In an earlier post Analytical Boussinesq Solutions for Strip, Square and Rectangular Loads, we discussed elastic solutions for these types of foundations. Most of the results shown were for perfectly elastic foundations. In this post we will concentrate on a) circular foundations and b) the difference between rigid and elastic foundations, and those which find themselves in between.
Engineers have been familiar with charts such as this, from NAVFAC DM 7.01:
The influence coefficients (see diagram above) for the vertical, horizontal and shear stresses respectively directly under the centre of the load (x = 0) are not difficult to compute, being
For just about everywhere else (except for the edge) closed form solutions are hard to come by. Why is this? Because, for most every other point, the solution for the influence coefficients involves the use of elliptical integrals. An illustration of the values of these is shown below.
Although mathematical packages such as Maple and Matlab are certainly capable of evaluating these, they are still not the common companions of engineers. Fortunately most of the questions about the stresses under circular foundations centre (sorry!) around the point under x = 0, so the above formulae are useful.
Foundations Rigid and Flexible
Up to this point, we’ve considered for the most part the response of the soil–both stress and deflection–to a purely flexible foundation. For these foundations at the soil-foundation interface the pressure exerted on the foundation and the pressure the foundation exerts on the soil is the same. If there is any rigidity in the foundation–and virtually any foundation has some–then both deflections and stresses in the foundation are redistributed.
It’s probably useful to note that, for deflections in general, the formula we use is
- settlement of the foundation at the point of interest
- influence factor
- uniform pressure on the foundation
- smaller dimension of rectangle or dimension of square side
- Poisson’s Ratio of the soil
- Modulus of elasticity of the soil
The values for are given below.
Turning to the flexible circular foundation, the value for for all of the radii can be computed using the formula (Timoshenko and Goodier (1951)):
This still has a complete elliptical integral of the second kind, but it more manageable. It can be solved by applying the trapezoidal rule and using small integration increments over the interval. Values for for ratios of various values of x (see diagram above) to the actual radius of the circle are shown below.
Although many elastic calculations assume the flexible foundation, as noted earlier in reality foundations have rigidity. For a perfectly rigid foundation, the deflection of the entire foundation under a concentric load is uniform. The effect of this on the stresses can be seen below.
For the foundation in (a), if it is a circular foundation, the vertical stresses at the base can be computed by the formula
At the corners of the foundation, the stresses are theoretically infinite. This means that the lower bound solution for such a foundation is zero stress. In reality it is reasonable to assume that a) no foundation is perfectly rigid and b) the soil will proceed into plastic deformation, which will redistribute the stresses.
It is interesting to note that, for the strip loads we discussed previously, the distribution for a rigid strip is similar to the circle, thus
where the notation is as it was in that discussion.
The figure (b) above shows different vertical stress distributions for different flexibility ratios, given by the variable . This can be approximated by the formula (Tsytovich (1976))
- modulus of elasticity of the soil
- half-width of the foundation, shown above
- modulus of elasticity of the foundation
- height of the foundation
To get a better approximation would require plate theory, where the mathematics are very involved; an example of this, using a pile toe where the circle is loaded around the edge, can be found here.
The rigidity of the foundation also influences the distribution of stresses under the foundation, as shown below.
There’s certainly life beyond elastic theory. In reality the type of soil affects the distribution of contact pressure on the foundation, even with a uniformly loaded foundation, as shown below.
The clay distribution most resembles that of an elastic response of a soil with a rigid footing, as discussed earlier. The purely flexible foundation will have the same uniform reaction as the load, as noted earlier.
- Foster, C.R. and Ahlvin, P.G. (1954) “Stresses and Deflections Induced by Uniform Circular Load.” Highway Research Board Proceedings, Highway Research Board, Washington, DC.
- Jahnke, E. and Emde, F. (1945) Tables of Functions with Formulae and Curves. New York: Dover Publishers.
- Timoshenko, S., and Goodier, J.N. (1951) Theory of Elasticity. New York: McGraww-Hill Book Company, Inc.