Thursday, August 26, 2010

TBM Now Mining Faster Than Planned

This just in --

The TBM under Second Avenue mined a total of 92.74 feet (in single day) yesterday, which I believe is the maximum achieved production on this project to date.

This single-day production level is greater than the 50 feet/day estimate that MTA Capital Construction is using for planning purposes, and I understand that it's even further than the 65 feet/day that the contractors (S3 Tunnel Constructors) said would be possible when they bid the job.

The machine has now mined a total of 1,760 feet of TBM Run No. 1 (i.e. the west tunnel) as of yesterday - which by my calculations would put it at about 85th Street.

Next stop (for this TBM), is 65th Street.

A Footnote:
One naturally might wonder how it is that they know the machine moved exactly 92.74 feet on Wednesday.

The answer... They measure the distance by shooting an invisible infrared laser, from something called a Total Station, at a pair of retroreflector prisms that are mounted on the machine's head. The Total Station calculates the distance using an optical technique known as phase measurement. The data from the Total Station is then fed to a computer inside the cab of the TBM, so the operator knows exactly where he is at all times.

I'm told that this method of measuring distance is accurate to about 1/1000 of a foot (which is 0.012 inches).

1 comment:

Joshua said...

Maybe this is why they put the average rate at 50 ft/day knowing that they would start out with slow 10 ft/day days at the start...I can't remember when they started the actual TBM, but if you assume they've been mining for 50 days and have mined 1760 feet, then they're averaging 35.2 ft/day and with each day higher than that average, it's bringing it closer to 50 that they estimated. Now, if they can continue to stay at 90/day for the next few months until they reach 65th street, we might be able to say that they actually beat expectations, including our own.