顶极下沉指发生在采场空间的围岩(指顶板)的变形现象。发生原因是：采场空间虽然处于应力降低区范围内，但由于其上方的顶板仍然受自重和上覆岩层的作用，故也会发生变形。顶板下沉是采场矿山压力显现的重要形式之一，是反映顶板状态好坏的客观标志。一般用顶板下沉量和顶扳下沉速度来表示。
vt. crush
misc. roof settling
通过分析现场监测结果所划定的顶板下沉期及—系列临界值，为地压管理提供了依据。
A serial of roof sagging time and critical value, measured by field-monitoring, has provided basis for pressure control.
用分形对3604S4工作面和回采巷道顶板进行了研究，得到了该工作面下沉速度的几种分维值。
Study was conducted on the roof of No. 3604S4 coal face and the actual mining roadway with fractals, and a few kinds of functional values of subsidence velocity of this face were obtained.
首先利用相似模拟实验研究了急斜特厚煤层顶煤的冒落特征、顶板的破坏规律，以及地表下沉的特点。
First, based on similar material model test, top coal cave-in characters in steep and thick seam were studied as well as failure law of main roof and features of surface subsiding.
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