BandA wrote:...They did freezing in Boston's Fort Point Channel to get through soft mud and under? a subway tunnel...If the tunnel is too shallow, it could be damaged by ships or shifting sediments. If it is too deep, the approaches will be difficult to design.
I'm not sure that Ft. Point Channel -- whatever they did -- is applicable to the Hudson River. About the only thing they have in common is water. Considering factors such as width, depth, the tidal action, the sheer volume of water in the Hudson, they are very different. I'm sure the tunnel will be buried in the river bed, well under the level where it could be struck by passing ships. One hundred years ago the Pennsylvania Railroad did have some concerns that a ship dropping an anchor directly on top of their tunnel might potentially pose some risk. However, being that the tunnels seem to be at all points at least twenty-five deep in the river bottom I don't think this was ever a problem.
Don't forget, the Hudson is a navigable river and thus under the control of the U.S. Coast Guard. So however Amtrak decides to build it's tunnels -- and there is no plan yet that I'm aware of (an EIS doesn't even have funding yet) -- the USCG will have to approve it before they can start building. In fact there are probably a number of federal and state agencies that will have oversight as well.
Ridgefielder wrote:I'm sure it's technically possible. However, aren't the sediments in the North River contaminated w. PCB's and other nastiness? Not sure the EPA/Army Corps/whomever would bless a dredging project of this magnitude if it involved stirring up contamination from the riverbed muck.
Apparently the US EPA and the Corps of Army Engineers have already ruled out this method of tunnel construction in the Hudson for the reasons Ridgefielder mentions: the amount of environmentally hazardous material buried in the river bed. I believe it will have to be a bored tunnel. Just to add, I think this information came from a Parsons Brinckerhoff engineer speaking at a symposium that was held last year. There is nothing printed or on line that I know of so I can't post a link. But I think dredging a bed for the tunnel has been ruled out.