Construction Technology and Key Points of Bored Cast-in-Place Pile Positive and Reverse Circulation Foundation!

(I) Preparation work
1. Before construction, the engineering geological data and hydrogeological data should be mastered, and the quality inspection reports of various raw materials and products used should be available.
2. During construction, relevant regulations should be followed to formulate safety production and environmental protection measures.
3. The construction of cast-in-place piles should have complete and effective construction records.

Mud wall drilling
1. Mud preparation and casing burial
(1) Mud preparation is designed according to the construction machinery, process and soil layer conditions. High plasticity clay or bentonite should be selected.
(2) The casing burial depth should comply with relevant regulations. The top surface of the casing should be 2m higher than the construction water level or groundwater level and 0.3m higher than the construction ground. Its height should also meet the requirements of the mud surface height in the hole.
(3) Before pouring concrete, the relative density of the mud after cleaning the hole should be less than 1.10; the sand content should not be greater than 2%; and the viscosity should not be greater than 20pa.S.
(4) Mud pools and mud collection facilities should be set up on site, and waste mud and slag should be treated without polluting the environment.

 

2. Positive and negative circulation drilling
(1) When drilling with mud wall, control the drilling speed according to the mud supply situation; keep the drilling rig stable.
(2) If inclined holes, collapsed holes, slurry leakage around the casing, instability, etc. occur during drilling, the drilling should be stopped first, and drilling should be resumed after taking corresponding measures.
(3) When the drilling reaches the designed depth and before pouring concrete, the thickness of the sediment at the bottom of the hole should meet the design requirements. If not required by the design, the sediment thickness of the end-bearing pile should not be greater than 100mm; the sediment thickness of the friction pile should not be greater than 300mm.

3. Impact drilling
(1) When drilling holes with impact drilling, the hammer should be low and dense, and the wall should be repeatedly impacted to keep the mud surface in the hole stable.
(2) Effective technical measures should be taken to prevent disturbance of the hole wall, hole collapse, hole expansion, drill jamming, drill drop, and mud loss.
(3) The hole should be checked once every 4 to 5 meters of drilling. Before replacing the drill bit or at places where the hole is prone to shrinkage, the hole should be checked and records should be made.
(4) Mud should be replenished in time during the slag removal process.
(5) When encountering inclined holes, plum blossom holes, collapsed holes, etc. during punching, measures should be taken before construction can continue.
(6) For hole walls with poor stability, mud circulation or slag extraction tubes should be used for slag removal. The mud indicators before pouring concrete after cleaning the hole meet the requirements.

Impact drilling
[Basic principle of impact drilling] is to use a heavy hammer to continuously lift and then freely fall down to smash the rock or soil layer at the bottom of the hole, and then fill it with mud to float out the fine stone debris. The impact drill is mainly used to drill holes in the rock layer.

4. Rotary drilling
[Construction steps of rotary drilling rig] Rotary drilling rig is in place → burying casing → the drill bit touches the ground lightly and rotates to start drilling → when the drill bit is full of soil and sand, it is lifted out of the hole → the rotary drilling rig rotates back and dumps the soil and sand inside on the earthmoving vehicle or the ground → close the drill valve, rotate the rotary drilling rig back to its original position, lock the drilling rig rotating body → put down the drill bit → drilling is completed, the hole is cleaned and the depth is measured → put in the steel cage and guide tube → concrete is poured → the casing is pulled out and the pile head sediment is cleaned and backfilled to form a pile.

Drilling
(1) Different drilling techniques should be used for drilling cast-in-place piles according to different strata and groundwater depths.
(2) The mud preparation capacity should be greater than the mud demand during drilling, and the mud reserve of each drilling rig should not be less than the volume of a single pile.
(3) Before drilling and each time the drill bucket is raised, the condition of the drill bucket and drill rod connection pins, the drill bucket door connection pins and the wire rope should be checked, and the debris on the drill bucket should be removed.
(4) The drilling rig should adopt the jump excavation method, and the mud should be replenished synchronously according to the drilling speed to keep the required mud surface height unchanged.
(5) The control index of the sediment thickness at the bottom of the hole meets the requirements.

(IV) Dry drilling

1. Long spiral drilling
(1) After the drilling rig is positioned, it should be re-checked. The deviation between the drill bit and the pile position should not be greater than 20mm. The drilling speed should be slow when drilling. During the drilling process, it is not advisable to reverse or lift the drill rod.
(2) If the drill is stuck, the drill rig shakes, deflects, or abnormal noise occurs during the drilling process, the drilling should be stopped immediately, the cause should be found, and the corresponding measures should be taken before continuing the operation.
(3) After drilling to the designed elevation, concrete should be pumped in first and paused for 10 to 20 seconds, and then the drill rod should be slowly lifted. The drilling speed should be determined according to the soil conditions and ensure that there is a certain height of concrete in the pipe.
(4) After the concrete is poured, the steel cage should be immediately inserted to the designed depth, and the residual concrete in the drill rod and pump (soft) pipe should be removed in time.

2. Borehole expansion
(1) The drill rod should be kept vertical and stable, and the position should be accurate to prevent the hole diameter from being enlarged due to the shaking of the drill rod.
(2) The thickness of the loose soil in the bottom hole of the drilled pile should meet the design requirements.
(3) When pouring concrete, the first pouring should be to the top surface of the bottom expansion position, and then vibrated to compact it; when pouring concrete within 5m below the top of the pile, it should be vibrated as it is poured, and the pouring height should not exceed 1.5m each time. (Note) It is generally used in the construction of piles with expanded bottoms. The upper part does not require expansion. When drilling to the bottom of the pile, increase the drilling pressure to open the bottom expansion drill bit radially to form expansion.

3. Manual excavation
1) Manual excavation piles must be selected under the premise of ensuring construction safety.
(2) The cross-section of the excavated pile is generally circular, and there are also square piles; the hole diameter is 1200~2000mm, and the maximum can reach 3500mm; the excavation depth should not exceed 25m.
(3) Concrete or reinforced concrete support hole wall technology is used. The thickness of the retaining wall, the connecting steel bars, the reinforcement, and the concrete strength grade should all meet the design requirements; the deviation between the center line of the well ring and the design axis shall not exceed 20mm; the lap length of the upper and lower sections of the retaining wall concrete shall not be less than 50mm; each section of the retaining wall must be vibrated and compacted, and the construction should be completed on the same day; the quick-setting agent should be used according to the water seepage of the soil layer; the template should be removed after the concrete strength is greater than 2.5MPa.
(4) After the excavation reaches the designed depth, the bottom of the hole should be treated. There must be no loose slag, mud, or sediment on the bottom surface of the hole.

(V) Key points of steel cage and cast-in-place concrete construction
1. The processing of steel cage should meet the design requirements. Appropriate reinforcement measures should be taken during the production, transportation and hoisting of steel cage to prevent deformation.
2. When hoisting the steel cage into the hole, it must not collide with the hole wall. After it is in place, reinforcement measures should be taken to fix the position of the steel cage.
3. The inner diameter of the steel cage of the pipe-sinking cast-in-place pile should be 60-80mm smaller than the inner diameter of the casing. The pile that uses the conduit to cast underwater concrete should be more than 100mm larger than the outer diameter of the conduit connection.
4. The underwater cast concrete used for cast-in-place piles should be premixed concrete, and its aggregate particle size should not be larger than 40mm.
5. The various processes of cast-in-place piles should be constructed continuously, and concrete must be poured within 4h after the steel cage is placed in the mud.
6. After the concrete pouring of the pile top is completed, it should be 0.5-1m higher than the design elevation to ensure that the concrete on the pile foundation surface reaches the design strength after the floating slurry layer at the pile head is removed.

7. When the temperature is below 0℃, insulation measures should be taken when pouring concrete, and the temperature of concrete during pouring shall not be lower than 5℃. When the temperature is higher than 30℃, retarding measures should be taken for concrete according to specific circumstances.

8. The actual amount of concrete poured for cast-in-place piles shall not be less than the calculated volume; the ratio of the average diameter of any section of cast-in-place piles with casing holes to the design diameter shall not be less than 1.0.

(VI) Underwater concrete pouring

1. After the pile hole is inspected and qualified and the steel cage is hoisted, the conduit is placed and concrete is poured.
2. The concrete mix ratio should be determined through tests and must have good workability. The slump should be 180-220mm.
3. The conduit should meet the following requirements: (1) The inner wall of the conduit should be smooth and round, the diameter should be 20-30cm, and the section length should be 2m. (2) The conduit must not leak. It should be tested and pressure tested before use. (3) The deviation of the conduit axis should not exceed 0.5% of the hole depth and should not be greater than 10cm. (4) The conduit should be equipped with a conical loop when using a flange joint; when using a spiral thread joint, there must be a device to prevent loosening.
4. The water-blocking ball used should have good water-blocking performance and should ensure smooth discharge.
5. When pouring concrete, the distance from the bottom of the conduit to the bottom of the hole should be 300-500mm; the first burial of the conduit below the concrete pouring surface should not be less than 1m; the depth of the conduit buried in the concrete should be 2-6m.

6. The pouring of underwater concrete must be carried out continuously, and the speed of lifting the pipe should be controlled. It is strictly forbidden to lift the pipe out of the concrete pouring surface. Faults during the pouring process should be recorded and filed.

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