Seismic Performance of Silty Soil Sites

Transcription

1 Seismic Performance of Silty Soil Sites Jonathan Bray, Ph.D., P.E., NAE, & Christine Beyzaei, Ph.D., P.E. Univ. of California, Berkeley With Contributions From: M. Cubrinovski, M. Riemer, C. Markham, J. Zupan, M. Stringer, S. van Ballegooy, M. Jacka, R. Wentz, etc. Sponsors: National Science Foundation, Pacific Earthquake Engineering Research Center, Ministry of Business, Innovation & Employment, and Earthquake Commission New Zealand

2 LIQUEFACTION EFFECTS 1906 San Francisco EQ (Lawson et al. 1908) 1964 Niigata, Japan EQ (from H.B. Seed) 1989 Loma Prieta EQ

3 LIQUEFACTION EFFECTS Flow Liquefaction Cyclic Mobility (strain-softening large strain) (strain-hardening limited strain)

4 LIQUEFACTION EFFECTS Cyclic Mobility C S R Liquefaction Effects Observed at Ground Surface Flow Liquefaction FS = FS =1.2 CRR No Liquefaction Effects Observed at Ground Surface FS = CRR / CSR Idriss & Boulanger 2008

5 Liquefaction Flow Slides when q c1ncs-sr < Idriss & Boulanger 2008

6 New Zealand Wellington Australian plate Alpine Fault MCE M w = 8 Christchurch Pacific plate

7 Canterbury Earthquake Sequence MW = Sept 10 MW = Dec 11 MW = Feb 11 MW = June 11 GNS Science

8 Canterbury NZ EQs: Widespread Liquefaction Cubrinovski et al. 2011

9 Liquefaction Effects in Christchurch From M. Cubrinovski

10 Liquefaction from 3 + EQs (Cubrinovski 2011) Base Map 22 Feb 2011 M w = 6.2 White Areas 4 Sep 2010 M w = 7.1 Black Areas 13 Jun 2011 M w = 6.0 CBD

11 Repeated Liquefaction Events 4 Sept Feb April June 2011: Part 1 (Mark Quigley: Avonside) 13 June 2011: Part 2

12 Age of Christchurch Soils 0 After Liquefaction from M. Cubrinovski

13 Liquefaction in Christchurch (van Ballegooy et al. 2014)

14 Capturing Liquefaction Effects

15 Post-Liquefaction Volumetric Strain (ε v ) Ishihara & Yoshimine 1992

16 Post-Liquefaction Volumetric Strain (ε v ) Decreasing D r Increasing ε v Decreasing FS Ishihara & Yoshimine 1992

17 Capturing Liquefaction Effects LSN considers when FS > 1 LSN limited by max ε v LSN affected by D r LSN weights heavily shallow layers van Ballegooy et al. 2014

18 Capturing Liquefaction Effects van Ballegooy et al. 2014

19 Liquefaction in Christchurch (van Ballegooy et al. 2014)

20 Land Damage LSN Estimates (van Ballegooy et al. 2014)

21 Observations of Liquefaction Ejecta q c (MPa) I c 2010 Darfield EQ Ejecta Observed Depth (m) No Ejecta I c = 1.8 van Ballegooy et al. Tonkin & Taylor for the EQC

22 CPT Profiles Increasing PGA Increasing Manifestations Beyzaei et al.

23 Liquefaction of Silty Soil Sites Site 23 site where no liquefaction effects were observed; yet simplified procedures indicate liquefaction was expected (from R. Wentz, Wentz-Pacific)

24 Liquefaction Assessment at Stratified Site DEPTH BELOW GROUND SURFACE (m) GWT GWT GWT CRR CSR Settlement ~ 13 cm LSN = 29 BUT no liquefaction effects observed CRR & CSR Factor of Safety Settlement (cm) Riccarton Road Site Feb 2011 EQ: PGA = 0.37 g, GWT = 0.6 m BGS, P L =50%, LPI = 19, CPT_36420 (Beyzaei et al.; CRR and FS plots exported from CLiq)

25 Cyclic Triaxial Testing Program 100 PERCENT FINER (%) Range of silty soils tested DIAMETER ( m) Site 21 Site 23 Site 33 (silty soil) Site 33 (sand) S33-DM1-5U-B (top) S33-DM1-5U-B (mid) S33-DM1-5U-B (bot) S33-DM1-6U-B (ends) S33-DM1-6U-B (mid) S21-DM1-3U-B S23-DM1-3U-A S23-DM1-7U-B EQC-4 Beyzaei et al.

26 Cyclic Triaxial Test Results (a) Cycle 1 (b) Cycle q/(2p' 0 ) 0.0 q/(2p' 0 ) Axial Strain (%) Axial Strain (%) 0.6 (c) 3% SA 0.6 (d) 5% DA q/(2p' 0 ) 0.0 q/(2p' 0 ) Axial Strain (%) Axial Strain (%) Beyzaei et al. EQC4-DM1B-7U-A (Clean Sand) S21-DM1-3U-A (Silt, NP) S33-DM1-8U-A (Silt, PI=10) S33-DM1-8U-B (Silt, PI=10)

27 Liquefaction Assessment Comparison Depth (m) Laboratory Data: CRR TX,field 0.19 Boulanger & Idriss 2014: CRR B&I 0.16 CSR B&I CRR & CSR Lab or Field FS for Liquefaction Triggering: FS Beyzaei et al.

28 Post-Liquefaction Reconsolidation VOLUMETRIC STRAIN (%) 0 EQC4-DM1B-6U-A EQC4-DM1B-6U-B EQC4-DM1B-7U-A EQC4-DM2-3U-A EQC4-DM2-3U-B EQC4-DM2-4U-A Clean Sand and Silty Sand TIME (sec) VOLUMETRIC STRAIN (%) 0 S23-DM1-3U-A S23-DM1-3U-B S23-DM1-4U-B S23-DM1-5U-A S23-DM1-7U-A S23-DM1-7U-B S23-DM1-8Ub-A Silt (PI=8-10) and Sandy Silt (PI=4-7) Beyzaei et al TIME (sec) 10000

29 Cyclic Triaxial Test Results Beyzaei et al. Clean Sand (EQC3-DM1-5U-A) Non-plastic Silty Sand/Silt (S33-DM1-6U-B) PI=10 Silt (S33-DM1-8U-A)

30 Liquefaction Case Histories from Boulanger & Idriss (2014) Mostly clean sand sites Not many silty soil sites Liquefaction Effects Observed at Ground Surface No Liquefaction Effects Observed at Ground Surface Boulanger & Idriss (2014) CPT-based method 253 cases in total 20 cases FC > 35% 97 cases (no lab FC) 15 cases est. FC > 35%

31 I c Fines Content (FC) Correlations Robinson, Cubrinovski, & Bradley 2013 R² = CBD Data from Zupan 2014, Taylor 2015, & Markham 2015 Apparent FC Ic Fines Content (%) 2.0 I c Fines Content (%) Data R&W(98)-General Correlation Idriss & Boulanger 2008

32 I c Fines Content Correlation 4.0 SOIL BEHAVIOR TYPE INDEX (I c ) bot ends top S33-DM1-6U-B mid mid S/H = Sieve/Hydometer L = Laser Site 37 (Logging, S/H) Site 14 (Logging, S/H) Site 33 (Logging, S/H) Site 2 (CTX, L) Site 14 (CTX, L) Site 21 (CTX, L) Site 23 (CTX, L) Site 33 (CTX, L) BI16 (C_FC=0) BI16 (C_FC=0.2) Best-fit (Lab data) FINES CONTENT (%) S33-DM1-5U-B Closely spaced continuous sampling with thin defined layers (Beyzaei et al.)

33 Grain-Size Composition of Soils Sand ejecta samples from areas in Christchurch #200 (Courtesy of M. Pender, Univ. of Auckland) Clean fine sands and non-plastic silty sands Does soil know that the #200 sieve exists?

34 Particle Shape of Soils Monterey 0/30 sand, FC = 0% S33-DM1-5U-A sand, FC=10% S23-DM1-8Ub-A silt, FC=63%: coarse fraction (> 75 µm) & fines fraction (< 75 µm) SEM photographs (200x) by Beyzaei et al.

35 Importance of Depositional Environment Recognized but often not explicitly considered Youd & Perkins (1978): factors that affect ground failure susceptibility include sedimentation process, age of deposition, geologic history, depth of water table Seed (1979): method of placement or soil structure and age since deposition or placement a single layer of relatively impervious fine sand or silt in such a deposit would completely invalidate the results of pore pressure dissipation computations for vertical flow

36 Canterbury Plains Christchurch

37 Buried Streams in Central Christchurch (from 1850 s Black Maps ) Madras St. Kilmore St. Armaph St. Hereford St. Cathedral from Christchurch: Swamp to City (photo from 1880, 30 years after Christchurch was founded) from M. Cubrinovski

38 Depositional Environment of Silty Sites WAIMAKARIRI RIVER RAKAIA RIVER Canterbury Plains PORT HILLS 1918 Photo from Christchurch: Swamp to City Beyzaei et al.

39 Regional Assessment (a) (b) Beyzaei et al.

40 Beyzaei et al. Thin-Layer Stratigraphy: Standard CPT vs. Mini-CPT Site 33 - Cashmere Site 37 - Clarence

41 Site Characterization Tools 3.65 DEPTH BELOW GROUND SURFACE (m) (a) q c (MPa) CPT_36421 Mini-CPT_01 Mini-CPT_02 I c (oxidized) fine sand, some medium sand, trace silt silt band very fine sand with silt laminations organics band organics band silt parting fine sand silty fine sand Graphic Log Sample Photograph DM High-Quality Sample (b) Sonic Boring Core Sample (c) Beyzaei et al.

42 Groundwater Table Effects Continuous sampling Crosshole seismic testing (UT-Austin) Site 21 Beyzaei et al. Piezometer (NZGD)

43 Over-Estimation of Liquefaction Triggering Cyclic testing data does not explain the discrepancy Other possible explanations? Groundwater table fluctuation & clayey crust Highly stratified subsurface profile At-depth suppression of ejecta movement & reconsolidation time Angular particles/borderline soil types Inherent conservatism in analysis approach Combination of all the above? System Response macro-scale system response as opposed to element/specimen level response

44 Natural Shoal Deposits of Treasure Island, San Francisco Pedro Espinosa, Phil Stuecheli, Stefanos, Papadopoulos, Joe Tootle, Uri Eliahu, Shah Vahdani, Bahareh Heidarzadeh, Steve Dickenson, Michael Beaty, Juan Pestana, Michael Riemer, Chris Markham, Jonathan Bray, & Nick Sitar

45 ORIGINAL CONDITION, CONSTRUCTION, AND DEVELOPMENT

46 IMPROVE FILL-SHOAL WITH VIBRO-COMPACTION USING DPC Site Location Sourced from Google Earth

47 Fill Shoal TEST PROGRAM CPT RESULTS Full-scale DPC test results indicated that no appreciable densification can be obtained within the shoal deposits.

48 Undisturbed Soil Sampling & Testing D&M Thin- Walled Piston Sampler ASTM D

49 FILL Fill consists largely of sand and non-plastic silt

50 SHOAL DEPOSIT Close grain packing and clay films bridging pores Shoal deposit is heterogeneous natural deposit often with interlocking sand grains with clay bridges Weakly developed clay bridges and few fines in pores

51 SUBSURFACE GEOLOGY

52 Silty Soil Liquefaction Effects on Hospital 2010 Chile EQ (M w = 8.8)

53 SILT LIQUEFACTION EFFECTS ON STRUCTURES 1999 Kocaeli, Turkey EQ: Adapazari

54 Evaluation of I c < 2.6 Criterion Soils at two sites in Adapazari

55 Conclusions Loose shallow sand & nonplastic silt deposits led to much damage in Christchurch, especially in areas with ejecta Interbedded silty soil sites differ from typical clean-sand sites upon which liquefaction procedures are largely based Depositional environment distinguishes between sites that did or did not liquefy; CPT-based simplified procedures did not Understanding system response is key Depositional environment should be explicitly considered in liquefaction assessments. Historical maps, geologic studies, and continuous sampling can provide key information Silty soil sites could exhibit liquefaction manifestations if heavy buildings were present or if shaken harder

56 RECOMMENDATIONS Liquefaction triggering procedures, which have been developed for sands and nonplastic silty sands, should be applied with judgment. Plasticity Index Susceptible to Liquefaction Moderate Susceptibility Not Susceptible w c /LL Perform cyclic testing on fine-grained soils that can be sampled effectively to assess their seismic response characteristics. Consider depositional environment & system response which may be missed by simplified methods (e.g., thin-layer stratigraphy, & groundwater fluctuations)