Is expiation (kaffara) or charity (sadaqat) obligatory from the principal amount of money deposited in a usurious bank, in addition to disposing of the interest, given that the deposit contributed to forbidden usurious investment?

It is obligatory to dispose of the forbidden usurious gains only. Your principal capital is permissible for you, and there is no need to dispose of any part of it, as Allah Almighty says: "And if you repent, you will have your principal sums. Wrong not, and you shall not be wronged." If one receives the value of a usurious transaction along with its profit, he owns only his original capital and should spend the usurious profit on charitable causes.