What is compaction? A simple ground improvement technique, where the soil is densified through external compactive effort. + water = Compactive effort

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.

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

Compaction Curve Water content Dry density (  d ) optimum water content  d, max Soil grains densely packed - good strength and stiffness - low permeability

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

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%) S<100% S>100% (impossible)

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 )

Compaction and Clay Fabric Higher water content or higher compactive effort gives more dispersed fabric. more dispersed fabric Water content Dry density (  d )

Line of Optimum Water content Dry density (  d ) Compaction curves for different efforts Line of optimum

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

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

simple stress (Axial stress) Stress (Pascal) = Force (Newton) Area (square metre) Mass x gravity Area Reaction force

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) 33 33 22 22 Horizontal stresses Stress ellipsiod

Soil mechanics: stress Stress due to the weight of soil above  v =  h 33 33 22 22 Horizontal stresses  v = vertical stress (kPa)  = unit weight of soil (kN/m 3 ) h  = depth (m)

Circular failure surface due to shearing of the soil Soil mechanics: stress & strain

Barham River valley Apollo Bay 1987 Moorabool River valley Gheringhap 2001

shear stress Normal stress (  ) Shear stress (  ) Coulomb Equation  = c +  tan   = shear stress c = cohesion  = normal stress  = angle of shearing resistance Charles-Augustin de Coulomb

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

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 soil’s shear strength and causes a shear failure

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) 33 33 22 22 Horizontal stresses Stress ellipsiod

Soil mechanics: strain Strain is the change in shape caused by the application of stress

Kinds of strain Strain ellipsoid Oblate (  1 >  2 =  3 ) Prolate (  1 =  2 >  3 ) Triaxial (  1 >  2 >  3 )

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).

This leads to settlements occurring over a long time, which could be several years. time settlement Soil Consolidation

In granular soils… Granular soils are freely drained, and thus the settlement is instantaneous. time settlement

Laboratory testing consolidation