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Tough challenge below the streets of Manhattan

July 9, 2015

It has been described as the most complex subway project in the world in the heart of one of the most densely populated cities on the planet. New York’s long-awaited Second Avenue extension is all that and more.

Manhattan working crew

New York’s long awaited Second Avenue subway extension is now well under way on Manhattan’s East Side and will be a welcome addition to the city’s mass transit system.

The first decision to build the new line was taken as long ago as 1920 and ever since then the project has been repeatedly postponed due to a long series of events ranging from The Great Depression through World War II to a prolonged series of financial and political obstacles.

In recent years however, renewed efforts to get the project back on track have succeede­d and New Yorkers will finally get their much needed subway – although not before 2019.

The new line under Second Avenue will ultimately be 8.5 km long, running from 125th Street in uptown Manhattan all the way down to Hanover Square in the financial district. Along the way, there will be three new stations – at 96th Street, 86th Street and 72nd Street – plus several branches linking to other sections of the city’s transport systems.

All in all, the project will cost a stagger­ing USD 17 billion, but considering the relief it will bring (more than 4.3 million people ride the NY subway every day), most people agree that it will be money well spent.

First phase on track

Among the many contractors engaged in the project is Skanska USA and Traylor Brothers who have teamed up to excavate two construction shafts, North and South, and a rock cavern in between which will eventually house the new 86th Street station.

For the drilling operations, the team is using state-of-the-art equipment from Atlas Copco. This includes a FlexiROC T30 R surface rig (formerly known as ROC D3), two Boomer E2 C tunneling rigs and shotcreting equipment from Atlas Copco MEYCO.

The work began with the sinking of a 10 m x 7 m shaft where the FlexiROC T30 was used to great advantage. Thanks to the rig’s compactness, the operator was able to rotate the rig within the extremely confined space to drill the 45 mm blast holes. In addition, the excellent boom coverage meant that the rig did not have to be frequently moved on the benches.

Dust was a major issue, but the contractors kept it well under control with the help of the FlexiROC’s dust control unit as well as blast mats and steel curtains to contain the flyrock and constant water spraying to dampen down the work areas.

Another challenge was to underpin the high rise building on the east corner of Second Avenue and 83rd Street. As a part of this building is directly above the future portal to the 86th Street Station, it had to be underpinned before excavation of the construction and escalator shafts could begin.

Tom O’Rourke, Skanska’s Project Manager, said: “It was very close work at times. We had to watch the overhead utilities and surround everything in blast mats. Some blasts were as small as two meters by two meters in diameter.”

After drilling its holes, the FlexiROC rig was hoisted from the shaft for blasting and then, after mucking out and scaling of the walls, it was lowered back into the hole to start drilling the next round.

Drilling the crown

Once at the bottom of the shaft, the next challenge was to start drilling the crown of the cavern with the Boomer E2C tunneling rig. However, there was not enough room to set it up at the right angle and a second Atlas Copco rig, a Boomer T1 D, was brought in to open up the shaft’s lower level in order to expose the top heading of the cavern.

The Boomer T1 D is normally used in narrow-vein mining applications and is compact and versatile. It has a carrier length of 4.8 m and a boom length of 4 m with the BMH 2825 feed system. It can also be used with the BUT 4B heavy duty boom system which provides a 900 mm extension and a 1 500 mm feed extension. Furthermore, the feed rollover is a full 360 degrees with a boom swing angle of 30 degrees which allowed good maneuverability.

The rig was used to drill short cuts at 90 degrees into the shaft wall and it took several rounds before the area was big enough for the Boomer E2 C. In addition, the shaft floor had to be lowered by about 3 m, and slightly angled, to enable the boom to reach the cavern’s top heading. After blasting, the Boomer E2 C could finally be lowered into position.

The cavern is located under Second Avenue with the crown only 12 m below street level and with an overhead rock cover of approximately 9 m. It will be 286 m long and has ancillary sections at each end, 74 m and 88 m long, with a so-called Public Cavern in between, 124 m long. The ancillar­y caverns were excavated by top heading, intermediate bench and bottom bench.

The Public Cavern, being 3.6 m lower at the crown, was excavated as a top heading and bottom bench only. The top headings were split into a center pilot and two side slashes which were followed by the bench excavation.

The top heading center pilots were 7.3 m high and the total width of the cavern is 21 m. Two Boomer E2 C rigs were used, one operating from the North shaft and the other from the South Shaft and mucking was carried out by wheel loaders bringing the muck from the face to the shaft area and thereafter hoisted in boxes up to street level.

Squeezing in the Boomer

The Boomer E2 C was used to drill most of the blast holes in order to remove 140 000 m3 rock (108 000 m3 the cavern with a further 32 000 m3 from the escalator tunnels and all other underground excavations) and was also used for drilling bolt holes.

But calculating how much space was needed in the shaft to get the Boomer in place presented the biggest challenge. Kip McCalla, Atlas Copco Area Sales Manager, says: “Having the dimensions of the rig and hole were not enough. We needed to know how the rig would react when articulating the booms in the shaft.”

Joe Mela, Atlas Copco Area Salesman, was on site when the Boomer E2 C made its descent. “It was very, very tight and seemed like there was barely a coat of paint to spare,” he says. “The crew really showed their expertise in getting the rig into position.”

Lars Jennemyr, Skanska Director of Underground Tunneling Operations, shared the sentiment: “We knew the Boomer would be able to drill within the area but making that happen was critical. By sending our crews to Atlas Copco’s Clarks Summit location, they were able to practice the maneuver repeatedly with the Boomer to get the routine down.”

Jennemyr explains: “When the booms travel up and down vertically, they spread wider from the center. By repeating this action and studying the booms’ movements the crews were better able to understand what would happen in the shaft.”

Once the Boomer E2 C was installed in the tunnel, face drilling advanced steadily and on schedule throughout the drilling part of the project,” said O’Rourke.

The cavern advanced on three faces starting with the top heading, then the bench drilling. The center face of the top heading was drilled up to 7.3 m wide at the crown and 5.5 m to 6 m high on each side. Some 120 to 150 holes were drilled for the center and 70 to 90 holes for the sides. The tunnel face advanced 2.5 to 3.5 m with each round.

The rock here is competent Manhattan schist and granite with wide fracturing and is easily drilled. Typically, the drilling progresse­d at the rate of 3 m/min.

More than 210 people were on site during M&C’s visit. Boomer operator Sean Keeffe has worked on several different Boomer models and said he especially likes the controls of the Boomer E2 C. “I think the controls are smoother and the Rig Control System makes it really easy to drill the pattern,” he said.

The process for each drill pattern started with surveyor Paul Stogner. Skanska Superintendant John Kierman explained: “Paul navigated the entire cavern. He would set up the transom and line up the first hole. It would go pretty fast from there.”

Stogner said: “It takes about two minutes for me to set up and locate the first hole. The driller would line up the rig and set up on the first hole and drill the pattern according to the computer’s direction.”

Drill operator Kevin Mari has worked on three projects in New York City using the Boomer E2 C. He says he appreciates the BUT 45 boom rotation device which allows 190 degrees of rotation in both directions. “I like having the steel on top because it’s easier to see and line up,” he says.

Atlas Copco made a variety of consignment bits, steel and adapters available so that the crew had access to whatever they needed. The primary bit used was a 48 mm T32 flat face ballistic bit, although a range of sizes were available for the various drills and formation transitions.

The drill steel used was R32 × T38 Hex rod with a 57 mm coupler and R32 × T38 round rod. Both were available in 3 – 4.8 m lengths with round rod in 5.5 and 6 m lengths. A shank adapter for the T38 was necessary for the COP 1838 rock drills used on both the FlexiROC T30 and the Boomer E2 C drill rigs.

Round the clock schedule

The engineers worked round the clock, five days a week in three shifts – the first shift for drilling, charging and blasting, the second for mucking and scaling and the third for bolting, shotcreting and initiating the next cycle.

O’Rourke said: “The same process was carried out with each slash; drill, blast, muck, bolt, shotcrete. For various reasons, the plan wasn’t always followed but we tried to keep it consistent. It was an intense five-day schedule. If we were drilling on one face, we’d be mucking on another and loading on the third.”

The plan was originally set up to drill benches with horizontal blast holes, but once they tried benching vertically with success, that’s the way the project continued with both the FlexiROC and the Boomer. The benches were blasted in 4–5 m rounds and the muck from the first bench dropped into the existing TBM tunnels from where it was hoisted to the surface in connection with the excavation of the second bench.

The MEYCO factor

Rock stabilization was carried out with 6 m resin rock bolts above the springline. Working from the rigs’ service platforms, the bolts were placed in a 1.8 m pattern and pre-stressed to 133.5 kN. Six meter dowels were installed below the springline in a grid pattern of 1.8 m vertical by 3.5 m horizontal.

Shotcreting was done in three phases using shotcrete and equipment from Atlas Copco MEYCO. Before bolting, the specifications called for a layer of steel fiber reinforced shotcrete with a minimum thickness of 50 mm. After bolting, a further layer of 100–150 mm of steel fiber reinforced shotcrete was applied. Lastly, a smooth coating without fiber and with a minimum thickness of 25 mm was applied to cover the fibers before the PVC water membrane was installed.

Gary Almeraris, Project Executive Manager, explained: “The key to shotcreting is high quality, not high quantity of the mix. We used 400 bar steel fiber reinforced shotcrete with a superplasticizer. An accelerator was added to give rapid support in just 10 hours and full support after 28 days. Every day, 60 to 90 m3 was sprayed using two Suprema shotcrete pumps and two Potenza spraying robots.

Almeraris commented: “With the MEYCO we could spray 18 cubic meters an hour with the guys in the cavern communicating with the guys on the surface by radio.” A tender on the Suprema pump unit on the surface controlled the ready-mix trucks so that the Suprema operators knew when the material was ready. “We applied shotcrete 80 to 100 mm at a time with a minimum of 175 mm overall. One layer could go on right after the other, and there was not really any waitin­g. The MEYCO equipment worked really well.”

Throughout the project an Atlas Copco service technician was on site, day and night – Jim Mattila on the day shift and Scott Streichenwein on the night shift. Almeraris said: “The support we received from Atlas Copco was unbelievable. They were with us all along the way. Whenever we needed them, they were there – really part of the team.”

On schedule

The project has continued on schedule with the removal of the muck from the downtown streets being the most complicated part of the project. Each day between 7 am and 10 pm a total of 40–60 muck boxes were hoisted up the shafts to the surface and loaded onto dump trucks to be hauled away.

Much of the drilling and blasting on this section is now complete and in a few months the cavern will be down to the final invert level. Then the construction of the cast-in-place concrete lining of invert, walls and arch concreting will begin. Lastly, the escalator and adit tunnels will be completed and the project will be handed over to the city’s Metro Transit Authority by September 2014 for track laying, mechanical and electrical installations.

This first section of the new line, between from 96th St. and 63rd St., is due to be opened in December 2016 and carry some 200 000 passengers per day.

Operational excellence North America Ergonomics FlexiROC T30 R surface rig Boomer E2 C tunneling rig Rock drills Productivity MEYCO shotcreting equipment Value creation USA Construction