At present, the Domus Aurea consists of a complex underground structure above which lies the Oppian Hill park with the visible remains of the Baths of Trajan.
The rooms that previously belonged to Nero’s vast palace, originally designed as an overground complex, are now concealed by the park and have thus become subterranean structures, though they were not intended to be so.
Holes in the vaults and apertures in the vaults have formed preferential paths for the percolation of water and caused the structures to become far weaker than they originally were.
Examples of these situations, widespread throughout the monument, can be found in Room 33, where rebuilding and conservation work has already taken place, in Gallery 20E and Room 94.
Numerous attempts have been made to waterproof the ancient structures to intercept rainwater and prevent it seeping into the underground monument, though these have covered limited areas.
We have recently completed the Definitive Project for the rehabilitation of the area above the Domus Aurea.
It is obvious that water infiltration, resulting from increasingly frequent heavy rains, continues to cause damage to the monument, even in areas where the conservation of internal surfaces is underway and where work has already been completed.
Considering the importance and urgency of limiting the percolation of water into the monument, in order to reduce the risk of compromising the conservation work already undertaken, the continuation of planned interventions and the potential opening of a limited area of the monument to visitors, we are in the process of laying a drainage layer on the surface of the park above the Domus Aurea.
The aim of this intervention is to significantly reduce the intensity of surface water percolation rapidly and at modest expense before the autumn rains that usually affect Rome in the first third of November.
We aim to achieve this objective by draining the surface waters in a way that does not hinder or endanger the work underway in the park.
We believe that draining 50% of rainwater is sufficient to re-establish a degree of security, or at least to slow down the decay of structures and paintings sufficiently for the definitive project, in the form of the Integrated Protection System, to be “in time”.
Drainage will be achieved by laying a geocomposite in a channel just over 4 metres wide, at the centre of which will be a drainage tube supported by gravel, with a gradient of 0.5%. The drainage geocomposite and the tube will be buried by the earth dug out, so as to ensure that the surface is passable as at present, allowing spontaneous plants to grow.
The depth of the geocomposite varies from a minimum of 20 cm to a maximum of 80 cm, whilst that of the drainage tube ranges from a minimum of 0.50 to a maximum of 1.20 metres. The tube will have a diameter of 160 mm for the first 20 metres and subsequently of 200 mm; it will flow into the park’s sewer system.
The purpose of the geocomposite is to drain and filter rainwater. It consists of a three-dimensional polyamide core, heat-treated to give it a v-shape configuration, particularly suited to resisting the confining pressures exerted by the surrounding soil, enclosed in two non-woven heat-sealed filters integral with the drainage core. The geocomposite chosen has a vertical transmissivity (hydraulic gradient i = 1) at 20 kPa of no less than 2.5 l/s m (equal to 9000 l/hm).
Rainwater seeps into the ground vertically. When it meets the drainage geocomposite it will tend to flow into it, as the resistance to motion is several degrees of magnitude lower than the surrounding soil, where a small quantity of water will nonetheless remain. The water will thus drain away inside the geocomposite until it reaches the drainage pipe that channels the water towards the sewer system. The route of the drainage pipe may run through highly permeable soils or soils with voids or fractures, where the water collected may percolate downwards, thus compromising drainage and making the concentration of water created by the drainage process dangerous. To avoid this, a geosynthetic liner with an impermeable outer membrane will be laid next to and underneath the drainage pipe, so as to form a waterproof channel through which water can pass only in the direction of the pipe before being discharged into the sewer. Building this system over the 16,000 m2 of the Oppian Hill above the Domus has a high cost in terms of time and expenditure. We have thus opted for a compromise, implementing these drainage systems over about 20% of the surface. However, the system can be extended to complement the work already carried out without compromising its effectiveness.
Drainage far exceeding 20% of surface water can be achieved by adopting some expedients such as digging a dense network of small drainage channels about 7 metres long, with a slope of 1%, a depth of 20 cm and a very low side gradient of 1/3 (18°). These small channels, which will not hinder movements on the surface, will run into the main drainage system; where the two systems meet small drainage pipes will be placed crosswise in the main channel to facilitate the capture of the waters collected on the surface.
These expedients will allow us to capture more rainwater, especially during very heavy and prolonged rains.
We should also consider that when large quantities of water are not captured by these systems, soaking the soil and thus creating a suspended water table, when the latter comes into contact with the underneath of the geocomposite it will be drained away. In this case, the system behaves like a drainage ditch.
All these considerations allow us to estimate that over 50% of rainwater will be drained away, especially during heavy and prolonged rains, which represent the greatest danger.
Finally, we should stress that not using waterproof sheaths helps transpiration and prevents the soil rotting, since it remains naturally aerated as before the implementation of the system; the capillary fringe is not significantly affected.
The drainage system is subdivided into 14 drainage channels positioned so that progress in building the 22 drainage basins belonging to the Definitive Project does not compromise the effectiveness of the channels not directly affected by this work.
The quantity of water drained by each channel will be measured to evaluate the efficiency of the drainage system.
Work has already begun and is scheduled for completion by the end of this October.