Bearing Capacity of Layered Ground 6

[PaulMarkham]

I am a general civil engineer working for a construction company.
We often use mobile cranes on the sites and my task is to design suitable foundations for the outriggers of the cranes.
Often the ground is layered with for example 2m of firm clay over soft clay. To improve the trafficability and bearing capacity we place say 300mm of compacted stone over the firm clay.
We assume a simple 45 degree spread of the load into the ground and check the bearing pressure on each layer.
I have searched the foundation engineering text books but they do not deal with this sort of situation.
Is this a valid approach?
Is there a recognised way of deaing with layered ground?

 

[gandersen]

Dear PaulMarkham:

Braja Das' book "Principles of Foundation Engineering" has a section on the Bearing Capacity of Layered Soils where there is a stronger soil underlain by a weaker soil. In my edition (4th), the section is 3.10.

I am a little perplexed by the question that you are asking and how you pose it in terms of "checking the bearing pressure on each layer". I am not sure what you mean by that.

The bearing capacity formulation is meant to model a condition where the shear strength of the soils underneath a shallow foundation is reached and the entire mass of soil fails by being pushed downward and outward. The actual stress in the failing soil mass under these circumstances are not constant but vary dramatically across the failure zone.

The actual geometry of the failure zone varies based upon the assumptions that are made, but it is relatively shallow and will generally involve those soils to a depth approximately equal to the width of the foundation.

Could you give us some more details about how you are actually running your analysis?

Glen Andersen
BBC&M Engineering, Inc.
Willoughby Hills, Ohio

 

[BigH]

Glen - sorry to but in (I'll answer question later) but I grew up in Willoughby! Drop me an email: bohica@indiatimes.com .

As to layered theory - we had the same thing when we were looking at large draglines sitting on top of highwalls in Fort McMurray Alberta. In this, we looked at the load transferred down to the "next layer" especially if the upper layer was stiffer - and presented a "ficticiuos" footing at that softer layer to see if it were overstressed. There are times that you should be checking to ensure that the shear stress in a layer doesn't exceed the shear strength of the soil at "any" single zone - not taking the "average" zone. This was brought to roost by Tschebotarioff on a large grain elevator where the elevator failed at much less than the ultimate - they found that the zones of first yield (about 3.3xSu) put sufficient zones of clay past their peak and into fully softened or residual strengths - if the clays are sensitive this can be critical.

There have been a number of theories used in layered systems - perhaps the most "traditional" is that of Button (circa 1954) - then there was Srinivasan and others. You can check out "Fang" (the old Winterkorn and Fang) Foundation Engineering Handbook.

 

[Focht3]

[blue]PaulMarkham[/blue]'s approach is pretty traditional - it's not a lot different than using Boussinesq or Westergaard to do pressure distribution with depth, then check the bearing capacity of each layer. As I recall, that's the basis for the procedure in Das' book. (This is how we checked for these kinds of bearing capacity problems in layered soils "in the old days" before computers were widely available.)


The 45&[ignore]deg[/ignore]; angle is too "flat" in my view - a 30&[ignore]deg[/ignore]; (from vertical) projection (or 2V:1H) is more widely used, and somewhat more conservative.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[dirtsqueezer]

I say that although crushed rock does provide strong shear value for soil, that compared to the dynamic forces used in lifting, they will not help distribute your weight, believe, at all. One method of lessening the stress on your clay layer is to set your outriggers on steel plate. I believe you're generally right about the angle of stress. If you distribute your weight over a greater area, the depth and force of your load will decrease, putting less stress on your soft clay layer, and possibly reducing it below the danger zone for your factor of safety. I just recently read an article about a landslide over a 50,000 ft2 area on a 2% slope that occured due to quick clay. I would wary whether you could trust a clay you could call 'soft'. My company usually goes by the strength of the weakest layer when more than one is involved.

I don't think I really answered your question, I'm just talking about one subject discussed in crane picks from the past. It seems there are more elements here that you're not asking about. You may want to consider this before taking on liability for your company. Soils engineering is risky even for those that understand their specialty and locality the best. I believe in any project that proceeding to spend the money to build can have disasterous consequences, much more than the cost of one's salary. Just a word of caution. I hope the best for you.

-JW

 

[tree1]

Paul,

You will never be the hero but you may get to be the goat. HAHA. If you work is in an area that has this typical 2m of firm clay over the soft clay your best short term bet is to grab the oldest crane operator at your company and ask him what works and what doesn't. If you have variability try to get a look at a soils report for the property that you are specifically working on. There is a chance that they will have near-surface shear testing (UU) if you are dealing with soft clays so that they can consider the impact of the soft clay on whatever you are bringing the mobile crane out to build. With this information you can better establish an actual necessary base improvement thickness. I think focht3 and dirtsqueezer gave you pretty good advice. If the thickness that you need for crane operation (outrigger loads) is significantly larger than what you need for trafficability use the steel plate.

 

[AlohaBob]

I don't have an answer for you, however, I can appreciate the importance of stable bearing for your crane.

Several years ago, big blue was rigged to lift a roof section in place for the county stadium. The outriggers bearing in front failed. A wind gust may have contributed to the failure. Several men were killed.

 

[iwudficud]

This is probably a dead issue by now but, here's my two-cents. I have had a similar experience in WA. We had a medium sized crane that sat on weathered bedrock that was fairly strong but, very sensitive.

I used the crane outrigger max load (inluding the lift materials) and a layered system as you mentioned. I remember using a boussinesque distribution to check the pressure at the lower layers. For simplicity I would definately use the 2:1 stress distribution if you have a weaker lower layer.

A shallow granular base is incapable of distributing stress because of it's mobility. We used 3/4 inch plywood with 4x4's both ways and a half-inch steel plate on top. Due to the sensitive nature of the soil duration was a factor but we oversized it and consulted with the senior crane operator (who was familiar with that specific soil type) before finalizing the design. The idea was for the wood to flex and distribute the stress over the surface and the steel prevented the pad from damaging/penetrating the wood.