Admiral Miguel Grau Freeway project has provided Peruvian historians a rare opportunity to look at the engineering techniques of another age. The excavation for the project has uncovered a wealth of public works that date back to Lima's colonial period including sewage systems and the city's original wall.

"One of the most important things we are learning is exactly where this and other works were located exactly," says Guillermo Cock Carrasco, the director general of ConsultPatCu E.I.R.L. The Lima-based consultant is overseeing the archeological aspects of the project. "Formerly we had an idea but couldn't tell for sure."

That uncertainly created headaches for the construction of the new freeway as the archeologists were unable to tell work crews exactly where they could expect to find historically significant items. The stop-start process that followed has delayed the completion of the road for several months, says Armando Molina, the project manager for the City of Lima.

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Every construction project in Peru requires an environmental assessment that includes an archeological portion. In recent years, a number of private companies such as ConsultPatCu have begun to capitalize on the growing number of projects in the country by offering contracting services by performing the archeological impact studies and handling the excavation once the project progresses.

Typically, this is aimed at understanding if there will be any effect on the country's rich pre-Spanish heritage but, in the case of the Grau Freeway, it is the history of the city that is being uncovered, Cock explains.

The bulk of their work on the Grau project has been the excavation of ceramics. The old city wall was Lima's de-facto dump and, to date, Cock's group has excavated more than 30,000 metric tons of ceramic pieces providing a wealth of information about the city's late colonial and early republican period.

"This is a by-product of the disasters that have struck the city through the centuries," Cock says. "Lima is known for its earthquakes and one of the things that earthquakes tend to be rough on is ceramics."

The biggest surprise has been the discovery of portions of the base from the outer protective wall that had been built during the colonial period to protect the city against attacks by pirates.

An adobe wall built in 1687 stood almost two centuries and protected colonial Lima. Individual adobe bricks were placed on the base large rounded stones held in place with a clay mortar.

In the second half of the 19th Century, Peru was reaping the rewards of the guano age. The sale of the bird droppings that covered the islands off the coast brought in millions of dollars and led to a population boom. The sudden largess unleased a spate of projects that were intended to provide amenities for the burgeoning populace.

In 1872, the city hired an American railroad entrepreneur, Henry Meiggs, to demolish the wall to make way for the public works boomlet. The original Grau Avenue was constructed over the layout of the original wall.

Another key part of the original city infrastructure being uncovered is the water and sewage system. Until the 1870s, this consisted of a series of rude canals that pre-dated the Spanish and were originally used to irrigate the crops grown in the region. The largest, known as Canal Guatca, ran directly through Lima.

The public works projects that followed the razing of the city wall required that the canals be covered so engineers decided to create a series of three tunnels in the existing waterway. The channel bottom was filled with stones and then brick walls were built dividing the channel into three portions. Each was covered with an arch.

While stone sections were held together with the traditional clay mortar, the brick mortar was calicanto, a mix of lime, sand, stone and bird eggs - the last readily available from the millions of birds that inhabited the guano islands.

The brick and mortar channel - portions of which are still in use today - has ceramic piping from the turn of the 20th century connected to it. These in turn have metal piping from the 1940s. These are connected to PVC pipe that came into use in the late 1970s.

"It is really incredible when you stop to think about what you are actually looking at," Cock said. "Basically, you can see the entire history of the city when you uncover these things," said.