Tuesday, June 3, 2008

Building a bridge... 4

We are still talking about installing the piles that form the foundation for this bridge. I know, it seems like it's taking forever, doesn't it? Trust me - I know! It truly is taking forever! But, like any good craftsman/woman knows, a good setup makes the final job a better product. I have to hand it to my piling contractor - they take their time to get a good setup nailed before applying hammer to pile.

Speaking of hammers, let's take a look at one:What you can see here is a diesel hammer and the pile driving leads. When in operation, the hammer rides up and down on the upper steel members of the leads. The purpose of the leads is to hold the hammer in position as it works. In addition, they are used to control and align the pile while it is being driven.

A diesel hammer is a pretty simple piece of kit. The functional parts are the helmet, the cylinder, the piston, the mechanism for spraying diesel into the chamber, and the exhaust. The helmet is the piece that actually rests on the top of the steel pile. It looks like an over-sized bottle cap. The pile fits inside the helmet. In this way, the pile is kept in place and is unable to fall over if there is no other support in place for the pile. However, this contractor has some serious support, as you will see in a moment or three. The cylinder assembly rests on top of the helmet. The cylinder is a heavy duty steel jacket that contains the combustion chamber. Picture taking an diesel engine, removing a single cylinder with the piston, and then super-sizing it. The piston doubles as the actual hammer. On this unit it weighs around 1800 kg. The whole assembly is around 20' / 6 m long, and with the leads it weighs roughly 9,000 kg.

How does it work? Glad you asked. Once the leads with the hammer are in place over the pile, the cylinder is manually raised up and mostly out of the cylinder (about 50% stays inside the unit. The method for this varies, either the crane lifts it or a hydraulic assembly internal to the hammer is used to raise it. Once at the extent of normal travel, the hammer is tripped and it falls to the bottom of the cylinder. As it reaches the bottom, a spray of diesel is injected into the chamber. The air is superheated by the compression, which then ignites the diesel, which fires sending the piston up and out. The process then repeats until it is stopped. The only difference between this and the diesel engine in my Jetta is that the cylinder head and the piston in my car don't make contact. When the piston hits the bottom of the cylinder in the hammer, the force of that impact is transmitted directly to the pile. This is where the "hammer" part of the name comes into effect. More on the actual hammering in a future post.

Now, we were talking about the setup for pile driving. In this shot, taken at max zoom from my little camera from across the river, you can see the contractor setting up temporary piles. These piles will be used as supports for the cross members of the pile driving frame. The piles are being installed with a vibro-hammer. Instead of a hammer actually hitting the piles, this grasps the top of the piles in a clamp. A hydraulically actuated eccentric weight in the hammer then rotates at a good rate. This has the effect of making rapid small impacts on the pile, vibrating it and the ground below, vibrating it into the ground. If you've ever seen the effect on granular material (silt/sand/gravel) when a vibration is applied to it - this is the same thing, up-sized.

So, I was mentioning the driving frame. Earlier you saw it getting prepared for install. Here you can see it in place, with a couple of piles stood up in it. They are resting on the ground. About 34' of pile is hanging in the wind above the frame. Even with that, they are held securely in place prior to the commencement of driving operations. Our friend the welder is working on the angle iron brackets that hold the piles from moving laterally during driving. The advantage of this sort of system is that there is very little movement of the piles, and their position can be managed quite effectively. How accurate do they need to be? Well, our friends in the Alberta government allow a tolerance of +/- 50 mm. Yes, they allow the piles to be out of position in the horizontal plane by a total just shy of 2 whole inches. This would be part of the reasoning for getting a surveyor to locate the pile locations. You don't want them in the wrong place!

1 comment:

JB said...

Hey Spit, love the 'Building a Bridge' posts. I'm learneding! :D

Keep 'em coming!