Seismic Numerical Simulation of Breakwater on a Liquefiable Layer: IRAN LNG Port

Seismic Numerical Simulation of Breakwater on a Liquefiable Layer: IRAN LNG Port Yaser Jafarian Assistant Professor Semnan University, Semnan, Iran Email: yjafarianm@iust.ac.ir Hamid Alielahi Islamic Azad University-Zanjan branch, Zanjan, Iran Email: h.alielahi@seiau.ir Alireza sadeghi Abdollahi, Rouzbeh Vakili Iran University of Science and Technology, Tehran, Iran Email: alireza_sadeghi61@yahoo.com roozbehvakili@hotmail.com ABSTRACT Dynamic analysis of earth embankments is even now a matter of concern among geotechnical engineers due to the difficulties exist in the appropriate selection of analysis method, constitutive model, and material properties. The LNG port, that is currently being constructed at the northern bank of the Persian Gulf at Tombak, involves an "L" shaped (top-down view) breakwater. Site investigations showed that there is a liquefiable sandy silt layer beneath the breakwater. Although, simplified liquefaction analyses revealed that the susceptible layer will not experience complete liquefaction during the design earthquake, earthquake-induced generation of pore water pressure and the subsequent softening of the layer has been a major concern in the design procedure. In the present study, several fully nonlinear analyses were performed by FLAC finite difference program in order to estimate the permanent deformations of breakwater and to study the effects of input material properties on the amounts of deformations. The Mohr-Coulomb constitutive model coupled with Byrne (1991) pore pressure buildup model has been employed in the analyses. KEYWORDS: Earthquake; Liquefaction; Numerical simulation; Breakwater. INTRODUCTION Soil liquefaction involves the generation, redistribution, and eventual dissipation of excess pore water pressure. The role of pore water pressure generation in the softening and weakening of liquefiable soils have long been recognized by geotechnical engineers. Liquefaction-induced displacements have caused severe damage to marine structure (such as quay walls, piers, dolphins, breakwater, buried pipelines, etc) during past earthquakes. Therefore a reliable procedure for the prediction of liquefaction and resulting displacements is necessary for the

Vol. 13, Bund. B 2 rational design of earth shore structures resting on liquefiable soils. Whereas conventional seismic impact analyses yield unacceptable deformation; consideration may be given to perform a more sophisticated liquefaction induced deformation analysis. State-of-the-art procedures for evaluating liquefaction induced deformation involve dynamic finite element or finite difference effective stress analyses using various constitutive models coupled with fluid flow effects. These analyses can estimate the displacements, accelerations and excess pore water pressure induced by a given input motion. The Iranian LNG project involves offshore and onshore facilities to be constructed on the north - western banks of the Persian Gulf, at Tombak area. Figure 1 illustrates the general layout and construction stages of the LNG port. The LNG plant includes a breakwater with the height varying, approximately, from 4.5m to 20m. The crest elevation changes from 5m to 10m with respect to the Chart Datum (CD). Based on the complementary site investigations, two subsurface soil layers were suggested to be separable as uniform subsoil layers. Moreover, a site-specific seismic hazard study was shown that Level 1 and Level 2 earthquakes have peak ground accelerations of 0.15g and 0.26g, respectively. Therefore the loose to medium sandy silt (ML) (upper layer) that has the maximum thickness of 13.5m is very susceptible to liquefaction with SPT blow count lower than 10. In this paper the commercial finite difference program FLAC has been employed to perform static and dynamic analyses in two selected cross sections of the mentioned breakwater. Technical procedures regarding the numerical deformation analysis such as geometrical and geotechnical parameters, earthquake input motion, numerical modeling procedure, and the obtained results are addressed. Figure 1: a) Photograph of breakwater, b)general layout of the LNG port

Vol. 13, Bund. B 3 GEOTECHNICAL PARAMETERS Seabed Soil Layers Preliminary site investigations were performed by Sahel Consulting Engineers (SCE) to recognize the seabed layers located beneath the breakwater construction site. Based on the geotechnical surveys, two subsurface soil layers were found as uniform subsoil layers. The upper layer having the maximum thickness of 13.5m is a loose to medium dense gray to light green sandy silt (ML) containing shell fragments. The lower thick layer is a dense composite soil layer (GC-GM). Table 1 shows the estimated geotechnical characteristics of these layers based on the results of in-situ index tests including SPT and CPTU and also laboratory tests (such as direct shear and triaxial) conducted on the taken undisturbed and reconstituted specimens. Table 1: Geotechnical parameters of the seabed soil layers Upper layer Lower layer Parameters Depth (m) 0-13.5 >13.5 Classification ML GC-GM Relative density (%) 30 >75 Fines content (%) 71 25 Dry density (tons/m 3 ) 1.46 2.04 Saturated density (tons/m 3 ) 1.9 2.3 Specific gravity 2.68 2.69 Cohesion (tons/m 2 ) 0 0 Internal friction angle (deg) 28 >39 Elastic modulus (tons/m 2 ) 700 14000 Poisson's ratio 0.35 0.25 Breakwater Characteristic Geotechnical properties of the breakwater materials were determined using available marine and offshore construction codes (e.g. OCDI, CUR, ICOLD, CEM) and also the earlier experiences achieved during previous similar works and case studies in Persian Gulf. This decision has been made due to the impossibility involved in the conventional laboratory testing on this type of large size materials. Table 2 presents the estimated geotechnical parameters of the breakwater which were incorporated in the analyses. It is required to note that based on the tender document the LPG jetty facilities were designed to be constructed above the breakwater. A plot of the breakwater and the position of two selected cross sections (B5 and B9) are illustrated in Figure 2.

Vol. 13, Bund. B 4 Figure 2: Geometrical positions of the selected cross sections of the breakwater Table 2: Proposed geotechnical parameters for different parts of the breakwater Core material Armor and filter material Design parameters Recommended Recommended Range Range Value for Analysis Value for Analysis Dry density 1 (kg/m 3 ) 1500-1800 1600 1500-1900 Armor: 1800 Filter: 1700 Saturated density 2 (kg/m 3 ) 1700-2000 1900 1600-2200 Armor: 2000 Filter: 1900 Internal friction angle 3 (degree) 35-46 38 38-48 Armor: 44 Filter: 41 Cohesion 4 (kg/cm 2 ) 0-0.4 0 0-0.3 0 Elastic Modulus 5 (kg/cm 2 ) 400-1000 800 400-1200 1000 Poisson Ratio 6 (kg/cm 2 ) 0.25-0.35 0.30 0.25-0.35 0.30 1,2- Based on results presented in CEM, ICOLD 3,4- According to proposed values in OCDI, CUR, ICOLD and Advances in Rockfill Structures 5,6- Based on parameter range collected in Advances in Rockfill Structures EARTHAUAKE INPUT MOTION Time Histories Selection The selection of an earthquake acceleration time-histories for applying in the numerical analysis was adopted by the following criteria (Green and Ebeling 2002):

Vol. 13, Bund. B 5 A real earthquake motion is desired, not a synthetic motion; The earthquake magnitude and site-to-source distance corresponding to the motion should be representative of design ground motions; and The motions recorded on rock or stiff soil should be used. Therefore, according to these criteria, three input motions were selected to be introduced to the base of the numerical models. The vertical coefficient is not recommended to be taken into account by OCDI since observations for the sites subjected to probable far field strikes have showed that the vertical component of earthquake is not so large compared with the horizontal component. Thus, the vertical coefficient is rationally neglected in the analyses (OCDI) because the breakwater is not vulnerable to a near field motion. In fact, the local seismological studies (GEO-TER, 2002) indicate that the LNG project site is only vulnerable for a far field earthquake strike. Figure 3 illustrates the time histories of Bandar Abbas, Bam, and Suza strong ground motions that were introduced to the base of the numerical models. Figure 3: Bam, Bandar Abbas, and Suza strong ground motions scaled to 0.15g The peak ground acceleration (PGA) values derived from the local seismological study are presented in Table 3. In this table, level 1 and level 2 earthquakes are referred to as the seismic motions addressing 75 and 475 years return periods, respectively. It is important noting that the recommended PGA values were predicted for the ground surface with implicit accounting for site amplification effects. The numerical models, however, involve natural soil layers beneath the breakwater. In order to being in conservative side and avoiding the uncertainties involved in deconvolution numerical analysis, the PGA values cited in Table 3 are used as target peak accelerations for the input accelerograms applied beneath the models. Table 3: Seismic coefficients of the LNG site LEVEL I LEVEL II PGA 0.15g 0.26g

Vol. 13, Bund. B 6 Significant Duration, Baseline Correction, and Filtering Researchers recommended several methods to optimize the duration of dynamic analyses, which can consume considerable times in complicated numerical simulations. One of the most applicable methods was proposed by Trifunac and Brady (1975) who considered the most energetic portion of accelerogram as significant duration. Actually, this is the interval of time over which a proportion (e.g. between 5% and 95%) of the total Arias intensity is accumulated. The significant durations of the used accelerograms were estimated by SeismoSignal software as illustrated in Figure 4. In order to eliminate linear drifts in displacement time histories obtained from double time integration of the input motions, baseline correction was applied to accelerograms using linear modifier function. In addition, band-pass frequency cutoff was carried out by FFT procedure to remove low and high unwanted frequency components, which do not significantly influence the performance of civil engineering structures. In this project, lower and upper limits of frequency cutoff processing were set to 0.1Hz and 15Hz, respectively. Figure 4 show the scaled, filtered, baseline corrected, and duration-modified input motions. The modified input motions were applied to the base of both sections B5 and B9. At any section, the maximum values of earthquake induced deformations among the deformations obtained from all the input motions are considered as the estimated seismic deformations under design earthquake. NUMERICAL MODELING Constitutive Model The conventional Mohr-Coulomb constitutive model has been used in the dynamic analyses. This elastic - perfectly plastic model was implemented in the FLAC program and can represent shear failure of soils and rocks. The Mohr-Coulomb yielding criterion can involve either associated or non-associated flow rule. In the present work, the latter one was employed since the associated flow rule commonly shows continuous and exaggerative dilation tendency which is not consistent with the actual behavior of the simulated soils. An effective stress analysis approach was initially proposed by Martin et al. (1975). Their proposed model is an equation linking the increment of the volumetric strain per cycle of loading to the shear strain occurred during that particular cycle. Subsequently, Byrne (1991) proposed a modified and simpler volume change model with two calibration parameters. In order to simulate the seismic behavior of liquefiable layer (seabed layer 1), the Finn built-in constitutive model was used. This is a loosely coupled effective stress model that works with combining Mohr-Coulomb criterion and the mentioned volume change-based pore water pressure buildup models. Since there is not currently any correlation to obtain the calibration parameters of the Martin et al. (1975) model, Byrne (1991) s model was used in the current project to predict pore pressure buildup of the liquefiable layer. In fact, all of the breakwater and the lower seabed materials have been simulated by Mohr-Coulomb model except for the liquefiable layer that has been simulated by Finn model.

Vol. 13, Bund. B 7 Figure 4: Illustration of the effective duration analysis and the modified of Bam, Bandar Abbas, and Suza records during its effective duration Grid Generation Grids size was selected small enough to satisfy the required level of precision. Moreover, it must satisfy the size criterion recommended for proper wave transmission. Kuhlemeyer and Lysmer (1973) showed that for accurate representation of wave transmission through a model, the spatial element size, Δ l, must be smaller than approximately one-tenth to one-eighth of the wavelength associated with the highest frequency component of the input wave: λ λ Δ l to (1) 10 8 Where λ is the wavelength associated with the highest frequency component that contains appreciable energy.

Vol. 13, Bund. B 8 Boundary Condition The grids located at the base of model were fixed against both horizontal and vertical movement in static and dynamic analyses. Right and left side grids were horizontally fixed for static analysis. In dynamic analyses, enough distance between the structure and lateral sides should be considered to suppress the reflection of waves contacting the boundaries. In the present work, lateral boundaries were considered as free-field, a built-in boundary condition in FLAC for dynamic analysis. This is an alternative procedure rather than using the mentioned large distance to enforce the free-field motion in such a way that boundaries retain their non-reflecting properties. Mechanical pressure of water enforcing both lee and sea sides of the breakwater were applied as additional boundary condition before the static analysis run. Other Dynamic Parameters Damping: Rayleigh damping was used in the dynamic analyses of the breakwater. It was originally used in the analysis of structures and elastic continua, to damp the natural oscillation modes of the system. For geological materials, damping commonly falls in the range of 2 to 5% of critical. Accordingly, damping ratio of 5% and f min of 2Hz has been used in the present project. Shear modulus: Values of maximum shear modulus of seabed and breakwater materials that are needed for dynamic analysis have been estimated from elastic modulus and Poisson ratios cited in Tables 1 and 2. RESULTS Prior to any dynamic analysis, static analysis is required to obtain static stress and strain regime in the geomaterial structure. Such regime is, in fact, the initial condition of the subsequent dynamic analysis. Sections B5 and B9 have been analyzed under their body loads, mechanical pressures of water enforced at their lee and sea sides, and sea water level condition. The filtered and corrected input motions were applied beneath the second seabed layer at the depth of 25 m with respect to breakwater base. This depth was assumed to be firm enough to be considered as bedrock since SPT tests were refused even several meters above which. Figure 5 shows the exaggerated deformed meshes of the analyzed sections under Suza input motions. Table 4 summarizes all the displacement results. It is seen that maximum horizontal and vertical displacements in Section 9 have induced by Suza input motions, respectively, equal to 0.85m and 2.1m. In Section 5, these values are equal to 2.8m and 1.55m. Although these values are considerably high, the deformed shape of the sections, that were exaggeratedly shown in the previous figures, show that they may be repairable after probable design earthquake. On the other hand, conservative selection of the 0.15g in the bedrock rather than ground surface has increased the final deformation values.

Vol. 13, Bund. B 9 Figure 5: Exaggerated deformed mesh of Section 5 and 9 under Suza motion Table 4: Earthquake induced deformations of the LNG breakwater Ground Motion Displacement direction Position Bam Bandar Abbas Suza Sec. 5 Sec. 9 Sec. 5 Sec. 9 Sec. 5 Sec. 9 Lee side of crest 0.68 0.33 0.77 0.02 2.8 0.82 X disp. (m) Crest center - - - - - - Sea side of crest 1.2 0.25 0.33 0.04 1.2 0.85 Lee side of crest 0.58 1.35 0.78 0.52 1.55 2.1 Y disp. (m) Crest center 0.19 1.5 0.1 0.51 0.55 1.9 Sea side of crest 1.1 1.35 0.37 0.48 1.0 1.8 X disp. = Horizontal permanent displacement Y disp. = Vertical permanent displacement

Vol. 13, Bund. B 10 SUMMERY AND CONCLUSION This paper addresses the methodology suggested in the earthquake induced deformation analysis of the Iran LNG breakwater. Two cross sections have been selected to be analyzed as the most critical sections. FLAC finite difference program has been employed to perform the static and dynamic analyses. The static deformations of the breakwater have been neglected since it would be compensated during construction. As seen in Table 4, the resultant values of estimated displacement are large in some cases and may significantly affect the safety of project during its lifetime. Several issues are evolved as follows: 1. Large deformations of breakwater were expected even before dynamic analysis because a thick liquefiable soil layer is located beneath the breakwater. Although simplified liquefaction analyses showed that the layer will not experience liquefaction, generation of pore water pressure during seismic loading and the subsequent deterioration of strength and large deformations are unavoidable. 2. In order to improve vulnerability of the project, it was decided to detach any important structures from the breakwater. An overview to the exaggerated deformed mesh reveals that the major portions of deformations occur at lee and sea side slopes while the central body of the breakwater persists against the design earthquake strikes. It means that in the absence of important facilities only the slopes of breakwater should be repaired provided the probable earthquake would happen during its lifetime. 3. The acceleration records applied to the models were scaled with recommended PGA values at the level ground surface (i.e. 0.15g). This is a conservative decision since the maximum acceleration at bedrock seems to be lower than 0.15g regarding the existing soft soil layer and its amplification effect. Thus the estimated displacements are somewhat conservative. 4. Results of slope stability analyses showed that the breakwater is stable while the values of factor of safety are very close to their allowable values. The differences between the finding of current paper and slope stability analyses emphasize on the importance of performance based analysis. In fact, the limit equilibrium methods used for slope stability analyses cannot appropriately consider the effect of pore water generation on the softening of liquefiable layer. REFERENCES 1. Byrne, P (1991) A Cyclic Shear-Volume Coupling and Pore-Pressure Model for Sand, Second International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics (St. Louis, Missouri, March, 1991), No. 1.24, 47-55. 2. CUR (1995) Manual on the use of Rock in Hydraulic Engineering, Netherlands. 3. Eurocode 8 (1994) Design Provisions for Earthquake Resistance Structures, Part5: Foundations, Retaining Structures and Geotechnical Aspects. 4. FLAC 4 User's Guide (2002) Itasca Consulting Group, Inc.

Vol. 13, Bund. B 11 5. GEO-TER (2002) Geological survey of Parak and Tombak onshore LNG sites, Report GTR/TFE/0602-172. 6. Green, RA, and Ebeling, RM (2002) Seismic Analysis of Cantilever Retaining Walls, Phase I, Earthquake Engineering Research Program, U.S. Army Corps of Engineers Washington, DC 20314-1000. 7. Hansen, KD, (1986) Soil-Cement for Embankment Dams, ICOLD Bulletin 54, International Commission on Large Dams, Paris, France. 8. Kuhlemeyer, RL, and J. Lysmer (1973) Finite Element Method Accuracy for Wave Propagation Problems, J. Soil Mech. & Foundations, Div. ASCE, 99(SM5), 421-427 9. Martin, GR, WDL. Finn and HB. Seed (1975) Fundamentals of Liquefaction Under Cyclic Loading, J. Geotech., Div. ASCE, 101(GT5), 423-438. 10. Neves, EM, (1991) Advances in Rockfill Structures, NATO ASI Series. 11. OCDI (2002). Technical Standards and Commentaries for Port and Harbor Facilities in Japan,. 12. Trifunac, MD, and AG. Brady (1975) A study of the duration of strong earthquake ground motion, Bulletin of the Seismological Society of America, 65, 581-626. 2010 ejge