But this, in turn, invites soil erosion. But in the runoff process, cap soil will be carried with the runoff, causing sheet and rill erosion and, ultimately, gullying of the cap. When you get gullies in the cap, it's all over. Other physical forces working constantly to destroy a landfill cap are freezethaw and wet-dry cycles. Soil shrinkage during dry weather can cause cracks.
Rain penetrates the cracks.
In winter, rain freezes to ice and expands, widening the cracks. And so on, year in, year out, century after century. The cracks not only let in water, they also provide pathways for plant roots and for burrowing animals. However, plants maintain their physical stability, and they gather water and nutrients, through roots, which can penetrate a landfill cap, destroying the cap's integrity. Burrowing animals of concern include woodchucks, badgers, muskrats, moles, ground squirchipmunks, gophers, prairie dogs and badgers.