Why does Gordimer mention that her house is located above a mine?

In "Once Upon A Time," Gordimer notes that her house is built on "undermined ground." Underneath the foundations, she explains, are the remnants of a gold mine which causes the structure of her house to shift slightly.
The reason that Gordimer mentions this fact is because she wants to make clear that her house is creaking not because of an intruder but because of the hollow ground beneath it.
Note in the paragraph above, for example, how Gordimer notes that two people have been murdered in her neighborhood in the past year. When she wakes up suddenly in the middle of the night, her mind immediately recalls these incidents and she becomes afraid. But she quickly realizes that she is not about to become another victim: the noises she can hear are nothing more than the natural movements of her house.
By mentioning this, Gordimer encourages the reader to stop and think before jumping to conclusions. This is significant given the story that she is about to tell, a story in which a family becomes so obsessed with personal security that their son is ripped apart by the barbed wire that they have recently installed.