Modified Schmertmann’s method

Modified Schmertmann’s method

The original Schmertmann’s equation can be rewritten as 

       (29)

where, 

is the net induced vertical stress at layer i calculated based on Boussinesq's equations. 

The original Schmertmann’s method is only applicable to a single footing. With the modification shown in Eq. (25), it is capable of being applied for multiple footings or overlapped footing loads such as those in a fluid tank which may include fluid pressure, column loads, and ring-wall load.

For foundation settlement analyses, a representative constrained (confined) modulus (M) of the supporting soil medium is usually sought (Mayne et al., 2007) and used in the calculation.  However, Equivalent Modulus of Elasticity, Es, implicitly reflects the lateral strains in the soil. Thus, it is larger than the modulus of elasticity, E (also known as Young's modulus), but smaller than the constrained (confined) modulus, M (Coduto, 2001). 

For practical problems, an average value may be used.

               (30)

Considering the modulus degradation of soils (see Nonlinear Compression), an average value of 0.94 is obtained to relate to initial shear modulus, G0, for degree of loading of 0.2 to 0.5 and Poisson's ration of 0.25 to 0.4. 

       (31)

Subdividing layers

The accuracy of the Schmertmann method improves when the strain profile shown in Figure 7 is sampled more densely. If the soil profile is fairly homogeneous, it is tedious to specify many layers with the same properties in order to improve the accuracy. For these reasons, the Schmertmann calculator will automatically subdivide layers so that each sub-layer has a thickness of approximately B/10.