Thursday, July 16, 2009

Retroreflectors Everywhere

95th Street on the SW corner
For the longest time I've been wondering what in the world this thing is. If you have been wondering too, now you're going to find out.

It's known as an Automatic Target Recognizing Station, and it's made by the Swiss company Leica Geosystems. This robotic device, and about 8 others just like it, are being used to survey the exact position of buildings and objects in the Second Avenue Subway construction zone.

The unit automatically surveys the area (over and over again) using special retroreflector targets (example shown below) that have been installed in and around on the work zone.

Once a target is acquired, the unit then precisely measures the horizontal direction, elevation angle and distance, with an angle measuring accuracy of 1" of arc, according to what I read in the specifications on the manufacturers website. The data collected can then be compared with a historical set of data to check for any movement (e.g. the wall of a building) over a period of time.

The data from these units will be used to monitor and check for any movement of the older buildings in the work zone -- in particular when the controlled blasting, in the southern end of the launch box, starts.

This is one of the retroreflector targets, up close.

If I had to guess, I'd say that there are about 100 of these targets on the buildings in the launch box work zone at the moment.

A side comment:
Similar retroreflector targets were placed on the Moon during the Apollo 11, 12 and 15 missions. Using this technology, scientists have been able to measure the distance to the Moon to an accuracy of about 3 centimeters (a little more than an inch), over an average distance of 385,000 kilometers (about 239,000 miles.)

94th Street, near the SW corner
Here you can see 2 more of the retroreflector targets.

1830 Second Avenue
A walk-up apartment building, with 10 units, built around 1920.

In this image the location of each retroreflector target is marked with a blue ring.

Left-click on the image for a close-up view.

The green rings mark the location of crack monitors that have been glued to the facade of this building. These gauges will show whether or not a particular crack is moving.

Although it is cutoff in the image, you can see that sidewalk sheds have been put up in front of the building. This was done, I've heard, because at least one large piece of the facade has fallen from the building. (Sidewalk sheds have been erected in the past year in front of almost all of the older buildings in the work zone.)

1814 2nd Avenue
on 94th, about 15 feet from the SE corner
A close-up look at a crack monitor, which is also known as a crack gauge, that has been glued to to the building.

92nd, NE corner
And here we have an image of 1768 Second Avenue / 301 East 92nd Street - a mixed use building, with 31 residential units and 3 commercial establishments.

An emergency Vacate Order was issued by the NYC Department of Buildings (DOB) on 6/29/09 for this building because it was found to be leaning to the north, 18" out of plumb.

Again, the location of each retroreflector target is marked with a blue ring.

The orange rings mark the location of vibration monitoring equipment.

With all of the monitoring equipment that has recently been installed here, one could assume that there is a great deal of concern about the structural integrity of this building.

btw. 91st and 92 - W side of the avenue
This is the robotic unit that is directly across the street from the building shown above.

92nd, NE corner
And this is as close a view as I could get of one of the vibration monitors that has been installed on this building.

301 East 92nd (the building shown above)
I found this taped to the police barrier that has been placed in front of the entrance to this building.

Left-click on the image for a view that is readable.

The paper on the left is an e-mail, from Council Member Daniel Garodnick, to the residents of 1772 Second Avenue -- the other residential building in the work zone that was issued an emergency Vacate Order by the DOB during June. I suppose that it was put up so that the residents of this building will know what Council Member Garodnick is doing to help with the hardship they now face.

What is even more interesting is the copy of the 2-page letter from the Coalition of East Side Elected Officials. This letter was sent by the Coalition on 6/17/09 to Michael Horodniceanu, President of MTA Capital Construction, and Magdi Mossad, The Manhattan Borough Commissioner at the Department of Buildings.

In the letter, the elected officials express concern about the structural stability of the buildings in the launch box work zone, between 92nd and 96th Streets. They also state that the building at 1774 Second Avenue has moved 3 1/2" in the past year and a half.

There's clearly a developing story here, that goes beyond the engineering and construction of a new subway line in New York City. As more details surface, I'll let you know.

Here's a listing of the recent additions
to the right-hand column of The Launch Box

"More Concrete Testers Said to Be Under Investigation"
The New York Times - 7/13/09

Contract Packaging Information
An unofficial listing of
Second Avenue Subway (Phase One) contracts.

Announced Completion Dates
A summary of the completion dates that the MTA has announced,
over the years, for Phase One of the Second Avenue Subway.


Anonymous said...

Looking at the detail photo of the overlap mechanical strain gauge, I'm somewhat surprised that seemingly random wood blocks were used to space it from the surface of the brick wall. Surely one would want to use a spacer block that is less likely to give a false reading if it gets wet, or changes temperature. Those scraps of lumber just can't be dimensionally stable.

Anonymous said...

Another note: I don't think the device pictured is really a "strain gauge" so much as a displacement gauge.

Ben said...

Thanks for the comments.

You're right -- I didn't use the correct label for the device that is being used to monitor the cracks in the building.

I've now relabeled it as a "crack monitor."


Jerrold said...

Today (July 21), Benjamin Kabak tells us on his site about how the anticipated date for completion of "Phase 1" has been moved back to 2016, or possibly 2017.
Maybe you should cross out 2015,like you did with the other years on that list.
I guess the end of the list could look like THIS:


Ben said...


Thanks for the this. I see (now) that the Daily News ran a big article on the 2nd Avenue Subway today too - with the possible new 2017 completion date.

I'll updated the blog this evening with all the latest news.