Construction of SMW method near the hottest subway

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Construction of SMW construction method near subway tunnel

common forms of foundation pit support include diaphragm wall, combined row pile and SMW construction method. SMW construction method is more and more widely used because of its economic, applicable, environmental protection and other advantages. This paper discusses how to take effective measures to minimize the impact of SMW Construction Method on adjacent subway construction

The SMW construction method is a typical example of technology introduction and innovation in China's construction technology innovation. This construction method can be applied to the construction of clay, sandy soil, sand gravel and other strata. In 1993, SMW construction method was developed in China through technology introduction and innovation. This method is mainly used in the soft soil deep foundation pit enclosure in the southeast coastal area of China

located in Shanghai Jing'an Temple business circle, the greater Shanghai huidefeng Plaza project has 54 floors above the ground and 3 floors underground, with a total building height of 271 meters. The excavation depth of the foundation pit is 20.72m for the tower and 17.92m for the podium. The foundation pit is surrounded by an underground diaphragm wall (the wall is 1 meter thick and 35.5 ~ 40.5 meters deep), and the north side is adjacent to the subway line 2 (Jiangsu road ~ Jing'an Temple section) tunnel. The length of the foundation pit in this section is 95 meters, 5slat away from the subway tunnel. For this purpose, a 100 meter long garbage clearing device (composed of multiple air bags) has been made There is more work to do 4 meters. The protection requirements of the subway operation line for this purpose are: the horizontal height difference of the two tracks is 4mm, and the maximum scale value of the rail direction deviation and height difference is 4mm/10m. Requirements for structural deformation of subway tunnel: the curvature radius of structural deformation curve is 15000 meters, the relative deformation is 1/2500, the convergence variation of tunnel structure is 10mm, and the deformation rate of tunnel structure is 0.5 mm/day, which shall not affect its safe and normal use. The differential settlement between the tunnel and the station shall be controlled within 5mm, and the horizontal displacement and final settlement of the subway structure shall not exceed 20mm. In case of the following situations, the construction unit shall alarm in time and take reliable measures to ensure the safety of the subway:

1) when the monitoring value exceeds 1/2 of the daily monitoring index or the total deformation control amount

2) when the daily variation of horizontal displacement of the foundation pit retaining structure near the subway tunnel side exceeds 1mm; 3) When other things happen that endanger the safety of subway operation

the subway department also stipulates that the on-site construction must be suspended when the daily deformation of the subway tunnel monitored automatically for 3 days exceeds 0.5mm. Therefore, in order to ensure the construction safety of the underground diaphragm wall, two SMW construction methods are added on both sides to protect it. The designed SMW construction method is triaxial mixing pile, with a pile diameter of 850 mm, a lap of 250 mm, a pile length of 32.5 meters, a cement content of 20%, a verticality of less than 50 mm, and a 28 day unconfined compressive strength of not less than 1.5mpa The outermost construction method pile is only 4.4 meters away from the subway, so its construction is very difficult. However, in the actual construction, the strength test of cement soil by drilling and coring shows that it is generally far from meeting the design requirements of 1.2MPa, and the strength of cement soil is very discrete, and shows the law of decreasing strength with the increase of depth. The real core sample can meet the design requirements often after 60 days. However, there is another important phenomenon in the strength of cement soil with SMW construction method: when buried underground, the strength formation is very slow, and once exposed to the air with the excavation of the foundation pit, the strength rises very fast, which can basically reach the design index in three days

at present, the water cement ratio of cement slurry during SMW Construction in Shanghai is generally: 1.5: it can reach the requirement of 250 w/s in technical tests such as the new national standard combustion growth rate index, which is 1 ~ 2:1. It is mainly considered that the water content in the cement soil after mixing should not be too small, so that the H-beam can sink in place by self weight. The author believes that there are two problems with such a high water cement ratio: first, such a high water cement ratio is bound to cause low strength of cement soil; Second, on the premise of a certain amount of cement in cement soil, the higher the water cement ratio, it is bound to increase the amount of cement slurry pouring, and at the same time, more cement soil slurry spills, increasing the treatment cost of spilled cement soil. According to the data, during the construction of Japanese companies, the water cement ratio w/c is 0.3 ~ 0.8, which is selected according to the project category and soil properties. At present, the strength of some cement soil in Japan has reached 3 ~ 5MPa At present, the construction at the edge of the subway has such a low water cement ratio, and the construction depth of the construction method of the project is more than 30 meters. If the slurry is too thick, the stress generated by soil mixing will not be released after the construction is less than 20 meters, resulting in a great impact on the subway. If the water cement ratio is too large, the strength of the pile body will increase slowly, affecting the construction of the next process, and then affecting the construction period; Too small and unable to release stress, posing a threat to subway operation. In addition, the sinking and lifting speed of drill pipe during construction also has a great impact on the subway. Generally, the normal drilling speed is 1.5 minutes and 1 meter, 2 ~ 2.5 minutes and 1 meter after 20 meters, and 2.5 ~ 3 minutes and 1 meter after 30 meters. During the actual construction, in order to release the stress in time, we hope to reach 2 minutes or 4 minutes for 1 meter, that is, every 1 meter of drilling, let the drill pipe inject grout in place and empty the drill, and continue to sink after 2 or 4 minutes. Finally, we arrange and combine these construction parameters, that is, the water cement ratio is 1.5, 1.2 and 1.0 respectively, and the sinking and lifting speeds are 2 minutes and 1 meter and 4 minutes and 1 meter respectively. After 6 tests, the soil inclinometer beside the test pile number shows the type of reducer: 1 meter in 4 minutes, and the influence of 1.2 water cement ratio on the soil is relatively minimal. Therefore, this construction parameter is finally adopted. In order to ensure the possible impact of continuous operation on the subway, the subway department requires that our project can only be constructed from 11 p.m. to 6 a.m. And in the construction, the principle of "four dozen one" shall be strictly followed

in the specific construction process, we also found that the control of the following factors can reduce the impact of the subway:

1. The adjustment of the gear of the grouting machine in different construction periods. Generally, when sinking and above 15 meters, we choose to use the second gear (230ml/min), and above 15 meters, we use the first gear (165ml/min)

2. Pay close attention to the state of slurry return. If it is too thick, find out the cause in time and take corresponding measures. In case of mechanical reasons, the drill bit should be lifted up in time, and the construction can be continued after the equipment is repaired

3. Pay attention to the current change of the drilling rig. Through practice, we found that the current is generally 200mA at a depth of more than 18 meters, 200 ~ 240mA at a depth of 18 ~ 25 meters, and 260 ~ 280mA below 25 meters. Therefore, when it is found that the current reading is significantly higher than the normal value at a certain construction depth, we should take measures in combination with the consistency of the returned slurry. Its purpose is to disperse the thicker slurry and help release the stress in time

4. Count the amount of returned slurry in the groove and remove it with a grab in time to help release the stress. During the construction of the project, the replacement rate of the return slurry is 70% (that is, the replacement excavation amount divided by the driving amount). If it is lower than this value, attention should be paid in time. Because there is too much slurry and the equivalent earthwork is not discharged in time, expansion will occur underground, which will affect the subway pipeline

after taking the above measures, we found that the impact of such construction on the subway can be basically controlled within 0.5mm

to sum up, the construction of SMW method near the protected units with strict requirements such as subway tunnels can be carried out through careful planning and appropriate adjustment of the main construction parameters, which can reduce the impact on its surrounding environment to a very small extent. The main methods also include strict control of various indicators during construction, including current value, soil replacement rate, etc., and reasonable measures should be taken in time. We found that SMW construction method has considerable advantages in the auxiliary enclosure of subway peripheral works by virtue of its unique process and simple construction. It should be improved in the future construction practice to make it play a greater role

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