Ancient Harbor at Caesarea
The Harbor at Caesarea
Perhaps one of the most impressive parts of ancient Caesarea was its harbor, Sebastos. At the time it was built in the 1st century BC, Sebastos Harbor ranked as the largest artificial harbor built in the open sea, enclosing around 100,000 m2. King Herod built the two moles, or breakwaters, of the harbor between 22 and 15 BC, and in 10/9 BC he dedicated the city and harbor to Caesar (Sebastos is Greek for Augustus). The speed of the harbor’s construction is stunning considering its size and complexity. The moles were made of lime and pozzolana, a type of volcanic ash, that would set into concrete underwater. Herod imported over 24,000 m3 of pozzolana from Puteoli, Italy, in order to construct the 500 meter long southern breakwater and 275 meter long northern breakwater. At a conservative estimate, a shipment of this size would have required at least 44 shiploads of 400 tons each. Herod also had 12,000 m3 of kurkar quarried to make rubble and 12,000 m3 of slaked lime produced to mix with the pozzolana. However, constructing the moles was just as complicated as obtaining the materials.
In order to build the moles, architects had to devise a way to lay the wooden forms for the concrete moles underwater. One technique was to drive stakes into the ground to make a box and then fill the box with pozzolana concrete bit by bit. However, this method required lots of divers to spend large amounts of time underwater hammering in the stakes and it also used a lot of the valuable pozzolana. Another technique was a double planking method used in the northern breakwater. On land, carpenters would construct a box with beams and frames on the inside and a watertight, double-planked wall on the outside. This double wall was built with a 23 cm gap between the inner and outer layer. Although the box had no bottom, it was buoyant enough to float out to sea because of the watertight space between the inner and outer walls. Once it was floated into position, pozzolana was poured into the gap between the two walls and the box would sink into place on the seafloor and be staked down in the corners. The flooded inside area was then filled by divers bit by bit with pozzolana-lime mortar and kurkar rubble until it rose above sea level.
On the southern breakwater of Sebastos Harbor, another type of construction, called barge construction, was used. The southern side of Sebastos is much more exposed to harsh waves than the northern side, leading it to need sturdier breakwaters. Instead of using the double planked method filled with rubble, the architects constructing the southern breakwater sank barges filled with layers of pozzolana concrete and lime sand mortar. The barges were similar to boxes without lids, and they were constructed using mortise and tenon joints, the same technique used in ancient boats, to ensure they remained watertight. The barges were ballasted with 0.5 meters of pozzolana concrete and floated out to their position. Alternating layers of pozzolana based and lime based concretes were hand placed inside the barge to sink it and fill it up to the surface.
During its height, Sebastos Harbor was one of the most impressive harbors of its time. It had been constructed on a coast that had no natural harbors and it served as an important commercial harbor in antiquity, even rivaling Cleopatra’s harbor at Alexandria. The ancient historian Josephus was so impressed with the harbor at Caesarea he wrote, "Although the location was generally unfavorable, [Herod] contended with the difficulties so well that the solidity of the construction could not be overcome by the sea, and its beauty seemed finished off without impediment." However, while the harbor was impressive on the surface, it had some underlying problems that would soon lead to its demise. Studies of the concrete cores of the moles at Caesarea have shown that the concrete is much weaker than similar pozzolana hydraulic concrete used in various ancient Italian ports. For unknown reasons, the pozzolana mortar did not adhere as well to the kurkar rubble as it did to other rubble types used in Italian harbors. Small but numerous holes in some of the cores also indicate that the lime used was of poor quality and was stripped out of the mixture by strong waves before it could set. Also, large lumps of lime were found in all five of the cores studied at Caesarea, which shows that the pozzolana-lime mixture was not mixed thoroughly, perhaps due to the incredibly rapid construction of the harbor. These structural deficits probably would not have seriously affected the harbor’s stability, except for one other detail – the harbor had been constructed over a geological fault line that runs along the coast of Israel. Seismic action gradually took its toll on the breakwaters, causing them to tilt down and settle into the seabed. Also, studies of seabed deposits at Caesarea have shown that a tsunami struck the area sometime between the 1st and 2nd centuries AD. Although it is unknown if this tsunami simply damaged or completely destroyed the harbor, it is known that by the sixth century AD the harbor was unusable and today the moles rest over 5 meters underwater.