The key stratum is not easy to cave, but once it is caved, there will be a sudden collapse of a rather thick stratum.
关键层较难崩落，但在崩落时具有突发性，会产生较大厚度岩层的突然垮塌。
The key stratum is not easy to cave, but once it is caved, there will be a sudden collapse of a rather thick stratum.
关键层较难崩落，但在崩落时具有突发性，会产生较大厚度岩层的突然垮塌。
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