Physics

Ancient Roman Road Construction Lessons for Modern Engineers



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Ancient Roman road construction lessons
For modern engineers



Can it be possible that ancient Roman engineers were ahead of their modern day counterparts in some areas? I've read recently that we have not been giving ancient societies enough credit for their technology development. Let's explore the idea expressed in our title. To discover what the Romans did in their road construction let me recommend "The Ancient Engineers" by L. Sprague De Camp, 1960. The 1993 Barns & Noble edition that I have is in stores now.

What was the primary reason the Romans would have for building roads over 2000 years ago? I'm sure it was to get their army to the battle zone quickly. The army was mostly marching troops. Sometimes there was heavy infantry, siege towers and other engines of war.

The local civilian population provided a secondary reason. Wear and tear on these roads by peasants and their horse drawn carts was minor. Except for roads near the center of Roman control it seems to me that their impact upon the paved roads would also have been slight.

Roman roads were laid out as straight as the land and the environs would allow. They went over a hill instead of around it or cut through it. This sometimes left some very steep grades. As much as possible the road would be put on an embankment to keep travel surfaces above the level of the surrounding land. That helped to keep it clean and let the solders keep an eye out for the enemy.

The main paved roads were massive. Starting with a trench several feet deep, they would search for bedrock or at least soil which had a high bearing strength. If it proved to be too soft, they would resort to driving piles. These were basically poles made from trees driven vertically. Structures called piers may have been installed on top of the piles to transfer the load from the road to the foundation.

The road was built up in layers using local material when available. As an example, a fully paved road would have 4 or 5 layers. Altogether the layers were about 4 feet thick and 6 ft to 20 ft wide. The bottom was a layer of sand, mortar or both. Cement or lime mixed with sand and water made a good binder. And yes, they had a form of cement back then.

Next was a layer of flat, squared stones set in cement. Then came a layer of gravel set in clay or concrete. A layer of rolled sand concrete would follow. Finally a crowned pavement that was higher in the middle than the sides. It was of large, many sided blocks or hard rock set in concrete with tops dressed. This was a provision for drainage.

Now my first impression of this construction is that it was over-built. I don't know the weight of their war engines and siege towers but they were not part of the regular traffic. Mostly foot and horse traffic would not warrant a 4 foot thick foundation. Coming up with this design was likely a trial and error process over many years. If a 1ft foundation is good so 4 ft has got to be better. Traffic had to be light over most of the roads. No wonder they lasted so long.

The construction and collection of material would have required a lot of time and certainly lots of slave labor. I'm sure they had abundant quantities of material over the empire and much travel may have been necessary to bring it to the site. Had the Roman army not had access to everything, the cost of such a project would have been enormous.

The modern day engineer's road building goes forward only after considerable study, planning and engineering analysis. In the first place, while they are not built to last 100 years, they are designed to last a reasonable length of time. Projections of growth and development of the part of the country the road is to serve are important. A major part of the engineering process is not only making the roads durable and safe but to do so economically because they will have to operate with a budget. They won't be able to build the roads as the crow flies. This may mean changing the route to avoid property that we can't get or that is too expensive to acquire. If the new right of way is important enough we may have to go to court and basically take the land through a law known as eminent domain. Land can be condemned by a government body so the road can continue if it is for the public benefit. A reasonable amount must be paid for the land and the courts will oversee the process.

Today's roadway generally has only three layers and they can be designed so that they are thinner than the Romans. Based on carefully selected design parameters and economics, the material requirements are established. Hardness and compaction values are set and tested during construction. The road bed is first. It's often soil but smoothed out and compacted. The base course is laid in next and highly compacted. The wearing course may be asphalt or reinforced concrete.

Nowhere in the Roman methods did I see anything about thermal expansion. Perhaps this was included with their trial and error techniques. Modern roads and bridges have expansion joints of various kinds that allow a slab of concrete to expand and relieve stress brought on by the heat. This prevents cracking and buckling.

No, I definitely do not think the ancient Roman construction methods provide modern day engineers with any useful lessons. When appropriate laws, building codes, design standards and basic engineering principles are followed, I believe modern methods are superior to the ancient Romans.

 

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