Constitutive Modeling of Cross-Anisotropy in Frictional Soils

Cross Anisotropy Effects on Shear Strength of Fictional Soils

Geotechnical engineers rely on determining the frictional behavior of the soils primarily based on triaxial testing. In this test set-up, the orientation of the applied vertical stress is fixed and the magnitude of both the intermediate and the minor principal stresses are the same. However, in the field applications, complexities associated with (1) orientation of the soil grains in comparison to the applied stress and (2) ratio corresponding to the differences between principal stresses vary. These conditions create cross-anisotropy in soils where the predictions of displacements in field conditions become very complex. When these predictions are attempted with soil constitutive models based on isotropic conditions, the results become inaccurate. An example can be given from an unpublished study where the horizontal deformations of a flexible retaining wall were predicted based on isotropic soil constitutive models. The study showed that the predictions were consistently 50-60% different from those measured in the model experiments that were performed by the Geotechnical Engineering Department at NTH in Trondheim, Norway. This is believed to be due to the employment of isotropic constitutive models with the ABAQUS finite element program and not accurately consider the cross anisotropy in soils.

Previous laboratory studies conducted with torsional shear and triaxial devices have made attempts to quantify the effects of cross anisotropy on frictional behavior of soils. A study conducted by Lade et al. with the torsional shear device showed that the friction angle of the same sand varied from 57 to 32 degrees. These experiments were conducted by simulating different ratios between principal stresses and inclining the applied vertical stress. Another study conducted by Siddharath et al. demonstrated the cross anisotropy due to the orientation of the soil grains in comparison to the applied stress. In that study, the researchers used the triaxial apparatus but prepared their soil samples to be oriented vertical and horizontal to the direction of the applied major principal stress. In both of the examples provided above, the researchers have tested a single type of sand. In this study, these results will be expanded and also recycled materials will be tested.

Laboratory test results obtained in this study will then be used to improve the existing Single Soil Hardening model developed by Lade to incorporate the effects of cross anisotropy. Also, a model to relate the observed behaviors from torsional and triaxial tests will be developed to allow future users to rely on conventional testing methods to quantify cross anisotropy. Once the Single Hardening Model is improved to incorporate cross anisotropy, the predicted soil behavior will be compared against the predictions obtained from other existing constitutive models that are based on isotropic conditions. This comparison will be achieved by both based on numerical analyses and field/large scale laboratory experimentation. The envisioned outcome of the study will be to create a model where the cross anisotropic behavior of frictional materials can be captured accurately.