Why do you have to scrap 10 kilometers? It is easy to be damaged because it has been used for a long time. The extremely hard shield machine also has its weaknesses. Such expensive equipment may soon be scrapped. Generally, the design life of shield machine is only 10 to 15 km (the design life of its main bearing is 10000 to 15000 hours, and its service life is a little over a year). Shield machine is a kind of precision equipment. After its service time exceeds the design range, it cannot guarantee its stable performance and is prone to failure. However, the shield machine can only enter and cannot retreat, which is very difficult to maintain. In foreign countries, the shield machine will be scrapped directly and thrown underground after overuse. In China, due to the low labor cost, most construction enterprises will dig it out for maintenance and reuse. Even if it can't be used, scrap iron and some equipment can be sold for more than one million yuan. However, it is risky to dig it out and reuse it. During the construction of Zhujiang section of Guangzhou Metro Line 3, the cutter head was damaged and the shield machine failed halfway. Because the shield machine can only move forward but not backward during construction, the staff had a great effort to repair the cutter head of the shield machine. When it is repaired, it reaches the other side of the Pearl River, digs a deep well from the ground and then drags it away.