Soil Flashcards
A pile dissipates pore water pressure as a function of time according to its equivalent diameter. What is the equivalent diameter for a hollow pile of diameter D, thickness t?
Maintain the area, so Deq = 2 sqrt{D*t}
A CPT is used to predict the BASE resistance of a pile. If the CPT cone stress is q_c, what is the base resistance?
This will only be mobilised for lots of settlement, so calculate how much of q_c is normalised at a settlement w/D [Charts exist for this conversion]
A CPT is used to predict the SHAFT resistance of a pile. If the CPT cone stress is q_C, what is the shaft resistance?
There is a figure in the DB that takes qc for a CPT test, and predicts the corresponding Friction ratio. tau_s = q_c * Friction ratio. Typially, this is about 100x smaller.
A pile is driven into clay. As it passes, the clay experiences cyclic shearing and friction fatigue. After driving, what is the pore pressure state?
Positive pore pressures nearly alwasys everywhere
In what kind of soil might negative pore pressures be generated in pile driving?
Very stiff clays, close to the pile shaft where dilatant shearing was highest.
In very stiff clays close to the pile shaft, negative pore pressures can be generated. What is the pore pressure in the far field?
The far field pore pressure is still positive.
What is set-up?
In most clays, positive pore pressures exist after driving. As these dissipate, the pile stiffens.
What is set-down?
Set-down is rare, but happens in heavily over-consolidated clay. The effective horizontal stress falls shortly after driving.
Why might set-up not happen in stiff clays? (The pore pressure is still positive.)
The pore pressures dissipate, but the true horizontal stresses also fall as the soil contracts away from the pile. The result is no change in effective stress, and no set-up.
A pile has been driven into soft clay. What will happen to the pile stiffness after driving?
Set-up will occur as positive pore pressures dissipate after driving.
A pile has been driven into very stiff clay. What might happen to its stiffness after driving?
Set-down could occur as negative pore pressures dissipate and so effective horizontal stresses fall shortly after driving.
The API method in clay uses seperate correlations for sigma’ > su and sigma’ < su. Why is this?
Highly OC clays (su»_space; sigma’) are dilatant, and so they create large sigma_h’ after driving. This means tau_sf is very large. Such piles are very stiff.
Under pile driving in sand, the axes are initially sigma’ / tau. What are they after the pile head has passed?
p’ / q
Under pile driving in sand, the axes are initially sigma’ / tau. At what point do they change to p’ / q?
After the pile head has passed. The in-situ condition, up to the first failure are in tau / sigma space.
How does the API shaft resistance and true shaft resistance compare in sand?
API is linear up to a point, and then reaches a constant limiting value.
True shaft resistance is very large near the base (where friction fatigue hasn’t occured) and smaller at the top (where friction fatigue has occured).
Why does the API shaft resistance have a limiting value in sands?
The limiting value accounts for friction fatigue, which reduces the strength of very long piles.
What is plugged penetration?
The soil column inside the column moves with the column.
What is unplugged penetration?
The pile plug doesn’t move, the pile is filled with soil.
What is partial penetration?
The soil columns moves downwards, but slower than the pile.
What TWO conditions need to be checked for pile failure?
Plugged and unplugged failure.
Are driven piles normally plugged or unplugged?
Unplugged (the soil level remains at the surface).
Do piles normally fail in a plugged or unplugged manner?
Plugged (the soil moves with the pile).
Soils are normally driven in an unplugged manner, but fail in a plugged manner. Why is there this discrepancy?
There is an additional inertia term in the driving of piles that helps the soil to ‘keep going’.
How does the soil forces on a laterally-loaded pile vary between a free-headed pile and a rigid-headed pile.
The rigid head means the entire height has Pu acting against H; H = Pu * L
The free head means there is a point of rotation. Finding this point is found by taking moments about the head of the pile.
How would you calculate the settlement at the centre of a compliant rectangular foundation?
This is the equivalent of 4 rectangles, with theire corners all touching. The settlement is 4 * w_c, where w_c is the settlement due to q of each of the smaller retangles.
What is the specific volume, v_o of a soil?
v_o = 1 + w * G_s if Sr = 1
Why are deep displacement foundations undesirable?
They are noisy and create vibrations.
Why are deep foundations good?
Give 3 reasons.
They can access deep, stiff soils, can handle M / H better, and can handle scour.
Deep foundations access deep, stiff soils and can handle M/H loading well. Why else are they good?
They can handle scour well (eg a bridge)
A raft has piles coming off it. Why might this be an issue?
The two will interact, increasing settlement and reducing capacity.
What is the zone of influence of a pile?
ln(r_m / R) = 4
r_m = R exp(4)
What is the capacity of a non-displacement pile?
Neglect base stiffness due to disturbance.
What is the horizontal capacity due to embedment in sand?
Kp gamma’ z (integrate over z)
Kp = 1 + sin(phi) / 1 - sin(phi)
Which is larger, Kp or Ka?
Kp = 1 + sin(phi) / 1 - sin(phi) is larger.
This is for something pushing on the soil.
What are the disadvantages of driven piles Vs bored piles?
Noisy, friction fatigue, soil consolidation away from pile.
What are the advantages of driven piles Vs bored piles?
Sigma_h increased by driving, Base preloaded.
Name 4 pile tests.
Maintained Load Test
Constant Rate of Penetration Test
Statnamic Test
Dynamic Test
