See figure 1 for the general situation of shield construction. The main contents of shield construction are: firstly, build a shaft or foundation pit at one end of a tunnel section for shield installation. The shield starts from an opening in the wall of a shaft or foundation pit and proceeds along the design axis in the stratum to the design hole of another shaft or foundation pit. The ground resistance during shield propulsion is transferred to the lining structure of prefabricated tunnel at the tail of shield through shield jack, and then to the back wall of shaft or foundation pit. In this construction method, shield is the most important and unique construction tool. It is a circular, rectangular or horseshoe-shaped steel tube structure that can withstand formation pressure and move forward in the formation. Various devices for supporting and excavating soil are arranged in front of the steel cylinder, and jacks required for jacking are installed on the inner surface of the circumferential ring in the middle section of the steel cylinder. The tail of the steel cylinder is a shell with a certain space, and one or two prefabricated tunnel lining rings can be assembled at the tail of the shield. Every time the shield advances, it will assemble a circle of lining under the support of the shield tail, and timely inject enough slurry into the gap between the surrounding of the excavated tunnel and the surrounding of the lining ring directly behind the shield tail to prevent the tunnel and the ground from sinking. In the process of shield propulsion, a proper amount of earthwork is continuously discharged from the excavation face.
When shield method is used in construction, it is often necessary to supplement other construction technical measures according to the engineering geological and hydrogeological characteristics of crossing the soil layer. Mainly includes:
Measures to drive groundwater in soil layer by drainage;
Soil reinforcement measures to stabilize strata and prevent tunnel and ground settlement;
Waterproof and plugging technology of tunnel lining;
Monitoring technology to cooperate with construction;
Labor protection measures in pneumatic construction;
Transportation and treatment methods of excavated earthwork.
Figure 1 Overview of Shield Construction
1- shield; 2- Shielded jack; 3- shield the front grille; 4- Unearthed turntable; 5- excavating belt conveyor; 6-stage assembling machine;
7 paragraphs; 8- grouting pump; 9- Grouting hole; 10- excavator; 1 1- segment composed of tunnel lining structure; 12- Grouting at the tail of shield; 13- support segment; 14 axis.
Shield method is a safe and effective construction method, but it is not a universal construction method. Therefore, it is necessary to fully grasp the characteristics of shield construction.
Second, the main advantages of shield method
Except for shaft construction, all construction operations are carried out underground, and the pollution caused by noise and vibration is less, which neither affects ground traffic nor reduces the impact of noise and vibration on nearby residents.
The main processes such as shield propulsion, excavation and lining assembly are carried out in cycles, and the construction is easy to manage, with few construction personnel, low labor intensity and high production efficiency.
The amount of earthwork transported abroad is less.
When crossing the river, it will not affect shipping.
Construction is not affected by weather conditions such as wind and rain.
The construction cost of the tunnel is not affected by the amount of soil covered, and it is suitable for the construction of deep-buried soil covered tunnels. Shield method has good technical and economic advantages in building deep-buried tunnels in places with poor soil quality and high water level.
When the tunnel crosses the river bottom or other buildings, it will not affect the construction.
As long as the shield excavation face is as stable as possible, the deeper the tunnel, the worse the foundation and the more buried objects in the soil that affect the construction. Compared with cut and cover, it is more advantageous in economy, construction and progress.
Third, the shortcomings of shield method.
When the curve radius of the tunnel is too small, the construction is more difficult.
When building a tunnel on land, it is difficult to stabilize the excavation surface or even to construct it. But underwater, if the soil layer is too shallow and the shield construction is not safe enough, it is necessary to ensure a certain soil layer thickness.
Noise and vibration exist in the shaft for a long time, and measures should be taken to solve them.
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.
It is difficult to completely prevent the surface settlement in a certain range above the shield tunnel, especially in the saturated soft soil layer. Strict technical measures should be taken to limit the settlement to a small range, and it is still impossible to completely prevent the surface settlement of the soil layer centered directly above the shield.
In saturated aquifer, the assembled lining used in shield construction has high technical requirements for realizing the waterproof performance of the whole structure.
When pneumatic construction is used, there is danger of hypoxia and dry well around, so corresponding measures must be taken.
Section 2 Development History of Shield Tunnel
1. Development history of shield tunnels abroad
The shield construction technology has a history of 170 years since Bruner initiated it in the Thames underwater tunnel project in London, England in 823. In the ups and downs of 170 years, after several generations' efforts, shield method has developed from a special technology that can only be applied in a few developed countries in Europe and America to a tunnel construction technology that is extremely common in developed countries and gradually applied in developing countries in China.
It is said that the earliest idea of inventing shielding method is an interesting discovery of the inventor. Bruner, England, found that there was a moth in the board of a ship. It drilled a hole and covered the wall with the liquid it secreted. 18 18 years, inspired by drilling, bruner first put forward the idea of building a tunnel by shield method, and obtained a patent for this construction method in Britain. 1825, Bruner made a shield with his own ideas and built an underwater tunnel on the Thames for the first time. The section of this highway tunnel (1 1.4m×458m) is quite large. During the construction, the tunnel was damaged by landslides and floods. At that time, it was in a state of difficult progress. The tunnel was flooded twice during the construction because the method of controlling the mud pouring into the tunnel was not mastered at first. Later, with the cooperation of the East London Subway Company, the shield construction was improved, and air pressure was used to assist the construction, which cost 6500 yuan.
1865, Barrow used circular shield for the first time, and cast iron segments were used as the lining of underground tunnels. 1869, he built a tunnel with an outer diameter of 2.2 1m under the Thames with a circular shield. The pneumatic method of applying compressed air to prevent water gushing when shield tunneling through saturated aquifer was first invented by Lord Colins in 1830. 1874, when building a tunnel with an inner diameter of 3. 12m in the clay and water-bearing gravel stratum of the South Line of London Metro, Henry the Great Head (1844 ~ 1896) integrated all the technical characteristics of shield method and pneumatic method, and put forward the construction technology of pneumatic shield method completely. In 1930s and 1940s, these countries successfully built a number of underground railways and river-crossing highway tunnels with an inner diameter of 3.0~9.5m. In new york alone, there are 19 important underwater tunnels built by pneumatic method. Shield construction covers a wide range, including highway tunnels, underground railways, sewers and other municipal public pipelines. In the early 1940s, the Soviet Union began to build subway tunnels and stations in Moscow, Leningrad and other cities with 6.0~9.5m diameter shields.
Since 1960s, the shield method has developed rapidly in Japan, and has been widely used in underground railway construction in Tokyo, Osaka, Nagoya and other cities, as well as in the construction of municipal public facilities such as sewers. In the 1970s, Japan, the Federal Republic of Germany and other countries developed various new lining and waterproof technologies, such as local air pressure shield, slurry pressure shield and earth pressure balance shield, and corresponding technologies and supporting equipment, which solved the technical problems such as surface settlement, precast high-precision reinforced concrete lining and joint waterproofing caused by shield construction in soft aquifers in urban construction areas.
It is worth mentioning the development of shield in Japan. Japan is the first country outside Europe and America to introduce shield construction technology. The Guanmen Tunnel of 1939 is the first tunnel project in Japan to adopt shield construction technology. Due to the war and the difficult period after the war, this technology has not been developed. It was not until 1957 that a tunnel was built in Marunouchi inner line of Tokyo Metro and 196 1 Nagoya Metro used this method to build a tunnel in Desperate Mountain and achieved satisfactory results that the shield construction technology developed rapidly in Japan. In just over 20 years, * * * has made more than 2,000 shield machines, which is in a leading position in the world. Japan's mechanical shield was developed at the same time as hand-dug shield. 1963 The mechanical shield with an outer diameter of 2.592m (the outer diameter of the tunnel is 2.35m) was applied for the first time in Osakanoe Waterway Dadian Water Pipeline Project (with a total length of 227m). 1964 the tunnel (with a total length of 447m) in Tanmachi work area of Osaka Metro Line 2 adopts a large-section mechanical shield with an outer diameter of 6.97m (the outer diameter of the tunnel is 6.8m). In the same year, a mechanical shield with an outer diameter of 3.4m (the outer diameter of the tunnel is 3.30m) was used in Shengu 3 2 District of Tokyo Sewage Bureau, and the standard monthly construction progress reached 360m m. The machinery with an outer diameter of 10.04 1m (the inner diameter of the tunnel is 9.90m) was used in the section of 1967. Especially for small-section shield, great progress has been made in the research of shortening the construction period. At the same time, extrusion shield is also developing in soft foundation.
The English Channel Tunnel, built in 1993, connects Britain and France, with a total length of 48.5km and a seabed length of 37.5km. The deepest elevation of the tunnel is 100m. This tunnel is all constructed by shield method. The British side * * * adopts 6 shields, 3 construction shore sections and 3 construction submerged sections. The shield for the construction of the submarine section should be pushed 2 1.2km in one direction to enter the waterway and dock with the shield pushed from France to Britain. The French side * * * used six shield machines, two for the onshore section and three for the subsea section. The Strait Tunnel consists of two single-track railway tunnels with an outer diameter of 8.6m and a 1 auxiliary tunnel with an outer diameter of 5.6m. As the maximum depth of the submarine section is 100m, both the shield machine and the precast reinforced concrete segment lining structure have to bear the water pressure of 10 ATM, and due to the unidirectional propulsion of 2 1.2km, The advancing speed of shield must reach 1000m per month before it can be completed in about three years. Therefore, the shield structure and its subsequent equipment must adopt high-quality wear resistance and corrosion protection. Therefore, the construction of this tunnel marks the latest level of shield construction technology.
In recent years, Japan has improved the mechanical shield, and developed a slurry pressure shield with pressurized slurry to stabilize the excavation surface and an earth pressure balance shield with excavated soil as the balance excavation surface.
2. Classification and applicable conditions of shield machine
The forms of shielding can be classified from many aspects.
According to labor and machinery, it can be divided into three categories: hand digging, semi-mechanical and mechanical.
According to the way of retaining soil in working face, it is divided into open and closed.
According to air pressure and mud pressure: air pressure, mud pressure, earth pressure balance, water addition, high concentration mud pressure and mud addition.
1. Dig the shield by hand. Hand-dug shield is the basic form of shield, and there are still projects in the world that use hand-dug shield, as shown in Figure 2. According to different geological conditions, the excavation face can be fully opened for manual excavation; You can also use all or part of the front support to excavate in layers independently according to the soil quality of the excavation surface, and support it with the excavation. The amount of excavation is the amount of soil discharged from all tunnels. This kind of shield is easy to observe strata and clear obstacles, easy to rectify deviation, simple and cheap, but it has high labor intensity and low efficiency, and it is easy to endanger personal and engineering safety once frontal collapse occurs. In water-bearing strata, precipitation, air pressure or soil reinforcement are needed.
This shield is excavated from top to bottom. During excavation, the front support jacks are replaced in turn, and the excavated soil is loaded into the excavation vehicle from the lower part by the belt conveyor. The basic condition for adopting this kind of shield is that the excavation surface can't collapse at least in the excavation stage, because the front of the shield is open when excavating the stratum.
Hand-dug shield is suitable for stratum: There are various supporting methods for excavation face by hand-dug shield, ranging from sandy soil to cohesive soil layer, so it is more suitable for complex stratum, with the most construction examples so far. When obstacles appear on the excavation surface, this form of shield is easier to remove because the front is open. This kind of shield has low cost, few faults and is the most economical. When constructing in the stratum with poor self-bearing face, it can be used simultaneously with auxiliary construction methods such as wind pressure, precipitation and chemical grouting to stabilize the stratum.
Figure 2 Hand-dug shield
2. Squeeze the shield. When the open-cut shield is constructed in silty sand and clay with poor geological conditions, soil will flow into the shield from the excavation surface, causing the excavation surface to collapse and unable to continue excavation. At this time, a chest plate should be set in front of the shield to close the front, and a small hole for excavation should be opened on the foot plate. This form of shielding is called squeeze shielding (see Figure 3). When the shield is pushed forward, as the paste is extruded from the nozzle, the soil will be squeezed into the shield from the dug hole. The opening ratio is determined according to the propulsion speed. When the porosity is too large, the excavation will increase, which will cause the settlement of the surrounding strata; On the contrary, it will increase the cutting resistance of the shield and make the ground uplift. When using extrusion shield, it is very important to set a certain opening rate according to certain geological conditions and control the excavation amount.
Squeeze shield is to close the breast plate of hand-dug shield and block the front soil. This kind of shield can be divided into full extrusion type or partial extrusion type, which is suitable for soft cohesive soil. When the shield body is fully squeezed and pushed forward, all breastplates will be closed and do not need to be unearthed, but it will cause considerable surface deformation. When using the partial extrusion shield, it is necessary to partially open the breast plate, squeeze the soil to be discharged into the shield from the opening, and then load it for transportation. This kind of shield construction has also caused great ground deformation.
Applicable stratum of extrusion shield: the applicable scope of extrusion shield depends on the physical and mechanical properties of stratum. In the specifications (shield) and specifications (version 1977) of Japanese tunnels, its application scope is determined according to the relationship between sand content-cohesion and liquid index-cohesion. According to the construction experience, even if the cohesion exceeds this range, it may be applicable in the stratum with small sand content. According to the construction experience so far, when the sand content of soil is below 20%, the liquid index is above 60%, and the cohesion is below 0.5kg/cm2, the aperture ratio of shield is generally 2 ~ 0.8%, and it is as small as 0.3% in extremely weak stratum. In the construction area of extrusion shield, if the foundation of buildings or strata has been reinforced by chemical grouting, it will affect the advancement of extrusion shield, so the shield chest plate should be detachable in advance.
Fig. 3 Squeeze shield
3. Grille shield is often used in Shanghai soft soil area. It has the characteristics that the water inflow is close to or equal to that of all tunnels, and often has the nature of local compression. The front of the shield is equipped with a steel grating, which can cut the soil during the propulsion process and stabilize the excavation surface when the propulsion stops. The cut-in soil can be transported out by turntable, belt conveyor, mine car or hydraulic machinery, as shown in Figure 4. If this shield method is carefully constructed in the stratum with suitable soil quality, the surface subsidence can be controlled to a moderate or small degree. Construction in water-bearing strata needs to be supplemented by drainage measures.
Figure 4 Grid Shielding
1- shield jack (used to propel shield); 2- excavation face support jack; 3. Heavy lifting arm (used to assemble the assembled reinforced concrete lining);
4- soil piling platform (the clods at the lower part of the shield are lifted by the turntable and then fall into the soil piling platform); 5- Scraper conveyor, where clods enter from the soil piling platform and then output; 6- assembling reinforced concrete lining; 7- Shielded steel shell; 8- Steel mesh on excavation face; 9- turntable; 10- earth loader.
4. Semi-mechanical shield. Semi-mechanical shielding is shown in Figure 5. Semi-mechanical shield is a form between hand-dug shield and mechanical shield, which is closer to hand-dug shield. It is based on the open shield to install a mechanical excavation device to replace manual labor, so it has the characteristics of labor saving and high efficiency.
The mechanical digging device can move back and forth, left and right and up and down. It has three forms: bucket type, cutting head type and both. Its top is the same as that of hand-dug shield, and it is equipped with movable front eaves and front support jacks.
The mechanical equipment of shield has the following forms:
(1) The lower part of the shield face is equipped with a bucket and a cutting head.
(2) The upper part of the shield working face is equipped with a bucket and the lower part is equipped with a cutting head.
(3) The shield center is equipped with a cutting head.
(4) The shield center is equipped with a bucket.
Form ①: The upper half of the shield working face is provided with a front supporting jack and a working platform. The upper part of the working face is excavated manually, and the excavated soil and sand fall to the lower part, which is excavated by bucket and loader.
Form ②: The upper working face of shield adopts bucket or loader for excavation, and the lower working face adopts cutting head or bucket for excavation.
Form ③: Excavation and cutting head excavation.
Form ④: Digging bucket for excavation.
Semi-mechanical shield is suitable for strata: semi-mechanical shield is more suitable for good strata than hand-dug shield. Form ① is suitable for strata that need excavation face support, and forms ② ~ ④ are suitable for strata that can stand on their own feet. ② Formula is most suitable for interlayer between loam and gravel. ③ Formula is most suitable for consolidation of cohesive upper layer and hard sand layer. Type ④ is most suitable for mixed layers of clay and gravel.
Fig. 5 Semi-mechanical shielding
5. Open the chest mechanical cutting shield. When the ground layer can stand on its own feet, or can stand on its own feet after taking auxiliary measures, install a large knife disc suitable for the diameter of the shield at the cutting part of the shield, and carry out full-face mechanical cutting and excavation, as shown in Figure 6. Mechanical shield is a kind of shield that uses rotating cutter head near the excavation face to excavate the whole section. It has the function of continuous excavation of soil layer. Can be unearthed, while advancing, continuous operation.
The cutting mechanism of mechanical shield is mostly in the form of big knife head, including single axis, double rotation and multi-axis, among which single axis is the most widely used. The cutting head with a plurality of spoke-shaped notches rotates around the central axis, and the soil cut by the cutting head enters the turntable arranged on the outer ring from the notches, then is lifted into the soil leakage bucket by the turntable, and then is sent to the excavating vehicle by the conveyor belt.
The advantages of mechanical shield can not only improve the working environment and save labor, but also significantly improve the advancing speed and shorten the construction period. The problem is that the cost of shield is high, and there are many follow-up equipment. In order to improve work efficiency, the base area is large. Therefore, if the tunnel length is short, it is not economical enough. Compared with hand-dug shield, it is more difficult to construct and rectify the deviation of shield under the condition of small curvature radius.
Mechanical shield is suitable for stratum: Mechanical shield can be constructed in stratum which is easy to collapse, because the big knife head of shield itself can prevent the excavation face from collapsing. However, when constructing in cohesive soil layer, the excavated soil is easy to stick to the turntable, and it will be difficult to dig out after compaction. Therefore, mechanical shield is mostly suitable for sandy soil layer with little geological change.
Fig. 6 Mechanical cutting shield for thoracotomy
Local pressure shield. A partition is installed in front of the support ring of the mechanical shield, and a sealed cabin is formed between the incision and this partition. The sealed cabin is filled with compressed air to stabilize the soil on the excavation surface. In this way, tunnel builders will not work under air pressure. Under appropriate geological conditions, it is undoubtedly better than full pressure shield. However, this kind of shield is prone to air leakage at the joint of sealed cabin, shield tail and segment, as shown in Figure 7.
Fig. 7 Local Pneumatic Shielding
7. Mud pressure shield. Mud-water pressurized shield is to inject mud with appropriate pressure into the sealed chamber to support the excavation face with a baffle and a support ring in front of the front face of the shield, and cut the soil with a big knife mounted on the front face. After the soil is mixed with mud, it is transported to the ground for treatment through mud pump and pipeline (see Figure 8).
Figure 8 Mud Pressurized Shield
(a) German style (b) Japanese style
Specifically, the slurry pressurized shield is to set a partition behind the big knife head of the mechanical shield, and the space between the partition and the big knife head is used as a slurry chamber, and the excavation face and the slurry chamber are filled with pressurized slurry, so that the stability of the soil on the excavation face is ensured by the pressurizing and maintaining mechanism. When the shield advances, the excavated soil will enter the mud chamber. Stirring is carried out by a stirring device, and the stirred high-concentration slurry is sent out of the ground by means of fluid transportation, and the sent slurry is separated from water and soil, and then the separated slurry is sent to the slurry chamber, and the slurry is continuously pressurized, so the excavation face cannot be directly observed inside the shield, so it is required that the shield should be systematically operated from propulsion, sludge discharge to slurry treatment. Through the measurement of mud pressure, mud flow and mud concentration, the excavated amount is calculated, and the whole operation process is comprehensively managed by the central console. Mud pressure shield uses the characteristics of mud to stabilize the excavation face, and mud has the following three functions at the same time.
Balance between mud pressure and water and soil pressure on excavation face.
After the mud acts on the stratum, it forms an impermeable mud film, which makes the mud produce effective pressure.
Pressurized mud can penetrate into a certain area of the stratum and make the excavation surface in this area stable.
As far as the characteristics of mud are concerned, the higher the concentration and density, the better the stability of excavation face, and the lower the concentration and density, the higher the mud conveying efficiency. Therefore, considering the above situation, the values widely used as mud management standards at present are as follows:
(1) unit weight: 1.05 ~ 1.25 (g/cm3) clay, bentonite, etc.
(2) Viscosity: 20 ~ 40 (s), funnel viscosity 500/500ml.
(3) Dehydration amount: q < 200ml, (APL filtration test 3 kg/cm2, 30min).
Mud pressure shield has Japanese system and German system, as shown in Figure 8. The difference between the two is that the German sealed cabin is equipped with a buffer air pressure cabin, which is convenient for manual control of the front mud pressure and has a simple structure; The Japanese sealed cabin is full of muddy water, so there should be a set of automatic control device for muddy water balance. Generally speaking, slurry shield has the least disturbance to stratum and the smallest ground settlement, but the highest cost.
In 1966, Japan first adopted the slurry pressure shield method. After the construction of Haneda Tunnel 1970 on Beijing-Yeh Railway under the canal, the slurry pressure shield method attracted attention. 1974 was polluted by chemical grouting liquid, the types of grouting liquid were controlled, and the slurry pressure shield construction method without chemical grouting was newly evaluated. After 1975, the projects constructed by this method increased sharply, and the slurry pressure shield was almost hot. By 198 1 year, the number of projects constructed by slurry pressure shield method accounts for 1/3 of the total number of shield construction projects. Most people think that slurry pressure shield has strong adaptability to different soil layers and is convenient for automatic management. On the other hand, at the 4th Japan Tunnel Technology Seminar in February 1983, the adaptability of slurry pressure shield to different soil layers was denied, and it was considered that at least the shield was not suitable for construction in gravel layer and pebble layer with little cohesive soil without auxiliary construction. It is generally believed that it is more advantageous to use slurry pressure shield in alluvial layer dominated by sandy soil, but the construction in alluvial layer dominated by cohesive soil requires higher slurry treatment cost. After the construction of slurry pressure shield, the surface settlement can be controlled within 10 mm, and the problem is how to reduce the cost of slurry treatment and subsequent equipment (the cost of slurry pressure shield is higher than that of earth pressure shield).
Suitable stratum for slurry shield: slurry pressure shield was originally used in special stratum with alluvial clay and alluvial sand. Due to the obvious influence of mud on the excavation surface, soft muddy soil layer, loose sand layer, gravel layer, pebble gravel layer and the interbedded layer of gravel and hard soil are all adopted. Mud-water pressurized shield is widely used in strata. However, using slurry pressure shield construction in loose pebble layer and hard soil layer will cause water leakage, so a certain amount of binder should be added to the slurry to stop the leakage. It may also fail when digging in a very loose pebble layer. In addition, when excavating in hard soil, not only the soil particles will reduce the quality of muddy water, but also the soil will often adhere to the cutter head and notch, which brings difficulties to excavation and should be paid attention to.
Applicability of slurry pressure shield;
The content of (1) fine-grained soil (particle size below 0.074mm) is above 10% of the particle size accumulation curve.
(2) The content of gravel (particle size greater than 2mm) exceeds 60% of the particle size growth curve.
(3) The natural water content is above 65438 08%.
(4) No coarse gravel of 200 ~ 30 omm.
Permeability coefficient k < 10-2 cm/s 。
8.EPB shield Earth pressure shield is also called closed earth cutting shield or earth pressure shield. There is a full-face cutting head at the front end, a sealed cabin for storing cutting soil at the back of the cutting head, and a long cylindrical spiral conveyor is installed at the lower part of the center line of the sealed cabin, with an inlet and an outlet at one end of the conveyor, as shown in Figure 9. The so-called earth pressure balance means that the cut soil and muddy water in the sealed cabin fill the sealed cabin, which can have appropriate pressure to balance the earth pressure on the excavation surface, so as to reduce the disturbance to the soil and control the surface settlement. This kind of shield can save the necessary mud-water balance and a lot of mud water treatment equipment costs, and is mainly suitable for cohesive soil or silty sand with certain viscosity. At present, there is a new type of water or soil pressure balance shield, which can be applied to various soil layers.
Fig. 9 EPB shield
EPB shield was first used in 1974. It is a hydraulic tunnel shield with an outer diameter of 3.72m m. Later, due to the improvement of the performance of the dumping mechanism of the earth pressure shield, various mechanisms were developed to stabilize the excavation face. Since 1978, the number of shields made in Japan has risen sharply to 19865438+.
The basic principle of EPB shield is that the cutter head cuts the soil layer, and the cut soil enters the soil cavity (studio). The soil in the soil cavity is excavated by the screw conveyor in the soil cavity at the same time when the pressure on the excavation face is balanced. When the excavation amount and the propulsion amount are balanced, the soil discharge device installed at the soil discharge port continues to be unearthed. The product names of EPB shield are different, even for similar shields, the names are different because of the methods of surface stability excavation by companies and the development process of dumping mechanism. Among the shields that keep the excavation surface stable under different conditions, the shield unearthed from the soil cavity by screw conveyor is different from the slurry pressurized shield. Earth pressure balance shield can be divided into four categories: cutting pressurization, earth pressure balance water addition, and high-concentration mud pressurization and mud addition.
Stability mechanism of excavation face;
According to geological conditions, the stability mechanism of EPB shield excavation face can be divided into two types: one is suitable for cohesive soil layer with small internal friction angle and easy flow; The other is suitable for sandy layers such as sand and gravel. Large internal friction angle, difficult to flow and high water permeability.
Stability mechanism of excavation face in (1) cohesive soil layer
Silty clay, silty sand, silty sand and other cohesive soil layers. The soil discharge mode of the excavation surface stabilizing mechanism is: the soil cut by the cutter head enters the soil cavity first, and when the soil pressure in the soil cavity and the soil pressure on the excavation surface (in cohesive soil, the mixture of soil pressure and water pressure on the excavation surface and the pressure effect) reach a balance, the excavated soil is sent to the rear by the screw conveyor, and then unearthed from the excavation door. This mechanism first fills the soil cavity with excavated soil. In soft cohesive soil, the strength of soil cut by cutter head is generally lower than that of undisturbed soil, so it is easy to flow. Even in the soil layer with high cohesion, the stirring action of the cutter head and the conveying action of the screw conveyor will disturb the soil and increase the fluidity of the soil, so the earth pressure of the soil filled in the soil cavity and the screw conveyor can be equal to that of the excavation face. Of course, the earth pressure of the soil cavity and the fill in the conveyor must be discharged by the screw conveyor under the condition that the earth pressure on the excavation surface is equal, and the excavation volume and discharge volume should be balanced. However, when the sand content in the stratum exceeds a certain limit, the fluidity of the soil cut by the cutter head becomes worse, and the soil in the soil cavity is too full and too fixed, which will make the soil compacted, making it difficult to dig and discharge the soil, forcing the propulsion to stop. In this case, the commonly used methods are: adding bentonite, clay and so on. Stir in the soil cavity, or spray water or air to increase the fluidity of the soil in the soil cavity.
(2) Stability mechanism of excavation surface in sandy soil layer.
In the sandy soil layer with gravel, the friction resistance of the soil is large, the groundwater is rich, and the water permeability coefficient is also high. Therefore, it is difficult to achieve a balance by relying on the earth pressure of the excavated soil and the pressure between the soil removal mechanism and the excavation surface (groundwater pressure and earth pressure). Moreover, the fluidity of the soil cut by the cutter head can not be guaranteed. For such a soil layer, it is difficult to stabilize the excavation face only by mechanically controlling the soil discharge mechanism. Therefore, a mixture of water, bentonite, clay, high-concentration slurry, slurry materials, etc. Pressure should be poured into the excavation face, and the mixture should be constantly stirred to change the composition ratio of the excavated soil, so as to ensure the fluidity and water sealing of the soil and make the excavation face stable.
The stability mechanism of excavation face can be divided into the following ways:
① Soil cutting and pressure mixing method: water, air or mixed materials are sprayed into the soil cavity to ensure the fluidity of soil and sand in the soil cavity. A rotary feeder (rotary valve or rotary funnel) capable of stopping water is installed at the soil discharge port of the screw conveyor, and the isolation function of the feeder can stabilize the excavation face.
(2) Water adding method: add pressure water to the excavation face to ensure the fluidity of the excavated soil and balance the pressure of pressure water and groundwater. The earth pressure on the excavation face is balanced by the pressure of the mixed soil in the soil cavity. In order to ensure the function of pressure water, a soil discharge regulating groove is installed at the back of the screw conveyor, and the excavation face is stabilized by controlling the opening of the regulating groove.
(3) High-concentration mud pressurization method: High-concentration mud is added to the excavation face, the fluidity of the excavation soil is ensured by mixing the mud and the excavation soil, and the earth pressure and water pressure on the excavation face are balanced by the pressure of the high-concentration mud. The discharge port of the screw conveyor is equipped with a rotary feeder, and the isolation function of the feeder makes the excavation surface stable.
④ Mud-adding type: clay and mud are injected into the excavation face, and the excavated soil is mixed and stirred by the spoke cutter head and the stirring mechanism, so that the excavated soil has water-stopping property and fluidity. The earth pressure of this improved soil is in balance with the earth pressure and water pressure of the excavation face, so the excavation face is stable.
EPB shield is more suitable for advancing in soft alluvial soil layer, but it needs to be added when advancing in gravel layer or sandy soil layer.