Soils and Rocks
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Soils and Rocks
SOIL MECHANICS
Simplifying
<Elastic - perfectly plastic model>
▣ Elastic Problem
Linear elastic theory: Hook’s law (E =  / )
- Elastic settlement (§7)
▣ Stability Problems
Perfect plastic theory:
▣ Seepage Problems
-
Shear strength (§9)
-
Lateral earth pressure (§10)
-
Bearing capacity (§11)
-
Slope stability (§12)
Steady-state flow (§5)
Transient flow --- Consolidation (§8).
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Soils and Rocks
§ 1. Soils and Rocks
• Soil Mechanics: Principles dealing with physical properties and behavior of soil (K.
Terzaghi, 1925)
․
• Soil engineering: Application of the principles
․
• Geotechnical engineering: Application of the principles of soil mechanics and rock
mechanics
▣ Rock Cycle and the Origin of Soil
▨ Rock cycle
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Soils and Rocks
◇ Igneous rock: Volcanic rocks (extrusive rocks) – rhyolite, andesite, basalt
Plutonic rocks (intrusive rocks) – granite, diorite, gabbro
◇ Sedimentary rock
: formed by compaction, cementation and crystallization of sediments
• Detrital sedimentary rocks: conglomerate, breccia, sandstone, mudstone, shale
• Chemical sedimentary rocks: limestone, chalk, dolomite, gypsum, anhydrite
◇ Metamorphic rock
: changed the composition and texture of rocks by heat and pressure
(eg) gneisses, schists, marble, quartzite
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Soils and Rocks
Composition of Rock Minerals
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Soils and Rocks
Mechanical weathering  single grain
▨ Weathering
Chemical weathering  clay minerals
▨ Formation of soils:
• Residual Soils --- formed in place by the weathering
• Transported (sedimentary) soils
- Glacial soils
- Marine soils
- Alluvial soils
- Aeolian soils
- Lacustrine soils
- Colluvial soils
• Fill --- man-made soil deposit
▣ Soil Particle Size (< 60mm)
clay
silt
BS
0.002
USCS
(fines)
sand
gravel
0.06 0.2 0.6 2
(0.075)
(4.75)
(#200)
fine-grained
Mechanical
Hydrometer analysis
analysis
coarse-grained
Sieve analysis
[ KS and JIS: clay size < 0.005 mm]
▣ Mechanical Analysis of Soil
▨ Sieve analysis  for coarse-grained soils
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cobble
60
(76.2)
boulder
200 (mm)
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Soils and Rocks
▨ Hydrometer analysis  for fine - grained soils
- Stokes’ law is used for this analysis.
▨ Particle-size distribution curve (Percentage grading)
[Example] Total dry wt. = 100 g.
# 10
# 16
# 30
# 40
# 60
# 100
# 200
passing (g)
95
82
70
60
45
35
30
Fines  hydrometer analysis
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retained (g)
5 (5)
13 (18)
12 (30)
10 (40)
15 (55)
10 (65)
5 (70)
% finer
95
82
70
60
45
35
30
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Soils and Rocks
▨ D10, Cu & Cc [e.g., 2.1]
• D10 = effective size  diameter corresponding to 10% finer
• Uniformity coefficient: Cu = D60/D10
• Coefficient of gradation (or coefficient of curvature): Cc = (D30)2 / (D10D60)
- Cu >6 for gravels, Cu > 4 for sands
- 1  Cc  3
“well-graded”
- If not satisfying the above, the soil will be "poorly graded (or uniform)".
▣ Clay Minerals
: a product of chemical weathering of feldspars, ferromagnesians and micas.
: Complex aluminum silicates
= (silica tetrahedron) + (alumina octahedron)
with different forms of bonding between the combined sheets.
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Soils and Rocks
▨ Typical clay minerals:
Lateral dimension
Thickness
Specific surface (m2/g)
kaolinite
1,000~20,000 Å
100~1,000 Å
15
illite
1,000~5,000 Å
50~500 Å
80
montmorillonite
1,000~5,000 Å
10~50 Å
800
Note: 1Å = 10-7 mm; Specific surface = (surface area)/mass.
▨ Characteristics of clay minerals:
- Absorbed water  viscous
- Double - layer water
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Soils and Rocks
▨ Structure of clay:
- Flocculated structure --- deposited under saline water
- Dispersed structure --- deposited under fresh water
▣ Specific Gravity
Gs = (unit weight of material) / (unit weight of water) =
(e.g.) sand: Gs = 2.65
clay, silt: Gs = 2.6~2.9
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WS
s
(=
)
w
Vs  w
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