Cast-in-place pile: a pile formed by drilling holes directly on the pile site by machinery or manpower, placing steel bars in the holes and pouring concrete.
Compared with precast piles, cast-in-place piles are not limited by stratum changes, and there is no need to connect and cut piles, which saves steel and has low vibration and noise.
According to the pore-forming method, cast-in-place piles can be divided into mud-protected cast-in-place piles, immersed cast-in-place piles, dry cast-in-place piles, and hand-dug cast-in-place piles.
1. Construction technology of hand-dug cast-in-place pile?
Large-diameter cast-in-place pile is a pile foundation formed by manual excavation, placing steel cages and pouring concrete, also known as pier foundation. It consists of cap, pile body and enlarged head (Figure 2-3 1), which passes through the deep soft soil layer and sits directly on the hard rock layer.
Advantages are large pile diameter and high bearing capacity; During construction, the quality of the hole can be directly checked in the hole to observe the geological and soil changes; The depth of pile hole is controlled according to the actual situation of foundation soil layer, and the hole cleaning and slag removal at the bottom of pile are completely clean, which is easy to ensure the quality of concrete pouring. ?
Figure 2-3 1? Digging pile with concrete retainer
1) construction technology
The hand-dug pile wall protection often adopts cast-in-place concrete wall protection, steel casing or open caisson wall protection. When using cast-in-place concrete retaining wall, the construction process is as follows:
(1) Determine the pile position and set out.
② Earthwork excavation. Excavation is carried out in sections, and the height of each section depends on the vertical bearing capacity of the earth wall, which is generally 0.5 ~ 1.0.
M, the excavation diameter is the design pile diameter plus twice the thickness of the retaining wall. The excavation sequence is from top to bottom, first in the middle, then at the edge of the hole.
(3) Supporting wall formwork. The height of formwork depends on the height of each excavation earthwork, which is generally 1.
M, consisting of 4-8 active templates. The thickness of the retaining wall should not be less than 100mm, and it is generally taken as D/ 10? + ? 5cm(D is the pile diameter), and the thickness of the retaining wall of the first well circle should be increased 100 ~ 150 compared with the following sections.
Mm, the upper and lower retaining walls can be tied with φ6-φ8 steel bars about 1 m long.
(4) Place the operating platform on the top of the template. The platform can be made of angle steel and steel plate into a semicircle, which together form a whole circle for temporary placement and pouring of concrete.
⑤ Pouring retaining wall concrete. The strength grade of retaining wall concrete should not be lower than that of pile concrete, and attention should be paid to pouring and tamping.
Real. According to the seepage situation of soil layer, accelerator can be considered. It is not allowed to pour concrete for retaining wall when the formwork is submerged by water in the pile hole. Each retaining wall shall be continuously constructed on the same day. The lap length of upper and lower retaining walls shall be not less than 50 mm
6. Dismantle the formwork and continue the construction in the next section. Generally, when concrete 24 is poured.
H, can dismantle. If honeycombs, holes and water leaks are found in the retaining wall, they should be reinforced and blocked in time to prevent water outside the hole from flowing into the pile hole through the retaining wall. When the retaining wall meets the quality requirements, the next section of earthwork can be excavated, and then the formwork is poured into the retaining wall concrete, and so on, until the depth required by the design is dug and compacted.
Design and expand the bottom.
⑦ Place reinforcing cage and pour concrete. When there is water at the bottom of the hole, it should be removed before pouring concrete. When concrete is poured on steel bars,
When the design elevation of the bottom surface is reached, the reinforcing cage is placed again, and the pile body concrete is continuously poured.
2) Construction precautions
(1) pile hole excavation, when the pile spacing is less than 2 times the pile diameter and less than 2.5.
M, interval excavation should be adopted. The minimum construction clear distance of row pile jump excavation shall not be less than 4.5 m, and the hole depth shall not be greater than 40 m.
(2) After each excavation, messenger wire must be used to check whether the center line position is correct, and the deviation of the center line plane position of the pile hole shall not exceed 50? Mm, the verticality deviation of the pile shall not exceed 65438 0%, and the pile diameter shall not be less than the design diameter.
(3) Prevent soil wall collapse and quicksand. If the excavation encounters loose or quicksand soil layer, the excavation depth of each section can be reduced (take 0.3-? 0.5
M) Or adopt steel casing and precast concrete open caisson, etc. As a retaining wall, it will be constructed in the general way after passing through this soil layer. When quicksand phenomenon is serious, well point precipitation should be adopted.
(4) When pouring concrete into pile body, attention should be paid to hole cleaning and water accumulation prevention. Pile concrete should be poured continuously at one time, and construction is not allowed.
In order to prevent concrete segregation, concrete should be poured in tandem. If the inflow of groundwater through the retaining wall is too large to be discharged, the underwater concrete should be poured by conduit method.
⑤ Safety measures must be formulated.
A. construction personnel must wear safety helmets when entering the hole. When someone is working in the hole, someone must supervise and protect the hole.
B an emergency ladder must be set in the hole for personnel to go up and down the well; Electric hoist, hanging cage, etc. Use should be safe and reliable, and equipped with automatic control equipment.
Dynamic clamping safety device; Hemp rope and nylon rope shall not be used to hang or trample the sidewall flange up and down; Electric hoist must be tested for its safe lifting capacity before use.
C poisonous and harmful gases in the underground must be detected before starting work every day, and adequate safety protection measures must be taken. When the excavation depth of the pile hole exceeds 10m, there should be special equipment to supply air to the underground, and the air volume should not be less than 25 L/s.
D the retaining wall should be 200 ~ 300mm above the ground to prevent sundries from rolling into the hole; A guardrail with a height of 0.8m should be set around the hole.
E safety lights or miner's lamps below 12V shall be used for lighting in the tunnel. Electrical appliances used must have strict grounding, zero connection and leakage protector (such as submersible pump, etc.). ).
2. Construction technology of bored pile
1) Dry bored pile
(1). The construction technology of dry bored pile is shown in Figure 2- 17.
(2) Dry hole-forming generally adopts spiral drilling machine to drill holes. The outer diameter of the auger bit is 400mm, 500mm and 600mm respectively, and the drilling depth is 12m, 10m and 8m respectively. It is suitable for general clay layer, sandy soil and artificial fill foundation without groundwater within the pore-forming depth, but not for soil layer and muddy soil with groundwater.
(3) After the drilling rig is in place, the drill pipe should be vertically aligned with the center of the pile position, and the drilling should be slow first and then fast, so as to reduce the shaking of the drill pipe and correct the deviation or displacement of drilling in time.
(4) After drilling to the required depth, remove the soil at the bottom of the hole. The purpose of hole cleaning is to take out the floating soil and virtual soil in the hole and reduce the settlement of the pile. The method is that the drilling rig idles at the original depth to clear the soil, then stops rotating and lifts the drill to unload the soil.
(5) The main reinforcement, stirrup, diameter, number, spacing and protective layer of main reinforcement of steel skeleton shall comply with the design regulations, and shall be firmly bound to prevent deformation. Use guide steel bars to send into the hole, and at the same time prevent soil and sundries from falling into the hole. After the steel skeleton is in place, concrete should be poured immediately to prevent hole collapse. When pouring, it should be cast in layers, and the thickness of each layer is 50 ~ 60 cm.
Figure 2- 17? Schematic diagram of construction process of bored pile with spiral drilling machine
(a) drilling holes with a drilling machine; (b) placing a steel skeleton; (c) pouring concrete
2) Bored cast-in-place pile with mud wall protection
Mud wall protection is a mechanical drilling method to protect and stabilize the hole wall with mud. It suspends the cut mud debris by circulating mud and discharges it out of the hole, which is suitable for soil layers with and without groundwater.
Hole-forming machinery includes submersible drilling rig, impact drilling rig, punching and grasping cone, etc.
Construction technology of bored pile with mud wall protection: determining pile position, burying casing, positioning pile machine, mud preparation, mechanical hole formation (submersible drilling rig, impact drilling rig, etc.). ), mud circulation slag cleaning, hole cleaning, placing steel skeleton, underwater concrete pouring.
?
(1). Bury lining and prepare mud.
(1) Before drilling, pay-off and locate at the site, dig out the surface soil of the pile hole according to the pile position, and bury the casing. See fig. 2- 18 for the embedding of bored piles and liners. The casing is about 2m high, and there are 1 ~ 2 overflow holes in the upper part. It is a cylinder made of steel plate with a thickness of 4 ~ 8 mm, and its inner diameter should be 200 mm larger than the diameter of the drill bit. The function of casing is to fix the position of pile hole, protect the orifice, prevent the inflow of surface water, increase the water pressure in the hole, prevent the hole from collapsing, and guide the direction of the drill bit when the hole is formed.
(2) During drilling, mud with relative density of 1. 1 ~ 1.5 is injected into the hole, so that the pores in the soil layer of the hole wall in the pile hole are densely filled, water leakage in the hole is avoided, and the water pressure in the casing is kept stable; The relative density of mud increases the water pressure in the hole, which can stabilize the hole wall and prevent the hole from collapsing. By circulating mud, the cut mud residue can be suspended and discharged, which plays the role of carrying sand and discharging soil. ?
Figure 2- 18? Bored pile casing embedding
(2) Pore formation
1) drilling with submersible drilling rig
① Schematic diagram of hole formation of DTH drilling rig
The hole-forming schematic diagram of DTH drill is shown in Figure 2-20. ?
Figure 2-20? Schematic diagram of submersible drilling rig
1 bit; 2- submersible drilling rig; 3- cable; 4-cylinder engine; 5- water pipe;
6- Roller (fulcrum); 7- drill pipe; 8- cable reel; 9-5kn hoist;
10- 10kn hoist; 1 1- current voltmeter; 12- start switch
② Working mode
Submersible drilling rig is a kind of rotary drilling rig. Its waterproof motor speed-changing mechanism is sealed with the drill bit, and after being positioned by the pile frame and drill pipe, it can dive into the water and drill holes in the mud. After the mud is injected, the soil particles and stone slag cut in the hole are discharged out of the hole by positive circulation or reverse circulation slag discharge method.
③ Slag discharge mode
There are two ways to discharge slag from DTH drilling rig: direct circulation and reverse circulation, as shown in Figure 2-2 1.
A. positive circulation slag removal method: during drilling, after the rotating drill bit cuts the mud into mud, the high-pressure mud is pumped out by the mud pump, sent to the bottom of the drill bit through the central pipe and bifurcation pipe of the drilling rig, forcefully sprayed, mixed with the mud cut into mud, carried up along the hole wall and discharged from the overflow hole of the casing.
B. Reverse circulation slag discharge method: the sand pump dives into the hole with the main engine, and directly draws the cutting mud residue out of the hole with the mud. ?
A) positive circulation slag discharge; (b) reverse circulation slag discharge
1- drill pipe; 2- water supply pipe; 3- host; 4- drill bit; 5- sedimentation tank;
6- Submersible mud pump; 7- mud pump; 8- sand pump; 9- slag extraction pipe; 10- slag discharge hose
(2) impact drilling to form holes
① The impact drilling rig lifts the heavy bit (impact hammer) with blades to a certain height through the frame and winch, and uses the impact force of free fall to cut the broken rock stratum or impact soil layer to form holes (Figure 2-22).
(2) The forms of impact drills are cross, I-shaped and herringbone. Generally speaking, cross impact drills are usually used (Figure 2-23).
(3) Steel liners should be buried before punching, and the wall protection materials should be prepared. ?
Figure 2-22? Schematic diagram of simple impact drilling rig
1- auxiliary pulley; 2- main pulley; 3- main pole; 4- Front stay cable; 5- Rear stay cable;
6- brace; 7- double drum hoist; 8- guide wheel; 9- stow wood; 10- steel pipe;
1 1- slurry supply pipe; 12- overflow port; 13- silt aqueduct; 14- Backfilling of lining; 15?
Figure 2-23? Schematic diagram of cross punch
(4) After the impact drilling rig is in place, the center of the impact hammer should be aligned with the center of the casing, and intensive impact should be lowered within the stroke range of 0.4~0.8m, and stones and mud should be added to protect the wall in time until the casing sinks for 3 ~ 4m, and the stroke can be increased to 1.5 ~ 2.0m, then normal impact can be performed, and the relative density of mud can be measured and controlled at any time.
⑤ During construction, the damage of wire rope, the tightness of clamping mechanism and the flexibility of steering device should be checked frequently to avoid drilling.
(3) punching and grabbing the cone to form a hole
① The cone head of the grab cone (Figure 2-24) is equipped with a heavy iron block and a movable grab piece. The grab cone is lifted to a certain height by the frame and winch. When falling, release the brake of the drum and open the bucket blade, so that the cone head can fall freely and rush into the soil. Then start the winch to lift the cone head and close the grab bucket blades to grab the soil. The grab cone is hoisted to the ground as a whole, and the soil residue is removed, and the holes are circulated in turn.
(2) The punching and grabbing cone hole-forming construction technology, installation requirements of liners and mud wall protection circulation are the same as those of impact hole-forming construction.
(3) It is suitable for punching holes in soft soil (sand, clay), but in case of hard soil, impact drill should be used for construction. ?
Figure 2-24? Punch and grab cone head
Grab the soil; (b) soil upgrading
1- grab the film; 2- connecting rod; 3- weight; 4 pulley block
The drilling construction of bored pile is shown in Figure 2- 19. ?
Figure 2- 19? Bored pile drilling construction
(3) hole cleaning
(1) Hole inspection is to check the pile position, diameter, depth and passage with a detector; Hole cleaning is to remove the silt and silt floating soil at the bottom of the hole, so as to reduce the settlement of the pile foundation and improve the bearing capacity.
(2) When drilling for mud retaining wall is cleaned, for the pile hole with good soil and not easy to collapse, a suction fan can be used to clean the hole, and the air pressure is 0.5MPa, so that a strong high-pressure air flow is formed in the pipe, and at the same time, fresh water is continuously replenished, and the stirred mud is discharged from the nozzle with the upwelling of the air flow until fresh water is ejected.
(3) For the hole wall with poor stability, the mud circulation method should be adopted to clean the hole or pump the cylinder for slag discharge, and the relative density of mud after hole cleaning should be controlled at 1. 15 ~ 1.25.
(4) pouring underwater concrete
(1) Pouring concrete for hole-forming of mud retaining wall is carried out in water or mud, so it is called pouring underwater concrete.
(2) Underwater concrete should be one strength grade higher than the design strength, and must have good workability, and the mixture ratio should be determined through experiments.
(3) The conduit method is often used for underwater concrete pouring (Figure 2-25).
(4) When pouring, first fill the conduit and funnel with concrete, and the quantity shall ensure that the lower end of the conduit is buried below the concrete surface for more than 0.8m at one time, then cut the steel wire hanging the waterproof plug, and the concrete mixture will quickly discharge the ball plug into the water under its own weight. Bored pile-concrete pouring is shown in Figure 2-26.
Figure 2-25? Underwater pouring concrete
1- feed hopper; 2- storage hopper; 3- slideway; 4- crane;
5- funnel; 6- catheter; 7-liner； 8- Water plug?
3. Construction technology of immersed tube bored pile
(1). Piling method
Cast-in-place pile with immersed tube is to use hammer piling equipment or vibration piling equipment to sink the pile tip (or steel boots) with reinforced concrete or the steel pipe with flip pile boots into the soil (the diameter of the steel pipe should be consistent with the design size of the pile), so as to create a pile hole, then put it into the steel skeleton and pour concrete, then pull out the casing, and vibrate and compact the concrete in the process of pulling out the pipe, thus forming the required cast-in-place pile. See Figure 2-27 for pile boots of immersed cast-in-place piles.
Figure 2-27? Pile boots for immersed cast-in-place piles
(2) Classification of piles
Using hammer sinking equipment to sink pipes and pull them out into piles is called hammer sinking cast-in-place pile (Figure 2-28); Vibrate the immersed tube with a vibrator and pull out the catheter to form a pile, which is called vibrating immersed tube cast-in-place pile (Figure 2-29).
① Hammer sinking cast-in-place pile
See Figure 2-28 for the schematic diagram of mechanical equipment for hammer sinking cast-in-place pile. ?
Figure 2-28? Schematic diagram of mechanical equipment for hammer sinking cast-in-place pile
1- pile hammer wire rope; 2- Pile-pipe pulley block; 3- skip wire rope; 4- Pile hammer; 5- Pile cap;
6- concrete funnel; 7- Pile tube; 8-pile frame; 9- Concrete bucket; 10- return rope;
1 1- steel pipe for driving; 12- precast pile tip; 13- hoist; 14- sleeper
② Vibrating immersed cast-in-place pile
See Figure 2-29 for the schematic diagram of vibration sinking pile machine. ?
Figure 2-29? Schematic diagram of vibration sinking pile machine
1- guide pulley; 2- pulley block; 3- Vibration exciter; 4- concrete funnel; 5- Pile tube;
6- Pressurized steel wire rope; 7- Pile frame; 8- Concrete bucket; 9- Return rope; 10- valve pile tip;
1 1- cable wind rope; 12- hoist; 13- steel pipe for driving; 14- sleeper
(3) pore-forming sequence
During the construction of immersed tube cast-in-place pile, it has soil squeezing effect and vibration influence, and the hole-forming sequence should be considered in combination with the site construction conditions.
Forming holes every other or two pile positions;
Forming holes before initial setting or after final setting of adjacent pile concrete;
If there are more than five piles under a bearing platform, drill the middle pile first, and then drill the outermost pile.
(4) Construction technology
In order to improve the quality and bearing capacity of piles, single pile method, multi-pile method and inverted insertion method are often used in cast-in-place piles.
One-stroke method (also known as one-time tube drawing method): when drawing the tube, lift the tube for 0.5 ~ 1.0m at a time, vibrate for 5 ~ 10s, and then draw the tube for 0.5 ~ 1.0m, and so on until it is completely pulled out.
Repeated play: play singles twice in a row in the same pile hole, or repeat the play partially as needed. During construction, ensure that the axes of the two immersed tubes coincide before and after the initial setting of concrete.
Overturning insertion method: for every 0.5m steel pipe lifting, insert 0.3m again, and so on until it is pulled out.
1) hammer sinking cast-in-place pile
(1) construction process of hammer sinking cast-in-place pile
Hammer sinking cast-in-place pile is suitable for general cohesive soil, silt soil and artificial fill foundation, and its construction technology is shown in Figure 2-30. ?
Figure 2-30? Construction technology of immersed cast-in-place pile
(a) in position; (b) steel pipes immersed in water; (c) starting pouring concrete;
(d) lowering the steel skeleton and continuing to pour concrete; (e) tube drawing and forming
1- pile tip; 2- steel pipe; 3- Strengthening
(2) Key points of hammer sinking cast-in-place pile construction
(1) A hemp (or straw rope) washer should be placed at the joint between the pile tip and the pile pipe to prevent groundwater from infiltrating into the pipe and make a buffer layer. When sinking the pipe, hammer it with a low hammer first, and then knock it normally after observing that there is no deviation.
(2) Before extubation, the casing should be hammered or vibrated first, and the concrete to be tested can not be extubated until it has indeed flowed out of the casing.
(3) The concrete in the pile tube should be filled as much as possible, and the tube drawing should be uniform, and the continuous hammering should be kept to control the tube drawing speed. Generally, the soil layer should be no more than 1m/min, and the soft soil layer and soft-hard joint should be controlled within 0.8m/min.
(4) Before pulling the pipe bottom to the design elevation of the pile top, knocking or knocking shall not be interrupted. Pay attention to keep the concrete in the pipe slightly higher than the ground and keep it until the whole pipe is pulled out.
⑤ When the distance between pile centers is less than 5 times the outer diameter of pile pipe or less than 2m, the jumping method should be adopted for construction; Piling shall not be started until the concrete of adjacent piles reaches 50% of the design strength.
(2) Vibrating immersed tube cast-in-place pile
1) Vibrating immersed tube cast-in-place pile adopts vibration exciter or vibrating immersed tube.
2) Its construction process is as follows:
(1) Pile driver in position.
② Immersed tube
③ Feeding.
④ extubation
(3) The quality problems and treatment methods of immersed cast-in-place piles.
1) necking
(1) necking: refers to the phenomenon that the local diameter of the pile body is less than the design requirements.
(2) When sinking the pipe in silt and soft soil, the squeezed soil wall will generate interstitial water pressure, and then it will be squeezed into the newly poured concrete after the pipe is pulled out, and the local area of the pile will be squeezed to form a necking. ?
(3) When the pipe drawing speed is too fast or the amount of concrete is small, or the workability of concrete mixture is poor, the surrounding silt soil will take the opportunity to fill and also form necking.
(4) Treatment method: When pulling out the pipe, the concrete surface in the pipe should be kept higher than the ground to make it have enough diffusion pressure, the concrete slump should be controlled at 50 ~ 70 mm, and multi-stroke should be adopted when pulling out the pipe, and the speed of pulling out the pipe should be strictly controlled.
2) Broken pile
① Broken pile: refers to the partial separation or fracture of the pile body, and what is more serious is that a section of pile has no concrete.
② Cause: The distance between piles is too close, and the concrete of adjacent piles is not strong enough during construction, so the formed piles are crushed.
③ Treatment method: During construction, the distance between control centers shall not be less than 4 times the pile diameter; Determine the piling sequence and piling route to reduce the impact on newly poured concrete piles. The construction of the next pile shall be carried out after the pile concrete reaches 60% of the design strength.
3) Hanging foot pile
(1) Hanging-foot pile refers to the phenomenon that the concrete at the bottom of the pile is empty or soft, and it is not implemented on the foundation soil layer at the bottom of the hole.
② Causes: The groundwater pressure is high, or the precast pile tip is damaged, or the gap of the pile shoe flap is large, water and mud enter the sleeve steel pipe, or the pile tip flap is subjected to soil pressure, and the pipe is not opened until it is pulled out to a certain height, so that the concrete falls, resulting in the pile foot is not dense and a soft layer is formed.
③ Treatment method: In order to prevent the valve from being unable to open, the method of tight pulling and slow pulling can be adopted when pulling out the pipe, and the bottom of the pile foot can be turned over several times to pull out the pipe normally. The interface between the pile shoe and casing should be lined with a better liner to prevent the infiltration of groundwater and mud.
4) concrete overflow
If the concrete consumption of cast-in-place pile is more than 65,438+0 times higher than normal, it may be because there is a hole at the bottom of the hole, or the soil is disturbed and the strength is greatly reduced when working in saturated silt. Under the lateral pressure of concrete, the pile expands and the concrete consumption increases. Therefore, before construction, we should know the geological conditions of the site in detail, and when using immersed cast-in-place piles in saturated muddy soft soil, we should first test the piles. If too much concrete is found, you should contact the design unit and use other pile types. ?