A bulking agent must be added to lighten the mix. Structure is the physical property of the mix that refers to its ability to hold shape in a pile and resist settling and compaction. Stiff, rigid particles with uniform shapes provide good structure. Bulking agents like wood chips, bark and straw are used to establish and maintain structure. However, over time, particular materials used as bulking agent can lose their stiffness as they absorb water and decompose.
Paper is a good example of a bulking agent that loses structure quickly.
When a pile loses structure, it collapses and becomes dense. It becomes difficult to turn and anaerobic conditions follow. The remedy is to add more bulking agent, or a different one that maintains good structure, like wood chips or shavings. Moisture is also a factor. A given material has more structure as it dries.