That is the depth at which workers are likely to hit compact soil or bedrock.
I've had the slaves dig down in the sand until they hit bedrock.
They went down on the right and hit bedrock at eleven feet.
In fact, the consensus is that poor Horis has gone around his last corridor and hit bedrock, if you know what I mean.
When it hit bedrock, it couldn't penetrate any farther.
It hit bedrock in 1932 and 1933, and so did the sunspots.
"We've hit bedrock down there," he told the men on the pier.
Parts of the pit are 60 feet deep, and with workers having hit bedrock, pools of water have collected.
How many hours it took him, he didn't know, but he hit bedrock four feet down: dry bedrock with no sign of water.