NUTS to bolts, girders and beams. In Roelof Schuiling鈥檚 book, the best way to build is with good old-fashioned rock. A geochemist at Utrecht University in the Netherlands, Schuiling has spent 15 years developing an extraordinary construction technique that could see rock bridges, walls and dams sprout from the seabed, driven by the power of chemistry on an unprecedented scale.
Anywhere, at least, that there is a solid bedrock of limestone. Drill a row of holes into it, pump sulphuric acid down them and the acid will trigger a reaction that doubles the limestone鈥檚 volume. With enough acid injected at precisely the right spots, a ridge of solid rock will rise slowly from the seabed and emerge, Old Testament-fashion, to part the waves 鈥� or so Schuiling has calculated.
He is already eyeing up the perfect spot to construct a prototype. For at least 3000 years, a channel of shallow water about 65 kilometres wide has separated India and Sri Lanka. Called the Palk Strait, it is crossed by a line of submerged limestone reefs known as Adam鈥檚 Bridge that in many places are less than 5 metres from the surface. In May, Schuiling published details of how his technology could convert this reef into a solid causeway (Current Science, vol 86, p 1351).
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A rock bridge would mean that cars, buses and trains could travel between the two countries. Lined with wind turbines, and with tide turbines placed in narrow channels in the base of the causeway, the structure could generate enough energy to power a city. The project could even help to dispose of millions of tonnes of waste acid from India鈥檚 chemical industry by locking it up in the fabric of the bridge. Schuiling hopes similar rocky structures could one day be popping up all around the world. From the English Channel to the Pacific Ocean, rocky barriers, bridges or sea walls grown from limestone outcrops in the seabed could link islands, protect vulnerable coastlines and save low-lying nations from inundation as sea levels rise. He even has plans to part the waters of the Red Sea once and for all (see Map).
Rather appropriately the seeds of the idea came to Schuiling one night in 1988, as he lay dreaming. When he woke the following morning, he had worked out a complete scheme for making limestone grow. 鈥淎t first I laughed at myself. Then I started trying to shoot holes in my idea,鈥� he says. 鈥淔inally I started to think about it seriously, and eventually I decided to try a small experiment in my office.鈥� He took a cylinder of limestone, drilled a hole in the centre and placed it in a rigid perspex tube. Each day he poured in small amounts of dilute sulphuric acid. After a few days the rock began to grow up and out of the tube, rather like a plant.
The science is remarkably straightforward. When sulphuric acid is added to limestone, they react to form the soft greyish mineral gypsum, plus carbon dioxide. What struck Schuiling was the considerable increase in volume that occurs as the reaction takes place. This experiment showed that if the limestone is contained in all but one direction, the gypsum that forms will 鈥渋nflate鈥� it to about twice its original volume.
Having confirmed that the process worked, Schuiling started to get excited about the possibilities. 鈥淚 started to think about how this reaction could be used to grow coastal defences and extra land out at sea,鈥� he says. Limestone is a common sedimentary rock and underlies many areas of shallow seawater. Schuiling realised that many underwater limestone formations could be made to rise up above the sea surface, forming surfaces for all sorts of uses, from coastal defences to extensions for airport runways and ports.
Confirmation of the power of limestone expansion has arrived from two unlikely sources. A few years ago, after presenting his results at a conference in Rio de Janeiro, Brazil, Schuiling discovered that his experiment wasn鈥檛 the first time that limestone had become bloated after a dose of acid.
For many years a Brazilian fertiliser company had been struggling to explain why one of its factories had been destroyed by upheaval from underneath. Over a few months, cracks had started to appear in the factory floors and the ground buckled and bulged. Eventually, even the steel girders became twisted and the factory was completely destroyed.
When the company heard about Schuiling鈥檚 experiment, everything became clear. 鈥淚t turned out that the company had been a little careless with their acid disposal, and unfortunately for them the factory was built on limestone,鈥� says Schuiling. Although it was too late to save the factory, he was able to confirm that limestone reacting with acid had caused its demise.
Back in the Netherlands, a retired oil company employee realised that Schuiling鈥檚 work could explain another baffling incident. The tanks that oil refineries use to store sulphuric acid are usually partly embedded in sand. One day an acid tank at the company鈥檚 refinery in Rotterdam started to leak. Rather than soaking up the acid, the sand swelled up. As the tank was forced upwards, connecting pipes snapped, causing a major acid spill. The former oilman recalled that the sand had been full of limestone shells. The acid, he realised, had reacted with the shells, turning them into gypsum and making them expand. Schuiling was told the story in confidence, so he hasn鈥檛 discussed it with the oil company, but as far as he knows, it still stores its acid this way.
Both these accidents confirmed to Schuiling the potential power of this simple chemical reaction. So, funded by a grant from the Netherlands Technology Foundation, he decided to patent his method and scale up his experiments.
In the mid-1990s he and his team tested different concentrations of acid, different ways of injecting it, and differently shaped chunks of limestone. But a recurring problem dogged their efforts 鈥� the sulphuric acid tended to react so fast that the pore spaces in the rock became clogged with gypsum and blocked the route for the remaining acid. The team tried injecting it very slowly, then pre-injecting the rock with hydrochloric acid. 鈥淭his means that a calcium chloride mush forms, which is easy to push the sulphuric acid through,鈥� Schuiling says. This discovery makes the process viable on a large scale, he adds.
After refining the technique in the lab, the team was ready to test it in the field. In an abandoned limestone quarry, they drilled shallow holes and over a period of about 6 hours, injected several thousand litres of sulphuric acid into the stone. Electronic tilt meters placed around the holes monitored the limestone鈥檚 movement. After a few days they were disappointed to discover that the surface had only risen a tiny bit 鈥� by just 1 millimetre. To find out why, they dug out the limestone and cut sections through it to take a closer look at the transformation.
It turned out that this particular limestone was extremely porous. As expected, the acid had spread outwards radially from the holes, turning the porous limestone solid, but beyond a 2.5-metre radius the reaction had fizzled out and the rock remained full of holes. They realised that since all the pores were filled with air, the reaction was simply closing up the pores rather than generating uplift. Clearly limestone expansion only works when the rock doesn鈥檛 have too many holes in it, and when it is constrained on all sides but one 鈥� just as in Schuiling鈥檚 own original experiment. And even if the limestone is very porous it may not matter, providing it lies beneath the water table or under the sea. 鈥淚f the pores are filled with incompressible water, rather than compressible air, then the limestone will still expand.鈥�
If a limestone bank is going to be used to build coastal defences, Schuiling鈥檚 home country seems an obvious place to start. He has his eye on a limestone formation 500 metres beneath the bed of the North Sea. Half the Netherlands is below sea level and although the country is protected by a complex system of sand dunes, dams and dikes, the combination of a severe storm and high tide could cause devastating floods if sea levels rise as predicted. In fact there was a near miss in 1995 and the Dutch are having to think seriously about how best to protect their country. 鈥淚f we expanded this formation it would create sandbanks and shallows at the surface,鈥� he says. Another idea is to extend Rotterdam harbour and create a new offshore international airport. Rather than use up existing land, Schuiling suggests creating a new island around 10 kilometres out from the harbour which could be specially designed for the airport buildings and runways.
So far Schuiling has discussed his idea with politicians and the director of Rotterdam harbour. But the Dutch government has yet to take him up on these projects, so he has been looking for possibilities in other, more receptive, countries.
Earlier this year, Schuiling read a scientific paper that suggested placing wind turbines on the string of reefs and islands between Sri Lanka and India called Adam鈥檚 Bridge. Schuiling was struck by one word: the paper said the islands were atolls, 鈥渨hich to a geologist means they consist of limestone鈥�, he says. So he began work on his most ambitious idea to date: growing a limestone land bridge across the Palk Strait, based on Adam鈥檚 Bridge.
This submerged limestone ridge breaks the surface periodically but for most of its length, it lies just a few metres below the surface. Large ships cannot pass up the strait and have to go around the far side of Sri Lanka. Meanwhile, deep underneath all of this lies an older formation called the Jaffna limestone.
Schuiling calculated that pumping acid into the Jaffna limestone would make Adam鈥檚 Bridge rise out of the water at a rate of almost a metre a month. In just six months, he could create dry foundations for a bridge on which engineers could mount wind turbines. If small gaps were left in the limestone, these would be a perfect place to site tide turbines to harness the power of currents as they rush through the strait. Combined with the wind turbines, they could become a significant source of power for Sri Lanka.
Initially, Schuiling would like to carry out a small pilot project in the strait, drilling a single hole in one of the islands. 鈥淲e can play with injection rates and acid strength to achieve the results we want,鈥� he says. And to get the acid to the right spot, he proposes using the latest technology from the oil industry: drills that can bore holes at any angle and change direction as they go. 鈥淲e would drill a series of holes and direct the acid to exactly where we want it,鈥� he says.
Finding a source of sulphuric acid is no problem: many Indian industries produce sulphuric acid as waste. Schuiling calculates that raising a 500-metre stretch of limestone by around 2.5 metres will take about 1 million cubic metres of acid, about a year鈥檚 waste from a large chemical plant. Given that the Palk Strait is around 65 kilometres wide, raising Adam鈥檚 Bridge could consume around 130 million cubic metres of acid in all.
But will a raised limestone ridge really make the creation of a road or rail bridge any easier? Mark Ketchum, a bridge engineer and vice-president of OPAC Consulting Engineers in San Francisco, is not convinced Schuiling鈥檚 project is necessary. 鈥淚f the water is shallow all the way across, it would not be too difficult to set up an automated drilling rig which would roll out across the strait and drill underwater shafts for the foundations,鈥� he says. The Seven Mile Bridge in the Florida Keys in the US was built in this way.
Schuiling acknowledges this, but says there are some significant benefits to lifting the limestone out of the water: 鈥淚f the process can be done cheaply, using waste acids, then I think it will cost less to construct a road on land than a bridge on pillars. It is also a good way of disposing of hazardous material safely.鈥� He adds that concentrating tidal currents in a few small gaps, as would be possible with a raised bridge, would increase their force and so boost their capacity for generating electricity.
Ketchum has another concern: the strength of the gypsum. 鈥淚 am not sure how sound a foundation material gypsum will be compared to the limestone that was there before.鈥� Schuiling, however, doesn鈥檛 think this is a problem. 鈥淭he transformed rock will be deep and it will assume a sausage shape around each borehole, surrounded on all sides by unreacted limestone,鈥� he says. 鈥淢any medieval German towns are built on small hills underlain by gypsum.鈥�
How these hills would fare if placed in the strong currents in the Palk Strait is impossible to tell. But despite his reservations, Ketchum is supportive of the idea. 鈥淚t seems clever 鈥� the kind of 鈥榦utside the box鈥� thinking that shouldn鈥檛 be discouraged. I would be keen to see it tested on a medium-scale project, before being used in the Palk Strait.鈥�
Technical worries are one thing, but what about the environmental effects of pumping sulphuric acid into underwater limestone? Schuiling is confident that it doesn鈥檛 pose a risk, even though waste acids are often polluted with heavy metals. He says his experiments have shown that these metals become immobilised in the transformation zone, where the limestone is turning into gypsum. 鈥淭here is no direct effect on flora and fauna, since the rock transformation takes place underground and is separated from the biosphere by a layer of unreacted limestone.鈥�
Perhaps the most important question of all is whether India and Sri Lanka actually want such a bridge. Victor Rajamanickam, a geologist at Tamil University at Tanjore in the Indian state of Tamil Nadu, says they do. 鈥淭he governments are interested in the idea. If Sri Lanka gives the okay, then I think India would accept it.鈥� In fact the Indian and Sri Lankan governments have been in talks for a number of years on constructing a bridge across the strait. A conventional six-lane road and rail bridge is predicted to cost about $880 million, so a cheaper way to link the two countries would certainly be welcome.
This is not to say the project would have a smooth ride. An Indian project to cut channels in the reef to ease navigation has been delayed for years by rising costs and political arguments. Schuiling鈥檚 bridge would certainly improve transport links between the two countries, but Rajamanickam for one, is concerned about the political implications of bridging the strait. 鈥淭he construction of a bridge could help smugglers and increase the drug traffic through Sri Lanka,鈥� he says.
Nevertheless, a bridge would strengthen relations and boost trade between India and Sri Lanka. And the power supplied by the bridge would be a huge bonus for an energy-poor nation like Sri Lanka.
Schuiling now faces his biggest challenge 鈥� convincing the two governments to steam ahead with his idea. If he succeeds, it could spark a new era of geochemical engineering. From the Palk Strait to the North Sea, parting the waves and making the land rise may become an everyday solution.
