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I am studying geotechnical engineering and specifically the chapter in B.M. Das book of the same name, 6th edition.

On the topic of shear forces, reference to internal friction angle, $\phi$ is made. My understanding is that the friction angle is the angle in a right triangle with legs of magnitude $F_N$,force normal to a specific plane, and $\mu F_N$, the friction force parallel with the plane, where $tan(\phi)=\frac{F_N \mu }{F_N}=\mu$.

My question is if $\phi$ is constant for all planes in a specific soil, i.e. isotropic, or can it be anisotropic?

Further, is the answer the same for rocks and minerals?

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No, research has shown inherent soil anisotropy, such as naturally deposited sand's bedding angle, has a substantial correlation with the stress-strain response of sand. research

For instance, Oda and Koishikawa (1979), Siddiquee et al. (2001), and Azami et al. (2010) showed that the bearing capacity of shallow foundations resting on inherently anisotropic sand decreases with the increase in the inclination of the bedding plan

Quoted from the linked document!

Rocks of a sedimentary nature, as well, are showing major differences in shear friction angle. Slates is a good example!

shear strength and bedding angle

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