Total vs Effective stress design

Question 1

For SLS, the difficulty is is selecting a _____ value representing ______ behaviour, and a _____ ______ for _____ in order to create reasonable predictions for pile performance.

Answer 1

Stiffness value

Non-linear

Question 2

______ is of major concern to foundation engineers

Answer 2

Soil variability

Question 3

Variability of soil properties ______ in importance as the size of the foundation ______

Answer 3

Increases

Decreases

Question 4

Procedure for modelling the soil shear strength variation for pile design purposes:

1) Firstly, the soil….
2) _____ of data removed
3) line, different quantities of data
4) Cu base

Answer 4

1) Firstly the soil needs to be divided up into layers based on geology. Inspection of BH records
2) Outliers of the shear str data removed based on inspection, knowledge of local conds and trial regression analyses
3) Cu avrged down pile shaft. Shaft friction - 50th percentile (avrg line).
- Lots of data –> linear regression
- Little data –> ‘cautious estimate’ or ‘worst credible’
4) Cu base more uncertain - lowest data value. 5th percentile est used

Question 5

How do you determine the 5th percentile?

Answer 5

Draw a lower bound line and shift it until 5% of the values are on the other side. Doesn’t necessarily have to be the same slope as the average line

Question 6

What is the rationale of ULS calcs?

Answer 6

All compatible components of resistance are included. Base capacity added to shaft capacity
F(G+V) <=Qb + Qs

Question 7

The beta method uses _____ ______ to determine the ____ ______

Answer 7

Effective stress
Skin friction (shaft resistance)

Question 8

Typical range of Ks for London clay

Answer 8

0.7Ko < Ks < 1.2Ko

Question 9

Advantage of the b method over the alpha method?

Answer 9

b shows more mechanisms than a and is more useful when you have little/no load/test data.

If you have loads of test data, might aswell use a

Question 10

Parry and Swain (1977) points of comparison between total and effective stress analysis?

1) _____ exceeds ______
2) Since failure occurs….
3) Alpha values in LDN clay….
4) Which is more reliable

Answer 10

1) Lateral ES is likely to exceed vertical ES
2) Since failure occurs on surfaces inclined to the soil-pile interface, full Ko cannot be used in the determination of skin friction due to rotation of principal stress planes at failure
3) alpha values in LDN clay can be explained using EF theory - generally considered appropriate
4) TS probs more reliable for design than ES - lots of test data available to determine alpha values

Question 11

Which angle of friction should be used for ES analysis (for beta method)?

Answer 11

Residual for driven piles and critial for bored

Question 12

_______ of stiff clays must be considered when designing using a drained soil parameter

Answer 12

Fissuring

Question 13

What is Ko

Answer 13

Coefficient of earth pressure at rest Ko=1-sin(phicrit)

Horiz/Vert stress

Question 14

Uncertainties due to construction sequence, why is the beta method preferred?

• fully drained soil str more relevant, why?
• original UD str altered how? and what does this mean we need?
• no clear relationship…..

Answer 14

• major shear distortion confined to a thin zone around the pile shaft and any drainage following loading must be rapid - fully drained soil strength more relevant
• driving remoulds soil adjacent to pile, boring permits swelling and softening. Both alter original UD str hence need for adjustment factor
• no clear relationship between UD str and drained str at pile/soil interface

Question 15

Sequence of stresses during construction and loading of a bored pile? 8
1)

Answer 15

1) Initial stresses total and effective, horiz and vertical stress plus pwp factored by reduction factor Fu
2) UD cavity contraction from initial horiz stress to sigma(m) in the bore
3) Local swelling and softening around the bore due to the reduction in radial ES
4) UD cavity expansion from mud pressure sigma(m) to the wet concrete pressure sigma(c)=unitWconcz
5) radial transient flow to regain the initial pwp (uo) in clay, together with shrinkage of the set concrete, will cause small changes in the radial stress on the shaft sigma(s)=UWconcz
6) Drained axial shearing may causes dilation of the clay at the pile interface, leading to a further small change in the radial stress sigma(s) on the shaft as peak shear stress develops (eqn)
7) Pile axial displacement, w, initially occurs by dragging down the adjacent soil, but at some limiting value, w=wslip it begins to slip at the peak shaft shear resistance
8) as axial displacement continues to increase, with w>wslip the shear stress falls

Question 16

How to calculate Ks

Answer 16

Use equation with fu factor, calculate at top(water table) and bottom then average

Question 17

5 states of soil

Answer 17

Intact, destructured, remoulded, resedimented and reconstituted.

I DON’T REALLY RATE ROONEY