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[SSS05-01] Formation simulation of imbrication and décollement under uniform compression based on the elasto-plastic soil mechanics
Keywords:Compression, Thrust fault, Imbrication, Décollement, Elasto-plasticity, Finite element analysis
A wide variety of topography is known to be formed in the ground subjected to lateral compression, depending on the constituent materials and boundary conditions. In terms of a single displacement discontinuity, there exist "reverse faults" over 45 degree, “thrust faults” under 45 degree, and horizontal "décollements". A partitioned ground by the faults exhibits a “pop-up” structure with conjugate slip surfaces or “imbrication” with parallel slip surfaces. To simulate their irreversible and multi-step formation process, (1) the elasto-plastic constitutive model for describing material nonlinearity and (2) finite deformation scheme for tracing changes in shape and state of the ground are necessary. In this abstract, we introduce the results of the formation analysis of these structures based on our analysis code GEOASIA [1] that can consider (1), (2), the presence of pore water, and dynamics.
First, an elasto-plastic deformation analysis was performed with three plane-strain finite element meshes with different boundary conditions shown in Fig. 1 (a)-(c). Here, we conducted a single-phase elasto-plastic analysis considering the self-weight of the model. SYS Cam-clay model [2] was incorporated as an elasto-plastic constitutive model. The overconsolidated parameters in Noda et al. [3] exhibiting remarkable softening behavior after the peak strength (Fig. 2) were used. The shear strain distributions in Fig. 3 indicate that the analysis without fixation of the bottom displacement in (a) exhibited pop-up, whereas the analysis with the prescription of the linear bottom displacement distribution (uniform compression) in (b) obtained imbrication. Furthermore, the separation of the imbrication and the thrust sequence, i.e., piggyback in (b) and overstep in (c), were affected by the presence of the end friction. We successfully simulated characteristic structures with the irreversible and multi-step strain localization in the compressed ground based on the elasto-plastic finite deformation analysis and confirmed the effect of boundary condition.
Next, the soil-water coupled elasto-plastic analysis for the saturated ground was conducted. Material constants of semi-consolidated ground were adopted conforming to Yamada et al. [4] and the identical mesh of Fig. 1(c) was used. The lateral compressive behavior obtained by analysis of one-element width is indicated in Fig. 4. Figure 5 shows the transition of the shear strain distribution. At the beginning of the analysis, the appearance of the imbrication was confirmed in (a). However, in (b), the occurrence of décollement with horizontal slip was solved at the central depth of the model. Then, a second slip occurred on the lower side of the first décollement and the region II accreted to the bottom of the region I in (c)-(d). The occurrence of the positive excess pore water pressure (Fig. 6) and the generation of a seismic wave due to brittle deformation (Fig. 7) were also solved.
(Acknowledgement) We received Grant-in-Aid for Scientific Research (Grant-in-Aid for Scientific Research (A): No. 17H01289).
[1] Asaoka, A., Noda, T., Yamada, E., Kaneda, K. and Nakano, M. (2002): An elasto-plastic description of two distinct volume change mechanisms of soils, Soils and Foundations, Vol.42, No.5, pp.47-57.
[2] Noda, T., Asaoka, A. and Nakano, M. (2008): Soil-water coupled finite deformation analysis based on a rate-type equation of motion incorporating the SYS Cam-clay model, Soils and Foundations, Vol.48, No.6, pp.771-790.
[3] Noda, T., Yamada, S., Toyoda, T. and Asaoka, A. (2015): Effects of initial imperfection on the Riedel shear bands in surface ground due to strike-slip fault (in Japanese), Proc. of JSCE, Vol.71, No.2, I_463-I_474.
[4] Yamada, E., Nakai, K. and Asaoka, A. (2021): Soil-water coupled elasto-plastic analysis on the formation process of normal faults in submarine ground due to its uplift and inclination, Proc. 20th Int. Conf. on Soil Mechanics and Geotechnical Engineering (Accepted).
First, an elasto-plastic deformation analysis was performed with three plane-strain finite element meshes with different boundary conditions shown in Fig. 1 (a)-(c). Here, we conducted a single-phase elasto-plastic analysis considering the self-weight of the model. SYS Cam-clay model [2] was incorporated as an elasto-plastic constitutive model. The overconsolidated parameters in Noda et al. [3] exhibiting remarkable softening behavior after the peak strength (Fig. 2) were used. The shear strain distributions in Fig. 3 indicate that the analysis without fixation of the bottom displacement in (a) exhibited pop-up, whereas the analysis with the prescription of the linear bottom displacement distribution (uniform compression) in (b) obtained imbrication. Furthermore, the separation of the imbrication and the thrust sequence, i.e., piggyback in (b) and overstep in (c), were affected by the presence of the end friction. We successfully simulated characteristic structures with the irreversible and multi-step strain localization in the compressed ground based on the elasto-plastic finite deformation analysis and confirmed the effect of boundary condition.
Next, the soil-water coupled elasto-plastic analysis for the saturated ground was conducted. Material constants of semi-consolidated ground were adopted conforming to Yamada et al. [4] and the identical mesh of Fig. 1(c) was used. The lateral compressive behavior obtained by analysis of one-element width is indicated in Fig. 4. Figure 5 shows the transition of the shear strain distribution. At the beginning of the analysis, the appearance of the imbrication was confirmed in (a). However, in (b), the occurrence of décollement with horizontal slip was solved at the central depth of the model. Then, a second slip occurred on the lower side of the first décollement and the region II accreted to the bottom of the region I in (c)-(d). The occurrence of the positive excess pore water pressure (Fig. 6) and the generation of a seismic wave due to brittle deformation (Fig. 7) were also solved.
(Acknowledgement) We received Grant-in-Aid for Scientific Research (Grant-in-Aid for Scientific Research (A): No. 17H01289).
[1] Asaoka, A., Noda, T., Yamada, E., Kaneda, K. and Nakano, M. (2002): An elasto-plastic description of two distinct volume change mechanisms of soils, Soils and Foundations, Vol.42, No.5, pp.47-57.
[2] Noda, T., Asaoka, A. and Nakano, M. (2008): Soil-water coupled finite deformation analysis based on a rate-type equation of motion incorporating the SYS Cam-clay model, Soils and Foundations, Vol.48, No.6, pp.771-790.
[3] Noda, T., Yamada, S., Toyoda, T. and Asaoka, A. (2015): Effects of initial imperfection on the Riedel shear bands in surface ground due to strike-slip fault (in Japanese), Proc. of JSCE, Vol.71, No.2, I_463-I_474.
[4] Yamada, E., Nakai, K. and Asaoka, A. (2021): Soil-water coupled elasto-plastic analysis on the formation process of normal faults in submarine ground due to its uplift and inclination, Proc. 20th Int. Conf. on Soil Mechanics and Geotechnical Engineering (Accepted).