A stone on stone cathedral, designed to have a 445 ft. stone tower, needs an awesome foundation. Considering the difficulties encountered as excavation began, the solution turned out to create artificial bedrock. Unlike the site of St. Luke’s Hospital across the street where solid rock lay a few feet below the surface, there were swales and striations of rock between pockets of soft shale and decomposed rock. The foundations of the Cathedral would need an engineering solution.
Irregular masses of stone, like waves interspersed with pockets of shale and clay lead to solid bedrock at depths of 20 to 45 feet below grade. They decided to concentrate excavation to the areas of the primary piers (shown in the shaded areas of the first image of this post) down to solid rock which varied between 15 and 45 ft below grade. A uniform mass of concrete filled the void.
These pits and their surrounding areas rose to elevation 100 feet above sea level, the floor level of the crypt. The setting of the granite piers would begin at that elevation. One of the pits, meant for the four large piers that would hold up the arches and subsequently the tower, reached a depth of 40 feet without hitting bedrock. Work stopped for several months contemplating a solution which included moving the Cathedral. Eventually, they excavated five more feet and bedrock showed up. Heins and Lafarge ordered core samples of 20 ft. below that level and the core showed solid rock.
The Concrete Process
The solid rock was leveled where needed and the faces roughly dressed, thoroughly cleaned with wire brushes, and washed with hoses and brooms. The rock was allowed to dry and small drippings were removed with sponges. The concrete consists of one part Portland cement, two parts sharp sand and three parts gravel (quartz gravel 1.5 to 2 in). This was mixed quite dry and was rammed by 20 lb. rammers from 10 in. layers down to 8 in. layers. When work resumed the next day the top surface received a plastered mortar of one part cement and two parts sand. The next course commenced in the same fashion. The process continued up to the desired elevation.
The magnitude of this job beginning in April 1895 was staggering with over 200 men working on it. Six steam-operated derricks and a central narrow gauge rail track centered in the area delivered the concrete bins from the two mixing locations. By August 1895 11,000 cubic yards of concrete had been laid with another 2,000 cubic yards to go.
The Cut Granite Piers Begin
Once the concrete foundations for the Cathedral reach the correct elevation, the cut granite blocks for the piers begin to arrive.
J.D. Crimmins and J.J. Hopper. were the earth and rock excavation contractors. John Peirce was the dimensional granite contractor. General William Sooy Smith of Chicago acted as the consulting engineer of the Cathedral. Sooysmith & Co., contracting engineers of New York performed the concrete work described here. Below, all the granite piers have risen to their complete height.
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- Its Foundations Rising, The New York Times, August 13, 1895
- Construction of the Foundations of the Cathedral of St. John the Divine, The Engineering Record, C. 1, v. 32, August, 10, 1895
- Cathedral Builders Puzzled, The New York Times, September 10, 1893