A couple years ago, I reviewed a book for the ASCE Jnl of Hydraulic Engineering. It may be useful.
Detection and Prevention of Leaks from Dams
by Antonio Plata Bedmar and Luís Araguás Araguás
A.A. Balkema Publishers, 2002.
The authors are both Spanish, and there are occasional little glitches in the English that you need to watch for. It contains nothing at all about prevention and the chapter on dam engineering is pretty weak, but I gave it good marks on what it does cover.
Quoting myself, "Where the book really shines is in its detailed explanations of natural and manmade tracers and some geophysical methods for locating and quantifying seepage, both familiar (rhodamine dye), and more exotic (stable-isotope ratios and self-potential). It describes
their use in great detail and provides guidance on advantages and disadvantages of each, and on which is appropriate in a given situation. The book also contains a good section on borehole permeability and gradient measurements."
It ain't cheap, but $129 can probably be justified for a new dam, or an existing one with real seepage issues.