There are five main construction methods for xx underwater tunnel: dike excavation method, pneumatic caisson method, rock drilling method, shield method and immersed tube method. Shield method is an important construction method in tunnel construction, especially in soft soil stratum. Since 1843, the first shield tunnel was built under the Thames in London, the design and construction technology of shield tunnel have made great progress, and mud-water pressure shield and earth pressure balance shield appeared. The lining is made of cast iron and reinforced concrete or steel.
The main characteristics of shield construction are: earthwork excavation and lining support can be carried out safely under the cover of shield; The underwater tunnel construction will not affect the navigation of the waterway; The main processes such as shield propulsion, excavation and lining assembly are carried out in cycles, so the construction is easy to manage and there are fewer construction personnel; Construction is not affected by weather conditions such as wind and rain.
Some problems existing in shield method are: when the curve radius of tunnel is too small, the construction is more difficult; When working underwater, it is unsafe to cover the soil too shallow; When the whole air pressure method is used to drain the stable stratum in shield construction, the requirements for labor protection are high and the construction conditions are poor. In saturated aquifer, the assembled lining used in shield construction has high technical requirements for realizing the waterproof performance of the whole structure.
2. The safety of shield tunneling in shallow overburden area.
2. 1 Shallow overburden is prone to roof caving and seepage.
In order to reduce the slope of the line, the bottom section of the river is generally covered with extremely shallow soil. In shallow overburden area, it is difficult to establish earth pressure balance in front of the big knife head under the condition of high head pressure. The river often enters the shield machine from the disturbed soil gap through the opening of the broadsword and the tail of the shield, resulting in the shield being flooded. This kind of machine is damaged and fatal accidents happen from time to time.
2.2 Tunnel floating
The shield advancing in the shallow overburden under the water area is uneven in upper and lower stress, the shield posture rises, the slope pressure is difficult, the tunnel floats, and the axis is difficult to control. After the assembled tunnel ring is separated from the shield tail, the upper ballast and self-weight can't resist the buoyancy generated by groundwater, and the tunnel floats. If the corresponding reinforcement measures are not taken, it is easy to cause local cracking and water leakage in the tunnel.
When the tunnel passes through the saturated soil layer, it will be subjected to the buoyancy of water. When the buoyancy exceeds the weight of the covering soil on the tunnel and the self-weight of the tunnel and the equipment in the tunnel, the tunnel will float. When the segment leaves the shield tail, the tunnel is surrounded by the slurry injected into the wall, and the buoyancy of the slurry is much greater than that in saturated soil. At the same time, the excavation of earthwork by shield tunneling will lead to the unloading of foundation, and the assembled tunnel will deviate upward from the central axis due to the rebound of foundation. Under the combined action of buoyancy and foundation rebound, the overlying soil on the tunnel will expand. If the maximum uplift pressure is not effectively controlled, the overlying soil will crack and appear permeable cracks, and the river will flow into the shield tail along the permeable cracks, which will seriously affect the tunnel and tunnel construction safety.
3 Guangzhou Metro Line 3 Lida section shallow excavation construction difficulties
The shield tunnel project from Lijiao to Dashibei of Guangzhou Metro Line 3 is divided into two sections: Dashibei originating well to Xiajiao station, with tunnel length of1429.7438+0m; From Xiajiao Station to Lijiao Station, the tunnel length is1621.75m. * * There are two xx's.
Among them, the tunnel passes through three perfume channels YCK13+780 ~ YCK14+098, with a channel width of about 300m·m m. The tunnel includes one left line and one right line, and two mud-water balanced shield machines are used for construction. First, the right line of the tunnel will be constructed, and the shield machine will advance from the south bank of Sanzhi Xiangshui Road to the north bank. When the right line shield advances xx, the left line of the tunnel advances from the south bank of three Xiangshui Road to the north bank.
There are silty mudstone composite strata such as strongly weathered rock, moderately weathered rock and slightly weathered rock in the cross section of the tunnel crossing Sanzhi Xiangshui Island. Above the vault, there are mainly silty soil, silty fine sand and clay. The total length of the tunnel in xx section is about 3 12m, with weathered interlayer, high strength, broken rock mass, close contact between surface water and fissure water in sand layer and bedrock, and weak stratum above the tunnel, which requires higher rock breaking ability and mud prevention ability of shield. The shield machine passes through three perfume passages, and the hydrogeological conditions of the surrounding rock of the tunnel body are greatly affected by it. The water level of Sanzhi Xiangshui Channel is mainly affected by the flood and tide of Beijiang River, and the tide level and duration of two adjacent high tides or low tides are not equal. Generally, the highest tide level appears in flood season and the lowest tide level appears in dry season, with short high tide duration and long low tide duration, with an average high tide level of 5.85m and an average low tide level of 4.33m m. ..
The tunnel passes through three perfume lanes, YCK13+780 ~ YCK14+098, and the river width is about 3 12m. The rock strata in the cave body are all weathered rock zones with uniform geology and rich groundwater. In addition, the xx section has thin overburden thickness, weak soil layer and high river water pressure, so it is risky in shield tunneling.
The geological conditions of xx section of Sanzhi Lane are complex and changeable, and the risk is great. The thickness of covering soil in xx section is very thin (only 7.4m), and the thickness of covering soil from ring 780 to ring 840 at the bottom of the river is generally within the range of 6 ~ 7m, and even the thickest part is only 8.6m, which is slightly larger than 1 times the diameter of the shield. The geological conditions are poor, and the upper part is sandy soil, so the water permeability is poor. The risk is greatest near the 840 ring road. The top of the tunnel is full of extremely unstable silt layer, which is easy to collapse. Shield machine is easy to raise its head, and a little carelessness will lead to serious safety accidents such as spewing, river bottom collapse and river flooding into tunnels, with disastrous consequences. In addition, it is difficult to stabilize the tunnel due to the influence of tides. Xx section is the key section, which directly affects the success or failure of shield construction. In order to ensure the shield machine to pass through the third branch canal of Xiangjiang River smoothly, it is necessary to adopt information construction, control tunnel deformation and dynamically manage various construction parameters in shield tunneling during the whole construction process.
In order to ensure the success of xx and avoid the occurrence of major accidents, Metro Corporation organized a number of well-known experts at home and abroad to review, demonstrate, analyze and study the safety of xx shield special scheme. After scientific, serious and serious argumentation, the experts at the meeting put forward some targeted opinions such as "protecting the head and tail", "xx monitoring", "operation plan" and "subsection floating monitoring".
The construction unit overhauled the shield machine and replaced the cutter before xx, thus avoiding the risk of replacing the cutter at the bottom of the river. In the process of construction, the monitoring technology of sensor method and sonar method is adopted to protect the river safety. In addition, combined with the actual situation, the emergency plan for xx was formulated to ensure foolproof.
4. Safety measures for shield tunneling in shallow overburden area at river bottom
4. 1 Measures to prevent slurry leakage at the tail of shield (leakage prevention)
(1) Improve the quality and management of synchronous grouting.
Before each cycle of propulsion, a small sample test should be carried out for the slurry injected simultaneously, and the initial setting time should be strictly controlled, and the initial setting time should be 13 ~ 15 seconds. In the process of synchronous grouting, the grouting pressure should be controlled reasonably, and the grouting outlet pressure = incision water pressure +60 ~ 100 kPa, so that the construction parameters such as grouting quantity, grouting flow rate and advancing speed can achieve the best matching.
(2) Strengthen the management of the shield tail cabin.
In the process of propulsion, the water pressure of the incision often fluctuates due to equipment failures and operational errors. After increasing the water pressure of the incision every time, it is necessary to try to advance, and arrange special personnel to observe the slurry leakage at the tail of the shield, and then formally increase the water pressure of the incision for normal excavation. At the same time, we should also pay attention to the protection of shield tail brush itself, and strictly control the pressure injection of shield tail grease; When in use, the tail cabin of the shield should be inspected regularly, and a comprehensive inspection should be conducted every 30 laps on average; Before segment assembly, the sundries in the shield shell must be cleaned to prevent damage to the shield tail brush.
(3) Countermeasures for slurry leakage from shield tail
When it is found that the slurry leakage at the tail of the shield is serious, double liquids (liquid A and liquid B) with short initial setting time are prepared for grouting behind the lining (section) wall, and the grouting position is 5~8 laps later, so as to appropriately reduce the incision water pressure, but the adjustment amount is not more than 0.5 Kg/cm2. If the slurry leakage has not been effectively controlled at this time, the last 6~8 rings will be plugged with polyurethane injection. In order to prevent slurry leakage, water leakage and water accumulation in the tunnel, a large water pump is added in the tunnel for use when necessary.
4.2 Measures to prevent the settlement of river bottom strata
(1) Control the hydraulic stability of the incision.
In the process of propulsion, the incision water pressure should be set according to the design value, and more importantly, it should be adjusted according to the changing tide level in the process of propulsion. Therefore, in the process of shield machine propulsion, it is necessary to strictly control the fluctuation range of incision water pressure, which is generally controlled within 5% of the set value to ensure the stability of the cutting surface. Especially when the shield machine crosses the middle section of the river with a thickness of only 7.48m, the incision water pressure should be controlled manually, and the incision water pressure value should be set to 120KPa. If necessary, manually adjust the construction parameters, set the upper and lower limit control parameters of incision water pressure when switching VE valve and steering bypass, and make records. Before advancing construction every day, check the opening and closing of VE valve and the working state of PLC system. On the premise of ensuring that the system controls the water pressure up and down the incision, normal excavation will be carried out again. At the same time, it is planned to measure the tidal level near the axis of the designed tunnel, record the tidal level every 20 minutes at ordinary times, and encrypt it to 10min during the rapid rise and fall of the tidal level to ensure the accuracy of the tidal level data.
(2) Increase grouting quantity
If the river bottom settlement is found to be large (> 5cm), the synchronous grouting amount should be appropriately increased, and supplementary grouting should be carried out behind the lining (section) wall when necessary. The grouting quantity should reach more than 130% of the theoretical building gap.
4.3 Measures to prevent the tunnel from floating
(1) Strictly control the posture of shield machine and segment.
In the process of shield tunneling, strengthening tunnel monitoring is an active measure to prevent the tunnel from floating. The attitude control of shield machine and segment can effectively control the tunnel floating;
A, measuring the rotation of the tunneling direction of the shield machine, and controlling the rotation angle to be 0.3 degrees;
B, measure the jack with the jack travel table, calculate the measurement data, analyze the direction correction value, and select the jack;
Measure vertical and plane positions. In order to accurately measure the position of the shield machine, check and correct it every 2 laps, and constantly set the current position at the known points of the new coordinates. The vertical and plane deviation of shield machine should be controlled within 50mm, and the shield posture should not change too much or too frequently. When advancing, do not rectify the deviation urgently or violently, and pay more attention to the gap between segment and shield shell, so as to reduce the influence of shield construction on the sealing effect of shield tail;
D segment posture must be strictly controlled, the elevation and plane deviation should be controlled within 50mm, the flatness of adjacent segments in each circle should be controlled within 4mm, and the flatness of longitudinal adjacent segments should be controlled within 5 mm. ..
(2) Be responsible for division selection.
Using the guide system of shield machine and manual measurement method, the gap between shield tail and the last ring segment is accurately measured from four directions: up, down, left and right. In shield xx, strictly control each value less than < 40mm, and then select the segment type of the next ring by combining the measurement data and the tunnel line data of the guidance system to ensure that the segment posture follows the shield machine.
(3) Improve the quality of synchronous grouting.
Improve the synchronization of grouting and shield propulsion, so that the slurry can fill the gap in the building in time. Synchronous grouting position is in the fourth ring section 1 1 and 5, 1 and 7. The specific grouting sequence can be determined according to the measurement results of this section. When the grouting filling amount is less than 1 10%, it is adopted.
4.4 Prevent the formation of mud cake
(1) Strengthen the personnel responsibility system in the central control room.
Based on the existing geological data, the area where mud cake may form is analyzed in detail in advance, and the personnel in the central control room are required to strictly monitor the parameters of the shield machine when the shield machine travels to this area. If there is any abnormality (increase in mud discharge ratio, decrease in flow rate, increase in mud inlet pressure, change in thrust and torque), it is necessary to judge whether mud cake is formed in time.
(2) Strictly control the operation specifications.
It is strictly forbidden to operate savagely just for the sake of progress, and to conduct central control scientifically and reasonably in strict accordance with the operating specifications. In the process of excavation, especially in areas where mud cakes may form, the specific gravity of mud should be controlled to prevent the formation of mud cakes from decreasing.
(3) Technical measures
If abnormal signs are found, the specific gravity of mud should be reduced in time, and high-pressure water should be used to impact, so as to reduce the chance of mud cake formation in time and dissolve the mud cake that has been initially formed.
5 concluding remarks
The construction of shallow coverage area is the key to the success of shield xx tunnel project. Making a strict construction plan, strengthening information management in the construction process, and focusing on four aspects: leakage prevention, sinking prevention, floating prevention and collision prevention, can ensure the construction safety of shallow coverage section. On July 17, 2004, the shield section from Lijiao to Dashibei of Guangzhou Metro Line 3 passed through Sanxiang Lane smoothly, and no major safety problems occurred during the nearly 30-day construction. The difficulty and speed of S.H.I.E.L.D. crossing Xiang Shui San Lane are rare in China. The left and right lines of Li Daduan's shield engineering need xx twice. The success of this xx has laid a good foundation for the shield to cross the South Pearl River waters with a width of 5 17 meters in the next stage, and is also of great significance to the opening of Line 3 in 2006.
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