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Slurry Walls

The Central Artery/Tunnel Project included the largest use of slurry walls anywhere in North America. What's a slurry wall? It was the single most important construction technique on this gigantic project, the primary tool of the designers to fulfill the most important promise to the people of Boston: Keeping the city open for business and traffic moving during more than a decade of construction. 

A slurry wall is a concrete wall that runs from the surface of the ground down to bedrock. It defined the area to be excavated for the underground highway and eventually formed the actual walls of the new Central Artery. "Slurry" refers to a clay-water mixture that is pumped into the excavation for the wall to keep the sides intact until concrete is poured in.

How a slurry wall was constructed:   

First, a trench about three feet by ten feet was dug by an excavating machine down to bedrock, which on the Central Artery project was more than 120 feet down in some areas. 

These images are close-ups of equipment used for slurry wall trenching:

As the earth was removed (either by a clamshell excavator or by a continuous milling machine that grinds away obstructions with rotating wheels) the liquid slurry was pumped into the hole.

The clay-water slurry mixture was just heavy enough to keep the walls of these very deep excavations intact before huge reinforcing steel beams were lowered into the trenches and concrete pumped in to fill the hole. The concrete displaced the slurry, which was pumped away and re-used. Each slurry "panel" took about two days to dig, reinforce, and fill.

The side-by-side panels form the walls of the underground Central Artery. A total of more than 26,000 linear feet of slurry walls - about five miles worth - went into the project, one ten-foot trench at a time. Once the walls were in, huge steel beams were placed between them at ground level and concrete decking placed on the beams. Traffic and construction equipment moved on the decking as soil was excavated below and removed through "glory holes" where sections of decking were removed.

As excavation proceeded, large steel beams called struts were installed between the walls to counter pressure from the ground and nearby buildings. Once the excavation reached the proper depth, the reinforced concrete roadbed was laid down, the struts removed as the roadbed built up, and the excavation filled in to the surface once the roadway was finished.

Slurry walls were essential to the success of the Central Artery project because the special excavating equipment can work in confined spaces in a dense old city such as Boston (the machinery was first developed in Europe), particularly under the elevated highway where there was no headroom for tall conventional excavators. The walls produced a rigid work area for excavating the tunnel without the need for a much wider conventional trench with sloping sides, which would be impossible in the narrow corridor where the elevated highway stood.

But perhaps the most crucial use of the walls involves the old highway itself. The existing six-lane elevated highway had to remain in service throughout construction even as a wider, eight-to-ten-lane expressway was built directly underneath it. Placement of the decking and excavation of the tunnel required that all of the footings that supported beams holding up the old road be removed. The entire weight of the elevated highway shifted onto new supports resting on the slurry walls of the new highway.

So the slurry walls made construction of the new highway possible in the tight confines of old Boston, and they let the project keep its key commitment to the city, with surface traffic moving on the decking and the walls holding the crowded old highway aloft as construction proceeded below.