Materials: Soil

Description

Soil material is available for 2D plain strain, axisymmetric and 3D analysis. The following soil models are available for these analysis types:

The Duncan-Chang Soil model is widely used for the modelling of soil as a nonlinear material. This model, as one of the first models used in finite element analysis, has been shown valid in a range of practical applications. Two fundamental relations used in the Duncan-Chang model are the Mohr-Coulomb failure criterion and the hyperbolic stress-strain relation.
The Modified Cam-Clay Soil model is a widely used plasticity model. The model includes features such as pressure sensitivity, shear induced dilatancy and hardening/softening responses which are in good agreement with experimental observations. The advantage of this model is that it requires relatively few parameters, which can be obtained from conventional laboratory tests.
The Mohr-Coulomb Soil model is a generalised form of the Coulomb friction failure law which considers additional effects from the water content, void ratio, and in-situ stresses. This soil material model is usually easy to define because of the availability of its parameters.
The Drucker-Prager Soil model is an approximation to the Mohr-Coulomb Soil model which also considers additional effects from the water content, void ratio, and in-situ stresses. This soil material model is usually easy to define because of the availability of its parameters.
The Linear Elastic Soil model is an isotropic soil material that never yields. It does consider the effects of water content, void ratio, and in-situ stresses, so it is useful in conjunction with other soil materials in a finite element model. This model requires the least number of material parameters.

Below are some notes regarding the application of soil material models:

Pre-stress effects on all soil materials is described using the in-situ attribute (see Plate Attributes: Soil, Brick Attributes: Soil, and Utility: Soil In-situ).
Soil materials ignore the pre-stress, pre-strain, and thermal strain attributes available for other material models.
Soil materials consider the effect of pore (air/water) in the soil in that they can be either Drained or Undrained.
- Drained: liquid in a saturated soil is free to flow when external pressure is applied, with no change in pore pressure.
- Undrained: liquid in a saturated soil is fully confined when external pressure is applied, with corresponding changes in pore pressure.
The uplift pressure that the water generates due to buoyancy effect is considered in all soil materials.
In a linear analysis such as linear static analysis, the soil stress is reported as the total stress. In a nonlinear analysis such as nonlinear static, quasi-static and nonlinear transient dynamic analysis, the soil stress can be reported as either the total or the effective stress, depending on the selected option under Result Settings: Entity Tabs.
If the void is filled with fluid with non-zero density, the mass of the fluid will be included for both static and dynamic analyses. In other words, the effective mass density of the element will include mass due to the soil bulk mass density plus mass due to the fluid density and element void ratio (for elements that are below the fluid level).
If an element is completely below the fluid level, it is assumed to have all of its voids filled with fluid. In contrast, if an element is completely above the fluid level, it is assumed to have no fluid within its voids. If an element has some Gauss point locations below and some above the fluid level, a proportional amount of fluid is smeared over the entire element.

See Results Interpretation: Result Quantities for a description of available soil results quantities.

See the Straus7 Theoretical Manual for more information.

Literature

Duncan, J.M. & Chang, C.Y. 1970, "Nonlinear Analysis of Stress and Strain in Soils," Journal of the Soil Mechanics and Foundations Division, ASCE, vol. 96, no. SM5, September.

Zienkiwicz, O.C. & Taylor, R.L. 1991, The Finite Element Method, 4th ed., vol. 2, McGraw-Hill, London.

Kondner, R.L. 1963, "Hyperbolic stress-strain response: cohesive soils", Journal of Soil Mechanics and Foundations Division, ASCE, vol. 89, no. SM1, February.

Kondner, R.L. & Zelasko, J.S. 1963, "A hyperbolic stress-strain formulation for sands", Proceedings 2nd Pan-American Conference on Soil Mechanics and Foundations Engineering, Brazil, vol. 1, pp 289-324.

Duncan, J.M., Byrne, P., Wong, K.S. and Mabry, P. 1980, Strength, stress-strain and bulk modulus parameters for finite element Analysis of stresses and movements in soil masses, report No. UCB/GT/80-01, University of California, Berkeley, California.

Naylor, D.J., Pande, G.N., Simpson, B. & Tabb, R. 1981, Finite Elements in Geotechnical Engineering, Prineridge Press, Swansea.

Yetimoglu, T., Wu, J.T.H. & Saglamer, A. 1994, "Bearing Capacity of Rectangular Footings on Geogrid-Reinforced Sand", Journal of Geotechnical Engineering, ASCE, vol. 120, no. 12, December.