Two thousand years ago, divers whose only air supply was their lungs pushed and pulled at a 50-foot-long rectangular wooden frame suspended in the water from cranes. First one, then another of the divers swam to the surface to breathe, then dove again. Bit by bit, the wooden frame descended, finally hovering inches above the ocean floor. The divers edged away. All at once, the ropes holding the frame were cut and it sank deep into the silt and sand on the ocean bottom. The divers swam ashore. Large wood and leather tubes like elephants’ trunks were lowered into the water inside the frame. Roman hydraulic concrete, three times denser than water, poured through the tubes, gradually displacing the water in the frame. When the pouring was done, the concrete hardened underwater to form a huge foundation block for a harbor breakwater.a
In the summer of 1982, scuba divers from the Caesarea Ancient Harbor Excavation Project (CAHEP)1 uncovered two intersecting beams that had been the bottommost pieces of the wooden frame, or form, into which this concrete was poured. The sand and silt covering these beams had protected them both from rot and from the destructive appetite of shipworms, thus preserving the timbers where they had been positioned 2,000 years ago.
Caesarea Maritima and its harbor (called Sebastos) were the vision and product of one man, Herod the Great.2 He was a prodigious builder, keenly aware of the political advantage of embellishing the cities within his kingdom and those in neighboring lands. A list of his works recorded by the first-century A.D. Jewish historian Josephus3, included temples, theaters, amphitheaters, gymnasia, baths, porticoes, aqueducts, fountains and colonnaded streets. The variety of these projects indicated not only the generosity (or ego) of the man but also reflected the religious, cultural and political needs of his varied constituencies. The Temple in Jerusalem was rebuilt for the Jews; theaters, gymnasia, porticoes, and colonnaded streets satisfied those who wanted cities to have a Hellenistic appearance; and temples dedicated to Roma and Augustus, baths, amphitheaters and aqueducts reflected the strong influence on Herod of Roman technology and institutions.
The harbor at Caesarea was an engineering marvel. On a stretch of eastern Mediterranean coastline known for its dangers to mariners and lacking sheltered anchorage, Herod built a harbor as large as Piraeus, the port of Athens. Two breakwaters, one on the north and one on the south, with a 60-foot entrance between them, enclosed a protected anchorage. The breakwaters extended as much as 1,500 feet from the shore. Within the main harbor was a sheltered inner harbor.
This past summer, CAHEP divers uncovered two massive concrete blocks, as well as parts of the wooden forms into which they had been poured. One block was located at the northwest corner of the north breakwater. It measured 39 × 49 feet with a maximum preserved height of 5 feet and an irregular upper surface. The remains of the wooden form into which the concrete had been poured were located on the north and west sides of the concrete block and at its northwest corner. These timbers, 11 inches wide, which ran along the lower edge of the block, intersected in a simple lap joint at the corner of the block. Cut into the upper surface of the beams were slots to receive a series of uprights with cross-sections averaging 6 inches by 9 inches. Fragments of these uprights were found in the beams.
When the forms were constructed, horizontal planking was nailed to two sides of the vertical posts, creating an inner and an outer wall. Then the hollow space between the inner and outer walls was filled with a blue-green mortar mixture containing large particles of tufa, pumice,b and lime, the same mixture as that of the block itself. Remains of the double wall filled with mortar were found along the west side of the block. The upper portion of the formwork was reinforced with additional vertical posts and horizontal tie beams. The tie beams and upper vertical posts decayed, but left their impressions, which appeared as square holes, in the concrete block, as seen in the drawing in the sidebar “How Herod Built a Harbor.” The original height of the block was not preserved, but we assume that another layer of horizontal tie beams was present at the top of the form, as shown in the reconstruction drawing in “How Herod Built a Harbor.”
Herod’s harbor was completed in 10 B.C. Two important events occurred just before this time; one definitely influenced the final form of Herod’s great harbor at Caesarea. This was the invention of hydraulic concrete by Roman engineers. The significant property of hydraulic concrete was that it hardened underwater. The special ingredient for Roman concretec was a volcanic sand known as pozzolana, a bonding agent similar to our portland cement.
The second event was the publication of a treatise in 25 B.C. called On Architecture by a Roman architect and engineer named Vitruvius. Although we cannot assume that Herod read Vitruvius, this book became the “state of the art” of its time. Vitruvius describes structures ranging from harbor installations to private dwellings. Vitruvius writes of pozzolana:
“There is also a kind of powder which from natural causes produces astonishing results. It is found in the neighborhood of Baiae and in the country belonging to 011the towns about Mt. Vesuvius. This substance, when mixed with lime and rubble, not only lends strength to buildings of other kinds, but even when piers of it are constructed in the sea, they set hard under water.”
On Architecture II.6.1
The process of pouring concrete underwater was probably much the same as it is today. The semi-liquid mass of concrete was poured into a tube (most likely made of wood with flexible leather joints); the other end of the pouring tube was held close to the ocean bottom. As pouring continued, the mouth of the tube was kept below the surface of the concrete already in place, so that the newly poured concrete remained in a proper mixture and gradually displaced the sea water within the form. (If the concrete had simply been dumped into the water, the heavier materials would have fallen and the lighter sand and cement would have remained in suspension.) The texture of the first-poured concrete on the bottom of the block was uneven until enough concrete was poured to cover the mouth of the pouring tube; then the concrete acquired a uniform consistency. A block about 50 feet by 40 feet by 5 feet required more than 10,000 cubic feet of concrete. Multiple tubes, each manned by a separate crew, were no doubt necessary for each pouring.
The second concrete block discovered this past summer projected from the sand 20 feet north of the first block, clearly separated from the breakwater. This block was built of the same materials and in the same manner as the first block and both blocks had approximately the same orientation, with their west faces aligned. Fragments of wooden formwork were found on the west side of the second block near its south corner. This block was not part of the breakwater but was probably the foundation of a tower standing northeast of the harbor entrance. (Previously, in 1981, the bases of two other towers southwest of the entrance had been identified by divers.) The positions of these tower foundations match those described by Josephus in the Caesarea harbor:
“The entrance to the port faced northwards, because in these latitudes the north wind is the most favorable of all. At the harbor-mouth stood colossal 014statues, three on either side, resting on columns; the columns on the left of the vessels entering port were supported by a massive tower, those on the right by two upright blocks of stone clamped together, whose height exceeded that of the tower on the opposite side.”
Jewish War I, 413.
No statues have been found near the Caesarea harbor towers, but contemporaneous depictions of sculpture at harbor mouths are known. A marble relief was found in 1863 or 1864 near the Torlonia villa on the northeast side of the port at Ostia (the port serving Rome). This relief depicts a colossal statue on the third story of the four-story lighthouse built by the Emperor Claudius. It also shows two other large statues on pedestals at the harbor. These statues on towers bear witness to the widespread popularity of this type of architecture in ancient harbors.
Towers with statues are also depicted on a small commemorative medal about the size of a dime, which was found by the Link Expedition during the first underwater excavations at Caesarea in 1960. This medallion contains a representation of the entrance to a port. Two letters, KA, found on the medallion might be an abbreviation for Caesarea. Perhaps we are looking at the Caesarea harbor, but it is also possible that the scene depicts a similar harbor at Alexandria.
Concrete blocks and wood fragments are not as exciting visually as the marble statues, mosaic floors and huge vaults found on land at Caesarea. But mundane as these concrete blocks and wood frames are, our discovery of them confirms important details about the plan and technology of the world’s first great artificial harbor.
Two thousand years ago, divers whose only air supply was their lungs pushed and pulled at a 50-foot-long rectangular wooden frame suspended in the water from cranes. First one, then another of the divers swam to the surface to breathe, then dove again. Bit by bit, the wooden frame descended, finally hovering inches above the ocean floor. The divers edged away. All at once, the ropes holding the frame were cut and it sank deep into the silt and sand on the ocean bottom. The divers swam ashore. Large wood and leather tubes like elephants’ trunks were lowered into […]
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This is an imaginative, but plausible, reconstruction of a technique for pouring hydraulic concrete underwater.
2.
Tufa and pumice are volcanic rocks. The tufa and pumice used in the construction of Caesarea’s harbor may well have come from the Bay of Naples.
3.
The term “Roman concrete” is also used to refer to opus reticulatum walls (see “Herod’s Family Tomb in Jerusalem”), walls built with uniformly cut small stones laid in a diagonal or other pattern. The concrete that was the core of opus reticulatum walls had the same composition as the concrete used in the breakwaters at Caesarea.
When used with respect to opus reticulatum,“Roman concrete” refers to the entire wall including stones and binding concrete; but as it is used to describe the material poured into forms at Caesarea, Roman concrete refers to the concrete blocks.
Endnotes
1.
The Caesarea Ancient Harbor Excavation Project is sponsored by the Center for Maritime Studies at the University of Haifa and led by Professor Avner Raban. Other co-directors and representatives of their supporting institutions are Robert L. Hohlfelder of the University of Colorado, John P. Oleson of the University of Victoria, and Lindley Vann of the University of Maryland.
