Determing required foundation type

sas

What is the prefered method for determing what type of foundation is required for a one off house e.g. strip, raft, piled?

We have planning permission on a site which to appears to be very boggy land. A build started a few months ago on a site 40 metres from mine and they did have to use a raft. I realise that I could have solid ground 2 feet down and hence would get away with regular strips. However lets assume it isn't.

The reason I ask is the architect (technician) is part of a small firm. He suggested we get some trial holes dug at various points on the footprint of the house.

My wife happened to be talking to the head guy in the firm and he reckoned we should dig out the full foundations for a strip and then decide if the ground is good enough.

Who's right? What's the best way to go about it?

I'm (most likely) going the direct labour route and hence the question.

Thanks
SAS

smashey

Get advice from a structural engeneer.

muffler

The boss man is correct. Trial holes are a good idea to the extent that they give you an idea in advance of what you are going to encounter. Normally you would only do this before agreeing to buy the site. But more often than not they are not dug along the line of the walls and thus cause problems when you finally get round to excavating the trenches.

Start digging for a normal strip foundation and if you encounter soil weakness then you will have to look at the possibility of a raft foundation. Either way you will need, as smashey said, a structural engineer at that stage anyhow. If the ground is solid then you have avoided the cost and time of digging trial holes.

Even if you hit "the hard" at 400 - 500mm keep digging to a depth of 900m from undisturbed ground level

Johnniep

Kinda unusual but I am going to disagree with muffler :eek:

I would ALWAYS recommend that you dig trial holes on site to determine the underlying ground conditions. It is vital to design the foundations of a building to suit the site conditions as if you get them wrong, it is very hard to rectify. The cost of renting a mini-digger and getting a Structural Engineer to look at the ground is well worth it (I know people find it hard to justify the money paid for someone to look at a hole in the ground!!!)

If the ground has suitable bearing capacity and is the same over the plan are of the site then strip footings would be the way to go. However, if the ground is poor or varies over the plan, a raft / piles may be a suitable and economic solution.

For the trial holes, distribute them about the perimeter of the plan of the proposed site, just be sure to keep them outside the plan of the site as there is no point digging in this area as it will have to be filled with leanmix / concrete afterwards!!!

Hope this helps.....

muffler

I wouldnt call it disagreeing.

My point was that instead of setting out with the intentions of digging trial holes to just proceed with digging the strip trenches. In other words let the strip excavations act as trial holes - same thing really. Whatever is uncovered when digging out the trenches will be the same as will be unearthed (I just love the puns ) with trial holes.

less labour involved.

Oh, I see smashey logged in - should you not be at work :rolleyes:

sas

Thanks for the advice.

SAS

galwaytt

Can I disagree with Muffler too ?

As an example of just how ground can vary - I had the opposite problem - I dug nice'n'easy for my basement.........for 2 feet. The next 2m+ were all solid rock. Then, a layer of bedrock.........:( 28 truck loads of it, in fact....... http://www.galwaytt.smugmug.com/photos/75038363-M.jpg http://www.galwaytt.smugmug.com/photos/75038278-M.jpg

So, at treatment plant time, we had all the big guns lined up, rock breakers etc, for the tank, 8m away from original dig. Bucket went straight down with no problem at all........in fact, I could have sold the spoil as 804 it was so good.......


Geology, huh?

OP, let your engineer make the call - it's he who has to stand over it, but I'd have thought on a site like that that you'd clean the entire area, and that a raft would be much more suitable.