Soil in the terrarium of snails should be required.
Firstly, because it is the soil that helps maintain the required level of humidity.
Secondly, because eggs are laid in the soil of a snail.
Thirdly, because the soil protects the snail shell from damage when it falls from the ceiling or walls of the terrarium (if the bottom is glass or metal, then the shell may crack, which will not benefit the mollusk’s health).
Fourth, snails need to be dug in whole or in part for some time. It is believed that in this way snails wait for daylight hours. In nature, they avoid direct sunlight, because in the sun they can "dry out." In addition, burrowing into the ground, achatines may regulate not only humidity, but also body temperature. As the soil, you can use:
1) mulch from tree bark;
4) coconut substrate;
5) land (without fertilizers and other chemicals);
I consider the use of the first two materials optimal; all others, in comparison with them, have a number of disadvantages. For example, moss is less accessible: you need to go to the forest for it, and even choose a sphagnum that grows in humid places along the outskirts of the swamps; sphagnum is the most hygroscopic in comparison with other species. Coconut substrate is expensive. Sand and sawdust are not so aesthetic and less functional. Although, perhaps you and your snails will just like some of this, or you will find other options.
All my snails "sit" on the mulch of the tree bark: it holds moisture very well, for a long time without getting wet and without sticking together. The bark is of different consistencies: chopped into small even pieces; with the predominance of large pieces of various shapes; with a lot of powder mass, etc.
The first option is most convenient: such soil is better ventilated, excess liquid flows directly to the bottom of the terrarium, not “polluting” the soil, and small snails hatching from eggs are easier to climb to its surface. In this case, you don’t have to worry about drainage at all. (Drainage is a layer of stones, sand, etc. materials that pass liquid well through themselves, which are placed on the bottom of the terrarium under the ground so that the excess moisture does not linger in the soil, but seeps down.)
In general, I recommend not to do drainage, but more often to replace the whole soil (as soon as it begins to deteriorate, i.e., become dirty, waterlogged, etc.). If there is egg laying in the terrarium, and the soil is in critical condition, only part of it can be replaced without affecting the laying or transfer eggs to another terrarium, creating a suitable environment in it.
Soil, before laying in the terrarium, it is good to disinfect, having subjected to heat treatment, in order to destroy bacteria, fungi and small invertebrates. You can scald it with boiling water and dry it (peat, substrate from bark, sawdust), calcine in the oven or boil in water (sand).
The amount (level) of soil you can determine experimentally. To my snails I lay from 4 to 7 cm of soil, depending on their size.