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Foundation Question - Compacted Sand vs Bedrock Close to the Surface

Continuum

If you were asked which is 'more problematic' to build a foundation - between 'compacted sand' and 'bedrock that is close to the surface,' which would you select?

The question does not provide more information.

From my understanding, compacted sand can support buildings but it can be problematic if the sand loses frictional strength. And while bedrock is typically very good for foundations, if it is too close to the surface there may be some issues for infiltration required.

Thanks for your thoughts.

 
Feb 15, 21 1:55 pm

Bedrock.  

Feb 15, 21 1:58 pm  · 
1  · 
Continuum

Is it for any other reason than infiltration issues?

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Non Sequitur

mostly infiltration from the Queen's army. Not really an issue these days.

1  · 
mightyaa

Agreed. Bedrock can be difficult assuming you have to get into it, it may need blasting. Also, water doesn't drain through it, and it has it's own subsurface water channels like valleys. You may have put your structure in that flow and dug out a nice bathtub. You can sonar map, but most don't want to pay for it beyond looking for voids (like caves) that can shift or collapse with the weight of a structure on top if you are in an area known for it. Sand has its own issues; I'm not as familiar. But it can shift, so it isn't stable. Normally, I thought you punch through sand with a pilaster or pier to get down far enough to avoid that surface shifting.

1  · 

I wouldn't put a slab on grade into bedrock but instead do a raised floor with columns. We had to work with exposed bedrock quite a bit in Duluth, MN. The real challenge comes when you're forced to bear part of the foundation on bedrock, and part on clay soil. Eeeep!

1  · 
Continuum

Thanks for your input! I am purchasing a steep sloped land with lots of bedrock, and will have to do some thinking..

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Non Sequitur

Chad, I have a series of buildings on the very condition you describe. Solution: a 600mm deep raftslab (it's a 2400sq.m building) and the exposed bedrock was drilled and scraped clean (p.eng even insisted on it being broom swept... that site guy was kicked off the job site btw).

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Sounds like what we would do. TAS (Thick Ass Slab)

1  · 
Non Sequitur

that baby did in fact got back.

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Wood Guy

The answer is the same as for any building science question: "It depends." Building on sand is a dream when it can support the necessary loads, because it drains freely. Bedrock can support a lot more weight and you don't have to worry about landslides after it's rained for two weeks straight. But pinning a shallow foundation to ledge is time-consuming, bringing in underground utilities is a pain, and if you want or need a basement, you have to blast and you risk water getting in. 

Most of my projects are built on either sand or bedrock. Clay is the next most common. Sometimes I get to build in a nice, flat field of gravel but it's pretty rare. 

Feb 15, 21 2:15 pm  · 
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Continuum

Thanks Wood Guy

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JLC-1

sand doesn't compact, try again.

Feb 15, 21 4:00 pm  · 
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Wood Guy

I've heard people say that before but I disagree. If sand particles were perfectly round and all the same size, it's true that they would not compact. But the particles vary in size and shape; when you dampen the sand and run a vibratory plate over it, it compacts. It's just more challenging than with other soils because the particles tend to slide around. At least that's my understanding, based on observation and some study. I'm not a soils scientist and have rarely worked on projects that require geotechnical engineers, and I'm open to new information.

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joseffischer

we put a relatively light load on sand, 2 stories with a lot of medical equipment near Myrtle Beach Florida. A geotech was involved and the structural engineer had to thicken the slab on grade to the point it was basically a foundation. The geotech had a perf pipe system under the building and 10' out from the perimeter (forget what it's called now) that kept the sand around the building dry via drainage. This was all required from the mix of shallow foundation, sand, and earthquake zone (C as I recall).

1  · 
JLC-1

basically you built a concrete tub that floats in the sand

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t a z

Assuming we are talking about shallow foundations?


In typical cases soil with adequate bearing capacity is ideal (assuming you don't have a high water table).


A high bedrock elevation can be problematic if any excavation is required ($$$).


So what study guide did this question come from?

Feb 15, 21 4:01 pm  · 
2  · 
b3tadine[sutures]

I'd you're going to create threads for every single question on the ARE, you're going to piss people off, and get your threads scrapped.

Feb 15, 21 5:47 pm  · 
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Continuum

Haha well, sir, I need to know these things to 'protect the public.' But it also goes to show how ambiguous some things can be, and whether that is fair or not.

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caramelhighrise

If it truly is for the ARE, NCARB has a community where others actually are looking for help on test questions just like this.

1  · 
SneakyPete

Read your bible more carefully.

Feb 15, 21 10:47 pm  · 
3  · 

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