A footwear system is disclosed, which includes a shell having an inner cavity and at least one recessed area offset from an inner surface of the shell, and a chassis having one or more components adapted to protect a user's foot from injury. In one embodiment, the chassis is insertable into the inner cavity of the shell, such that the one or more components of the chassis are securely retained (e.g., permanently affixed) within the at least one recessed area of the shell. Methods of forming the aforementioned footwear system using expandable materials are also disclosed.