4 - Compressibility Flashcards
What 3 components does ground settlement consist of?
- Consolidation settlement
-Secondary compression settlement - Immediate settlement
What is the total settlement?
The consolidation settlement + secondary compression settlement + immediate settlement
What does consolidation settlement result from?
A decrease in volume of the voids as pore water is squeezed out.
What is the speed at which consolidation settlement occurs a function of?
The permeability.
What does consolidation settlement tend to be associated with?
Clays and silts.
What does secondary compression settlement occur due to?
Particle reorientation, creep and decomposition of organic materials.
Does secondary compression settlement occur quickly or slowly?
Very slowly, and can only be considered to start after the primary consolidation is completed
When does secondary compression settlement tend to be important?
When dealing with soft clays and organic soils.
What us immediate settlement caused by
The elastic deformation of the soil at constant volume, (no change in water).
Does immediate settlement occur quickly or slowly?
Rapidly during the application of applied loading, and is quite small in dense sand/gravels and stiff/hard clays.
Why is settlement of soil unavoidable?
Becuase when you load a material, it is going to deform in some way.
How bad is uniform settlement for a building?
Not too bad, except for utilities under the building.
How bad is tilting with no distortion for a building?
Not great, structure might survive and remediation works used to fix the problem
How bad is distortion for a building?
Disaster for the engineer with a demolish and rebuild required.
What 3 mechanisms is compression of soil due to?
1 - Rearrangement of grains
2 - Fracture and rearrangement of grains
3 - Distortion and rearrangement of grains.
On unloading, what will happen to grain that have been fractured and rearranged?
Grains will no ‘un-arrange’, so volume change on unloading and reloading will be much less than volume change on first loading.
During compression, what are the 2 main properties of soil behaivour?
- Non linear
- Most irrecoverable
Consider a volume (V0) of saturated soil
in equilibrium under a constant total
stress. Initially the total stress is σ0 and
the pore pressure is u0 thus the effective
stress is: σ
0
0 = σ0 − u0
What happens immediately after loading?
Immediately after loading, the total
stress is increased by ∆σ. If there is
no change in volume then ∆σ
0 = 0.
Therefore ∆σ
0 = ∆σ − ∆u = 0. Hence
∆σ = ∆u. The extra load is (initially) carried by the pore water.
Consider a volume (V0) of saturated soil
in equilibrium under a constant total
stress. Initially the total stress is σ0 and
the pore pressure is u0 thus the effective
stress is: σ
0
0 = σ0 − u0.
What will happen after intitial loading drainage will occur?
Water will flow out of the soil and the excess pore pressure starts to dissipate, therefore change in stress > change in pore pressure, and the effective stress increases.
Consider a volume (V0) of saturated soil
in equilibrium under a constant total
stress. Initially the total stress is σ0 and
the pore pressure is u0 thus the effective
stress is: σ
0
0 = σ0 − u0.
What will happen once drainage is compltete?
Change in pore pressure = 0, and u = u0. The effective stress has increased by change in stress, and the volume has decreased by change in volume.
Describe the series of events that happens to an embankment constructured on a soil layer:
1 - On loading, the total stress initially starts to increase, at the same rate as the pore pressure increases, therefore the effective stress remains the same. The volume remains unchanged (UNDRAINED)
2 - After, the total stress remains the same and the effective stress begins to increases at the same rate the pore pressure decreases as water dissipates. The volume begins to decreases as water leaves the pore space. (CONSOLIDATION)
If the rate of drainage is quicker than the rate of loading, what does this mean?
Effective stress and volume chnages occur quickly = drained loading conditions
If the rate of drainage is slower than the rate of change of loading, what does this mean?
The pore pressure increases and the effective stress ad volume remain unchanged = undrained loading conditions.
Under fully drained conditions, what happens to the pore pressure?
It does not change, thus volume changes will occur simultaneously with the increase in loading, and stress.
Under fully undrained conditions, what happens to the pore pressure?
The increase in total stress produces an equal increase in pore pressure, change in effective stress = change in pore pressure.
What is the definition of consolidation?
The dissipation of excess pore pressure, accompanied by volume change.I
If the embankment is wide compared to the depth of the soil layer, what will happen?
Due to the restraining effect of adjacent columns of soil, there will be no horizontal movement, thus there is vertical strain but no horizontal strain.
What is the equation of vertical strain of a wide foundation?
Change in volume / initial volume
=
Change in H / initial height
= change in voids ratio / (1 + initial void ratio )
As the vertical from a foundation is raised or lowered, what happens to the layer at the top of the sample?
It contracts or expands.
When a sample is loaded for the first time, on a voids ratio (y) vs log effective stress (x) graph, what does it track?
Along the ‘normal compression line’ NCL
e = eN - Cc*log(effective stress)
What is Cc on an NCL graph?
The slope
When a sample is unloaded for the first time, on a voids ratio (y) vs log effective stress (x) graph, what does it track?
e = ek -Cs*log(effective stress)
What is Cs on a swelling recompression line (SRL)
The slope
What is the symbol for the compression index?
Cc
What is the symbol for swelling index?
Cs
What is the symbol for recompression index?
Cr
What is the symbol for voids ratio of NCL at effective stress = 1?
eN
What is the symbol for voids ratio of SRL at effective stress = 1?
eK
What is the one-dimensional stiffness modulus?
The slope of the stress/strain curve,
M’ = change in vertical stress / change in volume strain, as horizontal strain = 0
What does E’0 equal?
change in vertical stress / change in vertical strain
What is the reciprocal of the stiffness?
The coefficient of volume compressibility
What does the coefficient of volume compressibility, mv equal?
1 / E’0
Change in vertical strain / change in vertical stress
(initial voids ratio - new ) / 1 + e0 * 1 / change in stress.
Why is the coefficient of volume compressibility not a constant?
It varies with effective stress. It depends on the increment of loading so is not a soil constant.
What is the coefficient of compressibility, av defined as?
mv ( 1 + e0 )
If the current stress of a soil is the maximum stress to have ever been experienced by the soil in its history, what is the soil said to be on?
The Normal Compression Line
What is the largest previous effective stress the soil sample has experienced called?
The pre consolidation stress
What is the Overconsolidation Ratio (OCR) ?
OCR = pre consolidation stress / stress
If OCR > 1, what does this mean?
The sample is overconsolidated
If OCR = 1, what does this mean?
The sample is normally consolidated.
In order to determine the 1-D compression behaviour of soil, what 3 things do we need to know?
Cc, Cs and OCR
What apparatus do we use to determine the 1-D compression behaviour of a soil?
The Oedometer test
What happens in the Oedometer test?
A pressure is applied, and the settlement, change in H is measured.
The vertical strain can then be calculated and subsequently the change in voids.
Repeat for different stress increments and plot on a e - log (effective stress) axis.
After, unload to find the SRL line
From the graph of vertical strain vs log effective stress, what can we obtain?
The Modified Compression Index (C CE)
Cce = change in vertical strain / (log B - log A)
What equation relates Cc to Cce?
Cc = Cce ( 1+e )
The value of pre consolidation stress is identified in the Oedometer test as what?
The turning point - the point that separates the SRL and NCL behaviour. It is the greatest vertical stress that the soil has ever experienced
Casagrande (1936) proposed a standardised method for determining the (Effective stress)p from Oedometer Data, how do you do this?
1 - Determine by eye the point of max curvature on the compression curve (point A)
2 - Draw horizontal line from point A
3 - Draw a line tangent to point A
4 - Bisect the angle made by steps 2 and 3
5 - Extend straight-line portion of the virgin compression curve until it meets the bisector line
6 - The effective stress where the line from step 4 and 5 meet is the (effective stress)p
When soil has fill placed on top, what is the change in voids ratio due to?
The change in effective stress.
What will the change in voids ratio depend on when a soil is compressed?
Whether the clay is normally consolidated (NC) or over-consolidated
If a clay sample is over consolidated and the ratio of the final effective stress to the initital effective stress, what will the sample initially undergo?
Compression under the SRL line until the pre-consolidation effective stress is achieved, after which the compression will follow the NCL.
Do voids change linearly with effective stress?
No, we take the log of the effective stress to calculate changes in void ratios.
What is secondary compression due to?
Changes in soil structure, particle rearrangements and decomposition of organic matter.
What types of soils is secondary compression significant in?
Soft clays and organic soils, but negligible in organic soils.
What is the secondary compression index?
C(alpha) defines the rate of secondary compression that can be defined in either terms of void ratio or strain.
What does the secondary compression index, C(alpha) equal?
Change in e / log time
What do we assume about sand and its drainage?
That it drains immediately
How would you answer this question?
How would you answer this question?
How would you answer part A to this question?
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How would you answer part b and c to this question?
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