1
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Foreword SMW is the abbreviation of soilmixing wall, which came out in Japan in 1976
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It is a patent owned and developed by a medium-sized enterprise in Japan, Chengxin Industry Co., Ltd
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now it is widely used in the retaining structure of open cut tunnel and the waterproof curtain of deep foundation pit
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In this method, a multi axis drilling and excavation mixer is used to drill to a certain depth on site
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At the same time, cement fortifier is sprayed at the bit to mix with the foundation soil repeatedly
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Overlapping construction is adopted between each construction unit, and then H-shaped steel or steel plate is inserted as its stress reinforcement material before the cement soil mixture is hardened until the cement is hardened, Then a continuous and complete underground wall without joints with certain strength and rigidity is formed; Also known as the rigid cement soil mixing pile method, it combines the bearing load with the anti-seepage and water retaining, and makes it a retaining wall with both soil retaining and anti-seepage functions
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If only impermeability is needed, the process of inserting H-beam or steel plate can be cancelled, so as to reduce the investment and speed up the construction progress
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The most commonly used type of SMW construction method is triaxial drilling mixer, in which the drill pipe can be used for cohesive soil, gravel soil and bedrock
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In addition, some other machines have been developed for construction under urban viaduct, space limited occasions, or submarine wall construction, or soft foundation reinforcement
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Both sides of the drill pipe high-pressure shotcreting, the middle drill pipe air compressor jet, at the same time rotating mixing, make each cement soil cement content more uniform
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2
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The main characteristics and application scope of the construction method
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The traditional deep mixing pile construction method is to use the traditional double shaft mixing rig
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During the construction, the cement slurry is injected into the original soil gap, while the new triaxial mixing rig adds high-pressure air when filling the cement slurry
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At the same time, the rig fully mixes the cement soil and replaces a large amount of undisturbed soil, The new type of triaxial drilling machine has better pile strength and uniformity than the traditional biaxial drilling machine
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The verticality of pile, the parallelism of pile and pile and the degree of overlap are very good, which ensures the excellent and reliable waterproof performance, and is also conducive to the insertion and recovery of section steel
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Compared with the commonly used construction methods such as diaphragm wall and cast-in-place bored pile, it has the following characteristics: (1) little interference to residents: basically no noise during construction（ 2) Small impact on the surrounding environment: the construction does not disturb the adjacent soil, and will not cause the adjacent ground subsidence, house tilt, road crack damage and underground facilities displacement and other hazards（ 3) Strong reliability: insert H-beam or steel plate, have certain composite structure strength（ 4) Strong water retaining property: the drill pipe has the characteristics of alternate arrangement of screw propulsion wing and mixing wing
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With the repeated drilling and mixing, the cement fortifier and soil can be fully mixed, and there is no joint in the whole length of the wall, so that it can have more reliable water sealing property than the traditional continuous wall, and its permeability coefficient K can reach 10-7cm / s（ 5) Low cost: this enclosure construction method can generally recover H-beam and other bending materials through certain construction measures to reduce the construction cost, and the transportation volume of waste soil is far less than other construction methods; Generally, the price of each shift is about 7000 yuan, and the comprehensive unit price based on 25% cement content is about 220 yuan / m（ 6) Fast construction progress: the required construction period is shorter than other construction methods
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Under general geological conditions, each shift can form a wall of 70-80m2
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（ 7) Multi purpose (can adapt to all kinds of strata): can be used in clay, silt, sand, gravel soil Φ Application of pebble above 100 and rock stratum with uniaxial compressive strength below 60MPa（ 8) Large excavation depth: the wall thickness is 550-1300mm, and the common thickness is 600mm; At present, the maximum depth of the wall is 65m, which can be further constructed depending on the geological conditions; Generally, it is used as foundation pit retaining structure with depth less than 15m（ 9) Disadvantages: high requirements for the site, flat site, with steel plate, a small hook machine, the width of the site generally needs at least 15 meters
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Equipment composition, classification and performance index 4
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SMW construction sequence (1) guide ditch excavation: determine whether there are obstacles and make mud ditch（ 2) Place the guide rail（ 3) Set up construction signs（ 4) SMW drilling and mixing: drilling and mixing, repeated mixing, mixing when lifting（ 5) Place the stress reinforcement material (H-beam) (6) and fix the stress reinforcement material（ 7) SMW is completed
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The lapping of triaxial mixing pile and the verticality correction of formed mixing pile depend on the single hole repeated casing drilling of mixing pile to ensure the waterproof curtain effect of mixing pile
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Generally, the triple axis mixing pile is constructed by double hole full set compound mixing, but under special circumstances (such as the construction of mixing pile at a corner or construction interruption), it can also be constructed by unilateral extrusion
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The average speed of shotcreting and running in the triaxial mixing pile is controlled at 1m / min when it is running through the 4th, 6th and 7th layers of silty sand
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When it passes through the 4th, 6th and 7th layers of silty sand, it shall be stirred to the design depth at a speed of no more than 0.8m/min
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After 20 seconds of static shotcreting, it shall be re stirred
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It shall be lifted 5m at a speed of no more than 1m / min before sinking
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When it reaches the bottom of the pile, it shall be stirred and lifted to the top of the pile at a speed of no more than 1.2m/min
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Design requirements (1) hot rolled H-beam is used, H-beam specification is 488 × three hundred × eleven × 18 (HM) site 700x300x13x24, the upper end of the section steel is 500mm above the crown beam
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When the section steel is welded and lengthened, the groove equal strength welding shall be adopted, and the weld quality grade shall not be lower than grade II
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There are no more than two welded joints in a single section steel, and the position of welded joints should not be located at the supporting position or near the excavation face where the section steel is under great stress; The vertical positions of adjacent section steel joints shall be staggered with each other, and the staggered distance shall not be less than 1.0m（ 2) The perpendicularity of the pile and the inserted section steel shall not be greater than 1 / 200, and the allowable deviation of the section steel top elevation ± 50mm； Allowable deviation of profile steel plane position: 50mm parallel to foundation pit sideline and 10mm perpendicular to foundation pit sideline; The allowable deviation of section steel corner is 3 °。（ 3) The section steel should be inserted within 30 minutes after the completion of pile construction, and its flatness and joint weld quality should be checked before insertion（ 4) Before applying antifriction agent on the surface of section steel, the rust or dust on the surface must be removed, and the thickness of application shall be greater than 1 mm, and the application shall be uniform, so as to ensure the bonding quality of antifriction agent layer（ 5) Triaxial mixing pile, pile diameter Φ It is suggested that 42.5 ordinary portland cement should be used with 850mm, 600mm wheelbase and 250mm adjacent pile engagement
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The cement content should not be less than 30% of the natural mass of soil, and 10% bentonite can be added into the cement
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The water cement ratio of cement slurry of mixing pile is 1.0, and the 28 day unconfined compressive strength of cement soil shall not be less than 1.0MPa（ 6) The average speed of shotcreting and running in the triaxial mixing pile is controlled at 1m / min when it is running through the 4th, 6th and 7th layers of silty sand
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When it passes through the 4th, 6th and 7th layers of silty sand, it shall be stirred to the design depth at a speed of no more than 0.8m/min
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After 20 seconds of static shotcreting, it shall be re stirred
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It shall be lifted 5m at a speed of no more than 1m / min before sinking
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When it reaches the bottom of the pile, it shall be stirred and lifted to the top of the pile at a speed of no more than 1.2m/min（ 7) The pumping volume of slurry should match the sinking or lifting speed of mixing to ensure the uniformity of cement parameters in mixing pile（ 8) The slurry with initial setting due to long shelving time shall be treated as waste and shall not be used
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6
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Common problems and Countermeasures in construction (1) the construction site is soft, the pile machine is unstable, or the trench wall is unstable due to the deep excavation of the guide groove, which leads to the inclination of the mixing pile machine and affects the verticality of the mixing pile; It is necessary to do a good job of “three connections and one leveling” and positioning survey of enclosure pile, so as to ensure the walking route of drilling rig and the firmness of ground surface.
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