Active Earth Pressure Formula:
σ’h = ka * σ’v - 2 * c’ * SQRT(ka)
where,
ka = (1-sin(Φ))/(1+sin(Φ))
Passive Earth Pressure Formula:
σ’h = kp * σ’v + 2 * c’ * SQRT(kp)
where,
kp = 1/ka
What is expansive soils
- Swelling of wetted soil
- Shrinkage of dry soil
- Unsaturated to saturated and saturated to unsaturated
Clay Minerals in descending order
- Montmorillonite
- Vermiculite
- Illite
- Kaolin
Where does most movement occur
Expansive Soil
1-4m
Most susceptible structures are
Expansive Soil
- Pavements
- Canals
- Pipelines
- Retaining walls
- House Footings
- Light industrial Buildings
What is Soil suction
Expansive Soil “…”
The soil’s affinity for water
The 3 types of suction
Expansive Soil
- Matric Suction
- Solute Suction
- Total Suction
Total = Matric + Solute
Affect of a smaller diameter for matric suction
Expansive Soil
- The greater the curvature
- The higher the liquid tension
What is the formula for effective stress in saturated soil
Expansive Soil
σ’ = σ - Ua + χ(Ua - Uw)
Ua = pore air pressure
Uw = pore water pressure
(Ua-Uw) = Matric suction
What is the parameter “χ”
Expansive Soil - Used for Soil suction
- It varies between 0 and 1
- =1, Saturated
- Empirical estimates of variation of χ based on ratio of Um to Uae
What is pF given as,
Expansive Soil suction in pF = ….
pF = log(suction in kPa) +1.01
When is it active earth pressure
Retaining wall
When the wall is leaning away from the soil
When is it passive earth pressure
Retaining wall
When the wall is leaning into the soil.
