300 people
(B)？ celebrity
500 people
Answer: When sinking a shaft by freezing method, when the freezing depth is less than 500m, freezing can only be stopped after the construction of the permanent shaft wall is completed. So, the answer is option (C) 500.
Freezing method is a special construction technology that uses artificial refrigeration technology to freeze the water in the stratum, make natural rock and soil become frozen soil, increase its strength and stability, isolate the connection between groundwater and underground engineering, and carry out excavation construction of shaft or underground engineering under the protection of freezing wall. The patented technology was first used in Russian gold mining, and later used by German engineers in coal mine construction. The technology tends to be mature and has been widely used in subway, deep foundation pit, mine construction and other projects. The origin, basic principle, refrigeration system, reinforcement mechanism and construction method are introduced in detail.
Freezing method originated from natural freezing. Due to the development and application of artificial refrigeration technology, engineering freezing occurs from time to time. 1862, in the foundation construction of buildings in South Wales, England, artificial refrigeration was used for the first time to strengthen the soil. 1883, Pottshu, a German engineer, used freezing method to dig a shaft with a depth of103m in a coal mine in Albary, Germany, and obtained a patent for freezing sinking technology. After that, this technology spread to many countries in the world. The Soviet Union began to use freezing method from 1928, and the number of shaft sinking by freezing method has exceeded 400 so far, making it one of the largest countries in the world. Freezing depth is an important symbol of freezing sinking construction technology. In China, the freezing method was adopted in the west wind well 1955 in Kailuan, with a net shaft diameter of 5m and a freezing depth of 105m. Since then, freezing sinking technology has been gradually extended to Northeast China, North China, East China and Central South China. By 1990, there were about 300 frozen wells, with a cumulative freezing depth of 50km and a maximum freezing depth of 435m m. China is already one of the countries with the thickest topsoil in the world. Since 1992, freezing technology has been widely used in urban subway construction.