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Soil and Rock Properties

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Soil and Rock Properties Soil and rock properties W.A.C. Bennett Dam, BC Hydro 1 1) Basic testing methods 2) Soil properties and their estimation 3) Problem-oriented classification of soils 2 1 Consolidation Apparatus (“oedometer”) ELE catalogue 3 Oedometers ELE catalogue 4 2 Unconfined compression test on clay (undrained, uniaxial) ELE catalogue 5 Triaxial test on soil ELE catalogue 6 3 Direct shear (shear box) test on soil ELE catalogue 7 Field test: Standard Penetration Test (STP) ASTM D1586 Drop hammers: Standard “split spoon” “Old U.K.” “Doughnut” “Trip” sampler (open) 18” (30.5 cm long) ER=50 ER=45 ER=60 Test: 1) Place sampler to the bottom 2) Drive 18”, count blows for every 6” 3) Recover sample. “N” value = number of blows for the last 12” Corrections: ER N60 = N 1) Energy ratio: 60 Precautions: 2) Overburden depth 1) Clean hole 1.7N 2) Sampler below end Effective vertical of casing N1 = pressure (tons/ft2) 3) Cobbles 0.7 +σ v ' 8 4 Field test: Borehole vane (undrained shear strength) Procedure: 1) Place vane to the bottom 2) Insert into clay 3) Rotate, measure peak torque 4) Turn several times, measure remoulded torque 5) Calculate strength 1.0 Correction: 0.6 Bjerrum’s correction PEAK 0 20% P.I. 100% Precautions: Plasticity REMOULDED 1) Clean hole Index 2) Sampler below end of casing ASTM D2573 3) No rod friction 9 Soil properties relevant to slope stability: 1) “Drained” shear strength: - friction angle, true cohesion - curved strength envelope 2) “Undrained” shear strength: - apparent cohesion 3) Shear failure behaviour: - contractive, dilative, collapsive - pore pressure response, stress path 4) Time-dependent changes: - softening 5) Mechanical changes: -pre-shearing, residual strength 6) Moisture-dependency: - loss of strength on wetting 10 5 Basic soil strength model: Mohr-Coulomb (“drained”) peak shear strength : S = cu'(+ σ − )tan'φ Where: (σ-u) = “effective stress” c’ = “drained” cohesion φ’ = drained friction angle Strength envelope unstable S φ’ stable c’ (σ-u) 11 Types of cohesion: 1) Electrostatic forces (in stiff overconsolidated clays): 5-24 kPa May be lost through weathering S c’ (σ-u) pre-consolidation stress, p0’ 12 6 Types of cohesion: 2) Cementing by Fe2O3, CaCO3, NaCl etc.: up to 500kPa May be lost through 3) Root cohesion: 2-20kPa solution May be lost Lateral (high) through logging or fire Basal (low) 13 3) Apparent cohesion in unsaturated soil: As soil dries out, water menisci form at grain contacts. These are under negative capillary pressure. When capillary forces are netted over a unit area, we refer to “matric suction stress”, which creates interparticle forces→apparent 200 cohesion Problem: cohesion will disappear on wetting 14 (Fredlund and Rahardjo, 1993) 7 5) Apparent cohesion due to pore-pressure response during relatively fast (“undrained”) loading total stress σ τ “Direct Shear Test” T σ normal stress 15 Apparent cohesion: total stress σ pore-pressure, u τ T u σ normal stress 16 8 Apparent cohesion: Increased total normal stress increased total stress τ σ+∆σ T σ+∆σ normal stress σ 17 Apparent cohesion: increased total Undrained stress loading→∆σ=∆u τ σ+∆σ increased pore- pressure, u+∆u T σ+∆σ normal stress u+∆u 18 9 Apparent cohesion: Decreased total normal stress decreased total stress τ σ-∆σ T σ-∆σ normal stress 19 Apparent cohesion: decreased total stress σ-∆σ Undrained τ loading→∆σ=∆u decreased pore- pressure, u-∆u T σ-∆σ u-∆u normal stress 20 10 Apparent cohesion (Undrained shear strength): Clay will have a constant short-term strength (Undrained Shear Strength, Su) May be lost through time (delayed failure) T “φ = 0” S u condition σ 21 Softening: short term long term S (σ-u) c’ 22 11 Friction angle: LOOSE DENSE 23 Friction angle: LOOSE (contractant soil) DENSE (dilatant soil) 24 12 Friction angle: Dilatancy angle δ φ'= φ +δ negative basic LOOSE (contractant soil) Dilatancy angle δ positive DENSE (dilatant soil) 25 Dependence of friction angle on density in sands: Relative density from SPT test: N Dr 0-4 0-15 4-10 15-35 10-30 35-65 30-50 65-85 >50 >85 NAVFAC, 1971 26 13 Dependence of friction angle on plasticity in clays: φ’ 40º sinφ’ 30º 20º Plasticity Index (%) Lambe and Whitman (1979) 27 Pore-pressure response: LOOSE DENSE 28 14 Pore-pressure response: Pore space decreased LOOSE (contractant soil) Pore space increased DENSE (dilatant soil) 29 Results of a typical CONTRACTANT DILATANT triaxial test on soil Axial strain % Terzaghi and Peck, 1967 Axial strain % 30 15 Soil structure collapse Soil collapse: sudden change from loose to dense packing, volume change. If soil is saturated, volume change cannot occur and pore- pressure increases, reducing Loose Dense effective stress (“liquefaction”) packing packing What causes collapse: 1) loose, saturated soil (N<8) SE PE AP O E L COLL AC 2) Static overstress (caused by VE RF N SU E added loading, or increase in TH G pore pressure) EN TR S CYCLIC LOADING 3) Earthquake shaking (cyclic STATIC OVERSTRESS loading)\ Effect more dramatic, (Mc Roberts and Sladen, 1990) if accompanied by cohesion loss 31 Turnagain Hts. Slide, Anchorage, Alaska 1964 earthquake, Mag. 8.4 (Seed and Wilson, 1967) 32 16 Remolding of highly-sensitive (“quick”) clays Usually leached clays of marine origin, may be overconsolidated and low plasticity peak remoulded 33 Pre-shearing: Shear Stress Displacement peak strength residual friction S angle φr no cohesion (σ-u) 34 17 Pre-Shearing • The friction angle of clay decreases from peak to • residual value due to particle alignment, when sheared. Peak strength Residual (ultimate) strength 35 Friction angle Shear stress (degrees) Displacement 20 15 10 Peak 5 Residual Shear Stress Normal stress (Kenney, 1967) 35 Reasons for pre-shearing of clay surfaces 1) Tectonics: flexural slip 2) Glacier drag (in sedimentary rocks) (lacustrine clays, shales) 3) Relict landslides 36 18 Loss of soil strength, summary a) Long-term strength loss, leading to delayed failure: Process: Time frame Materials Increase of pore-pressure through drainage Weeks to clay, silt years Softening of fissured clay, loss of cohesion Decades Stiff, fissured clay Reduction in friction angle through shearing Variable Stiff clays (progressive failure) b) Sudden strength loss, leading to extremely rapid failure Process: Materials Sudden remoulding Extra-sensitive (quick) clay Static or earthquake liquefaction Loose, saturated sand, silt Loss of apparent cohesion on wetting Unsaturated silt Loss of true cohesion due to overstress Weakly cemented sand, silt 37 A problem-oriented classification of soils. Morgenstern, 1985 special properties Soils Natural Processed Coarse Fine Compacted Fills Mine Waste Dense Loose Structured Saturated Unsaturated Coarse Fine Coarse Fine Contractant Dilatant Unstructured Structured Unstructured Structured Unstructured Structured 38 19 A problem-oriented classification of soils. Morgenstern, 1985 Soils Natural Processed -We have no control -Can control properties > less reliable and placement > more reliable 39 A problem-oriented classification of soils. Morgenstern, 1985 Soils Natural Processed Coarse Fine Compacted Fills Mine Waste sand, gravel, >50% silt and clay -select material -uncompacted boulders -designed placement -random material -controlled properties -poor foundations - rapid drainage -slow drainage -cohesive > reliable > unreliable 40 20 A problem-oriented classification of soils. Morgenstern, 1985 Soils Processed Compacted Fills Mine Waste Coarse Fine Coarse Fine Design Design Waste pile Tailings dyke problems: problems: problems: problems: -compaction -deformability -loose, possibly -loose, possibly -stable drainage -shear strength collapsive (flow collapsive (filters) -internal erosion slides) (liquefaction) (filters) -poor -internal erosion foundations -poor drainage -complex (overtopping) drainage 41 A problem-oriented classification of soils. Morgenstern, 1985 Soils Natural Coarse Dense Loose Structured - May be collapsive Rd>35% -Cemented N>10-20 in undrained failure (liquefaction) -Weak cement may - Few problems if saturated sustain loose - erodible? (submarine flow structure! slides?) - may be collapsive -erodible? - erodible? 42 21 A problem-oriented classification of soils. Morgenstern, 1985 Soils Natural Fine Unsaturated Unstructured Structured -Apparent cohesion -Non-linear strength -May be cemented, envelope, dependent on fissured (secondary moisture content permeability) or collapsive. -Stability strongly >very low reliability dependent on infiltration >low reliability 43 A problem-oriented classification of soils. Morgenstern, 1985 Soils Natural Fine Saturated Contractant Dilatant Unstructured Structured Unstructured Structured -Insensitive soft -Intact stiff clays clays -Sensitive clays -Fissured stiff clays -Collapsive (flow -Cohesive tills -Cohesion and -“Textbook” -Few problems materials: slides) friction loss due to: -“Quick clays” -(rare) -Softening -Problem: -But: Watch for accurate -Pre-shearing pre-sheared -Progressive failure measurement of horizons and undrained shear Also: secondary progressive permeability strength failure -Watch for loose sand lenses! 44 22 A problem-oriented classification of soils. Morgenstern, 1985 collapsive behaviour? Soils Natural Processed Coarse Fine Compacted Fills Mine Waste Dense Loose Cemented Saturated Unsaturated Coarse Fine Coarse Fine Fissured Contractant Dilatant Unstructured Cemented Insensitive Sensitive Massive Fissured 45 A problem-oriented classification of soils. Morgenstern, 1985 long term strength loss? Soils Natural Processed Coarse Fine Compacted Fills Mine Waste Dense Loose Cemented Saturated Unsaturated Coarse Fine Coarse Fine Fissured Contractant Dilatant Unstructured Cemented Insensitive Sensitive Massive Fissured 46 23 A problem-oriented classification of soils. Morgenstern, 1985 dependence on moisture changes? Soils Natural Processed Coarse Fine Compacted Fills Mine Waste Dense Loose Cemented Saturated Unsaturated Coarse Fine Coarse Fine Fissured Contractant Dilatant Unstructured Cemented Insensitive Sensitive Massive Fissured 47 24.
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