Effect of foundation width of shear based bearing capacity
Effect of foundation width of shear based bearing capacity
(OP)
As a general concept explained in geotechnical engineering textbooks, shear based bearing capacity is explained to be increasing with foundation width. Since the 'width' component in Terzaghi's Bearing capacity equation increases, hence is the corresponding increase in ultimate bearing capacity in shear. Physical understanding of this phenomenon is also simple that with increase in foundation width length of failure path increases; longer failure path offers greater resistance to failure (i.e., 'unit resistance' multiplied by 'length of failure path' would be larger). A typical figure is shown in the attachment.

The use of Mayerhof or Hansen bearing capacity equations (i.e., by incorporating shape, depth, inclination factors, etc.) however, gives result which are opposite to the trend explained in previous passage. By using Mayerhof or Hansen factors ultimate bearing capacity appears to decrease with foundation width. A close look into the data show that main reason for this decrease are the values of depth factors 'dc' and 'dq' which decrease with foundation width. I have been searching for a physical explanation of this decrease in ultimate bearing capacity with foundation width, but could not find any. I have even gone through original papers Mayerhof (1963) and Hansen (1970) but they are also silent in this regard. Any valuable input in this regard is appreciated.

The use of Mayerhof or Hansen bearing capacity equations (i.e., by incorporating shape, depth, inclination factors, etc.) however, gives result which are opposite to the trend explained in previous passage. By using Mayerhof or Hansen factors ultimate bearing capacity appears to decrease with foundation width. A close look into the data show that main reason for this decrease are the values of depth factors 'dc' and 'dq' which decrease with foundation width. I have been searching for a physical explanation of this decrease in ultimate bearing capacity with foundation width, but could not find any. I have even gone through original papers Mayerhof (1963) and Hansen (1970) but they are also silent in this regard. Any valuable input in this regard is appreciated.





RE: Effect of foundation width of shear based bearing capacity
LeonhardEuler (Structural
Look at my post and the reference by Hough.
RE: Effect of foundation width of shear based bearing capacity
RE: Effect of foundation width of shear based bearing capacity
RE: Effect of foundation width of shear based bearing capacity
If we keep the depth fixed and increase the width, these factors appear to keep on decreasing since D/B ratio would decrease (see sample calculations in the image below).
RE: Effect of foundation width of shear based bearing capacity
Trial-1
Sand
phi=31
c=0
Df=2m
Result: Qult (more or less) increases with foundation width
Trial-2
Sand
phi=31
c=0
Df=7.5m
Result: Qult decreases with foundation width
Trial-3
Clay
phi=0
c=50kPa
Df=2m
Result: Qult decreases with foundation width
Trial-4
Clay
phi=0
c=50kPa
Df=7.5m
Result: Qult decreases with foundation width
So a conclusion I have reached from the above trials is that,
- Qult for sands decreases with width as long as D/B>1, and for D/B<=1, it increases with width.
- Qult for clays decreases with width irrespective of D/B ratio.
RE: Effect of foundation width of shear based bearing capacity
Nonetheless, it's important to remember that, while the stress is decreasing, the overall load that can be taken is increasing considerably (force = stress * area).
RE: Effect of foundation width of shear based bearing capacity
As an idea I wonder if it has anything to do with the concept that the foundation "recruits" an area of soil outside of its planar boundary. what I have seen is roughly an additional 2 feet on each side. This number is essentially fixed and doesn't vary with foundation width. In that way the percentage of additional soil recruited for bearing capacity is greater for a more narrow foundation than it is for a wider one. I have no reference to back this up, but maybe its good for thought and others can weigh in.
RE: Effect of foundation width of shear based bearing capacity
Anyway, I think that one of the reasons for the bearing capacity to decrease when using the Depth Factors is because the effective angle of friction reduces with depth (increasing of confining stress). For example, look at the non-linear hyperbolic soil model from Duncan and Chang. Since the bearing capacity factors depend on the friction angle, perhaps this was taken into the account. Appears that Brinch Hansen depth factors and the hyperbolic model by Duncan and Chang were published in the same year (1970) so perhaps Brinch Hansen considered the hyperbolic model when he developed the depth factors.
I was looking at my school notes and recalled that increasing the depth or effective confining pressure suppresses the dilation that occurs. So at high effective confining pressures, all that is measured is the sliding friction (constant volume shear strength). As the effective confining pressure decreases, the tendency for dilation (particles "climbing over one another") increases. Note that the effective friction angle is the sum of the sliding friction angle plus the dilation angle.