Geology; Rocks Soil Mineralogy Soil Formation Soil Soil Structure Deposits
Engineering Soil Classification Site Exploration Characterization of Soils
Phase Soil Permeability Stresses in Soil Relationships Seepage 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 fine-grained
Particle/Grain Size Soil Plasticity Distribution Particle Shapes (?) 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
D2γ (G − G ) v = w s L 18η
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,
)2 Cc= (D30 /(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 = 49.2 Si = 25.7 82.4% Al = 7.5 Fe = 4.7 Ca = 3.4 Na = 2.6 K = 2.4 Mg = 1.9 other = 2.6 12500 km dia 16 17 Basic Structural Units
Clay minerals are made of two distinct structural units.
hydroxyl or oxygen oxygen
aluminium or silicon magnesium
0.26 nm 0.29 nm
Silicon tetrahedron 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
Al Si Typically 70-100 Al 0.72 nm layers Si Al joined by strong H-bond ∴no easy separation Si Al joined by oxygen sharing Si 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
Si Al 0.96 nm ∴easily separated Si by water Si joined by weak van der Waal’s bond Al Si 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 Si joined by K+ ions Si fit into the hexagonal Al 0.96 nm holes in Si-sheet Si
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 face-to-face contact
Flocculated 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
50 montmorillonite illite A-line 40 dex n I y
t 30 ci
i kaolinite
ast 20 l P halloysite 10 chlorite 0 0 102030405060708090100 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 1 mm cube spec. surface = 222.2 mm2/g 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+ >> NH + > K+ > H+ > Na+ > Li+ 4 39 A Comparison
Mineral Specific surface C.E.C (meq/100g) (m2/g) 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 -- free water -- --double layer --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