Answer
Burrowing substrate needs enough structure to hold a tunnel. A compacted soil-based mix usually works better than a deep layer of very loose dry fibre.
Texture
The substrate should compress into a stable wall without becoming concrete-hard. Mixes based on clean topsoil/coco components can be adjusted to the species' moisture needs.
Depth
Give fossorials substantial depth and terrestrials enough substrate to extend a hide or shallow burrow. Measure usable depth from the actual surface after compaction.
Moisture
Slight moisture can improve structure in species that tolerate it. Do not keep a dry-adapted species saturated simply to make the substrate hold shape.
Hardscape
Place heavy cork or décor so excavation cannot undermine it and cause a collapse.