machine to the position in the direction of the earthwork transport vehicle. The loader is used to load the drill cuttings into the earthwork vehicle and transport them to an appropriate location for disposal.

During the drilling process, the penetration speed is controlled according to the geological conditions: when drilling from a hard formation to a soft formation, the drilling speed should be appropriately increased; when the soft formation becomes a hard formation, the drilling speed should be reduced; in the formation that is easy to shrink, the number of hole sweeps should be appropriately increased to prevent shrinkage; for the hard plastic layer, high-speed drilling is used to improve the drilling efficiency; for the sand layer, slow drilling is used and the mud density and viscosity should be appropriately increased.

As drilling progresses, mud continuously flows into the pile hole, ensuring that the water level in the pile hole is consistent with the water level outside the steel casing.

S5. Cave positioning and measurement. Please refer to Figures 1 and 2. Preliminary positioning is carried out based on the location of the cave in the geological survey report and the drilling depth, and the mud on the ground is blocked from flowing into the hole. When drilling to the specific location of the cave, the mud surface in the hole will drop rapidly. At this time, the hole depth can be approximated to the top elevation of the side wall cave. According to the height of the mud surface drop, the minimum capacity of the cave is calculated. Then, the approximate bottom elevation of the cave is determined based on the elevation of the cave bottom shown in the geological survey report and the drilled rock samples, and the cross-sectional size of the cave on the side wall of the hole is approximately determined.

S6. After the drilling rig has drilled to the final hole, refer to Figure 1. After the hole has reached the designed depth, verify the geological conditions. Compare the drill slag with the geological columnar diagram and the rock sample of the first pile acceptance seal to verify whether the geological conditions meet the design requirements; if they do not match the survey and design data, confirm and deal with them in time; if they meet the design requirements, immediately check the hole depth, hole diameter, and hole shape.

A hole gauge is used to test the hole diameter, hole wall, verticality and other inspection items.

Hole depth and sediment thickness detection: After the hole is formed, use a measuring rope to measure the hole depth L2 according to the drilling depth L1 displayed on the rotary drill interface. Compare the two. If L2 is less than L1, replace the bottom cleaning drill bit, clean the bottom, and re-measure the hole depth.

After confirming that the design and verification requirements are met, acceptance is carried out. After passing the acceptance, the hole is cleaned immediately.

S7. Hole cleaning and acceptance, please refer to Figure 1. Wet hole cleaning adopts slurry replacement method to clean the hole. When cleaning the hole, it is necessary to pay attention to maintaining the water level in the hole; dry hole sediment is cleaned with a flat-head drill bit after sedimentation. The purpose of hole cleaning is to remove drilling debris and sedimentation layer, reduce the sediment thickness at the bottom of the pile foundation, and prevent excessive sediment from remaining at the bottom of the pile to reduce the bearing capacity of the pile.

The hole cleaning is carried out in two times. The first hole cleaning is carried out after the drilling depth reaches the designed depth. The first hole cleaning should meet the requirements of the specifications, otherwise the steel cage will not be lowered.

After the steel cage is installed in place, the hole will be cleaned for the second time before the guide tube is lowered. The hole cleaning before pouring concrete must meet the following standards: the mud discharged or extracted from the hole has no granular feeling when touched, the mud specific gravity is greater than 1.03, the sand content is less than 2%, and the viscosity is 17 to 20s; the thickness of the sediment at the bottom of the pile foundation before pouring underwater concrete is not more than 5cm.

Measurement of sediment at the bottom of pile foundation: Use a measuring rope with a flat weight suspended at the front end to measure the hole depth around the hole wall. There should be no less than 4 measuring points. The difference between the two bottom elevations is the sediment thickness. Use a steel ruler to calibrate the measuring rope before each measurement.

S8. For the production, transportation and installation of the steel cage, please refer to Figures 1 and 3. The steel cage is produced according to the reinforcement diagram and divided into two sections according to the depth of the pile hole. The first section is a fixed-length steel cage (combined with the lifting height of the crane), and the second section is determined by the actual depth of the pile. Ensure that the position, spacing and number of steel bars meet the requirements of the drawings and specifications; the main bars are welded staggered so that the number of joints within 35d (d is the diameter of the steel bar) does not exceed half of the total number of steel bars.

The joints of the spiral hoops or stiffeners of the steel cage are welded, the connection between the stiffeners and the main bars is spot welding, and the connection between the spiral hoops and the main bars is tied. In order to prevent the steel cage from deforming during transportation and hoisting, the lifting points are strengthened and the stiffeners are encrypted.

If the depth of the cave exceeds 4m or the size of the cave cannot be determined, a quick-closing net is fixedly wrapped on the outside of the side wall of the cave corresponding to the steel cage, and the size of the quick-closing net is larger than the size of the cave entrance; a quick-closing net with folded edges on the top and bottom is used, and after the steel cage is lowered into the hole, the folded edges are made close to the hole wall; a quick-closing net with a top and bottom that is at least 50cm larger than the size of the side wall cave is used; U-shaped steel bars are used to tie the quick-closing net, and the U-shaped steel bars are denser, and the U-shaped steel bars are evenly arranged along the quick-closing net.

In order to ensure the thickness of the protective layer of the steel cage, U-shaped reinforcement supports are welded in advance on the outside of the steel cage, with a spacing of 2m along the length of the pile and no less than 4 locations on the horizontal circumference.

After the steel cage is manufactured in sections and accepted at the processing site, it is transported to the hole mouth and hoisted into the hole. The two sections of the steel cage are connected by single-sided welding, and the welding length is 10d (d is the diameter of the steel bar). During transportation and hoisting, it is necessary to prevent deformation, and the placement must be aligned with the hole position to avoid collision with the hole wall. The steel cage top must be fixed immediately after reaching the design elevation to prevent the steel cage from floating when pouring concrete. After the entire steel cage is manufactured, it is transported to the construction site in sections in the steel processing yard. Before installing the steel cage, remove the adhering dirt and oil stains to ensure that the steel bars and concrete are tightly bonded.

S9. For lowering the conduit, please refer to Figures 1 and 4. When installing the conduit, measure the actual length of each section and number them in sequence. Keep records to control the depth of the buried pipe during concrete pouring. Pay attention to whether the rubber ring is installed and check whether there are any broken threads at both ends of each section of the conduit to avoid water entering the conduit during the pouring process.

S10. Secondary hole cleaning. Please refer to Figure 1. After the guide tube is lowered into place, immediately check the sediment at the bottom of the pile foundation. If the sediment thickness does not meet the design requirements, use air lift reverse circulation for secondary hole cleaning. During the circulation, pay attention to maintaining the mud head and adding high-quality mud to prevent collapse. After the hole cleaning is completed and the inspection is qualified, immediately remove the mud suction elbow.

S11, underwater concrete pouring, please refer to Figure 1 and Figure 4, before pouring concrete, sprinkle water on the concrete delivery pipeline and container to moisten it, then install water-proof facilities in the filling duct, and after the storage hopper is full of concrete, start pouring underwater concrete.

The quantity of the first batch of poured concrete must be able to meet the needs of the initial buried depth of the conduit of at least 1m and the height of the bottom of the filling conduit. When sealing the bottom, the depth of the conduit buried in the concrete is at least 1m. The volume of the first batch of concrete is determined according to the pile diameter, the buried depth of the conduit and the volume of concrete in the conduit. The mixed concrete is transported to the pile foundation mouth by a concrete transport vehicle and injected into the hopper lifted by the drilling rig. The volume of concrete in the vehicle is about 8m³, which is commanded by one person. After both parties are ready, the watertight plug and the valve are opened at the same time to seal the bottom. The isolation plug is made of steel plate, which is pulled by fine steel wire rope and lifted by a tower crane or a car crane.

① After the first batch of concrete falls, the concrete is poured continuously. During the pouring process, the buried depth of the conduit is controlled at 2 to 6 meters.

②During the process of pouring concrete, use a hammer with a weight of not less than 4kg to frequently measure the height of the concrete surface in the hole. After the guide tube reaches a certain buried depth, quickly dismantle the guide tube step by step, and detect the height of the concrete surface in the hole before lifting the guide tube each time. The measuring rope used for measurement should be checked with a steel ruler once before and after each pile pouring to avoid errors.

③ The top elevation of the pile to be poured should be controlled to be at least 0.3m higher than the design elevation to ensure the strength of the concrete. The excess part of the pile head should be chiseled off to ensure that there is no loose layer on the pile head.

④ After pouring concrete, clean and inspect the conduit, funnel, etc. in time for use in the next hole.

⑤ Before pouring underwater concrete, the "Related Inspection Form" for checking the conditions of bored piles and steel cages should be filled in, and during the process of pouring underwater concrete, the "Underwater Concrete Pouring Record" should be filled in.

Those skilled in the art will readily come up with other embodiments of the present application after considering the specification and practicing the technical solutions disclosed herein. The present application is intended to cover any variations, uses or adaptations of the present application, which follow the general principles of the present application and include common knowledge or customary technical means in the art that are not disclosed in the present application. The specification and examples are intended to be exemplary only, and the true scope and spirit of the present application are indicated by the claims.

It should be understood that the present application is not limited to the precise structures that have been described above and shown in the drawings, and that various modifications and changes may be made without departing from the scope thereof. The scope of the present application is limited only by the appended claims.

Claims (9)

1. The construction method of the bored pile foundation in the karst area is characterized by comprising the following steps of:

s1, measuring lofting, precisely positioning the position of a pile hole by adopting an instrument, pulling a cross line nail according to pile positioning points to put four control piles, and controlling the burying position of a steel pile casing and the accurate positioning of a drilling machine by taking the four control piles as references;

S2, manufacturing and burying a steel pile casing, and rolling and forming the steel pile casing by adopting a steel plate to ensure that the inner diameter of the steel pile casing is 0.2-0.4 m larger than the diameter of the designed pile; after the steel pile casing is embedded, controlling the top of the steel pile casing to be 0.3m higher than the original ground for positioning a drill bit and protecting a pile hole;

S3, positioning the drilling machine, and checking whether the performance state of the drilling machine is good in advance when the drilling machine is positioned, so that the drilling machine is ensured to work normally; accurately determining the position of the drilling machine through the tested pile position, ensuring the stability of the drilling machine, and adjusting the drilling rod to keep a vertical state by utilizing an automatic control system after the drilling rod is basically vertical through manual rough leveling;

s4, drilling holes by a drilling machine, setting a mud pit on site, and then carrying out drilling operation;

S5, locating and measuring karst cave, primarily locating according to the position of the karst cave reported by the geological survey and the drilling depth, and blocking the inflow of ground mud into the hole; when the hole is drilled to a specific position of the karst cave, the mud surface in the hole can rapidly descend, at the moment, the hole depth position can be approximate to the elevation of the top of the karst cave on the side wall, and the minimum capacity of the karst cave is calculated according to the descending height of the mud surface; determining the approximate bottom elevation of the karst cave according to the karst cave bottom elevation displayed by the geological survey report and the drilled rock sample, and approximately determining the cross section size of the karst cave on the side wall of the hole;

S6, drilling a final hole by a drilling machine;

S7, hole cleaning and acceptance checking, wherein a wet hole cleaning method is adopted for hole cleaning, and the water level in the hole needs to be kept when the hole is cleaned; the dry hole sediment is cleaned by a flat bit after sediment is adopted;

s8, manufacturing, transporting and installing the reinforcement cage, and if the karst cave depth exceeds 4m or the size of the karst cave cannot be judged, fixedly wrapping a quick and easy closing-up net on the outer side of the reinforcement cage corresponding to the side wall of the karst cave, wherein the size of the quick and easy closing-up net is larger than the size of a hole opening of the karst cave;

S9, descending the guide pipe, taking the actual length of the guide pipe section by section and numbering the guide pipe in sequence when the guide pipe is installed, making a record, and checking that threads at two ends of each guide pipe section have no broken threads;

s10, secondary hole cleaning is carried out, sediment detection is carried out immediately at the bottom of the pile foundation after the guide pipe is lowered in place, if the sediment thickness does not meet the design requirement, gas lift reverse circulation secondary hole cleaning is adopted, and after the hole cleaning is finished and the inspection is qualified, the suction elbow is removed immediately;

s11, pouring underwater concrete.

2. The method for constructing a pile foundation in a karst region according to claim 1, wherein in step S2, stiffening rings are welded to the peripheries of the top and bottom ports of the steel casing.

3. The construction method of a bored pile foundation in a karst region according to claim 1, wherein in step S3, drilling is performed using a rotary drilling machine.

4. The construction method of a bored pile foundation in a karst region according to claim 1, wherein in step S4, for a rock layer with good cohesiveness, a dry or clean water drilling process is adopted, a slurry retaining wall is not used, and crushed stone soil with an orifice in a range of 0.5m is cleaned; for a loose stratum which is easy to collapse or a bottom layer with underground water distribution, a static mud wall protection drilling process is adopted.

5. The construction method of a bored pile foundation in a karst region according to claim 1, wherein in step S4, a footage speed is controlled according to geological conditions during the boring process: when drilling from a hard stratum to a soft stratum, the drilling speed is properly increased; when the soft stratum is changed into a hard stratum, the speed is reduced and the speed is slowly increased; in the stratum easy to shrink, the hole sweeping times are properly increased to prevent the shrinkage; drilling the hard plastic layer at a high rotating speed to improve the drilling efficiency; for sand formations, slow drilling at slow speeds is used and the mud weight and viscosity are increased appropriately.

6. The construction method of a bored pile foundation in a karst area according to claim 1, wherein in step S7, hole cleaning is performed twice, the first hole cleaning is performed after the depth of the drilled hole reaches a designed depth, and the second hole cleaning is performed before a guide pipe is lowered after a reinforcement cage is installed in place.

7. The construction method of a bored pile foundation in a karst area according to claim 1, wherein in step S8, reinforcement is applied to the hoisting point to strengthen the reinforcement in order to prevent deformation of the reinforcement cage during transportation and hoisting.

8. The construction method of a bored pile foundation in a karst area according to claim 1, wherein in step S8, a fast-easy-closing net with folded edges at the top and bottom is used, and after a reinforcement cage is lowered into a hole, the folded edges are tightly attached to the wall of the hole; a quick and easy closing-up net with the top and the bottom being at least 50cm larger than the karst cave size of the side wall is used; and binding the quick and easy closing-up net by using the U-shaped steel bars, encrypting the U-shaped steel bars, and uniformly distributing the U-shaped steel bars along the quick and easy closing-up net.

9. The method of construction of a bored pile foundation in a karst region according to claim 1, wherein in step S11, the concrete conveying pipe and the container are watered and wetted before the concrete is poured.