Application of compressed air operation in subway shield construction?

The following is the application of compressed air operation brought by Zhong Da Consulting in subway shield construction for your reference.

Combined with the shield construction example of Guangzhou subway, this paper introduces the operation principle and geological conditions of air compressor, briefly expounds the maintenance and safety devices of manhole, probes into the situation of changing tools in unstable stratum during the operation of air compressor, and expounds the common problems and treatment methods in the operation of air compressor.

Guangzhou is called "geological museum". The geological conditions of the north-south line are very complicated, such as underground quicksand layer, geological fault zone and high-strength granite boulder. All unfavorable conditions for construction can be seen everywhere in Guangzhou, and its difficulty is praised as "rare in the world" by experts at home and abroad. Shield tunneling in soft and hard strata on sandy pebbles causes serious tool wear, so it is an unavoidable task to choose the right time to enter the cabin to check and replace the tools.

In shield construction, there are roughly three methods for inspection and treatment of tools entering the cabin: 1) entering the soil cabin under pressure for inspection; 2) After the soil in front is reinforced, enter the cabin for inspection under normal pressure; 3) Make a shaft down from the ground to the front of the cutter head to realize the inspection and maintenance of the shield cutter.

EPB shield machine is mainly used for shield construction of Guangzhou Metro. In the process of shield construction, the flowing soil in the soil bin is used to maintain the balance of earth pressure on the working face. In construction, we often encounter unstable strata with soft top and hard bottom or buildings above tunnels, so it is very difficult to check and replace tools. The direct opening of the cabin may lead to the collapse of the excavation surface or even the ground settlement, which is very risky. It takes a long time to reinforce the soil in front; From the ground to the front of the cutter head, the ground is completely unconditional. In this case, it is undoubtedly the best choice to use manhole inspection and tool replacement in compressed air environment. In February, 2004, Tianhua Section of Guangzhou Rail Transit Line 3 successfully carried out the air compression operation for the first time in the domestic subway construction, and opened the carriage to check or replace tools, which achieved good results. The construction method of compressed air operation has been actively promoted in Guangzhou Metro Line 3, and this method has gradually matured in China.

1 compressed air operation principle

The principle of compressed air construction method is to inject compressed air into the tunnel. When there are tiny cracks in the improved soil around the tunnel, the pressure of compressed air can be used to suppress the groundwater pressure, prevent groundwater from infiltrating into the excavation face, keep the excavation face stable, and provide conditions for workers to enter the soil cabin of the shield machine safely.

The stabilizing effect of compressed air on the excavation face can be roughly divided into the following three types: 1) It can prevent the excavation face from water gushing and collapse; 2) Because the air pressure acts on the tunnel face, the stability of the tunnel face can be enhanced; 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.

2 Geological conditions of compressed air operation

Compressed air construction is not suitable for any geological conditions. Due to the different conditions of overburden thickness, soil quality and groundwater, sometimes the compressed air construction method cannot achieve the expected effect, so it must be carefully studied before adopting the compressed air construction method.

According to the buried depth of subway tunnel, the following strata are more suitable for air compression:

1) Silty foundation: poor water permeability and good compressed air effect, suitable for compressed air construction. As long as we pay attention to the relationship between soil cover and air pressure, water inrush will not occur and water inrush can be basically prevented. The supporting function of compressed air pressure itself and the strength produced by dehydration of surrounding rock will also be greatly enhanced.

2) Clay foundation: When the soil is weak and the excavation surface is unstable, the foundation can be reinforced by the retaining effect of air pressure itself and precipitation, so the air pressure method is often used.

3) Sandwich foundation: Generally, the surrounding rock is composed of various soil layers, which are complicated, and the groundwater pressure is often separated by impermeable layers. For example, if there is a stratum with poor permeability such as silt or clay above the excavation face, even gravel or sand layer is an ideal compressed air construction stratum.

Three-person brake

3. 1 Summary

In the maintenance and inspection work, the relevant personnel and the materials they need are transported to the earth cabin by using the manhole, and the place is kept under pressure.

Take herrick shield machine as an example, its manhole consists of a main cabin and an auxiliary cabin, which are separated by a pressure hatch. The main cabin is connected with the transition cabin directly welded on the bulkhead plate of the forebody through a flange; You can enter the earth capsule through the door on the bulkhead; The auxiliary cabin is connected with the side of the main cabin. Therefore, to enter the main cabin, you must first pass through the auxiliary cabin, which is used for the step-up and step-down process during compressor operation and emergency (emergency rescue).

According to the volume of the engine room, the main cabin can accommodate up to 3 people and the auxiliary cabin can accommodate 2 people. Both cabins can work independently and are equipped with the following instruments: communication system, exhaust valve and intake valve, timer, pressure gauge, thermometer and heater. In order to facilitate the operator's operation, the following instruments are installed: exhaust valve and intake valve leading to two cabins, communication system, belt recording system, flowmeter for recording the pressure of cabin and earth cabin, and air inlet of brake. Exhaust valve and intake valve can only be used by cabin personnel under abnormal conditions. The step-up and step-down processes are usually performed by the manual brake administrator.

3.2 Maintenance/Control

Check the door seal before closing every time, and clean or replace it if necessary; Lubricate the connecting mechanism of the door once a month; Before using manhole, check whether all pressure gauges work normally and whether the connecting mechanism is in good condition; Check the function of the recorder according to the specifications.

3.3 Safety device

Each engine room has a safety valve to limit the maximum pressure to 3 bar; There are also telephones and emergency calls.

4 compressed air operation

4. 1 Enter the Earth capsule

The operator enters the earth cabin through the manhole to check the cutter head and cutter, and at the same time check the sensor and the tunnel excavation surface. The excavation chamber is designated as a dangerous area, and the following safety measures must be taken during operation.

Workers in the earth cabin should pay attention to the collapse of rocks and soil and the sudden surge of groundwater. Therefore, according to the geological conditions (gravel and sandy foundation, groundwater), it is very necessary to use bentonite-added mud and stir it evenly with a knife disc to stabilize and seal the tunnel excavation surface after the excavation is stopped.

The use of bentonite suspension can support and improve the osmotic pressure of soil. This sealing form of the excavation face is called "mud cake", which keeps balance with the compressed air system and acts as a film to ensure the sealing of the tunnel excavation face.

4.2 Changing tools

Preparations for changing tools

1) transport the required materials into the earth cabin according to the geological conditions and operation mode (cabin opening mode/earth pressure balance mode).

2) Use a crane or transport platform to transport the tools to the main cabin (working cabin).

3) Control the cutter head to rotate to the required position through the control panel.

4) The fixed points of the lifting platform, the grid platform and the lifting equipment are all arranged on the shielding shell.

Tool replacement

In order to change the cutter, the cutter head must be rotated several times through the control panel.

After unscrewing the respective bolts, all tools can be disassembled. After each tool change, the bolt must be tightened again to the specified pre-tightening torque.

In order to lift and adjust tools, a crane is needed. Especially when replacing hob, lifting equipment with gripper will be used. No special tools are needed to replace the scraper and the edge scraper.

5 Matters needing attention in compressor operation

1) adopt the method of soil abandonment and pressurization in stages for pressurization; 2) The final working face pressure is about 0.1bar ~ 0.2 bar higher than the set earth pressure; 3) Keep the pressure as much as possible when opening the cabin, and control the change value of air pressure within the controllable range through the operation of the air inlet valve; 4) Safety must be confirmed before entering the Earth capsule for work; When the excavation surface is abnormal, immediately organize the staff to exit the earth cabin and close the hatch; 5) More than two communication lines should be set up to keep the internal and external of the pressure chamber, the ground and external related units and departments unblocked, and contact in time if there is any abnormality; 6) Designate a special person to monitor and inspect the ground buildings and underground pipelines; 7) Before the first operator enters the Earth capsule, he should continue to inflate for 10 minute to ensure the fresh air in the Earth capsule; 8) Operators must undergo training and physical examination. No drinking before the operation, no smoking during the operation, and no alcoholic drinks. 9) Within 6 hours after operation, the operator should not take intensive physical exercise. You are not allowed to fly or dive within 24 hours after the operation. You must stay nearby. 10) compressor running time record must be filled in carefully.

6 Common Problems and Solutions

1) If the tool accidentally falls into the earth cabin when changing the tool, it is difficult to locate the tool and salvage it because there is a lot of mud in the earth cabin. The tool changer should bring a shovel and a woven bag when entering the cabin, and bag the mud in the earth cabin.

2) If there are problems such as trachea burst and air compressor failure during operation, first calm down, try to stabilize the air pressure, and inform the operator to enter the manhole as soon as possible so as to decompress as soon as possible.

3) All personnel safety measures and disaster prevention measures should be taken. Employees should have a comprehensive physical examination, and those who fail the physical examination are forbidden to enter. During the operation of air compressor, attention should be paid to the possibility of fire caused by welding, leakage and grinding. All kinds of emergency equipment, such as hyperbaric oxygen chamber and single stand, should be in a ready state.

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