lateral spread in soft soil overlying stiff soil layer
lateral spread in soft soil overlying stiff soil layer
(OP)
In general the stress distribution below a footing spreads laterally at a 2V:1H ratio. I know that if the footing was resting on a soft layer of soil overlying a stiffer soil layer that the lateral spread would be greater than the 2V:1H generalization. Can anyone provide some numbers to the amount of lateral spread or can someone recommend a reference where I can read about the topic? Thank you in advance for your help.





RE: lateral spread in soft soil overlying stiff soil layer
Why ask about "spreading the spread"? Are you calculating possible settlement? If so, reducing the spread will give you a safer result, so why risk it? What about shear bearing failure in soft stuff? That may control what you are working with.
Also, I'll bet others will question being so sure about that 2 to 1 figure and being so sure about it changing for soft soil.
If I had to guess, I'd say the reverse is true.
RE: lateral spread in soft soil overlying stiff soil layer
f-d
¡papá gordo ain't no madre flaca!
RE: lateral spread in soft soil overlying stiff soil layer
Oldestguy: Sorry,I phrased my question wrong. What I meant to say is that it is generally accepted that the load spreads horizontally from a single edge of a strip footing at a rate of 2V:1H.
Fatdad: I will look into the Westergaard equations to see if they deal with varying layers of soil. I have been using Bousinesq's in the past but it would really be valuable if Westergaard provides this added advantage
What I was looking for is advice on how the combination of soil layers with extreme variations in stiffness affects the stress distribution below the layer. I remember reading a section of a book that describes this effect and seeing drawings depicting how the combination of layers effect the spread of the load. I can not remember the source of the information.
I had hope to provide this information to some of my coworkers who are working on a downdrag problem in soft clay underlain by a layer of sand which is underlain by another layer of soft clay. The neutral axis of the pile is located in the upper layer of soft clay. The pile is tipping in a sand layer. The settlement is calculated for the equivalent footing located at the neutral plan using a Bousinesq stress distribution. There is a lot of settlement predicted in the clay layer underlying the sand layer. I was wondering if the stress distribution would flatten out when it hit the sand layer thus reducing the stress in the underlying soft clay.
Thanks again everyone for taking the time to respond.
RE: lateral spread in soft soil overlying stiff soil layer
RE: lateral spread in soft soil overlying stiff soil layer
He states that for square or circular foundations, the bearing pressure on the soft (lower layer), (q1) is equal to:
q1 = qn (B/(B + d))^2
or for strip footings:
q1 = qn (B / (B + d))
where B is the breadth of the foundation, qn is the bearing pressure from the foundations and d is the thickness of stiff soil(upper layer) below the base of the foundaiton above the soft layer (lower layer). This is based upon a 1:2 or 30º. I have also seen the french method of calculating working paltforms, where they use the angle of internal friction for granular layers to calcuate the corresponding 'footprint' of a loaded area on underlying layers of soil.
This does all seem to be based around a global poissons ratio of 0.5, although accroding to Tomlinson (trying to keep to the same reference here), only undrained clays have a v of 0.5, stiff undrained clays are in the order of 0.1 to 0.2, silt = 0.3, sands = 0.1 to 0.3 and rocks are around 0.2.
On the point of assessing the settlement to a layer which can be deemed to be 'relatively incompressible' Tomlinson also suggests limiting the effect of settlemnt analysis to where the applied vertical pressure reduces to less than 205 of the overburden pressure.
RE: lateral spread in soft soil overlying stiff soil layer
RE: lateral spread in soft soil overlying stiff soil layer
http://www.ce.ncsu.edu/usucger/PandD/PandD.htm
RE: lateral spread in soft soil overlying stiff soil layer
RE: lateral spread in soft soil overlying stiff soil layer
RE: lateral spread in soft soil overlying stiff soil layer
RE: lateral spread in soft soil overlying stiff soil layer
RE: lateral spread in soft soil overlying stiff soil layer
RE: lateral spread in soft soil overlying stiff soil layer
RE: lateral spread in soft soil overlying stiff soil layer
RE: lateral spread in soft soil overlying stiff soil layer