What the $%#@$$#@!

Here's one for the builders out there. Perhaps Mick can explain this to me.
The block at the rear of my place was recently cleared to prepare for what I think will be 4-5 units. After clearing, they dug trenches about 600mm deep, and I presumed they were for the foundations. However, along came a pile driver and proceeded to drive 7 metre long treated pine posts into the ground in what looked like random positions in the trenches. On average they were about 3 metres apart.

Having driven them in to the point where they were level with the top of the trench, they then took a chainsaw and cut them off so there was only about 150mm still showing above ground. They then placed reinforcing mesh in the trenches and poured the concrete. They are now doing the brickwork.

I have never seen this before and I really can't work out what purpose the pine posts serve. Any ideas anyone ? :confused:

so do the posts stick up out of the concrete ? or are they buried in it?

last night the lecturer failed to turn up at an evening class I went to and they showed us videos to keep us quiet til he got there. There was one on waffle pod foundations ... it mentioned that if the design required it then pier foundations should be dug and poured before the main waffle pod foundation concrete was poured. In the video this looked like they dug a deep hole and filled it with concrete - ... maybe the posts are someone doing a cheap and cheerful pier? Do you know if the building surveyor was on site before they poured the concrete?

Ken,

I don't know why they did it near your place but it is a common method for foundations in Holland.

The whole of Amsterdam is built on timber piles which are rammed into the ground as it is built on sand and there is no solid ground.
Whilst the timber is in the ground and not in contact with the air the piles don't rot. BTW the Royal Palace in A'dam ( not a spelling mistake but a common dutch abbreviation :D ) was built over 350 years ago and they used 13655 piles to support it.


Peter.

I have seen this done in an area that is known to be unstable. They used railway tracks in this case and lots of them. They did the same as you indicated by pile driving them down and only having a shorrt peice sticking out then pouring the slab over the top. Do you know if the area has been back filled at an earlier time .

Timber piles are indeed a simple and effective method of providing solid footings in unstable conditions.

These work very much the same as drilled concrete piers, the concrete footing becomes an effective pile cap, and it is easy to determine the bearing pressure by observing the distance the pile travels after each blow of the driver. Because they are not used for uplift situations, it is possible to dowel the ends and keep adding to the depth until the necessary pressure is achieved.

Take away oxygen, and using treated timbers the timber is apparently very stable.

Much cheaper and easier than other methods of providing solid bearing in some situations.

Don't try this without proper engineering advice!

Cheers,

P

Thanks guys. To answer your questions, the posts are cut off and concrete then covers them completely. There was no sign of any building surveyor or anyone else for that matter - just the two guys driving the piles. I don't believe the area has been backfilled, as this is an old established suburb.

I suspect it is a form of support for the foundations, but a strange one as I thought the piles would rot eventually.

Quote:

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Originally Posted by Sturdee

Ken,
they used 13655 piles to support it.Peter.

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Ewww!, wish I was selling those donut-shaped pillows at the time. :eek: :eek:


Yes......I know. :o

Ben ;) :D ;)

Ken,
a prior soil test would have told them that there wasn't sufficient soil strength for a raft slab. Like the others have indicated the timber posts are piles, increasing the bearing capacity through their friction in the soil. Usually when piles are driven the assistant holds a staff (or level) up next to the post with a pencil on top. As the hammer is being hauled up he strikes an arc on the post. This continues, pile gets driven, hammer goes up and pencil mark goes on pile. When they think that it might be driven in far enough they pause and measure the distance between the last set of pencil marks. When there is enough friction the post moves less with each hammer blow. For each job/soil type/buiding mass there will be a pre calculated amount of friction required and thus distance between pencil marks. In the case of concrete piles the tops are jackhammered to expose the steel which is then tied into the steel for the slab or beam which is formed up over the pile. With the timber piles they are simply lopped off and concreted over. Timber fares quite well when buried. There's a boardwalk in the mangroves near Cairns airport that is supported by lengths of pvc pipe driven into the mud. The friction is enough for support. Like Sturdee says most of Amsterdam is built on timber piles driven into the sand and mud of a river delta.

Mick

Thanks, Mick. The interesting thing is that they used a huge auger to drill holes first and then drove them in, and they were all pounded until they were below the level of the trench. One suspects that drilling holes would reduce the friction you speak of. Anyway they are in now and the first 15 courses of bricks are up. Hope they do a good job, because I will buy one if they look ok. ;)

Ken,
could be that there was a hard "crust" which would have splintered a wooden pile if they had tried to drive it through, hence the "pilot" hole. I would say that if there are to be any problems because of faulty engineering or pile driving that they will probably come to light during the warranty period of the building (then you've just got to hope that the company is still around to rectify them :( )

Mick