Emergency welding repair of the hottest shield cut

Emergency welding repair of shield cutterhead

Abstract: it is required to complete the task in the shortest possible time to weld and repair the cutterhead on the excavation surface in front of the shield. This paper expounds the adoption of maintenance measures to maintain the stability of the excavation surface, the selection of repair materials, the optimization of repair process and the arrangement of repair process flow. Jing put forward his own opinions and views on some problems to be solved

key words: shield; Cutter head; Welding repair; Process

0 preface

s-169 subway shield tunnel construction project, during the process of TBM propulsion, when stopping for routine inspection, it was found that the peripheral knife and its installation base and some tooth knives and their bases close to the peripheral knife were seriously worn. Therefore, it was decided to carry out emergency welding repair on the cutter head body and the cutter mounting base. At the same time of repair, some of the surface of the cutter head body was overlaid with wear-resistant layer. The author reviews, analyzes and summarizes the repair work of the cutter head, and analyzes and discusses the related problems

1 Determination of excavation size of working face

under normal circumstances, the cutterhead is closely dependent on the soil mass of the excavation face. To weld the cutterhead on the excavation surface, a hole needs to be dug in front of the cutterhead to ensure that operators have appropriate working space when welding the cutterhead. The approximate size of the hole is a hemispherical hole with a diameter of 1500 mm and a depth of 400 mm. See Figure 1 for specific location and size

2 maintenance measures to maintain the stability of the excavation surface

2.1 control of the amount of soil discharged from the earth bunker

the height of soil discharged from the shield earth bunker is mainly considered comprehensively according to the needs of welding operations and the characteristics of the stratum on the excavation surface. Because the upper part of the stratum is clay layer and the lower part is sand layer. The dumping height is controlled at about 1/3 of the cutter head diameter, that is, 2m, to minimize the impact on the excavation surface

2.2 replace the earth pressure with air pressure to maintain the stability of the excavation surface

when the operation is stopped for a long time, in order to maintain the stability of the excavation surface, the earth silo must be inflated and pressurized. When pressurized, the water in the stratum of the excavation face is pushed out and flows to other places. However, gas pressurization cannot be maintained for a long time, and some places will leak if the time is a little longer; In order to control air leakage, we add bentonite into the soil bin to form a mud film on the surface of the excavation surface, which can maintain the pressure in the soil bin for a long time

2.3 vault support

the partial collapse of the vault is supported by woven bags and boards containing clay to control the scope of collapse and maintain the stability of the excavation surface

2.4 shorten the repair time

implement 24-hour continuous work to reduce the occurrence of collapse in a large area and scope

through the above measures, the stability of the excavation surface is effectively guaranteed, and the surface settlement is effectively controlled during the cutter head repair, Fig. 2 and Fig. 3. It can be seen from Figure 3 that with the extension of shutdown time, the land subsidence tends to increase, but the slope is significantly reduced. By taking the above measures, the land subsidence of R can be better controlled. During the week of repair construction, the maximum settlement shall be controlled within 12mm. Because with the extension of time, the settlement is increasing. Therefore, it is particularly important to speed up the repair and shorten the maintenance time

3 welding repair of cutter head body and cutter installation base

welding operation on the excavation surface in the soil bin is limited by many factors such as operation space and ventilation conditions. The operation is difficult, and there are also high requirements for the operation level of welders. Because the stratum condition of the excavation surface is relatively hidden during this operation, and the measures are effective, the welding operation is carried out smoothly

3.1 steel selection

cutter head repair work, time is tight and the task is heavy. However, we still pay close attention to the treatment of packaging waste and the recycling and reuse of resources. If the import volume of waste paper continues to decrease after 2019, we will investigate, analyze and prepare. First, adopt st steel grade according to the cutter head body. Equivalent to domestic low carbon steel. The repair width of the cutter head body and the cutter installation base is 350mm, the thickness is 100mm, and the height is 150mm. Considering the mechanical properties, weldability and other factors of the material, and after studying with the welding experts of relevant units, it is finally decided to use domestic 20# steel to repair the cutter head and installation base

3.2 optimization of cutter head repair scheme

how to ensure that the performance of the repaired cutter head body is not lower than the level of the original design and manufacture, we have conducted in-depth analysis and research, and processed sample blocks for simulated assembly analysis on site

3.2.1 considering the welding process, the scheme of left and right blocks plus support blocks is adopted.

in the original design repair scheme, two 150mm thick steel blocks are used to weld and repair the left and right sides of the cutter head boom. Through the analysis and the simulation installation of the sample block, we found that in the soil bin, due to the limitation of the operation space, the welding operation can only be carried out from two sides, and the cutter head cannot be welded from the front. Two 150mm steel blocks are welded on the width of 345mm. The first one can be tested: contraction test for welding, while the weld between the part adjacent to the second one and the first one and the cutter head body cannot be welded. Therefore, two 100mm thick steel blocks plus intermediate support plates are used instead; Weld the left and right pieces first, and then weld the intermediate connecting plate. This not only ensures the accessibility and convenience of welding operation, but also ensures a high connection strength between the restoration and the cutter head body

3.2.2 determination of the number of connecting bolts of the peripheral knife

the middle bolt connection hole on the knife installation base around the knife head is ground off by 1/2, and half of the bolt hole cannot be welded and repaired. We initially decided to use only the connecting holes at both ends for bolt connection and fixation, and the middle one is not fixed. After our analysis and research, we believe that: if the middle bolt hole is not connected, first, the connection strength between the peripheral cutter and the cutter head is reduced by 1/3; Second, if the bolt connection hole is not blocked, sediment will pass through the hole during the rotation of the cutter head, and because this part is located at the edge of the cutter head, its linear speed is large, which is easy to cause the wear of the cutter head and the tool; Therefore, it is decided to connect and block the bolt hole. The specific method is: process a smooth sleeve with internal thread hole, weld it with the cutter head residue at the position of the original sleeve, and connect and fix the bolt with the internal thread of the sleeve. This not only blocks the holes on the cutter head, but also improves the mechanical connection strength between the surrounding cutter and the cutter head body

3.2.3 increase the weld height to improve the connection strength between the restoration and the cutter head matrix

confirmed by the equipment manufacturer: when the cutter head is made, the weld height between the peripheral cutter mounting base and the cutter head body is 3 layers of 5mm. During the repair, we specified the weld height of 5 layers of 8mm. It ensures that the repair quality is not lower than the welding strength in the original manufacturing process

3.2.4 take a number of measures to ensure welding quality

(1) select welders with rich experience and high welding technology for continuous operation

(2) choose DC welding machine to ensure the stable combustion of welding arc, so as to improve the quality of weld formation

(3) select j506 welding rod, dry the welding rod before welding, and use an incubator to keep the welding rod warm during welding operation. So as to reduce the possibility that there are pores in the weld, so there is no sampling and holding circuit

(4) polish the welding surface with sandpaper and grinder, and wipe the welding surface with acetone to ensure that the welding surface is clean

so as to ensure the weld quality

(5) strictly take waterproof measures to ensure the quality of welds

in order to shorten the downtime of the shield, we have adopted the method of 24-hour continuous operation. The operation is divided into two shifts, each of which is 12 hours, and the two welders rotate within 12 hours. In this way, the repair of the cutter head was successfully completed within a week, and the worn tools were replaced

4 technological measures to improve the repair accuracy

because the peripheral cutter is fixed to the cutter head with three connecting bolts. Therefore, how to ensure the positioning accuracy of the bolt hole on the restoration in the welding operation has become the key to judge the quality of this repair. The technicians thought carefully and studied carefully, formulated the detailed construction process, and compiled the construction process flow chart, as shown in Figure 4; The welder is required to weld symmetrically and cross during the welding operation to ensure that the weldment is heated evenly and avoid displacement of the workpiece due to uneven heating. Due to the specific and reasonable formulation of various measures and the active cooperation of operators, the positioning accuracy after welding is ensured, and there is no rework. While ensuring the quality, it also ensures the operation progress

5 some suggestions and thoughts

from this welding repair work, we found and thought of some problems

5.1 several times of warehousing inspection should be strengthened

welding operations should be carried out on the excavation surface, which has high requirements for the stability of the formation. The excavation surface of our shutdown stratum is very stable, and we can dig out the working space forward. If the self stability of the soil on the excavation surface is poor, we need to strengthen the soil first, which will take a long time. This requires us to enter the warehouse in time in the future to check the wear of tools and cutterhead, and make inspection records

5.2 the cutter head overhaul process specification should be formulated as soon as possible

from the situation of this emergency repair, the cutter head is seriously worn. When completing the TBM hole out of the next section, the cutterhead must be overhauled. How to repair and how to ensure the quality? We have no idea about these problems. Therefore, we must invest a certain amount of human and financial resources to carry out this work as soon as possible, carry out the necessary scheme research and test, and formulate a reasonable cutter head repair construction process and surface strengthening treatment scheme

5.3 brief analysis of the causes of sharp tool wear

from the analysis of the construction process, the sharp wear of tools and cutterhead occurred after the 500th ring. Before the 500th ring, each shift can normally complete the tunneling of the 4th ring in 8h, while after the 500th ring, the propulsion speed is significantly slowed down. The obvious increase of thrust and torque results in the aggravation of tool wear; The wear of the cutting tool is aggravated, which leads to the increase of the torque of the cutter head. The two are cause and effect, which makes the situation worse and worse. This is only the author's preliminary analysis and judgment. The real reason still needs to be analyzed and studied by experts in depth based on the factors of construction records, construction parameters and so on, so as to give a correct conclusion. I won't go into details here

5.4 selection of TBM localized cutter head materials

this repair work also provides a valuable reference for our TBM localization in material selection. And because the carbon content of 35 # steel is higher, the hardness is higher and the wear resistance is better; In addition, the weldability of 35 # steel is acceptable, and the Cutterhead of domestic shield machine can choose 35 # steel as the parent material

references

[1] Li Zhicheng, et al. Manual of steel grades commonly used in the world, Beijing: China Materials Press, 1993

[2] Guide to the selection of welding electrodes for Wu shudui. Beijing: Chemical Industry Press, 1996 (end)