Is tayammum (dry ablution) allowed if there's no clean water available at work?

Yes — tayammum (dry ablution using clean earth, sand, stone, or a similarly dust-covered surface) is a valid substitute for wudu or ghusl when water genuinely isn't available, isn't accessible in time before the prayer window closes, or using it would cause real harm. Simply finding the office washroom occupied or a queue at the tap isn't enough — but situations like working at an outdoor site, in a vehicle, on a factory floor with no functioning water supply, or genuinely running out of prayer time while searching for water are exactly the kind of case tayammum is meant for.

The method is straightforward: make the intention, strike both palms once on a clean dust-covered surface (the Hanafi school also permits any clean part of the earth's surface, including stone, brick, or a clean wall; the Shafi'i school is stricter and requires actual earth/dust with visible particles that transfer to the hands), wipe the face once, then strike again and wipe the hands/forearms up to the wrists. Once genuine access to water returns before the prayer, any tayammum performed for a missed prayer doesn't need to be redone retroactively, but you should use water for the next prayer onward.

Source: Quran 4:43 and 5:6 (tayammum verses); general Sunni fiqh on the conditions permitting tayammum, with Hanafi/Shafi'i difference on the qualifying surface.