CN117905050A - Karst area bored pile foundation construction method - Google Patents
Karst area bored pile foundation construction method Download PDFInfo
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- CN117905050A CN117905050A CN202410320807.7A CN202410320807A CN117905050A CN 117905050 A CN117905050 A CN 117905050A CN 202410320807 A CN202410320807 A CN 202410320807A CN 117905050 A CN117905050 A CN 117905050A
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- 230000002787 reinforcement Effects 0.000 claims abstract description 41
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- 238000013461 design Methods 0.000 claims description 10
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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
Technical Field
The application relates to the technical field of bored piles, in particular to a construction method of a bored pile foundation in a karst area.
Background
The bored pile is a pile foundation type which is widely applied at present, the degree of mechanization is high, the single pile bears large load, and the pile body rigidity is strong.
When a karst cave is encountered in the drilling construction process, the karst cave is often filled with rubble, concrete, clay and the like, and a good retaining wall is formed together with karst filler. However, this method still has the following problems:
The karst area drilling pile foundation construction can not judge the karst cave size and the concrete overfilling amount is difficult to control, the pile forming quality is poor, the influence on the natural environment is large, and the green construction level is low.
Disclosure of Invention
The application provides a construction method of a bored pile foundation in a karst area, which efficiently solves the problem that the concrete overfilling amount is difficult to control because the karst cave size cannot be judged in a pile foundation, has high pile forming quality, small influence on natural environment, high green construction level and greatly shortened construction period.
The application provides a construction method of a bored pile foundation in a karst region, which comprises the following steps: 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.
In the above-mentioned alternative technical scheme of a karst area bored pile foundation construction method, in step S2, stiffening rings are welded on the top and bottom port peripheries of the steel casing.
In the above-mentioned alternative technical scheme of a karst area bored pile foundation construction method, in step S3, a rotary drilling rig is used to drill holes.
In the above-mentioned alternative technical scheme of a karst area bored pile foundation construction method, in step S4, for the good cohesiveness rock soil layer, adopt the dry-type or clear water to drill into the process, do not use the mud to protect the wall, clean up the broken stone earth in the range of 0.5m of the orifice; 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.
In the above-mentioned alternative technical scheme of a karst area bored pile foundation construction method, in step S4, the rate of approach 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 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.
In the above-mentioned alternative technical scheme of a karst area bored pile foundation construction method, in step S7, clear the hole is carried out twice, clear the hole for the first time after the drilling depth reaches the design depth, clear the hole for the second time before the pipe is put down after the reinforcement cage is installed in place.
In the above-mentioned alternative technical scheme of a karst area bored pile foundation construction method, in step S8, in order to prevent the steel reinforcement cage from deforming during transportation and hoisting, the hoisting point position is reinforced, and the stiffener is encrypted.
In the above-mentioned alternative technical scheme of a karst area bored pile foundation construction method, in step S8, use the top, bottom have fast easy closing up net of the hem, after the reinforcement cage is lowered into the hole, make the said hem hug closely the hole wall; 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.
In the above-mentioned alternative technical scheme of a karst area bored pile foundation construction method, in step S11, before pouring concrete, the concrete conveying pipeline and the container are sprayed with water for wetting.
According to the construction method for the bored pile foundation in the karst area, the step S8 is a key step, the quick and easy closing-up net is fixedly wrapped on the outer side of the side wall of the karst cave corresponding to the reinforcement cage, the size of the quick and easy closing-up net is larger than the size of the opening of the karst cave, the problem that the concrete overdrinking amount is difficult to control due to the fact that the karst cave size cannot be judged in a pile foundation is effectively solved, pile forming quality is high, influence on natural environment is small, green construction level is high, and construction period is greatly shortened.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the application and together with the description, serve to explain the principles of the application.
FIG. 1 is a flow chart of a method for constructing a foundation of a bored pile in a karst region according to an embodiment of the application;
FIG. 2 is a diagram showing a step of determining a karst cave position;
FIG. 3 is a schematic view of the installation of a reinforcement cage with a fast-easy-closing-in net;
fig. 4 is a schematic view of a concrete pouring process.
Specific embodiments of the present application have been shown by way of the above drawings and will be described in more detail below. The drawings and the written description are not intended to limit the scope of the inventive concepts in any way, but rather to illustrate the inventive concepts to those skilled in the art by reference to the specific embodiments.
Detailed Description
Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. When the following description refers to the accompanying drawings, the same numbers in different drawings refer to the same or similar elements, unless otherwise indicated. The implementations described in the following exemplary examples do not represent all implementations consistent with the application. Rather, they are merely examples of apparatus and methods consistent with aspects of the application as detailed in the accompanying claims.
As described in the background art, the bored pile is a pile foundation type which is widely applied at present, the degree of mechanization is high, the single pile bears large, and the pile body rigidity is strong.
When a karst cave is encountered in the drilling construction process, the karst cave is often filled with rubble, concrete, clay and the like, and a good retaining wall is formed together with karst filler. However, this method still has the following problems:
The karst area drilling pile foundation construction can not judge the karst cave size and the concrete overfilling amount is difficult to control, the pile forming quality is poor, the influence on the natural environment is large, and the green construction level is low.
According to the karst area bored pile foundation construction method, the outer sides of the side walls of the reinforcement cages corresponding to the karst cave are fixedly wrapped with the quick and easy-closing-up net, so that leakage ports of the karst cave can be plugged, and the technical problems that concrete overspray is difficult to control, pile forming quality is poor, influence on natural environment is large and green construction level is low due to the fact that the size of the karst cave cannot be judged in karst area bored pile foundation construction are solved.
The following describes the technical scheme of the present application and how the technical scheme of the present application solves the above technical problems in detail with specific embodiments. The following embodiments may be combined with each other, and the same or similar concepts or processes may not be described in detail in some embodiments. Embodiments of the present application will now be described with reference to fig. 1-4.
The embodiment of the application provides a construction method of a bored pile foundation in a karst region, which comprises the following steps:
S1, measuring and lofting, please refer to FIG. 1, the position of a pile hole is accurately positioned by adopting an instrument, the embodiment can adopt a total station, four control piles are placed by pulling a cross line nail according to pile positioning points, the burying positions of the steel pile casings and the accurate positioning of a drilling machine are controlled by taking the four control piles as references, and the control piles need to achieve good protection work and prevent disturbance in the construction process.
S2, manufacturing and embedding a steel pile casing, referring to FIG. 1, rolling a steel plate to form a steel pile casing, specifically, rolling a steel pile casing by using a steel plate with the thickness of 6mm to form the steel pile casing, wherein the inner diameter of the steel pile casing is 0.2-0.4 m larger than the diameter of a designed pile; welding stiffening rings at the periphery of the top and bottom ports of the steel casing; when the steel pile casing is installed, a drilling machine operator enlarges a pile hole by using a reamer, and then the steel pile casing is pressed into a designed elevation by a high-torque drill bit. Before and after the pile casing is pressed in, the vertical position of the steel pile casing is adjusted through an accurate level meter leaning against the steel pile casing; after the steel pile casing is buried, the top of the steel pile casing is controlled to be 0.3m higher than the original ground for positioning the drill bit and protecting the pile hole.
S3, positioning a drilling machine, referring to FIG. 1, specifically, the embodiment can adopt a rotary drilling machine for drilling; when the drilling machine is in place, checking whether the performance state of the drilling machine is good in advance, and ensuring that the drilling machine works normally; and 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 the 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, referring to FIG. 1, setting a mud pit on site, and then performing drilling operation.
① Slurry preparation
For a rock-soil layer with good cohesiveness, a dry or clear water drilling process is adopted, a slurry protection wall is not used, and broken stone soil with an orifice 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.
The mud pit comprises a sedimentation pit and a mud storage pit for back-grouting, which are generally 1.5-2.0 times of the volume of a drilled hole, and plastic cloth is paved at the bottom and the periphery of the mud pit or other sealing measures are adopted to prevent the outflow of mud.
Specifically, there are two types of equipment for preparing mud: a mud stirrer and a hydraulic stirrer. When the clay powder is used for pulping, a hydraulic stirrer is preferably used; when bentonite is used for pulping, a mud stirrer is preferably used.
Testing all performance indexes after slurry production, checking the specific gravity and sand content of slurry at any time in the drilling process, filling a slurry test record table, paying attention to geological changes at any time, adjusting the performance indexes of the slurry at any time according to the changes of geological conditions, and ensuring that all indexes of the slurry meet the standard requirements.
The recovered slurry pool is arranged on the drilling construction site and used for recovering the wall-protecting slurry, the slurry is conveyed into the slurry storage pool after being precipitated and purified, is further processed in the slurry storage pool (proper amount of calcined soda and CMC are added to improve the slurry performance), and is reused after being tested to be qualified.
② Drilling operation
When the drilling machine is used for drilling, the self weight of the drilling bucket and pressurization are used as drilling power, the one-time length short bar column displays the drilling depth of the current drill bit, the long bar column dynamically displays the movement position of the drill bit, and the figure of the hole depth displays the total depth of the hole. When the drilling bucket is squeezed and filled with drilling slag, the drilling slag is lifted to the ground surface, the rotary operating handle is operated to enable the machine to rotate to the position in the direction of the earthwork transport vehicle, the drilling slag is loaded into the earthwork vehicle by using the loader, and the drilling slag is cleared to a proper place for disposal.
Controlling the footage speed according to geological conditions in the drilling 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.
Along with the drilling, the mud continuously flows into the pile hole, so that the water level in the pile hole is ensured to be consistent with the water level outside the steel casing.
S5, locating and measuring karst cave, referring to fig. 1 and 2, carrying out preliminary locating according to the position of the karst cave reported by a geological survey and the drilling depth, and blocking the inflow of ground mud into a 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; and then 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, referring to FIG. 1, and checking geological conditions after the drilling hole reaches a designed depth. The drill slag is compared with a rock sample of a geological histogram combined with a first pile acceptance seal sample to verify whether the geological condition meets the design requirement; if the data does not accord with the survey design data, carrying out confirmation processing in time; if the design requirement is met, the hole depth, the hole diameter and the hole type are immediately checked.
And detecting detection items such as aperture, hole wall, verticality and the like by adopting a hole measuring instrument.
And (3) detecting the hole depth and the sediment thickness: after the hole is formed, measuring the hole depth L2 by using a measuring rope according to the drilling depth L1 of the display interface of the rotary drilling machine, comparing the two, and if L2 is smaller than L1, replacing a bottom cleaning drill bit, cleaning the bottom and re-measuring the hole depth.
And after the design and standard inspection requirements are confirmed to be met, checking and accepting are carried out, and after the checking and accepting are qualified, hole cleaning is carried out immediately.
S7, hole cleaning and acceptance checking, referring to FIG. 1, cleaning holes by a slurry changing method, wherein the water level in the holes needs to be kept when the holes are cleaned; and (3) cleaning the dry hole sediment by using a flat bit after sediment. The purpose of clearing the hole is to clear away drilling slag and a sediment layer, reduce the sediment thickness at the bottom of a pile foundation, and prevent the pile bottom from remaining too thick sediment so as to reduce the bearing capacity of the pile.
The hole cleaning is carried out twice, the first hole cleaning is carried out after the drilling depth reaches the design depth, the first hole cleaning meets the standard requirement, and otherwise, the reinforcement cage is not lowered.
And after the reinforcement cage is installed in place, the second hole cleaning is carried out before the guide pipe is lowered, and the hole cleaning before concrete pouring must reach the following standard: the slurry discharged or pumped out of the holes has no granular feel when touched by hands, the specific gravity of the slurry is more than 1.03, the sand content is less than 2 percent, and the viscosity is 17 to 20 seconds; the thickness of sediment at the bottom of the pile foundation before pouring the underwater concrete is not more than 5cm.
Measuring sediment at the bottom of a pile foundation: measuring the hole depth around the hole wall by adopting a measuring rope with a front end suspended with a balance weight, measuring at least 4 points, wherein the difference of the bottom elevations of the measuring rope and the hole wall is the sediment thickness, and checking the measuring rope by adopting a steel rule before each measurement.
S8, manufacturing, transporting and installing the reinforcement cage, referring to fig. 1 and 3, manufacturing the reinforcement cage according to a reinforcement diagram, manufacturing the reinforcement cage according to the depth of a pile hole, wherein the first section is a fixed-length reinforcement cage (combined with the lifting height of a crane), and the second section is a manufacturing length determined by the actual depth of the pile. Ensuring that the positions, the intervals and the number of the reinforcing steel bars meet the requirements of the specification and the standard of the drawing; the main bars are welded and staggered, so that the number of joints in the range of 35d (d is the diameter of the steel bars) is not more than half of the total number of the steel bars.
The joints of the spiral stirrups or the stiffening ribs of the reinforcement cage are welded, the stiffening ribs are connected with the main ribs by spot welding, and the spiral stirrups are connected with the main ribs by binding. In order to prevent the steel reinforcement cage from deforming during transportation and hoisting, the hoisting point positions are reinforced, and the stiffening ribs are encrypted.
If the karst cave depth exceeds 4m or the size of the karst cave cannot be judged, fixedly wrapping a quick and easy-closing net on the outer side of the side wall of the reinforcement cage corresponding to the karst cave, wherein the size of the quick and easy-closing net is larger than the opening size of the karst cave; a quick-easy closing net with folded edges at the top and the bottom is used, and after a reinforcement cage is lowered into a hole, the folded edges are tightly attached to the hole wall; 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.
In order to ensure the thickness of the protective layer of the reinforcement cage, U-shaped rib supports are welded on the outer side of the reinforcement cage in advance, the distance along the pile length is 2m, and the transverse circumference is not less than 4 positions.
And (3) carrying out sectional manufacturing on a processing site, transporting the steel reinforcement cage which is qualified in inspection to an orifice, hanging and putting the steel reinforcement cage into a hole, and adopting single-sided welding when two sections of steel reinforcement cages are connected, wherein the welding length is 10d (d is the diameter of the steel reinforcement). During carrying and hoisting, deformation is required to be prevented, the hole site is required to be aligned for placement, the hole wall is prevented from being collided, and the reinforcement cage top is immediately fixed after reaching the designed elevation, so that the reinforcement cage floats upwards when concrete is poured. After the whole reinforcement cage is manufactured, the reinforcement cage is transported to a construction site in a segmented manner in a reinforcement processing field. And the adhered soil and oil stains are removed before the reinforcement cage is installed, so that the tight adhesion of the reinforcement and the concrete is ensured.
S9, taking the actual lengths of the pipes section by section and numbering the pipes in sequence when the pipes are installed as shown in fig. 1 and 4, and recording the actual lengths so as to control the depth of the buried pipes in the concrete pouring process, wherein attention is required to be paid to whether rubber rings are arranged or not, and whether the threads at two ends of each section of the pipes are broken is checked so as to avoid the phenomenon of water inflow of the pipes in the pouring process.
S10, secondary hole cleaning, please refer to FIG. 1, after the guide pipe is lowered in place, immediately detecting sediment at the bottom of the pile foundation, if the sediment thickness does not meet the design requirement, adopting gas lift reverse circulation secondary hole cleaning, taking care of maintaining a mud head and supplementing high-quality mud to prevent collapse during circulation, and immediately removing the mud suction elbow after the hole cleaning is finished and inspected to be qualified.
S11, pouring underwater concrete, please refer to fig. 1 and 4, before pouring the concrete, sprinkling water to wet the concrete conveying pipeline and the container, then installing a water-proof facility in the filling conduit, and after the storage hopper is full of the concrete, starting pouring the underwater concrete.
The quantity of the first poured concrete can meet the requirements that the first embedding depth of the guide pipe is at least 1m and the bottom of the guide pipe is filled with the concrete, the embedding depth of the guide pipe in the concrete is at least 1m during bottom sealing, the quantity of the first concrete is determined according to the diameter of piles, the embedding depth of the guide pipe and the quantity of concrete in the guide pipe, the mixed concrete is transported to a pile foundation opening by a concrete transport vehicle, the mixed concrete is injected into a hopper lifted by a drilling machine, the quantity of the concrete in the vehicle is about 8m, the mixed concrete is uniformly commanded by one person, and after both sides are ready, a water-stop bolt and a valve are simultaneously opened for bottom sealing, a steel plate is adopted for the isolation bolt, and the steel plate is pulled by a thin steel wire rope and lifted by a tower crane or an automobile crane.
① After the first batch of concrete falls, the concrete is poured continuously. In the process of perfusion, the embedding depth of the catheter is controlled to be 2-6 m.
② And in the process of pouring concrete, a weight of not less than 4kg of a measuring hammer is adopted to constantly measure the rising height of the concrete surface in the hole, after the guide pipe reaches a certain burial depth, the guide pipe is quickly disassembled step by step, and the height of the concrete surface in the hole is detected once before each lifting of the guide pipe. And the measuring rope for measurement is checked by a steel ruler for each time before and after each pile is poured, so that errors are avoided.
③ The elevation of the poured pile top is controlled to be higher than the design elevation by more than 0.3m, so that the strength of concrete is ensured, the excessive pile heads are chiseled, and the pile heads are ensured to have no loose layer.
④ After the concrete is poured, the guide pipe, the funnel and the like are cleaned and checked in time to prepare the next hole for use.
⑤ Before the underwater concrete is poured, a 'related detection form' for checking the conditions of the bored pile and the reinforcement cage is filled, and a 'underwater concrete pouring record' is filled in the process of pouring the underwater concrete.
Other embodiments of the application will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed technology. This application is intended to cover any variations, uses, or adaptations of the application following, in general, the principles of the application and including such departures from the present disclosure as come within known or customary practice within the art to which the application pertains. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the application being indicated by the following claims.
It is to be understood that the application is not limited to the precise arrangements and instrumentalities shown in the drawings, which have been described above, and that various modifications and changes may be effected without departing from the scope thereof. The scope of the 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.
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