Obviously, there are gaps in our knowledge of Roman construction techniques and tools because we lack either written or physical evidence of these. Non-permanent materials have eroded away over two millennia and their characteristics elude us. Great structures like Hadrian’s Wall survive because of the use of stone or concrete.
There is, however, one clear example of a non-permanent structure which we know a lot about – Caesar’s first bridge over the Rhine. We know the story because Caesar tells us in volume four of The Conquest of Gaul. But construction techniques are only part of the story. Caesar built this bridge to show the Germans they were never safe from him because he had the skills to cross the river and attack them.
In 55 B.C, the German tribes Usipetes and Tenctheri crossed the Rhine to the Roman occupied west bank and plundered corn they found there. Crossing back over they joined forces with the Sugambri tribe. When Caesar sent an embassy to the Sugambri and asked that the culprits be returned to him, the response was that the Rhine was the limit of Roman authority. Simultaneously, the Ubii tribe, an ally of Rome, asked for help against the Suebi tribe who were harassing them.
Caesar, deciding he had enough reasons to cross the Rhine, planned an expedition to attack the Sugambri.
Downstream from Koblenz, there are two small towns named Andernach and Neuwied. Historical scholarship suggests Caesar planned to cross the Rhine in between these two locations. The distance between the shorelines in this area is 800 to 1000 feet. The average depth of the river is 16 feet.
Before we talk about the construction of the bridge, it will be useful to show a photograph of the model that sits in The Museo Della Civilta Romana in Rome.
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.
Once the support structure was in place, logs were laid across the piers and then boards were used to form the roadway.
I estimate that the piers were about twenty feet apart so a thousand foot river span would have required 50 piers. The bridge was built in ten days which meant workers would have placed five piers per day assuming the roadway was built as a parallel process over piers already completed.
The Romans had a variety of familiar tools at their disposal, including adzes, hammers, saws for cutting boards, and block and tackle. They used 9 inch nails to fasten boards together.
Once the bridge was complete, the army crossed over the Rhine. They spent 18 days in the land of the Sugambri, burning abandoned villages because the enemy had fled their homeland in fear of the Roman juggernaut. After treating with the Ubii, Caesar crossed back over the Rhine and destroyed the bridge behind him. Ten days in the making, Caesar’s Rhine bridge lived for eighteen days.
A footnote to the story. There have been endless arguments about this event among scholars. Some reinterpret Caesar’s Latin and challenge the construction methods described. Others question the engineering saying that the bridge would not have held. As an engineer, I am comfortable with the construction methods described and think these debates are a waste of time. Scholars will be scholars, however, and they’ll debate any issue until each has had his say. How many engineer historians are there, anyway?