Thesis
Geostatic Stress Calculation in ABAQUS for unsaturated sands
Dilan J Robert and Arul M Britto
Engineering Department Cambridge University
Introduction
The aim of this paper to study the effect the initial stresses has on the geostatic step and the subsequent analysis.
In section 01 constant pore pressure profile is specified and where the effect of specifying the correct and incorrect initial stresses on the geostatic step are explored.
In section 02 pore pressures are varied linearly with depth and the initial effective stresses are correctly specified from the total stresses.
Section 01: Geostatic step stress calculation
• ABAQUS uses the initial stresses specified by the user (*INITIAL CONDITIONS, TYPE=STRESS, GEOSTATIC) as an initial guess (or as a start in the process of getting a converged stresses for the start of the analysis). This initial stress specification has a great impact on the subsequent steps. i.e. Deviation of the initial stress values from the actual, would result in incorrect higher soil displacements of the model which in turn leads to instabilities and analysis termination. • ABAQUS calculates the stresses (total stress) which is in equilibrium with the external loading (in this case it is gravity) and boundary conditions. However it should be pointed out that the argument holds even in the absence of any gravity force as in the case of a triaxial sample in which case the problem becomes a trivial one. • The displacement that occur during the geostatic step is not due the external loading but due to the difference between the user predicted initial stresses and the converged stresses calculated by ABAQUS which is in equilibrium with the external loading. Eg: Assume an unsaturated sand model (depth=1.115m) with moisture content 17.7% (saturation 60%) having a dry unit weight of 1520 kg/m3.
Dilan J Robert and Arul M Britto
Engineering Department Cambridge University
Introduction
The aim of this paper to study the effect the initial stresses has on the geostatic step and the subsequent analysis.
In section 01 constant pore pressure profile is specified and where the effect of specifying the correct and incorrect initial stresses on the geostatic step are explored.
In section 02 pore pressures are varied linearly with depth and the initial effective stresses are correctly specified from the total stresses.
Section 01: Geostatic step stress calculation
• ABAQUS uses the initial stresses specified by the user (*INITIAL CONDITIONS, TYPE=STRESS, GEOSTATIC) as an initial guess (or as a start in the process of getting a converged stresses for the start of the analysis). This initial stress specification has a great impact on the subsequent steps. i.e. Deviation of the initial stress values from the actual, would result in incorrect higher soil displacements of the model which in turn leads to instabilities and analysis termination. • ABAQUS calculates the stresses (total stress) which is in equilibrium with the external loading (in this case it is gravity) and boundary conditions. However it should be pointed out that the argument holds even in the absence of any gravity force as in the case of a triaxial sample in which case the problem becomes a trivial one. • The displacement that occur during the geostatic step is not due the external loading but due to the difference between the user predicted initial stresses and the converged stresses calculated by ABAQUS which is in equilibrium with the external loading. Eg: Assume an unsaturated sand model (depth=1.115m) with moisture content 17.7% (saturation 60%) having a dry unit weight of 1520 kg/m3.