Cement block? good question. The hard part of traveling to other world lines is just that.
There is a system of clocks and gravity sensors that sample the environment before dropping out.
Its called VGL, (variable gravity lock).
If a cement block were there, the machine would backtrack until it sensed relative congruity to the original gravity sample.
A great deal of time and effort goes into picking just the right spot since you can not physically move during a displacement.

これタイムマシンが地上に下りるときの話だよね？