Sunday, December 4, 2011

Caesar’s Rhine Bridge – Rome Showcases its Technical Prowess

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?


Anonymous said...

I'm an engineer, and a Classic PhD candidate, but my engineering was electrical, then computer science (where I worked all my working life), and I don't work in the area of ancient engineering. But its interesting to hear from other engineers in the field!

Geoff Carter said...

On the subject of credibility; it is inconceivable that Caesar would have made up any of his account, it was written for an audience, many of whom may have been there.
Those aspects of the account which pertain to the general facts of the campaigns I would consider as accurate, to be caught obviously fabricating or exaggerating recent history would surely amount to a serious loss of face among his peer group.
Clearly there is room for maneuver in his account of his own thoughts and motivations - but the bridge is unquestionable. Like aspects of this unique account, translation can be not straight forward, as translators are seldom engineers.

Scott McCulloch said...

I recently met a Chemical engineer who was completing PhD in ancient history.

So they are around :)

Anonymous said...

Your account was very informatve. I am a Civil Engineer and I design foundations. I find the bridge very feasible. One issue I found out about driving wood piles at a batter is that driving them vertical with a drop hammer as you described would be easy in soft mud. The trick to batter the piles is to drive vertical an bend them into position and secure them with wire rope. The Romans had rope and their nails could have been bent over like a stape we use today. The trick to making this bridge work is to spread out the troups while crossing so you don't overload it. I have designed and built three pile clusters that supported power poles with 25 mph currents. What they did not stand up to was tornados and barges running in to them.

Anonymous said...

Bolches yarboclos pa todos.


Anonymous said...

Hey I'm also Michael. Thanks for the blog looks great can't wait to read it!

Griff Rkt. said...

Were wedge shaped structures upstream of every pier used to divert the force of the current around the piers? It seems that those would have been just as important to the success of this bridge design as many of the details mentioned above but I have never heard anyone explain them. I would think that the diverting and redirecting of the current around the piles would take at least a third of the burden off the piers. Anyone know more about these breakwaters?

Ned Bergerson said...

In Caesar's account he states that the wedge-shaped structures were meant to divert logs from upstream from hitting the piers, if the enemy tried to topple the bridge by ramming it with floating logs. The current was said to be swift there.
With only the slightest connection to the current discussion - my father was in a combat engineering unit in WWII and had to put a pontoon bridge across the Rhine near Koln, a fair distance downstream from where the Roman bridge is believed to have been. He was in his 80s when I showed him Caesar's account, and he was captivated by it. Different war, different goal, but to soldiers bridging the river, they had a shared purpose two thousand years apart.