Therefore, the conventional staggered distance model of ultra-close thin coal seam group during combined mining is put forward; and the theoretical research and physical simulation are carried out.

因此提出极近距离薄煤层群联合开采常规错距开采模型，并进行理论和物理模拟试验研究。

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Burn through is the main problem of longitudinal seam welding of ultra-thin tube with small diameter. Assembly of the joint and welding technology are the key to solve the problem.

小直径超薄壁管纵缝焊接的主要问题是烧穿，解决这个问题的关键是严格控制焊接接头的装配质量及焊接工艺。

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