If I remember correctly, I believe you have a timor monitor as a captive, therefore I will reply in response to Timors and what I think they need in captivity. In my opinion, you shouldn't worry about how burrowable your substrate is, etc, as timors are much different in their ecology than their sister taxa that many people here keep from australia.
Areas come from areas that are relatively dry and have inorganic soils, where burrowing most likely plays an important role of their everyday lives. Such australian species(aside from those residing in the Cape York/N Queensland), are not normally subjected to flooding/high water tables, where burrowing would become problematic.
With tropical species, such as timorensis, and others such as indicus, doreanus, etc., they come from completely different environments, which are usually laden with large trees, dense forests, and a wet/often times flooded forest floor. In most cases, burrowing in this forest floor would be unfeasable due to the constant rainfall and accumulation of water/high water table in coastal/lowland areas(you can't dig a burrow or sleep in a burrow when there is water covering the ground, or water an inch or two beneath the surface).
While I am not doubting that indo species will not seek refuge beneath the substrate on the forest floor(leaves/detritus), they will most likely not be digging burrows in the same fashion that we see with most other odatria, namely those originating from the drier areas of australia. The soil/ground in most cases is unsuitable for digging burrows to seek refuge in, therefore they use other methods of concealing themselves.
Timors come from areas where there are a plethora of trees> tight fitting spaces and nooks and crannies of varying sizes, temperatures, moisture levels, etc.> above the ground in trees, that are much more suitable retreats than burrows in the wet/often times flooded ground.
I think instead of worrying about what type of substrate to use so that your captive can burrow, etc, I think you should spend more time thinking about providing him/her/ with as many ample and usable hiding spots as possible; within logs, crevices, etc.- those which offer different conditions, to allow the captive to choose where he/she wants to retreat to, at that given point in time..
For substrate, i would offer it a layer of leaves. Leaves retain humidity very well, and are a more natural ground covering(perhaps not tree species, but natural in the sense that forests floors are covered with fallen leaves and other tree debris) that the monitors will most likely relate to and associate as a potential retreat.
Offer it lots of hiding spots up off the ground, and I think that he/she will feel right at home. Best of luck to you.