SOIL-BASEMENT STRUCTURE INTERACTION ANALYSIS ON DYNAMIC LATERAL EARTH PRESSURE ON BASEMENT WALL

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1 International Conference on Earthquake Engineering and Disaster Mitigation, Jakarta, April 1-15, SOIL-BASEMENT STRUCTURE INTERACTION ANALYSIS ON DYNAMIC LATERAL EARTH PRESSURE ON BASEMENT WALL Nurrachmad Wijayanto 1 and I Wayan Sengara 1 1 Civil Engineering Department, Institut Teknologi Bandung Jl. Ganesha 1 Bandung, Indonesia 13 nuno@si.itb.ac.id, iws@geotech.pauir.itb.ac.id ABSTRACT: Dynamic soil-basement structure interaction using time-domain approach to recommend dynamic lateral earth pressure on basement wall is presented. The analyses consider various parameters involved in the soil-basement structure system. These parameters are soil condition or site-class, peak base acceleration, frequency content of input motion, depth of basement, depth of base-rock, and stiffness of the basement structure. The variations in soil condition are homogenous clay deposit with reference to IBC site classification S C, S D and S E for hard soil, medium-stiff, and soft class, respectively. The study focused on identifying sensitivity of these parameters on dynamic response of the soil-basement structure system. The dynamic soil-structure interaction analyses were conducted using dynamic finite element software, adopting non-linear elasto-plastic Mohr-Coulomb soil model with Rayleigh damping. Result of the analyses indicated that distribution of the dynamic lateral earth pressure reach its maximum value on the basement wall with almost linear pressure increments with depth on soft soil and non-linear pressure increments on medium-stiff and stiff soil. The highest pressure increment is identified to occur near bottom of the basement wall. The sensitivity analysis on the peak base acceleration results in higher maximum lateral earth pressure with higher peak base acceleration with some variation depending upon the site-class. The maximum lateral earth pressure and its point of application are increasing proportionally with depth of basement wall. of seismic lateral pressure distribution considering the site-class and peak base acceleration is provided. 1. INTRODUCTION During the last decades, the basement structures were extensively built in most big cities in Indonesia. As it has been widely known that most of these big cities are located on the perimeter of tectonically active region, these basement structures were often subjected to severe earthquake loading. On the other hand, the behavior of basement structure during the earthquake loading is not well understood as it involves the interaction between soil and basement structure. This study is conducted in order to analyze the behavior of basement wall subjected to earthquake loading which incorporating the effect of soil-structure interaction. In particular, the result of this study is to provide recommendations of dynamic lateral earth pressure on basement wall considering several parameters, such as soil condition or site-class, peak base acceleration, frequency content of input motion, depth of basement, depth of base-rock, and stiffness of the basement structure.. METHODOLOGY In order to obtain the recommendation of the distribution of dynamic lateral earth pressure on basement wall, the analyses were performed with PLAXIS Dynamics Finite Element Code software (Brinkgreve et al., ). This software uses time-domain approach to model the -D earthquake wave propagation from the base rock to the basement structure. In PLAXIS Dynamic time-domain formulation, the damping matrix is formed by means of Rayleigh damping scheme (Rayleigh et al., 195). This damping scheme uses Rayleigh Alpha and Beta coefficients as multipliers to mass matrix and stiffness matrix, respectively, as described by Cook (199). The input values of these coefficients are obtained and verified with DEEPSOIL software (Hashash et al., ). Back to Table of Contents 39

2 International Conference on Earthquake Engineering and Disaster Mitigation, Jakarta, April 1-15,.1 Soil Dynamic Properties The analyses were conducted on a basement structure which is constructed on single thick homogeneous clay layer based on the soil dynamic properties classification described by Uniform Building Code (UBC), 1997, which are: 1. S C, soft rock or stiff clay, with shear wave velocity of 1 ft/s < v s 5 ft/s (3 m/s < v s 7 m/s, or (N 1 ) > 5, or S u psf (1 kpa).. S D, firm clay, with shear wave velocity of ft/s < v s 1 ft/s (1 m/s < v s 3 m/s, or 15 (N 1 ) 5, or 1 psf S u psf (5 kpa S u 1 kpa). 3. S E, soft clay, with shear wave velocity of v s ft/dt (1 m/dt), or 15 (N 1 ) 5, or clay with PI >, w %, or S u < 5 psf (5 kpa).. Earthquake Input Motion The input motions used in the analyses are synthetic input motions generated from earthquake target spectra of Jakarta with subduction and shallow crustal earthquake mechanisms using EZFRISK software (Risk Engineering, Inc, ). The target spectrums for Jakarta were previously obtained from Probabilistic Seismic Hazard Analysis (PSHA). The target spectrums for subduction and shallow crustal earthquake mechanisms are scaled to.1g,.g and.3g at PGA period. Figure 1 Finite element model of homogeneous soil layer, elastic bedrock and basement structure in PLAXIS..3 Dynamic Finite Element Modeling The -D dynamic finite element analyses were performed with PLAXIS software, in which the soil and the baserock are modeled as solid elements and the basement structure is modeled as concrete beam and column elements without the presence of upper structure. The left and right boundaries are modeled as wave absorbent boundaries and placed at considerable distance from the basement structure, in order to avoid the earthquake wave reflected perfectly as in the case of full fixities boundaries. The earthquake wave is propagated from the baserock with various depths from 3 meters to 5 meters. The finite element model of soil, baserock and basement structure is schematically drawn in Figure 1. The soil shear strength parameters as inputs to the PLAXIS model based on IBC 199 site classification are given in Table 1. The dimension and material parameter of concrete and beam column are shown in Table. Back to Table of Contents 39

3 International Conference on Earthquake Engineering and Disaster Mitigation, Jakarta, April 1-15, Table 1 Soil shear strength parameter input in PLAXIS based on UBC 1997 site classification. Site Class γ (kn/m3) c u ref (kn/m) c u increment (kn/m) K o C D E Table Dimension and material parameters of concrete beam and column in basement structure. Structural Element b (m) h (m) EA (kn/m) EI (knm/m) ν ρ (kn/m3) Beam Column In order to analyze the effects of local soil condition, earthquake mechanism and peak base acceleration (PBA) level on distribution of dynamic lateral earth pressure, a set of analyses is divided into 1 scenarios where the depth of basement and the depth of baserock are kept constant at 1 meters and 3 meters, respectively. These scenarios are summarized in Table 3. Table 3 Scheme of analyses with constant basement depth of 1 meters to analyze the effect of local soil condition, earthquake mechanism and PBA level to the distribution of dynamic lateral earth pressure. Case Site Classification Earthquake mechanism PBA Basement Baserock Case 1 Subduction.1g 1m 3m Case Subduction.g 1m 3m Case 3 Subduction.3g 1m 3m Case Shallow crustal.1g 1m 3m Case 5 Shallow crustal.g 1m 3m Case Shallow crustal.3g 1m 3m Case 7 Subduction.1g 1m 3m Case Subduction.g 1m 3m Case 9 Subduction.3g 1m 3m Case 1 Shallow crustal.1g 1m 3m Case 11 Shallow crustal.g 1m 3m Case 1 Shallow crustal.3g 1m 3m Case 13 Subduction.1g 1m 3m Case 1 Subduction.g 1m 3m Case 15 Subduction.3g 1m 3m Case 1 Shallow crustal.1g 1m 3m Case 17 Shallow crustal.g 1m 3m Case 1 Shallow crustal.3g 1m 3m Another different scheme of analyses is also conducted to analyze the sensitivity of basement structure depth on the distribution of dynamic lateral earth pressure of basement wall. In this case, the depth of basement varies by meters, 1 meters and 1 meters. This scheme of analyses is shown briefly in Table. Finally, the analyses are also performed at different depth of baserock with shallow crustal earthquake input motion at PBA level.1g and the depth of basement is kept constant at 1 meters. These analyses are summarized in Table 5. Back to Table of Contents 391

4 International Conference on Earthquake Engineering and Disaster Mitigation, Jakarta, April 1-15, Table Scheme of analyses with subduction mechanism input motion, PBA.g and constant baserock depth of 3 meters to analyze the effect of basement depth to the distribution of dynamic lateral earth pressure. Case Site Classification Earthquake mechanism PBA Basement Baserock Case 19 Subduction.g m 3m Case Subduction.g 1m 3m Case 1 Subduction.g 1m 3m Case Subduction.g m 3m Case 3 Subduction.g 1m 3m Case Subduction.g 1m 3m Case 5 Subduction.g m 3m Case Subduction.g 1m 3m Case 7 Subduction.g 1m 3m Table 5 Scheme of analyses with shallow crustal fault earthquake mechanism input motion, PBA.1g and constant basement depth of 1 meters to analyze the effect of baserock depth to the distribution of dynamic lateral earth pressure. Case Site Classification Earthquake mechanism PBA Basement Baserock Case Shallow crustal.1g 1m 3m Case 9 Shallow crustal.1g 1m m Case 3 Shallow crustal.1g 1m 5m Case 31 Shallow crustal.1g 1m 3m Case 3 Shallow crustal.1g 1m m Case 33 Shallow crustal.1g 1m 5m Case 3 Shallow crustal.1g 1m 3m Case 35 Shallow crustal.1g 1m m Case 3 Shallow crustal.1g 1m 5m 3. THE RESULT OF ANALYSIS 3.1 Surface Response Spectra for Different Local Soil Condition due to Effect of PBA Level Figure and Figure 3 show the surface response spectrum for stiff clay, firm clay and soft clay as results from wave propagation originated at base rock. It can be seen clearly that soft clay amplifies the earthquake wave at greatest level compared to stiff clay and firm clay. Surface Response Spectra for PBA.1g Surface Response Spectra for PBA.g.7 1. Acceleration (g) Stiff Clay Firm Clay Soft Clay Acceleration (g) 1... Stiff Clay Firm Clay Soft Clay Period (s) Period (s) Figure Surface response spectra for input motion with PBA.1g and.g. Back to Table of Contents 39

5 International Conference on Earthquake Engineering and Disaster Mitigation, Jakarta, April 1-15, 1. Surface Response Spectra for PBA.3g Acceleration (g) 1... Stiff Clay Firm Clay Soft Clay Period (s) Figure 3 Surface response spectra for input motion with PBA.3g. 3. Distribution of Dynamic Lateral Earth Pressure on Basement Wall due to Effect of Local Soil Condition Variation In this scheme of analyses, the input motions used are shallow crustal earthquakes with PBA.1g,.g and.3g, while the basement depth and the baserock depth are kept constant at 3 meters and 1 meters, respectively. During the earthquake, the basement wall shows relative movements towards and away from the adjacent soil. The typical minimum and maximum movements of the basement wall for different types of soil are shown in Figure. These dynamic movements induced dynamic lateral earth pressure as function of time. When the wall moves away from the soil, the total lateral earth pressure is decreasing and vice versa. The maximum decrement and increment of dynamic lateral earth pressure for PBA.1g and.3g are shown in Figure 5 and Figure, respectively. Horizontal wall movement (m)..5.3 Horizontal wall movement (m) Horizontal wall movement (m) Figure Horizontal maximum basement wall movement for basement depth 1 meters, baserock depth 3 meters and shallow crustal input motion with PBA.1g (left),.g (middle) and.3g (right). It can be inferred from figures below that the maximum dynamic pressure decrement is the largest in stiff clay, followed by firm clay and soft clay. These decrements reach their maximum values at the base of the wall. For the maximum dynamic pressure increment, the basement which lies on the soft clay shows an almost linear distribution along the wall, while the stiff clay shows nonlinear distribution. For stiff clay and firm clay, the dynamic increment pressures reach their maximum at the base of the wall, while soft clay shows different behavior which approximately at.75 H from top of the wall. It is also shown that at the level of basement beam, there are significant increment and decrement of dynamic pressure compared to surrounding wall level. This is due to additional stiffness caused by beams and plates. Back to Table of Contents 393

6 International Conference on Earthquake Engineering and Disaster Mitigation, Jakarta, April 1-15, Δσdyn minimum (kn/m ) Δσdyn maximum (kn/m ) Figure 5 Dynamic lateral earth pressure distribution for shallow crustal input motion with PBA.1 g, basement depth 1 meters and baserock depth 3 meters. Δσdyn minimum (kn/m ) Δσdyn maximum (kn/m ) Figure Dynamic lateral earth pressure distribution for shallow crustal input motion with PBA.3 g, basement depth 1 meters and baserock depth 3 meters. 3.3 Distribution of Dynamic Lateral Earth Pressure on Basement Wall due to Effect of Earthquake Mechanism and PBA Level Variation This scheme of analyses is focused on earthquake characteristic by its mechanism and PBA level. For this reason, the input motions used are subduction and shallow crustal earthquake with PBA level.1g,.g and.3g, while the basement depth and the baserock depth are 1 meters and 3 meters, respectively. The distribution of maximum dynamic lateral earth pressure is shown in Figure 7. From the figure, it can be seen that as the PBA level increases, the dynamic lateral earth pressure increases for any soil type. It is found that for stiff clay, the maximum pressure occurred at the base of the wall, while for firm clay it occurred at depth approximately.7h from the top and for soft clay at.h from the top. The subduction earthquake mechanism gives slightly greater dynamic pressure than shallow crustal earthquake, especially in the case of stiff clay. From this analysis, it is also found that the point of application of total dynamic force occurred at depth.5-.7h from the top regardless of the PBA level and earthquake mechanism. Back to Table of Contents 39

7 International Conference on Earthquake Engineering and Disaster Mitigation, Jakarta, April 1-15, Δσdyn maximum (kn/m ) Subduction.1 g Subduction. g Subduction.3 g SCF.1 g SCF. g SCF.3 g Δσdyn maximum (kn/m ) Subduction.1 g Subduction. g Subduction.3 g SCF.1 g SCF. g SCF.3 g Δσdyn maximum (kn/m ) 1 Subduction.1 g Subduction. g Subduction.3 g SCF.1 g SCF. g SCF.3 g (a) (b) Figure 7 Maximum dynamic lateral earth pressure distribution of basement wall embedded at stiff clay (a) firm clay (b) and soft clay (c) for different mechanism of earthquake and PBA level. 3. Distribution of Dynamic Lateral Earth Pressure on Basement Wall due to Effect of Baserock Depth Variation To analyze the effect of baserock depth, this scheme of analysis varies the baserock depth into three depths, which are 3 meters, meters and 5 meters. The input motion used is shallow crustal earthquake with PBA.1g and the basement depth is 1 meters. Maximum total lateral earth pressure (kn/m ) Maximum total lateral earth pressure (kn/m ) Baserock depth 3 m Baserock depth 3 m Baserock depth m Baserock depth m 1 Baserock depth 5 m 1 Baserock depth 5 m (a) Maximum total lateral earth pressure (kn/m ) Baserock depth 3 m Baserock depth m (b) Baserock depth 5 m (c) Figure Maximum dynamic lateral earth pressure distribution of stiff clay (a), firm clay (b) and soft clay (c) with shallow crustal.1g input motion, basement depth 1 meters and variation of baserock depth. Back to Table of Contents 395

8 International Conference on Earthquake Engineering and Disaster Mitigation, Jakarta, April 1-15, 3.5 Distribution of Dynamic Lateral Earth Pressure on Basement Wall due to Effect of Basement Depth Variation In this case, the basement depths are varied within range of to 1 meters, the input motion used is subduction earthquake with PBA.g and baserock depth constant at 3 meters. Δσdyn maximum (kn/m ) m 1 m 1 m (a) Δσdyn maximum (kn/m ) m 1 m 1 m (b) Δσdyn maximum (kn/m ) m 1 m 1 m (c) Figure 9 Maximum dynamic lateral earth pressure distribution of stiff clay (a), firm clay (b) and soft clay (c) with subduction earthquake.g input motion, baserock depth 3 meters and variation of basement depth. Figure 9 shows that for each soil type, the variation of basement depth gives same typical shape of distribution along the basement wall. For stiff and firm clay, the dynamic pressure reaches its maximum at the base of the wall with nonlinear distribution, whereas the maximum pressure gradient occurred between.5h-h from the top of the wall. In the case of soft clay, it gives different behavior, in which the maximum value occurred at approximately.5h from the top and decreases beyond that depth. 3. of Normalized Maximum Dynamic Lateral Earth Pressure s for normalized distribution of dynamic lateral earth pressure in the case of basement depth 1 meters and baserock depth 3 meters are show in Figure. These recommendations are based on the result of dynamic finite element simulation at various PBA level. Back to Table of Contents 39

9 International Conference on Earthquake Engineering and Disaster Mitigation, Jakarta, April 1-15, (Δσdyn/γH) maximum (Δσdyn/γH) maximum z/h w ith PBA.1g w ith PBA.g w ith PBA.3g for PBA.1g for PBA.g for PBA.3g z/h w ith PBA.1g w ith PBA.g w ith PBA.3g for PBA.1g for PBA.g for PBA.3g (a) (b) (Δσdyn/γH) maximum z/h with PBA.1g with PBA.g with PBA.3g for PBA.1g for PBA.g for PBA.3g.9 1 (c) Figure 1 for distribution of dynamic lateral earth pressure on basement wall with depth 1 meters and baserock depth 3 meters on stiff clay (a), firm clay (b) and soft clay (c).. CONCLUSION From the result of analyses, it could be concluded that the local soil condition will affect the movement behavior of basement wall whereas the maximum wall movement occurred at soft clay, followed by firm and stiff clay. This movement will induce greater dynamic lateral increment and decrement pressure at stiffer soil condition. The soil-structure interaction effect under lower level of PBA, is more affected by soil mass factor, while at the higher level, is more affected by the soil stiffness. As the PBA level of the earthquake increases, the dynamic lateral earth pressure increment will increase, which is caused by the greater translation wall movement at higher acceleration. The narrow variation of baserock depth does not significantly give the difference on distribution of dynamic lateral earth pressure. Under more extreme depth of baserock, such as -5 meters, it may give a considerable increment in the magnitude of dynamic pressure, since the amplification from the baserock to the soil surface will influence the dynamic load on basement structure. The variation of basement depth does not influence the distribution shape of dynamic lateral earth pressure, but affects the point of application of lateral earth force. Increasing the basement depth will shift the point of application downward relative to overall height of the wall. In this case, for basement depth meters, the lateral force acts at approximately.7h, while for basement depth 1 meters, at.h. Back to Table of Contents 397

10 International International Conference Conference Earthquake on Earthquake Engineering Engineering and Disaster and Mitigation, Disaster Mitigation Jakarta, April 1-15, 5. REFERENCES Brinkgreve, R.B.J., Vermeer, P.A., Bakker, K.J., Bonnier, P.G., Brand, P.J.W., Burd, H.J. and Termaat, R.J. (). PLAXIS Dynamics D version : Finite Element Code for Soil and Rock Analyses, A.A. Balkema, Rotterdam, Netherlands. Cook, R.D. (199). Concept and Applications of Finite Element Analysis (3 rd edition), John Wiley & Sons, New York. Hashash, Y.M.A. and Park, D. (). Viscous damping formulation and high frequency motion propagation in nonlinear site response analysis, Soil Dynamics and Earthquake Engineering, (7), pp Rayleigh, J.W.S. and Lindsay, R.B. (195). The Theory of Sound, 1st American ed., Dover Publications, New York. Risk Engineering, Inc. (). EZFRISK, Software for In-depth Seismic Hazard Analysis, Boulder, Colorado, USA. Uniform Building Code (UBC) (1997). Volume, Structural Engineering Design Provisions, International Conference of Building Officials. Back to Table of Contents 39