If good material, well compacted and well graded, I can see it being 3ksf and up. Due to aggregate interlock, well compacted limestone is pretty solid material. Too many variables.
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If well compacted and reasonably graded (like a road subbase or base course material), 3 ksf would be quite low - likely 300 to 400 kPa would be more appropriate. Usually, though, one will pick the bearing "capacity" with respect to settlement, hence allowable bearing "pressure" would be better terminology.
Yes, usually road base crushed rock has a CBR of 80+ and typically in the 100+ range . . . CBR is designed for roads NOT,in my view, for structural foundations.
This isn't a direct answer but, we're a conservative bunch and I'd guess 90% of the geotechnical engineers I've worked with wouldn't give more than 400-500 kPa ultimate for a shallow foundation without very good reason (extensive site investigation program, load testing, etc.), and certainly not more than 1000, regardless of what the equation tells you. I don't know if this is actually true since I haven't been to court myself but I was told if you give 100 - 150 kPa allowable it's very difficult to get to messed up in a court unless you gave 100 kPa allowable for some thawing muskeg.
Long answer, not enough information. Do you have 300 mm of crushed limestone over top of 20 m of clay? What about 5m of crushed limestone over 15m of sand? Is the footing 0.5mx0.5m footing for a lamp post on 300mm of crushed limestone, or an array of 3mx3m spread footings for a 10 storey brick building, or is it a 30mx30m slab for a warehouse? Allowable bearing capacity is really about settlement and for that I'd need quite a bit more information.
Thanks for explaining.
I was aware of gravels achieving up to 1000kPa. And rocks in the thousands.
With crushed limestone, it feels hard when you walk on it.
I don't yet believe the value from the equation above but wanted to know why the large difference from the suggested 300-400kPa and 10x this from the equation.
With regard to the specifics:
- Crushed limestone (unknown thickness so far)
- 600mm sand layer
- natural soil consisting of clayey sand.
- Plinth size 600x600x200.
Your crushed limestone is not carrying all the load itself - it is distributing the load down onto your natural soil consisting of clayey sand. That is the reason you wouldn't design to the bearing capacity of the crushed limestone.
For stability reasons, your plinth or footing is normally a minimum size anyway, so it generally doesn't make much difference to small foundation design whether you adopt a bearing capacity of 300 - 400kPa (which is probably reasonable) and 1 - 2MPa (which is ambitious and could come with problems- settlement, punching through of your plinth).
So there is good value to keeping the design bearing pressures within a proven range. It's not that you can't go higher, it's just that by doing so you open up a number of potential problems that need to be carefully analyzed by the geotechnical guys.
Thanks for commenting.
Perhaps I could consider load dispersed to the layer/s underneath.
As a hypothetical situation, given a single layer of crushed limestone, is the bearing capacity actually as high as mentioned above?
If you had a 10 meter thick layer of crushed limestone and underneath was basalt or something then yeah probably. Even if the friction angle was 45 degrees you should be around 3000 - 4000 kpa
Crushed limestone is the most common base material used in my area. It has excellent stability and as BigH noted, a CBR value that usually exceeds 100. It is usually placed in thin lifts, not more than 12 inches, so bearing capacity is a bit of a misnomer. The limestone in my area is relatively impervious when compacted. When placed over a competent, compacted subgrade, I would expect bearing capacities in the 4ksf to 5ksf range.
think the answer to this question depends on what you are using it for. Bearing pressure alloawable is related to settlement. is this for a building, a road or a temp crane mat? If its a building the allowable bearing will unlikely be governed by your fill material. Unless you have several meters of it! If you are looking at a temp working platform its more important as its short term load v long term.
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Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
