Rigid Insulation to replace overburden in strip footing design

[nicholi]

I am doing a strip footing design for a high load buildig structure is soils with very low soil bearing pressure. This new building is an add on to an existing building also with a strip footing. The original soils report only recommends either driven timber or steel piles and I can not do that because the existing building is finished in brick. Thus I want to use strip footings. However to negate the soil overburden on the acting on the footing and keep the footing a reasonable width (because of low soil bearing) I am thinking about replacing the soil overtop of the footing toes with rigid insulation up to about 1/2 meter below grade. Has anyone done this or seen this done before and if so what are some of the pro's & con's of this approach and any specific construction detailing to make it work? Thanks in advance for your assistance.

 

[Ginger]

I would be very careful about using the strip footing approach. Given that the structure you are tying into has been there some time then using a strip footing is likely to give you a major problem with differential settlement. Why not use piles with a reinforced concrete capping beam onto which you can found your brickwork?

With the best will in the world, a strip footing on a low capacity formation will give you settlement problems. Andy Machon

 

[Ron]

nicholi...I agree with Ginger. Your issue is not what you might put over the footings, but what your footings will rest upon. Comparatively, the 1/2 meter of soil you will be replacing with some other material (cellular concrete works well for this...low unit weight) is only a small portion of the load on the strip footing, this using this technique to reduce settlement is not likely to work.

If the existing building is performing well on strip footings, why does your geotech. recommend piles? This disparity needs to be resolved. Maybe your issue is poor soils evaluation rather than poor soils.

 

[Ron]

ps...Andy, good to see you active again. Why the hiatus?

 

[nicholi]

To all thanks for the advice, but here's a bit more background. The site is part of an old flood plain, with mostly silts and some clays. The soils report that was completed was focused on a huge department store that was eventually built on the site. The original geotech's recommendations were either driven steel or timber piles or a huge mat foundation. He briefly mentioned skin friction piles but gave no values for design, where as he gave values for the other types of piles. The consultant who designed the smaller store (the one that I am adding onto and is only a 132m2 building c/w a basement) had contacted the original geotech and got from him a value of allowable soil bearing presssure with which to design a strip footing.

With respect to the issue of differential settlement I am aware of this issue and am looking at jointing details that will allow the new building slide down with out impacting the other. I do not plan to tie the two buildings together so that the settlement of new structure will not impact the original.

The soils report is a good one and very detailed, just the soils are plain lousy!

 

[Rowland33]

nicholi, is the statement in your second to last paragraph correct? The bulb of pressue of the new footings may well exert influence under the existing footings and thus causing further settlement under the existing footing. It's worth checking.

 

[Ginger]

Nicolai

If the building is a "high load" building as you state, then Ron's advice is correct and the weight of 0.5m of soil will be insignificant when compared with the load applied to the footing from the structure above. Although I have not designed such a structure, I know that polystyrene blocks have been used as low weight fill to the rear of bridge abutments due to its low compressibility and low weight. This may be one option you could consider.


Ron - good to speak to you again. I've been keeping quiet because as the site has grown a there are many more engineers offering advice. The site seems to cater mostly for US engineers and due to the time difference, by the time I visit the site the query has usually been addressed.
Good to see you're still offering "Top of the Range" advice. Best Regards Andy Machon

 

[mfrad]

If you can determine that the existing building will not settle further based upon the pressure bulb that has already been established and has not been compromised as stated by by Rowland33 after further geotechnical review, you must then assume that the new addition will settle some significant amount regardless of reduction in soil weight from sytrofoam blocks. A possible solution to investigate is the use of drilled piles ie,minipiles rather than driven piles for this problem to reduce vibration and brick cracking that you have a concern for. The new addition will not settle significantly with this approach.

 

[Wopshistos]

mfrad has nailed it. you dont appear to have all the facts here to judge the efficiency and effects of a different foundation solution. Drilled or mini piles seem the safest way to go if you cant do further SI of your own at shallow depth.
The basement in the other building might say something about the shallow soils where the original designer just wanted them out of the way!

 

[nicholi]

Thanks to all who replied. After finally getting hold of the author of the original report and talking to him, I am going with drilled piles, and have asked the client to order up another soils report to confirm skin friction values for me.