Ground Bearing Pressures/Piling Mats 3

[markbrowny12]

Hi

Can anybody point me in the right direction for the design of simple pile mats.

We are utilising crawler cranes that are inducing ground bearing pressures in the region of 50T/m2. We are operating on a crushed concrete piling mat (600mm deep) on top of London clay. What theoretical ground settlement would be expected under loadings of this magnitude?

We have conducted plate bearing pressure tests and are experiencing settlements in the region of 10mm - how can I calculate the theoretical settlements that are expected under loadings of this magnitude?

 

[Focht3]

Are you looking at settlements due to the construction equipment?



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[markbrowny12]

Hi - thanks for your reply.

Yes we are looking at settlements due to construction equipment.

We will be using a 600T crawler crane lifting a load of approx 90T at a radius of 50m. This would induce a pressure of 65T/m^2.

We have conducted plate bearing pressure tests up to 70T/m^2 and have experienced a settlement of 10mm.

Not coming from a soil engineering background (I'm mechanical) I am curious to find out information on how pile mats are designed.

 

[Focht3]

You are primarily concerned with consolidation settlement - due to water being squeezed from the pore spaces between the soil particles. Think heat flow equation with nonlinear conductivity, temperature-dependent behavior, spatial variations in properties - this is not a simple subject.

The biggest hurdle you will have to cross is determining the sustained load. The area where the crane is stored will see the greatest settlement.

You should seek out an introductory text on geotechnical engineering for more information. I would suggest Ronald F. Scott's text; avoid Bowles' or Das' books.

There is no simple answer to your question -



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[markbrowny12]

Hi

If I know from empirical data the strength of the underlying clay (say 45kPA) and the thickness of the crushed pile mat (600mm) is there a formula to determine the allowable pressure on the mat?

 

[Focht3]

Okay, you're talking bearing capacity now. Still no simple formula to provide - sorry. But I'll walk you through the "quick" evaluation. You need to discuss this problem with your geotechnical consultant to get a firm answer.

A 45 kPa compressive strength results in a shear strength of 22.5 kPa - pretty weak soil. (It's still weak even if the shear strength is 45 kPa.) A straight bearing capacity calculation would indicate a bearing capacity failure at an applied pressure of about (22.5)5.14 = 115 kPa. Apply a factor of safety of at least 2.5, and your allowable bearing capacity is no more than 46 kPa. And you may need a much larger factor of safety ( because of "guessed" soil properties, risk of loss of life, etc.) That's a far cry from the 490 kPa your equipment needs...

You'll need a lot more than a 600 mm layer of stone to spread your load - probably need a geogrid reinforcement to spread the load and reduce the risk of a bearing capacity failure when simply moving the equipment. And use large timber mats under the tracks/wheels when picking up loads to spread the load and reduce the applied pressure. Depending on your loads, site layout, etc. you may also have to do a lot more site preparation to use that equipment.





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[markbrowny12]

Thanks for your advise Focht3 - they have been extremely helpful and it is very much appreciated.

I will try and keep you posted on the outcomes and thanks again!!

 

[DRC1]

A 6oo Tonne crane w/ 90 tonne pick at 50 m is a significant pick. Note that the geometry of the crane and the amount of counterweight (assuming it is a varrible CW rig) will greatly effect the distribution of the load. Under maximum pick conditions, the bearing pressure at the front of the crane will be much greater than the average. Although bearing pressure will probably control, Elastic settlement of the clay should also be checked, which may require oedometer testing. Note that that most allowable bearing pressures are based on 25 mm of settlement. Check that the differential settlement is not more than required to maintain stability (usually 1 degree out of level). Also note that the controlling condition may be with full CW, no load and boom up and max bearing on the rear of the crawlers. Checking crane stability is a tedious task with few shortcuts. This is a big pick with a big rig and you should engage a soils engineer to review it. I can't say with out checking, but 600 mm of crushed concrete may be a little light. If you are renting the crane, talk to the crane company. They should be glad to get involved in insuring the site is satisfactory.
Good Luck