Compressibility and 1D settlement Flashcards
What happened to in an embankment settlement in the USA?
20th-century engineers constructed a railway embankment over glacial clays, silts and sands in Minnesota. The problems began shortly after construction of the embankment. This extra weight of the embankment on the soil below caused a lot of settlement and the embankment sunk. Three years later, the project was $700,000 over the original $2,100,000 budget and approximately 5,000,000 m^3 fill had been placed with only 1,000,000 m^3 still showing.
Why does the leaning tower of Piza lean?
It was built on two different types of soils so there was differential consolidation settlement over many years. which means one soil settled after than the other and made the tower lean.
what is the 1-D compression concept? and name some examples of this.
- Soil elements deform only in the direction of the applied load.
- soil is constrained laterally in the other directions.
For example,
- natural deposition & erosion of soil.
- the foundation of a larger plan area in compression with depth (raft foundation).
- preloading to strengthen the soil.
What is swelling?
When soil is loaded it compressed and the volume decreases. Then when this applied force is removed the soil will then expand and increase on volume. This is called swelling.
Describe the set up of the oedometer.
-the sample is thin so side friction is low and drainage can occur quickly.
-Soil is constrained laterally by a steel ring, so only 1D settlement.
-Drainage supplied at the top and bottom.
-There is a water bath surrounding the sample so it remains saturated during the test.
Deformation is in the direction of loading, no lateral strain.
- a piston is placed on top of the soil to apply the load in the test.
What are the two types of test data given by the oedometer test:
- Ultimate settlement data: taken after the system has come to equilibrium.
- Transient state data: taken during the consolidation process.
What happens in the oedometer ultimate settlement tests?
Engineers typically apply several vertical loading increments:
- Each increment results in additional final settlement of the piston, which is plotted against applied stress.
- Each decrement results in a slight reduction in settlement which is also plotted against applied stress.
What results can we get from the oedometer test?
- Soil becomes harder to compress as effective stress increases.
- Soil is also stiffer upon unloading and reloading than upon first-time loading (normal compression).
- The swelling is when the soil sample is unloaded.
Young’s modulus (E) = ?
E = (/\F / A) / (/\x/x)
Constrained modulus (Eo’) = ?
Eo’ = sigmav’ / /\Ev = 1 / Mv
Mv= modulus of compression.
( the subscript o is to show it is 1D settlement)
Stiffness parameters: what is the same in the field and in the oedometer test?
- same density at the start of loading
- Same load/unload history
- Same stress-range during loading.
What does vertical strain equal in an oedometer test?
What does:
Volume =
/\V =
Ev = /\h / hi = (/\e*Vs) / ((1+ei)*Vs) = (/\V*Vs) / (Vi*Vs) i=initial h= height e= void ratio Vs= volume of solid
V = 1 + e /\V = /\e
Initial volume equals? (in terms of hi, hf, Wf, Gf)
Vi = hi/hf * ( 1 + Wf*Gf)
Gs =. specific gravity
Ws = water content
Sr = 1 usally
What happens on the first time loading (normal compression)?
- Particles of soil rearrange
- So new deformation is due to slippage of particles into new positions more efficient at resisting higher loads.
- This is PLASTIC deformation
- The higher the applied stress, the denser the packing and the harder it becomes to compress anymore.
What happens on unloading/reloading?
- Particles of soil do not need to rearrange to take higher stress.
- So deformation is due to expansion/compression of solid particles themselves and small reversible particle rotations.
- this is ELASTIC deformation.
