Meyrhof's Bearing Capacity equation 1

[CT9904]

I'm a fresh graduate, not my first time here but my first time posting, so bear with me. Here's my problem I was trying to calculate the bearing capacity of a soil for a project, I was working on. The ultimate bearing capacity equation used was Meyrhof's. I noticed that the bearing capacity would drop as the width of the footing increased. This seemed contrary to what I have studied; therefore, I tried to plot Ultimate Bearing capacity Vs. Width for a square isolated footing resting on a soil with C = 0 KPa, Φ = 11°, γ = 18KN/m³ and foundation depth(Df)=3m. The result was a non-linear plot with Bearing Capacity decreasing with width of the footing. I have attached the plot and the python(numpy+scipy+matplotlib)code used to produce the plot. Now, my question is shouldn't ultimate bearing capacity of an isolated footing increase with increase in width?

 

[oldestguy]

First off remember there are many different users here with different unit systems. You say nothing about that or show that bearing capacity usually is a pressure as unit of load per unit of area. So as you increase the area, by the abscissa, the total load increases and thus more settlement or tendency to shear results. And to keep that within failure limits, you reduce the unit pressure. Forgetting to designate what the numbers mean leaves you with the question.

I might add, to help clear this up to design a series of footings for equal settlement, the larger total loads require a larger footing area, but at a lesser bearing capacity than for smaller loads.

 

[geomane]

See the attached figure from the FHWA Soils and Foundation manual Vol II.

What type of soil are you dealing with that has those properties (i.e. Φ=11° and c=0kPa)?

 

[BigH]

jmcc3265 has presented a nice generalized graph - I developed similar graphs back in the 70s for Sarnia Ontario clays.

However, it irks me - perhaps not others, when geotechnical engineers refer "Allowable Bearing Capacity" - when, in fact, shear does NOT govern but settlement governs. In my view - allowable bearing capacity is a "shear/strength" issue; allowable bearing pressure would be best used if it is a serviceability issue. In this way there is no confusion.

 

[oldestguy]

Hey BigH: Right on the money with that. In his classic paper in 1959 my old boss and college prof B.K.Hough uses the term "Soil Bearing Value" and he tied it to settlement. His first sentence of the abstract reads: "The value and practicality of rupture theory as the chief basis for bearing value determination is questioned. The paper title: "Compressibility as the Basis for Soil Bearing Value". It resulted in shaking up discussors such as Peck and Ireland.

 

[Okiryu]

MathewosEndeshaw, the issue may be due to the depth factors values. Look at the term "Df/B". If you increase the width of your footing "B", you will be getting smaller depth factor values.