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Cavern concerns

Beneath the streets of Naples is a labyrinth of caverns that are collapsing. Filling the holes is straightforward; plotting them is another story

Neapolitans either thrive on or are inured to danger. Home of the famously violent Camorra criminal organisation, Naples nestles between two live volcanoes. Its drivers rank as some of the most anarchic and reckless anywhere.

And every year, completely without warning, two or three yawning voids open up, swallowing people, vehicles, buildings, and often large parts of urban blocks.

Two people died in 1996, the last time a sinkhole - measuring 8.1m across - claimed lives. The city has lost count of fatalities chalked up over the last century.

Systematic recording of the chasms has only been carried out since 1968, when six people were killed in a single collapse.

Central Naples is literally undermined by a network of caverns carved out of the volcanic tuff bedrock on which it is built.

'Cavities vary from small shafts 1m to 2m wide and 1m to 2m deep, to monsters that are 40m to 50m in span and 8m deep, ' says professor of foundation engineering at Naples University, Carlo Viggiani.

Excavation began in the first century AD, around the time that the Roman Imperial fleet was moored in Naples Bay, and continued to the end of the 19th century. Wells and aqueducts were excavated to supply houses in the city with fresh water. Quarrying was carried out around the city limits to supply stone for construction.

Activity was particularly acute in the 16th and 17th centuries.

Nobody recorded positions of either the aqueducts or quarries.

And as the city grew rapidly from the 18th century onward, buildings were erected directly above.

Volcanic tuff is a soft rock with a compressive strength of between 20N/m 2. Porosity ranges from 30% to 50%. It was deposited in strata that are seldom more than 10m to 20m thick. 'Tuff is good for engineering but highly erodable, ' says Viggiani.

Bedrock is typically overlain by 5m to 10m depth of pyroclastic materials with excellent friction and cohesive properties. Layers of soft clay are found in low lying areas.

Meanwhile, a tuff outcrop extends from the south west to the north of the city, covered by 5m to 30m of cohesionless soils.

Ground, and the caverns, remain reasonably stable when dry, says Viggiani. However, water swiftly wears the materials away, producing piping, opening up new cavities and underming pillars and walls. To make matters worse, roofs in the tuff quarries are normally no more than 3m thick, in bad quality rock, says Viggiani.

Worried by the mounting death toll, the spiralling costs of damage to the city's streets, buildings and infrastructure, and disruption to transport the Commune di Napoli or city council, convened an emergency Committee for Naples Subsoils in 1998 to gauge the extent of its problems.

Investigation is in the main physical; 'like caving', Viggiani says. The Committee will report finally at the end of next year. But so far it has plotted cavities with a total volume close to 6M. m 3. A combined surface area of 600,000m 2puts 3,000 of central Naples'1M population at risk from building collapse, Viggiani reckons.

And 'there are at least two to three times more cavities than we already know about', he warns.

In the last century subterranean water flow and resulting erosion has increased dramatically as Naples' sewer network, installed in 1878, has fallen apart, says Viggiani. Up to 50% of the city's wastewater is escaping into surrounding ground the survey shows. Dangers of collapse are particularly acute after periods of heavy rainfall when sewers are overloaded. Constrictions in the main drain, right in the cavityriddled city centre, result in a head of water tens of metres high. Water is jetted into surrounding tuff under its own gravitational pressure.

'The whole sewer network needs to be rebuilt or adapted, ' Viggiani comments. 'In the last decades a number of new sewers have been constructed, but without a united approach.'

The Committee is prioritising the remediation of cavities. They will be filled with very lean concrete or with expanded clay, says Viggiani. Key issues are cost - perhaps 9M. m 3offill will be needed - and pumpability.

Some of the caverns can only be reached via lengthy underground passages.

At the same time, the Committee is drawing up a remediation strategy for more than 200km of retaining walls which are no longer fit to hold back rockfall and mudslides on the city's steep hillsides. Observation combined with penetrative investigation is used to gauge whether walls can be stabilised or need to be rebuilt to modern standards.

Filling Naples' cavities and securing its slip-prone slopes will cost Lira 5,600bn ($2.55bn), the Committee estimates. Though national government agrees that urgent action is needed it is doubtful whether so much cash can be found even over a couple of decades, says Viggiani. No danger of Naples becoming too safe, too soon.

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