What is a cofferdam and do you know a large project that is built with this principle?

I agree with Mr. Jetley on Cofferdam.

Concur with Mr. Jetley !!

A cofferdam on the Ohio River near Olmsted, Illinois, built for the purpose of constructing the Olmsted Lock and Dam

A cofferdam (also called a coffer) is a temporary enclosure built within, or in pairs across, a body of water and constructed to allow the enclosed area to be pumped out, creating a dry work environment for the major work to proceed.

Correct mr. Jetley, the are also used as permanent structures for under quays.

here is an example: http://www.whalingcity.net/picture_hurricane_barrier.html