But where`s the hole?
Sidewalk superintendents have been scratching their heads as they watch the construction of a seven-story-deep underground garage being built next to Post Office Square in Boston.
In the months since the 34-year-old Boston Parking Authority garage was leveled last November, passersby have seen clamshell power shovels digging trenches deep into the soil. They have seen large cranes lowering 90-foot carpets of steel and plywood into the trenches, and other cranes sliding tall steel girders gingerly into seemingly bottomless holes that dot the site.
What passersby have not seen, even though concrete for the top floor of the garage is about to be poured, is any sign of the huge open pit one would expect to find to hold an underground structure that will hold 1,400 cars.
And they never will.
The garage is being built with an unusual but increasingly popular ”top- down” construction method pioneered in Italy for the Milan subway in 1950 and later refined for subway projects in Japan. (It sometimes is called the
”up-down,” or ”Milan method”).
Chicago`s Olympia Center, the first American development to use the technique, took the concept a major step further in 1984 by building simultaneously an underground garage as an office building rose skyward above it.
In Boston, the technique has been used in three other buildings built since 1985-Rowes Wharf, 75 State St. and 125 Summer St.-but it was not as obvious to the public because in those cases a building was going up overhead at the same time.
Because the 1.7-acre Post Office Square garage will have only a street-level park above it, the unusual nature of the project has been much more visible.
It is especially striking in contrast to the construction going on three blocks away at 125 High St., where excavators dug a 70-foot-deep pit on the 2.5-acre site before beginning to erect twin office towers that will be the third-largest office complex in Boston.
Soil conditions were a major reason for the decision to use the top-down technique at Post Office Square.
125 High St., once part of Ft. Hill, an 80-foot-high hill leveled in 1860 to fill in a swamp and extend the area to create Atlantic Avenue, represents one of the best building sites in Boston for an office tower.
Underground is a 50-foot layer of glacial till, a greenish, claylike material that is ideal for construction, said Peter J. Stankiewcz, senior construction manager for the project and vice president of Spaulding & Slye, the developer.
”The till is like digging in lean cement,” he said. ”The material is impervious to water and very stiff, making it ideal for load-bearing columns.” The thick layer of glacial till ”is rare for Boston.”
The Post Office Square area is more typical. It once was a swamp. Below the surface lie 12 feet of old fill, 40 feet of clay and 10 feet of sand, with just 20 feet of glacial till above bedrock.
Because so much of the soil is soft, loose material, it would be necessary to build expensive walls and braces to keep the sides of an open pit from caving in, said James Becker, president of Beacon Construction Corp., which pioneered top-down construction in Boston at its Rowes Wharf project. He chairs the construction committee overseeing the garage work.
The alternative is to build down, sinking the outer walls and inner support columns deep into the ground, then laying the first and topmost slab across the top, with only a shallow excavation beneath.
Once the top level is in place, bulldozers go to work excavating the ground beneath just enough to lay the floor for the next level, in a process something like digging a mine. As the earth is scraped away, cranes lift it to the surface through one of three openings in the floors above.
The process is repeated for each of the seven levels. As each floor goes into place and is connected to the walls and columns, it provides lateral support against shifts in soil pressure as the digging proceeds downward.
”Essentially, we excavate and pour a floor, with the floor serving as a brace as you go down,” said Robert Anthonyson, project manager.
When construction of the 500,000-square-foot garage is completed late next year at a cost of $40 million, a 1.5-acre public park will be built on top at street level. To enable trees to be planted, 1/2 feet of earth fill will be laid over the garage`s top level. The park is scheduled to open in 1991, with the total project costing $80 million.
Becker, who was a lecturer in the civil engineering department at Massachusetts Institute of Technology, said the top-down technique works best when underground construction must be very deep. The technique will be used for a similar-sized garage at North Station and for a proposed eight-level garage in Downtown Crossing for the new Bloomingdale`s store.
The huge project to replace Boston`s elevated Central Artery will use a variation of the method-digging in the walls and central supports, adding a top and then excavating the depressed highway.
It took workers 3 1/2 months to build the 1,150 feet of perimeter walls around the outside of the Post Office Square garage, a process that was completed in March. In 15-to-25-foot sections they dug 90 foot-deep trenches, then lowered cages of reinforcing steel into them and filled the trench with concrete. Seven plywood and plastic foam strips on each cage mark the level of each floor.
Crews have finished work on most of the 101 interior columns, called load-bearing elements by the engineers. For each one, shovels dug a 3-by-10-foot hole to bedrock, at a depth of 95 feet. They then lowered a steel beam into the hole and a shallow steel cage. Finally, 30 feet of concrete were pumped in, forming a heavy concrete footing below the lowest level.
This month, construction crews will pour the 25-inch-thick concrete roof slab, slightly ahead of schedule. Lower floors will be one foot thick.
Already, light-weight bulldozers are starting to clear away earth for the next level. The fill, clay, sand and till, more than 200,000 cubic yards in all, will be trucked to a landfill in Plainville, Mass., said C. Peter Erickson, construction administrator.
The top-down method has other advantages beyond its suitability for the loose soil of the Post Office Square site. For one, all the construction equipment can remain on the site itself, rather than jamming the streets around the perimeter.
And because most of the work is below the surface, there is less noise, an advantage in a densely built area surrounded by a hotel, office buildings and retail stores.
