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15 post(s) First 1 Last
Gravel under footings
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Re: Gravel under footings By  Lou Marks

Posts: 0

4/7/2002 4:11:00 PM

Gravel is compacted with two passes of a plate compactor. Is that the issue or do you believe something else needs to be done.


Re: Gravel under footings By  DWalker

Posts: 0

4/8/2002 7:24:00 AM

One part of this question is: would a foundation base of gravel ever not require compaction. The builder is telling me that "river rock" does not require compaction. I don''t know what he means by "river rock" but hope to find out today.

[This message has been edited by DWalker (edited 04-10-2002).]


Re: Gravel under footings By  Lou Marks

Posts: 0

4/8/2002 5:02:00 PM

River rock is round and he is correct, it does not need to be compacted. The second part is that you cannot use it because it cannot be compacted; crushed stone or gravel is the correct material to be used, depending on your area it should be around a number 57.
The round river rock is best used for roof ballast, as it does not have sharp edges to cut the roof membrane. Where used walk pads need not be used. It is also the medium used in filter tanks because it does not compact and block the flow and can be cleaned by back flushing.


Re: Gravel under footings By  Snickers

Posts: 0

4/9/2002 6:55:00 AM

If your frost depth is 42", how does the builder get away with the bottom of the footing only between 16" and 18" below grade? Lou Marks is right, river rock can''t be compacted since there is no fines to fill the voids. During frost upheaval, this would allow rolling and shifting of the rock that when the upheaval is over and thawed, can create voids (weak, unsuppported poits) under the footing. Akin to "building your house on sand." Good luck. Sounds like you are going to have one very unhappy builder on your hands.


Re: Gravel under footings By  wkh

Posts: 0

4/9/2002 7:26:00 AM


The thought that small rock/gravel does not require compaction is a common misconception. The ONLY backfill material which requires no compaction is concrete or other flowable cementitious backfill mix. Gravel will compact when subjected to vibration. Re-orientation of individual pieces will occur to create the lowest possible void ratio. Gravel as a footing subgrade material is a terrible idea. Gravel has a relatively high void ratio which will allow fine soil particles from the surrounding soils to migrate into the gravel voids. The migration of fines has been associated with foundation distress throughout the world.

If fost line is 42" then bottom of footing must be 42" below adjacent grade period. The presence of the gravel to a depth below the frost line should prevent frost heave but at the cost of many other potential problems.

If drainage tile is in gravel directly below footing, will footing load adversely affect pipe material?? What is bearing capacity of pipe material?? Crushed or deformed pipes don''t drain very well. This job sounds like a mess.

For Iowegians only:
Tell the guy to call Richard Handy at Iowa State University and ask him "When will the house fall?" The owner will need to have some serious levitating power to keep this one from shifting. The guy might also do some good by driving through Colona and seeking advice from the Amish builders around Iowa City whose creations have stood unharmed for over a century.


Re: Gravel under footings By  constructionarbitrator

Posts: 0

4/9/2002 10:54:00 AM

While I do not dispute what you say about compacting gravel, I am quite surprised. In the 50s and 60s I use to backfill garage slabs with on site material. In the 70s, I started importing 1? rock (gravel?), and just dumping and spreading it with *no* compaction, and pouring the garage slabs directly on top of the gravel. I have had *no* settlement problems, and I would have certianly have been notified if there had been a problem. BTW, all of my projects have had soils engineers on site, testing all of my fill, and none have ever questioned the rock fill under the garage slabs. Maybe I have just been lucky!


Re: Gravel under footings By  wkh

Posts: 0

4/9/2002 2:09:00 PM


I think we are talking about two very different scenarios.

With few exceptions, I have no problem whatsoever with gravel/rock under a lightly loaded slab. This same material under a footing can be disasterous depending upon the loading, soil type and available moisture.

What I gleened from the original description is an unfiltered french drain directly below the footing which is very bad news. Hope I interpreted this one correctly???


Re: Gravel under footings By  Lou Marks

Posts: 0

4/9/2002 4:19:00 PM

There generally is a substantial price difference between round river stone and crushed stone. We order round river rocks by size small, medium and large and there are no fines to fill voids. Apparently, there are some different grading standards around.
Compacted crushed stone can be used and is the only foundation used in all wood dwellings. Never caught on here but have 20 or so around for 15 years or more without apparent problems.
It will take and darn good engineer to convince me that round rocks can be used under a footing even compacted. Only way I can see it done is to contain the stone so it has no where to go laterally, that seems to be a lot of trouble to go through rather than compact crushed gravel.


Re: Gravel under footings By  constructionarbitrator

Posts: 0

4/9/2002 5:56:00 PM

Does crushed gravel with fines, give greater compaction than just ordering 1?" rock?


Re: Gravel under footings By  wkh

Posts: 0

4/10/2002 7:09:00 AM

I''ll probably regret saying this but "compaction" provides little direct information about a soils engineering parameters. Compaction is a relative term and may be used to correlate a soils density. Density is another relative term which may be used to correlate a soils strength parameters. Sounds like I''m making this up doesn''t it. Compaction removes excessive voids and provides a convenient means of "evaluating" the suitability of placed fill.

Example 1-1/2" rock with no fines will have a high void ratio. There will be a limited number of grain to grain contacts between individual rock particles due to their shape and size. (Example: how many bowling balls can you fit in the back of a volkswagon?).In a very simplistic approach, soil strength is primarily a function of the friction between soil particles where the grains contact. Remember air (voids) contribute nothing to the strength of the soil fabric. IF you were to take smaller soil particles (i.e. sand, silt etc)and fill the voids as much as possible you will significantly increase the soil''s strength by increasing the number of grain to grain contacts.

An ideal soil has a ballanced mixture of all particle sizes. This represents the lowest practical void ratio and the most desireable strength condition. The addition of fines to the pure rock/gravel will tend to make it more difficult to compact. However, once properly compacted, the mixture will be substantially stronger (more stable) than the rock/gravel would have been by itself.


Re: Gravel under footings By  DWalker

Posts: 0

4/10/2002 7:18:00 AM

Thanks to all of you who have taken the time to share your experience and knowledge.

The river rock used in this instance is smooth, irregular stone, varying in size from 3/8" to 1 1/2". It was set in a trench with a sloped bottom, from 24" to 48" deep. At the low point, a plastic drain tile intersects to take any water away from the house.

The exterior side of the trench has rigid styrofoam insulation, 2", at least on the shallow areas; I don''t know if it follows the slope of the trench to the full 4'', but it seems unlikely. the inside of the trench is earth.

At the low point of the trench, utility piping intersects the river rock, underneath the concrete. These are corregated plastic tubes, 6" in diameter, and there are four of them - part of an below ground heat exchange system. I imagine the river rock was poured on top of them.

The concrete footing turns out to be 24" wide, and was poured on top of the river rock and insulation using the sides of the excavation as forms. Therefore the edges of this footing are irregular and take the shape of the rough excavation. This footing will be 6" below the finished grade and the bottom will be 11 to 14" below grade. I am concerned that frozen soil will "grab" the irregular shape of the footing and move it.

I would particularily appreciate hearing from anyone who has experience with "rubble foundations" of any type.

[This message has been edited by DWalker (edited 04-10-2002).]


Re: Gravel under footings By  constructionarbitrator

Posts: 0

4/10/2002 9:44:00 AM


Interesting explanation, and I don''t challange what you are saying but I am curious: To take your bowling ball example, suppose we fill a pit with bowling balls (or river rock, or 1?" rock) and lay a structural slab on top of the bowling balls and exert compression loading on the slab. I don''t see the slab going anywhere (once the balls have fit together) and I don''t see that adding sand (or any other fine subsatance) to fill the huge voids between the balls is going to do anything at all. I know you''re right, and I know nothing about the science of soils (that''s why I hire them), but I am just curious.


Re: Gravel under footings By  wkh

Posts: 0

4/10/2002 3:07:00 PM

If you put nothing in the voids then you have no strength attributed to the air/water in the void space. If you achieve any grain to grain friction with fine material in the void space then you have something. You would be amazed at how a little friction can make alot of strength.

Performance of any soil is a function of three factors: soil structure, loading and disturbance/exposure.

Clarification, in my way of thinking a "structural slab" is one designed and constructed such that soil support is not required, so the soil below would make little difference.

Humor me and lets call it a nonstructural slab on grade over the pit full of bowling balls. We have defined two of the three factors already: Soil structure is an aray of closest possible packed spheres; loading is relatively light based upon typical slab loading. There should be no significant problem with performance of this subgade. But, let us entroduce a twist in the third factor of disturbance/exposure. Moisture from an undefined source is transporting finer soil particles from the pit walls into the voids between the bowling balls. This process continues and pretty soon there are now voids at the boundry between the bowling balls and native soils. These new voids allow the bowling ball matrix to shift and stability of the matrix is compromised. I''ve seen alot of this and it is generally referred to as migration of fines. This process is similar to what happens to a french drain without filter cloth or well clogs.

The presence of smaller soil particles within the void spaces of the larger particles will significantly reduce migration of fines. The increse in strength can be very substantial. Under a slab this is unlikely to be problamatic if continuous perimeter footings are founded below the bottom of the gravel layer. I personally believe that footings atop open gravel/stone is pure insanity.

Want a backyard bubba non-astm type test? Go out and take your truck to a site with a pure gravel surface and gas it from a dead stop. Rocks went flying everywhere didn''t they. The same thing tends to happen atop pure sand of uniform grain size. Now find a site with a well graded soil (sand/silt/gravel and maybe a little clay) and do the exact same thing. I bet not much, if any material went skyward. This is a very simplistic shear strength test. Try selling that one to your building official.


Re: Gravel under footings By  subgenius

Posts: 0

7/7/2004 7:43:00 AM

obviously, the foundation is shallow with regards to frost depth....however, the foundation is "otherwise protected" from frost. the insulation board is an indication of this, as well as the drainage course beneath the footing foundation, which is a common means to combat frost....moisture removal. the canadians have done quite a bit of research on the concept of "frost protection", it is worth researching on that end, as well as the "frost protected shallow foundation"



Re: Gravel under footings By  Boo1

Posts: 0

7/7/2004 11:28:00 AM

Foundation Basics

If the whole house settles slowly and evenly, some additional settlement is no big deal; but if settlement is uneven (differential settlement), there could be damage. A frame house with wood siding and drywall interiors can probably handle up to ? inch of differential foundation movement, but even ? inch of uneven settling is enough to cause cracks in masonry, tile, or plaster.

It?s the unusual situations that cause the most trouble. When the footing is laid out off-center so the wall misses its bearing, when you encounter a soft zone on site, or when the footing is undersized, the inspector faces a judgment call. If you think there?s a problem ahead, you know you should stop and call an engineer. But if the risk is low, you?d like to keep the job moving.

Lets take a systematic approach:
1. Is this system allowed by your code? Does your code require footing to rest on natural soil?

2. Is the shallow foundation frost protection adequate?

3. Location of French drain under a structural footing is not appropriate. The drains are to be located around the perimeter of the structure.

4. Is river rock acceptable filler under a structural footing? Agree with WKH that the low shear resistance of round river rock precludes their use in this area. Crushed rock is some times used in wet areas or soft soils, and always should be well tamped. This compacts them down into the soil reducing the voids or spaces between stones.

Conclusion: Even if it''s a small single-family house loading it will will have uneven settlement and the French drain will not function correctly. Will the footer totally fail? Who knows?


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