Basics of Soil Mechanics- Granular VS Cohesive Flashcards
State two examples of granular soils, stating their permeability range
- Gravel- 10-3 m/s - 10-1 m/s
- Sand- 10-7 m/s - 10-3m/s
Granular soils are highly permeable
State two examples of cohesive soils, stating their permeability range
- Silt- 10-9 m/s - 10-7 m/s
- Clay- <10-9m/s
Cohesive soils are less permeable
What does a permeability 10-1 m/s mean?
Consider a soil column that is 1m tall, it would take 1 second for water to flow from the top of the sample to the bottom.
What does the hydro-mechanical behaviour of soils strongly depend on?
Permeability
Describe how permeability affects granular soils and relate this to the saturated soil model
- Granular soils (highly permeable) exhibit large pores where water is easily drained under the effect of external loads
- This can be modelled by a large opening
For granular soils what are drained conditions?
The application of an external load does not induce pore water overpressure as water can drain easily.
(Δu = 0 and Δσ = Δσ)
Sketch a graph to show the variations in u, σ and σ’ when we are constructing, for example, a 5 story building on granular soils.
How many years can it take for water to drain from the top surface to the bottom in cohesive soils?
30 years
Describe the effect of permeability in cohesive soils and how this is related to the saturated soils model.
- Cohsive soils (low permeability) exhibit small pores where water cannot drain easily under the effect of external loads
- This can be modelled by a small opening
What is the consequence of low permeablity?
Water cannot drain easily and hence the application of external loads induces pore water overpressure
How many years can it take for the dissipation of these overpressures?
100 years
Sketch a graph to show how σ, σ’ and u relate when loads are applied on cohesive soils.
For cohesive soils when will drained and undrained conditions apply.
- Undrained conditions are short term
- Drained conditions are long term
