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Simple Hints on Characterizing Complex Bimrocks Simple Hints on Characterizing Complex Bimrocks Dr. Edmund Medley PE, CEG Consulting Geological Engineer,

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Presentation on theme: "Simple Hints on Characterizing Complex Bimrocks Simple Hints on Characterizing Complex Bimrocks Dr. Edmund Medley PE, CEG Consulting Geological Engineer,"— Presentation transcript:

1 Simple Hints on Characterizing Complex Bimrocks Simple Hints on Characterizing Complex Bimrocks Dr. Edmund Medley PE, CEG Consulting Geological Engineer, Belmont, California Noon Seminar Geotech Group, UC Berkeley November 10, 2004

2 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Who I am Prospector in Canada Prospector in Canada Educated in Geological/Geotech. Engineering Educated in Geological/Geotech. Engineering Geotechnical Eng. Consultant Geotechnical Eng. Consultant Principal Engineer at Exponent in Geology/Geotech/GeoEnvironmental Principal Engineer at Exponent in Geology/Geotech/GeoEnvironmental Licensed Geologist/Engineer in US, Canada and United Kingdom Licensed Geologist/Engineer in US, Canada and United Kingdom

3 BIG CONCLUSION 1: Remember this picture!!! Matrix Scale: 1:?????? Blocks, inclusions, lenses, etc Actual Distribution of Blocks Medley, 2000

4 Matrix Willis, 2000 Apparent Distribution of Blocks BIG CONCLUSION 2: Remember this picture as well!!!

5 Melanges and Similar Bimrocks Melanges: (French mélange) : UN CHILE mixtures of competent blocks composed of sedimentary/ metamorphic blocks in weaker matrix of sheared shale or serpentinite Melanges: (French mélange) : UN CHILE mixtures of competent blocks composed of sedimentary/ metamorphic blocks in weaker matrix of sheared shale or serpentinite Bimrocks: block-in-matrix rocks Bimrocks: block-in-matrix rocks – mixtures of rocks composed of geotechnically significant blocks within a bonded matrix of finer texture Similar Bimrocks: Saprolites, Breccias, Fault Zones, Lahars, Tillites, etc. Similar Bimrocks: Saprolites, Breccias, Fault Zones, Lahars, Tillites, etc.

6 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Bimrocks are NOT Bimbo Rock(stars), and have not yet made me a millionaire…

7 Mixture with weaker soil and strong blocks (decomposed granite at Hwy 50, California) Example of weathered rock bimrock Example of weathered rock bimrock soil Corestones, blocks, boulders

8 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Brecciated rock….

9 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer … or bathbim…

10 Fault Rocks and Shear Rocks Riedmueller et al, 2001 ExEx Fault zones and Shear zones may have blocks millimeters to 100s of meters wide: BLOCK SIZE DISTRIBUTIONS TEND TO BE SCALE INDEPENDENT

11 Limestone Olistolith (Block) Embedded in an Irregularly Foliated Matrix of Shale, Siltstone and Sandstone Egnatia Motorway, Greece Photo by Prof. Gunter Riedmueller

12 Global distribution of melanges Medley, 1994

13 Franciscan Complex melanges From Medley, 1994; after Ellen and Wentworth, 1995

14 Franciscan Complex melange (Point Delgada, California) shear block

15 Often severe spatial variability Often severe spatial variability Heterogeneity in properties Heterogeneity in properties Design and construction shortcomings in characterizations cost someone $$$$, (usually the Owner and/or Contractor) Design and construction shortcomings in characterizations cost someone $$$$, (usually the Owner and/or Contractor) $$Problems with Melanges

16 Geology 101 Subsurface characterization of layer cake geology Linear interpolation is geologically justified only SOMETIMES; extrapolation RARELY exploration borings Layer 2 Layer 3 Layer 1 ? ?

17 Difficult geology Difficult geology: lots of layers (“complex formations”) Layer 1 exploration borings Layer 2 Layer 1Layer 2 Layer 1Layer 2 Fault soil Layer 4 Layer 3 fault

18 The complexity of a turbidite sequence - interbedded sandstones and shales (Devil’s Slide, Pacifica, California)

19 Chaotic Geology: apparent “layers” are blocks, lenses, inclusions….. exploration borings soil Fault?? Characterization is NOT a piece of cake!

20 Bimrock – Mapped or cored Matrix Willis, 2000

21 The Benefits and Dangers of Drawing Straight Lines Matrix

22 Actual Distribution of Blocks Matrix Willis, 2000; after Medley, 2000

23 So What? Who Cares*? * Answer: Owners, Contractors and Lawyers

24 Characterization of Melanges and Similar Bimrocks

25 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Steps in Characterization of Bimrocks 1. Determine you are working in a bimrock 1. Determine you are working in a bimrock 2. Select the Characteristic Engineering Dimensions of interest 2. Select the Characteristic Engineering Dimensions of interest 3. Perform field mapping of geological and geotechnical features 3. Perform field mapping of geological and geotechnical features 4. Drill using careful drillers! 4. Drill using careful drillers! 5. Measure areal and linear block proportions 5. Measure areal and linear block proportions

26 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer More Steps in Characterization of Bimrocks 6. Select specimens for laboratory testing with blocks 6. Select specimens for laboratory testing with blocks 7. Test a variety of specimens with different block proportions 7. Test a variety of specimens with different block proportions 8. Adjust your linear block proportion to account for uncertainty 8. Adjust your linear block proportion to account for uncertainty 9. Estimate overall bimrock strength 9. Estimate overall bimrock strength

27 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer 1. Determine you are working in a bimrock

28 Really obvious melange…. Really obvious melange…. Gwna Melange, N. Wales photo Ed Medley

29 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Geomorphic clues: subtle!! Protruding blocks Vegetation contrasts=blocks

30 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer 2. Select the Characteristic Engineering Dimensions of interest

31 Q: What is Block Size?? A: Rarely the “diameter” maximum observed dimension

32 Blocks in Franciscan Melange Blocks range in size between mountains and sand and will always be found Blocks range in size between mountains and sand and will always be found Block size distributions are scale independent and fractal ( power law) Block size distributions are scale independent and fractal ( power law) Largest reasonable block is: Largest reasonable block is: 0.75  A aka d max 0.75  A aka d max Block/matrix threshold is 0.05  A Block/matrix threshold is 0.05  A Characterization must take blocks into account Characterization must take blocks into account

33 Use these guidelines at any scale of interest smallest blocks are: smallest blocks are: 0.05L c or 0.05  A or 0.05d max largest block is: largest block is: 0.75Lc or 0.75  A or 0.75d max 0.75Lc or 0.75  A or 0.75d max

34 When is a block not a block? depends on scale of interest 100 m R. of W. 20m 1 m block 100 mpipeline trench

35 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer 3. Perform field mapping of geological and geotechnical features

36 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Some elements of CHARACTERIZATION Block/matrix discrimination Matrix lithology, block lithology, block size, block shape, block orientation Block size distribution Block discontinuities Etc… ALL are straight forward geological engineering during field investigation…

37 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Systematic investigation of chaos Wakabayashi & Medley, 2004 Matrix strength

38 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Right and wrong way to map melanges Outcrops (blocks) WRONG!! Right!!

39 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Observe block/matrix contacts

40 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer 4. Drill using careful drillers! Log using your painfully learned Engineering Geology! (Bless Professors….)

41 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer right way and the wrong way… Wrong!!! Right!!! Wakabayashi & Medley, 2004

42 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer 5. Measure areal and linear block proportions

43 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Drilling and coring bimrocks boreholes

44 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Scott Dam melange (California) photo Prof. R Goodman Matrix- rich core Measure core/block intercept (chord) Measure block linear proportion

45 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Measuring Linear Proportion

46 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer 6. Select specimens for laboratory testing with blocks

47 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Scott Dam melange photo Prof. R Goodman Take samples of matrix

48 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer 7. Test a variety of specimens with different block proportions

49 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Different orientations and volumetric block proportions Lindquist, 1994

50 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Increase in friction angle with volumetric block proportion Lindquist, 1994

51 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Decrease in cohesion with volumetric block proportion Lindquist, 1994

52 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Scott Dam melange strength testing results Prof. R Goodman

53 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer 8. Adjust the linear block proportion to account for uncertainty

54 Determination of volumetric block proportions Application of stereological principle: Application of stereological principle: linear proportion = volumetric proportion Determine linear proportion from drill core Beware uncertainty! Beware uncertainty!

55 Warnings!!!! So, although stereology says that volumetric % = linear % So, although stereology says that volumetric % = linear % It is TRUE ONLY when you have sufficient linear measurements!! (lots of $$$ drilling!!) It is TRUE ONLY when you have sufficient linear measurements!! (lots of $$$ drilling!!)

56 Estimated volumetric properties on the basis of Scott Dam linear boring measurements x40% = 6%: use 34% block proportion Linear proportions assumed to be same as volumetric proportions Uncertainty: error in the assumption

57 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer 9. Estimate overall bimrock strength

58 Strength of Franciscan below Scott Dam Medley, 2002; after Goodman and Ahlgren, 2001 Approx 39% friction for 34% volumetric block proportion

59 Strength of bimrock depends on volumetric block proportion

60 Strength and deformation properties of Melange Bimrocks Strength and deformation of melanges are independent of block strengths (Lindquist and Goodman,1994) Strength and deformation of melanges are independent of block strengths (Lindquist and Goodman,1994) Overall strength is directly related to volumetric block proportion Overall strength is directly related to volumetric block proportion Blocks adds friction, stiffen the mixture and reduce cohesion. Blocks adds friction, stiffen the mixture and reduce cohesion. Must perform geotech tests with blocks in specimens Must perform geotech tests with blocks in specimens

61 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Slope Stability: Recent Research

62 Blocks added strength to matrix? Photo: Exponent

63 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer shears Tortuous Shears in Melange

64 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Tortuousity of Failure Surfaces Medley, 2004

65 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Developed profile of failure surface Medley, 2004

66 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Tortuousity is not Roughness Medley, 2004

67 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Measuring Tortuosity Medley, 2004

68 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Findings: Tortuosity depends most on block proportion (and orientation??) Width of shear zones approx to 0.15 L c Proportion block/shear contact closely related to volumetric block proportion Medley, 2004

69 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer What are the influences on tortuosity and slope stability in bimrocks? Block/matrix vol. proportion; matrix c, ϕ ?? Block shape, block&shear orientation?? Block size, location, orientation?? Bimrock weak zones, variability?? Medley & Sanz, 2004

70 Tortuosity depends most on volumetric block proportion and, intuitively, block orientations So: what are the effects of block proportion, block orientation on slope stability? Answer: let’s investigate!!

71 Simple Model Analysis Critical failure surface in matrix Φ = 25º c = 10 kPa FS ~ 1.26 Using Slope/W

72 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Simple, random arrays of blocks 50% 25% 13%

73 Analysis of Influence of Blocks on Slope Stability Using Simple Models horizontal rectangular blocks BSD= partial Franciscan Random arrays trial surfaces negotiate blocks, tortuosity reflects many factors A A =V V 5m

74 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Locate critical matrix-only and tortuous trial surfaces and compute F of S

75 50% block proportion: FS for a tortuous trial surface FS ~ 1.65

76 Slope stability of Boulder Colluvium Hong Kong GEO TN 4/92

77 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Trial tortuous failure surfaces with blocks oriented out-of-slope After Irfan & Tang, 1993

78 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Combining the results of analyses

79 Bimrock strength increases with block proportion (>~15%-25% V v )

80 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer Slope Stability Conclusions Increased tortuosity of failure surfaces results in increased slope stability Increases in tortuosity are most influenced by block volumetric proportion and block orientation Or: strength of bimrocks increases because of presence of blocks

81 BIG CONCLUSION 1: Remember this picture!!! Matrix Scale: 1:?????? Blocks, inclusions, lenses, etc Actual Distribution of Blocks

82 Matrix Willis, 2000 Apparent Distribution of Blocks BIG CONCLUSION 2: Remember this picture as well!!!

83 Other CONCLUSIONS Melange bimrocks and other bimrocks mixtures are chaotic but can be characterized in a disciplined fashion Melange bimrocks and other bimrocks mixtures are chaotic but can be characterized in a disciplined fashion Strength and deformation properties of bimrocks are determinable Strength and deformation properties of bimrocks are determinable Uncertainties in estimates must be considered Uncertainties in estimates must be considered

84 Conclusions continued: Bimrocks are NOT: Bimrocks are NOT: – “soil with boulders”, – “interlayered shale and sandstone” – “miscellaneous soils” Block volumes, sizes and lithologies are $important$ to Contractors/Owners and some effort should be made to determine them Block volumes, sizes and lithologies are $important$ to Contractors/Owners and some effort should be made to determine them Block sizes should be estimated very conservatively for construction Block sizes should be estimated very conservatively for construction

85 For more information, see contents of the CDROM and Felsbau Oct /2004 Characterizing melanges Characterizing melanges Medley, 1998 (IAEG, Vancouver) Medley, 1998 (ASCE ISC, Atlanta) Medley, 1999, 2001 (Feldsbau, Austrian J. Geomechanics) Medley, 2002 ( Eng. Geol. Practice No. Cal. ) Block size distributions in melanges Block size distributions in melanges Medley and Lindquist, 1995 (35th USRMC, Tahoe) Medley 2002 (NARMS Toronto) Uncertainties from boring data in melanges Uncertainties from boring data in melanges Medley, 1997 (IAEG, Athens)

86 Copyright: Dr. Edmund Medley, PE, CEG Consulting Geological Engineer


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