When to use a Raft ??

[Contraflexure74]

Hi All,

See attached borehole log for a site I'm working on. The project is the construction of a 2 storey rear extension to a mid terraced house where access is just non-existent (i.e. through the house). There are buildings on all 3 sides of the site where the extension is going in.

As you can see from the borehole log there is 3.8m of fill on the site. I have priced a piled solution down into the gravels but it's very expensive.

I'm trying to come up with a clever cost effective solution. I wonder is a raft solution up for consideration?? SPT's vary from 8-14 (which isn't great) in the fill and obviously I'm concerned about settlement and how I control it. I'm not sure how I would even be able to calculate it accurately. I have no information on ground water but the site is only 550m away from the City dock so ground water is likely to be tidal.

To keep loads down I could use hollow block/timber frame to avoid excessive settlements.

If I remove some of the fill and bring in some Engineered fill would this help in terms of surcharging the remaining fill and then lay a 400mm deep raft covering the entire extension footprint.

Any ideas/comments are welcome guys.

 

[dik]

With that much fill there is a really good chance for differential settlement... I'd avoid a raft and go for piles... also being 'closed in' it might be better to use as little concrete as possible.

Dik

 

[bhiggins]

In this situation I would use concrete piers to gravel and have a raft foundation spanning between piers. I'm in an area with highly expansive soils so this is very common but expensive. I'd avoid building the foundation on fill at all costs due to high settlement potential. Another option is to strip the site completely of unapproved fill and build an engineered pad but this is extremely expensive. I imagine concrete piers would be much cheaper. Is a pier and wood beam foundation an option? This would be cheapest I imagine.

 

[Contraflexure74]

I'm not familiar with the pier and wood beam system in this part of the world. We use CFA concrete piles with insitu concrete beams/slabs.....but similar principle I guess.

 

[SlideRuleEra]

Agree with Dik, piling. Without knowing where the water table is located, removing existing fill and replacing is a gamble (maybe a high water table is why existing fill's STP values are low). Piling ??may?? appear to cost more than a raft, but the piling are a reasonable length (4.5 m +), should be point bearing (virtually doing away with differential settlement), and quickly driven (saving labor cost compared to "fiddling" around with questionable excavation and backfill).

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[Contraflexure74]

Hi SlideRuleEra,

The piling contractors designer has specified 8m length piles, 4m going through the fill and a further 4m into the gravels. Is it really necessary to have them 4m in the gravels?? I'm guessing for restraint??

 

[dik]

Do you use spread bored piles in your local... almost an ideal use.

Dik

 

[SlideRuleEra]

Contraflexure74 - Get a local geotech engineer's opinion. Eight meter long CFA piles that are four meters into gravel seems a little deep for what I assume will be fairly light loading.

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[JStephen]

A couple of years ago, I was at a presentation about helical piles, and they used something about the size of a little bobcat loader to put them in. If access with equipment is a problem, that might be worth looking into.

 

[Contraflexure74]

Will do SlideRuleEra.

Must look into helical JStephen....are they suitable for gravels though?

 

[KennethCa]

Have you looked into ground improvement techniques such as rammed aggregate piers? Depending on the size of your site they may be suitable.

 

[WARose]

Another vote for helical piles.....not much point in a "raft" if it is at grade level.

 

[JStephen]

Contra, I don't know, that's out of my field, so just an option to investigate.

 

[SlideRuleEra]

Contra - Helical piles are a good suggestion, worthy of consideration. The "International Society for Helical Pile Foundations" has mixed advice on helical piles in gravel. For the "Dense Fine Gravel" (4.5 m to 10 m depth), seems they would be ok... ask the local geotech. If they say "yes" and they are available, give helical piles serious consideration.

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[spieng89]

bell piers might be an option in your area. They can be placed shallower then driven piles or traditional cast in place piers.

 

[Contraflexure74]

Thanks for all your comments. Really appreciate all your input/comments.

Really think this forum is brilliant.

 

[dik]

.are they suitable for gravels though?

Yes as long as you only have tiny rocks... they are good in gravels.

They are good for lite loads and heavy ones too... I've used them to support six 400,000 lb transformers... still standing after a couple of years.

Dik

 

[ajk1]

My vote is for helical piles (that would have been my first suggestion before reading this blog) provided the soil through which they penetrate is not very corrosive and are installed by an experienced and reputable contractor with a history of successful projects. Don't take any clever short cuts...house settlement is very expensive and difficult to rectify later.

 

[oldestguy]

Consider a basement, or at least a crawl space. Use a slab on that surface. That would unload the site some and any movement later is likely to be tolerated. Piles take big equipment, while small excavators can do this job. The connection to the old part can be flexible and likely the owner can tolerate it given the lower cost here. A geotech is mandatory here and likely can save many Euros or Dollars. I note no other geotech has commented yet.