Final answer:
Backfill layers around a drainage structure are typically between 1 to 10 cm thick, and this can vary based on material type and site conditions. Compaction guidelines such as 5-8 diameters side-to-side and up to 15 diameters deep offer a starting point for considering the appropriate layer thickness.
Step-by-step explanation:
The thickness of backfill material layers before compacting around a drainage structure should be tailored to the material type and project specifications. For alternating layers of medium- to coarse-grained sand and silty clay with root structures, an appropriate range might be from 1 to 10 cm thick depending on the specific engineering requirements and the conditions at the site.
Compaction is crucial for stability and to prevent settlement; thus the rule of thumb for the size of the backfill layer being related to the size of the drainage structure (e.g., 5-8 diameters side-to-side, and up to 15 diameters deep) can be applied when determining sufficient backfill thickness. It is essential to balance the compaction of layers with the need to create a stable base and sidewalls for the drainage infrastructure.