に～を重ねてCumulative repetition
This construction repeats the same action or event noun to emphasize accumulated rounds, layers, effort, hardship, or setbacks. The accumulation normally matters to a later result, but that result does not have to be successful.
Building layer upon layer
The repeated noun presents each new round as being added to the previous ones. Natural choices are nouns for actions, processes, and experiences, such as 検討, 改良, 研究, 努力, 苦労, and 失敗.
Accumulation leading to a result
The later clause shows why the accumulated rounds matter.
製品は改良に改良を重ねて、ようやく実用化に至った。
After improvement upon improvement, the product was finally put to practical use.
AnalyzeLiterally: Stacking improvement upon improvement
計画は検討に検討を重ねた末、白紙に戻された。
After round upon round of consideration, the plan was ultimately scrapped.
AnalyzeExtensive accumulation can still end in rejection or failure.
この素材は、研究に研究を重ねて開発された。
This material was developed through round after round of research.
AnalyzeCumulative layers versus vivid intensification
に～を重ねて explicitly presents repeated rounds as accumulating. By contrast, repeating a verb stem with に vividly intensifies how thoroughly or repeatedly someone performed that verb, without the same image of layers building up.
彼らは考えに考えて、応募を決めた。
They thought it over and over before deciding to apply.
AnalyzeThis is the neighboring verb-repetition pattern, not the cumulative repeated-noun construction taught on this page.
More examples
委員会は議論に議論を重ねて、最終案をまとめた。
The committee held round after round of discussion and finalized the proposal.
Analyze技術者たちは改良に改良を重ねたが、製品化には至らなかった。
The engineers made improvement after improvement, but the project never reached commercial production.
Analyze彼は失敗に失敗を重ね、ついに監督を辞任した。
After failure upon failure, he finally resigned as manager.
Analyze候補地は、調査に調査を重ねた上で決定された。
The site was selected only after repeated rounds of investigation.
Analyze苦労に苦労を重ねた末、ようやく店を軌道に乗せた。
After enduring hardship upon hardship, they finally got the business on track.
Analyze原因を突き止めるまで、研究チームは実験に実験を重ねた。
The research team conducted experiment after experiment until they identified the cause.
Analyze