CN111395322A

CN111395322A - Construction method for rotary digging cast-in-place pile under karst landform

Info

Publication number: CN111395322A
Application number: CN202010265492.2A
Authority: CN
Inventors: 田久英; 吕争魁; 王朝晖; 程英俊; 杜莎莎; 陈聪俊; 闫晓敏; 王美静
Original assignee: China Railway 12th Bureau Group Co Ltd; China Railway 12th Bureau Group Construction and Installation Engineering Co Ltd
Current assignee: China Railway 12th Bureau Group Co Ltd; China Railway 12th Bureau Group Construction and Installation Engineering Co Ltd
Priority date: 2020-04-07
Filing date: 2020-04-07
Publication date: 2020-07-10

Abstract

The invention discloses a construction method of a rotary digging cast-in-place pile under a karst landform, which comprises the following steps: according to the requirements of the designed aperture of the foundation pit support, the space between piles and the row spacing, firstly constructing the rear row of piles and then constructing the front row of piles by combining a construction site; piling, wherein the piles are constructed from two sides to the middle of a row of piles in sequence, simultaneously jumping and excavating all pile holes, constructing the middle at intervals, and jumping and constructing adjacent hole piles after concrete of pile bodies is poured until piling and hole forming are finished; selecting the length of the steel pile casing according to the soil property condition of field excavation, embedding the pile casing, and repeatedly circulating until a hole is formed by rotating, cutting soil, lifting and unloading the drill bucket; checking holes, cleaning the holes, manufacturing a reinforcement cage, hanging the reinforcement cage into the holes, and firmly positioning and ensuring the sound-measuring tube to be vertical; pouring concrete into the pile body; detecting the height of the concrete surface, and pulling out the pile casing after the concrete is poured; the method has low cost and good pore-forming quality.

Description

Construction method for rotary digging cast-in-place pile under karst landform

Technical Field

The invention relates to the technical field of construction of karst landform foundations, in particular to a construction method of a rotary digging cast-in-place pile under a karst landform.

Background

Karst landform characteristics and influence on construction karst landform are mostly formed by soluble limestone, the earth's surface is exposed to the wind and is extremely easily weathered by the rain, and the underground part is because the water that receives the erosion power is to the corrosion action of soluble rock, makes the partial structure of underground soft, and the rock stratum is more, and can also appear phenomena such as dark river and quicksand, and especially underground river influences greatly to the construction of building foundation, therefore the construction degree of difficulty is higher than other topography in the construction of karst landform foundation, especially because the soil body contains water the height and causes the collapse easily.

In the prior art, a steel pile casing is usually arranged on the whole length of a pile body in order to treat collapsed holes, and the length of the steel pile casing is more than 8m, so that the steel pile casing is difficult to pull out after concrete pouring of the pile body is finished, and waste of the steel pile casing is caused.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention provides a construction method of a rotary digging cast-in-place pile under a karst landform, and the method is low in cost and good in hole forming quality.

The invention is realized by the following technical scheme:

a construction method for rotary digging cast-in-place piles under karst landform comprises the following steps:

1) piling, namely, according to the requirements of the designed aperture of the foundation pit support, the space between piles and the row spacing, combining a construction site, constructing the pile in the rear row firstly, and constructing the pile in the front row secondly; piling, wherein the piles are constructed from two sides to the middle of a row of piles in sequence, simultaneously jumping and excavating all pile holes, constructing at the interval of 1-3 pile hole distances, and jumping and constructing adjacent hole piles after concrete of a pile body is poured until piling and hole forming are finished;

2) drilling by rotary digging:

embedding a pile casing, selecting the length of the steel pile casing according to the soil texture condition of field excavation, embedding the pile casing, and embedding the steel pile casing into a soil layer with the height less than 0.5m (the elevation of a soil-stone interface is-3.2 m) and 0.2-0.5m higher than the construction ground; the drilling bucket rotates, cuts soil, lifts and unloads the soil, and the circulation is repeated until the hole is formed;

the center deviation of the pile casing is not more than 10mm, the inclination is not more than 1%, the pile casing is 200-500mm higher than the ground, the depth of the pile casing foundation pit is 3.5-3.8m, and after correction, the periphery of the pile casing is embedded with clay;

drilling holes, excavating upper backfill, and burying the steel casing into the soil layer below 0.5m and 0.2-0.5m above the construction ground;

3) checking the formed hole, cleaning the hole: detecting and correcting the aperture, the shape of the hole, the vertical degree, the depth of the hole and the sediment at the bottom of the hole, and cleaning the hole to meet the thickness requirement of the sediment at the bottom of the hole;

4) manufacturing and hoisting a steel reinforcement cage: manufacturing a reinforcement cage with a lifting point, arranging a stiffening hoop, arranging a reinforcing rib in the stiffening hoop, binding an acoustic pipe on the inner side of the reinforcement cage, lifting the reinforcement cage into a hole, and firmly positioning and ensuring the acoustic pipe to be vertical;

5) pouring concrete into the pile body: the underwater concrete pouring adopts a conduit pouring method to initially pour the concrete, and the conduit is embedded into the concrete for more than 1.5 meters; continuously pouring concrete, lifting the guide pipe timely, and disassembling the guide pipe step by step, wherein the lower opening of the guide pipe is buried in the concrete for more than 2 meters; detecting the height of the concrete surface, wherein the detection times are more than or equal to the number of the sections of the used guide pipes; the pouring elevation of the pile top is increased by 0.3-0.5m compared with the designed elevation, and the pile casing is pulled out after the concrete pouring is finished.

Further, a drill bit of the guide structure is selected for rotary drilling, and a straight cylinder is selected for a drill bit cylinder.

Furthermore, 3 sound measuring tubes are embedded in each pile and evenly distributed along the circumference of the pile body in equal parts; the bottom of the sounding pipe is closed, the sounding pipe is 5-15cm above the bottom of the pile body, the top of the sounding pipe is 30-70cm higher than the top of the pile, and the interior of the sounding pipe is kept clean; the sound measuring pipe is connected by adopting threads, the joint is sealed and does not leak water or foreign matters, and the sound measuring pipe is bound at the inner side of the reinforcement cage and is welded and fixed by using the reinforcement.

Furthermore, the rotary drilling adopts a dry drilling method, and the method can better ensure the quality of the formed hole. And after the drilling is finished, cleaning the hole by adopting a flat-bottom double-door hole cleaning drill.

Further, the rotary drilling process specifically includes:

firstly, excavating an upper backfill with an open type drill bit with the diameter of 1.4 m, embedding a pile casing, and embedding a steel pile casing into a soil layer below 0.5m and 0.2-0.5m higher than the construction ground;

after the stone buds are dug, replacing a rock-socketed cylinder drill with the diameter of 1.2m for core pulling, and deslagging by using a slag removal drill with the diameter of 1.2 m;

and after the designed bottom elevation is dug, replacing a rock-socketed cylinder drill with the diameter of 1.4 m for reaming, and deslagging by using a slag removal drill with the diameter of 1.4 m.

Furthermore, longitudinal steel bars of the steel bar cage are mechanically connected by rolling straight threads or are overlapped by double-sided welding joints, the stressed steel bar connecting joints are arranged at positions with small internal force and are arranged in a staggered mode, and the number of the steel bar joints in the same section cannot exceed 50% of the total number of the joints; the distance between the stiffening hoops is 1-3 meters, the stiffening hoops are internally provided with reinforcing steel bars shaped like a Chinese character 'jing', and the stiffening hoops are reinforced by inclined struts.

Furthermore, after the reinforcement cage is hung in the hole, the deviation between the center of the reinforcement cage and the center of the pile hole is not more than 100mm, and the deviation of the height of the bottom surface of the reinforcement cage is not more than 100 mm.

Furthermore, in the process of adopting the guide pipe for pouring, when the water enters the guide pipe, immediately lowering the guide pipe again to a position 250-400mm away from the bottom of the hole, and adding enough reserved concrete again for bottom flushing or taking out the reinforcement cage and adopting a re-drilling to clean; then putting down the framework and the conduit again, and throwing in enough first batch of concrete for re-pouring.

Compared with the prior art, the invention has the following beneficial effects:

the method adopts the rotary drilling machine hole-forming technology, can well process the construction of karst landform, is basically not influenced by adverse geological conditions, has the advantages of low cost, good hole-forming quality, high hole-forming efficiency and the like, and can play a very great role in the construction of karst landform.

In the method, a pile foundation excavation construction sequence of two-driving-one-jumping is adopted during excavation. The hole collapse treatment adopts a construction technology combining the construction of embedding a 4m steel pile casing at the pile top part and secondary excavation after the hole collapse recharge of C15 concrete at the pile body part, and the steel pile casing is pulled out for secondary use after the concrete pouring of the pile body is finished and before the initial setting.

Drawings

FIG. 1 is a flow chart of the method of the present invention.

FIG. 2 is a schematic diagram of the relationship between the steel casing and the foundation pit.

FIG. 3 is a schematic view of the embedding manner of the steel casing.

Fig. 4 is a schematic view of a drilling process.

Fig. 5 is a schematic structural view of a reinforcement cage.

FIG. 6 is a cross-sectional view of a stiffener hoop.

Fig. 7 is a schematic diagram of the arrangement of the sounding pipes.

Fig. 8 is a flow chart of the method of pouring underwater concrete by a conduit method.

In the figure, 1-pile casing, 2-cross, 3-foundation pit, 4-pile diameter, 5-ground, 6-drill rod, 7-guide pipe, 8-sealing plate, 10-longitudinal steel bar, 11-stiffening hoop, 13-inclined strut, 14-spiral hoop, 15-sounding pipe, 16-reinforcement cage, 18- # -shaped reinforcing steel bar and 19-drill cylinder.

Detailed Description

In order to make the technical problems, technical solutions and advantageous effects to be solved by the present invention more clearly apparent, the present invention is further described in detail with reference to the embodiments and the accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. The technical solution of the present invention is described in detail below with reference to the embodiments and the drawings, but the scope of protection is not limited thereto.

Example 1

Topography, landform

The project is located in the sinoguan village of the mountain lake region in Guiyang city, and in the original basin region of the Guizhong mound in the middle of the plateau in Guiyang city, and is a karst depression landform. The proposed field is a construction land in the early stage, so that the overall terrain is relatively flat in the current situation, the elevation of the proposed building range is 1217.29-1223.51m, and the relative height difference is about 6.22 m.

Stratum, lithology

The ground covering soil layer is mainly provided with a fourth series of miscellaneous fill (Q)^ml) And red clay residual layer (Q)^el+dl) The underburden is a three-fold system big metallurgical group (T)₁d) Dark grey, thin to medium thickness limestone, containing local argillaceous matter.

Hydrogeology

The surface water field area is located on a plateau land in the west of a first-level branch cat jumping river on the right bank of the Wujiang river and a watershed of a Nanming river, is located about 3km east of a Baihua lake reservoir, belongs to a cat jumping river basin, the surface water in the project area grows, according to the survey, the original ground surface of the project area is mainly cultivated land, accumulated water with the depth of 0.2-0.5m is distributed in a ground surface paddy field in the middle of the project area, meanwhile, a brook in the south-north direction is developed in the east of the project area, the flow is 5-10L/s, the brook flows to Jinzhui road from the south to the north, and then is discharged to the Baihua lake along an artificial ditch, 3 ponds are distributed at the 150m positions (the left side and the right side of the Jinzhui road) on the west side of the proposed project area, and according to the survey, the 3 ponds are all.

Ground water

According to the occurrence conditions of the underground water of the test area, the characteristics of the water-containing medium, the lithology and the combination characteristics of the water-containing medium, the types of the underground water of the test area mainly comprise fourth-series loose pore water and bedrock fracture water.

① the fourth series loose pore water is buried in the fourth series (Q) miscellaneous fill layer and red clay layer or near the rock-soil contact surface, is distributed in lens shape, has large seasonal variation of water quantity, belongs to upper layer stagnant water, and is a weak aquifer in the region.

② basement rock fracture water is generated in the joint fractures and the rock stratum surface of the limestone under the ground, the water quantity and hydrological relation are controlled by the joint fractures and the rock stratum surface, the water is a medium water-containing rock group, the water-rich property is medium, the water is distributed in the whole to-be-built ground, the underground water receives the supply of atmospheric precipitation and upper layer stagnant water, permeates into the underground along the joint fractures and is transported along the rock stratum layers and among the fractures.

The underground water is mainly supplied by atmospheric precipitation, bedrocks near the site are exposed, which shows that surface water and underburden karst fissure water have close hydraulic connection, the observed water level elevation in a drill hole is 1213.45-1218.73m, the exploration period is a dry period, the underground water level elevation is about 1213.45-1218.73m, the underground water level elevation of the historic site is slightly higher than the water level of the current exploration period, the underground water level elevation in the water abundance period and the dry period is about 2-3m, and the amplitude of the corresponding water level elevation is 1215.45-1218.73 m.

1. Preparation for construction

1.1 Material preparation

And (4) carefully closing the raw materials, performing raw material incoming inspection, and enhancing the quality detection work of all raw materials, semi-finished products and incoming inspection. All the materials, finished products and semi-finished products entering the factory must have delivery qualification certificates, and are subjected to sampling inspection in batches according to the national regulation requirements, the materials which are not qualified in the sampling inspection are not used at all, and the quality accidents caused by the use of the unqualified materials are subject to the responsibility of inspection, acceptance and storage.

1.2 preparation of the technique

① the organization engineering technicians are familiar with the design and construction drawings of foundation pit support and special construction schemes, so that the site managers and constructors can fully understand the construction deployment, construction method, construction key points and cautions of the whole engineering, and can make a good deal of technical quality and safety.

② the related procedures of traffic, environmental protection and the like are handled before the start of work, so as to ensure the continuity of construction.

③, handling the measurement control point handover, and guiding the measurement control point to the site to establish a measurement control network.

1.3 job site preparation

① the construction road is paved to ensure smooth traffic, temporary construction electricity is led from north side of the foundation pit to the reinforcing steel bar processing field, and a secondary distribution box is arranged for field use.

② the lamp arrangement for night construction and illumination is prepared, the field is leveled, and the equipment of the steel bar processing field enters the field and is debugged.

③ concern rainy season changes and take safety precautions.

2. Measuring line

2.1 basis of paying-off

And designing a foundation pit supporting construction drawing and a foundation pit excavation drawing provided by a construction unit according to the design.

2.2 construction measurement Process

Verification and verification of a measuring instrument (GPS, a total station, a level gauge and a steel ruler → verification and verification of an initial basis → field control network measurement → foundation paying-off → vertical control of a building.

2.3 engineering positioning and paying-off and elevation control

1) Before basic construction, a business owner informs a planning department to recheck and check the given coordinate control points in the site, and before rechecking, the business owner sends a professional measurer to throw all the drawn control points back into the site, and arranges and measures the control angular points of the building according to a plane graph. And forming an paying-off record, and rechecking and signing by the owner or the supervision site representative. After closed inspection, the concrete is led to the outside of the building and is protected with concrete or led to the existing building, and other sizes and axes are separated as control lines of building axis and verticality.

2) The engineering adopts a GPS combined with a total station to pay off. Before the point is thrown, after the coordinate points of each pile position are rechecked, the rechecking is correct, and then detailed lofting can be carried out according to the size of the plane graph.

3) Elevation control: according to leveling point positions and elevation data (absolute elevation is converted into relative elevation) handed over by a construction unit, elevation points are guided to a construction site according to the requirements of three equal leveling measurements, two to three leveling points are arranged, obvious marks are made, and observation elevations of all the leveling points are rechecked periodically.

2.4 amount offset control

And (4) performing rechecking in the measurement and paying-off work, wherein all axes need to be closed and measured, the closing difference is less than 3mm, otherwise, the closing difference needs to be retested or adjusted, and a complete measurement record needs to be obtained after retesting and is submitted to a technical responsible person for auditing. Before each projection measurement, the instrument is strictly checked once, so that the measurement errors caused by the defects of the instrument are prevented, the accuracy requirements of the intersection angle of the axes are 180 +/-10 'and 90 +/-6', and the distance accuracy is +/-5 mm.

3. Rotary digging pore-forming slide-resistant pile construction

3.1 piling sequence

According to the requirements of the designed aperture of the foundation pit support, the space between piles and the row spacing, the construction site is combined, the piles are organized and constructed firstly, then the piles are arranged, and the piles are constructed in front. The piling sequence is constructed from two sides of a row of piles to the middle, all pile holes must jump to excavate, the middle is constructed at the distance of 2 pile holes, and after concrete of a pile body is poured, the pile jumps to construct adjacent hole piles until the piling and pore forming are completed.

3.2 Rotary drilling machine

3.2.1 embedding of pile casing

And 4m steel protective cylinders are arranged according to the soil excavation condition on site (other steel protective cylinders with different sizes, such as 8 meters, 2 meters, 10 meters and the like, can be selected in actual production). The technical requirements are as follows:

⑴ the casing is made of steel plate, the deviation of the casing center is not more than 10mm, the inclination is not more than 1%, and it is better to be L1 =300mm higher than the ground as shown in fig. 2 and 3.

⑵ the depth of the pile casing foundation pit L2 = 3.5-3.8m, and the pile casing foundation pit can be buried after being cleared away when obstacles are encountered.

⑶ after correction, the protecting cylinder is buried with clay to ensure the protecting cylinder has no leakage and displacement of slurry during drilling.

⑷ recorded for retesting.

3.2.2 drilling of holes

Taking an anti-slide pile with the diameter of 1.4 meters as an example, firstly, excavating upper backfill with an open type drill bit with the diameter of 1.4 meters, embedding a pile casing, and embedding a steel pile casing into a soil layer with the height of less than 0.5m (the height of an earth-rock interface is minus 3.2 m) and 0.3m higher than the construction ground; and after the stone buds are dug, replacing a rock-socketed cylinder drill with the diameter of 1.2 meters for core pulling and deslagging by using a slag-removing drill bit with the diameter of 1.2 meters, and after the stone buds are dug to the designed bottom elevation, replacing a rock-socketed cylinder drill with the diameter of 1.4 meters for hole expanding and deslagging by using a slag-removing drill bit with the diameter of 1.4 meters.

When the drilling is started, the drill rod 6 is ensured to be vertical, the drilling machine is stable, and then the drilling is carried out. When the drill bit is lowered to a preset depth, the drill bucket is rotated and pressure is applied to squeeze soil into the drill bucket, when the automatic display cylinder of the instrument is full, the bottom of the drill bucket is closed, and the drill bucket is lifted to discharge the soil to a stacking place. The general construction process is to repeatedly circulate the drilling bucket until the hole is formed by rotating, cutting soil, lifting and unloading the soil.

3.2.3 notes

(1) The construction sequence of the anti-slide pile is from two sides to the middle, so the pile must adopt jumping groove excavation with 2 piles at intervals.

(2) The site of the drilling machine is required to be leveled and compacted before entering the field, so that the stopping position of the drilling machine is ensured to be horizontal and stable, and uneven sinking is avoided.

(3) According to the back pressure backfill depth, a certain pile casing is buried to avoid hole collapse.

(4) The drill digs out the muck and loads the muck to the outside of the vehicle for transporting the waste soil.

(5) During the drilling process, the geological change condition should be noticed at any time, and the construction record should be checked with the geological profile map of the geological survey, if the construction record does not accord with the design condition, the construction record should be timely communicated with the supervision and the designer for timely processing.

(6) And after the drilling depth meets the design requirement, checking the aperture and the hole depth, and filling a drilling record table. The deviation between the drilling center and the pile position design center at the pile top is not more than 50 mm; the aperture is not less than the designed pile diameter; the gradient is not more than 1%; the hole depth must not be less than the design hole depth.

(7) The pile body concrete is self-sealing concrete, and the whole pile concrete is poured uninterruptedly at one time.

(8) The pile body sounding pipe embedding requirement is as follows: 3 acoustic pipes are embedded in each pile and evenly distributed along the circumference of the pile body in equal parts; the bottom of the sounding pipe is closed and should be 10cm above the bottom of the pile body, and the top of the sounding pipe should be 50cm higher than the top of the pile, so that the sounding pipe is kept clean. The sound measuring tube is made of steel thin-wall embedded pressure type SCG50 × 1.2-QY, and has an outer diameter of 50mm and a wall thickness of 1.2 mm. The sound measuring pipe is connected by adopting a thread, so that the joint is sealed, water leakage and foreign matter are avoided, and clear water is filled before monitoring. The sounding pipe is fixed by binding the inner side of the reinforcement cage and welding and fixing the sounding pipe by using a steel bar.

(9) The drilling speed of the rotary drilling rig is strictly controlled in the drilling process, so that the buried drilling accident caused by large drilling scale is avoided.

(10) And (5) withdrawing the drilling machine from the hole opening in time after the drilling is finished so as to prevent the pile hole from collapsing. The orifice is covered with a hard substance (wood balk, wood board) to prevent the potential safety hazard of falling of people or sundries.

(11) In order to ensure that the next process can be normally constructed, after the drilling is finished, the hole opening muck is timely cleaned to a fixed position by a prepared forklift, so that the field is flat and clean, and necessary conditions are provided for subsequent construction.

3.2.4 borehole anomaly handling

When the abnormal condition of the drilling hole is related to the structural safety or does not conform to the geological survey report, a supervision and a first party site confirmation are immediately reported.

⑴ when the collapse hole is found and not serious, it should be treated immediately to avoid serious collapse hole due to too long time, when it is found, the worker is informed to work on site and the drilling machine and personnel are organized to take place.

⑵ when the hole collapse is serious, a low grade C15 concrete is used for recharging:

① according to the hole collapse condition, if slight hole collapse occurs in the drilling process, adopting a recharging low-grade C15 common commercial concrete, recharging the commercial concrete with the height exceeding the top surface of the hole collapse by 500mm, and after the recharging concrete is finally set (not less than 24 hours), drilling a hole downwards from the recharging concrete surface by a drilling machine;

② drilling the first concrete layer, if not, then re-filling the low-grade concrete by the above method until all the collapsed hole sections are completely processed.

And (5) drilling downwards according to the construction drawing to meet the requirement of the designed rock-socketed depth, and rechecking the depth to obtain the final hole.

The earthwork excavated in the pile is piled in the range of 3.0 meters beyond the pile edge, the piling height is not more than 1.5 meters, the excavator is responsible for transferring the earthwork beside the pile in the yard to each yard every day, and according to the specific situation of each yard, the earthwork in the yard is timely transferred out of the yard to spoil by a 320-type excavator matched with manual work and an automobile.

3.3 inspection of pore formation

3.3.1 Aperture, hole shape, and verticality detection

The verticality control of the rotary digging pile is automatically controlled by a full-automatic numerical control rotary digging machine, and an operator needs to pay attention to the drilling quality at any time in the height accumulation.

The aperture detection is carried out after the hole is formed and before the steel reinforcement cage is put into the hole, and the cage type hole probing device is manufactured to detect the aperture according to the pile diameter. The hole detecting device is made of phi 20 steel bars, the outer diameter of the hole detecting device is not smaller than the designed aperture of the drilled pile, and the length of the hole detecting device is equal to 4-6 times of the diameter of the pile. During detection, the hole detector is lifted, the center of the hole is consistent with the lifting steel wire rope, the hole is slowly placed into the hole, and the hole diameter, the hole shape and the verticality meet the requirements in an unobstructed manner. If the hole is blocked in the middle, the hole is reduced in diameter or inclined, and measures must be taken to eliminate the hole bending and shrinkage in the hole. The drill bit of the drilling machine is lifted to a deflection position to repeatedly sweep the hole until the drilled hole is straight. When shrinkage occurs, slurry is added, slurry indexes are adjusted to repeatedly sweep the hole, and the hole diameter is enlarged. If serious hole bending occurs, the deviation is backfilled by small rubbles or pebbles and clay mixture, and the hole is drilled again for deviation correction after the filling is settled.

3.3.2 hole depth and bottom sediment detection

And detecting the hole depth and the sediment at the hole bottom by adopting a standard measuring rope. The measuring rope must be checked by a calibrated steel ruler. The measurement method is as follows: and (3) slowly sinking the cast-in-place pile into the hole by using a measuring hammer meeting the height of the cast-in-place pile, detecting the position of the top surface of the sediment by the hand feeling of a person, and obtaining the difference between the depth of the construction hole and the depth of the measuring hole, namely the thickness of the sediment. The thickness of the sediment is controlled within 100 mm.

3.4 cleaning of holes

And (4) after the drilled hole reaches the underground petrochemical rock formation reported by the land and reaches the rock embedding with the designed pile body length, clearing the hole after the report and supervision and the inspection of the Party A are qualified. The purpose of the hole cleaning treatment is to enable the thickness of the sediment at the bottom of the hole to meet the quality requirement and the design requirement, and then immediately enter the next procedure, so as to avoid the problem that the hole is collapsed due to overlong delay time after the hole forming is finished.

The geological conditions of the construction site are miscellaneous filling soil, primary red clay and a weathered rock stratum in primary limestone, and meanwhile, a rotary drilling hole-forming dry drilling method is adopted to ensure the working efficiency (quick hole forming), so that the method can ensure the hole forming quality. And after the drilling is finished, cleaning the hole by adopting a flat-bottom double-door hole cleaning drill. When the hole cleaning drill bit is used for cleaning holes, residual fine stone slag, powder and accumulated water in the holes can be completely removed, and the final thickness of the sediment at the bottom of the holes is smaller than 10 cm.

3.5 fabrication and hoisting of reinforcement cage

3.5.1 specific requirements for the Steel

The specification and quality of the steel bars entering the field are in accordance with the design requirements, and quality guarantee books are attached. And carrying out witness sampling and inspection on the raw materials and the welding quality according to the requirements of construction acceptance specifications.

The surface of the steel bar should be clean, and before the use, oil stain, scale rust and the like on the surface are removed;

the steel bars should be straight without local bending, and the coiled steel bars and the bent steel bars should be aligned.

3.5.2 fabrication of Reinforcement cage

(1) The manufacturing of the reinforcement cage is constructed strictly according to the design and specification requirements.

(2) Connecting steel bars:

① the specification of the longitudinal steel bar 10 of the steel bar cage is HRB335 phi 32 and HRB335 phi 25, wherein, the steel bar with the diameter phi 32 adopts a rolling straight thread mechanical connection joint, the steel bars with the diameter phi 25 and below adopt lap joint double-sided welding, the welding length is 5d, the connection quality must meet the specification requirement, the lap joint end of the two steel bars should be pre-bent, so that the axes of the two combined steel bars are consistent, the HRB400 grade steel bar adopts E55 type welding rod welding, the stress steel bar connection joint should be arranged at the position with smaller internal force and arranged in a staggered way, and the steel bar joint in the same section must not exceed 50% of the total number.

② the spiral stirrup 14 of the steel reinforcement cage is processed into single stirrup to be bound, lapped and spot-welded, and the lapping length is more than or equal to 1.05L abE and more than or equal to 300 mm.

(3) The stiffening hoops 11 are HRB335 phi 25, the distance is one at 2 meters, and as the weight of the steel reinforcement cage is large, in order to ensure that the steel reinforcement cage is not deformed during hoisting, the stiffening hoops are internally provided with # -shaped reinforcing steel bars 18 (the reinforcing steel bars adopt phi 16) at intervals and the stiffening hoops, and the stiffening hoops are reinforced by adopting HRB400 phi 28 as inclined struts 13 (detailed attached figures).

The steel reinforcement cage can not satisfy the hoist and mount requirement with wire rope hoist and mount steel reinforcement cage directly. According to previous experience, four hanging points are arranged at the first stiffening hoop at the top of the steel reinforcement cage, and the hanging points are firmly welded with the main reinforcement by using HRB335 phi 16 short steel bars with the length of 160mm and are propped against the stiffening hoops.

3.5.3 hoisting steel reinforcement cage

The tonnage of the reinforcement cage (about 2.8-3.3 tons) and the length of the reinforcement cage are about 15-17m, and the crane of a common model can not meet the use requirement, so that two wheeled cranes are adopted for hoisting the reinforcement cage in place, and two QY25 and 360-type excavators of the crane model are matched for on-site transportation.

When the steel bar cage is hung in the hole, the hole diameter is aligned, the steel bar cage is kept vertical, the steel bar cage is lightly placed in the hole slowly, the steel bar cage is slowly placed in the hole after the hole is inserted, the steel bar cage is not suitable for left-right rotation, and the steel bar cage is strictly prevented from swinging to collide the hole wall. If the transfer is stopped in case of obstruction, the transfer is processed by finding out the reason. The increase of the jerk and the forced lowering are strictly prohibited.

3.5.4 tolerance of reinforcement cage

(1) The tolerance after the steel reinforcement cage is hung and put into the hole is as follows:

① the deviation between the center of the reinforcement cage and the center of the pile hole is not more than +/-100 mm,

② the height deviation of the bottom surface of the reinforcement cage is not more than +/-100 mm.

(2) The tolerance of the steel reinforcement framework of the bored pile is shown in the following table 1:

TABLE 1

3.5.5 mounting of the sounding pipe

(1) The specification of the acoustic pipe is that the steel thin wall is embedded and pressed SCG50 × 1.2.2-QY, the external diameter is 50mm, the wall thickness is 1.2mm, the bottom pipe is sealed by a plug, firstly, the acoustic pipe is bound and fixed at the inner side of a reinforcing hoop of a reinforcement cage, then, a metal pipe is fixed on the reinforcing hoop in a spot welding way by an electric welder, the embedding depth of the acoustic pipe is 100mm above the pile bottom, the detection range is ensured to cover the whole pile length, in order to prevent the detection condition from being unsatisfied due to deformation, breakage and non-perpendicularity of the acoustic pipe, the acoustic pipe is made of steel pipe with the internal diameter of phi 50mm, the connection adopts threaded connection, the acoustic pipe 15 is arranged at the inner side of the reinforcement cage 16, the positioning is firm, the perpendicularity is ensured, the top end of the acoustic pipe is 500.

(2) The diameter of the engineering pile is 1400mm, and three sound measuring tubes are required to be arranged according to design requirements, as shown in fig. 7.

3.6 pile body pouring concrete

3.6.1 basic principle of pouring concrete: the method adopts a conduit pouring method, namely, a closed connecting steel pipe (or a non-metal pipe meeting the requirement of strength and rigidity) is used as a conveying channel of underwater concrete, and the lower part of the pipe is embedded into the concrete to a proper depth, so that the concrete continuously poured from bottom to top gradually forms a pile body.

3.6.2 main machines for pouring concrete

⑴ A guide pipe for underwater concrete delivery is made of 4-6mm thick seamless steel pipe or rolled steel plate and welded, the diameter of the guide pipe is determined by the diameter of pile and the amount of concrete to be poured per hour, but the minimum diameter is not smaller than 200mm, the length of each section of the guide pipe is determined by the technological requirements, the length of the guide pipe is generally 2.7-3m, several sections of short pipes with 0.5-1.5m are used at the top end to adjust the length of the guide pipe, the distance between the bottom of the pipe and the bottom of the hole is 300mm, the guide pipe is connected by flange plates, movable joint nuts and quick plug-in connection, and the guide pipe is sealed by rubber O-shaped sealing rings or rubber gaskets with thickness of 4-5mm, thus water and air leakage are prevented.

⑵ funnel and storage hopper can be made of 4-6mm steel plate, no slurry leakage and slurry hanging are required, discharging is smooth and thorough, and enough capacity is required to ensure that the first batch of poured concrete (initial pouring amount) can reach the required buried pipe depth (2-6 m).

⑶ the sealing plate used for the first concrete filling funnel for sealing the bottom of the funnel is made of steel plate, or steel plate or ball plug made of wood.

⑷ lifting and lowering the equipment for installing the pipe and funnel (crane, excavator or pile frame can be used on site).

3.6.3 construction process for underwater pouring concrete

⑴ sink the reinforcement cage 16.

⑵, the catheter was slowly sunk to a depth of 500mm from the well bottom L3 =300 with the bottom of the catheter open.

⑶ the sealing plate or ball plug is placed at the bottom of the funnel, and the sealing plate is led out by a thin steel wire rope.

⑷ the first batch of concrete is poured to fill the entire hopper.

⑸ the sealing plate or the ball plug is pulled out upwards, the concrete is poured for the first time, and the conduit is embedded in the concrete and is L4 =1.5 m or more.

⑹ the concrete is poured continuously, the guide pipe is lifted upwards, the lower opening of the guide pipe is buried in the concrete for more than 2m all the time, and the extraction is forbidden.

⑺ after the concrete is poured, the pile casing is pulled out.

3.6.4 essential point of underwater concrete pouring construction

⑴ when the first batch of concrete is poured, the distance between the lower end of the guide pipe and the bottom of the hole is controlled at 500mm, and the embedded length after the first bucket of concrete is poured reaches more than 1.0m (the initial pouring amount is 2.1-2.5 m)³）。

⑵ continuous pouring concrete, after the first batch of concrete is poured normally, the concrete should be poured continuously, stopping the machine in midway (the interval between two times of concrete pouring can not be more than 30 min), in the pouring process, a measuring hammer should be used to detect the rising height of the concrete surface, and the guide pipe should be lifted and dismantled step by step in due time, keeping the reasonable depth of the guide pipe, the detection times should not be less than the number of the used guide pipe sections, and the height of the inner and outer concrete surfaces of the pipe should be detected 1 time before the guide pipe is lifted each time, the detection times should be increased in special situations (local serious overdiameter, diameter reduction, leakage layer position and pile hole with large pouring amount, etc.), and the water return situation should be observed at the same time, so as to analyze and judge the situation in the hole correctly.

⑶ the buried depth of the conduit is that the buried depth of the conduit has great influence on the pouring quality, the buried depth is too small, so the floating slurry sediment on the concrete surface outside the conduit is often wrapped and rolled into the conduit to form an interlayer, the buried depth is too large, the pressure at the bottom opening of the conduit is reduced, the concrete in the conduit is not easy to flow out and is easy to block the conduit, the maximum buried depth is not more than 6m of the length of the conduit at the lowest end, and must be kept between 2m and 6m, and the bottom end of the conduit is strictly prevented from being lifted out of the concrete surface.

TABLE 2 buried depth selection of conduits

⑷ the concrete pouring time, the rising speed of the concrete pouring is not less than 2m/h, the pouring time must be controlled within the time that the concrete buried in the conduit does not lose the fluidity, and a proper amount of retarder can be added if necessary.

⑸ pouring elevation of the pile top and treatment of the pile top, wherein the pouring elevation of the pile top is at least increased by 0.3-0.5m compared with the design elevation so as to clear the floating slurry slag layer on the pile top.

⑹ when the air is not contained in the conduit, the concrete is poured into the hopper and conduit slowly through the chute, so that the concrete is not guided into the conduit from the top, and the high-pressure air bag is prevented from forming in the conduit, and the rubber pad between the pipe joints is prevented from extruding out to leak water from the conduit.

⑺ to prevent the reinforcement cage from being jacked up by the concrete when the concrete surface rises to the lower end of the reinforcement cage, the following measures should be taken:

① fixing the upper end of the reinforcement cage at the orifice;

② the concrete pouring time is continuous as much as possible to prevent the fluidity of the concrete from being too low when the concrete enters the reinforcement cage;

③ when the concrete in the hole approaches the reinforcement cage, the depth of the buried pipe should be kept, and the pouring depth should be slowed down;

④ after the concrete surface in the hole enters the reinforcement cage for 1-2m, properly lifting the guide pipe, reducing the buried depth of the guide pipe, and increasing the buried depth of the reinforcement cage in the lower layer of concrete.

⑻ the underwater concrete is vertically transported by automobile pump, the reinforcing steel bar is processed on site, if the site is limited and can not be processed, it can be processed on site by secondary transfer.

3.7 common problems of rotary digging pile and preventing and treating measures

3.7.1 treatment of collapsed hole:

⑴ slightly collapsing hole, namely backfilling cohesive soil with good plasticity into the hole by using an excavator, reversely turning and downwards pressurizing by a drilling machine, forwardly turning and taking soil, fully compacting the hole wall, and re-forming the hole;

⑵ severe collapse, pouring C15 concrete with low grade into the hole, re-forming the hole after the concrete is finally set for 24 hours (time is determined according to air temperature), and repeatedly replacing and filling C15 until the pile is finally formed, wherein the C15 measures tickets supplied by concrete companies.

3.7.2 hole shrinkage treatment:

⑴ plasticity weak layer, the weak soil is not easy to collapse in large area, the hole can be repeatedly swept, some dry soil is appropriately backfilled in the hole, the soil is positively drilled after back pressure, part of dry soil is pressed into the hole wall, the plasticity of the sludge layer is increased, the control of the drilling speed is noticed, if the hole bottom meets water, the depth of the hole opening from the water surface is recorded, the hole opening is stopped for a period of time after the water surface is lifted out when the drilling is lifted, the drilling is lifted after the water in the drill bit flows out of the drill bit, so as to reduce the washing of the water to the hole wall, thereby reducing hole collapse and hole shrinkage.

⑵ the weak layer with poor plasticity (or the weak layer is thick), stopping drilling when the drilling is impossible, repeatedly taking soil to form a cavity (5-8 buckets are suitable), filling C15 concrete into the hole, forming the hole again after 24 hours, and repeating the above steps when the weak layer is thick.

According to the actual situation of the project, the steel pile casing is arranged at the orifice of the pile position in the project, so that soil falling during the process of descending the reinforcement cage can be prevented.

Manufacturing and embedding a steel casing: the steel casing with the length of 4m is made of a steel plate with the thickness of 12 mm; the inner diameter of the steel casing is larger than the diameter of the drill bit, and the specific size is selected according to the design requirement; the embedding depth of the steel casing can meet the requirements of design and relevant specifications. When the pile casing is buried, the steel pile casing is buried in the soil layer by less than 0.5m (the elevation of the earth-rock interface is-3.2 m) and is 0.3m higher than the construction ground; before embedding the steel casing, accurately measuring lofting to ensure that the position deviation of the top surface of the steel casing is not more than 5cm, and ensuring that the verticality of the steel casing is not more than 1% in embedding;

3.7.3 prevention and treatment of perfusion accidents

⑴, water inlet of guide pipe

The water inflow of the conduit is mainly caused by the following three reasons:

①, the first concrete is not reserved enough, or although the concrete is reserved enough, the distance between the bottom opening of the guide pipe and the bottom of the hole is too large, and the concrete can not bury the bottom opening of the guide pipe after falling, so that muddy water enters from the bottom opening.

Prevention and treatment methods: if the water enters the guide pipe, the guide pipe is immediately lowered to a position 250-400mm away from the bottom of the hole, and enough stored concrete is added again to punch the bottom, so that the reinforcement cage needs to be lifted out and cleaned by adopting a re-drilling method. Then putting down the framework and the conduit again, and throwing in the first batch of concrete which is enough to be stored for re-pouring.

②, the joints of the pipes are not tight, the rubber pad between the joints is squeezed open by the high-pressure air bag of the pipe, or the welding seam is broken, and water flows into the joints or the welding seam.

③, the guide pipe is lifted too hard, or the detection is wrong, the bottom opening of the guide pipe exceeds the original concrete surface, and muddy water is poured into the bottom opening.

Aiming at accidents caused by ② and ③, the original conduit is pulled out and replaced and a new conduit is placed according to specific conditions, or the original conduit is inserted into the conduit for continuous irrigation, but water and settled soil entering the conduit are sucked out by a mud sucking and water pumping method before irrigation.

⑵, clip tube

The pipe clamping mainly has the following two conditions:

①, when pouring, the water-proof plug is stuck in the pipe, or the cement paste in the concrete is washed away and the coarse aggregate is concentrated to block the pipe because of the concrete itself, such as too low slump, poor fluidity, cobble, uneven mixing, segregation during transportation, water leakage at the pipe joint, and no covering during the transportation in rainy days.

The treatment method comprises the following steps: the concrete in the pipe is punched by a long rod, and the guide pipe is shaken by a lifting rope or an attached vibrator is arranged on the guide pipe to lead the water-stop bolt to fall. If the pipe still can not fall down, the pipe and the concrete in the pipe are lifted out of the borehole, cleaned and trimmed (care is taken not to make the concrete in the pipe fall into the borehole), and then the pipe is hoisted again and poured again. Once the concrete mixture has fallen into the hole, the mixture particles scattered at the bottom of the hole must be removed.

When the catheter is lifted, the weight of the upper part of the catheter is heavy and the weight of the lower part of the catheter is light, and reliable measures are taken to prevent people from being injured by overturning.

②, when the machine is out of order or the pouring time is too long, the concrete stays in the guide tube, or the pouring time is too long, the initial poured concrete is already set, the falling resistance of the concrete in the guide tube is increased, the concrete is blocked in the tube.

When the pouring time is long, the first concrete in the hole is initially set, the guide pipe is blocked with concrete, the guide pipe is pulled out, the drilling machine is installed again, the concrete in the reinforcement cage is drilled and sucked out by a small drill bit, and the reinforcement cages are pulled out one by using a punching and grabbing hammer. Then filling the well hole with clay and gravel, and drilling again to form the pile after settling.

⑶, collapse hole

During the pouring process, if the water (mud) level in the well casing suddenly rises to overflow the casing, and then suddenly falls and bubbles emerge, the collapse hole sign is suspected, and the collapse hole sign can be detected by a probe of a detector or a depth extending measuring hammer. If the depth measuring hammer is originally hung on the surface of concrete and is not taken out, the current concrete can not be lifted, or the surface depth measured by a probe of a depth measuring instrument can not reach the original depth, and the difference is very large, the occurrence of hole collapse can be confirmed. The reason for the collapse hole is that water leaks around the bottom of the pile casing, the water level in the hole is reduced, the original hydrostatic pressure cannot be maintained, and the collapse hole is possibly caused by stacking heavy objects or mechanical vibration and the like around the pile casing.

After the hole collapse occurs, the reason should be found out, and corresponding measures such as maintaining or increasing the water head, removing the heavy object, eliminating the vibration and the like are taken to prevent the hole collapse from continuing. Then sucking out the soil in the collapsed hole by a suction dredger; if the hole collapse is not continued, normal perfusion can be recovered. If the collapse hole still does not stop and the collapse position is deeper, the guide pipe is pulled out, the concrete is drilled out, the reinforcing steel bars are grabbed out at the same time, only the hole position is reserved, the C15 commercial concrete is backfilled, and the hole is drilled again after one day interval to form the pile.

⑷, burying the pipe

The reasons for the creation of a buried pipe are generally: the conduit is buried too deeply in the concrete, or the friction between the conduit and the concrete is too large because the concrete in and out of the conduit is initially set, or the conduit is broken because of too violent lifting of the conduit.

The prevention method comprises the following steps: the buried depth of the conduit should be strictly controlled within 2-6m, and the conduit should be frequently measured and lifted in time. The attaching vibrator is arranged on the conduit, and the concrete is properly vibrated before the conduit is pulled out or when the irrigation is stopped for a long time, so that the concrete around the conduit is not pre-set too early; the retarder is mixed into the first batch of concrete to accelerate the pouring speed; the pipe joint bolt should be checked for stability in advance; the catheter cannot be pulled violently when lifted.

If the pipe burying accident happens, the chain pulley and the jack can be used for trial pulling at the beginning. If the concrete can not be pulled out, the concrete can be inserted into a protective cylinder with small diameter until the concrete is poured into the concrete when the loss of the initial setting fluidity of the concrete is not overlarge, and the mud residue on the surface of the concrete is sucked out by a mud suction machine; sending a diver to the concrete surface, and cutting off the guide pipe flush with the concrete surface under water; the small protective sleeve is pulled out, and the catheter is placed again for perfusion. After the pile is poured, reinforcement is needed between the upper and lower broken layers.

⑸ rising up reinforcement cage

The steel reinforcement cage rises mainly because the concrete surface is close to the bottom opening of the steel reinforcement cage, and the bottom opening of the guide pipe is 3m below the bottom opening of the steel reinforcement cage to 1m above, and the pouring speed (m) of the concrete is higher than that of the guide pipe³Min) is too fast, so that the concrete falls down and is flushed out of the bottom opening of the guide pipe to be backflushed upwards, and the jacking force is greater than the gravity of the reinforcement cage.

In order to prevent the reinforcement cage from rising, when the bottom opening of the guide pipe is 1m-3m lower than the bottom of the reinforcement cage and the concrete surface is 1m above and below the bottom of the reinforcement cage, the concrete pouring speed is slowed down, and the allowed maximum pouring speed is 0.4 m ³①, properly reducing the number of stirrups at the lower end of the reinforcement cage to reduce the upward jacking force of concrete, ②, welding the upper end of the reinforcement cage on a casing to bear part of the jacking force and prevent the lifting of the reinforcement cage, ③, arranging 1-2 reinforced annular ribs with the diameter not less than that of the main ribs at the bottom of the hole, and firmly welding the reinforced annular ribs at the bottom of the reinforcement cage by proper number of traction ribs.

⑹ cast-in-place pile reinforcing method

Various quality accidents of the cast-in-place pile all lead to the reduction of the pile body strength, and can not meet the stress requirement of design, so that reinforcement treatment is needed, a cement paste reinforcement method is generally adopted, and the construction key points are as follows:

①, for the pile needing reinforcement, except drilling a coring hole by a geological drilling machine (two holes need to be drilled for the pile detected by ultrasonic and other non-damage depth detection methods), one hole is drilled for the grout inlet, the other hole is drilled for the grout outlet, the hole depth is required to reach 1m below the reinforcement position, and the pile is required to reach the bedrock.

②, pressing clean water into one hole with high pressure water pump, the pressure is not less than 0.5-0.7 Mpa, washing the mud and loose concrete slag out of the other hole until clean water is discharged.

③, grouting by a grouting pump, pressing pure cement slurry with water-cement ratio of 0.8 for the first time, inserting a slurry inlet pipe into the drilled hole for more than 1.0m, filling the periphery of the slurry inlet pipe with hemp wadding to prevent cement slurry from flowing out from a slurry inlet, pressing in concentrated cement slurry with water-cement ratio of 0.5 after the original clear water in the hole is pressed out from a slurry outlet, fully diffusing the slurry, stopping grouting once by once, stopping grouting after the concentrated slurry flows out from the slurry outlet, sealing and filling the slurry outlet by broken stones, and plugging by a hemp bag.

④, finally pressing in cement paste with the cement-cement ratio of 0.4, increasing the grouting pressure to 0.7-0.8 Mpa, closing the grout inlet gate, and closing the grout for 20-25 min under stable pressure, thus finishing the grouting operation.

The construction difficulty of the karst geology of the embodiment mainly lies in the complexity of geological conditions and more accidental accidents in construction. The geological upper layer of the standard section is silty clay, and the base layer is a weakly weathered limestone layer; some pile positions contain strong weathered layers and are easy to collapse; the pile position containing the rock cavern is more. The construction is difficult because the rainfall is large during construction, the underground water is abundant, and the underground water can emerge about 1m below the earth surface. The project mainly has the following construction difficulties to be solved.

1. Slurry leakage and hole collapse: due to the complexity of the karst cave stratum and the unpredictability of the shape and the size of the underground karst cave, the phenomena of slurry leakage and hole collapse are easy to occur in the construction process, particularly under the condition that the underground karst caves are communicated with each other.

2. Burying and clamping a drill: in the construction process of the karst cave stratum, the phenomena of drill burying and drill blocking are easy to occur when a drilling machine lifts a drill. When no filler exists in the karst cave, a top plate of the drilling tool is extremely easy to clamp with the top of the cave when the drilling tool is lifted, so that the drilling tool is clamped; when the karst cave is filled with fillers such as limestone, broken stone, soft-plastic or flow-plastic clay and the like, the buried drilling accident is easy to occur in the construction process.

3. Inclined holes: in the limestone layer at the top of the karst cave, most of the limestone at the pile position can present a large inclined plane due to large change of the rock surface of the limestone, and inclined hole accidents can be caused by careless drilling.

For the karst cave with a large amount of leaked slurry, concrete can be pumped inwards to fill the position 0.5m above the karst cave, and drilling is continued after the karst cave is initially set. For the mutually communicated super large karst caves, in order to reduce the concrete pouring amount, when the holes are drilled to the large karst caves, the holes are filled with low-grade concrete, and when the concrete has certain strength, the holes are drilled continuously. For larger karst caves, a method of following by a steel casing can be adopted. Manufacturing and embedding a steel casing: according to geological conditions, a steel casing with the length of 4m and a steel plate with the thickness of 12mm are adopted for manufacturing; the inner diameter of the steel casing is larger than the diameter of the drill bit, and the specific size is selected according to the design requirement; the embedding depth of the steel casing can meet the requirements of design and relevant specifications. When the pile casing is buried, the steel pile casing is buried below 0.5m of soil layer and 0.3m higher than the construction ground; before the steel casing is buried, accurate measurement and lofting are carried out, the position deviation of the top surface of the steel casing is not more than 5cm, and the verticality of the steel casing is not more than 1% in burying.

Under the condition that the rock surface of the limestone layer at the top of the karst cave is greatly changed and the limestone is inclined, the cylinder drill is required to be replaced when the limestone karst cave layer is drilled, the hole is drilled after being swept, and the pressure reduction drilling is required when the limestone layer is initially drilled. In actual operation, in addition to the situation that the position of the karst cave is required to be judged in advance by combining with a geological report, the drilling slag and the vibration condition of the drilling machine are required to be carefully observed to judge the stratum.

And selecting a proper drill bit, wherein a straight cylinder type is selected as a drill bit cylinder body, the height of the drill bit cylinder is increased as much as possible on the premise of allowable conditions, and the drill bit is provided with a guide structure.

And for the drilling of the karst cave bottom limestone with the karst cave height smaller than the barrel drilling height, the pressure reduction drilling is also started, and the conventional drilling method can be used after the karst cave bottom limestone passes through the inclined top layer. For the karst cave with the height larger than that of the barrel drill, as the bottom layer may encounter the stalagmite, the side wall rock of the karst cave, the half edge rock and the half edge soil and the like, the concrete and other fillers can be pumped into the oriented karst cave and are 0.5m higher than the top plate (more than the stone bud and the inclined plane) of the karst cave, and then the drilling is carried out by adopting a specially-made lengthened barrel drill.

While the invention has been described in further detail with reference to specific preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A construction method for rotary digging cast-in-place piles under karst landform is characterized by comprising the following steps:

2) drilling by rotary digging: embedding a pile casing, selecting the length of the steel pile casing according to the soil texture condition of site excavation, embedding the pile casing, and embedding the steel pile casing into a soil layer below 0.5m and 0.2-0.5m higher than the construction ground; the drilling bucket rotates, cuts soil, lifts and unloads the soil, and the circulation is repeated until the hole is formed;

2. The construction method for rotary excavating cast-in-place piles under the karst landform is characterized in that for treatment of hole collapse in the rotary excavating process:

slight hole collapse: backfilling cohesive soil with good plasticity in the hole by using an excavator, reversely turning the drilling machine to pressurize downwards, positively turning to take out soil, fully compacting the hole wall, and forming the hole again;

severe hole collapse: and pouring low-grade concrete into the holes, forming the holes again after the concrete is finally set, and repeatedly replacing and filling the low-grade C15 concrete until the holes are formed finally.

3. The construction method of the rotary digging cast-in-place pile under the karst landform is characterized in that 3 sound measuring pipes are buried in each pile and evenly distributed along the circumference of the pile body in an equal distribution mode; the bottom of the sounding pipe is closed, the sounding pipe is 5-15cm above the bottom of the pile body, the top of the sounding pipe is 30-70cm higher than the top of the pile, and the interior of the sounding pipe is kept clean; the sound measuring pipe is connected by adopting threads, the joint is sealed and does not leak water or foreign matters, and the sound measuring pipe is bound at the inner side of the reinforcement cage and is welded and fixed by using the reinforcement.

4. The construction method of the rotary excavating filling pile under the karst landform is characterized in that a dry drilling method is adopted for the rotary excavating drilling hole.

5. The construction method for the rotary excavating filling pile under the karst landform is characterized in that a drill bit of a guide structure is selected for rotary excavating drilling, and a cylinder body of the drill bit is of a straight cylinder type.

6. The construction method of the rotary excavating filling pile under the karst landform is characterized in that longitudinal steel bars of the steel reinforcement cage are mechanically connected through rolled straight threads or are overlapped with double-sided welding joints, stressed steel bar connecting joints are arranged at positions with small internal force and are arranged in a staggered mode, and the number of the steel bar joints in the same section cannot exceed 50% of the total number of joints; the distance between the stiffening hoops is 1-3 meters, the stiffening hoops are internally provided with reinforcing steel bars shaped like a Chinese character 'jing', and the stiffening hoops are reinforced by inclined struts.

7. The construction method of the rotary excavating filling pile under the karst landform is characterized in that after a reinforcement cage is hoisted into a hole, the deviation between the center of the reinforcement cage and the center of a pile hole is not more than 100mm, and the deviation of the height of the bottom surface of the reinforcement cage is not more than 100 mm.

8. The construction method of the rotary excavating filling pile under the karst landform is characterized in that in the process of filling the guide pipe, when the guide pipe enters water, the guide pipe is lowered to a position 250-400mm away from the bottom of a hole again, and enough reserved concrete is poured again to flush the bottom, or a reinforcement cage is lifted and removed by adopting a re-drilling method; then putting down the framework and the conduit again, and throwing in enough first batch of concrete for re-pouring.