In the "Murder On The Links" by Agatha Christie, how is the murderer able to dig the grave?

Question

I just finished the book "The Murder on the Links" by Agatha Christie. In the end it is revealed that

Marthe Daubreuil

was the murderer. But there is no explanation on how she did it. Poirot deduces that she overheard the Renauld's plan and killed him in the night. But since the beginning Poirot claimed that a woman can not dig a grave. Then how did she dig it? And if Renauld had already dug the grave, then how did she cover the body in soil?

Answer 1

The grave was dug, Poirot concludes, by M. Renauld himself:

‘That night Renauld will first bind and gag his wife, and then, taking a spade, will dig a grave in that particular plot of ground where he knows a—how do you call it?—bunkair? is to be made.’

_{Agatha Christie (1923). The Murder on the Links, chapter 21. London: Bodley Head.}

It did not matter whether the murderer had the strength to cover the body, because the body was not covered:

[M. Hautet, the Juge d’Instruction, said] ‘That is one of the most extraordinary features of the case. Monsieur Poirot, the body was lying face downwards, on an open grave.’

_{Christie, chapter 3.}