Well, it’s been a while since I’ve written. This is due, in part, to my distaste for cold weather. But also, work on the bridge has been largely repetitious over the winter. Most of the drilled shafts have been completed, and a good deal of progress has been made on the casting of the tower segments. We’ll take a look at all of this today.
First, though, I’d like to go back to December 9 for a moment. This was a historic day for the old Sarah Mildred Long Bridge. As you probably know, beneath the road deck, there is a railroad deck. The railroad was quite active until the line between Kittery and South Berwick was pulled up in 1952, and since then has only served the Portsmouth Naval Shipyard (which, for anyone who isn’t local, is actually in Kittery).
Most materials are trucked into and out of the shipyard these days, but the railroad remains the only means of moving nuclear materials off of the island. This primarily consists of spent nuclear fuel (SNF), bound for the Navy’s processing facility in Idaho. SNF is moved in special railcars that each carry a large, reinforced flask, and the trains containing them typically have have caboose carrying guards on the tail end. It’s a sight to see, and I never miss the opportunity to get some photos.
For the past decade or so, rail traffic to Kittery has been fairly predictable. A train brings some empty cars over, they get loaded, and another train retrieves them a couple weeks later. What was special about the outbound move on December 9 was that it was the last train that will ever cross this bridge. The photo below was taken from the NH State Pier, as the 650-ton train crawls across at walking speed with her radioactive payload.
More photos of the inbound and outbound moves can be found on my Flickr page:
https://flic.kr/s/aHskpMH3o1
https://flic.kr/s/aHskniQPDi
First, though, I’d like to go back to December 9 for a moment. This was a historic day for the old Sarah Mildred Long Bridge. As you probably know, beneath the road deck, there is a railroad deck. The railroad was quite active until the line between Kittery and South Berwick was pulled up in 1952, and since then has only served the Portsmouth Naval Shipyard (which, for anyone who isn’t local, is actually in Kittery).
Most materials are trucked into and out of the shipyard these days, but the railroad remains the only means of moving nuclear materials off of the island. This primarily consists of spent nuclear fuel (SNF), bound for the Navy’s processing facility in Idaho. SNF is moved in special railcars that each carry a large, reinforced flask, and the trains containing them typically have have caboose carrying guards on the tail end. It’s a sight to see, and I never miss the opportunity to get some photos.
For the past decade or so, rail traffic to Kittery has been fairly predictable. A train brings some empty cars over, they get loaded, and another train retrieves them a couple weeks later. What was special about the outbound move on December 9 was that it was the last train that will ever cross this bridge. The photo below was taken from the NH State Pier, as the 650-ton train crawls across at walking speed with her radioactive payload.
More photos of the inbound and outbound moves can be found on my Flickr page:
https://flic.kr/s/aHskpMH3o1
https://flic.kr/s/aHskniQPDi
Fast forward to late March. We’ll start by looking at the progress on the tower segment casting. About 1/3 of the 84 segments (21 per tower) are complete, and have been moved to the precast yard, seen here from atop the pile of salt that didn’t get used this season.
As you may remember from past posts, the large yellow structure visible from Market Street is used to cast these segments. Match casting allows for each segment to be cast directly on top of the one that will be below it in the finished tower. A segment is cast, the one below it removed, the new one dropped into place, and the next cast on top of that. It ensures that they mate perfectly when they are stacked 21-high to create the four lift towers.
The structure has been enclosed for the winter, so it’s a bit hard to see what’s going on. Here’s a look inside. You can see a completed segment, resting just above ground level. Note the red hydraulic jack at the center of the photo. These are on each corner, and allow the segment to be leveled before casting on top of it.
The structure has been enclosed for the winter, so it’s a bit hard to see what’s going on. Here’s a look inside. You can see a completed segment, resting just above ground level. Note the red hydraulic jack at the center of the photo. These are on each corner, and allow the segment to be leveled before casting on top of it.
Next, we’ll climb up to the top, where the forms have been assembled, and a concrete pour is imminent (it’s taken me so long to write this up that this segment is actually now finished). In the second photo below, we are looking down into the void between the forms that will be filled with concrete. There are a couple of things to note here.
The plastic tubes that run from the top to the bottom create voids in the concrete to accommodate the rod and tendons that will eventually hold the tower together. During assembly (stacking) of the towers, threaded hardened steel rods are inserted two segments deep, and nuts and washers are threaded on and tightened to hold the segments together. Once the entire tower is assemblies, steel tendons (essentially cables) are threaded through sets of tubes that run the entire height of the tower. These are tensioned, and then the void around them is filled with concrete to make them permanent. The temporary rods are also grouted in place, though they are no longer structurally significant.
The other interesting feature is the horizontal steel plate that is visible running along the top of the opening. This is convex on the bottom, and creates a trough in the top of the casting. We’ll look at that below.
The plastic tubes that run from the top to the bottom create voids in the concrete to accommodate the rod and tendons that will eventually hold the tower together. During assembly (stacking) of the towers, threaded hardened steel rods are inserted two segments deep, and nuts and washers are threaded on and tightened to hold the segments together. Once the entire tower is assemblies, steel tendons (essentially cables) are threaded through sets of tubes that run the entire height of the tower. These are tensioned, and then the void around them is filled with concrete to make them permanent. The temporary rods are also grouted in place, though they are no longer structurally significant.
The other interesting feature is the horizontal steel plate that is visible running along the top of the opening. This is convex on the bottom, and creates a trough in the top of the casting. We’ll look at that below.
The first photo below shows the top of a completed segment. You can see the troughs created by the plates seen in the photo above, and the tops of the plastic tubes. The tube on the right has a recess around it, as this will be for one of the temporary rods, and space is needed for the washer and nut. The tube on the left is for a tendon that will run continuously through all of the segments in the tower. As such, it is flush with the top of the casting.
The second photo below shows the bottom of a segment, and you can see the feature that is created by the trough below. When the segments are stacked, these will ensure horizontal alignment and increase horizontal stability. Like Legos.
The second photo below shows the bottom of a segment, and you can see the feature that is created by the trough below. When the segments are stacked, these will ensure horizontal alignment and increase horizontal stability. Like Legos.
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Let’s move now over to the river. At the time of my visit, there were still three drilled shafts to be completed. Below is a view of the future location of the Portsmouth side lift towers. There will be eight piers here, a large concrete “tub” will be placed on top of them. When it’s complete, it will look from above the water like a single pier (much like the old Sarah Long Bridge or the Memorial Bridge), but if you were to swim down a couple of feet, you would see that it’s actually eight separate piers. In the photo, you can see five of them. The steel tubes visible in the photo were used to guide and support the drill, but the top of the concrete inside only reaches the waterline.
If we look across to the Kittery side (below), it’s a bit more apparent how the eight piers are laid out. Six of them are arranged like a 6-pack of beer, and then there’s one on each end, each centered to form a point. The drill is seen perched atop the pier at the down-river point.
On the Kittery side, one of the piers is complete – the first of many. This is one of the railroad piers. Because of the weight of the trains that will cross the bridge, the railroad spans are half the length of the road spans, and require intermediate piers. This first completed pier is one of the railroad piers on the Kittery side. The first photo shows it close-up, and the second is the view along the alignment of the bridge. The two drilled shafts in the foreground will straddle the railroad, and support both it and the road above.
The steel tubes are cut away to just below the low water level, and remain in place permanently below that. Though exposed to salt water, they are engineered to endure for the 100-year lifespan of the bridge.
The steel tubes are cut away to just below the low water level, and remain in place permanently below that. Though exposed to salt water, they are engineered to endure for the 100-year lifespan of the bridge.
On the Portsmouth side, there are three piers that didn’t require drilling, and were constructed inside of cofferdams. They are thus known as the cofferdam piers. These won’t carry the railroad, as it curves off toward downtown before the bridge reaches this far. The photo below shows the most northern of these piers, which is very nearly complete, aside from some work around its base.
The center of the three, PV2, is still under construction. Below are two view down into the cofferdam. The rebar seen will form the base and pedestal.
Meanwhile, on the temporary trestle, the rebar cages are being assembled that will form the pier itself (actually two parallel piers).
Here’s a view down the bridge alignment showing the two completed cofferdam piers, and the one under construction in the center. If you were to drive by today, you’d see that the forms are now assembled at this location (between Market Street and the railroad track).
I wrote a bit about the Wirth drill in a previous post. What I didn’t explain was how the material that is drilled out (ground up stone) is removed from the shaft. The answer is that water is continuously pumped to the drill bit, which creates a slurry (water + stone) that is then pumped back out and into a barge. The photos below show to the two hoses (slurry in, water out) running into the barge, and a view down into it. The loader operating at the bottom of the barge is used to balance the load, since all of the material gets concentrated at one end.
That’s the news for now! Stay tuned.
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