So far, under soil strength section, we have explained total and effective stresses, pore pressure, angle of internal friction, shear strength, lateral earth pressure coefficient, and cohesion of clay.
Now we are ready to start seeing how these are actually used, when designing foundations.
In this subsection we will now learn about bearing strength, which is a direct result of shear strength, which in turn is a direct result of angle of internal friction, that we had introduced.
Soil under footings can fail in two ways:
- bearing failure.
- excessive settlement failure,
First, the bearing capacity is checked, but even if it is found to be sufficient, the settlement must also be checked regardless (which, in practice actually, is the governing criteria in majority of the cases). In other words, a soil can bear a stress applied by a foundation, but it may do so by so much settlement, which could be over acceptable limits, and this can still be considered as failure. If that is the case, then the bearing capacity of the soil or the stresses from foundation on that soil must still be adjusted, so that the settlement will be within acceptable limits (there is not just such thing to totally eliminate settlement, at least theoretically, so in practice, we limit it to what we consider acceptable). So we will cover bearing strength in this subsection, and soil settlement in the following subsections.
The bearing capacity of the soil means how much load can the soil below the foundation can totally carry, without undergoing shear failure and large settlements. Basically, the total load applied by the column above is divided by the area of the footing, to obtain applied stress, as stress equals: load divided by area, as we have seen before.
If this stress value is more than the bearing strength of the soil, then,
- either the load coming form the column must be decreased,
- or the area of the footing must be increased which means a bigger footing,
- or, the soil bearing capacity must be improved.
Bearing failure occur differently, depending on soil strength. The figure below shows the general shear failure and it occurs in soils with greater strength, such as dense sands, strong clays such as clays that were not subject to larger loads in the past (normally consolidated clays, which we will explain), or even some rocks:
In the figure above, it can be seen how shear strength and angle of internal friction is used, to find bearing strength. You can also see in first figure that, the footing is not on ground surface but at a certain depth. This is a good thing… Because, the soil above the footing presses the soil wedges on sides of the footing, that would otherwise fail and heave, as in figure 15b.
This type of failure as in the figure above happens in strong soil only. If the soil is loose or weak, the footing just sinks, as in figure below, as it does not have the strength to form wedges like above. This type of failure is called punching shear failure:
For soils in between, the shape of failure is something in between the two cases we showed, and that one is called local shear failure.
As we saw previously under the section Structural Basics > Strength of Materials, every material has an ultimate strength, and also an allowable limit that is actually used for design. This applies also to soils. The bearing failure (shear failure) figures you see above are not actually the state to which we design to. The figures above are the ultimate shapes, where total collapses occur, when the ultimate bearing capacity is reached. But the actual value we design to, which is called allowable bearing capacity, is far less than these ultimate states. We divide the ultimate value by a safety factor (which varies between 2-3), to reach to an allowable bearing capacity value. That allowable capacity value, is what we use as our criteria, when deciding if a soil is safe against bearing failure or not. Because before reaching figures above, there are already some local failure, which means bearing failure still. To be consistent with the level of this book, we will not go into further details of bearing capacity calculations here.
Asymmetric loading of a footing, and bearing of soil:
When horizontal loads, such as earth pressure, wind, earthquake etc.. acts on a structure, a turning effect (moment) is created on the foundation, where it can decrease the load or even create an uplift on one side of the foundation.
So, the foundation must be designed such that no part of it will loose contact with the soil, because if it happens, it means our foundation is now suddenly smaller. So then the same vertical load that always exist, must now be carried by a smaller soil area that is still in contact with foundation. As there is less area to resist the same load, this means increase in load per area, in other words, increase in stress. This can now mean that the bearing capacity of the soil be exceeded. So to prevent this loose of contact situation, the foundation itself must be wide and heavy enough.
In this figure, you can see that one side of the foundation is under more pressure. In fıgure 17a, the right side is still under pressure here, which is the desired situation. The design must be made such that the pressure will not fall to zero anywhere at the bottom of foundation. To achieve this, M<PB/6 and e<B/6 must be satisfied. “e” is the eccentricity of the resultant force relative to foundation geometric center.
The pressure may fall to zero, or worse, the zero point may even move leftward, which further reduces foundation contact area and increases stress on soil as in figure 17b above.
Of course we showed eccentricity in one plane, but it can also happen in two perpendicular planes or even as inclined loading in real life, which would have different equations but the main logic is that the footing must not loose contact with the soil.
In the next post of this series, we will discuss “Stress Increase in Soils”