Equate CBR value to kN/m2

[ArranB]

Hi folks,

I've been asked by a contractor to determine via CBR if the ground is suitable to take a crane load. The crane load is given by the manufacturer in kN/m2.

The information they want is 'if you achieve x CBR value the ground can support the crane load (135kN/m2).

How would I go about doing this? Its stopped me in my tracks.

Thanks for your help.

 

[GeorgeTheCivilEngineer]

No... CBRs aren't suitable for this. They need to do a plate loading test.

This is safety critical. Don't put your name to a kN/m^2 derived from a CBR.

 

[ArranB]

No... CBRs aren't suitable for this. They need to do a plate loading test.

This is safety critical. Don't put your name to a kN/m^2 derived from a CBR.

Thanks.

This what I thought when working it through, but they made me second guess myself.

 

[geotechguy1]

Per chance is the test to achieve CBR value one that the contractor is doing themselves? Or worse, are they doing something like a DCP or shear vane to CBR correlation and then trying to get you to give them CBR to bearing?

Inferring that's the case, I would be hesitant to trust contractors to mark their own homework for something high risk like this, and also avoid plugging correlations into correlations (I'm just inferring this is what's happening since this story sounds familiar to me).

In any case a crane is safety critical and there are lots of permutations and combinations where such a crude correlation could become unsafe, for example, your contractor takes the correlation and then uses it to park the crane a meter from a slope or trench or something and ends up in a river. You want to inspect the situation yourself (or some other engineer), understand the loading, understand where the crane is going and the site layout, and have some independent testing done. A PLT as suggested above is a great idea. Something else might be OK depending on the situation. With PLT, be careful about scale effects and non-linearity. They can be very misleading if extrapolated to a larger size / load.

 

[EireChch]

OP - you are based on the UK. There is a CIRIA standard for this. You need phi for granular and cu for cohesive soils to then work out bearing capacity.

 

[CJLCivilStruct]

Structural engineer here - so may be speaking out of term.

But.. CBR / plate bearing test with a 450 dia. plate (?) will only indicate strength of the soil very close to the surface (about 1.5d). It will NOT tell you if the ground is suitable for outrigger pads or similar. This will require analysis informed by appropriate site investigation.

 

[EireChch]

An outrigger foot maybe only be 600mm dia so its not far off. Even better if you use a 600mm dia plate. In cohesive soils, If you do a PLT and get a CBR you can approximately equate that to an undrained shear strength by multiplying by 23. For sand you can use your modulus to indicate if you are in loose/medium or dense SAND and adopt a suitable friction angle.

Typically you do an analysis and if its not passing then keep adding outrigger mats to spread the load until it eventually passes.

 

[CJLCivilStruct]

Good point, depends on the pad I suppose. I've had to spec some big ones in the past! Even as a minimum I would normally expect a 1.2x2.4m pad for any decent size crane.

 

[Lomarandil]

Exactly, and once you're specifying crane pads of any decent size under the outrigger, CJL's concern about depth of influence is valid. Some additional evaluation should be performed to ensure the near-surface capacity is similar to or less than the capacity deeper below grade.

CIRIA and TWf have some great resources for UK practice. We're less organized about it in the states.

 

[EireChch]

I dont disagree with you Lom - but being honest, I my experience no one does site investigation for a crane pad. Great if you have data but 90% of the time you dont. Its managed by using conservative parameters, factors of safety and some qualifying statements.

 

[Lomarandil]

Sure thing -- it's an exercise in managing risk. Some sites have naturally lower risk (the "can I drive my truck over it?" test). Some plants are small enough to naturally work out.

My niche means I tend to be evaluating 300t+ cranes swinging (and sometimes walking with) 75t picks. In those cases, we absolutely do a site investigation first. But the risks (to the crane/operator and project schedule) are higher.