THE IMPORTANCE OF CASE HISTORIES IN GEOTECHNICAL ENGINEERING

Transcription

1 THE 4 TH ANNUAL PEDRO DE ALBA LECTURE THE IMPORTANCE OF CASE HISTORIES IN GEOTECHNICAL ENGINEERING presented by I. M. Idriss, Professor Emeritus, Univ. of California at Davis Consulting Geotechnical engineer, Santa Fe, NM imidriss@aol.com Presented at The University of New Hampshire Durham, New Hampshire April 7, 2016 Professor Pedro de Alba ( ) 1

2 ROLE OF CASE HISTORIES Case Histories have always played a strong role in geotechnical engineering. They have been an essential means for: Improving understanding; Calibrating analytical procedures; Designing & interpreting physical model tests; and Developing semi-empirical procedures Under static as well as during earthquake and postearthquake loading conditions. SIGNIFICANT EARTHQUAKES SINCE Mexico City 1964 ALASKA 1964 NIIGATA 1966 Parkfield 1967 Caracas 1968 Tokachi-Oki 1971 SAN FERNANDO 1975 Oroville 1975 Haicheng 1976 Gazli (USSR) 1976 Tangshan 1978 Miyagiken-Oki 1978 Santa Barbara 1978 Tabas 1979 Coyote Lake 1979 IMPERIAL VALLEY 1980 Livermore 1980 Mammoth Lake 1982 Miramichi 1983 Coalinga 1985 Chile 1985 MEXICO CITY 1985 Nahani 1986 N. PALM SPRINGS 1987 WHITTIER-NARROWS 1988 Armenia 1988 Saguenay 1989 LOMA PRIETA 1990 Manjil 1990 Philippine 1991 Costa Rica 1991 Sierra Madre 1992 Turkey 1992 Joshua tree 1992 Petrolia 1992 Landers 1992 Big Bear 1994 NORTHRIDGE 1995 KOBE 1999 KOCAELI 1999 CHI-CHI 1999 Duzce 2001 Bhuj 2001 Nisqually 2004 Niigata 2015 Nepal 2

3 OUTLINE OF THIS TALK Plan A Case Histories involving triggering of liquefaction in cohesionless soils. Case Histories of large deformations involving soft cohesive soils. Case Histories involving lateral flows in cohesionless soils. OUTLINE OF THIS TALK Plan B Case Histories involving triggering of liquefaction in cohesionless soils. Case Histories of large deformations involving soft cohesive soils. 3

4 LIQUEFACTION OF COHESIONLESS SOILS Examples of Surface Evidence of Liquefaction LIQUEFACTION OF COHESIONLESS SOILS 1978 Miyagiken-Oki earthquake 4

5 LIQUEFACTION OF COHESIONLESS SOILS 1964 Niigata earthquake (photo: NISEE) LIQUEFACTION OF COHESIONLESS SOILS 1964 Niigata earthquake (photo: NISEE) 5

6 LIQUEFACTION OF COHESIONLESS SOILS 1971 San Fernando earthquake (photo: California DWR) LIQUEFACTION OF COHESIONLESS SOILS 1999 CHI-CHI earthquake 6

7 LIQUEFACTION OF COHESIONLESS SOILS Information needed for each case history A. Site information: 1. Location, adjacent topography; 2. Adjacent physical features; 3. Surface [Evidence/No Evidence] of liquefaction. B. Subsurface information: 1. Borings, samples methods used; 2. Water table measurements; 3. Standard penetration tests details used; 4. Cone penetration resistance data; 5. Shear wave measurements method(s) used. C. Earthquake & earthquake ground motions information 1. M w, distance, nearby recordings, site "classification". LIQUEFACTION OF COHESIONLESS SOILS Use of liquefaction case histories started in At that time, there were only 23 cases with observed surface evidence of liquefaction and only 12 cases with no observed evidence of liquefaction. These case histories were used in the development of the Seed-Idriss simplified liquefaction procedure, which was published in the Journal of ASCE's SM&FE Division in

8 LIQUEFACTION OF COHESIONLESS SOILS Since then, the number of cases has dramatically increased. While in 1968 correlation was made to relative density and SPT blow count only, correlations are now made with: SPT blow count; CPT tip resistance, and V s, shear wave velocity. More recently, correlations with dilatometer measurements have been proposed. Reference materials 2008 monograph 2010 SPT update 8

9 UPDATED DATABASE DISTRIBUTION OF CASE HISTORY PARAMETERS Total number of SPT-based case histories: US case histories: Y/N/M 28/30/1 Japan case histories: Y/N/M 74/75/1 Other case histories: Y/N/M 13/7/1 + 4/ update of the CPT-based procedure Total number of CPT-based case histories: 253 Y/N/M 180/71/2 US case histories: Y/N/M 65/35/1 Japan case histories: Y/N/M 24/13/1 New Zealand case histories: Y/N 53/16 Other case histories: Y/N 38/7 18 9

10 Reference materials Additional data from recent large earthquakes Improved understanding of duration (magnitude scaling) effects Desire for probabilistic CPTbased model that parallels our SPT-based model (Boulanger & Idriss 2012a) 2014 CPT & SPT update Soil Behavior Type (SBT) Index, I C & State Parameter, 20 10

11 Robertson (1990) developed a chart to identify soil behavior type (SBT), based on dimensionless parameters, Q and F (in percent), which are obtained using the following expressions (Robertson & Wride 1997): Q n qc v P a P a ' v n = 0.5 for sands & n = 1 for clays f s F 100 q c v Robertson & Wride (1997) introduced the SBT index, I C, which is calculated using the parameters Q and F: 2 2 C I 3.47 Log Q 1.22 Log F 21 STATE PARAMETER, e max CSL e = e c = e - e c Void ratio, e e min state parameter, is negative for soils dense of critical Mean effective normal stress, p'/p a 11

12 Dilative = Liquefaction failure case histories [Lateral Flows] (from Robertson 2010) 23 Normalized cone tip resistance, Q < I c < 1.64 [9] 1.64 < I c < 2 [80] 2.0 < I c < 2.2 [49] 2.2 < I c < 2.4 [29] 2.4 < I c < 2.6 [13] CPT Liquefaction Case Histories Normalized friction ratio, F (%) 24 12

13 Normalized cone tip resistance, Q State parameter = < I c < 1.64 [9] 1.64 < I c < 2 [80] 2.0 < I c < 2.2 [49] 2.2 < I c < 2.4 [29] 2.4 < I c < 2.6 [13] Normalized friction ratio, F (%) CPT Case Histories With surface evidence of Liquefaction 25 Normalized cone tip resistance, Q State parameter = < I c < 1.64 [21] 1.64 < I c < 2 [28] 2.0 < I c < 2.2 [12] 2.2 < I c < 2.4 [5] 2.4 < I c < 2.6 [5] Normalized friction ratio, F (%) CPT Case Histories No surface evidence of Liquefaction 26 13

14 Observations The Cone Penetration Test (CPT) has proven to be a very valuable tool for characterizing subsurface conditions and assessing various soil properties, including estimating the potential for liquefaction. The main advantages of using the CPT are that it provides a continuous record of the penetration resistance and is less vulnerable to operator error than is the SPT test. Its main disadvantages are the difficulty in penetrating through layers with larger particles (e.g., gravels) or very high penetration resistances (e.g., strongly cemented soils) and the need to perform companion borings or soundings to obtain soil samples. 27 OUTLINE OF THIS TALK Case Histories involving triggering of liquefaction in cohesionless soils. Case Histories of large deformations involving soft cohesive soils. 14

15 The 1964 Great Alaska Earthquake The great Alaskan earthquake of 1964 was the largest earthquake in North America; M W = 9.2. The 1964 Great Alaska Earthquake Extensive damage was caused by this earthquake over a large portion of Alaska. Several major landslides were triggered in the City of Anchorage. 15

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17 The 1964 Great Alaska Earthquake The 1964 Great Alaska Earthquake Some important observations regarding the landslides in Anchorage: Movements were either several feet or less than 6 inches; Movements started well after the start of shaking based on "eye witness" reports; and Movements stopped when shaking stopped. 17

18 The 1964 Great Alaska Earthquake The 1964 Great Alaska Earthquake Today, I will be discussing the three Cases Case No. 1: 4 th Avenue slide Moved northward movements of 19 and 11 ft. Analyzed in detail by Woodward-Clyde Consultants in Results summarized in a paper by Idriss (1985). Reanalyzed in 2004 Results summarized in a paper by Boulanger & Idriss (2006) 18

19 The 1964 Great Alaska Earthquake Case No. 2: L Street slide Moved westward movement of 14 ft. Analyzed in detail by Woodward-Clyde Consultants in Results summarized in a paper by Moriwaki et al. (1989). The 1964 Great Alaska Earthquake Case No. 3 Court House Site Suffered practically no movements (less than 2" westward & none northward). Analyzed in detail by Woodward-Clyde Consultants in 1986 No results have been published as yet. 19

20 The 1964 Great Alaska Earthquake 4 th AVENUE SLIDE 4 th Avenue Slide 20

21 4 th Avenue Slide 4 th avenue slide in the 1964 Great Alaska Earthquake (M w = 9.2) 4 th Avenue Slide 21

22 4 th Avenue Slide Properties of Bootlegger Cove Clay 4 th Avenue Slide The sand/silt layers in the in the upper 60 ft are too dense to liquefy during this earthquake (PGA 0.15 to 0.2 g). Therefore, it was judged that stability was controlled by behavior of the lower Bootlegger Cove Clay, which is moderately sensitive based on boring, sampling and insitu vane shear tests conducted in 1982 for this evaluation. A sensitivity of about 3 was established for this clay and the strength of the clay was reduced to 30% of its peak when a displacement of 6" is reached. Displacements were estimated using the Newmark type analysis. 22

23 4 th Avenue Slide 4 th Avenue Slide 23

24 The 1964 Great Alaska Earthquake 4 th Avenue Slide -- remediated section along B Street The 1964 Great Alaska Earthquake L STREET SLIDE 24

25 L Street Slide L Street Slide L Street Slide 25

26 L Street Slide in the 1964 Great Alaska Earthquake (M w = 9.2) The 1964 Great Alaska Earthquake THE COURTHOUSE SITE 26

27 The Courthouse Site The State Courthouse Site 27

28 The State Courthouse Site Court House site -- suffered practically no movements The Supreme Court engaged one firm to address the risk of building a court house at this site. The firm's opinion was: "the site did not move in 1964, but adjacent areas had significant lateral movements." Cannot dismiss the possibility of large movements at this site in future earthquakes. Therefore, DO NOT BUILD AT THIS SITE, was the recommendation of that firm. THIS IS A MISUSE OF CASE HISTORIES The State Courthouse Site The Supreme Court then engaged another firm to address the risk of building a court house at this site. The second firm's opinion was: "the site did not move in 1964, during an extremely large & powerful earthquake." Most likely will not move in future earthquakes. Therefore, YOU CAN BUILD AT THIS SITE. THIS IS AN ABUSE OF CASE HISTORIES 28

29 The State Courthouse Site After receiving two diametrically opposed viewpoints and recommendations, the Supreme Court then asked Woodward-Clyde for an opinion about the suitability of the site and about the two recommendations it had received from the other two firms. The State Courthouse Site Our response was: "we do not know and cannot begin to think about an opinion until we understood why this site had practically no movements while adjacent sites moved significantly." 29

30 The State Courthouse Site After a detailed field investigation [undisturbed sampling, CPT, in-situ torvane ], laboratory testing [consolidation, DSS ], and analyses similar to those completed for the 4 th Avenue and the L Street slides, the reasons for the difference in performance were attributable to the fact that: The undrained shear strength of the Bootlegger Cove Clay underlying the Courthouse site is much greater than the undrained shear strength of this clay underlying the other two locations. The higher undrained shear strength at the courthouse site is attributable to higher OCR. LARGE DEFORMATIONS INVOLVING SOFT COHESIVE SOILS 30

31 Properties of Bootlegger Cove Clay LARGE DEFORMATIONS INVOLVING SOFT COHESIVE SOILS VARIATIONS OF s ' u v 31

32 LARGE DEFORMATIONS INVOLVING SOFT COHESIVE SOILS 4 th Ave. Slide L Street Slide Courthouse Site Range of OCR Average OCR Range of s u / ' v Average s u / ' v Ratio of s u / ' v THE COURTHOUSE SITE BUT, why is the OCR in the Bootlegger Cove Clay beneath the Courthouse site higher than the OCR values at corresponding depths beneath the other two locations? Because 32

33 Why the higher OCR at the Courthouse Site? The Courthouse Site 33

34 The Courthouse Site The 1964 Great Alaska Earthquake Concluding Remarks: The landslides triggered during the 1964 Great Alaska earthquake have provided a wealth of information and insight regarding the behavior of moderately sensitive clays during earthquakes. Clays in the San Francisco Bay Area, in the Seattle area and other parts of North America and elsewhere in the world would probably exhibit similar characteristics and hence would have similar behavior during earthquakes. 34

35 The 1964 Great Alaska Earthquake Concluding Remarks (Cont'd): Liquefaction was not a controlling mechanism in triggering landslides in downtown Anchorage. It may have played a role in the triggering of the Turnagain Heights Slide, however. Terzaghi's advice (circa 1936) regarding the importance of geologic and subsurface details is well supported by the three cases in Anchorage. The 1964 Great Alaska Earthquake Concluding Remarks (Cont'd): Properly investigated, interpreted and documented case histories will always be the mainstay of geotechnical engineering research, teaching and practice. Case histories have been, and will continue to be, an essential means for: improving understanding; Calibrating analytical procedures; Designing & interpreting physical model tests; & developing semi-empirical procedures. 35

36 The 1964 Great Alaska Earthquake Publications re: 4 th Avenue Slide & the L Street Slide: The 4th Ave. slide was published in: "Evaluating Seismic Risk in Engineering Practice", by I. M. Idriss, Theme Lecture No. 6, Proceedings, XI International Conference on Soil Mechanics and Foundation Engineering, San Francisco, August 1985, v. 1, pp and again in: "Evaluating the Potential for Liquefaction or Cyclic Failure of Silts and Clays", by Boulanger, R. W., and Idriss, I. M., Report No. UCD/CGM-04/01, Center for Geotechnical Modeling, Department of Civil and Environmental Engineering, University of California, Davis. The L-Street slide was published in: "A Re Evaluation of the "L" Street Slide in Anchorage during the 1964 Alaska Earthquake", by Y. Moriwaki, I. M. Idriss, T. L. Moses, Jr., and R. S. Ladd, Proceedings, XII International Conference on Soil Mechanics and Foundation Engineering, Rio de Janeiro, Brazil, August The Courthouse site evaluation has not been published yet. 36