SMW is the abbreviation of Soil Mixing Wall. This construction method was introduced in Japan in 1976. It is a patent owned and developed by a medium-sized Japanese enterprise, Seishin Industry Co., Ltd. It is now widely used in open-cut tunnel retaining structures and deep foundation pit water-stop curtains.
This construction method uses a multi-axis drilling and mixing machine to drill to a certain depth on site, and at the same time, sprays cement-based strengthening agents at the drill bit to repeatedly mix and mix with the foundation soil. Overlapping construction is adopted between each construction unit, and then H-shaped steel or steel plates are inserted as stress reinforcement materials before the cement-soil mixture hardens. When the cement hardens, a continuous, complete, and seamless underground wall with certain strength and rigidity is formed; it is also called the rigid cement-soil mixing pile method, which combines load bearing with anti-seepage and water retention, making it a retaining wall of a supporting structure with both soil retaining and anti-seepage functions. If only anti-seepage is required, the insertion of H-shaped steel or steel plates can be canceled, reducing investment and speeding up construction progress.
The most commonly used SMW method is the three-axis drilling and mixing machine, of which the drill rods are divided into those for clay, gravel, and bedrock. In addition, some other models have been developed for construction under urban viaducts, in places with limited space, for building walls on the seabed, or for reinforcing weak foundations.
The traditional deep mixing pile construction method uses a traditional double-axis mixing drill. During construction, cement slurry is injected into the gaps in the original soil. The new three-axis mixing drill adds high-pressure air when filling the cement slurry. At the same time, the drill fully mixes the cement soil and replaces a large amount of original soil. Due to the different equipment used, the new three-axis drilling rig has significantly better pile strength and uniformity than the traditional double-axis drilling rig. The verticality of the pile body, the parallelism of the piles, and the degree of overlap are all very good, ensuring excellent and reliable waterproof performance, and also facilitating the insertion and recovery of steel sections. Compared with the currently commonly used underground continuous wall and bored pile construction methods, it has the following main characteristics:
1) Little disturbance to residents: There is no noise during construction.
(2) Little impact on the surrounding environment: The construction does not disturb the adjacent soil, and will not cause damage such as the adjacent ground sinking, house tilting, road cracking and damage, and underground facility displacement.
(3) Strong reliability: H-shaped steel or steel plate is inserted, and the composite structure has a certain strength.
(4) Strong water-retaining property: The drill rod has the characteristics of alternating spiral propulsion wings and mixing wings. As drilling and mixing are repeated, the cement-based enhancer and soil can be fully mixed, and the wall has no joints along the entire length, making it more reliable than traditional continuous walls. The permeability coefficient K can reach 10-7cm/s.
(5) Low cost: This enclosure method can generally reduce construction costs by recycling H-shaped steel and other bending-resistant materials through certain construction measures, and the amount of waste soil transported out is much less than other methods; the general shift price is about 7,000 yuan per shift, and the comprehensive unit price analyzed based on 25% cement content is generally about 220 yuan/m.
(6) Fast construction progress: The required construction period is shorter than other construction methods. Under normal geological conditions, each shift can build a wall of 70 to 80 m2.
(7) Multi-purpose (adaptable to various strata): It can be used in clay, silt, sand, gravel, pebbles above Φ100, and rock formations with uniaxial compressive strength below 60MPa.
(8) Large excavation depth: The wall thickness can be 550-1300mm, and the commonly used thickness is 600mm; the maximum wall depth is currently 65m, and it can be constructed to a deeper depth depending on geological conditions; it is generally used as a foundation pit retaining structure with a depth of less than 15 meters.
(9) Disadvantages: It has high requirements for the site, a flat site, self-contained steel plates, and a small hook machine. The site width generally needs to be at least 15 meters.
(1) Trench excavation: determine whether there are obstacles and make mud and water trenches. (2) Place guide rails. (3) Set construction signs. (4) SMW drilling and mixing: drilling and mixing, repeated mixing, and mixing during lifting. (5) Place stress reinforcement materials (H-shaped steel) (6) Fix stress reinforcement materials. (7) Construction of SMW is completed.
The overlap of three-axis mixing piles and the verticality correction of formed mixing piles are achieved by repeated drilling of single holes in the mixing piles to ensure the water-proof curtain effect of the mixing piles. Three-axis mixing piles are generally constructed by skip-slot double-hole full-set remixing, but in special cases (such as corner construction or construction interruption of mixing piles), single-side extrusion construction can also be used.
The average speed of grouting and drilling for three-axis mixing piles is controlled at 1m/min. When passing through the 4th, 6th, and 7th silt sand layers, the piles are mixed at a speed of no more than 0.8m/min to the designed depth. After 20 seconds of static spraying, the piles are re-mixed and lifted for 5m at a speed of no more than 1m/min. After reaching the bottom of the pile, the piles are mixed and lifted to the top at a speed of no more than 1.2m/min.
(1) Hot-rolled H-shaped steel is used. The H-shaped steel specifications are 488×300×11×18 (HM) and 700X300X13X24 on site. The top of the steel is exposed 500mm. When the steel is welded and extended, strong welding such as grooves should be used, and the weld quality grade should not be lower than level 2. There should be no more than 2 welded joints in a single steel. The position of the welded joint should not be located at the support position or near the excavation surface where the steel is subjected to greater force; the vertical positions of adjacent steel joints should be staggered with a staggered distance of not less than 1.0m.
(1) The steel section adopts hot-rolled H-shaped steel, the H-shaped steel specification is 488×300×11×18 (HM) and the on-site size is 700X300X13X24, and the crown beam is exposed 500mm at the top of the steel section. When the steel section is welded and extended, strong welding such as groove should be adopted, and the weld quality grade should not be lower than level 2. There are no more than 2 welding joints in a single steel section, and the welding joint position should not be located at the support position or near the excavation surface where the steel section is subjected to greater force; the vertical positions of adjacent steel section joints should be staggered, and the staggered distance should not be less than 1.0m.
(2) The verticality of the pile body and the inserted steel section should not be greater than 1/200, and the allowable deviation of the steel section top elevation is ±50mm; the allowable deviation of the steel section plane position is: 50mm parallel to the foundation pit edge line, 10mm perpendicular to the foundation pit edge line; the allowable deviation of the steel section angle is 3°.
(3) The steel section should be inserted within 30 minutes after the pile construction is completed. Before insertion, its flatness and the quality of the joint weld should be checked.
(4) Before applying the friction reducer on the surface of the steel section, the surface rust or dust must be removed, the thickness of the application should be greater than 1 mm, and the application should be uniform to ensure the bonding quality of the friction reducer layer.
(5) For three-axis mixing piles, the pile diameter is Φ850mm, the wheelbase is 600mm, and the adjacent piles are 250mm apart. It is advisable to use 42.5 ordinary silicate cement, and the cement content is not less than 30% of the natural mass of the soil. 10% of the cement mass of bentonite can be added to the cement. The water-cement ratio of the cement slurry of the mixing pile is 1.0, and the 28-day unconfined compressive strength of the cement soil should not be less than 1.0MPa.
(6) When the three-axis mixing pile is mixed and drilled, the average speed of the spraying and drilling is controlled at 1m/min. When crossing the 4th, 6th, and 7th layers of silt sand, the mixing is carried out at a speed of no more than 0.8m/min to the designed depth. After 20 seconds of static spraying, the mixing is started again, and the pile is lifted for 5m at a speed of no more than 1m/min and then sunk. After reaching the bottom of the pile, the mixing is lifted to the top of the pile at a lifting speed of no more than 1.2m/min.
(7) The slurry pumping volume should match the mixing sinking or lifting speed to ensure the uniformity of the cement parameters in the mixing pile.
(8) The slurry that has initially set due to being left alone for too long should be treated as waste and is strictly prohibited from being used.
(1) The pile driver is unstable due to the soft construction site, or the trench is excavated too deep and the trench wall is unstable, causing the pile driver to tilt, affecting the verticality of the pile driver; the positioning and measurement of the “three connections and one leveling” and the retaining piles should be done carefully to ensure the movement route of the drilling rig and the solidity of the surface.
(2) There are many underground obstacles that cannot be constructed; the underground obstacles along the guide trench must be cleaned up and replaced when necessary. When the obstacle cleaning range and replacement range are large, a special construction plan should be issued and the construction parameters of the SMW pile should be adjusted to ensure the construction quality and safety of the pile body.
(3) When constructing in sandy layers with medium or higher soil density, if the length of the mixing pile exceeds 18m, it is very easy to cause drilling accidents, and the steel pile cannot be inserted to the designed depth by its weight; before construction in thick and medium-dense fine sand layers, the drilling rig and grouting pump models should be appropriately adjusted according to local experience to ensure sufficient power, high grouting pressure, and graded start of the power head. If necessary, the air compressor model, drill pipe spiral blade form, and drill bit model should be adjusted. Under the premise of not affecting the strength of the pile body, high-quality bentonite accounting for 2%-5% of the cement dosage should be appropriately added. Drilling quickly and pulling slowly can avoid drilling accidents.
(4) The verticality of single-section steel processing is difficult to control and is easy to deform; when the root section steel cannot meet the requirements, it needs to be assembled on-site. Therefore, it is necessary to cast (process) a fixed pedestal on site, equip it with full-time welders and cranes, control the welding quality of the section steel joints, and ensure the quality of the section steel forming.
(5) Methods and measures for handling mechanical failures during shutdown and power outage: In the event of an unexpected shutdown, raise the drill rod by 100 cm and re-mix to prevent broken piles or interlayers. If the two piles are engaged for more than 24 hours, the second pile is increased by 20% in slurry volume, or additional piles are used. When the H-shaped steel cannot be fully inserted into place by its weight, use an excavator to pressurize it or use a vibrating hammer to vibrate and sink it so that the steel is inserted into the designed elevation. When the above scheme fails, cut off the exposed part of the steel, add a cement-soil mixing pile on the outside, and insert the steel for strength compensation. When resuming construction after a long shutdown, a single row of cement-soil mixing piles should be added on the outside to prevent the inner gear from leaking due to the gap between the contact surface of the new and old mixing piles due to excessive time.
(6) Water leakage treatment methods and measures: During the excavation stage, if leakage points are found in the retaining structure, they should be sealed in time.
A. Drainage pipe: Insert the drainage pipe at the seepage point of the foundation pit, and seal it with quick-setting waterproof cement mortar around the drainage pipe. After the cement mortar reaches the strength, tie the drainage pipe.
B. Double liquid grouting:
1) Prepare chemical slurry.
2) Send the prepared and mixed chemical slurry and cement slurry into the slurry storage barrel for standby use.
3) Start the grouting pump during grouting, connect the two pipelines of two grouting pumps to the Y-type joint at the same time and mix them from the H port to the reinforced soil part at the bottom of the hole.
4) During the grouting process, the flow rate and pressure should be controlled as much as possible to prevent the loss of slurry. The recommended construction parameters are grouting pressure: 0.3-0.8Mpa; grouting flow: 25-35L/min; grouting volume: 0.375 m3/meter; slurry ratio, A liquid: water: cement: bentonite: admixture = 0.7:1:0.03:0.03 (cement is ordinary Portland cement with a grade of 425#), B liquid: water glass with a wave width of 35-40°blA liquid: B liquid = 1:1; initial setting time 45 seconds; setting strength 3-4 Mpa/2h.
(1) Calculation of cement dosage: According to the design requirements, the cement parameter is 25% and the bentonite parameter is 10%; the formula is T=SXdXrX25%; T is the cement dosage of each pile; S is the cross-sectional area of a single pile; d is the length of the pile; r is the average weight of natural soil; the cement dosage of the first pile is 15.74t and the bentonite dosage is 1.57t; the cement dosage of a single pile is 10.50t and the bentonite dosage is 1.05t.
(2) Water-cement ratio control: The backstage workers were specifically instructed to control the water-cement ratio to 1:1. Each pile was mixed with 11 barrels of cement slurry, including 10 barrels of 1000kg cement mixed with 1000kg water and 1 barrel of 500kg cement mixed with 500kg water. The corresponding parameters were posted in the operation room. The construction team was required to arrange a special person to supervise the operation of the front and backstage workers, and the construction workers were required to supervise the whole process. The technical department was required to conduct spot checks on each three-axis mixing machine no less than 8 times a day. If a problem was found, the pile was immediately ordered to be re-drilled as if it had not been constructed and strict penalties were imposed.
(3) Control of grouting pressure: Strictly follow the grouting pressure used in the first two pile tests and do not adjust it without authorization. The grouting pump should ensure that its actual flow rate matches the speed of the mixer’s spraying drilling sinking or spraying lifting so that the cement content is evenly distributed in the cement soil pile. In actual construction, the spraying pressure should be controlled according to the soil characteristics and is usually controlled at 0.8MPa-1.0MPa. (The grouting pressure at each stage of our site is determined based on the first two pile tests)
(4) Control the drilling rod lifting speed: the mixing sinking speed is controlled at 0.5m/min-1m/min, and the lifting speed is controlled at 1m/min-2m/min, and the sinking or lifting speed is maintained at a uniform speed; the calculated cement consumption of 10.5t per pile is divided into 11 barrels for mixing, of which 10 barrels are 1000kg cement and 1000kg water, and 1 barrel is 500kg cement and 500kg water, and each pile is 5 barrels are used for pile sinking (including re-mixing sinking), 5 barrels are used for lifting (including re-mixing lifting), and 1 barrel (500kg cement mixed with 500kg water) is used for pile bottom reinforcement; the amount of subcontracted cement entering the site is controlled, and regular reviews are conducted based on the total number of pile widths; the construction team is required to arrange a dedicated person to supervise the operations of the front-end and back-end workers, and the construction personnel are required to supervise the entire process. The technical department will conduct random inspections on each three-axis mixing machine at least 8 times a day. If a problem is found, the piles in that section will be ordered to be re-driven as if they had not been constructed, and strict penalties will be imposed.
(5) Pile position control: Before construction, the pile position is located by laying out the line. After the drilling rig is in place, the pile position is determined by a steel ruler, plumb bob, and one-meter line.
(6) Verticality control: After the drilling rig is located at the pile position, the plumb bob on the drill rod is used to adjust the verticality.
(7) Steel top elevation control: After the first steel is inserted, the top elevation of the pile is located by theodolite and fixed by hanging rods. Then, during the construction process, the steel is pressed to the same top elevation as the previous one by the drilling rig.
(8) Control of the steel core angle: The steel positioning guide frame and the vertical positioning suspension component control the steel core angle to be no more than 3°.
(9) The mixing sinking and lifting speed must be uniform. When encountering obstacles, slow down to prevent equipment damage.
(10) When using the information method for construction, the backstage and pile driver must be closely connected and coordinated to ensure the continuity and integrity of the process.
(1) To ensure the strength and uniformity of the pile body, the following requirements must be met: strictly control the amount of cement and the liquid level in each mixing barrel, and adopt total water consumption control; the soil should be fully mixed, and the drilling sinking and lifting speed should be strictly controlled to fully break the original soil, which is conducive to the uniform mixing of cement slurry and soil; the slurry should not be segregated, and the cement slurry should be made strictly according to the predetermined mix ratio. To prevent the slurry from segregating, it must be stirred for 3 minutes before pouring (starting from the time of cement pouring) before pouring into the slurry storage barrel; the slurry delivery pipeline should not be blocked during the grouting stage, and slurry breakage is not allowed. The entire pile must be grouted evenly, and no soil slurry sandwich layer should occur; if the pipeline is blocked, the pump should be stopped immediately for processing. After the processing is completed, the mixing drill should be immediately lifted and sunk 1.0m before continuing the grouting, and wait for 10 to 20 seconds to resume upward lifting and stirring to prevent pile breakage.
(2) Quality assurance measures for inserting H-beams: The arrival of the steel must be confirmed by the supervision unit. After the supervisor checks the flatness and welding quality of the steel and believes that the quality meets the construction requirements, the H-beam can be inserted. The steel must be hoisted one by one when it arrives on site, and sleepers must be placed at the bottom of the steel to reduce its deformation. Before inserting the H-beam, the flatness of the steel must be checked to ensure that the steel can be inserted smoothly. Before inserting the steel, the positioning equipment of the steel must be accurately fixed and its level must be checked. After the steel is hoisted, the verticality of the steel is adjusted with a theodolite. After the verticality requirement is met, the H-beam is inserted. A level or other reliable measures are used to control the top elevation of the H-beam to ensure the insertion depth of the H-beam.
The grouting pressure, air pressure, sinking and lifting speed, slurry flow rate, and cement content are determined through test piles: grouting pressure 0.5~0.8Mpa, air pressure 0.5~0.6Mpa, sinking speed 0.22m/min, lifting speed 0.88m/min, slurry flow rate 460L/min.
5 people in the backstage cement mixing team, 2 people in the SMW drilling rig operation team, 8 people in the steel welding and hoisting team, 4 people in the friction reducer application team, and 2 assistant technicians.
Work efficiency: 400m3/d. platform piles in clay layer, 180 tons of cement (cement content is 25%, soil bulk density is 1.8 tons), 300m3/d. platform piles in a medium-dense sand layer.
The subcontract price of each cubic meter of cement soil is 100 yuan for labor and machinery (equivalent to 140 yuan for 1m) + cement fee (260 yuan/ton) + steel cost (including steel freight 200 yuan/ton, welding, hoisting, plugging and unplugging, etc. 1100 yuan/ton, rental 8 yuan/ton.day). The LT series anti-friction coating is a dark brown paste solid, with a dosage of 1kg/m2, a softening point of not less than 55°, and a bonding strength with A3 steel greater than 0.7Mpa. For example: the diameter of the cement soil mixing pile is φ850mm, the spacing is 600mm, and the overlap between the piles is 250mm. The design requires overlapping and overlapping construction, using 32.5 grade A slag silicate cement, and the cement content is 25%; the vertical deviation of the cement soil mixing pile is <1% and the position deviation of the mixing pile is <5㎝; it is roughly around 7,000 yuan per shift, and the comprehensive unit price is generally around 220 yuan/meter.
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