Caesarea Maritima - Sebastos Harbor

Sebastos Harbor

When 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 pace of construction was impressive considering its size and complexity. The moles were made of lime and pozzolana, a type of volcanic ash, set into a concrete underwater. Herod imported over 24,000 m3 pozzolana from Pozzuoli, Italy, to construct the 500 meter long southern breakwater and 275 meter long northern breakwater. 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 mixed with the pozzolana.

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 many divers to hammer in the stakes underwater and it used large quantities of 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 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, barge construction was used. The southern side of Sebastos was much more exposed than the northern side, requiring sturdier breakwaters. Instead of using the double planked method filled with rubble, the architects sank barges filled with layers of pozzolana concrete and lime sand mortar. The barges were similar to boxes without lids, and 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.

At its height, Sebastos was one of the most impressive harbors of its time. It had been constructed on a coast that had no natural harbors and served as an important commercial harbor in antiquity, rivaling Cleopatra’s harbor at Alexandria. Josephus wrote: “Although the location was generally unfavorable, 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, there were underlying problems that led to its demise. Studies of the concrete cores of the moles have shown that the concrete was much weaker than similar pozzolana hydraulic concrete used in 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 was of poor quality and 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 mixture was not mixed thoroughly. However, stability would not have been seriously affected if the harbor had not been constructed over a geological fault line that runs along the coast. 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 CE. Although it is unknown if this tsunami simply damaged or completely destroyed the harbor, it is known that by the 6th century the harbor was unusable and today the moles rest over 5 meters underwater.