The construction process of SMW pile is shown in the figure below.
1. Measure pay-off
1.1 According to the design drawings and measurement control points, the design pile position is released (the enclosure structure is considered to be placed 8cm), the plane deviation of the pile position is not more than 20mm, and the positioning frame on both sides is marked with red paint according to the design spacing to ensure that the mixing pile is accurate each time.
1.2 On both sides of the trench, set up the stakes that can restore the center line, so that in the case of the trench has been excavated, the strike center line of the trench can also be checked at any time.
2. Trench excavation and guiding positioning
According to the lofting out of the enclosure center line excavation work trench, the depth is 0.5~1m, and on both sides of the guide positioning steel is laid to make the drilling position and the H-beam insertion position on the guide positioning section steel according to the design requirements.
3. Spraying sinking
When the drill pipe of the mixing pile machine sinks to the design pile top elevation of the mixing pile, start the mortar pump, and after the pure cement slurry reaches the mixing head, sink the mixing head at a speed of less than 0.5m/min, while grouting, stirring, and sinking, so that the cement slurry and the original foundation soil are fully mixed, and the pile length mark on the drill pipe is observed to reach the design elevation of the pile bottom. After the drilling mixer sinks to the design depth, the bottom part of the pile should be continuously stirred and grouting for more than 30s or 1m above the bottom of the pile and repeated stirring, while spraying and rotating the mixing drill bit.
4. Spraying lifting
The lifting speed is controlled within 1.0m/min, and the pumping must be continuous. At the same time, the drilling mixer is lifted in strict accordance with the lifting speed determined by the design. The drill pipe needs to be injected with cement slurry when sinking and lifting.
5. Lifting and positioning of section steel
Before inserting the section steel, install the guide rail composed of section steel, and its side buckle is wrapped with rubber skin to ensure that the section steel can be inserted into the pile body more vertically and reduce the damage of the surface friction reducer.
6. Technical requirements and control points
6.1 Technical Requirements
The specific technical parameters of the SMW construction method mixing pile are detailed in the following table.
SMW construction method mixing pile construction parameter table
| SMW method mixing pile construction | Projects | Construction parameters |
| Slurry | Pressure(Mpa) | 0.3Mpa~0.8Mpa |
| Flow rate (L/min) | 145 | |
| Number of nozzles | 2 | |
| Slurry water-cement ratio | 1.5 | |
| Slurry specific gravity | 1.372 | |
| Construction equipment | Three-shaft mixer | JB160 |
| Grouting pumps | BW-250 | |
| Sinking speed | m/min | 0.4 |
| Lift speed | m/min | 1.0 |
| Stirring shaft speed | r/min | 40r/min |
| Spraying method | The spraying method of two stirring and two spraying is used for spraying | |
| The amount of cement incorporated | The cement content is 20% (550Kg per meter of cement) |
6.1.1 Preparation of cement slurry: P.042.5 grade of ordinary Portland cement is used, the water-cement ratio is 1.5, and the cement content is 20%. The stirring time of the slurry is greater than 3min, not longer than 2h, and the stirring method is used twice to make and use.
6.1.2 stirring sinking: sinking the mixing head at a speed of 0.4m/min, while grouting, stirring, sinking, so that the cement slurry and the original foundation soil are fully mixed, the amount of grouting is generally 70% of the rated amount of grout when drilling, through the observation of the pile length mark on the drill pipe, to reach the design elevation of the pile bottom. Stir the grouting for more than 3min or repeat the stirring at 1m above the bottom of the pile.
6.1.3 Stirring and lifting: while spraying, rotating the stirring bit, the lifting speed is controlled at 1.0m/min, and the pumping must be continuous.
6.1.4 Brush the anti-friction agent on the surface of the section steel more than 2 times, the thickness is controlled at about 3mm, and there can be no oil, rust or block rust spots on the surface of the section steel.
6.1.5 section steel hoisting and positioning: every 1~2 piles are stirred, the section steel will be inserted in time, and the mixing will be stopped until the pile insertion time is controlled within 30min, which cannot exceed 1h. The length of the H-beam is 500mm higher than the top ring beam, so that the section steel can be pulled out when it is recycled.
6.2 Control points
The construction of three-axis cement mixing pile strictly implements the "four non-construction methods" (the pile position is not biased, the drill pipe is not oblique, there is a lot of cement, and the pile body is not leaking).
Before the formal construction, the pile test should be carried out according to the design requirements.
The configured slurry should have good fluidity, no segregation, and be convenient for pumping and spraying. To pre-screen the agglomerates in the cement. The cement slurry should be stirred continuously in the mixer until it is poured into the storage hopper and should be stirred constantly. During the construction of the mixing pile, the slurry hydrometer should be used to sample and check the slurry density from time to time.
The slurry supply should be continuous, and the slurry breaking phenomenon is not allowed. The amount of cement and water, slurry supply, slurry stop time, slurry supply, and spraying lifting speed must be recorded by a special person.
After the drilling rig is drilled down to the design depth, the mixing head should be sprayed up and down within 1m of the pile end and stirred for about 3min, and then lifted. When the drill bit is lifted to 0.5m below the ground, the spraying should be stopped.
In the process of piling, when the slurry is stopped for any reason and the construction continues, the overlapping piles must be overlapped, and the length of the overlapping piles shall not be less than 0.5m. If the shutdown is more than 3 hours, the slurry pipeline should be removed and cleaned. In order to ensure the effect of repairing the pile, the lap thickness of the plain pile and the enclosure pile is about 20cm, as shown in the figure below.
Construction cold joint treatment
7. Quality standards and control measures
7.1 Quality Standards
The deviation of the mixing pile position is ≤50mm, the deviation of the pile diameter is ≤10mm, and the deviation of the verticality of the pile body is ≤3‰.
The left and right error of section steel insertion shall not be greater than 30mm, the verticality deviation shall not be greater than 1/150, and the bottom elevation error shall not be greater than 200mm.
7.2 Quality Control Measures
Before starting, detect and remove all underground obstacles, excavate the guide ditch, determine whether there are obstacles and make muddy ditches, and the drilling speed of the drilling rig is about twice as slow as the lifting speed to ensure full mixing. The lifting should not be too fast, to avoid vacuum negative pressure, hole wall collapse, the use of a drilling and lifting reinforcement method, the bottom part of the pile should be repeated stirring.
Strictly control the water-cement ratio of slurry to meet the design requirements, the mixing time of cement slurry is not less than 2~3min, pour it into the aggregate pool after filtering, and then stir continuously to prevent cement segregation. The grouting should be carried out continuously and without interruption.
During the construction process, the construction records and measurement records should be checked at any time, and the quality of the piles should be assessed.
The pile diameter and mixing uniformity of the machinery should be checked regularly, and the drill bit should be regularly reviewed and inspected, and its diameter wear should not be less than 10mm.
When the drill bit sinks to the elevation of the bottom of the pile, repeat the stirring at the bottom of the pile for more than 30s or 1m above the bottom of the pile, and repeat the stirring once in the range of 2~3m at the bottom of the mixing pile for the soil layer with large sand content.
Three-axis mixing pile construction process diagram
Schematic diagram of the construction sequence of the three-axis mixing pile
After each pile is drilled, it should be operated continuously, and the spraying should not be interrupted, if the slurry is stopped for any reason during the construction process, the mixer head should be lifted or sunk 0.5m before the spraying and mixing construction, the interval time of the lap construction of the cement-soil mixing pile should not be greater than 24h, when it exceeds 24h, the mixing speed should be slowed down during the lap construction, if the lap can not be lapped or the lap is poor, it should be recorded as a cold seam, and should be approved by the design unit, and remedial measures should be adopted at the lap joint. It is strictly forbidden to carry out drill pipe sinking (lifting) operations without spraying. The storage capacity of the slurry storage tank should not be less than the design amount of one pile.
The section steel must be processed by the factory and the quality is reliable, the welded joints in a single section steel should not exceed 2, the use of 45 ° groove welding, the position of the welded joint should be avoided near the support position or excavation surface, and the position of the joint should not be less than 2m at the bottom of the pit, the position of the adjacent section steel joint should be staggered, and the friction reducer should be brushed more than 22 times, and the thickness should be controlled at about 3mm.
Every time 1~2 piles are stirred, the H-beam is inserted in time, and the stirring is stopped until the pile insertion time is controlled within 30min. The length of the H-beam is 50cm higher than the top ring beam, so that the section steel can be pulled out when it is recycled. The left and right error of section steel insertion shall not be greater than 30mm, and it should be inserted on the side close to the foundation pit, and the verticality deviation shall not be greater than 1/150, and the bottom elevation error shall not be greater than 200mm.
8. Prevention of quality problems and precautions
8.1 The strength of the pile is insufficient
Reason analysis: the construction cement content does not meet the pile strength requirements, or the construction is not in accordance with the design requirements.
Take measures: reduce the water-cement ratio of the slurry, increase the amount of cement, reduce the sinking and lifting speed of the pile driver drill pipe, and make the grout and soil fully and evenly stirred.
8.2 The pile is discontinuous and the pile is broken
Cause analysis: During the construction process, the interruption caused by power failure or special circumstances caused by shutdown, the lifting or sinking speed of the mixing head is too fast, and the cement is not fully mixed
Take measures: in case of shutdown, the mixing drilling tool should continue to grouting after sinking (lifting) 1m, resume lifting (descending) mixing, reduce the sinking and lifting speed of the pile driver drill pipe, and make the grout and soil fully and evenly stirred.
8.3H-shaped steel insertion is not in place
Cause analysis: the consistency of the water-cement ratio of cement slurry is not suitable, the H-shaped steel friction reducer does not meet the requirements, and it is easy to fall off; the H-shaped steel is not vertical when inserted, and rests on the hole wall; the H-beam is not inserted in time after the pile is formed, and the stirring body has been hardened.
Take measures: under the premise of design requirements, adjust the consistency and water-cement ratio of the mud configuration to facilitate the insertion of H-beam; before the anti-friction agent is painted, the rust and dirt on the surface of the section steel shall be removed, and the surface of the section steel shall be melted by heating the friction reducer and evenly applied to the surface of the section steel more than twice, and the thickness shall not be less than 3mm; the H-beam shall be inserted in time within 30min after the construction of the three-axis mixing pile is completed, and the horizontal position and verticality of the H-beam shall be reviewed before insertion.