Unsaturated Soil Mechanics

In unsaturated soils, matric suction (negative pore pressure) not only influences the hydraulic conductivity but also contributes to effective stress, and strength. Several constitutive models, suction-stress coupling parameters, and hydraulic parameters are available to describe these relationships between mechanical and hydraulic behavior. This chapter discusses the general overview of the available methods to represent unsaturated soil response, while detailed formulations about the Soil behaviors in unsaturated zones can be found in the linked theory manual.

RS3 offers following methods to explicitly control the effective stress principle and constitutive behavior of soil with respect to the change in pore pressure under the Strength tab of the Define Material Properties dialog:

• Unsaturated Zone Calculations > Unsaturated Behavior: Single Effective Stress;
• Unsaturated Zone Calculations > Unsaturated Behavior: Unsaturated Shear Strength (only compatible with Mohr Coulomb model); and
• Failure Criterion: Barcelona Basic

For more explanations about the methods above, see Unsaturated Zone Calculations page.

Hydraulic Parameters

All hydraulic parameters exert a direct or indirect influence on groundwater flow in both saturated and unsaturated soils, though the magnitude of influence varies. Among these, water content controls and the Maximum Negative Pore Water Pressure parameter play a direct role in governing soil compressibility and consolidation behavior. For detailed information about these parameters along with other hydraulic property controls, see General Hydraulic Properties and Finite Element Hydraulic Properties.

Maximum Negative Pore Water Pressure

The Maximum Negative Pore Water Pressure parameter defines the upper limit on the matric suction (negative pore pressure) considered in effective stress (σ′) calculations for the selected material. This limit prevents unrealistic increases in shear strength in soil above the water table. The cutoff affects only the computation of effective stress and is not applied to the pore pressure field used in hydraulic (seepage) analysis.

Water Content and Degree of Saturation

The water content or degree of saturation describes the quantity of water within a pore space. They are only computed when Steady State or Transient analysis are enabled (Project Settings > Groundwater). Based on the hydraulic (permeability) model of choice, which governs the Soil Water Characteristic Curves (SWCC) definition, the water content and degree of saturation distributions are computed with respect to the matric suction. The computed magnitude of water content at each Gauss point of the solid elements is used to update the stress and/or strength of soil.