Geology; Rocks Soil Formation Soil Deposits Soil Mineralogy Soil Structure Engineering Characterization of Soils Soil Classification Site Exploration Phase Relationships Soil Permeability Seepage Stresses in Soil Masses Stress Strain Behavior of Soils Soil Consolidation Soil Shear Strength 1 Engineering Characterization of Soils Soil Properties that Control its Engineering Behavior Particle Size − Sieve Analysis − Hydrometer Analysis coarse-grained Particle/Grain Size Distribution Particle Shapes (?) fine-grained Soil Plasticity 2 Particle Size; Standard Sieve Sizes 3 ASTM Particle Size Classification 4 Sieve Analysis (Mechanical Analysis) This procedure is suitable for coarse grained soils See next slide for ASTM Standard Sieves No.10 sieve …. Has 10 apertures per linear inch 5 ASTM Standard Sieves 6 Hydrometer Analysis Also called Sedimentation Analysis Stoke’s Law D γ w (Gs − GL ) v= 18η 2 7 Grain Size Distribution Curves 8 Terminology C….. Poorly-graded soil D …. Well-graded soil E …. Gap-graded soil D10, D30, D60 = ?? Coefficient of Uniformity, Cu= D60/D10 Coefficient of Curvature, Cc= (D30)2/(D10)(D60) 9 Particle Distribution Calculations Example 10 Particle Shapes 11 Clay Formation Clay particles < 2 µm Compared to Sands and Silts, clay size particles have undergone a lot more “chemical weathering”! 12 Clay vs. Sand/Silt Clay particles are generally more platy in shape (sand more equi-dimensional) Clay particles carry surface charge Amount of surface charge depends on type of clay minerals Surface charges that exist on clay particles have major influence on their behavior (for e.g. plasticity) 13 Clay Minerals Kaolinite family Kaolinite (ceramic industry, paper, paint, pharmaceutical) Smectite family Montmorillonite (weathered volcanic ash, Wyoming Bentonite, highly expansive, used in drilling mud) Illite family 14 15 Elements of Earth 8-35 km crust % by weight in crust O Si Al Fe Ca Na K Mg other 12500 km dia = 49.2 = 25.7 = 7.5 = 4.7 = 3.4 = 2.6 = 2.4 = 1.9 = 2.6 82.4% 16 17 Basic Structural Units Clay minerals are made of two distinct structural units. hydroxyl or oxygen oxygen aluminium or magnesium silicon 0.26 nm Silicon tetrahedron 0.29 nm Aluminium Octahedron 18 Tetrahedral Sheet Several tetrahedrons joined together form a tetrahedral sheet. tetrahedron hexagonal hole 19 Tetrahedral & Octahedral Sheets For simplicity, let’s represent silica tetrahedral sheet by: Si and alumina octahedral sheet by: Al 20 Different Clay Minerals Different combinations of tetrahedral and octahedral sheets form different clay minerals: 1:1 Clay Mineral (e.g., kaolinite, halloysite): 21 Different Clay Minerals Different combinations of tetrahedral and octahedral sheets form different clay minerals: 2:1 Clay Mineral (e.g., montmorillonite, illite) 22 Kaolinite Typically 70-100 layers joined by strong H-bond ∴no easy separation Al Si Al Si 0.72 nm Al Si Al Si joined by oxygen sharing Kaolinite ¾ used in paints, paper and in pottery and pharmaceutical industries ¾ (OH)8Al4Si4O10 Halloysite ¾ kaolinite family; hydrated and tubular structure ¾ (OH)8Al4Si4O10.4H2O 24 Montmorillonite ¾ also called smectite; expands on contact with water Si Al Si ∴easily separated by water joined by weak van der Waal’s bond Si Al Si Si Al Si 0.96 nm 25 Montmorillonite ¾ A highly reactive (expansive) clay swells on contact with water ¾ (OH)4Al4Si8O20.nH2O Bentonite high affinity to water ¾ montmorillonite family ¾ used as drilling mud, in slurry trench walls, stopping leaks 26 Illite Si Al joined by K+ ions fit into the hexagonal holes in Si-sheet Si Si Al Si 0.96 nm Si Al Si 27 Others… Chlorite ¾ A 2:1:1 (???) mineral. Si Al Al or Mg Vermiculite ¾ montmorillonite family; 2 interlayers of water Attapulgite ¾ chain structure (no sheets); needle-like appearance 28 A Clay Particle Plate-like or Flaky Shape 29 Clay Fabric edge-to-face contact Flocculated face-to-face contact Dispersed 30 Clay Fabric ¾ Electrochemical environment (i.e., pH, acidity, temperature, cations present in the water) during the time of sedimentation influence clay fabric significantly. ¾ Clay particles tend to align perpendicular to the load applied on them. 31 32 Scanning Electron Microscope ¾ common technique to see clay particles ¾ qualitative plate-like structure 33 Others… X-Ray Diffraction (XRD) ¾ to identify the molecular structure and minerals present Differential Thermal Analysis (DTA) ¾ to identify the minerals present 34 Casagrande’s PI-LL Chart 60 U-line Plasticity Index 50 montmorillonite illite A-line 40 30 kaolinite 20 halloysite 10 chlorite 0 0 10 20 30 40 50 60 70 80 90 100 Liquid Limit 35 36 Specific Surface ¾ surface area per unit mass (m2/g) ¾ smaller the grain, higher the specific surface e.g., soil grain with specific gravity of 2.7 10 mm cube spec. surface = 222.2 mm2/g 1 mm cube spec. surface = 2222.2 mm2/g 37 Isomorphous Substitution ¾ substitution of Si4+ and Al3+ by other lower valence (e.g., Mg2+) cations ¾ results in charge imbalance (net negative) positively charged edges + + + _ _ _ _+ + _ + _ _ _ __ + _ _ _ _ _ __ _ _ _ _ _ _ negatively charged faces Clay Particle with Net negative Charge 38 Cation Exchange Capacity (c.e.c) known as exchangeable cations ¾ capacity to attract cations from the water (i.e., measure of the net negative charge of the clay particle) ¾ measured in meq/100g (net negative charge per 100 g of clay) milliequivalents ¾ The replacement power is greater for higher valence and larger cations. Al3+ > Ca2+ > Mg2+ >> NH4+ > K+ > H+ > Na+ > Li+ 39 A Comparison Mineral Specific surface (m2/g) C.E.C (meq/100g) Kaolinite 10-20 3-10 Illite 80-100 20-30 Montmorillonite 800 80-120 Chlorite 80 20-30 40 Cation Concentration in Water ¾ cation concentration drops with distance from clay particle clay particle + + + + + + ++ + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ ++ + + - + cations + - ++ + + + + + + + + + + + + + + - + + + + + + + -+ + + + + + -+ + + double layer + + + + + + free water41 + Adsorbed Water ¾ A thin layer of water tightly held to particle; like a skin ¾ 1-4 molecules of water (1 nm) thick ¾ more viscous than free water - - adsorbed water 42 Clay Particle in Water adsorbed water - 1nm 50 nm - double layer - water free water 43 44 Summary - Clays ¾ Clay particles are like plates or needles. They are negatively charged. ¾ Clays are plastic; Silts, sands and gravels are non-plastic. ¾ Clays exhibit high dry strength and slow dilatancy. 45 Summary - Montmorillonite ¾ Montmorillonites have very high specific surface, cation exchange capacity, and affinity to water. They form reactive clays. ¾ Montmorillonites have very high liquid limit (100+), plasticity index and activity (1-7). ¾ Bentonite (a form of Montmorillonite) is frequently used as drilling mud. 46
