First Term Checklist Flashcards
What are total stress, effective stress and pore water pressure?
Total stress is the sum of external stresses acting on the soil
Pore water pressure is the outward pressure exerted by incompressible water trapped in the space between soil particles
Effective stress is the stress experienced by the soil subtracting the pore pressure from the total stress
How is effective stress (sigma’) calculated?
Total stress (sigma) - pore water pressure (u)
What is the boundary particle size between coarse and fine grained soils?
75 microns
How does soil change volume?
Since soil particles are incompressible, volume change occurs by the reduction in size of voids between particles. Depending on the contents and sizes of the voids, the time for this process can vary
How do you plot a graph of e - log sigma’
Plot e on the y axis and sigma’ on a log scale on the x axis
Define consolidation
The delayed volumetric response to a change in pressure due to gradual movement of water
Define overconsolidation
A state in which a soil is under a lesser stress than it has previously experienced and therefore has a lower Void ratio than would be expected for its type and load
How is the overconsolidation ratio calculated?
The highest stress the soil has experienced divided by the current stress
What is the relationship between a Mohr’s circle for total and effective stress?
The effective stress circle is identical in size to the total stress but it is located closer to zero and closer to the failure line
What is dilation?
Where shear acting on a soil with uneven particles causes these particles to rotate in such a way that the volume of the soil increases
Why are fine grained soils assumed to remain saturated above the water table?
The small particles cause small voids wherein water surface tension causes a capillary effect which draws water above the water table
Is total stress is constant, how do pore pressure and effective stress change during consolidation?
Initially pore pressure rises, causing effective stress to decrease. Over time pore pressure settles back to its original value and effective stress increases
What does it mean if soil is drained?
Water is considered to flow out of a unit of soil instantaneously when pressure is applied
Why are undrained conditions only considered to exist in fine grained soils?
The small voids in fine grained soils prevent water from freely flowing out, whereas in coarse grains this is not the case
What is at rest Earth pressure?
Lateral pressure exerted due to the horizontal expansion of soil as it compresses vertically under its own weight
Why are overconsolidated soils less compressible than non consolidated soils?
The past consolidation has already decreased the volume of voids in the soil, meaning there is a smaller amount left by which the volume can decrease further
What is the difference between drained and undrained shearing?
In drained conditions, water flows in or out to allow volume change
In undrained conditions, volume cannot change so excess pore water pressure is generated
How does overconsolidation ratio affect horizontal effective stress calculation?
The formula for coefficient of Earth pressure K0 is multiplied by the sqrt of OCR
Why does a higher OCR result in higher horizontal stresses?
Particles rearrange during consolidation causing horizontal contact points to become stronger and locking in higher horizontal effective stress
Define critical state
The point at which shearing will occur without any further changes in effective stress, sheer stress or volume
Why does critical state occur?
After sufficient shearing all the initial packing geometry of the soil has taken place
How does vertical effective stress affect the critical state?
Soil with higher effective stress will finish at a denser critical state
How does the stress path travel on a graph of e - sigma’ in drained conditions?
Vertically up or down to the critical state line
How does the stress path travel on a graph of sheer stress against effective stress in drained conditions?
Vertically up. If the soil is initially densely packed, the path may travel up to the peak stress and then down to the critical state line
How does soil pack density affect volume change to critical state?
Loose soil is reduced in volume to the critical state. Dense soils increase in volume to the peak then decrease to the critical state
Where does peak strength occur
Only in dense soils
Why is peak stress reached?
Because additional energy is required to increase the soil volume during dilation
How is the angle of dilation psi defined?
tan psi = - (volumetric stain epsilon/shear strain gamma)
How does the angle of dilation affect peak strength calculation?
It is added to the failure angle phi’ crit
How does progressive failure occur?
As the load increases the stress at the toe of the slope reaches peak stress. After surpassing peak stress the stress at the toe is redistributed to the rest of the failure surface. The stress keeps being redistributed up the slope until the entire surface has failed
How does effective stress affect peak strength?
As effective stress increases the maximum dilation angle and hence the peak failure angle decrease
How are stress paths to the critical state different in undrained shearing?
They are horizontal on the E-sigma’ graph and curved on the tau-sigma’ graph
How is undrained strength calculated
The radius of the Mohr’s circle
Why is the ratio of undrained strength to effective stress constant?
The stress paths are parallel
How does over consolidation ratio affect undrained strength?
As OCR increases, undrained strength increases
What happens in undrained shearing when failure does not occur?
Consolidation, which moves the stress path away from the failure line
Why does strength change at very high shear strains in clay?
As strains get higher and higher the platy clay particles align and create a polished low friction surface with very low strength
