What is compaction? A simple ground improvement technique,
where the soil is densified through external compactive effort. +
water = Compactive effort
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Why compact soils? Increases strength Decreases permeability
Reduces settlement Reduces shrinkage Applications: Roads
Foundations Embankments Dams Aircraft runways Parking areas Paving
Retaining walls Rammed earth structures Etc. etc.
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Theory: Ralph R. Proctor (circa 1933) related compaction to
four variables: Dry density Moisture content Compactive effort Soil
type Laboratory tests Mould (standard dimensions) Hammer (standard
cross-section area, weight, drop) Method (standard number of layers
and number of drops for each layer) Mositure content
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Compaction Curve Water content Dry density ( d ) optimum water
content d, max Soil grains densely packed - good strength and
stiffness - low permeability
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Compaction Curve What happens to the relative quantities of the
three phases with addition of water? Water content Dry density ( d
) soil water air difficult to expel all air lowest void ratio and
highest dry density at optimum w
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Zero Air Void Curve All compaction points should lie to the
left of ZAV curve - corresponds to 100% saturation Water content
Dry density ( d ) Zero air void curve (S=100%) S100%
(impossible)
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Effect of Compactive Effort Increasing compactive effort
results in: E1E1 E 2 (>E 1 ) Lower optimum water content Higher
maximum dry density Water content Dry density ( d )
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Compaction and Clay Fabric Higher water content or higher
compactive effort gives more dispersed fabric. more dispersed
fabric Water content Dry density ( d )
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Line of Optimum Water content Dry density ( d ) Compaction
curves for different efforts Line of optimum
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Laboratory Compaction Test - to obtain the compaction curve and
define the optimum water content and maximum dry density for a
specific compactive effort. hammer Standard Proctor: Modified
Proctor: 3 layers 25 blows per layer 5 layers 25 blows per layer
2.7 kg hammer 300 mm drop 4.9 kg hammer 450 mm drop 1000 ml
compaction mould
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Compaction Control -a systematic exercise where you check at
regular intervals whether the compaction was done to
specifications. e.g., 1 test per 1000 m 3 of compacted soil Minimum
dry density Range of water content Field measurements (of d )
obtained using sand cone nuclear density meter
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simple stress (Axial stress) Stress (Pascal) = Force (Newton)
Area (square metre) Mass x gravity Area Reaction force
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Soil mechanics: stress Stress due to the weight of soil above v
= h v = vertical stress (kPa) = unit weight of soil (kN/m 3 ) h =
depth (m) 33 33 22 22 Horizontal stresses Stress ellipsiod
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Soil mechanics: stress Stress due to the weight of soil above v
= h 33 33 22 22 Horizontal stresses v = vertical stress (kPa) =
unit weight of soil (kN/m 3 ) h = depth (m)
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Circular failure surface due to shearing of the soil Soil
mechanics: stress & strain
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Barham River valley Apollo Bay 1987 Moorabool River valley
Gheringhap 2001
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shear stress Normal stress ( ) Shear stress ( ) Coulomb
Equation = c + tan = shear stress c = cohesion = normal stress =
angle of shearing resistance Charles-Augustin de Coulomb 1736 -
1806
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shear stress The groundwater in the pore spaces creates an
uplift pressure the pore water pressure to the shear plane. The
pore water pressure relates to the pressure head caused by the
weight of water and rock above Water table The normal stress ( )is
countered by the pore water pressure (u) and the result ( u) is
called the effective stress ( ) u Mohr - Coulomb Equation = c + tan
= shear stress c = effective cohesion = effective stress =
effective angle of shearing resistance
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Slope mechanics: rainfall as a trigger of instability Pore
water pressure time Rainfall event Water table Raising the
watertable increases the pore-water pressure and reduces the
effective stress, which in turn lowers the soils shear strength and
causes a shear failure
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Soil mechanics: stress Stress due to the weight of soil above v
= h v = vertical stress (kPa) = unit weight of soil (kN/m 3 ) h =
distance (m) 33 33 22 22 Horizontal stresses Stress ellipsiod
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Soil mechanics: strain Strain is the change in shape caused by
the application of stress
Introduction to Consolidation When a saturated clay is loaded
externally, saturated clay GL the water is squeezed out of the clay
over a long time (due to low permeability of the clay).
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This leads to settlements occurring over a long time, which
could be several years. time settlement Soil Consolidation
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In granular soils Granular soils are freely drained, and thus
the settlement is instantaneous. time settlement
