BEARING CAPACITY OF LAYERED SOILS
No simple, satisfactory, analytical method is currently available to determine the bearing capacity of layered soils. Analytical methods are available for two layered soils (see the author’s textbook Foundations and Earth Retaining Structures, John Wiley & Sons, NY). If the thickness, H1, of the soil below the footing in the top layer (Figure ) is greater than
the failure surface will be confi ned in the top layer and it is sufficiently accurate to calculate the bearing capacity based on the properties of the soil in the top layer. Otherwise, the failure surface
Footing on a two-layer soil
would be infl uenced by the bottom layer and may extend into it. Alternatively, Hcr can be calculated from
where (qu)t is the ultimate net bearing capacity of the top layer and (qu)b is the ultimate net bearing capacity of the bottom layer with a fi ctitious footing of the same size and shape but resting on the surface of the bottom layer. A geotechnical engineer can apply a set of practical guidelines or use numerical tools such as the fi nite element method for analyzing layered soils. The basic problem lies in determining and defi ning the soil properties for layered soils. We will resort to some practical guidelines for three common cases: a soft clay over a stiff clay, a stiff clay over a soft clay, and thinly stratifi ed soils.
Soft clay over stiff clay: In general, shallow foundations on soft clays should be avoided except for lightly loaded structures such as houses and one-story buildings. You should investigate removing the soft clay and replacing it with compacted fi lls. An inexperienced geotechnical engineer should calculate the bearing capacity using the methods described previously before making a decision to replace the soft clay. Stiff clay over soft clay:
The bearing capacity for this case is the smaller value of 1) treating the stiff clay as if the soft clay layer does not exist and (2) assuming that the footing punches through the stiff clay and is supported on the soft clay. The bearing capacity is the sum of the shear required to punch through a vertical plane in the stiff clay and the bearing capacity of the soft clay layer. Only a fraction, about 23 to 12 , of the undrained shear strength should be used in computing the shear resistance on the vertical plane in the stiff clay layer. Another method is to place an imaginary footing on the soft clay layer with dimensions (B 1 tsc) 3 (L 1 tsc), where B and L are the real width and length of the footing and tsc is the thickness of the stiff clay layer below the base, and then calculate the ultimate net bearing capacity using the bearing capacity equations.
Thinly stratifi ed soils: In this type of deposit, deep foundations should be used. If deep foundations are uneconomical, then the bearing capacity can be calculated by using the shear strength parameters for the weakest layer. Alternatively, harmonic mean values for su and f' can be calculated and then these values can be used to calculate the bearing capacity.