Back-Analysis of an Embankment Constructed in Soft Soil using the Cysoil Model

Back-Analysis of an Embankment Constructed in Soft Soil using the Cysoil Model

C. Detournay, A. Lucarelli & Varun
Itasca Consulting Group, Inc., Minneapolis, Minnesota, USA

The paper presents a friction hardening/softening elasto-plastic constitutive model (Cysoil) for soils characterized by a non-linear volumetric cap. The model includes built-in evolution laws for shear and volumetric yielding, and is capable of reproducing the hyperbolic type stress-strain behavior observed in drained triaxial tests. The model is used to analyze a practical case history of an embankment constructed on soft soil in rural Minnesota (Figure 1).

Figure 1 Embankment design along with existing ground surface and location of different sensors.

The construction involved excavation, soil replacement and a soil preload and surcharge. Earth pressure, pore water pressure and deformations were recorded for multiple cross-sections as part of the geotechnical performance monitoring and construction control program.

All the parameters for the constitutive model were obtained from cone penetration tests that were carried out as part of a geotechnical site investigation program (e.g., Figure 2 and 3).

Figure 2 Variation of in-situ density (left) and initial Young’s modulus (right) with depth estimated from CPT data.

Figure 3 Variation of friction angle (left) and cohesion (right) with depth estimated from CPT data.

The construction of seven lifts of 2-ft (0.6-m) thickness each was taken into account in the simulation. A close-up view of the FLAC model showing the embankment is presented in Figure 4. Simplified two-way hydromechanical coupling is used for placing the lifts and involves instantaneous undrained analysis (e.g., change in pore pressure due to undrained deformation), followed by simulation of pore pressure dissipation.

Figure 4. Numerical model showing embankment geometry, lifts and location of different sensors.

The pore pressures, displacements and total earth pressure predicted by the model are compared to field measurements in Figures 5 to 7.

Figure 5. Comparison between excess pore pressures obtained from numerical model and field data.

Figure 6. Comparison between vertical displacements at different location on top of loading platform obtained from numerical model and field data.

Figure 7. Comparison between earth pressures obtained from numerical model and field data.

The pore pressures, displacements and total earth pressure predicted by the model were found to be in good agreement with field measurements. While the model is flexible enough to capture different aspects soil behavior, it doesn’t suffer from the drawback of too many non-physical parameters, as most parameters can be calculated easily from field tests that are carried out routinely during site investigation, and default values can be used for the rest of the parameters.

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