General situation of shield tunnel of Nanjing Metro 1 Line
The first phase of Nanjing Metro 1 Line starts from the Olympic Sports Center Station in the south and ends at Maigao Bridge in the north, with a total length of 2 1.72km and an interval of * * * 15. Five and a half blocks are constructed by EPB shield, and the total length of shield propulsion is about 10.9km. The design is undertaken by Shanghai Tunnel Design Institute and China Railway Luoyang Tunnel Design Institute. The maximum coverage thickness of shield tunnel is 15m, and the minimum thickness is only 0.7m The longitudinal slope of the tunnel is V-shaped, with the maximum longitudinal slope of 33%, forming a high station and low interval. The minimum plane curve radius is 400m m. The main strata that the shield tunnel passes through are: plastic ~ soft plastic silty clay, silty soil, silty sand, silty sand mixed with fine sand. Among them, muddy clay has high compressibility, easy to produce soil flow, and the excavation surface is extremely unstable. Silty fine sand, silty sand mixed with fine sand has rich water content and strong water permeability, which is easy to produce water gushing and sand gushing; In particular, there is a section of tunnel with the length of 150m, which is in a serious liquefaction area, and the liquefaction effect is considered in the design and construction.
The downtown area through which the shield tunnel line passes has narrow streets and heavy traffic, and there are many high-rise buildings and underground pipelines on both sides of the road. The tunnel will cross Qinhuai River, Jinchuan River, ancient city wall, Xuanwu Lake highway tunnel under construction and many buildings. When the shield crosses Qinhuai River, the covering soil is only 0.7m, and the minimum clear distance between the shield and the floor of Xuanwu Lake Highway Tunnel under construction is only 1m, so the construction is very difficult.
2 Selection of shield machine
There are four shield machines in the shield tunnel of Nanjing Metro 1 Line, three of which are produced by Herrick Company of Germany and 1 by Mitsubishi Company of Japan. According to the geological and hydrological conditions in Nanjing, it is mainly muddy silty clay, silty clay, silty sand and silty soil. The groundwater level is below the surface 1 ~ 2m, and the permeability coefficient is 5× 10-3cm/s, so it is easy to liquefy. The adopted shield types can only be slurry shield and earth pressure balance shield. In the construction of slurry shield, it is necessary to separate slurry in slurry pool, which occupies a large area and will cause certain pollution to the environment. Moreover, the shield is expensive and the equipment technology is not easy to master. EPB shield is suitable for silty clay and water-bearing sandy silty soil layer. In addition, it is equipped with a mud adding device, which has a good effect on controlling surface subsidence. Therefore, EPB shield is selected as the four shields.
Taking the three-target shield machine as an example, the main parameters of the shield machine are introduced. The designed maximum buried depth of the shield machine is 18m, the maximum climbing rate is 35‰, and the minimum transition radius is 300m. The maximum thrust of the shield is 3560t, which consists of 16 pairs of 32 jacks. The outer diameter of the shield is 6340mm, the length of the main machine is 7400mm, and the total length of the shield is 60m. The maximum torque of cutter head is 469.4t·m, and the opening of cutter head is 40%.
3 shield tunnel construction
3. 1 Basic situation of shield tunneling
Nanjing Metro 1 Line 1 shield tunnel has a single-line propulsion length of 10.9km, which is divided into three sections, which are respectively propelled by four shields. Among them, the first bid section of shield is from the north working well of Zhonghua Gate Station-sanshanjie station (test section) to Xinjiekou-Zhujiang section, which is constructed by Shanghai Tunnel Company with Mitsubishi shield. The single-track advancing length of this bid is 3.206km, which is scheduled to be completed by the end of 2003 10, with a total construction period of 3 1 month; The top of this tunnel is covered with thin soil, and the test section is only 4 ~10m; When the shield crosses the Inner Qinhuai River, it needs anti-floating treatment, and the shield machine is only 0.8m away from the bottom of the anti-floating plate. The second bid section of shield is Sanshan Street-Zhang Fuyuan-Xinjiekou, with a single-line propulsion length of 3.06km, which is scheduled to be completed by the end of 2003 10. This section was constructed by Shanghai Foundation Company with the shield of Herrick Company in Germany. The third bid section of shield is Xuanwu Gate-Xufu Lane-Nanjing Station, with a single-track propulsion length of 4.57km, which is scheduled to be completed by the end of February 2003. The bid section was constructed by Luoyang Tunnel Bureau with two German Herrick shield machines. There are many difficulties in this bid. The shield needs to pass through Xuanwu Lake, Xuanwu Lake Tunnel under construction, ancient city wall, Jinchuan River and many buildings, and the shield partially passes through fine sand stratum. Judging from the current construction situation, the shield construction is relatively smooth, and now it has successfully crossed Xuanwu Lake Tunnel, Inner Qinhuai River and Jinchuan River under construction, and the settlement control has reached the expected requirements. The average advancing speed of shield is 8 ~ 10 cycle/day, and the highest speed of the third bid section of shield is 17 cycle/day.
3.2 Shield entry and exit hole reinforcement
There are 24 entrance and exit ends of shield tunnel. According to the analysis of geological conditions, hydrological conditions and ground environment, reinforcement is needed. The entrance and exit of shield tunnel is a construction stage with great technical difficulty and complicated procedures. Once handled improperly, the soil outside the tunnel entrance is easy to collapse or lose, and even the shield is out of control. Therefore, on the basis of careful investigation of geological environment, reasonable reinforcement scheme should be adopted, the operation of shield machine before entering the reinforcement area should be strictly controlled, bentonite mud should be properly injected into the excavation face, and the cutter head should be pushed at a low speed to prevent overload operation and avoid the unfavorable phenomenon that the cutter head is stuck by mixing pile or jet grouting pile and forced to push before the shield enters the receiving well.
The stratum reinforcement range of the inner and outer end wells of the tunnel is 3.0m outside the tunnel excavation contour line, the reinforcement length of the originating end is 6.0m, and the reinforcement length of the arriving end is 3.5m.. However, from the construction situation, the reinforcement length of the 3.5m shield arrival section in the sand layer section is short.
The selection of reinforcement method of shield working well should be based on geological, hydrological and surrounding environment factors. Due to the complicated geology, Nanjing Metro has adopted various reinforcement methods according to local conditions, such as deep mixing, high-pressure jet grouting, well point dewatering and freezing method. Sometimes a combination of methods can be used. Deep mixing method is suitable for cohesive soil layer and muddy soil layer; High-pressure jet grouting method is suitable for sand and silt. After reinforcement, the strength of the soil should be controlled at the unconfined compressive strength of about 0.5MPa, and the reinforced soil should be uniform and sealed with anti-flow sand, which is very important for the shield to enter and exit the tunnel safely.
Judging from the current construction situation of shield entering and leaving the tunnel, the construction of shield entering and leaving Xufu Lane and Xuanwu Gate in Bid Section 3 is relatively smooth, but there are some problems when shield entering and leaving the tunnel in Bid Section 2 and Bid Section 1. For example, there were two sand gushes when the shield of the second bid section of a station came out of the hole, and the sand gushing amount was 1 100 m3, mainly concentrated in the east and west sides of the hole center. The land subsidence in the east 20m2 area is about 1.5m, and that in the southwest of the reinforcement area is about1.5m. Because the concrete at the entrance has been dug up, it has partially cracked. In order to prevent the instability of concrete at the entrance, I-beam number. 18 is welded on the steel ring at the mouth of the cave as a crosspiece, and C20 concrete is poured with a wooden mold for reinforcement.
In order to prevent the recurrence of quicksand and ensure the safety of shield machine, it is necessary to supplement and strengthen the end well. So consider three methods: deep well dewatering method, jet grouting pile reinforcement method and freezing method. According to the two sand production flows, the sand production has not decreased for a long time, but has an increasing trend, indicating that the groundwater supply is relatively rich. The Inner Qinhuai River is about 50 meters away from the Toujing in the southern section of Zhang Fuyuan, and the groundwater and the Inner Qinhuai River may be interlinked, so the precipitation effect cannot be guaranteed. Theoretically speaking, the effect of jet grouting reinforcement in this stratum is good, but it is sandwiched between the continuous wall and the mixing pile plus solid, and the hard cement soil on one side and the reinforced concrete on the other side, which affects the pile-forming effect. In addition, the continuous wall of the entrance has cracked, and there may be side leakage during the construction of jet grouting pile, and there may be running water in the stratum, which has an impact on pile formation. Finally, it is decided to use freezing method to supplement and strengthen the end well. Freezing holes are arranged along the outer side of the continuous wall of the working well in the direction of shield exit, and low-temperature salt water is circulated in the freezing holes to freeze the water-bearing strata near the freezing holes to form a frozen soil wall. Shield tunneling under the protection of frozen soil wall. The frozen soil wall is designed with an effective thickness of 0.5m, an effective width of 8.7m and a freezing depth of 18.5m (the frozen soil lap width around the hole 1m, and the lower lap height of 2.5m).
Due to a large number of quicksand, the shield tunnel mouth could not be opened when the shield 1 bid section left the station. The soil around the original tunnel entrance was reinforced by single-pipe jet grouting, and the reinforcement scope was 4m in the upper part, 3m in the lower part, 3m in the left and right sides and 6m in the axial direction. Due to the existence of pure silt aquifer and serious quicksand in many places within the propulsion range, the reinforcement effect in many places is not obvious. Therefore, the groundwater level will be reduced to below 15m by precipitation, so as to ensure that sand gushing and quicksand will not occur when opening holes. Well points are set at 2m on both sides of the tunnel, and the spacing between well points is 2.5m along the shield advancing direction, with 5 *** 10 in each row, and the length of each well tube is 17.5m, in which the filter screen is 4.0m long.
3.3 Optimization of shield tunneling parameters
In shield construction, the key to minimize the disturbance to the surrounding soil is to maintain the stability of the shield excavation surface and fill the gap in time after the segment leaves the shield tail. Therefore, it is necessary to adjust the parameters of shield tunneling and do a good job of synchronous grouting and secondary grouting.
Shield tunneling is mainly controlled by the following 10 parameters: cutter head and earth bin pressure, excavated volume, advancing speed, screw speed, total jacking force of jack, grouting pressure, shield slope, shield posture and segment assembly deviation, etc. In order to reasonably select the shield tunneling parameters, the parameters are predicted and calculated according to the geological depth and environmental conditions, and the surface deformation above the shield axis is measured and fed back to verify the rationality of the selected parameters and optimize the construction parameters. In general, the range of 50 ~ 100 m from the working well is selected as the test section, and the parameters are optimized through the observation of surface subsidence in the test section. The purpose of EPB shield is to ensure the balance of the earth pressure on the excavation face. By controlling the advancing speed and adjusting the amount of soil discharge, the difference between the earth and water pressure in the stratum and the pressure in the soil bin can be minimized. This balance is a dynamic balance.
3.4 Handling of Special Construction Sites
There are many difficulties in this project. The following only introduces the treatment schemes of two construction sites.
3.4. 1 Shield passes through Xuanwu Lake Tunnel under construction.
The shield tunnel crosses the Xuanwu Lake Highway Tunnel under construction twice, and the minimum distance between the two tunnels is only1.0m..
The settlement at the bottom of Xuanwu Lake Tunnel is strictly controlled, which requires the settlement to be controlled within -20mm and the uplift to be controlled within 5mm, so the shield construction is very difficult. In order to ensure the safety of shield construction and the future operation of Xuanwu Lake Tunnel, according to the results of model test and numerical analysis during shield construction, the following technical measures have been taken:
(1) Because it is muddy silty clay and the interlayer is very thin, before the construction of Xuanwu Lake Tunnel, the subway at the lower part of the tunnel was reinforced by grouting (Q0> 0.5 MPa);
(2) In the design of Xuanwu Lake Tunnel, in order to enhance the longitudinal stiffness of the tunnel, the reinforcement of the bottom plate is strengthened, and uplift piles are added under the bottom plate, so that most of the load in operation is transferred to the lower foundation through the uplift piles, and at the same time, the shield tunnel can be prevented from floating;
(3) Strengthen the monitoring during shield construction, carry out secondary grouting in time, and control the earth pressure balance.
At present, the left line of shield tunnel has successfully passed through Xuanwu Lake Tunnel. According to the monitoring, the maximum settlement 1.9mm and the maximum uplift 1mm of Xuanwu Lake Tunnel meet the expected requirements.
3.4.2 Shield Crossing Qinhuai River
The No.1 shield is between Sanshan Street and Zhonghua Gate and needs to cross the Inner Qinhuai River. The overburden here is very thin, and the depth under the original riverbed stone foundation 1.5m is basically rubble, broken bricks and other construction wastes, and the shield is only 80cm away from the bottom of the anti-floating slab, which leads to the failure of reinforcement and compaction of the upper overburden, easy to cause water leakage and mud leakage, so that the upper pressure of the tunnel is too small, and the tunnel will drift upward, resulting in gaps in the lower part. In addition, shield tunneling is difficult to control, and the shield is prone to deviation. Therefore, the following measures were taken during the construction of shield crossing the Inner Qinhuai River: removing construction wastes such as gravel and bricks, covering the soil and backfilling, and pouring anti-floating plates with a thickness of 70cm on them; Under the roof, the soil in front of the shield is compacted and grouted. The grouting hole is made of PVC pipe with inner diameter 100mm, reinforcement depth of 7m, hole spacing and hole spacing 1m, and there are * * * 16 1 reinforcement holes, and the cement consumption of each hole is 0.684t: a row of bored piles are drilled on both sides of the shield.
4 Development of shield construction technology
With the continuous improvement of urbanization, the underground space will be as dense as the above-ground space, which will inevitably lead to the development of urban subway construction in the direction of large depth, long distance, automation, large section or arbitrary section. In order to adapt to the development of urban subway, shield construction technology has the following development trends.
(1) large depth
There are various underground pipelines in the metropolitan area, such as sewer pipes, gas, communication, power cables, subways, underground shopping malls, underground parking lots and other existing structures, which make the underground space more and more dense. The new rail transit planning will inevitably avoid these existing underground structures and develop in depth. When carrying out shield construction in deep underground, it is necessary to solve the problems of shield cutter head seal, shield tail seal and deep well construction.
1980, Japan developed a shield with cutterhead 100t/m2 and tail sealing water stop system, which can continuously drive more than 10km. Its sealing technology has been used in more than 40 shield tunnels with underground water pressure exceeding 30t/m2. In the English Channel shield construction, the maximum water pressure reaches 100t/m2 and the maximum excavation length reaches 20km. The development of sealing and water-stopping technology under high water and soil pressure makes it possible for shield construction in deep layer.
With the deepening of shield construction, the construction of shield shaft is becoming more and more difficult, and neither construction safety, construction period nor project cost can be effectively controlled. Therefore, it is necessary to study some new shield machines and corresponding construction methods to solve the contradictions in shaft construction. 199 1 year and 1993, respectively, Japan developed MSD shield (two shields meet through mechanical connection underground) and spherical shield, and accordingly developed underground meeting method and vertical and horizontal continuous excavation method. The underground meeting method is a method in which shield machines excavate from both ends of the tunnel, and two shield machines meet in the underground to mechanically connect and cancel the intermediate shaft. Vertical and horizontal continuous excavation method is a method of continuous excavation of vertical shafts and horizontal tunnels from the ground at right angles with billiards shield. Its main feature is that the shaft is also excavated by shield machine, which makes the shaft construction fast and safe.
(2) Long distance
Due to the increase of shield construction depth and the limitation of construction site, construction period and construction cost, long-distance shield construction is inevitable. During long-distance construction, due to great geological changes, the same shield machine may encounter soft soil, pebbles, sandstone, rocks and other strata when driving. This requires the development of the technology that the same shield machine can change the corresponding cutter head at will when it encounters different strata, and the tunneling technology in composite strata. Germany developed the KURUN shield machine, which can rotate the cutter head to the inner side of the shield machine to replace the cutter head, avoiding replacing the cutter head with grouting reinforcement in the middle shaft or stratum. When there are both soft soil and rocks or sandstone in the same excavation section, a dual-purpose shield machine was invented. The cutter head of shield machine is equipped with superhard cutting bit for excavating rock stratum, cutting edge for excavating soil and sand, and advanced geological drilling rig. During construction, excavation and segment installation are carried out at the same time, which greatly improves the construction speed.
After the depth and length of shield construction increase, it is inevitable to encounter sharp turns and steep slopes, which can not be completed by ordinary shield construction. 1980 developed omnidirectional hinged shielding. At present, articulated shield can overcome the sharp bend with the horizontal radius of 10m, and the longitudinal slope construction is within 30.
(3) Automation
In the shield construction with large depth, long distance, high speed and large cross section, the most important thing is to shorten the construction period, reduce labor intensity, ensure construction safety and improve quality, which requires improving the automation level of shield machine construction. At present, foreign shield machines for long-distance shield construction are generally equipped with the following systems: excavation management, safety control system of excavation face, automatic guidance control system, segment handling and supply system and segment automatic installation system.
(4) Diversification of excavation sections
Circular section has advantages in stress, and it is also suitable for shield construction, and it is easy to assemble segments, so circular section is often used in shield construction. But sometimes the circular section can't be used effectively. In order to save investment, it is ideal to excavate the underground part as needed. Therefore, Japan developed the first MF slurry multi-circle shield in 1987, and the first DOT EPB multi-circle shield in 199 1 year, and successively developed rectangular shield, elliptical shield and other special-shaped cross-section shields. The cutter head of point shield machine is located in the same plane and rotates synchronously; The cutter head of MF shield machine is staggered excavation, and the cutting surface is panel structure. MF or DOT shield can be used to build multiple tunnels and single-story multi-span stations. At present, there are 13 EPB shield machines in the world, and six blocks in Japan have been successfully constructed with double circular shield machines. Shanghai Tunnel Company cooperated with Japan to build a double circular shield tunnel on Shanghai M8 line.
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