Isolation and containment
This imagery is both concrete and abstract. Concrete instances include the buildings that comprises each gulag, and also the frozen tundra that surrounds the gulags, which are absolutely confining, because no one can escape, and if they did, no one could possibly escape the tundra. The abstract qualities include the prisoners' experience of emotional domination and powerlessness suffered at the hands of their jailers.
Paranoia and espionage
The gulags can be seen as expressions of paranoia obeyed absolutely. Since Stalin wants to maintain his power by any means necessary, he commands his administration to follow any and every suspicion. The prisoners are forced to betray their own people by spying on them, monitoring their calls through tapped phone lines. The state sends anyone (guilty or innocent) to the gulags without fair trial, leaving the gulags and the nation filled with paranoia.
Absolute power
Through fear and abuse, Stalin maintains a power than can be viewed as its own abstract imagery, because the jails are instances of Stalin's absolute dominion. He cannot be thwarted because he works tirelessly to protect his own power. He is absolutely concerned with his power, and although he himself isn't walking around the jails, his authority is clearly obvious to each and every prisoner. The prisoners, conversely, are powerless. Even their basic human rights are violated in the name of Stalin's power.
Horror and torture
This imagery is a difficult one to stomach. The gulags are not safe. They are enclosed by cement and brick, and they are hidden away from discovery or help by the tundra, so they are truly hopeless. The prisoners make the most of their opportunities, for fear of mistreatment or death, because if anyone disobeys or tries to incite some kind of counteraction, the guards can simply throw them out to die in the wilderness. The horror of mistreatment and the mental torture that these prisoners endure are all painted by Solzhenitsyn as if they are the main point of the novel. One could argue that this imagery is the main point of the novel.
