In shield construction, it is inevitable to disturb the surrounding soil and make the surface subsidence [1]. Many scholars have studied and analyzed the settlement of shield tunnel construction [2-3]. Urban subway engineering has the characteristics of dense buildings, small construction site, complex geological conditions, dense underground pipelines, large traffic volume and limited construction conditions, so the control requirements of surface subsidence for shield tunneling through the urban center are more stringent [4]. Aiming at the shield of Shenyang Metro 1 Line crossing underground commercial street, the technical measures adopted in the whole process are systematically expounded. 1 Project Overview The main body of a bid section of Shenyang Metro 1 Line is constructed by shield method. The total length of the section design right line is 778.82 meters, and the total length of the left line is 778.836 meters ... Mitsubishi 6 140 spoke EPB shield machine is selected for the excavation of the left and right lines in this bid section. The segment of the interval tunnel is 6 000 mm in outer diameter and 5 400 mm in inner diameter, which is composed of precast reinforced concrete segments with a ring width of 1 200 mm. Each ring consists of 3 standard A-shaped segments, 2 adjacent B-shaped segments and 1 capped C-shaped segment. The strength grade of precast lining reinforced concrete is C50, and the impermeability grade is S 10. M24 high-strength bolts are used for the longitudinal and circumferential connections of the lining. The commercial street along Zhonghua Road is about 102.65 m long and about 32 m long. The width is perpendicular to Zhonghua Road. The minimum distance from the bottom of the independent foundation under the column to the top of the tunnel is only 5.2 m, and the maximum distance is only 6. 1 m. 2 Geological conditions. The terrain of the construction site in this section is relatively flat, with the ground elevation of 42. 18 ~ 42.74m, high in the west and low in the east, and the relative ground elevation difference of 0.56 m. The site landform belongs to the new fan of Hunhe River in Quaternary. The interval soil layer from top to bottom is: 1) Quaternary Holocene artificial fill layer (Q4ml). Among them, the miscellaneous fill on the sidewalks on both sides of Zhonghua Road is mainly composed of construction waste, gravel soil, sandy soil and cohesive soil, which is loose-slightly dense; The miscellaneous fill of Zhonghua Road is mainly composed of pebbles, gravels, sandy soil and cohesive soil, slightly wet, slightly dense-medium dense, and the top is asphalt pavement. 2) Quaternary Hunhe new fan (Q4 1al+pl). Among them, silty clay is yellowish brown, containing iron nodules and mica flakes, slightly smooth, without shaking reaction, medium dry strength and toughness, plastic-hard plastic state, locally mixed with thin layers of powder and fine sand; Medium coarse sand is yellowish brown to grayish brown, poorly graded, timely feldspar, containing 5% ~ 10% gravel, occasionally pebbles, with the maximum particle size of 40 mm, slightly wet ~ saturated, and the upper part is generally medium sand, which gradually becomes coarse sand downward; Pebbles are yellowish brown and poorly graded. Its parent rock is mainly composed of moderately weathered granite and dike rocks. The rock is hard, with gravel accounting for 50% ~ 65%, with a general particle size of 2 ~ 20 mm and a maximum particle size of about 80 mm. It is filled with medium-coarse sand, very wet ~ saturated and dense. 3) Quaternary Upper Pleistocene Hunhe ancient fan (Q32al+pl). Silty clay is yellow-brown, containing a small amount of iron-manganese nodules, saturated, soft plastic, slightly smooth, without shaking response, with medium dry strength and toughness, partially mixed with sand and mixed with thin layers of fine sand, and horizontally distributed; Medium sand and coarse sand are yellowish brown and poorly graded feldspar, containing a small amount of clay and gravel, and occasionally pebbles. It can be seen that the maximum particle size is about 30 mm, saturated, medium-dense, and horizontally distributed; Round gravel is mainly composed of igneous rock, which is sub-round ~ round, hard, saturated, medium-dense ~ dense, with thin gravel sand locally, and the filler is medium-coarse sand and a small amount of cohesive soil, which is distributed in a lens shape. 3. Technical measures for construction control of shield crossing commercial street. Measures before crossing 1) Soil pre-reinforcement. In order to ensure the safety of shield tunneling, the east and west sides of the commercial street were pre-reinforced by double liquid grouting before tunneling. See table 1 for grouting parameters. Table 1 grouting parameters of soil pre-reinforcement 2) Add test section. New technical parameters are adopted for trial excavation within the scope of 100 ring before the tunnel goes under, so as to ensure that the shield can safely and smoothly cross the commercial street. In the test section of shield machine, every technological process, especially grouting technological process, is monitored for 24 hours during shield tunneling, and all data in the test process are recorded in time. Through the test and analysis of the propulsion parameters of the test section, the feasible technical parameters of the shield tunneling under the commercial street are determined, including earth pressure, propulsion speed, grouting pressure, synchronous grouting quantity, slurry ratio, location and frequency of secondary grouting, grouting quantity and slurry material selection. 3) Before the shield crosses the commercial street, adjust the shield posture to the best state, and retest the air guide control network and the measurement control points in the underground and tunnel. In the case of confirmation, the shield machine will adjust the axis error to less than 10 mm according to the measured data, and advance with an accurate attitude. 4) Advance grouting of shield. In order to ensure the safety of the commercial street structure, a special double-liquid grouting equipment for shield machine is equipped. Before the shield goes under the structure, the soil in front of the excavation face is reinforced by advanced grouting through the side wall of the shield machine 12 grouting port. The designed advanced grouting depth is 5 ~ 6 m, the maximum grouting reinforcement soil section diameter is not less than 12 m, the reinforcement scope is the upper side of the tunnel, and the included angle is 240. 3.2 Measures during crossing 1) Set the earth pressure reasonably to prevent over-excavation. In the process of shield tunneling, according to the theoretical calculation data, preliminary tunneling data and monitoring data, the earth pressure value is analyzed and adjusted in time, so as to set the earth pressure value scientifically and reasonably, prevent over-excavation, reduce the disturbance to the soil and control the surface settlement. Because shield tunneling in sandy soil is easy to lead to overbreak, which leads to large ground settlement, and the thickness of covering soil in commercial street is very small, and the settlement control standard is high, so in this case, it is necessary to appropriately increase the set value of positive earth pressure (generally, the set value of positive earth pressure is 65438+ 0.4 times of the static earth pressure in the cutter head center), and make appropriate fine adjustments according to the settlement monitoring results. 2) Improve sandy soil. After the sand in front is drained, it is very difficult for the cutter head of the shield machine to cut soil, and the thrust and torque of the cutter head will exceed the allowable range of normal work during shield tunneling. In order to ensure that the thrust of the shield and the torque of the cutter head are within the normal working range, and minimize the wear of the cutter head and cutter, the soil on the face of the tunnel is improved in advance, that is, fermented bentonite is injected into the soil in front of the cutter head to reduce the oil pressure and torque of the cutter head, so that the sand has proper workability, and carboxymethyl cellulose is added to the bentonite to increase the water retention and stability of the mud. 3) During shield tunneling, it is necessary to ensure the constant and stable tunneling speed, strictly control the direction of shield tunneling, and reduce rectification, especially a large number of rectification. In the process of crossing the commercial street, measure the attitude deviation 1 time of the shield machine every 60 cm, adjust the advancing direction of the shield machine in time according to the deviation, and close the overbreak. At the same time, during the shield tunneling, the shield machine will continue to advance at a constant speed, thus ensuring the shield machine to smoothly cross the commercial street. 4) Reasonable synchronous grouting. According to the calculation, the building gap per 1 ring is1.6m3. Considering the high settlement control standard of the commercial street under the tunnel, the synchronous grouting quantity is controlled within the range of 160% ~ 180% of the building gap, that is, 2.6 ~ 2.9m3, and the grouting loss rate is 10%. In addition, considering that the soil layer in this section is rich in water and the water pressure is high, the grouting pressure will be increased by 1 ~ 2 kg/cm2 on the basis of the water and soil pressure at the grouting site, so that the slurry will not enter the soil silo and crush the segment. The slurry ratio is cement (kg): fly ash (kg): bentonite (kg): sand (kg): water (kg) = 100: 450: 60: 600: 500. 3.3 Measures after crossing 1) should adopt reasonable secondary grouting. Synchronous grouting is a kind of flowing single slurry, which has no self-supporting force during injection and is easy to be lost to other parts of the tail gap, so it is difficult to fill the injection area, especially the top of the back of the segment (see figure 1). In addition, the synchronous grouting slurry takes a long time to consolidate and is easily diluted by groundwater, which leads to the early strength decline and the soil above the tunnel slides and collapses to the unfilled gap. Therefore, in order to limit the flow of synchronous grouting slurry, reduce the loss of slurry and achieve the purpose of filling in a fixed area, according to the monitoring data analysis of the shield test section, it is decided to adopt the circular grouting method for secondary grouting. See Figure 2 for circumferential grouting mode and Figure 3 for secondary grouting mode. 2) Strengthen monitoring, measurement and patrol, and reinforce them in time. After the completion of shield tunneling, we will continue to monitor and measure the area near the commercial street behind the tunnel and conduct 24-hour road patrol. Once abnormal phenomena are found, secondary grouting or multiple grouting shall be carried out in time, and ground grouting reinforcement measures shall be taken in severe cases. 4 Construction monitoring According to the formula, determine the influence range of ground settlement, and monitor the settlement of buildings within the influence range. Monitoring instruments include electronic precision level, bar code ruler, total station, etc. The mileage pile number of commercial street is k 12+ 416-k12+519, and the maximum allowable settlement is 7 mm. Choose pile numbers K 12+506 and k12+40. As can be seen from Figure 4, the maximum cumulative settlement of the three measuring points is 3.76 mm, 4. 14 mm and 4. 17 mm respectively, which are all less than the allowable values. Conclusion Based on the monitoring data, adjusting the shield construction parameters or adopting relevant technical measures can successfully avoid excessive differential settlement between building columns and internal surface uplift of buildings, and ensure the shield construction to pass through underground commercial streets smoothly.
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