Marcin Kasprzak, 26, attacked Michelina Lewandowska with a Taser, bound and gagged her and buried her under soil, leaves and an 88lb tree branch in woodland near their home in Huddersfield, West Yorkshire.
Miss Lewandowska, 27, escaped from the box after around an hour by using her engagement ring to cut herself free.
Kasprzak, of Penistone Road, was found guilty at Leeds Crown Court of attempted murder. His friend Patryk Borys, 18, of Rashcliffe Hill Road, Huddersfield, who helped in the kidnap, was cleared of attempted murder.
Both men admitted kidnap at a previous hearing. Kasprzak also admitted possessing a prohibited weapon.
Yesterday the Recorder of Leeds, Judge Peter Collier QC, said this was a “planned attempt to kill”.
He said: “It was your intention that she should not be found and it was your intention that she would die there.
“The death you intended would have been long and slow. It is mind-numbingly awful to imagine the sort of death you intended her to die.”