This had to be one of the most bizarre pieces of literature I have ever read -- but that's a good thing, really. It's a very claustrophobic work -- the setting is ultimately very very small and limited. I think this was a really cool effect -- it made us feel more "at home" with where the characters were.
To think that, according to Abe, sand -- only 1/8 mm in diameter -- can so oppress us... Maybe, he is saying, life is sometimes beyond our control.
The themes of living amidst even the worst circumstances are quite apparent, I think, and the sand pit being representative of the mind-numbing simplicity of every day life is a nice pessimistic vision for us all. This book is the story of a man who wants to escape from this mundane existence which he is forced into against his own will, like we all have no choice but, whether we earn an education or not, to work, every day, with little consolation or reward. This is a story of a man who lives out a pure human existence, though in captivity. He works, he eats, he sleeps.
Abe's point must be that there is no more to life than this. We should never expect too much from our lives. Like Jumpei does in this novel, we simply have to come to terms with our existence and find something worth devoting our time to -- like his little discovery in the end, which spurs him on in his work.
A note: in this translation, we are lead to believe that Niki Jumpei is single and living with a woman. This isn't true. In the Japanese version, Jumpei is married to Niki Shino. The author uses a Japanese pronoun to mean "woman" which is most commonly used by married Japanese men to refer to their wife. This novel is written in a very traditional Japanese manner, believe it or not, so the translator had to take a few liberties, I would assume. Since the story is told in third person, the use of this particular pronoun would confuse any transltor, really. Also, in the "missing person notice" at the end, the claimant is Jumpei's WIFE, not his mother. That final passage is translated word-for-word -- except for some reason the translator felt the need to put the word "mother" in parentheses as an attempt to clarify Niki's family life.
I think this might help the reader, because reading the Japanese version, one gets the impression early on that Jumpei left on his little trip partly as a result of a marital conflict.