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Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions (2012)

Chapter: CHAPTER FIVE Conclusions and Suggestions for Further Research

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Suggested Citation:"CHAPTER FIVE Conclusions and Suggestions for Further Research." National Academies of Sciences, Engineering, and Medicine. 2012. Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions. Washington, DC: The National Academies Press. doi: 10.17226/14660.
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Suggested Citation:"CHAPTER FIVE Conclusions and Suggestions for Further Research." National Academies of Sciences, Engineering, and Medicine. 2012. Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions. Washington, DC: The National Academies Press. doi: 10.17226/14660.
×
Page 25
Page 26
Suggested Citation:"CHAPTER FIVE Conclusions and Suggestions for Further Research." National Academies of Sciences, Engineering, and Medicine. 2012. Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions. Washington, DC: The National Academies Press. doi: 10.17226/14660.
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Page 26

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14 pwp generation and, in some models, of cyclic degradation clay is of much lower intensity than in sand; and that in over- are included by degradation of soil strength and stiffness. consolidated clay, both positive and negative (suction) pwp Some models use different factors for degradation of soil may develop (e.g., Matasovic and Vucetic 1992). Generic strength and stiffness. For example, Matasovic (1993) and sets of material parameters for this model are provided in Matasovic and Vucetic (1995b) proposed degradation index the D-MOD2000 package. functions that degrade strength and stiffness of sands at dif- ferent rates, whereas the concept of the degradation index (Idriss et al. 1978) is used to degrade strength and stiffness of soft clays. A number of pwp generation models have been devel- oped, starting with Martin and Seed (1978) and Martin et al. (1975). The Martin and Seed (1978) model was imple- mented in early iterations of FLAC by Dr. Wolfgang Roth (Roth and Inel 1993; W. Roth, personal communication, 2011). The Martin et al. (1975) pwp generation model was used in DESRA-2 (Lee and Finn 1978). A more recent example of the semi-empirical pwp model for saturated sand is the Dobry et al. (1985) model. This model was based on strain­controlled cyclic direct simple shear and cyclic triaxial testing. The model was later modified by Vucetic FIGURE 9 Stress-strain behavior modeling illustrating (1986) to allow for quasi-2-D shaking and further by Mata- stiffness degradation with the MKZ Constitutive Model sovic (1993) to more accurately model pwp-induced deg- (Matasovic 1993; Matasovic and Vucetic 1993). radation of shear modulus and shear stress. The Vucetic (1986) modification of this pwp model has been success- Advanced Effective-Stress-Based Models fully incorporated in DESRAMOD (Vucetic 1986), and the Matasovic (1993) modification has been incorporated in Another class of soil constitutive models used in site D-MOD (Matasovic 1993; Matasovic and Vucetic 1995b), response analysis is effective-stress models. In these D MOD_2 (Matasovic 2006) and D-MOD2000 (Matasovic models, the formulation of the constitutive law is devel- and Ordonez 2007), and DEEPSOIL (Hashash et al. 2011). oped in effective-stress space, and pwp is computed as the The pwp generation models described require the use of an difference between effective-stresses and total stresses equivalent number of cycles to represent earthquake shak- in the domain of interest. Examples of plasticity-based ing. Polito et al. (2008) introduced an energy-based model constitutive laws include Roscoe and Schofield (1963), (GMP model) for the generation of pwp based on a large Mroz (1967), Roscoe and Burland (1968), Prevost (1977), number of laboratory tests, which does not require the devel- Dafalias and Popov (1979), Pestana (1994), Whittle and opment of an equivalent number of cycles. This model has Kavvadas (1994), Byrne et al. (1995), Manzari and Nour been implemented in DEEPSOIL (Hashash et al. 2011) com- (1997), Beaty and Byrne (1998), and Elgamal et al. (2001). bined with the degradation index framework introduced by These advanced constitutive models are capable of simu- Matasovic (1993). With the exception of the modified Dobry lating complex soil behavior under a variety of loading et al. (1985) model, as implemented in D-MOD2000, there is conditions. Key elements of these models include yield limited information to guide the user in selecting the appro- surfaces, flow rules, and hardening (or softening) laws. priate pwp model parameters. A review of advanced constitutive models with appli- cation in site response analysis is provided in Potts and The effect of cyclic degradation on soil stiffness and Zdravkovi (1999). strength is illustrated in Figure 9 for the MKZ constitutive model (Matasovic 1993; Matasovic and Vucetic 1995b). The Generic material parameters for advanced constitu- initial hysteretic loop shown in the figure refers to the first tive models are often not available. Evaluation of material cycle of cyclic loading (i.e., at time t = 0). The subsequent parameters for these models requires significant expertise degraded hysteretic loop refers to any subsequent cycle (i.e., and detailed site-specific soil properties. Examples of site at time t) for which enough pwp has built up to degrade both response programs that incorporate advanced constitutive initial shear modulus Gmo and initial shear stress co at cor- models are DYNA1D (Prevost 1989), SUMDES (Li et al. responding shear strain co. 1992), SPECTRA (Borja and Wu 1994), AMPLE (Pestana and Nadim 2000), CYCLIC 1-D (Elgamal et al. 2004), An example of a pwp model for clay is the Matasovic and CyberQuake (Modaressi and Foerster 2000; Foerster and Vucetic (1995a) model. This model was based on the results Modaressi 2007; Lopez-Caballero et al. 2007) and the of cyclic simple shear testing. It can be noted that pwp in ground response module in the OpenSees simulation plat-

15 form (Ragheb 1994; Parra 1996; Yang 2000; McKenna and stress site response analyses (Kwok et al. 2007; Stewart et Fenves 2001). Other programs that have been used include al. 2008). Another interesting benchmarking exercise, the ABAQUS, ADINA, PLAXIS, and FLAC. The UBC sand evaluation of site response at Turkey Flat (Real et al. 2006; model (Byrne et al. 1995; Beaty and Byrne 1998) has Kramer 2009), highlighted the challenges in computing gained acceptance in the geotechnical earthquake engi- the response at a well-constrained relatively stiff soil site. neering community and is available in FLAC and more Hashash et al. (2010) provide a recent review of many of the recently in PLAXIS. issues associated with 1-D nonlinear site response analysis. Pore Water Pressure Dissipation and Redistribution Models GUIDANCE DOCUMENTS FOR SITE RESPONSE ANALYSIS The layers in a soil column have finite, saturated hydraulic The FHWA guidance document for seismic design of highway conductivities. Even though loading is relatively rapid dur- facilities (FHWA-NHI-11-032: LRFD Seismic Analysis and ing ground shaking, pore water redistribution may occur at Design of Transportation Geotechnical Features and Struc- that time as a result of differences in hydraulic gradients and tural Foundations, 2011) is posted at http://www.fhwa.dot.gov/ hydraulic conductivities between layers, following the prin- engineering/engineering/geotech/pubs/nhi11032/nhi11032. ciples of Terzaghi's theory of consolidation. pdf. In the meantime, an older document (Kavazanjian et al. 1997a, b) that focuses on geotechnical analysis and design of Martin and Seed (1978) introduced an early model for highway facilities, including dynamic site characterization for pwp dissipation and redistribution. Input parameters include site response analysis, is still available for download. Several constrained rebound modulus and (saturated) hydraulic con- other guidance documents are used in transportation engineer- ductivity. The incorporation of such a model in nonlinear ing and other seismic design practices. These documents and codes, such as CyberQuake, DESRA-2C, D-MOD_2, D their websites are listed in Table 1. Many of these documents MOD2000, DEEPSOIL, ABAQUS, PLAXIS, FLAC, and were referred to by survey respondents (see chapter four). OpenSees, allows for calculation of simultaneous genera- tion, dissipation, and redistribution of pwp during and after Most of the DOTs documents follow, in some way, the shaking. A pwp dissipation model for composite soil depos- general guidelines for conducting a site response analysis its (alternating sand and clay layers), developed by Mataso- outlined in AASHTO documents. Some documents discuss vic and Vucetic (1995a), is incorporated in D-MOD_2 and the use of equivalent-linear analysis while others discuss the D-MOD2000. use of nonlinear site response analyses with and without pwp generation. However, with the exception of NRC RG 1.208: A Performance­Based Approach to Define the Site-Specific CALIBRATION AND BENCHMARKING STUDIES Earthquake Ground Motion, these documents do not provide sufficient guidance on the mechanics and steps required for A number of individuals and groups have conducted eval- developing design ground motions, characterizing the site, uations of site response analysis procedures versus mea- or performing appropriate site response analysis. sured response from earthquake recordings and downhole vertical arrays. SOFTWARE USED IN PRACTICE Researchers have used a range of inverse analysis tech- niques to calibrate soil constitutive models in site response Table 2 lists and classifies currently available site response analysis procedures to discover soil behavior. These include analysis software. However, any listing of such programs is ad hoc, system identification (e.g., Zeghal and Elgamal 1993; likely to be incomplete and subject to interpretation. Glaser and Baise 2000; Assimaki and Steidl 2007), and evo- lutionary soil models (e.g., Tsai and Hashash 2007). Table 2 shows that geotechnical engineers have a variety of equivalent-linear and nonlinear software to choose from Kramer and Paulsen (2004) conducted an informal sur- for 1-D and multidimensional site response analyses. Some vey on the practical use of site response analysis models. of the software is widely used, for example SHAKE and They found that 1-D equivalent-linear site response analysis its numerous DOS- and Windows-based derivatives. Some is by far the most commonly used approach and that there is of the Windows-based equivalent-linear software operates a lack of guidance on use of nonlinear site response analysis, as pre-processors and post-processors for SHAKE91 (e.g., so it is not used often. ProShake and SHAKE2000), whereas others were written from scratch (e.g., EERA and equivalent-linear mode of A number of benchmarking exercises have also been DEEPSOIL). The Windows-based software generally offers conducted to evaluate site response analysis tools. A key convenient plotting of input data (soil profile and ground recent exercise is the PEER benchmarking exercise for total motions) and resulting output. More important, they guide

16 TABLE 1 GUIDANCE DOCUMENTS FOR SEISMIC DESIGN Document Website (if available) 1 Caltrans Seismic Design Criteria http://www.dot.ca.gov/hq/esc/earthquake_engineering/SDC_site/ 2 Washington State Department of Transportation, Geotechnical http://www.wsdot.wa.gov/Publications/Manuals/M46-03.htm Design Manual (2011) 3 Illinois Department of Transportation http://www.dot.il.gov/bridges/brmanuals.html 4 AASHTO Guide Specifications for LRFD Seismic Bridge https://bookstore.transportation.org Design (1st Edition) (2010a) Revision to AASHTO Guide Specifications for LRFD Seismic Bridge Design 5 NCHRP Report 611 http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_611.pdf 6 ASCE 7-05, ASCE 7-10 (pending) ASCE 4, ASCE 43-05 7 NRC RG 1.208: A Performance-Based Approach to Define http://www.nrc.gov/reading-rm/doc-collections/reg-guides/power-reactors/rg/ the Site-Specific Earthquake Ground Motion 01-208/01-208.pdf 8 U.S. Army Corps of Engineers EM110-2-6050: Response Spec- http://140.194.76.129/publications/eng-manuals/em1110-2-6050/toc.htm tra and Seismic Analysis for Concrete Hydraulic Structures 9 Oregon Department of Transportation, Geotechnical Design ftp://ftp.odot.state.or.us/techserv/Geo-Environmental/Geotech/GeoManual/ Manual FinalGDM12-1009/Volume1GeotechDesignManualFinal_Dec2009.pdf 10 South Carolina Department of Transportation, Seismic Design http://www.scdot.org/doing/bridge/bridgeseismic.shtml Specifications for Highway Bridges 11 South Carolina Department of Transportation, Geotechnical Manual http://www.scdot.org/doing/bridge/geodesignmanual.shtml 12 Georgia Department of Transportation, Bridge and Structures http://www.dot.state.ga.us/doingbusiness/PoliciesManuals/roads/ Policy Manual BridgeandStructure/GDOT_Bridge_and_Structures_Policy_Manual.pdf 13 Rhode Island LRFD Bridge Design Manual http://www.dot.ri.gov/documents/engineering/br/RILRFDBridgeManual.pdf TABLE 2 SITE RESPONSE SOFTWARE USED IN ENGINEERING PRACTICE Dimensions OI Equivalent-Linear Nonlinear DOS SHAKE/SHAKE91 AMPLE, DESRA-2C, DESRAMOD, D-MOD_2, DESRAMUSC, LS-Dyna, 1-D SUMDES, TESS, SIREN, Graphical User Interface ShakeEdit, ProShake, SHAKE2000, EERA, CyberQuake, D-MOD2000, DEEPSOIL, DEEPSOIL, WinMOC FLAC, NERA, VERSAT 1-D DOS FLUSH, SASSI, QUAD4/QUAD4M, DYNAFLOW, DYSAC2, TARA-2, FLUSH TARA-3 2-D / 3-D Graphical User Interface QUAKE/W, SUPER SASSI, SASSI2000 FLAC, PLAXIS, PLAXIS 3D, ABAQUS, OpenSees, VERSAT 2-D Notes: 1. Only the latest versions of a particular software are listed in Table 2. Full software reference is provided in Appendix C. 2. Several of the listed software have options to use more than one constitutive model, including equivalent-linear, bi-linear, and nonlinear models (e.g., FLAC, PLAXIS, ABAQUS, and D-MOD2000). 3. Several of the listed software have a pore water pressure generation option (e.g., D-MOD2000, DEEPSOIL, CyberQuake, DESRA-2C, DESRAMUSC, OpenSees, PLAXIS, TESS, SUMDES, DYSAC2, and FLAC). the user through the key elements of the input and reduce ates Inc., respectively). QUAKE/W is similar to QUAD4M, the number of errors. The importance of a user interface to but it does not calculate the average acceleration of sliding enhancing the quality of analysis cannot be overemphasized. mass as QUAD4M does and it has a Windows GUI. Nonlin- FLUSH, SASSI, and QUAD4M are well established and ear software for 1-D analyses with GUI are gaining popu- widely used 2-D, DOS-based, equivalent-linear programs. larity (e.g., as D-MOD2000 and DEEPSOIL in the United (Windows-based graphical user interfaces are available for States practice; Cyberquake in European practice). Nonlin- QUAD4M and SASSI, but they are proprietary to the United ear 2-D site response software with GUI, such as FLAC and States Army Corps of Engineers and Stevenson & Associ- PLAXIS, is also gaining acceptance.

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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 428: Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions identifies and describes current practice and available methods for evaluating the influence of local ground conditions on earthquake design ground motions on a site-specific basis.

The report focuses on evaluating the response of soil deposits to strong ground shaking.

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