Compaction & state variables [Lecture

Engineering use chart

Soil classification system

1. Particle size data

2. Plasticity data

→ should only be used for general guidance or preliminary assessment

Main scalar parameter used to quantify clays

Sr=Vw/Vv=wGs/e

Main scalar parameter used to quantify mixtures

Relative compaction

Main scalar parameter used to quantify sands

Relative density

Clay strength at liquid limit and plastic limit

Dry density

Ms/V

Porosity

n=Vv/V=(e/(1+e))

Specific volume

v=V/Vs=(1+Vv/Vs)=(1+e)

Mixtures dry density increases/decreases with void ratio?

decreases

pd=0 e=infinity

pd min → emax

pd → e

pd max → emin

Relative compaction spans from

0 to 100

between 80 & 100 → between emax and emin

Compaction

= process of increasing the dry density of soil by reducing the volume of air

• Partially saturated soils 

• Soil densification through application of external loads and mechanical mixing 

• Reduction in total volume through removal of air

• Can be done through mixing

Consolidation

• Saturated soils (Vv=Vw)

• Assume solid particles are incompressible

• Soil densification through application of external loads  

• Reduction in total volume through removal of water

Soil compaction effects

Improves soil properties

• Increase stiffness (reduce settlement)

• Increase strength (increase bearing capacity, liquefaction, resistance, etc.)

• Reduce permeability (impermeable layer)

Compacted soil examples

• retaining backwall fill

• road embankment

• earth dams

• offshore pipeline backfills

Field & lab compaction methods for fine-grained mixtures

1. Static load/pressure → ok or ineffective

2. Kneading

3. Impact

4. Vibration → ineffective

Field & lab compaction methods for coarse-grained mixtures

1. Static load/pressure → ok or ineffective

2. Kneading → ok or ineffective

3. Impact → ineffective

4. Vibration

Sand cone test (field)

1. Calibrate device/sand 

2. Choose testing spot (representative, flat) 

3. Prepare testing location (fix base plate) 

4. Dig hole and collect soil (keep moisture) 

5. Place and open sand cone 

6. Close valve and measure final weight

7. Calculate bulk and dry soil densities

Balloon test (field)

1. Choose testing spot (representative, flat) 

2. Prepare testing location (fix base plate) 

3. Adjust balloon pump and take ‘zero’ measurement 

4. Dig hole and collect soil (keep moisture) 

5. Adjust balloon pump and take volume measure 

6. Calculate bulk and dry soil densities

Nuclear density test (field)

Detailed comparison of field compactors

Optimum water content

at maximum dry density

Proctor compaction test

• manually or mechanically

• add water

• mix and incorporate water

• fill mold and separate each vertical slice

• weight part of compacted soil

• dry it and weigh again to find dry density

• start from low w

• minimum of 5 samples

• calculate rho_d

• plot compaction curve

• establish w_opt and rho_d

Effect of soil type on max dry density

decreasing particles sizes = increasing plasticity

→ max dry density decreases

→ optimum water content increases

Effect of compactive effort on max dry density

increasing compaction effort

→ max dry density increases

→ optimum water content decreases

(standard proctor vs modified proctor)

Standard proctor vs modified proctor

Typical field compaction specifications

Dry of optimum

• soils w/ small volume changes from change in wc (granular soils for ex) → use where you need higher strength

Wet of optimum

• soils w/ large volume changes from changes in wc (expansive & collapsible soils for ex) → use where you want to stop water flow