It began as a typical reconstruction project along New York City’s Sixth Avenue from 56th Street to Central Park South. Then, a problem was uncovered. A series of sink holes and pavement settlements had developed in the roadway and sidewalks of that area over the past 10 years, causing it to be unstable. All but one of the holes ranged in size from that of a basketball to being large enough to swallow up a car.
How large was that one hole?
“There was one hole buried below 57th Street and Sixth Avenue in the southeast corner 3 ft. below grade that had actually been a room at one time,” recalled Anthony Santoro, vice president of Trocom Construction Corporation, contractor for the $7.2-million project. “It was 8 ft. high, 10 to 12 ft. wide and about 100 ft. long. Part of an old structure belonging to the Transit Authority (TA) or some other facility, it had been there for a number of years, unknown to anyone. We had to fill that along with the others.”
The work, which began in April, presented several other challenges for Trocom, the first of which was determining the best manner of remediation for filling the voids. The contract contained a number of different methods available but ultimately it was up to the contractor to select the most effective.
“The contract called for conducting exploratory excavation to a depth of five feet and locating any existing utilities [electric, telephone, gas and public utilities],” explained Santoro. “Then, using a boring method, we were to drill 4-in. boring holes throughout the job site, develop a profile and get a sense of the nature of the sub-surface material. They [New York City Department of Design and Construction (DDC)] also wanted us to use a video camera to investigate the voids. But the final choice was ours. Trocom always felt the flowable grout was the only method to use for filling the voids. With all of the utilities, deep excavation was out of the question.”
The method employed used a flowable fill with no pressure that was placed through a 4-in., in-ground PVC pipe. According to Santoro, the boring method was revised on a daily, sometimes hourly, basis. “The methods specified in the contract were inadequate for the steel conditions. A standard specification called for being employed with a moist soil, not one for miscellaneous backfill material and shock rock. We had to come up with alternate ways of drilling through the material.”
The flowable fill was a mixture of sand, cement, ply ash and bentonite. Over time, a series of additives were added. After being mixed, the fill flowed from the back of a truck, through a bin into the 4-in. boring holes. The fill would seek its own level, filling any void it encountered. In some areas there were a series of small voids due to the layers of shot rock.
“We filled all of the holes with as much grout as they would take. In the end, we flowed 3,000 cu. yds. into the ground. That got us to the remediation process but the initial phase of the project was trial and error,” said Santoro. “We had to determine what worked in terms of borings, grout, filling holes, even getting the city to pull away from the idea of deep excavation.”
Selecting the proper equipment to drill the borings also presented a challenge for Trocom. Once again, it was a matter of trial and error, with a series of rigs being brought to the site, including professional augers, air tracks, air rotary and sonic drills.
“We tried a few different bits and machines to advance the holes but those are the machines we decided on. They would do the job and not impact the subway structure or the buildings. There was an issue about vibration and settlement. Any equipment used had to be low vibration, low impact. It had to be of such a nature that it wouldn’t damage any nearby facilities,” reported Santoro.
In contrast, the equipment used for the grouting was considered low-tech. Concrete drums were set up with hoses attached to them. The grout was fed into the drums, which in turn fed through a hose into the PVC pipe that had been left underground. The grout would flow into those pipes until reaching full capacity.
Another challenge came from the volume of utility interference found in the form of steam and utility systems that were high and in advanced stages of deterioration. As a result they needed to be replaced or upgraded.
“The contract indicates the presence of utilities, but the nature of the work and the conditions are unclear. That is all speculative at the time of bid,” explained Santoro. “Once you start exposing facilities and moving things around, grades change and you can see how other facilities are impacted. As it turned out, there was a great deal of asbestos located in ConEdison’s steam facilities. That significantly impacted the project. We had to dust the system with air forces and remove the asbestos in a satisfactory manner.”
Although the work performed cannot guarantee the problem won’t reoccur, that possibility has been significantly reduced. “We could only drill where we could place holes. Because of Transit Authority and steam facilities, private utilities and other underground structures, we couldn’t always drill in every location in any kind of pattern. We drilled in those areas that were available as a result of our pre-excavation so we may not have hit every location. Will this material wash out again? It’s possible but it shouldn’t be a problem for a long time,” said Santoro.
Trocom also replaced a water main; replaced the roadway with reinforced concrete consistent with the balance of Sixth Avenue; and installed new granite curbs, sidewalks and street lighting.
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