1. Shield construction is more and more widely used in urban infrastructure, such as urban subway, municipal public pipeline, underground oil pipeline and so on, because of its safety, rapidity and high efficiency.
However, the cutting tools on the shield machine are vulnerable parts, especially in Chengdu sandy pebble stratum, the cutting tools are particularly worn. Therefore, in the process of construction, it is necessary to check, maintain and replace the tools frequently.
From September 9 to June 2005 1 1, the National Development and Reform Commission entrusted Beijing Urban Construction Design and Research Institute Co., Ltd. to conduct an expert review on the feasibility study report of the first phase of Chengdu Metro 1 Line. According to one of the expert review results of the civil engineering group, Article 2.3: According to the specific conditions of different sections, the report analyzes and compares the construction methods of the interval tunnels, and the overall scheme is basically feasible. In the report, it is suggested that shield construction should be adopted in urban areas, and it is reasonable to adopt open excavation and cover excavation construction in high-tech zones. However, the geological conditions of the line are complex, and there are certain risks in technology, construction period and investment in shield construction, which must be paid enough attention to. It is suggested that the evaluation opinions on the shield test of the first phase project (civil engineering group) of Chengdu Metro 1 Line from February 4 to February 7, 2006 and the preliminary design evaluation opinions of the Urban Rail Transit Professional Committee of China Communications and Transportation Association should be issued as soon as possible. In the comprehensive review, domestic authoritative experts analyzed the special geological conditions of Chengdu's water-rich sandy pebble stratum and the actual situation of Chengdu's first underground tunnel construction by shield method. We fully considered the risks of shield method in Chengdu stratum construction, and fully communicated these situations with contractors and shield machine equipment manufacturers in time. After extensive discussion and demonstration, the herrick shield machine with small opening, hob as the main structure and some toothed knives and scrapers was finally adopted. Judging from the current advanced section, this shield machine configuration is conducive to maintaining the stability of the excavation face, crushing large-size sand pebbles, and preventing water inrush during excavation. The overall progress of excavation is basically within our planning scope. However, due to too many sandy pebbles in the stratum, tools with various configurations are greatly worn during excavation. According to our statistics, tools must be replaced at a distance of about140m, otherwise the excavation cannot be continued. At present, the tool changing methods adopted at home and abroad mainly include two ways: normal pressure warehouse opening and under pressure tool changing:
Earth pressure balance shield machine mainly uses the ballast soil in the soil bin to maintain the earth pressure balance on the working face. However, when checking and replacing tools, it is necessary to clean up the ballast soil in the excavated silo to provide working space, and the original earth pressure balance is broken. Therefore, it is an urgent problem to keep the balance of earth pressure and the stability of working face soil when changing tools.
In order to maintain the stability of the tunnel face soil, the commonly used methods are dewatering reinforcement, grouting reinforcement, retaining pile (such as manual digging pile) reinforcement and pressure reinforcement. Because each reinforcement method has its own advantages and disadvantages, it is necessary to choose a reasonable tool change method according to different construction conditions.
2. Commonly used tool changing methods in China There are a large number of shield tunnel construction projects in Shanghai, Guangzhou and Beijing. According to different geological conditions in different places, different tool changing methods are adopted.
Shanghai is a soft soil stratum, with low soil strength and little wear of shield cutter, so it is usually unnecessary to replace the cutter. Guangzhou, known as geological museum, has complex geological conditions, including granite layers with high self-stability and muddy siltstone layers with low self-stability. Therefore, there are many ways to change tools, such as direct opening, foundation reinforcement, under pressure, shaft excavation and so on.
Directly opening the silo to change the tool is to directly empty the soil in the silo and enter the silo to change the tool under the condition of high foundation bearing capacity and good hole surface stability without any reinforcement method. This method is suitable for working face with good stability.
Dewatering reinforcement is to build a dewatering well from the ground and realize soil consolidation through dewatering, thus improving the stability of the working face soil.
Surface grouting reinforcement is to inject cement slurry (or other reinforcement slurry) from the surface into the foundation to reinforce the soil and improve the stability of the working face soil.
The reinforcement of retaining piles is to build retaining piles around the working face to directly ensure the stability of the working face.
Reinforcement under pressure is to stabilize the working face by injecting compressed air into the soil bin and replacing the earth pressure balance with air pressure balance.
3. The principle of pressure tool change and its application at home and abroad.
3. 1 Working principle under pressure The working principle of checking, repairing and replacing cutters and cutterheads is: after the stratum in front of the cutterhead of the shield machine is improved or reinforced, under the condition of ensuring that the surrounding stratum and soil bin in front of the cutterhead meet the air tightness requirements, air is pressurized and injected into the soil bin, and the soil pressure is replaced by air pressure, so that the water and soil pressure in front of the cutterhead can be balanced by establishing reasonable air pressure in the soil bin, and the stability of the working face can be realized. Operators enter the earth cabin under air pressure to carry out inspection, maintenance, tool replacement and other technological operations.
The stabilizing effect of compressed air on the excavation face can be roughly divided into the following three types:
1) can prevent water gushing from the excavation face and prevent the excavation face from collapsing;
2) Because the wind pressure acts on the tunnel face, the stability of the tunnel face can be directly strengthened;
3) Because compressed air pushes water out of cracks in surrounding rock, the strength of silty sand, clay layer or sandy soil containing silty clay increases.
Because the pneumatic tool change is all carried out in the hole, it will not interfere with the ground traffic.
3.2 Looking at the development history of shield construction of similar projects at home and abroad, the fully open shield is suitable for cutting self-supporting strata, and the extrusion shield is easy to cause stratum uplift or settlement, which is only suitable for soft soil strata, while the closed shield, namely mud-water balance shield and earth pressure balance shield, can overcome the above shortcomings and are increasingly selected.
1) As far as China is concerned, at present, slurry shield and earth pressure balance shield are mostly used for the excavation of urban subway tunnels, railway and highway cross-river tunnels and oil (gas) and various municipal pipelines. At present, in the construction practice of slurry balance shield, there are usually three methods to check and deal with the storage of tools:
(1) Pressurize and enter the earth cabin for inspection;
(2) After the soil in front is reinforced, enter the cabin for inspection under normal pressure;
(3) From the ground to the front of the cutter head, the inspection and maintenance of the shield cutter head can be realized. In China's subway shield construction, because the key technology of shield loading operation has not been mastered or the shield machine configuration is insufficient, most construction units adopt the latter two methods. Only a few construction units carry out inspection, maintenance and tool replacement by warehousing under pressure. For example:
In June 2005, the underwater shield of the left line from Xiaoguwei to Xinzhuang of Guangzhou Metro Line 4 was replaced under pressure.
Yichang Yangtze River Tunnel Project (Sichuan-to-East Gas Transmission) started from June 5438, 2006 to February 2006 was constructed by German Herrick slurry pressure shield machine, loaded into the warehouse to repair the worn cutter head and replace the main bearing seal of the cutter head.
At the end of 2003, under the pressure of 1.6- 1.8bar, the boulder stuck on the cutter head was broken in the section from Tianhe Passenger Station to Huashi Station of Guangzhou Metro Line 3.
In the shield construction of Wuhan Yangtze River Tunnel, China Railway Tunnel Group independently realized the high water pressure and pressure operation technology (4.5bar pressure and pressure operation) for the first time.
In a word, domestic shield construction units and subway construction units are actively exploring and studying the under-pressure tool changing technology under different stratum conditions. Although some achievements have been made, they need to be further summarized and popularized.
2) As far as foreign countries are concerned, there are many engineering examples of inspection, maintenance and replacement of cutter heads under pressure [4] (table 1). As a routine operation, tool changing under pressure has been widely used in earth pressure balance shield machines and slurry shield machines, especially when the cross-river and cross-sea tunnels do not have the condition of normal pressure tool changing.
The fourth tunnel in Elbe4, Germany, was constructed in the Quaternary moraine layer composed of sand, marl and boulders by slurry balance shield machine. Excavation diameter 14.2m, water depth (maximum head of 4.2bar) and shallow burial (minimum depth of 7m). In the deepest part of the river, it is necessary to change the central knife into the warehouse for repair. Workers can work for up to 80 minutes under the pressure of 4-4.5bar, and the decompression after leaving the warehouse must be carried out in the oxygen chamber, which takes 2 hours. The whole line operated under pressure for 2738 hours, of which the pressure was greater than 3.6bar in 237 hours, and 2/kloc-0 decompression sickness was reported, all of which occurred when the pressure was less than 3.6 bar.
The Weser tunnel in Germany is constructed by slurry shield (diameter: 1 1.7 1m) in glacial sediments. The top arch of the tunnel is 40 meters from the sea. Crusher and cutterhead are kept at the maximum pressure of 5 bar and 4.5 bar, and divers also work in bentonite slurry with the maximum pressure of 5 bar. The working time of the whole line under pressure 1400 hours, in which the pressure exceeded 3.6bar for 600 hours, and only decompression sickness 15 times was reported, and the pressure was less than 3.6bar.
When the red line subway in St. Petersburg, Russia is constructed in soft and low-plastic clay, silty sand and fine sand, when the top arch of the tunnel is 65m from the ground, the clear water pressure is 5.6bar, and the air pressure of the earth warehouse is 5.5bar, people wear breathing masks to enter the warehouse, working for 1.5 hours, and the decompression time is about 5 hours. The warehouse only has 4.5 hours of working time a day.
Some special regulations for working in compressed air drafted by British Tunnel Engineering Association, Health and Safety Administration (HSE) and Anti-hypertension Group of Health Research Association.
The above engineering examples, as well as the relevant specifications and research results of using mud or earth pressure balance shield to carry out maintenance under pressure in tunnel excavation in foreign countries, show that foreign companies can not only achieve maintenance under pressure below 3.6bar, but also work under pressure as high as 6.0bar. On the one hand, it shows that operation under pressure, especially high-pressure operation, is quite common in foreign projects. On the other hand, it also shows that more attempts and research are needed to master the technology of shield operation under pressure in China.
3.3 Air pressure balance stability guarantee
3.3. 1 Determination of air pressure value In order to ensure the working face, sufficient air pressure must be provided to balance the water and soil pressure of the working face. Therefore, it is necessary to calculate the air pressure according to the buried depth of shield tunnel, water level conditions and stratum physical and mechanical parameters. When the earth pressure is safe, the air pressure value is calculated according to the static earth pressure.
3.3.2 The stability of gas pressure ensures the stability of Nevas pressure in the excavation bin, which needs to be guaranteed from two main aspects: geotechnical (air leakage) and electromechanical (air intake).
(1) geotechnical system
In order to evaluate the air loss degree of natural soil and the influence of additives on formation gas permeability before the tool changing operation under pressure, it is necessary to carry out field test of formation air compression near the expected shut-in location. According to the test data, the gas loss and its change when compressed air with a certain pressure is applied to the excavation face are calculated, and the gas seepage prevention effect of the stratum after adding additives when the excavation bin is semi-muck or empty is evaluated.
Make a complete and reliable excavation program. The excavation procedure shall be carried out in 5- 10 cycles before the estimated mileage of opening and repairing tools under pressure. Ensure the air tightness of the soil bin and the protective cover to prevent air leakage.
In the process of gas filling, send someone to check whether there is a boundary where gas leakage may occur on the ground such as engineering or municipal pipeline wells, evaluate the possibility and influence of gas leakage, and take effective measures in time. If no measures can be taken, it is necessary to avoid this area for pneumatic tool change.
(2) Electromechanical system
Before starting the operation procedure, the shield machine and its spare sleeve system should be inspected in detail to ensure the safe operation of the machine, and problems should be found and solved in time.
Check the compressed air system in detail before starting the operation procedure.
Check the air compressor to avoid being higher or lower than the set pressure value in the soil bin. Someone must be appointed to look after the air compressor, and report any problems immediately.
3.3.3 Evaluation of tunnel heading stability under wind pressure stability and abnormal conditions. The air pressure in the excavation bin replaces the earth pressure. After the slag is discharged to the set level, it is necessary to wait and observe for a period of time to see if the air pressure in the earth bin is stable.
When the wind pressure is stable and the amplitude does not exceed 0.02bar, it is necessary to organize experienced geotechnical engineers to enter the excavation bin with pressure to evaluate the stability of the tunnel face.
When the air pressure in the excavation chamber changes greatly and is unstable, it shows that the mud film around the tunnel face or shield is not well formed, and it needs to be excavated again, and then the test is carried out after leaving the existing disturbed area.
4. Comparison of tool changing methods
4. 1 Tool-changing reinforcement requirements In order to satisfy the efficient, safe and smooth construction of Chengdu Metro, the tool-changing reinforcement method must meet the following conditions.
① low cost;
② The reinforcement is effective and reliable;
(3) high efficiency, and can achieve the reinforcement effect in a short time;
(4) The steel bar will not solidify the cutter head and cutter;
(5) try not to occupy the surface;
To build a subway in the center of Chengdu, we must first minimize the impact on urban traffic. Therefore, when choosing the tool changing method, we also fully consider how to reduce the impact on the ground environment and avoid disturbing the people.
4.2 Comparison of tool changing methods According to the characteristics of water-rich sandy pebble stratum in Chengdu area, combined with the pre-excavation situation of four blocks, we have the following tool changing methods:
(1) Direct opening and tool changing mode
Due to the low stability of the sandy pebble stratum in Chengdu, the underground water level is higher than that of the shield tunnel, and it is impossible to directly enter the soil silo. This method is not feasible.
(2) Dewatering reinforcement method
The dewatering method is simple and feasible, but it faces the risk of stratum instability. Because of the low self-stability of Chengdu sandy pebble stratum, it can be self-stable in a short time, and once the time is too long, the stratum is in danger of collapse. Practice has proved that it is not feasible to change tools only by precipitation in urban construction.
(3) surface grouting reinforcement
Surface grouting can strengthen the stratum to a certain extent, and the reinforcement effect is better than precipitation, but it needs to be operated on the surface, so on the one hand, it is limited by the surface conditions. On the other hand, the stratum is dense and the grouting effect is not good. Many practices have proved that the surface may still collapse after reinforcement, and this method can not guarantee the safety and surface requirements of replacing tools.
(4) Reinforcement of retaining piles
Manual digging piles are usually used as enclosure piles in Chengdu. Manual digging pile reinforcement can ensure the safety and stability of tool changing. However, there are the following disadvantages:
The mileage of (1) pile is easy to determine. If the reinforcement position is determined slightly improperly, the tools will wear out and cannot be excavated before the shield reaches the mileage of the pile. In this case, forced driving, or grinding the cutter head, or excessive slag, will also cause the surface to collapse.
② High cost and long construction period. The construction cost of manual digging pile is relatively high and the construction period is long.
③ It involves a wide range of departments. Artificial excavation from the surface involves transportation, municipal pipelines, houses, surface restoration, etc. , and slag discharge and feeding are related to the environmental protection department.
④ It has great influence on the surrounding environment. According to the current situation, the tool changing distance is generally within 150m. Digging piles in such a short distance is undoubtedly opening the belly, which has no less impact on people's lives than surface collapse.
⑤ The surface conditions are limited. Because the subway shield tunnel lines are mostly in downtown areas, they do not have the conditions for manual digging piles.
⑥ The pile foundation left underground will cause unpredictable harm and risk to future municipal engineering.
⑦ Failure to reflect the advanced nature of shield machine has little impact on the environment.
(5) Air pressure enhancement
Air pressure reinforcement does not occupy an area and has the characteristics of low cost and high efficiency. However, due to the relatively high technical requirements for construction, and the particularity and risk of Chengdu stratum, it is necessary to continue to strengthen exploration and attempt.
4.3 Analysis and comparison
5. Conclusions and countermeasures
Tool changing under pressure is the main tool changing method for other similar shield tunneling, especially when crossing ditches, rivers, seabed and other special geographical environments. However, due to the high permeability of stratum sand and pebbles in Chengdu, the buried depth of subway line is shallow, it is very difficult to change tools under pressure, and there are many uncertain factors, especially on urban main roads, which are extremely risky and will bring disastrous consequences once they fail. In view of this, in the early stage, we all adopted the way of opening warehouses at normal pressure, mainly focusing on dewatering and foundation reinforcement. Through a period of practice, this method is very suitable for the formation characteristics of Chengdu in terms of operation safety and tool changing efficiency. However, one of the main disadvantages of this tool changing method is that it has brought influence to the ground environment and interfered with the ground traffic. At present, we are trying to explore the feasibility of changing the tool under pressure in Chengdu stratum in bid section 3 of shield (construction of China Railway Second Bureau), and we have achieved the success of 12, which can be said to be basically successful. In the next stage, we will continue to explore. In the process of changing knives, we plan to take some safety protection measures such as partially closing some traffic, so as to be foolproof in measures and emergency treatment. In the next stage, we will strive to organically unify the machine opening, tool configuration and screw conveyor type with the safety, quality, progress and investment of the project, and gradually reduce the number of tool changes.
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