Answered by Shaykh Muhammad ibn Adam al-Kawthari
Where the land gets easily water logged due to its clay composition and there is the possibility of the grave collapsing from above and below and the sides – is it permitted to reinforce below ground level the grave walls by:
a) use stone slabs/stone base/concrete or other material at the bottom of the grave
b) the same on the four side walls of the grave
c) dig the grave and then in effect by using a) and b) add a slab as a roof effectively entombing the body and then cover the rest of the grave to ground level with earth
d) have c) but with earth on the floor of the effective chamber so that the body is in contact with earth.
In the Name of Allah, Most Compassionate, Most Merciful.
When the land is soft and gets easily waterlogged and there is a possibility of the grave collapsing, then it is totally permissible to reinforce the grave by making walls on the side. Also putting a slab at the top is permissible as mentioned in (c). It is better to put soil at the bottom of the chamber as mentioned in (d). [See Ibn Abidin in his “Hashiya” v.2,p.234].
And Allah knows best.
Muhammad ibn Adam