Schematic diagram of continuous wall construction using SMW method
This article describes the basic concepts, origin and development, advantages and disadvantages, design points, construction points, engineering applications, research progress, and problem analysis of the SMW method.
SMW method piles are developed from mixing piles, but the two are different.
First, the mixing head of the mixing pile is generally cross-shaped, while the mixing head of the SMW method mixer is spiral-shaped;
Second, the mechanical power of the mixing pile is small, while the mechanical power of the SMW method pile is large;
Third, the cement content of the mixing pile is small, generally less than 14% in soft soil areas, while the cement content of the SMW method mixing pile is large, generally around 20%;
Fourth, the water-cement ratio of the mixing pile slurry is generally 0.5~0.7 (soft soil area), while the water-cement ratio of the SMW method mixing pile is 1.5~2.0;
Fifth, the mixing pile does not discharge soil during mixing, and the soil extrusion is large, while the SMW method mixing pile has a small amount of soil discharge during mixing, and the soil extrusion is small.
Advantages:
1. Small construction disturbance;
2. No mud pollution;
3. Low vibration and noise;
4. Good water-stopping performance;
5. Wide application range;
6. Short construction period;
7. Small construction site;
8. Less waste soil transportation;
9. High safety;
10. Low project cost.
Disadvantages:
1. Long cement soil curing time;
2. Compared with underground continuous walls, construction quality is more difficult to control;
3. Compared with the underground continuous wall, it lacks integrity;
4. Compared with underground continuous walls, it has poor impermeability.
Take the use of SMW method piles for underground retaining walls as an example.
1. Design principles
Safety (meeting stability conditions and strength conditions of each part of the material); Economic (ensuring that H-shaped steel can be recycled); Convenient construction
2. Determination of cement-soil mix ratio
The amount of cement and admixture added must be determined by on-site tests, generally 7%, 9%, 11%, 13%, and 15% are used for testing.
3. Determination of the depth of burial
The burial depth of steel: The burial depth of steel is generally slightly smaller than that of cement-soil mixing piles, which is mainly determined by the stability of the foundation pit against uplift, the internal force of the retaining wall, deformation, and pull-out of steel.
The burial depth of cement mixing piles: It is determined by three factors: ensuring that precipitation in the pit does not affect the environment outside the foundation pit; preventing pipe bursts; and preventing bottom drums.
4. Determination of cross-sectional form
H-shaped steel setting form
The calculation mode of the SMW retaining wall is similar to that of the wall-type underground wall. It is considered that the water and soil pressure are all borne by H-shaped steel, and the cement soil mixing pile only plays a water-stopping role. The specific calculation steps are as follows:
(1) Calculate the converted thickness of the wall-type underground wall according to the principle of stiffness equivalence;
There are two cases:
Considering stiffness improvement; Not considering stiffness improvement
Schematic diagram of thickness conversion of rigid pile and other rigid wall underground wall
(2) According to the equivalent thickness of the concrete wall underground wall, calculate
the internal force and displacement of each linear meter of wall;
(3) Convert the internal force and displacement of each steel section;
6. Strength verification
(1) Bending verification considers that the bending moment is all borne by the steel section to verify the strength;
(2) Shear verification
It is divided into two parts:
Steel shear verification; Cement soil local shear verification
7. Steel pull-out calculation
To ensure the smooth recovery of steel, pull-out calculation is required. It is best to conduct on-site tests to determine the maximum pull-out force of steel.
8. Check the strength of the cement soil at the bottom of a steel
The section at the bottom of the steel is a variable stiffness section, and the shear strength of cement soil must be checked.
1. Construction technology
SMW process flow chart
2. Construction points
(1) It is necessary to dig trenches to receive the return slurry and set up fixed frames to fix the H-shaped steel;
(2) It is necessary to reasonably determine the amount of cement slurry injected during downward drilling and upward lifting;
(3) It is necessary to reasonably determine the mixing sinking and lifting speeds according to the site conditions and reasonably determine the mix ratio of cement slurry;
(4) Control the verticality of cement soil mixing piles and H-shaped steel;
(5) It is necessary to take reasonable measures to ensure that the H-shaped steel can be recovered smoothly.
3. Construction quality assurance measures
(1) Ensure the quality of cement and steel, and strictly inspect the quality of steel processing;
(2) Check the positioning of the pile frame, the depth, and speed of the drilling hole, and check the mixing operation specifications and the water-cement ratio of the cement slurry;
(3) Ensure that the pile driver is stable, fixed, and upright, and the pile frame is vertical;
(4) Strictly control the water-cement ratio, mixing time, slurry quality, and control the grouting pressure and grouting speed during grouting;
(5) Control the speed of drilling and lifting the drill pipe to prevent broken piles and empty piles;
(6) When inserting H-shaped steel, it must be vertical and not tilted, control the insertion depth, and prevent dislocation, insertion deviation, and twisting;
SMW overlapping and overlapping construction method
The main scope of engineering application
Underground retaining wall in construction and civil engineering; anti-seepage water-stopping wall (dam, sewage pool, etc.); reinforcement of soft soil foundation.
Engineering application examples
(1) Foundation pit of “Global World” commercial building
The excavation area of the foundation pit is about 3000m2, the excavation depth is 8.65m, and the retaining structure adopts three rows of cement soil mixing pile wall, the diameter of the mixing pile is 700mm, the center spacing of the mixing pile is 500mm, the inserted steel is H800×400, the thickness of the flange and web is 10mm, the length of H-shaped steel is 13.6m, and the spacing is 1000mm. The support system adopts a reinforced concrete support, and grouting reinforcement is carried out in the pit.
SMW construction method construction achievements
(1) The horizontal displacement of the wall is controlled within 3cm;
(2) The maximum design bending moment of the H-shaped steel is 80% of the design value;
(3) The cost of the enclosure structure is 40% lower than that of the underground continuous wall;
(4) The construction period of the enclosure structure is shortened by 1/3.
Since 1998, China has successfully developed the ZLD series multi-axis SMW method continuous wall drilling machine, four-axis deep mixer, large depth and high torque four-axis deep mixer, and other construction machinery.
The “SMW retaining pile technology seminar” organized by the Underground Engineering Professional Committee of the Shanghai Civil Engineering Society was held in the Science Hall on December 8, 1999. The conference focused on the “Application of SMW method in Shanghai”, “H-beam recycling technology”, “Development of four-axis mixer” and other topics.
Experimental studies have shown that the effect of the SMW method on cement soil mixing piles in different soil layers is different. Generally speaking, the better the original soil properties, the greater the strength of the cement soil mixing pile; the worse the original soil properties, the smaller the strength of the cement soil mixing pile. For example, the effect of cement soil mixing piles in the interlayer of sandy silt and silty clay ⑤1 is much better than that in silty clay ③1, ④.
The SMW method of retaining structure has a very broad application prospect and has been recognized by the civil engineering community in Shanghai.
Construction of high-precision underground continuous wall (TRD method)
The main reasons for leakage, excessive deformation, ground subsidence, inability to pull out steel, and even foundation pit collapse in some engineering applications are:
(1) Insufficient cement content in cement soil, or failure to differentiate between different soil layers;
(2) Improper slurry mix ratio, too low slurry concentration, H-shaped steel prone to tilt or displacement, too high concentration, difficult to insert steel;
(3) Too fast sinking or lifting speed during cement soil mixing, resulting in uneven mixing;
(4) Insufficient overlap thickness of cement soil mixing piles;
(5) The verticality of cement soil mixing piles or H-shaped steel does not meet the design requirements;
(6) Excavation is carried out before the cement soil curing time is up, resulting in insufficient strength;
(7) Interruptions occur during construction, resulting in cold joints;
(8) Support is not set up in time during foundation pit excavation;
(9) Uneven application of drag reducer on the surface of H-shaped steel;
(10) The gap after the steel is pulled out is not backfilled in time;
(11) The quality of the construction team is uneven.
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