"Arbeit" is something that can't be divided in single items or parts so you could count it like e. g. "Personen". The same goes for sugar (Zucker), water (Wasser) etc. Now if you want to speak of a considerable amount of the (impartible) total you use "viel" only in singular and without flexion.

Of course there is an exception ;). If you want ti emphasize that considerable amount you use the correspondending definite article together with "viel", but now "viel" is inflected! E. g. "die viele Arbeit", "das viele Wasser".

Hopefully this helps! :)