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A church on stilts

Boston's Back Bay was a reeking fen in the mid-19th century, 450 acres of brackish mudflats and sewage runoff that flooded twice a day at high tide. It was a breeding ground for diseases and insects as well as a source of chronic stench at low tide.

But, for a growing city of 130,000 confined to a narrow peninsula, this fen was the western frontier, inspiring a massive land-reclamation project that went against a basic axiom of construction: You shouldn't build anything important in a swamp.

By the time famed architect H. H. Richardson was commissioned to build the new Trinity Church in 1871, the newly filled land beneath Copley Square was what one observer called "a desert of dirt, dust, mud and wind." On this soft ground, Richardson was to build a stone church whose central tower alone weighed an estimated 19 million pounds.

Yet, today, while other Back Bay and South End property owners battle rot that has sometimes dangerously weakened the pilings that hold their buildings up, Richardson's masterpiece shows nary a scratch from 130 years perched on swampland.

This summer, an extensive inspection of many of the 4,500 spruce logs driven into the ground to support the church revealed no significant deterioration or shifting of the church's weight. In fact, the engineers concluded that Richardson's piling system was twice as strong as it needed to be to hold up one of the most architecturally significant buildings in the country.

"[Trinity] is a hell of an engineering project and an unbelievable building," said Patrick Watson-Hogan, project executive for Shawmut, the construction company overseeing the piling inspection and building renovation.

The good news about Trinity Church is small comfort for people who own hundreds of other buildings built on the filled land. Over the decades, public works projects such as tunnel digging have caused sudden drops in underground water levels, leaving wooden support pilings exposed to air and rot. Rotten pilings can cause the building to sink, cracking foundations or even buckling walls -- and replacing the pilings can cost $250,000 for a single rowhouse.

But Trinity's success is not merely the result of good fortune that ground water never drained away. Instead, Richardson -- an architect known for his intricate Romanesque facades and careful attention to detail -- designed his church with the distant future in mind, building in backup systems that would support the church long after his generation was gone.

The filling of the Back Bay was a 30-year process, begun in 1857 and made possible by a railway built between downtown Boston and the gravel pits at Needham 9 miles away. For three decades, trains ran every 45 minutes, with men and steam shovels working 24 hours a day to fill the entire area with at least 20 feet of gravel, an effort that required almost twice as much earth as that being displaced by Boston's current Big Dig project.

Construction of the Trinity Church was part of a bold attempt to create a public space in the heart of the emerging neighborhood. Later, the Boston Public Library would join the church in Copley Square, but the builders of Trinity had few places to look for engineering tips on such a largescale project.

And Richardson was quite aware that he was a pioneer. Builders of modern Back Bay structures such as the Hancock Tower drive steel pilings right down into the bedrock 30 or more feet below the ground-water line, anchoring the building in stone. But engineers in the 1870s didn't have the technology for such construction methods, forcing Richardson to improvise.

"The nature of the ground on which the building was to stand brought problems for the solution of which no familiar precedent existed," writes Richardson in "A Description of Trinity Church," his account of the project.

Richardson decided to provide more support for Trinity than was theoretically needed, just to guard against any unforeseen difficulties, over-engineering the foundation to such a degree that little short of an earthquake could shake it. Workers drove 4,500 wooden piles -- each 35 feet long and 10 to 12 inches in diameter -- into the ground upon which the church would eventually rest, all sunk so that they were resting directly on the bedrock. The space around the top 2 feet of the pilings was then filled in with concrete to prevent shifting.

During construction, which began in 1873, Richardson noted even the minutest details to demonstrate to history the pains he went to in ensuring the stability of the church:

"Every pile was watched, numbered, its place marked on a plan at a large scale, and a record made of the weight of the hammer with which it was driven, the distance that the pile sank at the last three blows, and the height from which the hammer fell."

The biggest danger to the foundation of any old Back Bay building is a rotting away of the wood pilings, which is caused by oxidation when water levels drop and allow the wood to come into contact with air. For Trinity, workers leveled the tops of the piles well below the average water level to keep them wet. According to the structural engineers currently examining the foundation, Trinity's piling system is so overbuilt that more than half of the diameter of every piling could rot away and the church still wouldn't be in danger. In addition, Trinity has its own on-site monitoring wells, so if the ground-water level gets low, church officials simply leave garden hoses running into the wells for a few weeks to bring the levels back up.

"The theory is that when they first put this building together, they actually planned on the redundancy in their design, and that's what protected the building," said Watson-Hogan of Shawmut.

But the leadership of Trinity Church is taking no chances. About three years ago, 13 test pits were dug adjacent to different parts of the church to do an initial screening for piling-rot and to see how stable the water levels have been. The results showed that the water table varied by about 8 inches across the entire site, but only the pilings on the east end, where the water was the lowest, showed any kind of rot. Workers on the current Trinity excavation project are exposing every pile around the edge of the east wing and repairing any that show signs of trouble.

Knowing they have a solid foundation, church officials can focus on ongoing renovations, intended to beautify the visible parts of the magnificent building, which has been a national historical landmark for more than 30 years.

The church's construction "was all kind of state-of-the-art for 1877, and to do [the current renovations], it turns out that it's all state-of-the-art for 2003," said Trinity Church spokesman David Trueblood. "Getting ready for this project, they went through and used . . . a newly developed system that uses lasers to make exact measurements of structures so you get an absolute, infinitesimally detailed picture. We have much, much better drawings now of what the place is as built than the engineers had who built it."

The renovation-in-progress is uncovering all sorts of little peccadilloes that never would pass engineering standards today, but that don't seem to have had any adverse effects. For instance, there are numerous places where the foundation stones don't quite line up on top of the piles. The understanding is that most of the piles were pounded in before the builders were exactly sure where the building was going to go or what its final dimensions would be.

"My feeling is that the architects kind of waved their arms and said, `Make it like this,' and then the artisans would get up and they would actually make it, and you got somewhere in between vision and reality," Trueblood said. "But there are no cracks at all in this whole thing, so whatever is down there has worked perfectly for 126 years."

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