I set my automated BDSM survey algorithms off to scan a collection of stories that might match this description...
Sure enough... "Ever Increasing Bondage" is the perfect match...
Snippet:
"...There was only one crumb of comfort - & then only the tiniest of morsels - in an otherwise bleak outlook. And that was the fact that there was now very little else the twins could do to make things any worse.Unless, of course, they decided to seal the coffin in concrete, or something of that nature.Suddenly Lauren recalled that, on her arrival yesterday, she'd seen several bags of cement stacked up by the side of the house. She hadn't paid much attention to them at the time, but now, as her mind shifted into overdrive, their presence took on more sinister overtones...."
This post contains a few other candidates I tried out:
https://steelraptstories.blogspot.com/2020/09/10-classic-stories-2020-sep-25.html