状態・変化N3
〜一方だ
goes on increasing / keeps getting... (trend)
Structure
V-辞書形 + 一方だ；い形（普通形）+ 一方だ
Explanation
Indicates a trend or tendency that continues to develop, "keeps getting...". Attaches to the dictionary form of verbs, describing one-directional ongoing change.
Examples
都市の人口は増える一方だ。
The urban population keeps increasing.
仕事は忙しくなる一方だ。
Work keeps getting busier.
Note
一方だ indicates a one-way ongoing trend, often negative or markedly changing. Distinguish it from 一方（で） (contrast).