The Romans weren't exactly the first to use trenchworks. Aeneas Tacticus mentions them a whole lot of times, including disguised trenches and fancy stuff like that, and they show up in lots of authors going way back--Homer has the Achaeans at Troy dig a trench in front of their fortifications! Caesar even says that what "Oppidum autem Britanni vocant, cum silvas impeditas vallo atque fossa munierunt"--the Britons call it a town when they have fortified the difficult (that is, hard to get through) forests with a wall and ditch (fossa). The Romans' enemies were perfectly aware of how to deal with trenches, as they used them themselves. Caesar mentions his trenchworks being filled at least twice:

alii vallum manu scindere, alii fossas complere inciperent

Some began to tear open our wall, while others filled in our trenches

And actually twice when speaking of Commius' force at Alesia, once during preparations for their night assault:

Itaque productis copiis ante oppidum considunt et proximam fossam cratibus integunt atque aggere explent seque ad eruptionem atque omnes casus comparant.

And so with their troops arrayed they pitch camp in front of the town and they cover the nearest trench with wickerwork and they fill it with earth and they prepare themselves for the sally and for every possibility

And again during the actual night assault itself, this time of Vercingetorix's force on the other side of the wall:

dum ea quae a Vercingetorige ad eruptionem praeparata erant proferunt, priores fossas explent, diutius in his rebus administrandis morati prius suos discessisse cognoverunt, quam munitionibus appropinquarent

While they bring up the things which were prepared by Vercingetorix for the sally (i.e. various engines of war), first they fill up the trenches--being delayed for some time in overseeing these matters they learned that their earlier forces (i.e. Commius' men) had withdrawn, before they approached the fortifications