Before starting construction, the Romans built a crane on a raft and floated it out into the river. You can see the framework of this apparatus on the left of the picture. The piers supporting the bridge were constructed of one and a half foot thick logs. These tigna bina were tied together in pairs with a two foot gap between them. They were then driven into the river bed at an oblique angle using a pile driver. This was apparently a large stone, attached to block and tackle on poles, that could be swung at the posts once they were tied into position. Once two sets of these posts were secure a large log (fibulae) was placed between them in the slot formed by the two foot gap. Angled supporting posts were tied to the tigna bina to provide additional support.
Caesar says the piles were positioned forty feet apart before they were driven into the riverbed.