The oldest text about marine works we know of is Philon of Byzantium’s text that is unfortunately lost (ca. 250 BC). Vitruvius’ “de Architectura” dated around 20 BC, is the only ancient text left about marine works. In his time, ‘Puteolanus pulvis’ (often translated by ‘pozzolana’) is already in use for hardening concrete under water. The resulting mass of ‘marine concrete’ is “neither particularly hard nor strong” but provides an “extraordinary longevity in sea-water” (from Oleson, 2014).
Roman marine concrete ratios and properties are summarised below, from the extensive work of Marie Jackson (in Oleson, 2014).
|Concrete with carbonate rock|
|Unit weight dry mix (kg/m3)||1100 – 1250||1400 – 1550|
|Unit weight hardened concrete (kg/m3)||1500 – 1600||1600 – 1700|
|Compressive strength (MPa)||5 – 8.5||2.5 – 5|
This major invention in river and coastal hydraulics was introduced around 200 BC (acc. to Oleson, 2014) and further developed in the 1st century BC, when large blocks of hundreds of cubic meters of concrete were constructed under water under the name ‘pila’ (up to 1000 m3 in Nisida). The oldest known applications are at Agrippa’s naval base of Portus Iulius, near Pozzuoli, in 37 BC, and at Cosa (Oleson, 2014). This technology (and Puteolanus pulvis that goes with it) were exported to several places around the Mediterranean Sea, such as Caesarea Palaestinae Sebastos (Israel), Alexandria (Egypt) and Iol Caesarea Mauretaniensis (Algeria), but Vitruvius may not have been informed about the floating caissons used at Caesarea as it was built between 23 and 14 BC., i.e. just after he wrote his book.
Vitruvius described three methods for building port structures, but unfortunately, none of his sketches survived and this makes interpretation of his three methods quite hard.
The first method of Vitruvius consists of dumping pozzolana mortar with rubble inside an enclosure made of poles that are driven into the subsoil in order that these materials replace water by falling into the enclosure. This method is made possible by the use of hydraulic concrete (that hardens under water) which is made with pozzolana (provided materials are lowered with help of baskets and not just dumped into the water from the surface). This method supposes that piles can be driven into the subsoil and that they will resist the pressure of mortar before hardening (in the second method, Vitruvius mentions two months of hardening, while modern concrete would take less than one month). If needed, tie rods can be inserted between opposite faces of the enclosure. Such tie rods were made of wooden beams (today, they are made of steel) and have disappeared with time, leaving transversal cavities inside the structure.
In any case, the enclosure height could not be much more than 1.5 to 2 m, but this was an acceptable water depth for ancient ships.
On this model of the Môle de la Marseillaise (at La Nautique near Narbonne), the piles are juxtaposed and tied by horizontal tie rods with a system of tenon and mortise that can still be seen.
Bartocini describes a similar structure on the South breakwater of Leptis Magna.
According to C. Brandon (« How did the Romans form concrete underwater? », Historic Mortars Conference, Prague, 2010) this method was widely used: Anzio, Astura, Cosa, Circeii, Egnazia, Sapri, Santa Severa, San Marco de Castellabate, Portus Claudius, Misenum and Baiae (Italy), Marseille (France), Side (Turkey), Caesarea (Israel), Thapsus (Tunisia) and probably the eastern jetty of Leptis Magna where large masses of concrete are still submerged.
A similar method with an enclosure made of ashlar blocks instead of wooden piles was used, according to Brandon, at San Cataldo (Italy), and Pompeiopolis and Kyme (Turkey).
An alternative to this first method consists of prefabricating a rigid wooden enclosure, with or without a bottom, which is then floated to the desired location before being filled with hydraulic concrete. Such a structure is now called a “caisson” (modern caissons are made of concrete and have a bottom in order to float). This alternative method is well suited for hard (rocky) sea beds where piles cannot be driven.
This alternative seems to have been used for a quay of 23 x 2.20 m at the Port des Laurons (Martigues, France) and reached a summit at Caesarea (Israel).
In the latter case, Flavius’ description mentions blocks of 50 x 18 x 9 feet, that is nearly 600 tons (archaeology has even revealed blocks of 14 x 7 x 4 m, or 1000 tons). Archaeological excavations showed imprints inside and under the concrete mound, proving that the structure consisted of wooden caissons used as lost formworks for concrete to be poured in situ. Such caissons with a bottom could be built on a nearby beach and be floated to their final position. From there, one can easily imagine that an old ship could be sunk to build a manmade island like the one of Portus Claudius.
An older alternative consists of floating a block of concrete that was prefabricated in the dry without using hydraulic concrete. Such a method might have been used in places where pilae made of concrete with ‘opus reticulatum’ were found under water: Nisida and Secca Fumosa (both in front of Pozzuoli), Ponza, Egnazia (acc. to Oleson, 2014).
Vitruvius’ third method is close to the first method as it also requires an enclosure, albeit a water tight one (we now call this a “cofferdam”) allowing water to be pumped out in order to enable work in the dry. Hydraulic concrete and pozzolana are thus not required in this method. However, the walls must resist the pressure of water and shoring may be required (like in the first method, the height of the enclosure did not have to exceed 1.5 to 2 m which was a sufficient water depth for ancient ships). Moreover, large pumping capacity must be provided depending on the permeability of the subsoil. It would therefore be difficult to use this method on a sandy sea bed as water would seep into the enclosed area through the bottom and Vitruvius rightly recommends digging out the area down to the rocky substratum. He also indicates that the foundation must be wider than the planned jetty. This foundation can be a mound of concrete placed on top of the rocky bottom or on a series of wooden stakes if the subsoil is unstable (the use of coal for filling the space between the stakes is somewhat unclear. Did they believe that as fire hardens wood, coal would preserve it in the long term?). The jetty can then be completely built in the dry.
This method was mainly used to build bridge piers in rivers (and is still in use nowadays). Brandon nevertheless mentions some maritime applications: Marseille (Quays F.28 and F.120), Ponza and Nisida (Italy). The cofferdam of the Corne of the ancient port of Marseille may be mentioned also (Roger Guéry « Le port antique de Marseille », Collection Etudes Massaliètes, 3 (1992), pp. 109-121). This method was also used to build maritime arched breakwaters like at Civitavecchia, Terracina, Misenum, Pozzuoli, Nisida.
Vitruvius’ second method consists of building the structure from the shoreline and progressing in offshore direction.
If stones are to be dumped into the sea, the stone size must be sufficient to resist wave attack. Stones of tens and hundreds of kilos must be used for the core and covered by an armour layer made of stones of several tons: no technical problem but tricky logistics. This method was used by Alexander when besieging Tyr (in 322 BC, well before Vitruvius).
Floating barges can be used to dump stones further out of the coastline, e.g. to build a manmade island, but barges are exposed to waves and increase risk of down time. This was done at Civitavecchia to build an island at the entrance of the port.
If concrete blocks are to be built into the sea, as Vitruvius seems to suggest, one can think of blocks of several tens of cubic meters built on the beach on top of a small mound made of sand and contained by a small wall (Vitruvius mentions a height of no more than 0.50 m). After hardening of the block, the small wall is removed and sand can be eroded by the sea. The block will then tumble into the sea and the process can be started again. One must be patient … and no application of this method is known, except perhaps at Hereum Promontorium (Bosphorus).