What is the significance of the staircase in the novel Hard Times?

In chapter 20, Mrs. Sparsit is presented as having been at Mr. Bounderby's house for several weeks. When her stay ends, she will lose her free lodgings and ability to interfere in his life. Mrs. Sparsit, previously disappointed when Bounderby married the much younger Louisa, cannot approve of her. A severe, unpleasant woman, Sparsit is pleased to see that Louisa has apparently become involved with Mr. Harthouse.
The staircase is a metaphor for Louisa's desired ruin. Mrs. Sparsit imagines a physical counterpart for the younger woman's downfall. She imagines her inexorable descent. Every day, the mental vision of Louisa's imminent ruin entertains her.

Now, Mrs. Sparsit was not a poetical woman; but she took an idea in the nature of an allegorical fancy, into her head . . . She erected in her mind a mighty Staircase, with a dark pit of shame and ruin at the bottom; and down those stairs, from day to day and hour to hour, she saw Louisa coming.

Dickens emphasizes Mrs. Sparsit's limited creativity by having her think up a completely obvious symbol. He also emphasizes her malicious nature through showing her enjoyment in repeatedly contemplating the idea of another person's failure.