ESKISEHIR OSMANGAZI UNIVERSITY INSTITUTE OF SCIENCES GEOTECHNICAL EARTHQUAKE ENGINEERING Experimental Liquefaction Research Imtaz DEWAN 503320220023 1. Liquefaction behavior evaluation of a multi-layered site with finesdominated soils Purpose of Study To study an advanced NDA procedure to account for interaction of a multi-layered system which could improve the prediction of the liquefaction response of a site with plastic and non-plastic fines-dominated deposits. Test System and Materials Used Nonlinear Dynamic Analysis (NDA) methods besides their ability in accounting for nonlinearity of soil mechanical response arising from hysteretic behavior and porewater pressure generation, can capture the multi-layer effects of a site, as they are able to account for the interaction of porewater pressure between adjacent soil layers. As the first step, the geometry of the model was developed in FLAC based on a 2D crosssection, A-A, passing through the instrumentation array and Alamo River at the site as shown by Fig. 2(b). A level ground surface was deemed for the area according to observations and Alamo River was modeled in the center of the model. Having the river in the center of the model resulted in minimum undesirable effects induced by dynamic boundary conditions incompatibilities. Result and Conclusion The cyclic undrained shear resistance variation with the number of loading cycles at failure along with shear stiffness reduction and equivalent damping ratio variation with shear strains are those cyclic responses that if modeled rigorously can produce realistic liquefaction simulation of soils. The main challenge in this regard arises as these behaviors are significantly different for plastic soils versus non-plastic materials and therefore should not be modeled by a single constitutive. There have been extensive investigation efforts on the characterization of recorded ground motion, porewater pressure, and compatibility of these two sets of data for WLA in 1987 Superstition Hills earthquake. A number of these studies propose the piezometers at WLA site have not accurately recorded the generated excess porewater pressure. PM4 nonlinear dynamic analysis procedure was described and validated for liquefaction investigation of sites with fines-dominated materials using WLA site in 1987 Superstition Hills earthquake case history. Items listed below summarize the characteristics of this procedure along with the insights provided by this study in terms of WLA site case history and significance of multi-layer effects considerations in liquefaction evaluations. 2. Liquefaction analysis of sandy soil during strong earthquake in Northern Thailand Purpose of Study Northern Thailand has experienced several earthquakes which led to soil liquefaction in the past few decades. Traditional methods of Evaluating liquefaction potential involve standard penetration test (SPT) or cone penetration tests. This research augmented experimental results with numerical methods to evaluate the liquefaction potential of Mae Lao Sand in Chiang Rai province of northern Thailand. SPT and downhole seismic test data collected during a field investigation at the Mae Lao site were compared to a 1D site response model analysis of the site. Test System and Materials Used UBC3D-PLM is a modified constitutive model based on the UBCSAND model, which can simulate the development of liquefaction behavior over a course of dynamic loadings in sands and silty sands (Tesfaye, 2010). Several versions of UBCSAND currently exist and the model is evolving continually. The original UBCSAND model was developed at the University of British Columbia (Puebla et al., 1997) to analyze the Mochikos tailings dam in Japan under static and dynamic loading. Later, UBCSAND Version 904aR was developed to evaluate the Success Dam in California with improved model behaviors under certain types of loading. The implementation of UBCSAND in FLAC is described by Beaty and Byrne. . Result and Conclusion The material used in this study was clayey sand obtained from Mae Lao, whose initial dense and loose state void ratios were e0 = 0.618 and e0 = 0.976; these void ratios are typical of dense and loose state values observed from field density tests. Laboratory tests were conducted on this material at the Disaster Prevention Research Institute (DPRI), Kyoto University, Japan. The testing program involved monotonic and cyclic tests as illustrated in Table 2. Monotonic tests were conducted using a method of consolidated-undrained triaxial compression tests on soils with pore water pressure measurements (JGS 0523, 2009b). Sand specimens were saturated for the consolidation process at three initial confining pressures of 50, 150, and 200 kPa. Then, shearing was performed at a displacement-controlled loading of 1 mm/min, which was equal to an axial strain of 1% per minute. Cyclic tests were performed using a cyclic undrained triaxial test method (JGS 0541, 2009a), with initial confining stresses of 20, 50, and 100 kPa. After consolidation, symmetrical stress loading was applied with a frequency of 1 Hz. The cyclic axial stress, cyclic axial strain, confining pressure, and excess pore water pressure were measured during cyclic and monotonic loading. Cyclic loading was terminated when the number of cycles exceeded 200, or if higher than 5% axial strain was reached. 3. Liquefaction potential for the Kathmandu Valley, Nepal: a sensitivity study Purpose of Study An assessment of liquefaction potential for the Kathmandu Valley considering seasonal variability of the groundwater table has been conducted. To gain deeper understanding seven historical liquefaction records located adjacent to borehole datapoints (published in SAFER/GEO-591) were used to compare two methods for the estimation of liquefaction potential. Standard Penetration Test (SPT) blow count data from 75 boreholes inform the new liquefaction potential maps. Test System and Materials Used Seed and Idris (1971) proposed a method to assess liquefaction resistance of soils. In this approach, the factor of safety against liquefaction (FL) is determined by the ratio between the cyclic resistance ratio (CRR) and the earthquake-induced cyclic stress ratio (CSR) (see Sönmez (2003) for further commentary on the historical development of this approach). This method is widely used as a triggering model to evaluate the factor of safety against liquefaction FL (e.g., Gayen et al. 2020). FL is used to evaluate if a soil layer is susceptible or non-susceptible to liquefaction during an earthquake. Iwasaki et al. (1984) proposed a liquefaction potential index (IL) to evaluate the liquefaction potential in multiple layers of soil. The liquefaction potential index (IL) in Iwasaki et al. (1984) is referred to in this paper as the liquefaction potential (PL); this parameter assumes that surface manifestation depends on the thicknesses of all strata that can liquefy in the uppermost 20 m of a soil column, their proximity to the ground surface, and the amount by which the factor of safety against liquefaction in each stratum (FL) is less than 1.0 (Gayan et al. 2020). The methodology of Iwasaki et al. (1984) was also used in the work of Piya et al. (2004) who presented both qualitative and quantitative liquefaction potential assessments for the Kathmandu valley using the available SPT data at that time. Result and Conclusion The results are presented for a range of PGA values, obtained from an average representing several GMPEs, (GMPE AVERAGE of Stevens 2020) and a single GMPE (AB03 of Stevens). At each borehole location in the SAFER/GEO-591 database used for the liquefaction potential analysis, two different seasonal values of water table depth were used (1.6 m and 5.1 m) as discussed in Sect. 2. Figure 8a and b show the liquefaction potential map of the Kathmandu Valley based on PGA considering the AVERAGE GMPE assumption for 2% in 50 years probability of exceedance under a wet scenario and dry scenario, respectively. Among these, the PGA having 2% probability of exceedance in 50 years is around 1.2 g, and it is almost uniform across the valley. This represents the worst-case scenario of a wet season earthquake where both the sandy deposits and silty or fine-grained materials (characterizing the southern valley) are saturated and so have the potential to liquefy. 4. Effects of non-plastic fines on liquefaction properties of saturated silt using discrete element modeling Purpose of Study To present a numerical investigation using the three-dimensional discrete element method (DEM) to evaluate the macro-scale properties of saturated silt in a cyclic triaxial test. Test System and Materials Used The shape of soil particles directly affects the macroscopic mechanical properties of the soil. In DEM, there are mainly two methods to simulate the shape of particles, by Clump or by Rigid block. Clump particles are constructed by combining a series of ball particles of different sizes. In this way, compared with ball particles, clump particles can mimic various complex shapes. In this simulation, discrete element models of eight kinds of silt with different fines contents were prepared. The size of large particles is in the range of 0.075 mm - 0.25 mm. The size of fine particles is in the range of 0.01 mm - 0.075 mm. The grain size distribution curves of the DEM samples are shown in Fig. 4. The fines content of the samples is 0, 15%, 25%, 30%, 40%, 60%, 80% and 100%, respectively. Result and Conclusion The silt used in this paper is composed of silt and fine sand. The contact mode between silt and sand determines the macroscopic mechanical properties of silt. The analysis of liquefaction properties of silt is closely related to the study of the contact state of particles for the soil. In this paper, a three-dimensional discrete element model with rigid block particles built based on the SEM images of silt was used to study the effects of fines content on the liquefaction properties of silt. The cyclic triaxial test of saturated silt was also conducted and the results were compared with the numerical results to analyze its macroscopic behavior and characteristics under cyclic loading conditions.
