ISAHypoplasticity (for sand)
File: Presentation ISAHypoplasticity for cyclic loading. Viena 2018 Fuentes
File: Presentation How to make Umats for soils
Contact
For questions, please write to Prof. William Fuentes (fuentesw@uninorte.edu.co)
Courses on:
Abaqus for soils. Basic examples on Geotechnical Engineering.
Abaqus Umat (user material) implementation for beginners.
Abaqus UEL (user element) implementation for beginners.
Hypoplasticity and ISAhypoplasticity for cyclic loading.
Simulations of element test

Undrained triaxial, constant deviator stress amplitude
2. Undrained triaxial: constant axial strain amplitude
3. Oedometric test: with unloadingreloading cyle:
Papers
1) Evaluating the performance of an ISAHypoplasticity constitutive model on problems with repetitive loading. File: Download
2) An ISAplasticity based model for viscous and nonviscous clays. File: Download
3) On the modeling of multidimensional cyclic loading with an intergranular strain approach. File: Download
4) ISA: A constitutive model for deposited sand. File: Download
Note: All examples with ISAhypoplasticity for sand were simulated with the following parameters:
Example 1 Pile under cyclic axial loading simulated with the ISA model
Description:
On this analysis a concretefilled pile driven into a thick and homogeneous sand layer and subjected to a cyclic axial load is simulated. The pile has diameter D = 0.80 m and length L = 13.0 m. Taking advantage of the symmetry of the problem, an axially symmetric condition is considered, therefore fournode axisymmetric elements CAX4 are used. The ground water table lies on the top of the soil. An axial cyclic load with frequency f = 4 Hz is applied as a concentrated force at the top of the pile. Due to the cyclic loading the pore pressure increases and accumulates and the effective stress decreases causing zones in which liquefaction may occur.
Fig. 1 (a): Problem sketch (b): Overview of the FEM model
Two parts were considered for the 2DFE model: the soil part and the pile part. The pile material (concrete) is simulated as elastic with E = 25 e6 kPa and Poisson ratio of 0.3. The boundary and initial conditions are set as shown in Figure 1b. A surface to surface contact interaction is set for the soilstructure behavior, including a penalty friction formulation and the “Hard” Contact normal method.
 On the first step, the geostatic equilibrium is achieved. The initial geostatic stresses are calculated with the initialstate.for file. The total step time is 1 s.
 On the second step a vertical load of 600 kN is gradually applied at the top of the pile. The total step time is 0.1 s and the time increments 0.01 s.
 On the third step the pile is unloaded and reloaded several times. The step is solved under dynamic conditions. The total step time is 5 s and the time increments 0.01 s.
Fig. 2 Cyclic load at the top of the pile
Files:
 input files: input files – deep foundation
 Presentation: Pile under cyclic axial loading ISA hypoplastic model
Description of the constitutive model, element tests, FEproblem.
Results:
Video of the simulation – Liquefaction zones due to cyclic loading on granular soil
Fig. 3 Liquefaction zones at the end of the simulation – mean effective stress p’ values lower than 10 kPa
Fig. 4 Vertical displacements under the pile
Fig. 5 Mean effective stress p’ under the pile
Example 2 Pile under cyclic axial loading simulated with the ISA model
Description:
On this analysis a strip footing on a thick and homogeneous sand layer and subjected to a cyclic vertical loading is simulated. The strip footing has a base of B=1.0 m and its base is located 1 m below the surface. A 2DFE model is constructed with plainstrain condition, therefore fournode plane strain elements CPE4 are used. There is no ground water table near the surface.
Fig. 1 (a): Problem sketch (b): Overview of the FEM model
The boundary and initial conditions are set as shown in Figure 1b. A coupling constraint is set for the nodes below the footing to assure that they have the same vertical displacement.
 On the first step, the geostatic equilibrium is achieved. On this step, a constant pressure of 17 kN/m3 is applied on the top of the soil part to account for the 1m soil layer between the surface and the bottom of the footing. The initial geostatic stresses are calculated with the initialstate.for file. The total step time is 1 s.
 On the second step a vertical load of 20 kN/m is gradually applied as a pressure on the base of the footing. The total step time is 0.1 s and the time increments 0.01 s.
 On the third step the footing is unloaded and reloaded several times. The step is solved under dynamic conditions. The total step time is 5 s and the time increments 0.01 s.
Fig. 2 Cyclic load applied on the strip footing
Files:
 input files: input files – shallow foundation
Results:
Video of the simulation – Vertical displacements
Fig. 3 Final vertical displacements
Fig. 4 Displacements under the footing