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Lessons Learned and Need for NEES Instrumented Liquefaction Sites T. Leslie Youd Brigham Young University.

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Presentation on theme: "Lessons Learned and Need for NEES Instrumented Liquefaction Sites T. Leslie Youd Brigham Young University."— Presentation transcript:

1 Lessons Learned and Need for NEES Instrumented Liquefaction Sites T. Leslie Youd Brigham Young University

2 Purposes of Presentation To convince the members of this workshop that instrumented liquefaction sites are important and deserves our support To urge geotechical engineers in Taiwan, Japan and the US to actively identify sites and seek opportunities place instruments to add much needed instrumental data to the liquefaction case history data base

3 Need for Instrumented Field Sites Past instrumental records provide important information on pore pressure rise and site response These records provide field data for development and verification of empirical and analytical predictive procedures More instrumental records are needed to better understand and model pore pressure generation, ground deformation and ground failure

4 Wildlife Liquefaction Array (WLA) WLA was instrumented by US Geological Survey in 1982 Recorded two earthquakes in 1987: the Elmore Ranch (M=6.2), which did not generate significant pore pressures, and the Superstition Hills (M=6.6) which generated liquefaction at the site WLA is being redeveloped and reinstrumented under the NSF Network for Earthquake Engineering Simulation (NEES) program

5 Wildlife Site Regional map showing location of Wildlife site

6 General setting and recent earthquakes that have shaken the Wildlife site (WLA) (map from Holzer et al., 1989)

7 Liquefaction Occurrences Near Wildlife Site Liquefaction Effects observed following six earthquakes in past 72 Years Wildlife site YearArea of effects

8 1950 sand boil that erupted about 1.5 km northwest of Wildlife Site

9 1982 USGS Array The two accelerometers are still functioning and maintained by USGS The six piezometers failed sometime after 1987 earthquakes

10 View of Wildlife site After 1987 Superstition Hills Earthquake Sand boils in the foreground and instrument hut in the background. (USGS photo)

11 Wildlife Site Acceleration and Pore Pressure records generated during the 1987 Superstition Hills earthquake

12 A.Predicted and actual ground motions at WLA site from November 24, 1987 Superstition Hills earthquake B. Pore pressure ratios versus time (after Dobry et al., 1989) A B Peak Accel Pore pressure ratios, r u, range from 0.4 to 0.6 R u = 1.0 End of strong ground shaking

13 Reason for continued rise of pore water pressure: Although strong ground accelerations ceased at about 23 sec, ground displacements continued to rise with maximum of 22 cm (peak to peak) at about 35 sec. Cyclic shear strain, as a consequence of ground displacement, generates increased pore water pressures. Correlation of acceleration and pore water pressure spikes was due to dilatent arrest of ground movement producing a sudden drop of pore pressure and the acceleration spike. Movement then ensued in the opposite direction. These spikes are numbered on the upper plots (Zeghal and Elgamal, 1994) WLA Site Response – 1987 Superstition Hills Eq End of strong shaking

14 Shear stresses were calculated from measured ground accelerations and mass of soil above liquefied layer Shear strains were calculated from ground displacements, determined from double integration of acceleration records, and dividing by distance between accelerometers Note initial near-vertical stress strain loops that flattened and develop banana-type loops with time Analysis by Zeghal and Elgamal (1994)

15 Predicted and actual acceleration time histories from the Superstition Hills earthquake, WLA site, M = 6.6 Predicted Amax = 0.31 g Actual Amax = 0.21 g

16 Predicted and actual response spectra for Superstition Hills earthquake - WLA site Short-period (>0.7 sec) spectral accelerations were attenuated Long-period (> 0.7 sec) spectral accelerations were enhanced

17 Lessons from 1987 SH Earthquake Pore pressures continued to rise after strong ground accelerations ceased Repeated dilatent arrest of lateral ground displacement observed Stress-strain properties of softening soil calculated from site response Softened layer inhibited transmission of short-period (T<0.7 sec) strong motions Liquefaction enhanced long-period motions (T>0.7 sec) Test of viability of predictive tools

18 Principal Investigators Principal Investigators T. Leslie Youd, BYU Jamison Steidl, UCSB Robert Nigbor, USC The George E. Brown Jr. Network for Earthquake Engineering Simulation (NEES) Phase 2 Award PERMANENTLY INSTRUMENTED FIELD SITES FOR STUDY OF SFSI Cooperative Agreement No. CMS

19 Wildlife Liquefaction Array (WLA) Objectives: Provide a simple, well-characterized permanently instrumented field site for study of liquefaction, ground deformation, and ground failure Install new accelerometers, piezometers, inclinometers, etc., to monitor liquefaction and induced ground deformation and displacement Provide teleobservation and telepresence capabilities for remote monitoring and interaction with site

20 Reasons for Reinstrumenting WLA Many new and important lessons learned from old site; more lessons yet to be learned Old site has been disturbed and piezometers are no longer functional New research opportunities with expanded instrumentation and greater ground deformation potential Teleobservation and telepresence capabilities provide distributed research and educational opportunities

21 General view of wildlife area with locations of old and new sites (view looking east southeast) New Site Old Site Alamo River

22 New Site Old Site Stream erosion is cutting into bank adjacent to new site generating a free face that should facilitate ground deformation and lateral spread Alamo River

23 Map of wildlife area showing locations of 1982 and new sites

24 USGS CPT rig conducting soundings at new WLA site

25 Free face created by incised river

26 Enlarged view of new WLA site

27 clay sand Wildlife Liquefaction Array (WLA) Cross-section A-A’ Soil Behavior Types Interpreted by USGS from CPT Soundings River clay

28 Wildlife Liquefaction Array (WLA) Cross-section B-B’ – Soil Behavior Types from CPT Soundings River clay sand

29 Wildlife Liquefaction Array (WLA) Cross-section C-C’ – Soil Behavior Types from CPT Soundings

30 Enlarged view of new WLA site CPT 35

31 Liquefaction resistance of WLA CPT 35: M = 6.5 and various levels of peak ground acceleration (CPT procedure of Youd et al., 2001) 0

32 Enlarged view of new WLA site

33 Cross section A-A’ showing liquefaction resistance from analyses of CPT data for M = 6.5 earthquakes and Amax = 0.4 g River

34 Cross section B-B’ showing liquefaction resistance from analyses of CPT data for M = 6.5 earthquakes and Amax = 0.4 g River

35 Cross section E-E’ showing liquefaction resistance from analyses of CPT data for M = 6.5 earthquakes and Amax = 0.4 g

36 Cross section D-D’ showing liquefaction resistance from analyses of CPT data for M = 6.5 earthquakes and Amax = 0.4 g

37 1982 Site New Site Cross section C-C’ showing liquefaction resistance from analyses of CPT data for M = 6.5 earthquakes and Amax = 0.4 g

38 Enlarged view of new WLA site

39 Expected Contributions to NEES Fully instrumented site to monitor ground motions, pore pressures, and ground deformation as liquefaction and lateral spread develop during future earthquakes Well characterized site from which analytical and empirical tools can be developed and tested Site where new field measurement tools can be tested and calibrated

40 More instrumented sites are needed To increase the likelihood of timely recording of site responses To increase the number of recorded responses available To increase the variety of sites and site conditions To increase the variety of earthquake mechanisms and magnitudes To speed the development of predictive procedures


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