Infrastructure Projects - Construction Technology
A Practical Guide to Construction Techniques and Practice
Here is an excellent photo of a secant pile wall. You can see the carefully formed cylinders of concrete. You can see how they are plumb and aligned and interlock one with another and essentially form a continuous, almost monolithic wall. The area is below the water table. There is no seepage. The excavation is perfectly dry. So it is clear and obvious what an effective system this is and how strong and robust the system is.
Let me just go through some of the features. This is the exposed H pile that was cast into the secondary hole. The concrete in this area has been removed in order to get at the H piles and install rock anchors. This is a very nice illustration of a pair of rock anchors. It's one on either side of the H pile, which is a nice way to do this. Rock anchors, as I've said before, are extremely helpful, because they freed the sight of any pipe struts or any bracing and they give you this unobstructed area to construct your new structure.
Also, the rock is very high. This is the exposed rock and this is the exposed rock. You don't have to go very far in order to be in rock. In other words, this rock anchor is not very deep. It doesn't need much penetration in the rock in order to achieve a high capacity. By contrast, if you wanted to install rock anchors way up here to this upper whaler that rock anchor would have to travel this whole distance to get down in the rock.
What you see here is a carefully thought-out plan. We're at the upper level. This whaler is supported by a conventional brace that goes across the excavation, ten there's a system of whalers at a somewhat lower level. They also have braces that go across the excavation. Then, beyond that, it's a free-standing wall until you get down to the lowest level, which is supported by these rock anchors. This is not unusual to find this combination of techniques; The Rock anchors, because they're down at the bottom close to rock. They're relatively short and less costly. Up at the higher elevations, conventional whaler - and cross lot bracing is perfectly appropriate. This is a carefully thought-out solution, which combines the best features of the different techniques.
I'd like to talk about production rates. We have been looking at two different projects, and I would say the average production rate one secant pile per day. The piles varied in length in the original project. The one where we saw the videos, they were about 60 feet long and one pile per day, I think, is a good rate that you can use. The piles are approximately three feet in diameter, but because of the overlap, they don't develop three feet of wall. It might be more like two and a half feet of wall,
A good plug number to use for a cost would be $150 per square foot of secant pile wall (2009). Now, the piles illustrated here are supported on rock. The rock is very high and the piles are actually perched on the rock. When you see a condition like this, the tip of that pile has to be anchored into the rock and the cost and the time consumed developing that anchorage detail are significant. When I say that you can accomplish one secant pile per day. That does not include any extra work that would be involved, such as the case here, where you would have to anchor or somehow connect the tip of the pile to the bedrock.
This is another illustration on the right hand side. You have a wall constructed of secant piles. You can see the upper level of bracing. Here's the whaler and the cross lot braces are these pipe struts. Somewhat unusual, the left hand side actually braces against an existing subway tunnel and this is a live tunnel with traffic in it. But by Distributing the loads and bracing against, in this instance, the roof of the tunnel and down here, the invert of the tunnel. This can be very safely done, and it was done very effectively here.
I want to zero in on this area and talk about load transfer. As you excavate down, the load comes on the support system, whatever it is and the support system is likely to move a small amount as the load comes onto it. The whalers are likely to deflect. The pipe struts can compress. A certain amount of movement is unavoidable. To overcome this, you actually preload the strut. So there's a little shelf here. You can see this shelf and resting on top of the shelf is a hydraulic jack. You place a jack on this and then you jack against the whaler.
By jacking against the whaler, you transfer the load into the strut and take out any sloppiness that occurs. You now pushing the whaler hard up against the support wall. You're compressing the strut. You're taking the deflection out of the whaler. You do this by means of a pair of these jacks. There's obviously one on the other side of the pipe. When you finish doing this, you will have likely created a little gap and I hope you can see that there is a plate in here, which has been inserted to fill that gap.
You preload the strut, and then you shim out that space. You drive wedges, if necessary. You weld it. You do what you need to do in order to lock the load into the strut. This is a typical method for deep excavations. It's not at all exclusive to secant pile walls, but the need to do this arises in any system you're using, where you want to preclude any movement to the side wall. Of course, that's always extremely desirable.