Our daily correspondence sometimes includes letters from inmates, informing us of the situation within walls. Recently, several letters have arrived from Pelican Bay Prison, from inmates incarcerated at the prison's Security Housing Unit (SHU). A form of supermax incarceration, SHU consists of 22-23 hours of solitary confinement a day.
SHU has been designed for escape risks, violent or threatening inmates, and "members of disruptive groups", such as gangs. There are some challenges involved in appropriately classifying inmates into SHU or general population. Classification is a crucial issue, because research on solitary confinement consistently shows that SHU-like conditions adversely and severely impact inmates' mental health (here's a 1993 piece by Craig Haney on this and a 2009 New Yorker article reviewing research and including interviews).
The concept of solitary confinement is not new. Our visit notes from Eastern State Penitentiary in PA might be useful. However, the purpose has changed. While the original ideology behind solitary confinement was to provoke repentance, rebirth, and rehabilitation, current supermax isolation conditions are aimed at incapacitation and safety.
Here is Laura Sullivan's 2006 NPR story about SHU and the solitary conditions. The institution currently exceeds its design capacity by approximately 1,200 inmates.