一方（いっぽう） is a 名詞（めいし） (noun) that is often seen after verbs, in order to convey the meaning of ‘more and more (A)’, ‘continuing to (A)’, or ‘getting (A)er and (A)er’.
一方（いっぽう） will be paired with the 助動詞（じょどうし） (auxiliary verb)
だ (at the end of sentences), or で, だ’s 連用形（れんようけい） (conjunctive form) when it will be followed by some sort of explanation.
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アメリカとロシアの関係（かんけい）は悪化（あっか）する一方（いっぽう）だ。
The relationship between the US and Russia keeps on getting worse.
仕事（しごと）の量（りょう）が増（ふ）える一方（いっぽう）で、休（やす）みが取（と）れない。
Because the amount of work keeps on increasing, I can't take a day off.
As the literal meaning of 一方（いっぽう） is ‘single direction’, it may be used with things that are increasing, decreasing, intensifying, or weakening, so long as the shift is all in a single direction.
一方（いっぽう） may only be used with verbs that express some kind of change, as 一方（いっぽう） itself does not express change, just a continuing direction.
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高齢化（こうれいか）が進（すす）んで、人口（じんこう）が減（へ）る一方（いっぽう）だ。 (Natural)
Because of the aging society, the population continues to decline.
子供（こども）が怪我（けが）をして泣（な）く一方（いっぽう）だ。 (Unnatural)
The child cries in one direction because he got injured.