1. Overview
Rotary bored pile-concrete bored pile with rotary drilling rig.
rotary drilling rig-it is a kind of drilling rig which is powered by telescopic drill pipe to make a circular rotary bucket (referred to as drilling bucket) with soil cutting blades cut soil and drill holes to form piles.
rotary drilling is an advanced and effective hole-forming method for drilling holes in soil layer with rotary drilling rig. In this drilling method, a barrel-shaped drilling tool (also called a drill bucket) with rake teeth at the bottom is connected to the lower end of the drill string. With the self-weight of the drilling tool and the pressure of the drilling machine, the rake teeth cut into the soil layer, and the drill bucket rotates at the same time under the action of rotary torque to cut and dig the soil layer in front, and the cut clods are included in the bucket. When the soil in the bucket is loaded to a certain amount and lifted out of the hole by the drill, open the bucket and discharge the drilling slag. Then put the drill bucket into the manhole and repeat the above operation. Drill and dig again and again and lift the bucket to unload the slag, and the hole is deepened and finally formed.
At present, rotary drilling rigs are widely used in construction engineering, municipal engineering and traffic engineering, such as the Bird's Nest pile foundation of the National Stadium, the bridge pile foundation of the Qinghai-Tibet Railway, the bridge pile foundation of the Beijing-Tianjin express railway, the transportation hub of Tianjin and the Beijing-Shanghai high-speed railway under construction. Its maximum drilling depth L exceeds 8m, and its maximum diameter exceeds 2m.
2. construction technology of rotary bored pile
1) process flow
process flow chart of rotary bored pile construction
3. key points of process and supervision
1) before formal construction, it is advisable to try to make holes.
2) According to different stratum conditions and groundwater depth, the bored piles drilled by rotary drilling should be bored by dry operation, mud wall protection and casing wall protection.
hole formation by dry operation
hole formation by slurry wall protection
hole formation by casing wall protection
3) Installation of drilling rig: the self-weight of rotary drilling rig is heavy, so the required ground endurance is not less than 1kPa, and the position where the shoe is located should be flat and the slope should not be greater than. (because first, there is a limit to adjusting verticality only by oil cylinder; Secondly, it is easy to produce power loss, and the inclination angle is larger during the turning process, and the high center of gravity is easy to cause safety accidents; Third, prevent tilting displacement. )
If necessary, steel plate or cushion (road base plate) can be laid on the site to ensure its safe walking and operation.
4) Bolt pile: The pile point must pass the self-inspection according to the pre-inspection results, and only after Party A or the supervisor check the line can two mutually perpendicular straight lines intersect at the pile point, and cross control points are determined and marked.
5) Align the hole position: firstly, adjust the mast of the rotary drilling rig to make it in a vertical state, and align the center of the auger bit or the center of the drill bucket with the hole position to drill holes.
6) Each pile shall be equipped with steel liners, which shall meet the provisions of relevant specifications.
the role of the casing:
(1) fixing the pile position and guiding the drilling
(2) protecting the orifice to prevent the soil layer at the orifice from collapsing.
(3) isolate the surface water and keep the water level in the hole higher than the construction water level to stabilize the hole wall.
Embedding of the liners: After setting the cross control piles, bury the liners, measure the hole depth, guide the elevation to the top of the liners with a level, and make records. It shall comply with Article 6.3.4 of JGJ 94-28 Technical Specification for Building Pile Foundation.
7) inspection of liners: according to the deviation degree of liners with cross control points, if it exceeds the range, it should be re-buried.
(1) The deviation between the center of the pile casing and the center of the pile position shall not be greater than 5 mm.
(2) The pile casing can be made of steel plates with a thickness of 4-8 mm, and its inner diameter should be greater than the diameter of the drill bit by 1 mm. It is advisable to open 1-2 slurry overflow holes in the upper part.
8) The preparation of mud is the same as that of other mud-protected bored piles. (when the hole is formed by wet operation)
9) rotary drilling: the shape and specification of the cutting teeth at the bottom of the drill bucket, the type of drill pipe and the length of steel wire rope are selected according to the formation characteristics. Explanation:
(1) During the construction of rotary drilling rig, the machine should be stable and safe, and if necessary, a steel plate or cushion (road base plate) can be laid on the site to ensure its safe walking and operation.
(2) Before the hole is formed and every time the drill bucket is lifted out, the conditions of the connecting pin of the drill bucket and the drill pipe, the connecting pin of the drill bucket door and the wire rope should be checked, and the muck on the drill bucket should be removed.
(3) The rotary drilling rig should be used to drill holes by jumping, and the minimum distance between the soil poured out by the drill bucket and the pile orifice should be greater than 6m, and it should be removed in time. The mud should be replenished synchronously according to the drilling speed to keep the required mud level unchanged.
(4) In order to ensure the stability of the hole wall, the liners with appropriate length should be lowered into the hole according to the thickness of the loose layer of topsoil, and the height of the mud level should be ensured. With the increase of mud loss and hole depth, mud should be replenished into the hole in time to maintain the pressure balance in the hole.
(5) When drilling soft layers, especially cohesive soil layers, long bucket teeth and a bucket with large tooth spacing should be selected to avoid drilling paste. After drilling, the bottom cutting teeth should be checked frequently, the mud between teeth should be cleaned in time, and the dull bucket teeth should be replaced. When drilling hard soil, if it is found that the drilling depth is too small each time, the amount of slag in the drill bucket is too small. A drill bucket with a smaller diameter can be replaced. First, drill a small hole, and then use a drill bucket with a suitable diameter to enlarge the hole.
(6), drilling sand gravel layer, in order to strengthen the hole wall and facilitate the removal of sand gravel, an appropriate amount of clay balls can be put into the hole in advance, and a double floor sand fishing drill bucket can be used to prevent sand gravel from leaking from the bottom during drilling.
(7) If the drill bit is lifted too fast, it will easily generate negative pressure and cause the hole wall to collapse. Generally, the lifting speed of the drill bucket can be used according to the recommended value in Table 3-23.
(8) When drilling in the bearing stratum at the pile end, the bearing stratum may be loosened due to the lifting of the drill bucket, so attention should be paid to the lifting speed of the drill bucket when approaching the elevation at the bottom of the hole.
1) In unstable formation, the borehole wall is extremely unstable, and the whole casing is used for drilling.
11) hole cleaning and hole inspection: when the hole reaches the design depth, a hole cleaning drill should be used to clean the hole or other reliable methods. And according to the requirements of building pile foundation specification, check the hole location, hole depth, aperture, verticality and sediment thickness at the bottom of the hole.
12) inspection of reinforcing cage: the construction method is the same as other cast-in-place piles.
13) lowering the reinforcing cage: during transportation in the field, hoisting in place, lowering into the hole, etc., it shall be ensured that the reinforcing cage will not be permanently deformed.
14) catheter inspection: the catheter should be installed and pressure tested before use, and the minimum pressure is .6-1.MPa ..
15) Lower conduit: the conduit connection should be sealed and straight, the lower orifice of the conduit should be about 3～5mm away from the bottom of the hole, the pile with small diameter can be enlarged, the conduit platform should be flat, and the splint should be firm and reliable.
16) sediment measurement (secondary hole cleaning): sediment should not exceed the requirements in Technical Specification for Building Pile Foundation: end bearing pile ≤5mm, friction pile ≤1mm
*17) Underwater concrete pouring should meet the following requirements:
(1) When concrete pouring begins, appropriate conduit length should be provided according to the hole depth to ensure the conduit bottom to the hole bottom. When the pile diameter is less than 6mm, the distance from the bottom of the conduit to the bottom of the hole can be appropriately increased.
(2) Check the sealing condition of each joint of the catheter in the process of going into the manhole to prevent leakage.
(3) There should be enough concrete reserves, and the volume of storage hopper should meet the requirements when on-site mixing is adopted. When commercial concrete is adopted, the first tank of concrete should be continuously and evenly added so that the conduit is buried at least .8m below the concrete surface at one time.
(4) The buried depth of the conduit should be 2 ~ 6m. It is forbidden to raise the concrete surface of the conduit. There should be a special person to measure the buried depth of the conduit and the elevation difference of the concrete surface inside and outside the conduit, and fill in the underwater concrete pouring record.
(5) Underwater concrete must be continuously constructed, and the pouring time of each pile is controlled according to the initial setting time of the initial concrete, and all faults in the pouring process shall be recorded for the record.
(6) Control the last pouring amount, the pile top shall not be low, and the flood height to be chiseled must ensure that the exposed pile top concrete reaches the strength design value.
18) For dry hole formation, the bottom of the hole should be checked again before pouring concrete to ensure that the bottom of the hole is clean and free of impurities such as waste soil. When pouring concrete, string cylinders or conduits should be used, and the lower opening of string cylinders or conduits should not be more than 2m away from the bottom of the hole; When pouring concrete into the belled pile, it should be poured to the top surface of belled bottom for the first time, and then vibrated and compacted; When the concrete is within 5m below the top of the cast-in-place pile, it should be compacted with the plug-in vibrator as it is poured, and the pouring height should not be greater than 1.5m at a time. The pile top should be over-poured by .3m to ensure the quality of the pile top.
4. Advantages and application scope of rotary drilling rig construction
Advantages of rotary drilling rig construction
The main advantages of rotary drilling rig hole-forming method are as follows:
1. Rotary drilling hole-forming construction has the advantages of low noise, low vibration, large torque and fast hole-forming speed.
2. The rotary drilling technology can be suitable for all kinds of complex strata, and can be used for construction in strata with high water level and large pebbles, which cannot be constructed by reverse circulation and long auger drilling, thus greatly improving work efficiency and construction quality.
3. The crawler chassis bears the load, and the grounding pressure is small, so it is suitable for various construction conditions, walking freely in the construction site, being flexible, and convenient and fast for hole location.
4. The telescopic drill pipe not only transmits the rotary torque and axial pressure to the drill bit, but also realizes the rapid lifting and unloading of the drill bit by using its own flexibility, so as to shorten the auxiliary drilling time and improve the drilling rate.
5. High degree of automation, good pore-forming quality and high efficiency; The drilling rig is driven by full hydraulic pressure and controlled by computer, which can accurately position drilling, automatically correct drilling verticality and automatically measure drilling depth, and ensure drilling quality to the greatest extent. The drilling rig can efficiently drill in various stratum conditions, and it only takes one hour to construct a pile hole with a depth of 2m of φ 8 ~ 12, which is 8 ~ 1 times the efficiency of a general reverse circulation machine.
6. adopt a new hole-forming technology of dry hole drilled by rotary drilling or static wall protection without mud circulation to reduce mud pollution and realize civilized construction.
7. The rotary drilling rig uses the drill bucket non-circulating technology to drill. The cuttings excavated by the drill bucket can directly understand the changes of the construction stratum compared with the forward and reverse circulation drilling technology. In addition, its computer-controlled precise positioning system can accurately locate the squeezed branch pile in the designed position and effectively verify it, so as to ensure that the squeezed branch pile is located in the designed bearing stratum and effectively ensure the construction quality of the squeezed branch pile.
8. Using sand scoop to clean the sediment at the bottom of the hole can make the bottom of the hole cleaner than ordinary reverse and forward circulation drilling technology, make full and effective use of bearing stratum, reduce foundation settlement and save engineering cost.
9. Bring diesel power to alleviate the contradiction of power shortage on the construction site, and the potential safety hazards caused by power cables can be eliminated.
5. Scope of application
1) Suitable for clay, silt, sand, gravel, pebbles, strongly weathered bedrock, backfill soil, miscellaneous fill soil and other strata.
2) The pore-forming diameter ranges from 6 mm to 3 mm, and the pore-forming depth can reach 8m or even deeper.
3) It is mostly used for foundation piles of various structures, anti-floating piles and slope protection piles for foundation pit support.
4) The rotary drilling rig can also be used for the construction of large-diameter dewatering wells.
6. Quality inspection requirements
(1) Quality inspection standard for reinforced cages of concrete cast-in-place piles (mm)
(2) The quality inspection standard for concrete cast-in-place piles should comply with the regulations.
Several key links
1. Inspection of bearing capacity and pile integrity-relevant provisions on pile foundation inspection
1. Sampling inspection of single pile bearing capacity and pile integrity should be carried out for engineering piles (national standard-strong bar)
2. The selection of the inspected piles for sampling inspection of single pile bearing capacity and pile integrity acceptance should meet the following provisions:
1) Piles with questionable construction quality;
2) piles that the designer considers important;
3) Piles with abnormal local geological conditions;
4) Piles with different construction techniques;
5) Class III piles determined in integrity inspection;
6) In addition to the above provisions, piles of the same type should be evenly and randomly distributed.
3. Requirements for bearing capacity testing
1) For bored piles with a building safety level of Grade I and a diameter of no more than 6mm, the static load test method should be adopted on site. Bored cast-in-place piles in other situations and hand-dug cast-in-place piles in various situations can be verified according to the bearing layer inspection and pile quality inspection at the end of the hole. (Landmark)
*2) The bearing capacity of cast-in-place piles should be tested by static load test in the following situations: (Landmark)
a. Piles with questionable construction quality;
B. mechanical pore-forming cast-in-place pile located in complex strata such as thick fill and silt;
C. piles with the detection conclusion of Class III when detected by other methods;
D. Piles constructed with new technology, new methods and new materials
*3) For end-bearing large-diameter cast-in-place piles, when the vertical compressive bearing capacity of a single pile cannot be detected due to equipment or site conditions, the core drilling method can be used to measure the thickness of sediment at the bottom of the pile and drill a rock-soil core sample of the bearing layer at the end of the pile to test the bearing layer. Sampling number should not be less than 1% of the total number of piles, and should not be less than 1. (national standard)
(core drilling method-a method of drilling core samples with a drilling machine to detect the pile length, pile defects, sediment thickness at the bottom of the pile and the strength, compactness and continuity of pile concrete, and to judge the geotechnical properties at the pile end. )
4, integrity testing requirements
*1) The integrity of foundation pile concrete can be tested by low-strain geophysical exploration, acoustic wave transmission method or core-pulling method. For single pile in the column and cast-in-place pile with cross-section size greater than 8mm * 8 mm or diameter greater than 8 mm, acoustic wave transmission method or drilling core-pulling method should be adopted for detection.
(bottom strain method-a detection method that uses low-energy transient or steady-state excitation to excite the pile top, measures the velocity time-history curve or velocity admittance curve at the top of the pile, and judges the integrity of the pile through wave theory analysis or frequency domain analysis. Advantages: convenient and flexible, fast detection speed, suitable for the detection of precast piles and small diameter cast-in-place piles.
acoustic wave transmission method-a method of transmitting and receiving acoustic waves between embedded acoustic pipes, and testing the integrity of pile body by measuring the relative changes of acoustic parameters such as acoustic time, frequency and amplitude attenuation of acoustic waves propagating in concrete medium. )
*2) When low-strain geophysical prospecting method is used to detect the pile quality, the number of foundation piles detected under the same conditions in a unit project should not be less than 3% of the total number of this type and should not be less than 2. When using acoustic wave transmission or drilling core-pulling method to detect the pile quality, the number of foundation piles under the same conditions in a unit project should not be less than 1% of the total number of such piles, and all single piles in the column should be detected. (The number of pile caps with three or less piles under the column shall not be less than one)
*3) According to the relevant regulations of our city,
a) All pile foundations constructed by rotary drilling and hole-forming technology shall be tested by wave transmission method or core-pulling method.
b) For piles with a depth of more than 15m, the quality of finished piles shall be tested by wave transmission method or core-pulling method.
II. Inspection of hole depth
Purpose:
First, to prevent the pile end from being supported on the designed bearing layer
Second, to ensure that the rock-socketed depth must meet the design requirements
1. For manually dug piles and dry rotary bored piles with small hole depth, the rock-socketed depth can be inspected by directly lowering the hole with ruler and naked eye observation.
*2. The rotary excavation piles that cannot be inspected in the hole should be controlled and inspected according to the following procedures
Cause: uneven distribution of strata. For example, the distribution of strata is inclined or fluctuating.