Request for Proposal

 

The city of Nimes, France has outgrown its local water supply and is seeking proposals to supply 44 million gallons of fresh water per day.  There are a few constraints you will need to contend with.  There is no electricity or petroleum, thus no motors, engines or pumps, no steel, no concrete, no transits or laser levels.  You may use local resources like scrub timber and stone.

 

This was the problem faced by the Roman engineers around the year 19BC.  Reconnaissance located a strong spring about 20 kilometers from Nimes.  But the problem was getting the water over, through and around hills and valleys between the spring and the city.  There is a 12 to 17 meter elevation drop (depending which source of information you use) between the spring and the city.

 

I recently found myself in this area of France and had the pleasure of thinking what it would have been like to be faced with such a task.  As it turns out, after careful surveying and design, the engineers settled on a 50 kilometer aqueduct that would consist of tunnels, trenches and bridges. The most challenging part of the aqueduct was how to get it over the Gardon River at the proper elevation.  It needed to be approximately 50 meters above the river, the river was known to rise 25 meters in flood stage and the width of the riverbed was 142 meters – bank to bank.

 

I pondered, faced with this problem and a blank piece of paper, what would I have designed.  The Roman engineers, and dare I say architects, came up with an elegant solution now known as the Pont du Gard.

 

 

 

 

The aqueduct bridge has three levels of arches as shown in the pictures.  The aqueduct is a 1.4 meters wide x 1.9 meters tall channel at the top of the bridge.  The structure was built of dry stacked limestone blocks, the largest being approximately six tons.  The stone was quarried about 400 meters from the bridge site.  Each stone was cut to fit and numbered at the quarry.  Again I had to ponder, how did they cut the stone so precisely, how did they get the stone to the site, and how did they hoist them into position?

 

To keep the aqueduct from leaking it was lined with a “secret” material.  Plinio the Elder left a recipe in one of his texts: “lime extinguished in wine, pork fat, and figs.”  This turns into a very tenacious plaster that is still in evidence in some of the aqueduct’s inner surfaces.

 

Depending on whose research you use the aqueduct took between 15 and 100 years to complete.  It was constructed by skilled craftsmen such as masons, carpenters, lime kiln operators, etc., as well as slaves.  The aqueduct did carry 44 million gallons of water a day for approximately 800 years – another thing to ponder.  The water was delivered to a water tower in Nimes, and it was distributed to the Roman baths, and private homes through lead distribution pipes.  Evidence suggests that it operated into the 9^th century AD when the aqueduct became clogged with calcium deposits. With the downfall of the Roman empire there was no maintenance.  Subsequently local people took the stones from the aqueduct for building blocks for other structures.

 

Over the years, the Pont du Gard became more usable as a bridge for commercial traffic. During one period the second level arches were partially cut away so that donkeys laden with large loads could get by.  Fortunately the structure did not fall and societies over the years have seen the wisdom of keeping the bridge in decent repair.  In fact repair work was going on during my visit.

 

So as we go about our daily routine as engineers, it is nice remember that we come from a long line of special people who did important work. Maybe some of us will design and build something that will survive 2000 years.