Ultimate bearing capacity on Tarmac

[ac4u2nv]

Dear all
I have a site which is 200mm of tarmac overlying 400mm of hardcore, overlying 1800mm of made ground:gravelly, silty fine to coarse sand.
How do i work out the Ultimate bearing capacity of the tarmac, if i wanted to place a crane on it (base 1.5x1.5m)?

What dry density and internal friction angle should i use?
Thank you

ash

 

[oldestguy]

Assuming you don't care if there is some indentation due to compression of the pavement or its modulus of elasticity, you probably are planning to use a method that involves soil shearing. Thus, the strength characteristics of the pavement structure and the soil below can be estimated or determined from testing (field or lab). Such testing likely means some field evaluation and possible test in place. You may have some indication of that with your apparent borings, as related to blow count, or possible in-place shear tests. Density usually is found from field tests, usually involving large samples, but nuclear testing methods also may be applied. You may have to involve experienced consulting engineering firms for finding what you need.

Finally you need to apply some method of relating eccentric footing loads into your bearing capacity formula. It may involve some approximations however.

Another method may be considered. That is an actual bearing test, called plate bearing test, which then has to be extrapolated upward to the size of your foundation pad. That extrapolation is an estimate, not precise, due to the larger pad affecting more zones below.

If it were me, I'd start with that plate bearing test to get an idea of just what I am up against for the larger size pad. This extrapolation probably means your full size pad has less bearing capacity than the plate bearing test result.

 

[hokie66]

What sort of crane? With only a 1500 x 1500 base, it can't be a very heavy crane. Or is this an outrigger size?

 

[oldestguy]

I should probably add something. In my view the ultimate bearing capacity of the full size pad likely will be when excessive settlement has occurred, not necessarily due to a shearing of support soil. That would be then recognized when conducting and using the results of a plate bearing test.

 

[Ron]

Same question as hokie66.....size makes no sense unless outriggers. Your pavement section is substantial. Would support a very large crane with any reasonable footprint.

 

[racookpe1978]

Concur on crane loading: There "should" be 4x PADS under the 4x outrigger bases, each pad is likely to 1.5 m x 1.5m. A single outrigger is not going to be that large.

Now, your crane load + rigging load + crane weight + boom weight + counterweight will probably be divided between those 4x pads, right?

 

[ac4u2nv]

Sorry, let me add more details
I have logs, with no N values or triaxial data or density data.
I know the crane to be used will provide a load of 135kn/m2 and its base will be 1.5x1.5m in size. I just need to know how to calculate whether the tarmac(0.2m) over hardcore (0.4m) over Made ground: gravelly silty, fine sand (1.8m)will hold the weight of the crane or will a crane mat be needed?

how do i calculate this?

Ive been using the standard formula in tomlinson (7th edition) p.46 (2.8) - the parts im unsure about are the dry density values and phi values to use.

What do you all suggest?

thank you

Ash

 

[ac4u2nv]

oh and do i just work out the U.B.C of the tarmac or do i have to consider the layer beneath it too?
how do i work it out for both?

Thanks

 

[Lomarandil]

You'll want to consider the UBC of the tarmac and layers beneath.. however, since the layers beneath will see less load (per a Bousinessq stress check), you may be able to consider them good by inspection (I don't know what you mean by hardcore).

I don't know what kind of tarmac this is, but you can always compare the tire pressure of the vehicle it was designed to carry against the bearing pressure from your crane. Might be able to rule out local failure that way.

Also, the NAVFAC manuals have some generic allowable bearing capacities for different classifications of soil/pavements/rock. May be worth looking into if you don't have proper geotechnical information.

Finally, consider whether soil shearing is truly an acceptable failure condition, or if you'll be limited by settlement requirements of your crane. A lot of cranes get derated if they have a cross-slope of 1 or 2%.

 

[ukbridge]

You may be able to get some crude estimates if you trust your quantitative description of the underlying ground from various published papers, handbooks etc.

I mention this only as it hasn't been mentioned in your second post regarding loading. It is absolutely essential that you take into account the effects of wind which will result in a moment, and introducing an eccentricity into your foundation. The crane supplier should be able to give you this information. Is it anchored to the ground? Make sure you put a healthy FoS against overturning on the crane.

 

[hokie66]

I would not hesitate to put a 1500 x 1500 footing, with working load 135 kPa, so a total load of 30 tonnes, on the pavement as described...if that is the full story. But it still doesn't sound like a crane.

 

[racookpe1978]

There is no way that single load is a crane.

Unless the person who wrote the text book question/homework assignment didn't know what he was doing. (a 1.5 meter x 1.5 meter stack of "logs" ????? No way. Stack of square cut timbers maybe, but then you'd need a "nested" stack at least 3x timbers "high" to get the load back evenly spread between the cross-beams.)