Topic 3 - Consolidation and Settlement

compression

decrease in soil volume due to external load

consolidation

specific, long-term, time-dependent process where saturated soil decreases in volume due to the expulsion of water from void under sustained static loading

sands

high permeability, consolidation occurs immediately

clays

low permeability, consolidation takes years

consolidation test

find properties for magnitude and rate of settlement

1. sample is subjected to load increments

2. each load increment is applied after deformation under previous has stopped (full dissipation of excess u) (σ3 = σ’3)

3. double load every 24 hrs

4. changes in sample height w/ time

preconsolidation pressure Pp

max effective vertical stress an over consolidated clay had experienced in the past

causes of overconsolidation

erosion of overburden, past glaciers, past structures, changes in water table elevation [pore pressure changes]

Casagrande method

Method to determine preconsolidation pressure

1. Locate point of max curvature [min radius]

2. horizontal line and tangent at that point

3. bisect the angle between the two lines

4. extrapolate the linear virgin compression line to intersect

Terzaghi assumptions

1. homogenous and fully saturated soil

2. water and soil particles are incompressible

3. volume change only in the vertical direction [1D, lateral confinement]

4. drainage is vertical

5. Darcy’s law v=ki

6. k [coefficient of permeability] and mv [coefficient of volume compressibility] are constant

7. unique e - σ’, independent of time

8. principle of effective stress is valid σ’ = σ - u

Ut

average degree of consolidation